- [Voiceover] We've already talked about light having
wave-like properties, and the waves that we're familiar with
in our everyday life we consider to be
disturbances traveling through a medium.
We talked about dropping a pebble in water,
and the water's a medium,
and we see the wave travel outwards.
We think about sound waves, which is disturbance in the air.
We think about a wave traveling through this rope,
the medium there is the rope.
So, in the mid-19th century it was completely reasonable for
folks to say, "Well look, light has wave-like properties;
it must be a disturbance traveling through a medium."
And they said, "Well what do we call that medium,
even though we don't observe it directly?
Well, let's call it the luminiferous Ether."
So an obvious question that was facing folks
who had this reasonable assumption, they said
"Well, can we somehow detect the luminiferous Ether?
Can we validate the luminiferous Ether existing?"
And a key realization is, is "Well, we must be moving
quite rapidly relative to the luminiferous Ether."
How do we know that?
Well, we just have to remind ourselves that, obviously
the Earth is rotating, but not only is it rotating
on its own axis, but it's rotating around the sun.
So if this is the sun right over here, this is the Earth.
The Earth is rotating, and these are all rough figures,
the Earth is moving around the Sun at approximately
30 kilometers per second. 30 kilometers per second!
By our everyday standards, that's quite fast,
but we're not done yet.
'Cuz the Sun is also moving around the center of the galaxy.
And this isn't an actual picture of the Milky Way;
obviously we haven't gotten this far from our own galaxy
to actually get this type of a vantage point,
but if the Sun were right over there, the Sun,
estimates are, are moving with a speed of 200, roughly, 200,
let me write that in a better color so
you can actually see it, 200 kilometers per second.
200 kilometers per second
around the center of the Milky Way,
and then the Milky Way itself could be moving.
So we don't know our actual, kind of, our orientation
relative to the Ether, but we are,
we're constantly changing our orientation,
we're moving in these orbital patterns.
If there is some type of luminiferous Ether,
if there is some type of luminiferous Ether,
and I'm just gonna draw these lines over here
to kind of show our luminiferous Ether,
we must be moving relative to it
if we orient ourselves just the right way.
In fact, the odds of us being stationary relative to
the Ether are pretty close to zero,
especially if we wait a little.
If we're stationary relative to the Ether right now,
let's say at this point, since we're changing our direction,
we're not going to be stationary relative to
the Ether at that point.
And that's just when you consider
the Earth's orbit around the Sun.
It's even more true when you think about the
solar system's orbit around the center of the galaxy,
or even the movement of the galaxy.
So, we should be moving relative to the Ether,
or the Ether should be moving relative to us.
So we should be able to detect some type of,
some type of what's called an "Ether wind".
'Cuz it should be moving relative...
Ether wind.
Now how would you detect an Ether wind?
Well, let's think about some other type of medium
moving relative to us.
Let's say that we are sitting on an island,
let me do this in a better color for an island.
So let's say that we're on an island that's in
the middle of a stream.
So these are the shores of the stream.
These are the shores of the stream.
And there is some type of a current.
So the water is moving in that direction.
So that's the medium.
And now let's start a wave propagating through this.
So if I were to just take a pebble and drop it
right over here, what would happen?
Well the wave is going to propagate faster to the left
than it is to the right.
This is from our everyday experience,
and that's because to the left it's moving,
the medium is also moving to the left.
So as the medium moves,
and then you propagate through that medium,
you're going to move faster to the left than to the right.
So the wave is going to propagate, is going to propagate,
something is going to propagate,
so after a small period of time,
the crest on the right might be there,
but the crest on the left might be there.
So it might look something like this.
And then after another period of time,
it might look something, it'll look something like this.
So the general point is, for this little stream example,
you're going to see your wave propagate faster
in the direction of which the medium is moving.
So similarly, if you have an Ether wind,
if you have Ether wind and this luminiferous Ether
is the medium by which the light propagates,
the light is a disturbance in this medium,
then if this Ether wind has some,
let's say it has some speed,
let's just call it "S".
If you were try to propagate light in that direction
versus in this direction, versus in that direction,
it should go faster, we should notice it going faster
if it's going along the same direction as the Ether.
'Cuz it's propagating through something
that's also moving relative to us.
And likewise, if it's going in the opposite direction
of the Ether, even though it's propagating through
the actual Ether at that same speed,
the Ether is moving in the other direction, so the light,
based on our 19th century understanding of the universe,
the light should seem slower.
So you can imagine, people started to theorize,
"Well, maybe we can measure light in different directions
and see if, relative to us, if relative to us,
we see a different velocity for light."
Now the problem was, is that in the mid-19th century,
light is incredibly fast.
We now know that the speed of light is approximately
300 thousand kilometers per second.
And in the mid-19th century, we didn't have good tools
to measure this with a lot of accuracy.
Especially because the Ether wind itself,
even if you say "This is 30 kilometers per second,
maybe we're moving around the galaxy at, you know,
200 kilometers per second, maybe 300 kilometers per second",
that's still a small fraction of the actual speed of light.
So if you don't have a lot of accuracy when you're measuring
the speed of light in these different directions, and
the Ether wind is so slow relative to the speed of light,
well, with just traditional tools in the 19th century,
you're not going to be able to detect
this actual Ether wind, if it existed.
And that's what gets us to the famous
Michelson-Morley Experiment,
because there they didn't just directly try to measure
the speed of light in one direction or another,
instead they thought about, "Let's split some light into
two different directions, and then recombine them
and see the interference patterns.
And if the different directions traveled at different speeds
then we'll have different interference patterns."
And we're going to see that in the next video.