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MARKUS KLUTE: Welcome back to 8.20, special relativity.
So we're starting a new chapter in which
we look at tests and implications
of special relativity.
We started the entire discussion and evaluation
of Lorentz transformations and the description
of the paradoxes based on Einstein's postulates.
Those are not axioms, which means that we do actually
have to verify.
They are a prediction of how nature functions,
and experimental verification is needed to gain confidence
that those postulates are actually correct
or realized in nature.
And we have studied some of those tests already.
And this serves as a little bit of a review of the discussions
we had to this point.
One experimental test is stellar aberration,
which we discussed can be explained by special relativity
and by velocity addition.
So this gives us some idea about what light is.
Light isotropy is being tested in a variety
of different experiments, starting from Michelson-Morley,
which basically tests that the speed of light
is independent of the orientation of the apparatus.
We have not discussed in detail another experiment,
which is very similar.
It's the Kennedy and Thorndike experiment,
which tests that the speed of light
is also independent of the velocity
of the apparatus itself.
As one of the homework assignments,
we looked at de Sitter who tested that the speed of light
is independent of the speed of the source.
And that has been tested, for example,
with the motion of binary stars, as we did in our pset,
in our homework assignment.
We can look in particle physics at the decay of a pion
into two photons.
And those pions, they can have a lot of energy, for example,
with a beta of 0.999 times the speed of light.
And still the photons are of this decay.
They behave like any other photon.
They move with the speed of light.
Let me discuss the Doppler effect and relativistic Doppler
effect and analyze light with various frequencies.
And one of the hypotheses you could have
is that the photon actually is a massive particle.
This would directly modify Coulomb's laws, which
are tested experimentally.
And those results would be dependent on the frequency.
There is weird electromagnetic effect
if you introduce a mass to the photons like torque
on a magnetic ring Again, here precision measurements
have been performed.
And they're all in agreement with the hypothesis
that the photon is massless.
And we talked about the Doppler shift and redshift for light.
Another class of experiment is where
we look directly at time dilation,
for example, in the decay of the muon.
As we discussed, we have cosmic muons to study,
or we can produce muons in the laboratory as well
and study them.
Or we can put very precise clocks on planes,
fly them around the globe, and compare them with stationary
and just simply measure the effect of special relativity
on those clocks.
In all of this experimentation and experimental verification,
it's important to understand the importance of uncertainties
in the scientific process overall.
I think that-- remember, when one
has the historic perspective on science,
one often forgets that a specific measurement
comes with an uncertainty, often,
a statistical uncertainty or systematic effects, which
are quite important to quantify the level of verification
of the theoretical hypothesis.
One example here is-- and experiments
can have biases as well.
And one example of a biased experiment
is here one by Walter Kaufmann who
tried to measure e/m, the electric charge
over the mass of the electron.
But he had a rather strong theoretical bias.
And he conducted the experiment at the time Einstein
was proposing his theory.
The bias really came from the model of an electron
at the time.
And, as experiments, they were inconsistent with Einstein.
So he said Einstein is wrong.
Einstein and Lorentz are wrong.
Planck looked at this and said maybe,
but Einstein's conclusion immediately by looking at this
was, no, this cannot be.
And it took a little bit of time to tee up those experiments
until 1940.
You can read more about this in this Wikipedia article
about Walter Kaufmann's experiments,
but let me just read this to you.
"The prevalent results decidedly speak against the correctness
of Lorentz's assumption, as well as Einstein's.
If, on account of that, one considers this basic assumption
refuted, then one would be forced
to consider it a failure to attempt
to base an entire field of physics, including
electrodynamics and optics, upon the principle
of relative movement."
We now know this was wrong, but scientific process
happens in scientific environment.
And I started this class by explaining
that one needs to be open minded to learn and to study
and to grow scientifically.
And one has to question the assumptions,
understand the assumptions when then go into measurements.
So this is the first part of this chapter
where we talk about tests.
We will have a discussion of implications
of special relativity as we move on from here.