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- [Voiceover] We've already
talked about light having

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wave-like properties, and the
waves that we're familiar with

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in our everyday life we consider to be

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disturbances traveling through a medium.

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We talked about dropping
a pebble in water,

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and the water's a medium,

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and we see the wave travel outwards.

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We think about sound waves,
which is disturbance in the air.

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We think about a wave
traveling through this rope,

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the medium there is the rope.

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So, in the mid-19th century it
was completely reasonable for

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folks to say, "Well look,
light has wave-like properties;

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it must be a disturbance
traveling through a medium."

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And they said, "Well what
do we call that medium,

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even though we don't observe it directly?

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Well, let's call it the
luminiferous Ether."

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So an obvious question
that was facing folks

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who had this reasonable
assumption, they said

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"Well, can we somehow detect
the luminiferous Ether?

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Can we validate the
luminiferous Ether existing?"

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And a key realization is,
is "Well, we must be moving

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quite rapidly relative to
the luminiferous Ether."

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How do we know that?

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Well, we just have to remind
ourselves that, obviously

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the Earth is rotating, but
not only is it rotating

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on its own axis, but it's
rotating around the sun.

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So if this is the sun right
over here, this is the Earth.

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The Earth is rotating, and
these are all rough figures,

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the Earth is moving around
the Sun at approximately

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30 kilometers per second.
30 kilometers per second!

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By our everyday standards,
that's quite fast,

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but we're not done yet.

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'Cuz the Sun is also moving
around the center of the galaxy.

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And this isn't an actual
picture of the Milky Way;

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obviously we haven't gotten
this far from our own galaxy

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to actually get this
type of a vantage point,

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but if the Sun were right
over there, the Sun,

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estimates are, are moving with
a speed of 200, roughly, 200,

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let me write that in a better color so

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you can actually see it,
200 kilometers per second.

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200 kilometers per second

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around the center of the Milky Way,

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and then the Milky Way
itself could be moving.

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So we don't know our actual,
kind of, our orientation

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relative to the Ether, but we are,

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we're constantly changing our orientation,

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we're moving in these orbital patterns.

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If there is some type
of luminiferous Ether,

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if there is some type
of luminiferous Ether,

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and I'm just gonna draw
these lines over here

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to kind of show our luminiferous Ether,

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we must be moving relative to it

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if we orient ourselves just the right way.

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In fact, the odds of us
being stationary relative to

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the Ether are pretty close to zero,

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especially if we wait a little.

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If we're stationary relative
to the Ether right now,

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let's say at this point, since
we're changing our direction,

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we're not going to be
stationary relative to

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the Ether at that point.

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And that's just when you consider

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the Earth's orbit around the Sun.

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It's even more true
when you think about the

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solar system's orbit around
the center of the galaxy,

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or even the movement of the galaxy.

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So, we should be moving
relative to the Ether,

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or the Ether should be
moving relative to us.

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So we should be able
to detect some type of,

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some type of what's
called an "Ether wind".

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'Cuz it should be moving relative...

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Ether wind.

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Now how would you detect an Ether wind?

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Well, let's think about
some other type of medium

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moving relative to us.

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Let's say that we are
sitting on an island,

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let me do this in a better
color for an island.

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So let's say that we're
on an island that's in

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the middle of a stream.

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So these are the shores of the stream.

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These are the shores of the stream.

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And there is some type of a current.

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So the water is moving in that direction.

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So that's the medium.

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And now let's start a wave
propagating through this.

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So if I were to just
take a pebble and drop it

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right over here, what would happen?

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Well the wave is going to
propagate faster to the left

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than it is to the right.

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This is from our everyday experience,

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and that's because to
the left it's moving,

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the medium is also moving to the left.

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So as the medium moves,

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and then you propagate
through that medium,

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you're going to move faster
to the left than to the right.

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So the wave is going to
propagate, is going to propagate,

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something is going to propagate,

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so after a small period of time,

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the crest on the right might be there,

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but the crest on the left might be there.

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So it might look something like this.

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And then after another period of time,

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it might look something, it'll
look something like this.

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So the general point is, for
this little stream example,

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you're going to see your
wave propagate faster

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in the direction of which
the medium is moving.

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So similarly, if you have an Ether wind,

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if you have Ether wind and
this luminiferous Ether

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is the medium by which
the light propagates,

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the light is a disturbance in this medium,

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then if this Ether wind has some,

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let's say it has some speed,

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let's just call it "S".

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If you were try to propagate
light in that direction

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versus in this direction,
versus in that direction,

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it should go faster, we
should notice it going faster

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if it's going along the
same direction as the Ether.

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'Cuz it's propagating through something

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that's also moving relative to us.

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And likewise, if it's going
in the opposite direction

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of the Ether, even though
it's propagating through

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the actual Ether at that same speed,

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the Ether is moving in the
other direction, so the light,

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based on our 19th century
understanding of the universe,

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the light should seem slower.

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So you can imagine, people
started to theorize,

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"Well, maybe we can measure
light in different directions

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and see if, relative to
us, if relative to us,

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we see a different velocity for light."

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Now the problem was, is that
in the mid-19th century,

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light is incredibly fast.

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We now know that the speed
of light is approximately

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300 thousand kilometers per second.

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And in the mid-19th century,
we didn't have good tools

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to measure this with a lot of accuracy.

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Especially because the Ether wind itself,

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even if you say "This is
30 kilometers per second,

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maybe we're moving around
the galaxy at, you know,

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200 kilometers per second, maybe
300 kilometers per second",

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that's still a small fraction
of the actual speed of light.

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So if you don't have a lot of
accuracy when you're measuring

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the speed of light in these
different directions, and

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the Ether wind is so slow
relative to the speed of light,

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well, with just traditional
tools in the 19th century,

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you're not going to be able to detect

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this actual Ether wind, if it existed.

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And that's what gets us to the famous

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Michelson-Morley Experiment,

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because there they didn't
just directly try to measure

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the speed of light in
one direction or another,

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instead they thought about,
"Let's split some light into

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two different directions,
and then recombine them

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and see the interference patterns.

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And if the different directions
traveled at different speeds

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then we'll have different
interference patterns."

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And we're going to see
that in the next video.