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MARKUS KLUTE: Welcome back
to 8.20, special relativity.

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So we're going to continue
the discussion that

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makes a case against ether when
discussing stellar aberration.

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So the problem we
have in mind here

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is the ones where we
use a telescope to look

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at a distant star.

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When we do this,
the fact of ether

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might change the way we have
to orient our telescope.

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Let's have a look at this.

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So we are in a situation
that our hypothesis

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is that the ether exists.

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This carries-- is the medium
in which light travels.

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But it's dragged with earth.

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So this is our case number one.

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In that situation, the
telescope looks at the star.

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The light of the star enters
our telescope at the top

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and then tries to
go to the bottom.

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Because the medium in
which the light travels

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and the telescope move with
the same velocity-- remember,

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the ether is dragged
with the earth--

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the light appears to go
straight down the telescope.

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Without the--
[INAUDIBLE] case 2,

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without the ether being
dragged or even exist,

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we actually do have to
slightly tilt our telescope.

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Why?

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Because after the light hits
the top of our telescope,

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the telescope keeps
moving because it's

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in a different reference frame.

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And therefore, we have to
have this slight tilting.

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The value of the tilt
is equal to velocity

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of the Earth over C.

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This is a well-known effect in
astrophysics and, by the way,

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was already studied way earlier
by James Bradley in the 1720s.

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He actually developed the--

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not really supported
theory of light

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where he was talking about
particle nature of light.

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So his idea was very
similar to case 2,

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that the Earth is moving
in a different reference

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frame with respect to
the star and the sun,

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moving around the sun.

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And therefore, the
tilting angle needs

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to be the tangent of
the tilting and needs

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to be equal to the distance
of Earth to the Sun.

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And the distance is [INAUDIBLE]
this angle is very, very small

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because the nearest star
[INAUDIBLE] to Earth

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is about four light years away.

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So with this now, we can again
do our ether versus particle

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nature model comparison.

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So in our first discussion,
we concluded clearly

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that the wave and ether
hypothesis dominates or wins.

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But now we have studied at
least two further experiments.

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So, again, I invite you
to stop here and think

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about how stellar aberration
and Michelson-Morley would

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be answers to the particle
model or wave model [INAUDIBLE]

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explanation.

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As a thought experiment,
the Fizeau and Airy

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experiment that we haven't
and did not discuss this here.

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All right.

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So for stellar aberration,
the particle model actually

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seems to work, as it
was proposed apparently

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to solve this very problem.

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And the Michelson-Morey
experiment

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also is consistent with
the particle model.

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So now we're in this dilemma
that some experiments

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or experimental
evidence for particle

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ether nature and some for
the particle nature of light.

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So the question of how do we get
out of the dilemma-- and we'll

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discuss this in [INAUDIBLE]
the next section.