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[SQUEAKING]

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[RUSTLING]

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PROFESSOR: So I'm
going to come to 8.20--

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Special Relativity.

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It's a great pleasure to
introduce David Kaiser.

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David is a faculty in
the physics department.

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He's also a
historian of science.

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So he's in a super
great position

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to talk about Einstein
and special relativity

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and give kind of the
frame for this class.

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So please welcome David.

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And take it along.

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DAVID KAISER: Great.

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Well, hello, everyone.

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Happy New Year.

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Welcome back at least
virtually to MIT.

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It's really a great pleasure
to talk with you today

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about this material.

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I love this material.

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So hopefully you can all
see that first slide--

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smiling Einstein on the bicycle.

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OK.

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So I want to talk about
three main parts today

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for the material.

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And we'll talk about
how all of the most

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accomplished physicists were
thinking about motion of bodies

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through space and time
during the middle, late years

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of the 19th century, roughly,
say, 100 to 150 years ago,

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give or take, because
that really sets up

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a pretty sharp contrast
with how at the time

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a rather young and
very little known

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person named Albert Einstein
began asking similar questions

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but often in very
different ways.

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So we'll start with some
of this context of what

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was happening before Einstein
even came on the scene

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to help us better make sense
of why his approach seemed

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really so unfamiliar and
so surprising at the time.

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And then for the last part--

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I find it really
fascinating, drawing on work

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from some of my friends and
colleagues, other historians--

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we'll try to ask, what was
going on with Einstein?

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Why was Einstein's
approach so different?

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And can we make sense of
it since it wasn't just

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the ordinary routine that we
might have otherwise expected?

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That's that last part
about coordinating clocks.

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Well, let's jump right in.

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Let's start not
with Albert Einstein

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but with another very familiar
name, James Clerk Maxwell.

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I thought about growing a
beard like that during COVID.

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I haven't made much progress.

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But that's a typical 19th
century, fine Cambridge beard--

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Cambridge, England.

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So we all know Maxwell's name.

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Many of you probably
own a t-shirt

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with Maxwell's equations on it.

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If you do, I'm very jealous.

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I don't have one anymore.

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Anyway, so we still
use his approach

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to electromagnetism, as of
course you all know very well.

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And we've been able
to boil it down

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to basically a tweet
or a single t-shirt.

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It turns out, as
you may know, what

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we now call Maxwell's equations
were hammered out by Maxwell

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and actually some
other colleagues

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during the 1860s
and early 1870s,

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so approximately 150 years ago.

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And in fact when he
first wrote them down,

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they weren't in such
a nice, compact form.

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It was in a 900-page, two-volume
treatise that was first

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published in 1873--

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two volumes, a
total of 900 pages.

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It was a weapon.

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You could hurt someone
with these books,

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it was so fat, even though
now we can boil it down

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to a simple t-shirt.

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And so what's even
more interesting to me

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is that even though we still
use Maxwell's equations, what

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we think they say
about the world

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is really, really
different than what

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Maxwell and his
immediate circle thought.

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And the biggest
difference of all

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is that for Maxwell and really
for all of his contemporaries

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and his students, the equations
of electricity and magnetism,

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as far as they were
concerned, had everything

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to do with some
physical substance,

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a medium that they called
the ether that they assumed

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must be spread evenly
throughout the entire universe,

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filling every nook
and cranny of space.

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So all of the phenomena
that we would associate

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with electricity or magnetism,
the flow of currents,

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the splaying of iron
filings around a bar magnet,

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and even much more
complicated things,

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to Maxwell and his group,
these were all just

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evidence of the state of
stress of this underlying

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physical medium.

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It was like a springy,
substantial, resistive medium

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that they called the ether.

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And he says that in the
opening pages, for example,

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of his now very famous
treatise and throughout,

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that the whole point of studying
this field, as far as he

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was concerned, was to study the
behavior of the distribution

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of stress in this medium
extending continuously

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throughout the
universe, the ether.

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It wasn't just Maxwell.

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His a little bit more senior
colleague, William Thomson,

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who went on to be known as
Lord Kelvin, like the Kelvin

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temperature scale
and many things

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we still use from
Thomson's work,

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he wanted to give a
sense to non-scientists

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a few years later in
a popular lecture what

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all the excitement was about.

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He said, here's
what we're doing.

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We want you to get a sense
for what we're doing.

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We're studying the physics of
this elastic, physical medium

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called the ether.

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And he instructed his very
elite, very fancy audience

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who went to one of
these popular lectures,

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stick your hand in a bowl
of jelly, he tells them.

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And see how it
wiggles and vibrates

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as you move your hand around.

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That's the highest of high-end
physics in the 1860s and '70s.

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It was all about
understanding the behavior

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of this elastic medium
called the ether.

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And that had very specific
follow-on implications

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for this group.

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So one of the first things
that you all know already,

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one of the most exciting
features of Maxwell's work

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that really got himself excited
and solidified his reputation

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was this unexpected unification
that he put together

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in the 1860s.

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Not only was there
a deep relationship

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between electricity
and magnetism.

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That had been wondered about
but never really formalized.

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But even more surprising,
that these two areas,

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electricity and
magnetism, were also

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deeply associated with optics,
with the propagation of light.

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So it was Maxwell, as I'm
sure you know and probably

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have already done
this on problem sets

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by now yourselves--

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using Maxwell's
equations, you can

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derive the quantitative
behavior of light

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as it travels through space.

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Light, as Maxwell
intuited, was nothing

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but a certain pattern
of propagating

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electric and magnetic fields.

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Again, to Maxwell, these were
propagating in this material,

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physical ether.

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They were disturbances
propagating,

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skittering through this physical
stuff that filled all of space.

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And in fact, by trying
to calculate the speed

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with which disturbances would
travel through space based

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on other properties
of the ether that he

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and his colleagues
had already measured,

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basically sets up
like spring constants,

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he found that the
speed of propagation

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would be equal to the
value that had already

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been calculated many years
before for the speed with which

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light travels.

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And he says in his own words,
in this lovely old-fashioned

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phrasing, "The velocity of
transverse undulations"--

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a certain kind of wave--

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"in our hypothetical
medium agrees so exactly

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with the velocity
of light calculated

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from optical experiments"--
mostly astronomy at the time--

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"that we can scarcely
avoid the inference

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that light consists of
the transverse undulations

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of the same medium
which is the cause

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of electric and
magnetic phenomena."

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That's why his book
was 900 pages, right?

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Today we can put
in on a t-shirt.

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He was really just walking
through this very old fashioned

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language to come
to the point saying

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light was nothing but a certain
kind of pattern in the ether.

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And so in fact they often
called this not just the ether

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but an even fancier name.

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They called it the
luminiferous ether.

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And from the Latin, it
means "light bearing"

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or "light carrying."

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So "lumin", like "lumos" for
you Harry Potter fans, that part

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you probably recognize.

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And the "ferous" is
like a Ferris wheel.

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That means, like, to carry
or to move through space.

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So this was the light-bearing
or light-carrying ether.

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And that's what
Maxwell and really all

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of his contemporaries
were convinced

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was most exciting
about Maxwell's work,

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that this was a way
of characterizing

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the state of the ether including
these optical phenomena.

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OK, so that's where things
stood in the 1860s and 1870s.

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But that led to a whole
new set of questions.

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And the next generation
quite understandably

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wanted to kind of
follow up on that

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and ask ways to
generalize that framework.

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Maxwell's work
had really assumed

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that there was no relative
motion between either

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the source of light or
the receiver of light,

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that everything was analyzed
in what we would now

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call the same rest frame,
or frame of reference,

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as the ether itself.

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But of course that wasn't
the most general situation,

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as Maxwell's own contemporaries
or the next younger generation

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began to wonder about.

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One of the most influential
of that next wave

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is this gentleman shown
here, Hendrik Lorentz

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from the Netherlands,
from Leiden.

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So Lorentz wanted to ask about
the electrodynamics of moving

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bodies.

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What if either the emitter of
light or the receiver or both

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were moving with respect to
this all-pervasive medium?

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And so Lorentz,
as we'll see, had

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really two distinct
motivations in mind

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when he tried to think about
the electrodynamics of moving

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bodies, or let's say optics for
moving sources or receivers.

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He had a kind of mathematical
set of quandaries.

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And we'll talk
about those first.

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But he also had a series of very
puzzling experimental results.

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So he had an
empirical set of ideas

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that we'll come to in a
moment and also these more

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formal or mathematical.

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We'll start with the math.

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Lorentz was really one of
the best-trained, leading

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mathematical
physicists of his age.

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So he knew very well how
to handle relative motion.

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Galileo had derived
that in the 1610s.

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That was hardly
news in the 1890s.

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We would do what we still
call the Galileo-Newton

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transformation.

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For example, Galileo
wrote very famously

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in his charming dialogues
in the 17th century,

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if you watch your friend float
down a river on a boat that

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moves at a constant speed,
neither speeding up nor slowing

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down, then you
know very well how

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to compare the coordinates
between you, sitting

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at rest on the
bank of that river,

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and your friend as she
floats down on her boat,

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that what you call the
origin, the spatial origin,

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of your coordinate system, you
can fix at, say, x equals 0.

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But your friend could call
the center of her boat

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the origin for her
coordinate system.

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And then you can see how
her system coordinates will

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move with respect to yours.

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If the boat moves
at a constant speed,

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little v, then you
just have to calculate

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00:11:00,650 --> 00:11:04,430
the offset between where the
origin, the center of her boat,

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is as it floats further
and further down the river.

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So what she calls the origin
of her spatial coordinates

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will be offset from yours
because this relative motion,

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this relative drift.

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Meanwhile, as Newton
himself famously said

251
00:11:19,100 --> 00:11:22,520
in the beginning
of his Principia

252
00:11:22,520 --> 00:11:25,370
where we learn about
Newton's laws of mechanics,

253
00:11:25,370 --> 00:11:26,900
that time is time is time.

254
00:11:26,900 --> 00:11:28,460
There's some absolute time.

255
00:11:28,460 --> 00:11:30,950
He actually referred it
to the sensorium of God.

256
00:11:30,950 --> 00:11:34,080
He thought there was only one
possible time to consider.

257
00:11:34,080 --> 00:11:35,690
And so formally
there'd be no change

258
00:11:35,690 --> 00:11:37,940
to the rate at which
your clock ticked

259
00:11:37,940 --> 00:11:40,730
versus the rate at which
your friend's clock ticked

260
00:11:40,730 --> 00:11:41,810
on her boat.

261
00:11:41,810 --> 00:11:44,180
This is called the
Galileo-Newton transformation.

262
00:11:44,180 --> 00:11:45,380
I made a big deal out of it.

263
00:11:45,380 --> 00:11:47,380
You probably would have
done this in your sleep.

264
00:11:47,380 --> 00:11:49,260
Lorentz did this in his sleep.

265
00:11:49,260 --> 00:11:51,085
However, it drove
him to nightmares

266
00:11:51,085 --> 00:11:52,460
because when he
then applied this

267
00:11:52,460 --> 00:11:54,770
to Maxwell's really
beautiful and much newer

268
00:11:54,770 --> 00:11:56,780
set of equations for
things like optics

269
00:11:56,780 --> 00:12:00,020
or the propagation of electric
and magnetic disturbances

270
00:12:00,020 --> 00:12:02,180
in the ether, he
found that in that

271
00:12:02,180 --> 00:12:04,220
transformed reference
frame if you

272
00:12:04,220 --> 00:12:07,550
take into account that relative
motion between either emitter

273
00:12:07,550 --> 00:12:11,300
or receiver of light, then
Maxwell's beautiful description

274
00:12:11,300 --> 00:12:15,470
of light, his quantitative
description, looked very ugly.

275
00:12:15,470 --> 00:12:17,330
And in particular,
it would no longer

276
00:12:17,330 --> 00:12:20,300
suggest that light should
behave as oscillating

277
00:12:20,300 --> 00:12:21,770
sines and cosines.

278
00:12:21,770 --> 00:12:23,760
It worked beautifully
with Maxwell's equations

279
00:12:23,760 --> 00:12:29,090
if you look only at both sources
and receivers of light that

280
00:12:29,090 --> 00:12:31,880
are fixed in place and
not moving with respect

281
00:12:31,880 --> 00:12:32,870
to the ether.

282
00:12:32,870 --> 00:12:36,560
Once you apply the very, very
well-known transformation

283
00:12:36,560 --> 00:12:38,330
to take into account
relative motion,

284
00:12:38,330 --> 00:12:41,120
then your description of
light gets all literally out

285
00:12:41,120 --> 00:12:42,020
of whack.

286
00:12:42,020 --> 00:12:44,750
And this was a mathematical
conundrum for Lorentz.

287
00:12:44,750 --> 00:12:47,060
It didn't seem to make any
sense because on the Earth,

288
00:12:47,060 --> 00:12:50,762
on the moving Earth presumably
moving through the ether,

289
00:12:50,762 --> 00:12:52,220
he and his colleagues
could measure

290
00:12:52,220 --> 00:12:55,590
light to behave like beautiful
sines and cosines all the time.

291
00:12:55,590 --> 00:13:00,170
So we have an empirical reminder
that Maxwell's description

292
00:13:00,170 --> 00:13:03,260
seems really, really accurate
for describing optics

293
00:13:03,260 --> 00:13:06,020
even on our own moving
ship of the Earth.

294
00:13:06,020 --> 00:13:08,340
So this is a real
conundrum for him.

295
00:13:08,340 --> 00:13:10,760
He was a very, very
clever, mathematically

296
00:13:10,760 --> 00:13:12,860
gifted physicist.

297
00:13:12,860 --> 00:13:15,080
So he introduced a
mathematical clue.

298
00:13:15,080 --> 00:13:20,210
He introduced a mathematical
stopgap to address this.

299
00:13:20,210 --> 00:13:22,160
He called it local time.

300
00:13:22,160 --> 00:13:24,140
And he was actually very
clear in his writings.

301
00:13:24,140 --> 00:13:26,160
He said it was a
mathematical fiction--

302
00:13:26,160 --> 00:13:26,660
his word.

303
00:13:26,660 --> 00:13:29,780
It was literally just
a mathematical trick.

304
00:13:29,780 --> 00:13:31,970
If he introduced a
new transformation

305
00:13:31,970 --> 00:13:35,840
for the time variable, so when
you compare your coordinates

306
00:13:35,840 --> 00:13:37,770
with those of your
friend on the boat,

307
00:13:37,770 --> 00:13:41,600
what if you actually did make
a transformation of t prime,

308
00:13:41,600 --> 00:13:43,580
not just of x prime?

309
00:13:43,580 --> 00:13:45,650
And the transformation
he kind of worked out,

310
00:13:45,650 --> 00:13:47,690
he reverse engineered
the form it

311
00:13:47,690 --> 00:13:50,690
would need to have so
that Maxwell's description

312
00:13:50,690 --> 00:13:54,470
of light waves would be
restored to the very simple form

313
00:13:54,470 --> 00:13:57,600
that Maxwell himself had written
down a few years earlier.

314
00:13:57,600 --> 00:13:59,810
So this is where we still
call this the Lorentz

315
00:13:59,810 --> 00:14:00,590
transformation.

316
00:14:00,590 --> 00:14:01,550
Many of you might have seen it.

317
00:14:01,550 --> 00:14:03,717
If not, you'll have plenty
of time to learn about it

318
00:14:03,717 --> 00:14:05,480
and practice it during IAP.

319
00:14:05,480 --> 00:14:07,490
We call it Lorentz,
not Einstein,

320
00:14:07,490 --> 00:14:10,820
because Lorentz derived these
equations first in a series

321
00:14:10,820 --> 00:14:13,250
of papers in the 1890s.

322
00:14:13,250 --> 00:14:16,340
And he was responding to this
question of the electrodynamics

323
00:14:16,340 --> 00:14:17,300
of moving bodies.

324
00:14:17,300 --> 00:14:20,630
How do you preserve this
beautiful description of light

325
00:14:20,630 --> 00:14:24,080
from Maxwell even if there's
relative motion with respect

326
00:14:24,080 --> 00:14:25,100
to the ether?

327
00:14:25,100 --> 00:14:26,190
He said it was a trick.

328
00:14:26,190 --> 00:14:27,380
It was merely mathematics.

329
00:14:27,380 --> 00:14:31,180
But it would at least
get the right form back.

330
00:14:31,180 --> 00:14:33,540
So that was the
mathematical motivation.

331
00:14:33,540 --> 00:14:35,730
Lorentz, as we know,
was also concerned

332
00:14:35,730 --> 00:14:39,180
about an empirical or
experimental curiosity.

333
00:14:39,180 --> 00:14:41,850
He was following
very closely the work

334
00:14:41,850 --> 00:14:44,460
of a US-based physicist
named Albert Michelson.

335
00:14:44,460 --> 00:14:46,270
Michelson's story is
really fascinating.

336
00:14:46,270 --> 00:14:49,290
I won't take too much time
now, but super interesting.

337
00:14:49,290 --> 00:14:53,010
He was actually an immigrant
from what's now Germany,

338
00:14:53,010 --> 00:14:55,650
was often part of
Poland, central Europe.

339
00:14:55,650 --> 00:14:58,050
He moved to the US when he
was about two years old, very

340
00:14:58,050 --> 00:14:59,910
young, and was raised
right around the time

341
00:14:59,910 --> 00:15:01,932
of the gold rush in California.

342
00:15:01,932 --> 00:15:03,390
And before I get
carried away, I'll

343
00:15:03,390 --> 00:15:04,800
stop talking about
Michelson, although he's

344
00:15:04,800 --> 00:15:05,670
super interesting.

345
00:15:05,670 --> 00:15:08,400
So what Michelson
wound up doing was

346
00:15:08,400 --> 00:15:11,250
he was fascinated by this
question of Maxwell waves,

347
00:15:11,250 --> 00:15:14,730
of waves of electricity
and magnetism in the ether.

348
00:15:14,730 --> 00:15:16,920
And he was convinced that
if we are on this moving

349
00:15:16,920 --> 00:15:19,560
ship of the Earth moving
through the ether,

350
00:15:19,560 --> 00:15:23,370
that should have a measurable
impact on the light waves

351
00:15:23,370 --> 00:15:27,600
that we can produce and
measure here on Earth.

352
00:15:27,600 --> 00:15:30,250
So he set really his life's
goal starting from very,

353
00:15:30,250 --> 00:15:32,850
very early trying
to build devices,

354
00:15:32,850 --> 00:15:35,550
extremely sensitive
instruments with which

355
00:15:35,550 --> 00:15:37,530
he could measure the
fact that the Earth was

356
00:15:37,530 --> 00:15:40,080
moving through this ether.

357
00:15:40,080 --> 00:15:42,090
Like all his
contemporaries, Michelson

358
00:15:42,090 --> 00:15:44,820
believed without any
hesitation that there must

359
00:15:44,820 --> 00:15:48,030
exist the material, the ether.

360
00:15:48,030 --> 00:15:51,630
What else could light waves be
but disturbances in the ether?

361
00:15:51,630 --> 00:15:53,820
And then he figured
that much like

362
00:15:53,820 --> 00:15:57,540
if a bicyclist going
for a bike ride,

363
00:15:57,540 --> 00:15:59,040
we should be able
to tell that we're

364
00:15:59,040 --> 00:16:00,550
moving through that medium.

365
00:16:00,550 --> 00:16:03,390
So before I describe his device
called the interferometer,

366
00:16:03,390 --> 00:16:06,300
let me just give a little more
of an analogy for why Michelson

367
00:16:06,300 --> 00:16:09,690
was so convinced there should
be a measurable effect.

368
00:16:09,690 --> 00:16:12,900
If we walk outside our houses,
which is now a luxury--

369
00:16:12,900 --> 00:16:15,000
we can still do
it with masks on--

370
00:16:15,000 --> 00:16:18,930
walk outside on a still day when
there's no wind or no breeze,

371
00:16:18,930 --> 00:16:21,090
we won't feel any
particular wind on our face

372
00:16:21,090 --> 00:16:22,660
if we're standing still.

373
00:16:22,660 --> 00:16:24,460
So we're outside in
a physical medium.

374
00:16:24,460 --> 00:16:26,230
There's a physical atmosphere.

375
00:16:26,230 --> 00:16:28,740
But if we're at rest with
respect to that atmosphere,

376
00:16:28,740 --> 00:16:31,620
if there's no wind, we
don't feel it on our face.

377
00:16:31,620 --> 00:16:34,170
On the other hand, if
it's still a still day,

378
00:16:34,170 --> 00:16:36,780
there's no breeze, when
we get on our bicycle

379
00:16:36,780 --> 00:16:40,290
and pedal really quickly, we'll
now feel a breeze on our face

380
00:16:40,290 --> 00:16:43,290
because we're moving through
that physical medium.

381
00:16:43,290 --> 00:16:45,510
We're moving through
the Earth's atmosphere.

382
00:16:45,510 --> 00:16:47,670
Even if the atmosphere
seems to be perfectly

383
00:16:47,670 --> 00:16:49,920
at rest, no strong
breeze, no hurricane,

384
00:16:49,920 --> 00:16:52,560
whatever, if we're going
quickly enough through it,

385
00:16:52,560 --> 00:16:54,450
we'll feel a breeze on our face.

386
00:16:54,450 --> 00:16:55,950
And Michelson thought
the same thing

387
00:16:55,950 --> 00:16:57,930
should be happening
for light waves

388
00:16:57,930 --> 00:17:01,980
as they are carried by the
Earth through this medium, not

389
00:17:01,980 --> 00:17:05,400
the Earth's atmosphere but this
all-pervasive, light-bearing

390
00:17:05,400 --> 00:17:06,240
ether.

391
00:17:06,240 --> 00:17:08,430
So he really thought it
would be like the bicyclist.

392
00:17:08,430 --> 00:17:12,000
Now, his real genius was
devising this device,

393
00:17:12,000 --> 00:17:13,800
we now call it the
interferometer,

394
00:17:13,800 --> 00:17:15,390
with which he could
try to measure

395
00:17:15,390 --> 00:17:19,140
the impact of that motion of
the Earth, the entire Earth,

396
00:17:19,140 --> 00:17:20,640
through the medium.

397
00:17:20,640 --> 00:17:22,260
So he took a source of light.

398
00:17:22,260 --> 00:17:24,960
Today we use lasers
because they're awesome

399
00:17:24,960 --> 00:17:26,640
and they're also monochromatic.

400
00:17:26,640 --> 00:17:30,540
Their light shines basically
at one frequency, or very

401
00:17:30,540 --> 00:17:32,040
dominantly one frequency.

402
00:17:32,040 --> 00:17:34,530
Lasers of course weren't
available to Michelson.

403
00:17:34,530 --> 00:17:38,040
So he used very bright
sodium arc lamps.

404
00:17:38,040 --> 00:17:41,580
Sodium lamps will shine
mostly at one dominant color,

405
00:17:41,580 --> 00:17:43,770
not nearly so monochromatic
as modern lasers.

406
00:17:43,770 --> 00:17:46,600
But they were the industry
standard at the time.

407
00:17:46,600 --> 00:17:48,060
So he basically
took sodium lamps

408
00:17:48,060 --> 00:17:51,270
that mostly shine a particular
yellow color, shined them

409
00:17:51,270 --> 00:17:53,490
at what we now call
a beam splitter.

410
00:17:53,490 --> 00:17:56,340
It was a half-silvered
mirror, as its name implies,

411
00:17:56,340 --> 00:17:58,290
that would let half
the light through.

412
00:17:58,290 --> 00:18:01,240
It would act like a window
roughly half the time.

413
00:18:01,240 --> 00:18:03,850
But it would act like a mirror
the other half of the time.

414
00:18:03,850 --> 00:18:07,050
So half of this incident beam
will go right through this

415
00:18:07,050 --> 00:18:09,420
as if it were just
a plate of glass.

416
00:18:09,420 --> 00:18:11,010
It would be transmitted.

417
00:18:11,010 --> 00:18:13,470
It will then make its way to
a fully reflecting mirror--

418
00:18:13,470 --> 00:18:16,385
sorry-- a fully reflecting
mirror at the end of the path.

419
00:18:16,385 --> 00:18:17,760
It'll be reflected
and come back.

420
00:18:17,760 --> 00:18:20,400
And then, again, half
of that return light

421
00:18:20,400 --> 00:18:23,910
will be reflected by the mirror
and come out to a screen.

422
00:18:23,910 --> 00:18:26,190
Meanwhile, you can run
the same kind of story

423
00:18:26,190 --> 00:18:29,190
for light that gets
deflected towards path 2.

424
00:18:29,190 --> 00:18:32,400
From that same, single
incident source,

425
00:18:32,400 --> 00:18:33,900
half of the incident
beam will be

426
00:18:33,900 --> 00:18:36,990
reflected by that beam splitter,
that half-silvered mirror.

427
00:18:36,990 --> 00:18:40,300
It'll travel a path down to a
fully reflecting mirror, come

428
00:18:40,300 --> 00:18:40,800
back.

429
00:18:40,800 --> 00:18:43,900
And then half of that returning
light will be transmitted.

430
00:18:43,900 --> 00:18:46,350
So you have light coming
together on some detector

431
00:18:46,350 --> 00:18:49,500
screen, that it started
out as a single light wave

432
00:18:49,500 --> 00:18:52,350
from a single, nearly
monochromatic source.

433
00:18:52,350 --> 00:18:54,570
But you split it into
two beams so the beams

434
00:18:54,570 --> 00:18:56,490
travel different paths.

435
00:18:56,490 --> 00:18:59,640
And now the light starts out
fully in phase with itself.

436
00:18:59,640 --> 00:19:00,660
It's one light beam.

437
00:19:00,660 --> 00:19:03,720
So crests are with crests
and troughs with troughs.

438
00:19:03,720 --> 00:19:05,580
And the idea was
that you then split

439
00:19:05,580 --> 00:19:09,270
it to set up a race
to see if there

440
00:19:09,270 --> 00:19:11,820
was any difference in
the travel for light

441
00:19:11,820 --> 00:19:13,740
that traveled through
path 1 and out

442
00:19:13,740 --> 00:19:15,750
to the screen versus
light that traveled

443
00:19:15,750 --> 00:19:18,090
path 2 and back to the screen.

444
00:19:18,090 --> 00:19:21,420
All the light starts as a single
light wave in sync or in phase

445
00:19:21,420 --> 00:19:22,380
with itself.

446
00:19:22,380 --> 00:19:25,110
If there was any
difference in the travels

447
00:19:25,110 --> 00:19:26,970
of that light between
path 1 and path 2,

448
00:19:26,970 --> 00:19:30,210
you should see a shift in
the interference pattern.

449
00:19:30,210 --> 00:19:33,120
Crests should no longer
arrive lining up with crests

450
00:19:33,120 --> 00:19:34,210
or troughs with troughs.

451
00:19:34,210 --> 00:19:35,877
You should see a very
characteristic set

452
00:19:35,877 --> 00:19:37,140
of interference fringes.

453
00:19:37,140 --> 00:19:38,880
What is called an
interferometer,

454
00:19:38,880 --> 00:19:41,070
extremely sensitive.

455
00:19:41,070 --> 00:19:43,980
Now, his idea was if,
like the bicyclist,

456
00:19:43,980 --> 00:19:47,700
we're on the moving Earth, then
the path is heading directly

457
00:19:47,700 --> 00:19:48,720
into the ether.

458
00:19:48,720 --> 00:19:51,780
That is, say, the path that's
along the Earth's motion

459
00:19:51,780 --> 00:19:54,840
through the ether should be
just like the bicyclists feeling

460
00:19:54,840 --> 00:19:58,260
the direct headwind of
that breeze directly

461
00:19:58,260 --> 00:20:00,900
on her or his face.

462
00:20:00,900 --> 00:20:02,650
That means the effective speed--

463
00:20:02,650 --> 00:20:05,160
let's say this is the direction
of the Earth's motion.

464
00:20:05,160 --> 00:20:07,080
The effective speed with
which this light beam

465
00:20:07,080 --> 00:20:09,180
could travel toward
its mirror should

466
00:20:09,180 --> 00:20:13,530
be experiencing a direct
headwind for its upward path

467
00:20:13,530 --> 00:20:16,350
and a direct tailwind
for its downward path.

468
00:20:16,350 --> 00:20:19,290
It should have a different
speed than the light that

469
00:20:19,290 --> 00:20:22,530
follows the more orthogonal path
so that even the light starts

470
00:20:22,530 --> 00:20:24,750
out perfectly in
phase, you should find

471
00:20:24,750 --> 00:20:26,430
a shift in the
interference fringes

472
00:20:26,430 --> 00:20:30,270
because the light's effective
speed through the medium

473
00:20:30,270 --> 00:20:32,880
is different for
path 1 than path 2--

474
00:20:32,880 --> 00:20:35,100
again, an analogy to the
ether wind of a bicyclist

475
00:20:35,100 --> 00:20:37,060
through the atmosphere.

476
00:20:37,060 --> 00:20:39,820
Not only that-- a
little foreshadowing

477
00:20:39,820 --> 00:20:41,050
for what's to come later--

478
00:20:41,050 --> 00:20:43,240
this device is so
sensitive, it's

479
00:20:43,240 --> 00:20:47,240
sensitive to second order
in a very small quantity.

480
00:20:47,240 --> 00:20:51,790
So it could measure
differences to the second order

481
00:20:51,790 --> 00:20:55,120
in the relative speed of, say,
the Earth through the ether

482
00:20:55,120 --> 00:20:58,040
compared to the speed of light.

483
00:20:58,040 --> 00:21:00,220
So that's called a second
order because it goes

484
00:21:00,220 --> 00:21:02,200
this small quantity squared.

485
00:21:02,200 --> 00:21:04,450
That's how sensitive this
really ingenious device was.

486
00:21:04,450 --> 00:21:06,280
And as many of
you might know, we

487
00:21:06,280 --> 00:21:08,800
use interferometers all
the time now in all kinds

488
00:21:08,800 --> 00:21:10,060
of industrial applications.

489
00:21:10,060 --> 00:21:12,010
Even more, my
favorite example is

490
00:21:12,010 --> 00:21:16,510
from our friends who detect
gravitational waves with LIGO.

491
00:21:16,510 --> 00:21:19,060
That's essentially a
super-sized interferometer

492
00:21:19,060 --> 00:21:21,610
with very, very similar
principles of design.

493
00:21:21,610 --> 00:21:24,670
So Michelson was doing
this really cool work.

494
00:21:24,670 --> 00:21:26,350
It was among the
earliest work that any

495
00:21:26,350 --> 00:21:29,050
of the super fancy,
elite European scientists

496
00:21:29,050 --> 00:21:31,660
paid any attention to
coming out of the US.

497
00:21:31,660 --> 00:21:33,190
Michelson was
among the first who

498
00:21:33,190 --> 00:21:36,250
made even the people like
Hendrik Lorentz sit up and pay

499
00:21:36,250 --> 00:21:39,850
attention at a time when the US
was still otherwise pretty much

500
00:21:39,850 --> 00:21:41,680
a scientific backwater.

501
00:21:41,680 --> 00:21:43,060
So it's a really cool idea.

502
00:21:43,060 --> 00:21:46,690
It's a remarkably clever
design for a device.

503
00:21:46,690 --> 00:21:48,670
Michelson built a
small prototype.

504
00:21:48,670 --> 00:21:51,760
Then he super sized it with
his colleague Edward Morley.

505
00:21:51,760 --> 00:21:54,130
They built a device
11 meters long

506
00:21:54,130 --> 00:21:57,610
for each arm, so roughly
33 or 34 feet long--

507
00:21:57,610 --> 00:22:00,940
huge, huge for its
day, very long arms.

508
00:22:00,940 --> 00:22:03,290
They floated the whole
thing in a vat of mercury,

509
00:22:03,290 --> 00:22:04,450
which I don't recommend.

510
00:22:04,450 --> 00:22:06,760
But they were trying to
tamp down any vibrations

511
00:22:06,760 --> 00:22:09,040
from nearby cable cars.

512
00:22:09,040 --> 00:22:11,320
They wanted to have a clean
laboratory environment.

513
00:22:11,320 --> 00:22:13,000
Then they did this
for years and years

514
00:22:13,000 --> 00:22:15,340
and years, months and
months and years and years

515
00:22:15,340 --> 00:22:16,780
because, of course,
you never know

516
00:22:16,780 --> 00:22:21,040
what the ether's
actual rest frame is.

517
00:22:21,040 --> 00:22:22,690
Who's to say that
at this moment,

518
00:22:22,690 --> 00:22:24,348
this is the direction
of the Earth's

519
00:22:24,348 --> 00:22:25,390
motion through the ether?

520
00:22:25,390 --> 00:22:28,300
Maybe it's this direction or
some other angle in between.

521
00:22:28,300 --> 00:22:30,790
They would look for day
to night variations.

522
00:22:30,790 --> 00:22:32,590
They would look for
annual variations

523
00:22:32,590 --> 00:22:34,840
as the Earth went around
its orbit around the sun.

524
00:22:34,840 --> 00:22:38,890
They would try to find any
time, day or night, winter

525
00:22:38,890 --> 00:22:41,710
versus spring, year
after year when

526
00:22:41,710 --> 00:22:44,950
they could measure some
measurable offset or change

527
00:22:44,950 --> 00:22:48,220
in those interference fringes
because at some point,

528
00:22:48,220 --> 00:22:49,690
this direction
should have lined up

529
00:22:49,690 --> 00:22:51,232
with the Earth's
direction of motion,

530
00:22:51,232 --> 00:22:52,750
even it wouldn't be every time.

531
00:22:52,750 --> 00:22:56,270
That's why they kept doing this
over and over and over again.

532
00:22:56,270 --> 00:23:00,370
And as you might know,
they found nothing.

533
00:23:00,370 --> 00:23:06,010
So after years of
painstaking data collection,

534
00:23:06,010 --> 00:23:10,330
they found no compelling
empirical evidence

535
00:23:10,330 --> 00:23:12,640
of the Earth's motion
through the ether.

536
00:23:12,640 --> 00:23:15,640
Michelson won the Nobel Prize
for this and related work.

537
00:23:15,640 --> 00:23:19,150
He was an astoundingly
accomplished experimentalist.

538
00:23:19,150 --> 00:23:21,010
He was the first
US-based physicist

539
00:23:21,010 --> 00:23:22,660
to win the Nobel Prize.

540
00:23:22,660 --> 00:23:24,610
He lived 20 more
years after that

541
00:23:24,610 --> 00:23:27,440
and died considering
himself a failure.

542
00:23:27,440 --> 00:23:29,023
So I hope you all
win the Nobel Prize.

543
00:23:29,023 --> 00:23:30,732
And I hope you think
better of yourselves

544
00:23:30,732 --> 00:23:32,620
and give yourselves more credit.

545
00:23:32,620 --> 00:23:35,950
Michelson knew for certain
the ether must exist.

546
00:23:35,950 --> 00:23:38,140
And yet he had
failed to find it.

547
00:23:38,140 --> 00:23:40,970
So Lorentz was following
this back in Europe.

548
00:23:40,970 --> 00:23:43,300
And this was really his
second main motivation

549
00:23:43,300 --> 00:23:45,820
to think about these
funny ways of handling

550
00:23:45,820 --> 00:23:49,010
coordinates in what we now call
the Lorentz transformation.

551
00:23:49,010 --> 00:23:51,010
So I mentioned his
mathematical concerns

552
00:23:51,010 --> 00:23:52,660
about the transformation
properties

553
00:23:52,660 --> 00:23:54,550
of Maxwell's equations.

554
00:23:54,550 --> 00:23:57,783
He was also following
the Michelson-Morley work

555
00:23:57,783 --> 00:23:59,200
with great interest
and citing it.

556
00:23:59,200 --> 00:24:01,570
And we know he was really
following each update.

557
00:24:01,570 --> 00:24:03,730
So he was concerned
about this failure

558
00:24:03,730 --> 00:24:08,043
to measure a shift in
the interference fringes.

559
00:24:08,043 --> 00:24:09,460
But Lorentz said,
oh, but actually

560
00:24:09,460 --> 00:24:12,080
maybe there's a physical
reason to account for that.

561
00:24:12,080 --> 00:24:15,040
If there really is, as
he knew there must be,

562
00:24:15,040 --> 00:24:18,400
this physical, resistive
medium, kind of elastic medium

563
00:24:18,400 --> 00:24:22,150
through which the Earth and
everything else is moving,

564
00:24:22,150 --> 00:24:27,220
then there must be a force
exerted by that viscous medium

565
00:24:27,220 --> 00:24:29,530
on every single
atom and molecule

566
00:24:29,530 --> 00:24:32,350
making up all the stuff
in Michelson's device

567
00:24:32,350 --> 00:24:34,010
as well as in everything else.

568
00:24:34,010 --> 00:24:36,910
And so it would be like
picturing a beach ball.

569
00:24:36,910 --> 00:24:39,370
If you try to drag a
beach ball underwater,

570
00:24:39,370 --> 00:24:42,400
in a resistive medium,
the shape will be deformed

571
00:24:42,400 --> 00:24:43,730
along the direction of motion.

572
00:24:43,730 --> 00:24:46,600
If you drag that beach ball
fast enough underwater,

573
00:24:46,600 --> 00:24:48,400
it will shrink in the
direction of motion.

574
00:24:48,400 --> 00:24:51,830
It will be deformed
like this picture here.

575
00:24:51,830 --> 00:24:53,710
And so Lorentz said
that must be happening

576
00:24:53,710 --> 00:24:57,220
for every bit of matter
in the arm of Michelson's

577
00:24:57,220 --> 00:25:00,880
interferometer that's most
subject to that resistive force

578
00:25:00,880 --> 00:25:01,720
of the ether.

579
00:25:01,720 --> 00:25:05,013
There should be a contraction
along the direction of motion.

580
00:25:05,013 --> 00:25:06,430
So then, again
because he's a very

581
00:25:06,430 --> 00:25:08,470
gifted mathematical
physicist, he

582
00:25:08,470 --> 00:25:11,110
calculated exactly how
much shift must there

583
00:25:11,110 --> 00:25:17,440
be for the resultant path to be
shortened just enough to make

584
00:25:17,440 --> 00:25:19,610
it a tie race after all.

585
00:25:19,610 --> 00:25:22,540
So the arm of the
interferometer that's

586
00:25:22,540 --> 00:25:25,600
experiencing this
shrinking, this extra force

587
00:25:25,600 --> 00:25:27,880
due to the resistive
medium, would

588
00:25:27,880 --> 00:25:30,520
have to shrink by a
specific calculable amount.

589
00:25:30,520 --> 00:25:33,285
We now use the
Greek letter gamma.

590
00:25:33,285 --> 00:25:34,660
If you haven't
seen that already,

591
00:25:34,660 --> 00:25:37,620
you'll see it throughout IAP
and the rest of your life.

592
00:25:37,620 --> 00:25:40,050
It takes this somewhat
simple-looking form--

593
00:25:40,050 --> 00:25:44,790
1 over the square root of
1 minus v over c squared.

594
00:25:44,790 --> 00:25:46,800
If you plot it, you
can see it diverges.

595
00:25:46,800 --> 00:25:50,200
When the relative speed gets
close to the speed of light,

596
00:25:50,200 --> 00:25:51,990
you have 1 minus 1
in the denominator.

597
00:25:51,990 --> 00:25:53,400
You divide by 0.

598
00:25:53,400 --> 00:25:54,210
Your eyes bug out.

599
00:25:54,210 --> 00:25:55,230
And cats and dogs live together.

600
00:25:55,230 --> 00:25:56,290
Everything goes crazy.

601
00:25:56,290 --> 00:25:58,650
So it diverges at v equals c.

602
00:25:58,650 --> 00:26:00,240
On the other hand,
for small speeds,

603
00:26:00,240 --> 00:26:02,250
small compared to
the speed of light,

604
00:26:02,250 --> 00:26:05,130
this factor is
indistinguishable from 1.

605
00:26:05,130 --> 00:26:06,720
So here's a quick plot.

606
00:26:06,720 --> 00:26:09,450
For ordinary speeds that we
encounter on the highway,

607
00:26:09,450 --> 00:26:11,760
let alone on a
bicycle, our speed

608
00:26:11,760 --> 00:26:14,500
compared to the speed of
light is vanishingly small.

609
00:26:14,500 --> 00:26:18,150
So this factor gamma is
very, very close to 1.

610
00:26:18,150 --> 00:26:20,640
Only when you get to speeds
approaching the speed of light

611
00:26:20,640 --> 00:26:23,820
would you expect to measure
any shift from that.

612
00:26:23,820 --> 00:26:26,010
Nonetheless, Lorentz
said in principle this

613
00:26:26,010 --> 00:26:27,930
happens at any speed.

614
00:26:27,930 --> 00:26:30,630
And the shrink, the
amount of contraction,

615
00:26:30,630 --> 00:26:32,310
is governed by
that relative speed

616
00:26:32,310 --> 00:26:34,440
given by this
Greek letter gamma.

617
00:26:34,440 --> 00:26:37,170
So gamma is always greater
than or equal to 1.

618
00:26:37,170 --> 00:26:41,670
That means 1 over gamma
must be smaller than 1.

619
00:26:41,670 --> 00:26:43,950
And that's saying that the
contracted length of one

620
00:26:43,950 --> 00:26:45,450
of those arms of
the interferometer

621
00:26:45,450 --> 00:26:49,620
got shorter by a
certain amount depending

622
00:26:49,620 --> 00:26:51,930
on the relative speed.

623
00:26:51,930 --> 00:26:55,080
That would be enough to
account for this null result,

624
00:26:55,080 --> 00:26:56,010
Lorentz argued.

625
00:26:56,010 --> 00:26:58,080
It turned out that
exact form of gamma

626
00:26:58,080 --> 00:27:03,030
was the same form he'd found
for his mathematical local time

627
00:27:03,030 --> 00:27:04,050
manipulations.

628
00:27:04,050 --> 00:27:06,932
So here's how we can
live in a physical ether

629
00:27:06,932 --> 00:27:08,640
even though we don't
measure its effects.

630
00:27:08,640 --> 00:27:09,960
OK.

631
00:27:09,960 --> 00:27:12,870
Let me now shift to how
a different person began

632
00:27:12,870 --> 00:27:15,510
thinking about very familiar
questions by that point

633
00:27:15,510 --> 00:27:17,460
but coming at them
quite differently.

634
00:27:17,460 --> 00:27:19,650
That's young Albert Einstein.

635
00:27:19,650 --> 00:27:22,170
So Einstein was born in
the midst of all this.

636
00:27:22,170 --> 00:27:25,740
He was born in 1879
in kind of rural,

637
00:27:25,740 --> 00:27:27,510
roughly speaking,
nowheresville Germany,

638
00:27:27,510 --> 00:27:29,370
not near any of the big cities.

639
00:27:29,370 --> 00:27:32,670
So Lorentz was already
a practicing physicist

640
00:27:32,670 --> 00:27:34,230
when Einstein was born.

641
00:27:34,230 --> 00:27:36,030
He was much younger.

642
00:27:36,030 --> 00:27:37,710
Einstein's main
ambition was actually

643
00:27:37,710 --> 00:27:40,680
to become an electrical
engineer at a time

644
00:27:40,680 --> 00:27:43,860
when that term itself
was actually brand new.

645
00:27:43,860 --> 00:27:46,830
Einstein's father and uncle
had gone into business

646
00:27:46,830 --> 00:27:49,380
together in this
really brand new field

647
00:27:49,380 --> 00:27:50,530
of electrical engineering.

648
00:27:50,530 --> 00:27:52,560
This was the age
of electrification.

649
00:27:52,560 --> 00:27:55,260
Think about extended
street lighting,

650
00:27:55,260 --> 00:27:59,730
trolley cars with the
shared electric lines above.

651
00:27:59,730 --> 00:28:02,730
This was transforming
the face of everyday life

652
00:28:02,730 --> 00:28:05,520
in cities and eventually
even in more rural areas.

653
00:28:05,520 --> 00:28:08,520
And Einstein's father and
uncle were in on that.

654
00:28:08,520 --> 00:28:10,497
They were early professionals
in this new field

655
00:28:10,497 --> 00:28:11,580
of electrical engineering.

656
00:28:11,580 --> 00:28:12,497
And Einstein loved it.

657
00:28:12,497 --> 00:28:17,100
He loved to tinker with
electrotechnical gadgets.

658
00:28:17,100 --> 00:28:20,280
He had many strong
feelings about many things.

659
00:28:20,280 --> 00:28:22,020
Among them, he hated
what he considered

660
00:28:22,020 --> 00:28:25,650
the overly militaristic German
high school that he attended.

661
00:28:25,650 --> 00:28:28,157
He would argue and
insult his teachers.

662
00:28:28,157 --> 00:28:29,490
They didn't like him any better.

663
00:28:29,490 --> 00:28:31,110
They were delighted
when he dropped out,

664
00:28:31,110 --> 00:28:32,190
so they didn't have
to kick him out.

665
00:28:32,190 --> 00:28:34,290
He was really very
obnoxious, it turns out.

666
00:28:34,290 --> 00:28:36,600
So he was a high-school dropout.

667
00:28:36,600 --> 00:28:42,368
He dreamed of entering the Swiss
Federal Technical Institute.

668
00:28:42,368 --> 00:28:44,160
It was kind of like
the MIT of Switzerland.

669
00:28:44,160 --> 00:28:48,123
Except to be more proper, MIT
is the Massachusetts version

670
00:28:48,123 --> 00:28:48,790
of their school.

671
00:28:48,790 --> 00:28:50,640
Zurich's was founded first.

672
00:28:50,640 --> 00:28:54,020
It's often called the ETH, or
the Eidgenossische Technische

673
00:28:54,020 --> 00:28:55,080
Hochschule.

674
00:28:55,080 --> 00:28:57,330
It was really a very
elite technical university

675
00:28:57,330 --> 00:28:59,250
in Zurich, Switzerland.

676
00:28:59,250 --> 00:29:01,260
Einstein, by this
point, had renounced

677
00:29:01,260 --> 00:29:02,560
his German citizenship.

678
00:29:02,560 --> 00:29:03,810
He dropped out of high school.

679
00:29:03,810 --> 00:29:06,323
And one of the best things
about this technical university

680
00:29:06,323 --> 00:29:07,740
in Zurich was that
you didn't have

681
00:29:07,740 --> 00:29:09,000
to have a high school diploma.

682
00:29:09,000 --> 00:29:09,750
He said, perfect.

683
00:29:09,750 --> 00:29:11,490
That's the place for me.

684
00:29:11,490 --> 00:29:14,820
So they had an entrance exam,
which he very dutifully studied

685
00:29:14,820 --> 00:29:17,160
for and then failed
because, after all, he

686
00:29:17,160 --> 00:29:19,750
ignored the topics that
weren't of interest to him.

687
00:29:19,750 --> 00:29:22,560
He did well enough on
the physics and math

688
00:29:22,560 --> 00:29:27,413
portions of the overall exam
that a kindly physics professor

689
00:29:27,413 --> 00:29:28,830
took him aside and
said, you know,

690
00:29:28,830 --> 00:29:31,740
if you go to basically a
kind of regional, something

691
00:29:31,740 --> 00:29:33,360
almost like a
community college, go

692
00:29:33,360 --> 00:29:35,610
to a local, regional
school for a year,

693
00:29:35,610 --> 00:29:37,792
study up, retake the
exam, you might do better.

694
00:29:37,792 --> 00:29:39,000
And that's what Einstein did.

695
00:29:39,000 --> 00:29:40,680
So he went to a
little regional school

696
00:29:40,680 --> 00:29:44,610
in, again, in rural
Switzerland for a year,

697
00:29:44,610 --> 00:29:46,690
took the exam again, and passed.

698
00:29:46,690 --> 00:29:48,330
So now he's able
to enter his dream

699
00:29:48,330 --> 00:29:51,030
school, the ETH in Zurich.

700
00:29:51,030 --> 00:29:52,680
Once he then worked
so hard to get in,

701
00:29:52,680 --> 00:29:54,150
he proceeded to cut classes.

702
00:29:54,150 --> 00:29:56,040
This guy was a horrible student.

703
00:29:56,040 --> 00:29:58,110
Whatever you do, don't
be like Albert Einstein,

704
00:29:58,110 --> 00:30:00,213
at least while
you're in university.

705
00:30:00,213 --> 00:30:01,380
He worked so hard to get in.

706
00:30:01,380 --> 00:30:03,338
And then he, again, would
insult his professors

707
00:30:03,338 --> 00:30:05,430
as much as he'd insulted
his high school teachers.

708
00:30:05,430 --> 00:30:07,195
He thought everything
they did was boring.

709
00:30:07,195 --> 00:30:09,070
They didn't know what
was really interesting.

710
00:30:09,070 --> 00:30:10,862
So he would cut classes
and read on his own

711
00:30:10,862 --> 00:30:12,630
and then borrow notes
from his girlfriend,

712
00:30:12,630 --> 00:30:15,240
his long, suffering, very
patient girlfriend, Mileva

713
00:30:15,240 --> 00:30:17,970
Maric, who was doing very well
in her own physics and math

714
00:30:17,970 --> 00:30:18,470
courses.

715
00:30:18,470 --> 00:30:20,595
And likewise, he borrowed
notes from another friend

716
00:30:20,595 --> 00:30:21,690
of his, Marcel Grossmann.

717
00:30:21,690 --> 00:30:24,450
So he'd just cram and scrape
through for the exams.

718
00:30:24,450 --> 00:30:26,550
Not an ideal student.

719
00:30:26,550 --> 00:30:29,190
For some reason that Einstein
couldn't possibly fathom,

720
00:30:29,190 --> 00:30:31,650
because he actually
was a dolt frankly,

721
00:30:31,650 --> 00:30:34,320
he impressed none
of his professors.

722
00:30:34,320 --> 00:30:36,487
So when it came time to
graduate, none of them

723
00:30:36,487 --> 00:30:38,570
would write him a strong
letter of recommendation.

724
00:30:38,570 --> 00:30:42,860
Again, his life bears many
important lessons for us today.

725
00:30:42,860 --> 00:30:43,760
Attend class.

726
00:30:43,760 --> 00:30:46,760
And be slightly less
obnoxious to your instructors.

727
00:30:46,760 --> 00:30:48,390
And maybe things
will turn out better.

728
00:30:48,390 --> 00:30:49,730
So none of his teachers
would basically

729
00:30:49,730 --> 00:30:51,650
support him because
he had middling grades

730
00:30:51,650 --> 00:30:53,610
and he was rude to them.

731
00:30:53,610 --> 00:30:56,040
So he couldn't get a
job after graduation.

732
00:30:56,040 --> 00:30:58,250
So finally one of
his close friends,

733
00:30:58,250 --> 00:30:59,960
Marcel Grossmann,
one of the folks

734
00:30:59,960 --> 00:31:03,470
from whom he'd borrowed notes,
his father had connections.

735
00:31:03,470 --> 00:31:05,240
And so basically
through connections,

736
00:31:05,240 --> 00:31:08,720
he was able to get Einstein
an entry-level civil service

737
00:31:08,720 --> 00:31:11,810
job at the patent office
in Bern, Switzerland.

738
00:31:11,810 --> 00:31:13,320
So still in Switzerland.

739
00:31:13,320 --> 00:31:15,867
He was a patent
officer, third class.

740
00:31:15,867 --> 00:31:18,200
As I'm always fond of saying,
there was no fourth class.

741
00:31:18,200 --> 00:31:20,900
You were the lowest
entry-level gig.

742
00:31:20,900 --> 00:31:23,570
So even for young Einstein,
it wasn't what you know.

743
00:31:23,570 --> 00:31:24,980
It was who you know.

744
00:31:24,980 --> 00:31:27,320
What he then
proceeded to do was he

745
00:31:27,320 --> 00:31:29,060
had a day job at
what they called

746
00:31:29,060 --> 00:31:32,570
the electrotechnical desk, much
like what his father and uncle

747
00:31:32,570 --> 00:31:33,290
were doing.

748
00:31:33,290 --> 00:31:34,970
He was a patent
examiner for a lot

749
00:31:34,970 --> 00:31:37,490
of these cool, new
electrical gadgets.

750
00:31:37,490 --> 00:31:39,470
And then he would go
hang out with a bunch

751
00:31:39,470 --> 00:31:43,610
of friends and drink
beer a lot at the pubs.

752
00:31:43,610 --> 00:31:46,190
So they formed what they
called the Olympia Academy.

753
00:31:46,190 --> 00:31:47,780
And this was very ironic.

754
00:31:47,780 --> 00:31:49,275
They gave themselves
the most elite

755
00:31:49,275 --> 00:31:50,900
sounding name because
they're basically

756
00:31:50,900 --> 00:31:52,760
three semi-bums
hanging out, reading,

757
00:31:52,760 --> 00:31:54,680
and blowing off their families.

758
00:31:54,680 --> 00:31:57,318
So they gave themselves a
very, very prestigious name,

759
00:31:57,318 --> 00:31:59,360
even though it was literally
three recent college

760
00:31:59,360 --> 00:32:01,310
graduates hanging out.

761
00:32:01,310 --> 00:32:04,310
It was Maurice Solovine,
Conrad Habicht,

762
00:32:04,310 --> 00:32:06,200
and young Albert Einstein.

763
00:32:06,200 --> 00:32:09,620
And they would go and sometimes
drink coffee, often drink beer,

764
00:32:09,620 --> 00:32:11,750
and talk about
stuff they'd read.

765
00:32:11,750 --> 00:32:13,370
And they would read
a lot of physics

766
00:32:13,370 --> 00:32:16,290
and read a lot of philosophy
and talk about it.

767
00:32:16,290 --> 00:32:19,220
And one of the books that we
know from their correspondence

768
00:32:19,220 --> 00:32:21,410
at the time that they
were really interested in

769
00:32:21,410 --> 00:32:25,160
was this fascinating book
by an Austrian polymath

770
00:32:25,160 --> 00:32:26,120
named Ernst Mach.

771
00:32:26,120 --> 00:32:29,120
Here's the book translated
into English, known

772
00:32:29,120 --> 00:32:30,490
as The Science of Mechanics.

773
00:32:30,490 --> 00:32:33,710
It had come out in the 1880s.

774
00:32:33,710 --> 00:32:35,610
Mach was really remarkable.

775
00:32:35,610 --> 00:32:38,390
He was both a
mathematical physicist

776
00:32:38,390 --> 00:32:39,680
and experimental physicist.

777
00:32:39,680 --> 00:32:42,110
You might know the
terminology for Mach number,

778
00:32:42,110 --> 00:32:45,710
like speed of sound, Mach 1,
Mach 2, Mach 3-- same Mach.

779
00:32:45,710 --> 00:32:47,930
He did a lot of studies
of acoustics and optics

780
00:32:47,930 --> 00:32:50,960
but also of what we would
now call psychology,

781
00:32:50,960 --> 00:32:55,190
like sensory experiments
and so on, and eventually

782
00:32:55,190 --> 00:32:56,543
medical surgery.

783
00:32:56,543 --> 00:32:57,960
And then at the
end of his career,

784
00:32:57,960 --> 00:32:59,600
he was a professor of the
history and philosophy

785
00:32:59,600 --> 00:33:00,100
of science.

786
00:33:00,100 --> 00:33:01,650
This guy did it all.

787
00:33:01,650 --> 00:33:05,930
He wrote these very, very
dense, conceptual critiques

788
00:33:05,930 --> 00:33:08,790
of Newtonian physics,
among other things.

789
00:33:08,790 --> 00:33:12,200
And Mach was convinced
that Newton was really

790
00:33:12,200 --> 00:33:14,330
getting himself into
a horrible muddle

791
00:33:14,330 --> 00:33:16,370
because he had not paid
sufficient attention

792
00:33:16,370 --> 00:33:17,450
philosophically.

793
00:33:17,450 --> 00:33:20,120
And this is what the young
members of the Olympia Academy

794
00:33:20,120 --> 00:33:21,260
got really excited about.

795
00:33:21,260 --> 00:33:24,020
We know from their letters
and their notes at the time.

796
00:33:24,020 --> 00:33:27,050
Mach was advocating
a position that

797
00:33:27,050 --> 00:33:29,450
came to be known as positivism.

798
00:33:29,450 --> 00:33:32,360
According to Ernst Mach,
unlike Isaac Newton,

799
00:33:32,360 --> 00:33:36,320
or for that matter even some
folks like Maxwell, Mach

800
00:33:36,320 --> 00:33:39,360
argued that only quantities
that could become objects

801
00:33:39,360 --> 00:33:41,360
of positive experience,
that is to say things we

802
00:33:41,360 --> 00:33:45,530
could actively measure or
sense or feel or touch,

803
00:33:45,530 --> 00:33:48,860
only those things belonged
in our scientific theories.

804
00:33:48,860 --> 00:33:51,620
Anything else was just
like mere speculation,

805
00:33:51,620 --> 00:33:53,660
like counting the number
of angels that could

806
00:33:53,660 --> 00:33:54,785
dance on the head of a pin.

807
00:33:54,785 --> 00:33:57,290
He was scathingly
critical of Newton.

808
00:33:57,290 --> 00:33:59,960
And in particular,
Mach wrote in detail

809
00:33:59,960 --> 00:34:02,600
that Newton's notions
of absolute space

810
00:34:02,600 --> 00:34:05,000
and absolute time had no
meaning because how could you

811
00:34:05,000 --> 00:34:06,290
ever measure absolute time?

812
00:34:06,290 --> 00:34:08,960
Show me the clock that
could measure absolute time.

813
00:34:08,960 --> 00:34:11,449
That was the kind of
classic Machian response.

814
00:34:11,449 --> 00:34:14,540
I can measure the passage
of time by using a clock.

815
00:34:14,540 --> 00:34:17,239
But who says that's what
Newton wrote absolute time was?

816
00:34:17,239 --> 00:34:22,699
This had an amazing impact, as
we know, on the young Einstein.

817
00:34:22,699 --> 00:34:23,989
So he's in this milieu.

818
00:34:23,989 --> 00:34:26,239
He's now not in a
university setting.

819
00:34:26,239 --> 00:34:29,000
He scraped through his
undergraduate studies.

820
00:34:29,000 --> 00:34:30,949
He has a nice day job.

821
00:34:30,949 --> 00:34:34,025
But he's not doing particularly
well professionally.

822
00:34:34,025 --> 00:34:35,400
He's hanging out
with his buddies

823
00:34:35,400 --> 00:34:38,449
and reading some interesting
and hard, obscure philosophy

824
00:34:38,449 --> 00:34:40,580
science in the evenings.

825
00:34:40,580 --> 00:34:43,909
Well, he has what's now commonly
called his miraculous year,

826
00:34:43,909 --> 00:34:46,614
his annus mirabilis, in 1905.

827
00:34:46,614 --> 00:34:48,239
In fact, he only took
about six months.

828
00:34:48,239 --> 00:34:50,870
It was, like, half a year,
during which he submitted

829
00:34:50,870 --> 00:34:54,662
four really astonishing,
original papers to the leading

830
00:34:54,662 --> 00:34:56,120
journal of physics,
the Annalen der

831
00:34:56,120 --> 00:34:58,820
Physik in Germany at the time.

832
00:34:58,820 --> 00:35:01,130
They've since been
published and translated.

833
00:35:01,130 --> 00:35:02,690
There's a particularly
nice edition

834
00:35:02,690 --> 00:35:07,370
you can find edited by John
Stachel with very nice essays

835
00:35:07,370 --> 00:35:08,190
to accompany them.

836
00:35:08,190 --> 00:35:10,550
You can find them online.

837
00:35:10,550 --> 00:35:12,470
So for IAP and for
the rest of today

838
00:35:12,470 --> 00:35:14,570
and even throughout
the month, we're

839
00:35:14,570 --> 00:35:17,030
really interested in what was
the third of these articles

840
00:35:17,030 --> 00:35:17,960
that he submitted.

841
00:35:17,960 --> 00:35:20,600
He submitted it to the
journal in June of 1905.

842
00:35:20,600 --> 00:35:23,330
It's on what we would now
call special relativity.

843
00:35:23,330 --> 00:35:25,340
Its title, as you
can see up top,

844
00:35:25,340 --> 00:35:28,760
was actually, in translation,
"On the Electrodynamics

845
00:35:28,760 --> 00:35:32,480
of Moving Bodies," a thoroughly
familiar title, exactly what

846
00:35:32,480 --> 00:35:34,670
Lorentz and all of
Lorentz's colleagues

847
00:35:34,670 --> 00:35:37,380
have been talking about
for decades by that point.

848
00:35:37,380 --> 00:35:40,160
So the title of
Einstein's paper in 1905

849
00:35:40,160 --> 00:35:42,410
was not surprising, even
though, as we'll see,

850
00:35:42,410 --> 00:35:43,760
his approach was quite distinct.

851
00:35:48,580 --> 00:35:53,020
He begins this now very well
done paper not by saying,

852
00:35:53,020 --> 00:35:55,420
I found an error in
Lorentz's calculation.

853
00:35:55,420 --> 00:35:58,570
There was a missing factor
of 2 pi or whatever.

854
00:35:58,570 --> 00:36:00,910
He doesn't say, I conducted
my own measurements.

855
00:36:00,910 --> 00:36:04,180
And I found these results
with an experimental error.

856
00:36:04,180 --> 00:36:06,640
He starts out by saying
there's an asymmetry

857
00:36:06,640 --> 00:36:09,940
in the explanation which is
not present in the phenomena.

858
00:36:09,940 --> 00:36:12,820
It sounds very philosophical.

859
00:36:12,820 --> 00:36:15,220
He goes on in his
very opening paragraph

860
00:36:15,220 --> 00:36:19,180
to say that when we use
Maxwell's equations,

861
00:36:19,180 --> 00:36:22,240
we come up with very different
kinds of accounts for what

862
00:36:22,240 --> 00:36:24,010
should be the same phenomenon.

863
00:36:24,010 --> 00:36:24,760
Very simple.

864
00:36:24,760 --> 00:36:26,650
Nothing super fancy
like an interferometer.

865
00:36:26,650 --> 00:36:30,280
Just take a bar magnet and
a coil through which current

866
00:36:30,280 --> 00:36:31,510
could flow.

867
00:36:31,510 --> 00:36:34,330
And make sure you have a current
meter, an ammeter attached

868
00:36:34,330 --> 00:36:35,390
to the coil.

869
00:36:35,390 --> 00:36:38,627
So if electric current
flows through the coil,

870
00:36:38,627 --> 00:36:39,460
it will be measured.

871
00:36:39,460 --> 00:36:41,410
The ammeter needle
will be deflected.

872
00:36:41,410 --> 00:36:45,160
And we are free to move
either the magnet or the coil.

873
00:36:45,160 --> 00:36:47,980
In his opening paragraph,
Einstein says, physicists

874
00:36:47,980 --> 00:36:51,970
had treated this situation
completely orthogonally even

875
00:36:51,970 --> 00:36:54,770
though, as far as
Einstein was concerned,

876
00:36:54,770 --> 00:36:57,320
there's only one
explanation needed.

877
00:36:57,320 --> 00:36:59,210
And so with case
1, if you assume

878
00:36:59,210 --> 00:37:01,640
the bar magnet is moving,
you're shaking the magnet

879
00:37:01,640 --> 00:37:05,300
back and forth and keeping
the coil fixed in location,

880
00:37:05,300 --> 00:37:08,810
then you would appeal to one
set of Maxwell's equations.

881
00:37:08,810 --> 00:37:11,720
And again, on the t-shirt
it would be this one.

882
00:37:11,720 --> 00:37:14,910
You have a time-varying
magnetic field

883
00:37:14,910 --> 00:37:17,810
which will induce a spatially
varying electric field.

884
00:37:17,810 --> 00:37:21,890
And that will exert a push,
a force on the little ions,

885
00:37:21,890 --> 00:37:24,170
the little electric
charges within that coil.

886
00:37:24,170 --> 00:37:25,100
They'll feel a push.

887
00:37:25,100 --> 00:37:26,630
They'll move along the coil.

888
00:37:26,630 --> 00:37:29,030
Electric current, as
Maxwell himself had argued,

889
00:37:29,030 --> 00:37:33,650
was nothing but the motion of
these electric-charge bearers.

890
00:37:33,650 --> 00:37:36,200
So because you have a
time-varying magnetic field,

891
00:37:36,200 --> 00:37:39,650
the little bits of matter in
the coil will be pushed along.

892
00:37:39,650 --> 00:37:41,480
You'll induce a current.

893
00:37:41,480 --> 00:37:44,600
That's case 1-- moving
magnets, stationary coil.

894
00:37:44,600 --> 00:37:47,790
But if you want to analyze
the other situation,

895
00:37:47,790 --> 00:37:50,420
to hold the magnet
rigidly fixed in place

896
00:37:50,420 --> 00:37:52,460
and shift the coil
back and forth,

897
00:37:52,460 --> 00:37:55,580
then physicists would give an
entirely separate explanation.

898
00:37:55,580 --> 00:37:58,610
They would appeal to a different
one of Maxwell's equations.

899
00:37:58,610 --> 00:38:02,150
Now they'd say that there's
a static magnetic field that

900
00:38:02,150 --> 00:38:03,590
varied in space.

901
00:38:03,590 --> 00:38:06,800
It's spatial gradients
were non-vanishing.

902
00:38:06,800 --> 00:38:10,250
And that would exert a force,
like a kind of Lorentz force

903
00:38:10,250 --> 00:38:11,900
law, on those charges.

904
00:38:11,900 --> 00:38:15,170
So they have some velocity
in a magnetic field.

905
00:38:15,170 --> 00:38:16,700
They'll be pushed
along the wire.

906
00:38:16,700 --> 00:38:18,600
And you'll measure a current.

907
00:38:18,600 --> 00:38:21,710
Einstein said that's one
explanation too many.

908
00:38:21,710 --> 00:38:23,330
All that we could ever measure--

909
00:38:23,330 --> 00:38:26,660
again, you can almost hear the
kind of Mach coming through--

910
00:38:26,660 --> 00:38:29,810
all we can ever see as an
element of positive experience

911
00:38:29,810 --> 00:38:33,590
is some relative motion
between the magnet and the coil

912
00:38:33,590 --> 00:38:35,540
which induces an
electric current.

913
00:38:35,540 --> 00:38:37,580
Who's to say one
was actually still

914
00:38:37,580 --> 00:38:38,780
while the other was moving?

915
00:38:38,780 --> 00:38:40,940
So Einstein begins
this paper by saying

916
00:38:40,940 --> 00:38:42,980
that there's something
wrong with the stories

917
00:38:42,980 --> 00:38:44,660
we tell about the equations.

918
00:38:44,660 --> 00:38:46,310
He doesn't argue
about the equations.

919
00:38:46,310 --> 00:38:47,960
He argues about
our interpretation

920
00:38:47,960 --> 00:38:48,740
of the equations.

921
00:38:48,740 --> 00:38:50,530
Very striking.

922
00:38:50,530 --> 00:38:53,170
He then goes on, just in,
like, paragraph 3-- still

923
00:38:53,170 --> 00:38:57,640
very early in the introduction--
to posit two postulates.

924
00:38:57,640 --> 00:38:58,600
He doesn't prove them.

925
00:38:58,600 --> 00:39:00,340
He doesn't derive them.

926
00:39:00,340 --> 00:39:03,340
He doesn't say, I have
demonstrated these

927
00:39:03,340 --> 00:39:04,960
by doing experiments.

928
00:39:04,960 --> 00:39:08,170
He says, let me hypothesize
these and see what follows.

929
00:39:08,170 --> 00:39:11,800
And these are the two
in rough paraphrase.

930
00:39:11,800 --> 00:39:14,500
The first one was
actually already called

931
00:39:14,500 --> 00:39:16,840
the principle of relativity,
or sometimes simply called

932
00:39:16,840 --> 00:39:21,070
Galilean relativity, going
back again to the 17th century.

933
00:39:21,070 --> 00:39:24,040
Galileo had argued by
thinking about that boat

934
00:39:24,040 --> 00:39:27,550
floating down the river, that
to a person on the boat, as long

935
00:39:27,550 --> 00:39:30,070
as the boat is moving
at a constant speed,

936
00:39:30,070 --> 00:39:32,200
neither speeding up
nor slowing down,

937
00:39:32,200 --> 00:39:34,990
that all the laws of mechanics
should work perfectly well

938
00:39:34,990 --> 00:39:37,090
for that observer the
same as they would

939
00:39:37,090 --> 00:39:40,030
for us at rest on the shore.

940
00:39:40,030 --> 00:39:42,417
If you toss a ball, it'll
land back in your hand

941
00:39:42,417 --> 00:39:43,000
as you expect.

942
00:39:43,000 --> 00:39:45,850
Even though to us,
we see the ball trace

943
00:39:45,850 --> 00:39:49,060
a complicated parabola,
to the person on the boat,

944
00:39:49,060 --> 00:39:51,370
she's perfectly entitled to
say, I'm sitting at rest.

945
00:39:51,370 --> 00:39:53,050
The ball went straight
up and landed straight

946
00:39:53,050 --> 00:39:53,717
back in my hand.

947
00:39:53,717 --> 00:39:56,170
All the laws of mechanics
should work equally well

948
00:39:56,170 --> 00:39:59,320
in any reference frame that's
moving at a constant speed.

949
00:39:59,320 --> 00:40:01,600
Einstein just takes
that existing principle

950
00:40:01,600 --> 00:40:04,760
of relativity from
mechanics and just assumes,

951
00:40:04,760 --> 00:40:07,120
just hypothesizes
that that should

952
00:40:07,120 --> 00:40:10,390
apply not only to mechanics but
to every physical phenomenon--

953
00:40:10,390 --> 00:40:13,002
electricity, magnetism,
optics, thermodynamics.

954
00:40:13,002 --> 00:40:13,960
And that's just a leap.

955
00:40:13,960 --> 00:40:15,100
He just says, what if?

956
00:40:15,100 --> 00:40:18,100
What if this applies to any
kind of physical phenomenon, not

957
00:40:18,100 --> 00:40:19,660
just mechanics?

958
00:40:19,660 --> 00:40:22,270
And then he introduces
the second postulate

959
00:40:22,270 --> 00:40:25,970
which seems actually to be in
tension a bit with the first.

960
00:40:25,970 --> 00:40:27,940
His second postulate
is, the speed of light

961
00:40:27,940 --> 00:40:32,740
is a constant independent
of the motion of the source.

962
00:40:32,740 --> 00:40:33,280
What?

963
00:40:33,280 --> 00:40:34,540
That doesn't sound right.

964
00:40:34,540 --> 00:40:36,820
If you go back to Galileo
and watching his friend

965
00:40:36,820 --> 00:40:39,940
on the boat, if
she fires a cannon,

966
00:40:39,940 --> 00:40:42,370
the speed with which we
measure the cannonball

967
00:40:42,370 --> 00:40:45,760
is different than the speed
she measures it, right?

968
00:40:45,760 --> 00:40:49,030
If you watch her
lobbing a tennis ball,

969
00:40:49,030 --> 00:40:51,550
our measurement of the speed
of an object in her reference

970
00:40:51,550 --> 00:40:52,820
frame is not the same as ours.

971
00:40:52,820 --> 00:40:54,430
And Einstein here
is saying light

972
00:40:54,430 --> 00:40:58,000
is special, that unlike
tennis balls, ping pong balls,

973
00:40:58,000 --> 00:41:00,580
cannonballs, or trains,
or anything else,

974
00:41:00,580 --> 00:41:03,040
the speed of light,
as a postulate,

975
00:41:03,040 --> 00:41:05,500
will be constant
for any observer

976
00:41:05,500 --> 00:41:08,950
as long as they're in an
inertial frame of reference.

977
00:41:08,950 --> 00:41:11,437
And here's an excerpt from
the English translation.

978
00:41:11,437 --> 00:41:12,520
I won't read all this out.

979
00:41:12,520 --> 00:41:15,220
But he says, what follows
from these postulates

980
00:41:15,220 --> 00:41:18,860
is that the luminiferous ether
will prove to be superfluous--

981
00:41:18,860 --> 00:41:19,660
[GERMAN]

982
00:41:19,660 --> 00:41:21,380
He doesn't say, I've
disproven it, ether.

983
00:41:21,380 --> 00:41:23,290
He says it's just irrelevant.

984
00:41:23,290 --> 00:41:26,560
50 years' worth, by that point
100 years' worth of study

985
00:41:26,560 --> 00:41:29,950
by all of Europe's most
prestigious physicists was

986
00:41:29,950 --> 00:41:31,163
irrelevant.

987
00:41:31,163 --> 00:41:33,580
That's probably why he didn't
have many friends in physics

988
00:41:33,580 --> 00:41:34,280
at the time.

989
00:41:34,280 --> 00:41:35,860
So he doesn't
disprove the ether.

990
00:41:35,860 --> 00:41:38,170
He says we just won't even
need to refer to it anymore.

991
00:41:38,170 --> 00:41:40,210
It makes the entire
set of questions

992
00:41:40,210 --> 00:41:42,100
that had driven
people like Lorentz

993
00:41:42,100 --> 00:41:44,440
really kind of fall away,
at least as far as Einstein

994
00:41:44,440 --> 00:41:46,640
himself is concerned.

995
00:41:46,640 --> 00:41:49,180
So why does Einstein introduce
that second postulate?

996
00:41:49,180 --> 00:41:52,690
And again, historians and
physicists and philosophers

997
00:41:52,690 --> 00:41:54,700
have studied this
question a lot.

998
00:41:54,700 --> 00:41:56,560
And we have some good
documentary evidence

999
00:41:56,560 --> 00:41:58,570
because Einstein was
writing letters and diaries

1000
00:41:58,570 --> 00:41:59,530
and stuff all the time.

1001
00:41:59,530 --> 00:42:01,610
And a lot of those
have survived.

1002
00:42:01,610 --> 00:42:03,970
So we have some contemporaneous
documentation as well as

1003
00:42:03,970 --> 00:42:06,340
his own later
recollections and so on.

1004
00:42:06,340 --> 00:42:09,520
We know that actually 10
years before this paper, back

1005
00:42:09,520 --> 00:42:12,370
when Einstein was
a mere teenager,

1006
00:42:12,370 --> 00:42:14,590
he'd like to pose these
kind of thought experiments

1007
00:42:14,590 --> 00:42:16,133
or questions to himself.

1008
00:42:16,133 --> 00:42:17,800
And one of the questions
he would return

1009
00:42:17,800 --> 00:42:20,560
to really over the
course of a full decade

1010
00:42:20,560 --> 00:42:23,410
was, what would it look like,
what would you experience,

1011
00:42:23,410 --> 00:42:25,390
if you could catch
up to a light wave?

1012
00:42:25,390 --> 00:42:29,680
And he reasoned it would be like
a surfer riding along an ocean

1013
00:42:29,680 --> 00:42:32,200
wave, that to us
on the shore, we'd

1014
00:42:32,200 --> 00:42:33,700
see both these things moving.

1015
00:42:33,700 --> 00:42:38,380
We would see a dynamical
wave moving over time,

1016
00:42:38,380 --> 00:42:41,110
not just a frozen
waveform in space.

1017
00:42:41,110 --> 00:42:42,940
But to the surfer,
if she's really

1018
00:42:42,940 --> 00:42:45,880
moving at the same
speed as the wave,

1019
00:42:45,880 --> 00:42:50,090
then she would see the
wave frozen in time.

1020
00:42:50,090 --> 00:42:55,000
It would be a crest
here, a trough there.

1021
00:42:55,000 --> 00:42:56,980
At least for some
extended period of time,

1022
00:42:56,980 --> 00:42:59,140
the surfer would
see the wave frozen,

1023
00:42:59,140 --> 00:43:04,090
not dynamical-- a frozen
waveform varying in space

1024
00:43:04,090 --> 00:43:06,077
but frozen in time.

1025
00:43:06,077 --> 00:43:08,410
So Einstein said that doesn't
make any sense if we think

1026
00:43:08,410 --> 00:43:09,493
about Maxwell's equations.

1027
00:43:09,493 --> 00:43:11,320
When he got a little
more sophisticated

1028
00:43:11,320 --> 00:43:13,210
and learned more
about Maxwell, he

1029
00:43:13,210 --> 00:43:16,120
said there's no solution
to Maxwell's equations.

1030
00:43:16,120 --> 00:43:17,980
If you're in a
source-free region,

1031
00:43:17,980 --> 00:43:20,500
if there's no clump of
electric charge around,

1032
00:43:20,500 --> 00:43:24,520
no electric currents around,
if both rho and J vanish,

1033
00:43:24,520 --> 00:43:27,730
then there's no way to
have spatially varying

1034
00:43:27,730 --> 00:43:30,430
electric and
magnetic fields that

1035
00:43:30,430 --> 00:43:33,130
are nonetheless static, right?

1036
00:43:33,130 --> 00:43:34,820
That seems to be
a contradiction.

1037
00:43:34,820 --> 00:43:36,730
So how do you get out
of this contradiction

1038
00:43:36,730 --> 00:43:40,460
of thinking you would have
a static waveform of light

1039
00:43:40,460 --> 00:43:42,460
if you could catch up and
move at the same speed

1040
00:43:42,460 --> 00:43:44,063
as that light wave?

1041
00:43:44,063 --> 00:43:44,980
How do you avoid that?

1042
00:43:44,980 --> 00:43:47,650
You just make sure you can
never catch up to the wave.

1043
00:43:47,650 --> 00:43:50,170
How can you never ever
catch up to the wave

1044
00:43:50,170 --> 00:43:52,960
if the wave is always
traveling at the speed c

1045
00:43:52,960 --> 00:43:56,240
even if you're traveling
on a very fast train or now

1046
00:43:56,240 --> 00:44:00,040
a hypersonic jet or a
spaceship of your imagination?

1047
00:44:00,040 --> 00:44:03,130
That second postulate
for Einstein we now know

1048
00:44:03,130 --> 00:44:06,280
was really the endpoint of
a 10-year series of thought

1049
00:44:06,280 --> 00:44:08,200
experiments about,
what would it look

1050
00:44:08,200 --> 00:44:09,940
like to catch up
with a light wave?

1051
00:44:09,940 --> 00:44:13,030
And he said that would make
so many other things mutually

1052
00:44:13,030 --> 00:44:13,630
inconsistent.

1053
00:44:13,630 --> 00:44:16,210
Let's make sure no one could
ever catch a light wave.

1054
00:44:16,210 --> 00:44:18,130
And that's what
starts driving much

1055
00:44:18,130 --> 00:44:21,190
of the rest of his thinking, not
worrying about interferometers

1056
00:44:21,190 --> 00:44:22,400
and all the rest.

1057
00:44:22,400 --> 00:44:26,080
So what he does, unlike Lorentz
and really all the masters

1058
00:44:26,080 --> 00:44:28,990
in the field at
the time, Einstein

1059
00:44:28,990 --> 00:44:32,950
begins with kinematics,
with the force-free motion

1060
00:44:32,950 --> 00:44:35,330
of bodies through
space and time,

1061
00:44:35,330 --> 00:44:36,940
which is a very
Machian thing to do.

1062
00:44:36,940 --> 00:44:37,780
What can we see?

1063
00:44:37,780 --> 00:44:40,750
We measure objects moving
through space and time.

1064
00:44:40,750 --> 00:44:43,243
Whereas people like Lorentz,
and Maxwell for that matter,

1065
00:44:43,243 --> 00:44:45,160
have been starting with
forces, with dynamics.

1066
00:44:45,160 --> 00:44:47,290
Remember, Lorentz
has this great idea

1067
00:44:47,290 --> 00:44:49,570
that there's a force from
the ether on the matter.

1068
00:44:49,570 --> 00:44:51,430
And in Michelson's
instrument, it's

1069
00:44:51,430 --> 00:44:53,500
all about dynamics, forces.

1070
00:44:53,500 --> 00:44:55,125
Einstein inverts that order.

1071
00:44:55,125 --> 00:44:56,500
He says forces
will be important.

1072
00:44:56,500 --> 00:44:59,140
But first let's make sure we
understand force-free motion

1073
00:44:59,140 --> 00:45:00,620
kinematics first.

1074
00:45:00,620 --> 00:45:03,190
After all, that's
what could become

1075
00:45:03,190 --> 00:45:05,962
objects of positive experience.

1076
00:45:05,962 --> 00:45:07,670
And so he has these
wonderful quotations.

1077
00:45:07,670 --> 00:45:09,970
It's now from the
introduction of the paper.

1078
00:45:09,970 --> 00:45:12,280
It's on the bottom of page 2.

1079
00:45:12,280 --> 00:45:16,360
He's really redescribing
how to lay out coordinates.

1080
00:45:16,360 --> 00:45:17,500
It sounds childish.

1081
00:45:17,500 --> 00:45:21,170
It sounds thoroughly
unprofessional to the folks

1082
00:45:21,170 --> 00:45:22,660
of the time.

1083
00:45:22,660 --> 00:45:26,110
Hendrik Lorentz says don't
bother with 'what do we mean

1084
00:45:26,110 --> 00:45:26,920
by coordinates?'

1085
00:45:26,920 --> 00:45:28,337
But Einstein was
convinced we have

1086
00:45:28,337 --> 00:45:30,600
to think through how
do we describe motion

1087
00:45:30,600 --> 00:45:32,333
through space and time first.

1088
00:45:32,333 --> 00:45:33,750
And I won't bother
reading it out.

1089
00:45:33,750 --> 00:45:34,700
I'll share this slide.

1090
00:45:34,700 --> 00:45:35,000
And you can see it.

1091
00:45:35,000 --> 00:45:37,208
But that's really what he's
doing in the very opening

1092
00:45:37,208 --> 00:45:39,230
paragraphs of this paper.

1093
00:45:39,230 --> 00:45:42,090
And that leads him to other
follow-up conclusions,

1094
00:45:42,090 --> 00:45:44,270
one of which very
famously becomes

1095
00:45:44,270 --> 00:45:45,993
known as the relativity
of simultaneity.

1096
00:45:45,993 --> 00:45:47,660
And you might have
heard of that before.

1097
00:45:47,660 --> 00:45:51,140
You'll spend more time
with this in IAP, I'm sure.

1098
00:45:51,140 --> 00:45:53,450
If there's no such
thing as absolute time,

1099
00:45:53,450 --> 00:45:56,660
if time is what we measure
with time-measuring devices

1100
00:45:56,660 --> 00:45:58,910
like clocks, if
that's all time is,

1101
00:45:58,910 --> 00:46:02,120
is what can be measured by an
actual instrument like a clock,

1102
00:46:02,120 --> 00:46:05,840
then how can we compare the
times in different places?

1103
00:46:05,840 --> 00:46:08,150
After all, I'm sitting
here with my clock.

1104
00:46:08,150 --> 00:46:09,740
Well, he says one
thing we could do

1105
00:46:09,740 --> 00:46:12,470
is trade light signals
because, at least according

1106
00:46:12,470 --> 00:46:15,650
to his postulate number
2, light is special.

1107
00:46:15,650 --> 00:46:18,620
If I throw ping pong balls,
that's not so special

1108
00:46:18,620 --> 00:46:21,880
because we will disagree
on the speed with which

1109
00:46:21,880 --> 00:46:23,500
those ping pong balls travel.

1110
00:46:23,500 --> 00:46:25,870
If I send a light wave,
then we had better

1111
00:46:25,870 --> 00:46:29,080
agree on the speed with which
that light moved from point A

1112
00:46:29,080 --> 00:46:31,210
to point B because
of postulate 2.

1113
00:46:31,210 --> 00:46:33,460
So he starts thinking about
one of his favorite things

1114
00:46:33,460 --> 00:46:34,870
in the universe, trains.

1115
00:46:34,870 --> 00:46:36,190
He loved trains.

1116
00:46:36,190 --> 00:46:37,610
It came up all the time.

1117
00:46:37,610 --> 00:46:40,000
So he imagines a train
moving along a platform

1118
00:46:40,000 --> 00:46:43,600
at a constant speed, a nice,
inertial frame of reference.

1119
00:46:43,600 --> 00:46:47,320
Einstein is standing here on the
embankment, the train platform.

1120
00:46:47,320 --> 00:46:48,430
He has two friends.

1121
00:46:48,430 --> 00:46:53,260
We can call them Alice
and Bob, A and B. Einstein

1122
00:46:53,260 --> 00:46:56,470
has marked himself out to be in
the perfect midpoint of where

1123
00:46:56,470 --> 00:46:58,570
Alice and Bob are standing.

1124
00:46:58,570 --> 00:47:00,560
So they mark this all
out ahead of time.

1125
00:47:00,560 --> 00:47:02,590
He marks himself
perfectly in the middle.

1126
00:47:02,590 --> 00:47:07,030
Alice and Bob are each equipped
with a lantern and a watch.

1127
00:47:07,030 --> 00:47:09,010
And by prior
arrangement, they say

1128
00:47:09,010 --> 00:47:12,640
at 12:00 noon on the dot,
turn on your lanterns.

1129
00:47:12,640 --> 00:47:15,790
So the light will
travel from both A and B

1130
00:47:15,790 --> 00:47:17,560
toward the midpoint.

1131
00:47:17,560 --> 00:47:19,780
Point your lanterns
toward Einstein

1132
00:47:19,780 --> 00:47:23,590
at point M. Turn them
on at the same time.

1133
00:47:23,590 --> 00:47:27,100
Meanwhile on this
zooming train, there's

1134
00:47:27,100 --> 00:47:29,320
another friend of
Einstein's sitting

1135
00:47:29,320 --> 00:47:31,150
at the midpoint of the train.

1136
00:47:31,150 --> 00:47:33,920
At the point, this is
Marge, M prime here.

1137
00:47:33,920 --> 00:47:36,670
She knows that she's in the
exact midpoint of the train.

1138
00:47:36,670 --> 00:47:38,890
She knows by prior
arrangement that Alice and Bob

1139
00:47:38,890 --> 00:47:42,140
are standing one train
length apart from each other.

1140
00:47:42,140 --> 00:47:45,160
And again, she's expecting that
they will turn their lanterns

1141
00:47:45,160 --> 00:47:47,030
on at the same time.

1142
00:47:47,030 --> 00:47:49,870
So how do these different
folks describe this series

1143
00:47:49,870 --> 00:47:51,490
of events that follow?

1144
00:47:51,490 --> 00:47:53,710
Let's focus first
on what Einstein

1145
00:47:53,710 --> 00:47:55,630
sees when he's standing
still on the train

1146
00:47:55,630 --> 00:47:59,560
platform, the embankment,
watching the train go by.

1147
00:47:59,560 --> 00:48:01,360
The observer who's
standing still

1148
00:48:01,360 --> 00:48:05,680
on the platform at point M
was, by prior arrangement,

1149
00:48:05,680 --> 00:48:08,405
at equal distance
from Alice and Bob.

1150
00:48:08,405 --> 00:48:09,280
I guess I changed it.

1151
00:48:09,280 --> 00:48:09,940
Now it's a she.

1152
00:48:09,940 --> 00:48:13,060
So Mileva, his wife,
is standing there.

1153
00:48:13,060 --> 00:48:14,320
Maybe M stands for Mileva.

1154
00:48:14,320 --> 00:48:17,170
So she receives the light
waves from points A and B

1155
00:48:17,170 --> 00:48:18,560
at the same time.

1156
00:48:18,560 --> 00:48:22,090
So she can conclude that the
light flashes were emitted

1157
00:48:22,090 --> 00:48:26,350
at the same time, that the
event of person A and person

1158
00:48:26,350 --> 00:48:29,350
B shining the lights, those
events were simultaneous.

1159
00:48:29,350 --> 00:48:30,730
How do you know?

1160
00:48:30,730 --> 00:48:34,560
Because the light had
equal distances to travel.

1161
00:48:34,560 --> 00:48:37,920
And the light could only
travel at but one speed.

1162
00:48:37,920 --> 00:48:39,520
And they arrived
at the same time.

1163
00:48:39,520 --> 00:48:41,760
They must have started the
journeys at the same time.

1164
00:48:41,760 --> 00:48:43,530
The act of shining
those lanterns

1165
00:48:43,530 --> 00:48:46,480
must have been simultaneous.

1166
00:48:46,480 --> 00:48:50,080
But what about the person
who's on the moving train?

1167
00:48:50,080 --> 00:48:56,500
She sees the following as she is
zooming along at point M prime.

1168
00:48:56,500 --> 00:49:01,120
She sees the light from point
B arrive at her location first.

1169
00:49:01,120 --> 00:49:02,945
Now, Mileva or
Einstein would say, oh,

1170
00:49:02,945 --> 00:49:04,570
that's because you're
racing toward it.

1171
00:49:04,570 --> 00:49:06,430
She says, not so fast.

1172
00:49:06,430 --> 00:49:08,200
I'm in a perfectly
self-consistent

1173
00:49:08,200 --> 00:49:09,790
mechanical system.

1174
00:49:09,790 --> 00:49:12,610
All the laws of physics work
as well for me as for you.

1175
00:49:12,610 --> 00:49:14,140
And the light
couldn't have sped up

1176
00:49:14,140 --> 00:49:15,940
or slowed down by
your own postulate,

1177
00:49:15,940 --> 00:49:18,340
she would say-- she
would be entitled to say.

1178
00:49:18,340 --> 00:49:21,460
So if she's equal distant
between points A and B

1179
00:49:21,460 --> 00:49:24,160
and she receives the
light from B first,

1180
00:49:24,160 --> 00:49:28,210
the only possible explanation
is that the lights were not

1181
00:49:28,210 --> 00:49:31,810
shown simultaneously, that A
and B did not turn the lanterns

1182
00:49:31,810 --> 00:49:33,500
on at the same time.

1183
00:49:33,500 --> 00:49:36,940
So the person on the train
platform and the person

1184
00:49:36,940 --> 00:49:39,460
on the moving train
disagree about what

1185
00:49:39,460 --> 00:49:41,110
happens simultaneously.

1186
00:49:41,110 --> 00:49:43,840
This becomes known as the
relativity of simultaneity.

1187
00:49:43,840 --> 00:49:45,280
So who's correct?

1188
00:49:45,280 --> 00:49:48,460
I've already said, according
to Einstein, no one or both

1189
00:49:48,460 --> 00:49:51,430
of them because of postulate 1.

1190
00:49:51,430 --> 00:49:53,050
They're both
entitled to work out

1191
00:49:53,050 --> 00:49:57,610
a perfectly
self-consistent set of laws

1192
00:49:57,610 --> 00:50:00,700
involving electromagnetism,
optics, and mechanics.

1193
00:50:00,700 --> 00:50:02,260
They agree they were
at the midpoint.

1194
00:50:02,260 --> 00:50:04,385
They agree that light didn't
speed up or slow down.

1195
00:50:04,385 --> 00:50:06,430
So they are both
right, which is to say

1196
00:50:06,430 --> 00:50:08,470
there is no right answer
to the question, what

1197
00:50:08,470 --> 00:50:10,150
was really simultaneous?

1198
00:50:10,150 --> 00:50:13,600
Simultaneity becomes relative
to one's frame of reference.

1199
00:50:13,600 --> 00:50:16,900
That's a pretty deep change
from the Newtonian system.

1200
00:50:16,900 --> 00:50:19,330
And as Einstein says--
again, it's bottom of page 2,

1201
00:50:19,330 --> 00:50:20,950
very early in the paper--

1202
00:50:20,950 --> 00:50:23,830
"we see that we can
attribute no absolute meaning

1203
00:50:23,830 --> 00:50:27,100
to the concept of simultaneity,
but that two events which

1204
00:50:27,100 --> 00:50:28,660
examined from a
coordinate system

1205
00:50:28,660 --> 00:50:32,140
are simultaneous"-- like
him on the embankment--

1206
00:50:32,140 --> 00:50:35,142
"can no longer be interpreted
as simultaneous events when

1207
00:50:35,142 --> 00:50:36,850
examined from a system
which is in motion

1208
00:50:36,850 --> 00:50:38,020
relative to that system."

1209
00:50:38,020 --> 00:50:39,520
OK, that's about kinematics.

1210
00:50:39,520 --> 00:50:43,090
It's not about forces or
resistive medium or dynamics.

1211
00:50:45,690 --> 00:50:48,620
So now he goes on-- again,
very early on in the paper--

1212
00:50:48,620 --> 00:50:51,360
once you get to the relativity
of simultaneity-- again,

1213
00:50:51,360 --> 00:50:53,810
as you'll have a chance to
unpack with more patience

1214
00:50:53,810 --> 00:50:55,820
in the coming days and weeks--

1215
00:50:55,820 --> 00:50:58,370
other strange phenomena
seem to follow from that,

1216
00:50:58,370 --> 00:50:59,780
argues Einstein.

1217
00:50:59,780 --> 00:51:03,360
One of the first he talks
about is length contraction.

1218
00:51:03,360 --> 00:51:05,480
So this had been found
and published by Lorentz.

1219
00:51:05,480 --> 00:51:08,750
Einstein is about to give an
entirely different derivation,

1220
00:51:08,750 --> 00:51:12,147
even though quantitatively it's
the same form of the equation.

1221
00:51:12,147 --> 00:51:14,105
For Einstein, it has
nothing to do with forces.

1222
00:51:14,105 --> 00:51:17,150
It's all about kinematics.

1223
00:51:17,150 --> 00:51:19,600
In general, like a
good Machian, he asks,

1224
00:51:19,600 --> 00:51:21,350
how do we measure the
length of an object?

1225
00:51:21,350 --> 00:51:25,460
How do we make length an
object of positive experience?

1226
00:51:25,460 --> 00:51:28,322
Well, at the same time
measure the location

1227
00:51:28,322 --> 00:51:30,530
of the front of the object
and the back of the object

1228
00:51:30,530 --> 00:51:31,890
and take the difference.

1229
00:51:31,890 --> 00:51:33,350
So if you want to measure
the length of the train,

1230
00:51:33,350 --> 00:51:34,010
that's fine.

1231
00:51:34,010 --> 00:51:35,990
Just measure where the
front is and the back

1232
00:51:35,990 --> 00:51:38,390
is at the same time,
and then mark off

1233
00:51:38,390 --> 00:51:41,900
the difference between those
two locations in space.

1234
00:51:41,900 --> 00:51:44,510
Our friends Alice and Bob
can do that with the train.

1235
00:51:44,510 --> 00:51:52,010
And they find the answer capital
L. The train is length L long.

1236
00:51:52,010 --> 00:51:54,410
Meanwhile, Alice and Bob--

1237
00:51:54,410 --> 00:51:56,870
and I should say that's
when the train is at rest.

1238
00:51:56,870 --> 00:51:59,120
When they do this
with a moving train,

1239
00:51:59,120 --> 00:52:01,700
Alice and Bob measure the
length of a moving train,

1240
00:52:01,700 --> 00:52:03,950
they measure some
length shorter than what

1241
00:52:03,950 --> 00:52:06,470
the person on the
train was expecting.

1242
00:52:06,470 --> 00:52:10,190
The person on the train expected
the full length L. The person

1243
00:52:10,190 --> 00:52:12,920
who's moving with the object,
either at rest with it

1244
00:52:12,920 --> 00:52:17,090
or moving together with it,
says the train is L units long.

1245
00:52:17,090 --> 00:52:19,280
But when Alice and
Bob measure the train

1246
00:52:19,280 --> 00:52:21,590
by measuring the front
and back at the same time,

1247
00:52:21,590 --> 00:52:23,120
they find a shorter answer.

1248
00:52:23,120 --> 00:52:25,430
They found it shrunk along
the direction of motion

1249
00:52:25,430 --> 00:52:28,130
by some specific amount.

1250
00:52:28,130 --> 00:52:29,180
Now, how could that be?

1251
00:52:29,180 --> 00:52:32,070
Remember, we disagree
about "at the same time."

1252
00:52:32,070 --> 00:52:33,740
So if we disagree
about simultaneity,

1253
00:52:33,740 --> 00:52:35,900
we will-- perforce,
we must start

1254
00:52:35,900 --> 00:52:38,250
to disagree about lengths.

1255
00:52:38,250 --> 00:52:41,870
So the person on the train says,
you did your measurement wrong.

1256
00:52:41,870 --> 00:52:44,660
You measured the front
of the train first.

1257
00:52:44,660 --> 00:52:47,990
We know that event B
happened first, she says,

1258
00:52:47,990 --> 00:52:50,997
because we got the light
from event B first.

1259
00:52:50,997 --> 00:52:52,580
So you measured the
front of the train

1260
00:52:52,580 --> 00:52:55,880
and then waited while the train
slid and then later measured

1261
00:52:55,880 --> 00:52:57,620
the position of the
rear of the train.

1262
00:52:57,620 --> 00:52:59,810
So of course you have
a shorter distance

1263
00:52:59,810 --> 00:53:01,640
because you performed
a bad measurement,

1264
00:53:01,640 --> 00:53:03,140
because you didn't
measure the front

1265
00:53:03,140 --> 00:53:04,940
and back at the same time.

1266
00:53:04,940 --> 00:53:06,065
We say, don't be silly.

1267
00:53:06,065 --> 00:53:07,190
They were at the same time.

1268
00:53:07,190 --> 00:53:09,440
We checked by
trading light beams.

1269
00:53:09,440 --> 00:53:10,820
We know they were simultaneous.

1270
00:53:10,820 --> 00:53:12,560
And we got the answer L prime.

1271
00:53:12,560 --> 00:53:13,820
Your train is short.

1272
00:53:13,820 --> 00:53:14,630
Who's right?

1273
00:53:14,630 --> 00:53:18,170
Sort of both and neither,
that because we disagree

1274
00:53:18,170 --> 00:53:21,230
on simultaneity, we
disagree on the outcomes

1275
00:53:21,230 --> 00:53:25,310
of simultaneous events like
measurements of length.

1276
00:53:25,310 --> 00:53:27,950
And so he goes on with
just a few lines of algebra

1277
00:53:27,950 --> 00:53:30,170
to find the exact form
of what was already

1278
00:53:30,170 --> 00:53:32,960
known as the Lorentz
contraction, that same factor

1279
00:53:32,960 --> 00:53:36,800
gamma that you'll
be using all IAP,

1280
00:53:36,800 --> 00:53:40,010
that the measurement
of an object in motion

1281
00:53:40,010 --> 00:53:42,380
shrinks along the
direction of motion.

1282
00:53:42,380 --> 00:53:44,330
Whereas if we were at
rest with that object,

1283
00:53:44,330 --> 00:53:45,770
we'd find a longer length.

1284
00:53:45,770 --> 00:53:48,950
And the amount it shrinks
depends on that relative speed

1285
00:53:48,950 --> 00:53:50,045
v over c.

1286
00:53:50,045 --> 00:53:53,270
It depends, in particular,
on this form gamma.

1287
00:53:53,270 --> 00:53:56,180
This has nothing, for Einstein,
to do with forces or dynamics.

1288
00:53:56,180 --> 00:53:59,390
It's a totally different
derivation than Lorentz's.

1289
00:53:59,390 --> 00:54:02,570
It's simply a consequence of
kinematics and, in particular,

1290
00:54:02,570 --> 00:54:04,830
about simultaneity.

1291
00:54:04,830 --> 00:54:06,210
It's a very simple exercise.

1292
00:54:06,210 --> 00:54:09,030
You'll probably wind
up doing it very soon

1293
00:54:09,030 --> 00:54:12,000
to find a similar consequence
for the rate at which clocks

1294
00:54:12,000 --> 00:54:12,720
will tick.

1295
00:54:12,720 --> 00:54:15,300
This has now became
known as time dilation.

1296
00:54:15,300 --> 00:54:18,570
Imagine a very simple kind
of clock, the simplest one--

1297
00:54:18,570 --> 00:54:21,570
two highly polished
mirrors with a light beam

1298
00:54:21,570 --> 00:54:24,580
that just bounces straight
back and forth between them.

1299
00:54:24,580 --> 00:54:27,030
If the clock is at rest
with respect to us,

1300
00:54:27,030 --> 00:54:30,510
we hold the clock perfectly
still at a fixed height,

1301
00:54:30,510 --> 00:54:32,280
then the time it
takes for that light

1302
00:54:32,280 --> 00:54:35,430
to travel from the bottom
mirror to the top mirror

1303
00:54:35,430 --> 00:54:37,830
will count as one
tick of our clock.

1304
00:54:37,830 --> 00:54:39,030
[SNAPPING]

1305
00:54:39,030 --> 00:54:41,560
And we know that light can
only travel at a fixed speed.

1306
00:54:41,560 --> 00:54:44,190
So it's a really
great, uniform clock.

1307
00:54:44,190 --> 00:54:46,780
As long as we can hold
that height really fixed,

1308
00:54:46,780 --> 00:54:50,070
this is a great way of defining
a unit of time and therefore

1309
00:54:50,070 --> 00:54:51,870
our clock rate.

1310
00:54:51,870 --> 00:54:54,390
What happens if our friend
has an identical clock,

1311
00:54:54,390 --> 00:54:57,150
same mirrors, held equally
rigidly at the same height

1312
00:54:57,150 --> 00:55:01,350
apart but zooming past
us on that moving train?

1313
00:55:01,350 --> 00:55:04,230
Then we watch the light
have to travel this longer

1314
00:55:04,230 --> 00:55:08,585
path because after the light
beam leaves the bottom mirror,

1315
00:55:08,585 --> 00:55:09,960
in order to reach
the top mirror,

1316
00:55:09,960 --> 00:55:13,080
the whole assembly has moved
with the train to the right,

1317
00:55:13,080 --> 00:55:16,860
some distance v times t
in the time during which

1318
00:55:16,860 --> 00:55:19,410
that light beam was in transit.

1319
00:55:19,410 --> 00:55:21,690
So it has to travel
the hypotenuse

1320
00:55:21,690 --> 00:55:22,800
of this right triangle.

1321
00:55:22,800 --> 00:55:24,780
Instead of just this
simple up and down,

1322
00:55:24,780 --> 00:55:27,750
it travels perforce
a longer distance

1323
00:55:27,750 --> 00:55:29,550
and likewise back down.

1324
00:55:29,550 --> 00:55:32,970
So our observation
of the moving clock

1325
00:55:32,970 --> 00:55:36,870
is that it takes
longer between ticks.

1326
00:55:36,870 --> 00:55:38,910
That becomes known
as time dilation.

1327
00:55:38,910 --> 00:55:41,830
The time between
ticks has stretched,

1328
00:55:41,830 --> 00:55:46,170
which is to say the clock is
running slowly as we measure.

1329
00:55:46,170 --> 00:55:48,780
We measure the moving
clock to run slowly

1330
00:55:48,780 --> 00:55:51,270
by that same factor gamma.

1331
00:55:51,270 --> 00:55:53,160
The time between
ticks gets stretched.

1332
00:55:53,160 --> 00:55:56,010
Remember, gamma
is greater than 1.

1333
00:55:56,010 --> 00:55:58,950
So the time between
ticks gets stretched.

1334
00:55:58,950 --> 00:56:02,100
And that's the same with
the clock is running slowly.

1335
00:56:02,100 --> 00:56:04,558
Meanwhile, the person on the
train says, you're ridiculous.

1336
00:56:04,558 --> 00:56:06,183
I had the clock with
me the whole time.

1337
00:56:06,183 --> 00:56:07,110
It kept perfect time.

1338
00:56:07,110 --> 00:56:08,790
But your clock ran slowly.

1339
00:56:08,790 --> 00:56:10,920
I watched as I ran
by the station.

1340
00:56:10,920 --> 00:56:14,590
Perfectly symmetrical
conclusions.

1341
00:56:14,590 --> 00:56:17,610
So Einstein was working all
this out, very much inspired

1342
00:56:17,610 --> 00:56:21,810
by Ernst Mach, by this notion
of positivism, of what can we

1343
00:56:21,810 --> 00:56:24,840
hope to measure, not
starting with forces

1344
00:56:24,840 --> 00:56:27,570
but with the force-free
motion of objects

1345
00:56:27,570 --> 00:56:30,420
through space and time, and
really trying to bring it back

1346
00:56:30,420 --> 00:56:31,740
to, what would be measurable?

1347
00:56:31,740 --> 00:56:35,100
What would I see if I
were in this situation?

1348
00:56:35,100 --> 00:56:38,610
One of his teachers at
the ETH, a mathematician

1349
00:56:38,610 --> 00:56:42,450
named Hermann Minkowski, had
formed a deservedly low opinion

1350
00:56:42,450 --> 00:56:43,710
of student Einstein.

1351
00:56:43,710 --> 00:56:46,060
Einstein used to cut
class, was very rude.

1352
00:56:46,060 --> 00:56:49,290
But one of their mutual
friends, after Einstein's paper

1353
00:56:49,290 --> 00:56:52,710
came out, sent the paper to
Minkowski saying, you know,

1354
00:56:52,710 --> 00:56:54,370
it's actually kind
of interesting.

1355
00:56:54,370 --> 00:56:56,610
Take a look, to paraphrase.

1356
00:56:56,610 --> 00:56:59,700
And Minkowski said,
well, despite the fact

1357
00:56:59,700 --> 00:57:02,430
that I knew Einstein was never
going to amount to anything,

1358
00:57:02,430 --> 00:57:03,300
I read the paper.

1359
00:57:03,300 --> 00:57:05,310
And I was right, Minkowski says.

1360
00:57:05,310 --> 00:57:08,110
He really made a mess
of this, as well.

1361
00:57:08,110 --> 00:57:11,010
So Minkowski's just
interested enough

1362
00:57:11,010 --> 00:57:13,380
to redo Einstein's
work in a form that

1363
00:57:13,380 --> 00:57:15,090
made much more
sense to Minkowski,

1364
00:57:15,090 --> 00:57:18,060
who was, after all, a
professional geometer.

1365
00:57:18,060 --> 00:57:19,450
He wasn't just a mathematician.

1366
00:57:19,450 --> 00:57:21,660
He loved geometry most of all.

1367
00:57:21,660 --> 00:57:24,600
So Minkowski, not
Einstein, is the one

1368
00:57:24,600 --> 00:57:26,310
who actually brings
these pieces together

1369
00:57:26,310 --> 00:57:28,470
a few years later
in-- it was published

1370
00:57:28,470 --> 00:57:31,410
in 1908 instead of 1905.

1371
00:57:31,410 --> 00:57:34,080
And he says, all this
talk about moving trains

1372
00:57:34,080 --> 00:57:37,830
and polished mirrors, all of
that's just a distraction.

1373
00:57:37,830 --> 00:57:40,860
To a properly trained
mathematician,

1374
00:57:40,860 --> 00:57:43,350
which Einstein was
not, all we're doing

1375
00:57:43,350 --> 00:57:46,290
is performing rotations
or projections

1376
00:57:46,290 --> 00:57:49,200
in a certain kind of
space, in this case

1377
00:57:49,200 --> 00:57:53,940
a spacetime in which there's,
say, one direction of space

1378
00:57:53,940 --> 00:57:56,100
running along the
horizontal axis

1379
00:57:56,100 --> 00:57:58,990
and one dimension of
time running up the page.

1380
00:57:58,990 --> 00:58:01,140
We now call these
Minkowski diagrams

1381
00:58:01,140 --> 00:58:04,380
in his honor, or simply
spacetime diagrams.

1382
00:58:04,380 --> 00:58:06,720
None of this appears in
Einstein's early work.

1383
00:58:06,720 --> 00:58:10,020
This was done by his
former teacher in response

1384
00:58:10,020 --> 00:58:13,680
to what Minkowski considered
to be Einstein's continuing

1385
00:58:13,680 --> 00:58:15,810
confusion and sloppiness.

1386
00:58:15,810 --> 00:58:19,740
And there's a benefit from
doing that, Minkowski finds.

1387
00:58:19,740 --> 00:58:23,100
He agrees with Einstein
that we will disagree

1388
00:58:23,100 --> 00:58:24,760
on certain kinds
of measurements,

1389
00:58:24,760 --> 00:58:27,360
for example lengths
and durations of time,

1390
00:58:27,360 --> 00:58:29,730
because we disagree
on simultaneity.

1391
00:58:29,730 --> 00:58:31,800
But Minkowski finds
something that had not been

1392
00:58:31,800 --> 00:58:33,640
so clear to Einstein himself.

1393
00:58:33,640 --> 00:58:37,380
There are combinations of
those kinds of intervals

1394
00:58:37,380 --> 00:58:39,450
on which we will all agree.

1395
00:58:39,450 --> 00:58:43,080
This is what we would now
call a spacetime invariant.

1396
00:58:43,080 --> 00:58:46,590
Put together, a combination
of the time interval

1397
00:58:46,590 --> 00:58:51,630
minus the space interval, a
relative minus sign and each

1398
00:58:51,630 --> 00:58:53,880
of those quantities
squared, that combination

1399
00:58:53,880 --> 00:58:55,650
we will all agree
on as long as we're

1400
00:58:55,650 --> 00:58:59,790
each in states of
non-accelerating motion

1401
00:58:59,790 --> 00:59:01,500
even though we
disagree separately

1402
00:59:01,500 --> 00:59:04,530
on the duration between two
events, delta t, or the lengths

1403
00:59:04,530 --> 00:59:06,030
between them, delta x.

1404
00:59:06,030 --> 00:59:09,240
And that, from Minkowski, is
what any geometer should do.

1405
00:59:09,240 --> 00:59:11,130
Some relationships
remain invariant

1406
00:59:11,130 --> 00:59:12,600
even under changes
of coordinates.

1407
00:59:12,600 --> 00:59:16,440
And for Minkowski, that was
what any undergraduate geometry

1408
00:59:16,440 --> 00:59:18,430
student should know to do first.

1409
00:59:18,430 --> 00:59:21,060
So we actually have
this notion of one kind

1410
00:59:21,060 --> 00:59:25,095
of thing called spacetime coming
from Minkowski kind of redoing

1411
00:59:25,095 --> 00:59:26,340
Einstein's work.

1412
00:59:26,340 --> 00:59:29,160
And as Minkowski famously
writes in his own article--

1413
00:59:29,160 --> 00:59:31,360
it was published posthumously;
he died quite young.

1414
00:59:31,360 --> 00:59:32,340
And he gave a lecture.

1415
00:59:32,340 --> 00:59:34,560
And it was published
soon after he died.

1416
00:59:34,560 --> 00:59:37,420
And he wrote upon
introducing this new work,

1417
00:59:37,420 --> 00:59:40,500
"Henceforth space
by itself and time

1418
00:59:40,500 --> 00:59:44,070
by itself are doomed to
fade away into mere shadows

1419
00:59:44,070 --> 00:59:46,323
and only a kind of
union of the two

1420
00:59:46,323 --> 00:59:47,490
will preserve independence."

1421
00:59:47,490 --> 00:59:49,740
And by shadows, he really
meant projections, just

1422
00:59:49,740 --> 00:59:53,280
drop perpendicular to
the appropriate axis

1423
00:59:53,280 --> 00:59:55,380
and then think about
coordinate transformations

1424
00:59:55,380 --> 00:59:58,883
in that spacetime of x and t.

1425
00:59:58,883 --> 01:00:00,300
So that's, again,
something you'll

1426
01:00:00,300 --> 01:00:03,030
get much more practice with if
you haven't seen it already.

1427
01:00:03,030 --> 01:00:03,960
But that's a preview.

1428
01:00:03,960 --> 01:00:06,138
That comes in response
to Einstein's work.

1429
01:00:06,138 --> 01:00:07,680
It wasn't an Einstein's
own original.

1430
01:00:07,680 --> 01:00:10,440
Although, just to jump
ahead very briefly,

1431
01:00:10,440 --> 01:00:12,185
Einstein first thought
that was horrible

1432
01:00:12,185 --> 01:00:13,560
because he didn't
like Minkowski.

1433
01:00:13,560 --> 01:00:15,820
But then over the
next several years,

1434
01:00:15,820 --> 01:00:17,910
the better part of
a decade, Einstein

1435
01:00:17,910 --> 01:00:20,310
himself became more impressed
by that way of thinking

1436
01:00:20,310 --> 01:00:22,465
and really adopted it
for his own later work

1437
01:00:22,465 --> 01:00:24,840
on what became known as the
general theory of relativity.

1438
01:00:24,840 --> 01:00:26,010
That's for later.

1439
01:00:29,470 --> 01:00:31,998
OK, very briefly
here's the third part

1440
01:00:31,998 --> 01:00:33,790
of the material I wanted
to share with you.

1441
01:00:33,790 --> 01:00:36,123
And then hopefully there'll
be some time for discussion,

1442
01:00:36,123 --> 01:00:37,190
as well.

1443
01:00:37,190 --> 01:00:39,670
So this last part
is much more brief.

1444
01:00:39,670 --> 01:00:41,680
But it's, how do we
make sense of this?

1445
01:00:41,680 --> 01:00:44,500
Why was Einstein doing all
this seemingly unusual,

1446
01:00:44,500 --> 01:00:46,210
maybe even crazy
stuff, certainly

1447
01:00:46,210 --> 01:00:48,520
out of step with Lorentz,
with his own teachers

1448
01:00:48,520 --> 01:00:50,530
like Minkowski?

1449
01:00:50,530 --> 01:00:53,020
How can we account for
Einstein's quite idiosyncratic

1450
01:00:53,020 --> 01:00:56,350
approach to what was otherwise
a common set of questions

1451
01:00:56,350 --> 01:00:58,745
about the electrodynamics
of moving bodies?

1452
01:00:58,745 --> 01:01:01,120
Now, here's some of my favorite
work by other colleagues,

1453
01:01:01,120 --> 01:01:03,010
by other historians and
physicists who really

1454
01:01:03,010 --> 01:01:06,170
looked at this in great detail.

1455
01:01:06,170 --> 01:01:08,350
So for a long, long
time, I mean literally

1456
01:01:08,350 --> 01:01:10,180
for the better
part of 100 years,

1457
01:01:10,180 --> 01:01:13,010
since people began taking
relativity seriously,

1458
01:01:13,010 --> 01:01:17,555
which is a long time by now, it
had become very common to say,

1459
01:01:17,555 --> 01:01:19,180
well, Einstein must
have been motivated

1460
01:01:19,180 --> 01:01:21,370
much like Hendrik Lorentz
was, really like that

1461
01:01:21,370 --> 01:01:23,530
much of that generation was.

1462
01:01:23,530 --> 01:01:25,690
Einstein must have
been responding

1463
01:01:25,690 --> 01:01:28,780
to the null result from
Michelson and Morley

1464
01:01:28,780 --> 01:01:31,060
and their interferometer,
that Einstein

1465
01:01:31,060 --> 01:01:34,030
must have been trying to explain
why they couldn't measure

1466
01:01:34,030 --> 01:01:37,240
our motion through the ether
much as Lorentz, we know,

1467
01:01:37,240 --> 01:01:39,190
very explicitly
was trying to do.

1468
01:01:39,190 --> 01:01:42,130
And there's a fascinating--
one of the first really careful

1469
01:01:42,130 --> 01:01:45,040
examinations of that claim
was by Gerald Holton, who's

1470
01:01:45,040 --> 01:01:47,380
a real kind of hero of mine.

1471
01:01:47,380 --> 01:01:51,700
Holton did his first
PhD in physics,

1472
01:01:51,700 --> 01:01:54,460
low-temperature physics, and
then retooled many years later

1473
01:01:54,460 --> 01:01:55,750
in the history of science.

1474
01:01:55,750 --> 01:01:58,450
And he wrote a series of
really remarkable essays

1475
01:01:58,450 --> 01:02:02,320
about Einstein once a lot of
Einstein's papers and letters

1476
01:02:02,320 --> 01:02:04,330
and notes became available.

1477
01:02:04,330 --> 01:02:06,010
Holton was among the
first to really dig

1478
01:02:06,010 --> 01:02:09,040
through these with
great care and almost

1479
01:02:09,040 --> 01:02:11,650
like a detective going
through the evidence.

1480
01:02:11,650 --> 01:02:13,450
And what Holton
concluded was it's

1481
01:02:13,450 --> 01:02:15,580
not clear that
Einstein even knew

1482
01:02:15,580 --> 01:02:17,890
about the Michelson-Morley
interferometer.

1483
01:02:17,890 --> 01:02:20,268
Lorentz knew about it and
followed it carefully.

1484
01:02:20,268 --> 01:02:22,810
There's very little trace that
Einstein paid it any attention

1485
01:02:22,810 --> 01:02:23,310
at all.

1486
01:02:23,310 --> 01:02:25,780
If he did know about it,
it was kind of in passing.

1487
01:02:25,780 --> 01:02:29,560
It was certainly not front of
mind in the months and years

1488
01:02:29,560 --> 01:02:32,110
leading up to the 1905 paper.

1489
01:02:32,110 --> 01:02:34,000
He might have known
about it secondhand

1490
01:02:34,000 --> 01:02:36,340
from reading through all
the footnotes of Lorentz.

1491
01:02:36,340 --> 01:02:38,800
But it's not something he
seems to have been paying

1492
01:02:38,800 --> 01:02:42,110
much attention to at all.

1493
01:02:42,110 --> 01:02:43,780
And so there's nice
textual evidence

1494
01:02:43,780 --> 01:02:45,940
even in Einstein's
paper, that 1905 paper

1495
01:02:45,940 --> 01:02:48,910
that I keep showing some
brief excerpts from.

1496
01:02:48,910 --> 01:02:51,790
Again, this is on the
first or second page.

1497
01:02:51,790 --> 01:02:56,650
When he's trying to reason why
we should think about maybe

1498
01:02:56,650 --> 01:02:58,990
giving up on the
ether, he mentions--

1499
01:02:58,990 --> 01:03:00,730
without citing any
of them-- he mentions

1500
01:03:00,730 --> 01:03:03,700
several unsuccessful
attempts to discover

1501
01:03:03,700 --> 01:03:06,850
any motion of the Earth relative
to the light medium, the ether.

1502
01:03:06,850 --> 01:03:08,693
He doesn't say
which ones he means.

1503
01:03:08,693 --> 01:03:11,110
But he actually then goes on
to say in the same paragraph,

1504
01:03:11,110 --> 01:03:14,875
none of these have found any
deviation to first order in v

1505
01:03:14,875 --> 01:03:16,030
over c.

1506
01:03:16,030 --> 01:03:18,040
And what's so striking,
as Holton reminds us,

1507
01:03:18,040 --> 01:03:20,950
is Michelson-Morley was
actually second order.

1508
01:03:20,950 --> 01:03:23,650
Einstein seems to be talking
about a whole different class

1509
01:03:23,650 --> 01:03:26,133
of earlier experiments
even before Michelson

1510
01:03:26,133 --> 01:03:26,800
got in the game.

1511
01:03:29,500 --> 01:03:31,030
He's certainly
not obsessing over

1512
01:03:31,030 --> 01:03:32,822
the Michelson-Morley
interferometer the way

1513
01:03:32,822 --> 01:03:34,180
other people were.

1514
01:03:34,180 --> 01:03:37,060
And so some people then
read Holton's analysis

1515
01:03:37,060 --> 01:03:39,322
to say a kind of overcorrection.

1516
01:03:39,322 --> 01:03:40,780
Does that mean
Einstein didn't care

1517
01:03:40,780 --> 01:03:43,060
about any kinds of
experiments at all?

1518
01:03:43,060 --> 01:03:45,220
If he wasn't trying
to respond directly

1519
01:03:45,220 --> 01:03:46,750
to the Michelson-Morley
experiment,

1520
01:03:46,750 --> 01:03:49,635
was he just off in a kind of
philosophical dream world?

1521
01:03:49,635 --> 01:03:51,010
And that's where
I think we again

1522
01:03:51,010 --> 01:03:53,770
can now say quite firmly no.

1523
01:03:53,770 --> 01:03:55,330
In fact, it makes
sense to go back

1524
01:03:55,330 --> 01:03:59,770
to Einstein's own early
years, his early fascination

1525
01:03:59,770 --> 01:04:02,490
with electrotechnical
gadgets, and remind ourselves

1526
01:04:02,490 --> 01:04:07,660
what his day job was in the
early 1900s, including 1905.

1527
01:04:07,660 --> 01:04:11,200
Put him back in the patent
office in Central Europe

1528
01:04:11,200 --> 01:04:13,610
at this very specific moment.

1529
01:04:13,610 --> 01:04:16,870
So let's think about Einstein's
favorite technology, trains.

1530
01:04:16,870 --> 01:04:18,820
Trains and railroads
themselves were actually

1531
01:04:18,820 --> 01:04:21,170
fairly new in the 19th century.

1532
01:04:21,170 --> 01:04:24,640
They were introduced to
widespread commercial usage

1533
01:04:24,640 --> 01:04:28,090
for commercial
transportation really only

1534
01:04:28,090 --> 01:04:29,950
in the 1830s and '40s.

1535
01:04:29,950 --> 01:04:31,870
Remember, Einstein
was born in 1879.

1536
01:04:31,870 --> 01:04:35,650
They were relatively new
even in his childhood.

1537
01:04:35,650 --> 01:04:39,770
During this whole time until
very late in the 19th century,

1538
01:04:39,770 --> 01:04:42,610
there were no
coordinated time zones.

1539
01:04:42,610 --> 01:04:45,340
There was nothing like Eastern
Standard Time or Pacific

1540
01:04:45,340 --> 01:04:48,580
Time or Central European Time.

1541
01:04:48,580 --> 01:04:51,160
Each town set its
own local time.

1542
01:04:51,160 --> 01:04:52,930
And all the residents
could coordinate

1543
01:04:52,930 --> 01:04:56,200
by agreeing that we'll use
that clock tower, either

1544
01:04:56,200 --> 01:04:59,710
some municipal building or
often a tall church in the town

1545
01:04:59,710 --> 01:05:02,890
square, say that's
our local time.

1546
01:05:02,890 --> 01:05:04,630
We'll set our local
watches to when

1547
01:05:04,630 --> 01:05:07,810
that clock chimes 12:00 noon.

1548
01:05:07,810 --> 01:05:09,730
So each town kept
its own local time.

1549
01:05:09,730 --> 01:05:12,790
And before railway, who would
ever need anything else?

1550
01:05:12,790 --> 01:05:15,910
Because one was rarely
encountering more than one

1551
01:05:15,910 --> 01:05:18,250
town in a given day.

1552
01:05:18,250 --> 01:05:21,730
After the advent of railroads,
you still have this local time.

1553
01:05:21,730 --> 01:05:24,070
But that became more
and more of a problem.

1554
01:05:24,070 --> 01:05:26,020
It was true in North
America as in Europe

1555
01:05:26,020 --> 01:05:28,650
and other parts of
the world, as well.

1556
01:05:28,650 --> 01:05:32,480
So here, closer to
Boston, passengers

1557
01:05:32,480 --> 01:05:35,060
riding on the train
between Boston and New York

1558
01:05:35,060 --> 01:05:37,580
during this time would have
to change their watches

1559
01:05:37,580 --> 01:05:39,590
by an average of 37 minutes.

1560
01:05:39,590 --> 01:05:41,360
Today we're all in
the same time zone.

1561
01:05:41,360 --> 01:05:44,360
But that's how different
the local variations

1562
01:05:44,360 --> 01:05:47,180
tended to be just
up and down the US

1563
01:05:47,180 --> 01:05:49,280
portion of the
mid-Atlantic seacoast

1564
01:05:49,280 --> 01:05:51,290
and likewise throughout Europe.

1565
01:05:51,290 --> 01:05:53,960
That was becoming more
and more of a problem

1566
01:05:53,960 --> 01:05:56,480
when you have to coordinate
lots of trains traveling

1567
01:05:56,480 --> 01:05:59,330
across large distances.

1568
01:05:59,330 --> 01:06:01,820
It wasn't only a problem
for commercial railway

1569
01:06:01,820 --> 01:06:03,200
or for shipping.

1570
01:06:03,200 --> 01:06:06,350
It became a very potent,
added challenge in Germany

1571
01:06:06,350 --> 01:06:08,330
during Einstein's youth.

1572
01:06:08,330 --> 01:06:10,850
So again, as many
of you may know,

1573
01:06:10,850 --> 01:06:13,010
there only became
one single country

1574
01:06:13,010 --> 01:06:17,240
of Germany, a unified
Germany, in 1871 following

1575
01:06:17,240 --> 01:06:18,590
yet another war with France.

1576
01:06:18,590 --> 01:06:21,435
That was basically the story
of the previous 500 years,

1577
01:06:21,435 --> 01:06:23,810
German-speaking lands fighting
with French-speaking ones.

1578
01:06:23,810 --> 01:06:25,768
There was another one,
the Franco-Prussian war,

1579
01:06:25,768 --> 01:06:27,380
throughout 1870.

1580
01:06:27,380 --> 01:06:28,880
The German-speaking
folks prevailed.

1581
01:06:28,880 --> 01:06:31,220
And it was at that moment,
in the wake of that war,

1582
01:06:31,220 --> 01:06:34,160
that there was then
a first unification

1583
01:06:34,160 --> 01:06:38,660
of a country of Germany, now
a much larger expanse of what

1584
01:06:38,660 --> 01:06:42,470
had otherwise been independent
German-speaking territories.

1585
01:06:42,470 --> 01:06:44,420
So now there's an
added reason to worry

1586
01:06:44,420 --> 01:06:47,600
about trains that have to be
coordinated across distances.

1587
01:06:47,600 --> 01:06:49,700
There's now a new
country's worth

1588
01:06:49,700 --> 01:06:52,730
that has to defend its borders
all the way from basically

1589
01:06:52,730 --> 01:06:58,580
Poland or Russia on the
East and France on the West.

1590
01:06:58,580 --> 01:07:02,810
And there's a famous quotation
from a leading German general,

1591
01:07:02,810 --> 01:07:06,770
Count von Moltke,
who in the 1890s, 20

1592
01:07:06,770 --> 01:07:09,530
years into German unification,
is basically saying,

1593
01:07:09,530 --> 01:07:10,950
we have a problem on our hands.

1594
01:07:10,950 --> 01:07:13,760
We have to coordinate
trains for military purposes

1595
01:07:13,760 --> 01:07:19,710
as well as commercial, passenger
rail, and merchant shipping.

1596
01:07:19,710 --> 01:07:21,350
We have a problem
with the fact that we

1597
01:07:21,350 --> 01:07:23,390
don't have unified time zones.

1598
01:07:23,390 --> 01:07:25,470
This is all happening
in Einstein's childhood.

1599
01:07:25,470 --> 01:07:27,980
Remember, Einstein
was born in 1879,

1600
01:07:27,980 --> 01:07:30,620
very early in this
period of unified Germany

1601
01:07:30,620 --> 01:07:35,070
and a new focus on coordinating
clocks at a distance.

1602
01:07:35,070 --> 01:07:37,960
So one of the main ideas that
many clever people, inventors,

1603
01:07:37,960 --> 01:07:40,460
came up with throughout Europe
and other parts of the world,

1604
01:07:40,460 --> 01:07:43,460
as well, was to try
to coordinate clocks

1605
01:07:43,460 --> 01:07:46,160
and therefore help
coordinate train stations

1606
01:07:46,160 --> 01:07:47,960
in these different cities.

1607
01:07:47,960 --> 01:07:49,850
Now, much of continental
Europe was now

1608
01:07:49,850 --> 01:07:52,250
being connected by rail.

1609
01:07:52,250 --> 01:07:55,790
If the major hubs, if
the major train stations

1610
01:07:55,790 --> 01:07:58,820
could agree on the time, then
you could set your watch then.

1611
01:07:58,820 --> 01:08:02,720
And the trains could be better
coordinated throughout even

1612
01:08:02,720 --> 01:08:04,190
into the hinterlands.

1613
01:08:04,190 --> 01:08:06,830
And the main idea that many
inventors and entrepreneurs

1614
01:08:06,830 --> 01:08:11,180
kind of zeroed in on was
to install these so-called

1615
01:08:11,180 --> 01:08:13,970
[GERMAN] clocks, mother
clocks, central clocks

1616
01:08:13,970 --> 01:08:16,340
that you would say
were the standard ones.

1617
01:08:16,340 --> 01:08:19,729
And then connect those by
electromagnetic signal,

1618
01:08:19,729 --> 01:08:22,370
either telegraph or,
increasingly, radio waves,

1619
01:08:22,370 --> 01:08:24,410
literally traveling
electromagnetic waves

1620
01:08:24,410 --> 01:08:25,675
in the ether.

1621
01:08:25,675 --> 01:08:27,800
Some of you might know that
the Eiffel Tower, which

1622
01:08:27,800 --> 01:08:29,870
is under construction
in just this period,

1623
01:08:29,870 --> 01:08:32,510
was originally built to
aid radio communication.

1624
01:08:32,510 --> 01:08:34,939
It was a radio beacon before
it was a reason for all of us

1625
01:08:34,939 --> 01:08:35,899
to go visit Paris.

1626
01:08:35,899 --> 01:08:37,729
There were other
reasons before that.

1627
01:08:37,729 --> 01:08:39,290
The Eiffel Tower
was built really

1628
01:08:39,290 --> 01:08:42,950
in this period in large
part to become a radio

1629
01:08:42,950 --> 01:08:46,310
beacon to broadcast
standardized time signals.

1630
01:08:46,310 --> 01:08:48,319
And then the idea
was you would know

1631
01:08:48,319 --> 01:08:49,789
how far you were from Paris.

1632
01:08:49,789 --> 01:08:52,729
You know there'd be some
delay of when you'd expect

1633
01:08:52,729 --> 01:08:54,529
to receive the radio beacon.

1634
01:08:54,529 --> 01:08:57,229
If it's 12:00 noon in Paris
when that beam was sent,

1635
01:08:57,229 --> 01:09:00,180
you have an offset based
on how far away you are

1636
01:09:00,180 --> 01:09:02,580
and the speed of light that
it would take to get there.

1637
01:09:02,580 --> 01:09:04,279
So then these very
clever folks who

1638
01:09:04,279 --> 01:09:06,830
make all these gadgets
to both receive

1639
01:09:06,830 --> 01:09:10,340
the time, either
telegraph or radio wave,

1640
01:09:10,340 --> 01:09:13,939
and then implement the
offset and then reset

1641
01:09:13,939 --> 01:09:16,250
the clock in your next
train station clock

1642
01:09:16,250 --> 01:09:17,340
and down the line.

1643
01:09:17,340 --> 01:09:20,390
So imagine these coordinated
sets of coordinates

1644
01:09:20,390 --> 01:09:24,800
where you have local clocks at
every important location that

1645
01:09:24,800 --> 01:09:27,510
are sending and receiving
electromagnetic signals

1646
01:09:27,510 --> 01:09:29,729
to coordinate the local time.

1647
01:09:29,729 --> 01:09:31,725
That's exactly the
scenario that Einstein

1648
01:09:31,725 --> 01:09:33,350
mentions in the
abstract in the opening

1649
01:09:33,350 --> 01:09:36,240
pages of his 1905 paper.

1650
01:09:36,240 --> 01:09:39,649
He doesn't talk about
these patent applications

1651
01:09:39,649 --> 01:09:45,300
or about trains in this
more practical way.

1652
01:09:45,300 --> 01:09:49,550
But it turns out, as my
friend Peter Galison shows

1653
01:09:49,550 --> 01:09:51,609
in this beautiful book
Einstein's Clocks,

1654
01:09:51,609 --> 01:09:53,569
Poincare's Maps,
this is the subject

1655
01:09:53,569 --> 01:09:56,360
of an enormous tech spree.

1656
01:09:56,360 --> 01:09:59,210
This was the tech challenge
of the day for all these

1657
01:09:59,210 --> 01:10:01,400
very smart, young
electrical engineers

1658
01:10:01,400 --> 01:10:04,940
to build up and patent every
bit, every switch and gear,

1659
01:10:04,940 --> 01:10:07,550
for this way of coordinating
clocks at a distance often

1660
01:10:07,550 --> 01:10:11,740
using electromagnetic signals.

1661
01:10:11,740 --> 01:10:15,100
Not only that, Einstein was
at the electrotechnical part

1662
01:10:15,100 --> 01:10:17,920
of the Bern patent office,
where a lot of these

1663
01:10:17,920 --> 01:10:18,940
were flowing through.

1664
01:10:18,940 --> 01:10:22,000
So he was an examiner on
many of the little widgets

1665
01:10:22,000 --> 01:10:25,090
and gadgets as part of this
clock-coordination sequence.

1666
01:10:25,090 --> 01:10:27,700
Even better, my favorite part
of Peter's otherwise quite

1667
01:10:27,700 --> 01:10:31,090
fascinating book, was that Peter
and his research assistants

1668
01:10:31,090 --> 01:10:33,340
pieced together the
walk that Einstein

1669
01:10:33,340 --> 01:10:35,620
used to take from his
apartment to the office--

1670
01:10:35,620 --> 01:10:37,750
he had a lovely
stroll through Bern

1671
01:10:37,750 --> 01:10:39,370
during this time of his life--

1672
01:10:39,370 --> 01:10:42,970
and then finding out when each
of the clocks along his route

1673
01:10:42,970 --> 01:10:46,498
were wired up to this
now coordinated system.

1674
01:10:46,498 --> 01:10:48,040
So literally the
clocks that Einstein

1675
01:10:48,040 --> 01:10:50,500
would walk past
between his apartment

1676
01:10:50,500 --> 01:10:53,830
and his office were in this
exact moment being wired up

1677
01:10:53,830 --> 01:10:56,830
in this new, explicit,
electrotechnical clock

1678
01:10:56,830 --> 01:10:59,920
coordination system, not just
across distant train stations

1679
01:10:59,920 --> 01:11:03,220
but soon even across
the semi-urban regions

1680
01:11:03,220 --> 01:11:04,630
where he lived.

1681
01:11:04,630 --> 01:11:07,810
It was literally his
day job and the path

1682
01:11:07,810 --> 01:11:10,030
that he walked to get to work.

1683
01:11:10,030 --> 01:11:13,030
So this notion of using
electromagnetic signals

1684
01:11:13,030 --> 01:11:16,840
to coordinate clocks in a
measurable, repeatable way

1685
01:11:16,840 --> 01:11:19,450
was of philosophical interest.

1686
01:11:19,450 --> 01:11:21,130
It was certainly
inspired in part

1687
01:11:21,130 --> 01:11:22,810
by the writing of Ernst Mach.

1688
01:11:22,810 --> 01:11:26,080
But Einstein was also immersed
in a different set of realities

1689
01:11:26,080 --> 01:11:28,330
than someone like
Hendrik Lorentz

1690
01:11:28,330 --> 01:11:30,190
or the other Maxwellians.

1691
01:11:30,190 --> 01:11:33,640
And so when he comes to the
question of electrodynamics

1692
01:11:33,640 --> 01:11:35,680
of moving bodies,
Einstein's immersed

1693
01:11:35,680 --> 01:11:39,310
in different conversations
and day-to-day questions

1694
01:11:39,310 --> 01:11:43,780
compared to those of the
physics elite of his day.

1695
01:11:43,780 --> 01:11:48,370
So we come back to this very
iconic but kind of unusual

1696
01:11:48,370 --> 01:11:50,800
paper from 1905, "On
the Electrodynamics

1697
01:11:50,800 --> 01:11:52,510
of Moving Bodies."

1698
01:11:52,510 --> 01:11:55,720
I think we can make a bit more
sense of it, informed by work

1699
01:11:55,720 --> 01:11:59,590
by people like Peter Galison.

1700
01:11:59,590 --> 01:12:03,740
Einstein's paper has
almost no references.

1701
01:12:03,740 --> 01:12:06,410
Much like many
patent applications,

1702
01:12:06,410 --> 01:12:10,640
you want to emphasize priority
and downplay precedence.

1703
01:12:10,640 --> 01:12:13,098
Einstein focuses on the
operational details.

1704
01:12:13,098 --> 01:12:15,140
How would you actually
perform these measurements

1705
01:12:15,140 --> 01:12:17,540
and compare the answers
at some distance?

1706
01:12:17,540 --> 01:12:20,240
It starts to look sort of
like a patent application,

1707
01:12:20,240 --> 01:12:24,423
not like a fancy exercise
in mathematical physics.

1708
01:12:24,423 --> 01:12:26,090
So we can come back
to this question of,

1709
01:12:26,090 --> 01:12:27,840
why was Einstein doing
things differently?

1710
01:12:27,840 --> 01:12:28,910
And what was he up to?

1711
01:12:28,910 --> 01:12:31,970
And we see him really enmeshed
in just a different set

1712
01:12:31,970 --> 01:12:35,240
of ideas, of philosophical
conversations,

1713
01:12:35,240 --> 01:12:38,870
of mathematical techniques,
and also of actual gadgets

1714
01:12:38,870 --> 01:12:42,430
compared to some of the
other experts of his day.