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MARKUS KLUTE: Welcome back
to the last section of 8.20

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Special Relativity.

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So as we discussed for special
relativity itself, also

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general relativity
is a theory which

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requires experimental
evidence to be confirmed.

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And there's plenty of
experimental evidence

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for general relativity.

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We talked about a
few examples of them.

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But let's go through
this, one by one.

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Also, a little bit with
an historical context.

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So one of the first experimental
pieces of evidence pointed out

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by Einstein was the
procession of mercury

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and also of other planets.

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This was always this problem
that's a procession of mercury,

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deviates from
Newton's prediction

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was well known and first
recognized already in 1859.

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And it turned out that attempts
to correct this failed.

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You can think
about, maybe there's

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other objects in the
solar system which

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modify the trajectory of
mercury around the sun.

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But nothing really
added up correctly.

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And then when
Einstein calculated

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the effect of the
procession, he found

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that it's in good agreement
with the observation.

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This was already
very strong evidence

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for general relativity effects.

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And then there's
gravitational lensing.

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And here, this
was first measured

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by Dyson and Eddington,
1919, of light

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passing the sun in
a total eclipse.

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The observation was
in Brazil but also

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at the West Coast of Africa.

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This wasn't the first
attempt to measure this.

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It was an eclipse, a total
eclipse in Argentina in 1912.

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But unfortunately,
this expedition

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didn't lead to a result,
because it rained out.

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There was eclipse
shortly after in 1914,

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but that happened during
the Second World War,

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and there's long stories and
accounts of how this failed.

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But basically, one
of the expeditions

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wanted to travel to
Crimea in Russia.

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And because Russia was
in war with Germany,

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material was confiscated,
and people were imprisoned.

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So this was canceled,
if you want,

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due to the second
and first World War.

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But then in 1919, this
led to the observation

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the data was not as clear.

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I think there was a little
bit more hope than science

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in the interpretation.

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So there was-- there was
not a strong evidence,

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a strong significance
of the results.

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But the evidence,
nevertheless, was there.

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And as I was explaining
earlier, that

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led to the fame or the
triumph of Einstein,

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where really, his
fame resulted out

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of the reporting
of those events.

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There's more
experimental evidence.

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Light travel time
but around or close

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to massive object is modified.

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We talked about gravitational
time dilation, which can be

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measured or has been measured.

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Other tests of the equivalent
principle, but also

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the observation of
gravitational waves.

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Gravitational waves were
predicted by Einstein

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by the theory of
general relativity.

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And only very recently, we
were able to observe those.

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And then, in addition, there
is plenty cosmological tests,

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which require a
precise understanding

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of general relativity
in order to get

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to agreement between
the observations

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and the theoretical predictions.

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But let's talk about--

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let's talk about
gravitational waves.

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So those we predicted,
as I was saying,

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but they're very, very
difficult to measure.

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First, indirect measurement was
performed by Hulse and Taylor.

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They were able to study
binary neutron star system.

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And because the
orbits of those two

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decayed required lots of
energy, and that lots of energy

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needs to happen somehow.

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And it was theorized or
predicted by general relativity

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that that loss of energy
is due to the fact

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that gravitational
waves are emitted.

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And they received
for their findings

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the Nobel Prize in
physics in 1993.

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So how are gravitational
waves generated?

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You can ask-- I have a
spinning sphere, like our sun.

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Would that generate
a gravitational wave?

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The answer is no.

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It's a symmetrical situation.

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There's no change of
the metal distribution.

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And therefore, spacetime
is not modified.

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But if you have a sphere
with a little bump,

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that would create
gravitational waves.

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If you have a mass
which is moving by,

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maybe two passing galaxies,
that would not directly

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create gravitational waves.

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But if you have those
galaxies rotating,

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or two stars rotating, or
neutron stars rotating,

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or black holes rotating
around each other,

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those generate
gravitational waves.

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And the closer the
object, the higher

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the masses of the
objects, the stronger

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the gravitational waves are.

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So how can you measure
gravitational waves?

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Very similar to the
Michael Smalley experiment.

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What you want to do
is measure differences

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in arms of your interferometer.

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And you do this with
very powerful lasers

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and with very precise mirrors.

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So it's very clear the very same
experiment as Michael Smalley,

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just much, much bigger.

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So we're talking about
multiple miles of arms

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and very powerful
lasers in order

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to conduct those experiments.

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LIGO, which is one
of those measurement,

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of those devices,
experiments, measures

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the change in the length
of one arm with a precision

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smaller than the
diameter of a proton.

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So that's just really--

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it's mind-blowing, the
level of precision,

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the level of understanding
needed in order

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to measure gravitational waves.

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But nevertheless,
they succeeded.

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So here, you see
two experiments.

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LIGO has actually
two experiments,

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two of those devices
in the United States.

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And there's other experiments
similar worldwide.

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You see also highlighted
here, Caltech and MIT.

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Those are the leading
communities of the leading

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universities in this endeavor.

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And then the first observation
of gravitational waves

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happened in September 14, 2015.

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And this first observation
was rather spectacular

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because it was not
just any observation,

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but it was the observation of
two collapsing black holes.

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So you have two black holes.

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They get close to each other,
than they circle each other

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and create a new,
heavier black hole.

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So the collision of
those two black holes

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with masses around 30 times
the mass of the sun, it

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actually took place
1.3 billion years ago.

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So the gravitational wave
was traveling towards us

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for 1.3 billion years.

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The energy of about three
times the mass of the sun

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was emitted as gravitational
waves in fractions of seconds.

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So the huge amount
of energy released

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in form of gravitational waves.

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The collision happens with
both black holes moving

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with half the speed of light.

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So this is just a catastrophic
kind of event, in our universe.

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And researchers or
faculty at MIT and Caltech

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received the Nobel
Prize in physics in 2017

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for this discovery, only two
years after the discovery

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actually happened--

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and very deserved,
very deserved.

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Let me close this lecture
by just reminding you

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of a quote of Einstein,
which I use in order to start

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this very same lecture.

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It is true that we are living
through some difficult times,

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some turbulent times.

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But if you think about
the bigger picture,

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I think we're making
a lot of progress

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scientifically but
also as humanity.

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And I like this quote from
Albert Einstein a lot.

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"It is not the result
of scientific research

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that ennobles humans and
enriches their nature,

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but it's the struggle
to understand

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while performing creative and
open-minded intellectual work."

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I think if there's
one thing I want

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you to take away from this
lecture, it is this quote.

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I want you to be encouraged to
be creative, to be open-minded,

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to question, and to perform
high-level intellectual work.

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Thank you.