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MARKUS KLUTE: Welcome back
to 8.20, special relativity.

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So we're starting a
new chapter in which

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we look at tests
and implications

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of special relativity.

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We started the entire
discussion and evaluation

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of Lorentz transformations
and the description

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of the paradoxes based
on Einstein's postulates.

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Those are not axioms, which
means that we do actually

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have to verify.

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They are a prediction
of how nature functions,

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and experimental verification
is needed to gain confidence

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that those postulates
are actually correct

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or realized in nature.

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And we have studied some
of those tests already.

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And this serves as a little bit
of a review of the discussions

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we had to this point.

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One experimental test
is stellar aberration,

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which we discussed can be
explained by special relativity

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and by velocity addition.

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So this gives us some
idea about what light is.

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Light isotropy is being
tested in a variety

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of different experiments,
starting from Michelson-Morley,

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which basically tests
that the speed of light

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is independent of the
orientation of the apparatus.

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We have not discussed in
detail another experiment,

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which is very similar.

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It's the Kennedy and
Thorndike experiment,

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which tests that
the speed of light

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is also independent
of the velocity

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of the apparatus itself.

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As one of the
homework assignments,

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we looked at de Sitter who
tested that the speed of light

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is independent of the
speed of the source.

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And that has been
tested, for example,

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with the motion of binary
stars, as we did in our pset,

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in our homework assignment.

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We can look in particle
physics at the decay of a pion

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into two photons.

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And those pions, they can have
a lot of energy, for example,

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with a beta of 0.999
times the speed of light.

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And still the photons
are of this decay.

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They behave like
any other photon.

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They move with the
speed of light.

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Let me discuss the Doppler
effect and relativistic Doppler

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effect and analyze light
with various frequencies.

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And one of the
hypotheses you could have

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is that the photon actually
is a massive particle.

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This would directly modify
Coulomb's laws, which

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are tested experimentally.

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And those results would be
dependent on the frequency.

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There is weird
electromagnetic effect

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if you introduce a mass
to the photons like torque

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on a magnetic ring Again,
here precision measurements

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have been performed.

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And they're all in agreement
with the hypothesis

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that the photon is massless.

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And we talked about the Doppler
shift and redshift for light.

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Another class of
experiment is where

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we look directly
at time dilation,

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for example, in the
decay of the muon.

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As we discussed, we have
cosmic muons to study,

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or we can produce muons
in the laboratory as well

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and study them.

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Or we can put very
precise clocks on planes,

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fly them around the globe, and
compare them with stationary

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and just simply measure the
effect of special relativity

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on those clocks.

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In all of this experimentation
and experimental verification,

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it's important to understand
the importance of uncertainties

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in the scientific
process overall.

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I think that--
remember, when one

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has the historic
perspective on science,

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one often forgets that
a specific measurement

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comes with an
uncertainty, often,

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a statistical uncertainty
or systematic effects, which

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are quite important to quantify
the level of verification

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of the theoretical hypothesis.

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One example here
is-- and experiments

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can have biases as well.

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And one example of
a biased experiment

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is here one by
Walter Kaufmann who

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tried to measure e/m,
the electric charge

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over the mass of the electron.

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But he had a rather
strong theoretical bias.

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And he conducted the
experiment at the time Einstein

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was proposing his theory.

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The bias really came from
the model of an electron

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at the time.

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And, as experiments, they were
inconsistent with Einstein.

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So he said Einstein is wrong.

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Einstein and Lorentz are wrong.

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Planck looked at
this and said maybe,

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but Einstein's conclusion
immediately by looking at this

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was, no, this cannot be.

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And it took a little bit of
time to tee up those experiments

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until 1940.

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You can read more about this
in this Wikipedia article

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about Walter
Kaufmann's experiments,

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but let me just
read this to you.

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"The prevalent results decidedly
speak against the correctness

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of Lorentz's assumption,
as well as Einstein's.

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If, on account of that, one
considers this basic assumption

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refuted, then one
would be forced

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to consider it a
failure to attempt

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to base an entire field
of physics, including

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electrodynamics and
optics, upon the principle

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of relative movement."

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We now know this was wrong,
but scientific process

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happens in scientific
environment.

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And I started this
class by explaining

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that one needs to be open
minded to learn and to study

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and to grow scientifically.

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And one has to question
the assumptions,

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understand the assumptions
when then go into measurements.

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So this is the first
part of this chapter

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where we talk about tests.

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We will have a discussion
of implications

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of special relativity
as we move on from here.