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#1: Initial revision by user avatar Olin Lathrop‭ · 2021-10-02T15:10:15Z (12 months ago)
Back when what light is and how it propagates was poorly understood, people naturally related it to sound.  Sound propagates thru a <i>medium</i>, like air.  Light was therefore assumed to propagate thru a medium too.  This hypothetical medium was referred to as the <i>ether</i>.

While this line of reasoning is understandable, it turned out to be wrong.  Light isn't carried by stuff.  This may not sound like much, but is actually quite profound.  Since light isn't carried by some stuff, unlike with sound, propagation isn't dependent on the sender's and receiver's movement thru any stuff.  The experiments you describe seem to be intended to result in an observable difference whether sound or light propagates thru stuff.

For example, lets say you hold a sound transmitter and receiver 1 m apart.  In normal still air, there will be a delay of about 3 ms between the transmitter sending something, and the receiver receiving it.  If you take the same setup and mount it on the roof of a moving car, then that value decreases when the air is moving from the transmitter to the receiver, and increases when in the other direction.  If you do this on a rocket in space (vaccuum), then you won't receive anything at all because there was no medium to carry the sound.

If you do the same thing with a light transmitter and receiver on a rocket in space, you first notice that you do receive the transmission at the receiver.  That either means light travels without requiring a medium, or that some mythical medium exists anyway in what we think is the vacuum of space.  The latter is what this "ether" was assumed to be.

However, if you make the rocket go fast, then try the test again, you don't find any difference in the propagation time.  If you flip the transmitter and receiver with the rocket going the same speed as before, you still observe the same propagation delay.  If light was carried by some ether, then there should have been a difference.

This not only shows that light doesn't propagate thru a medium, but that there is no such thing as an absolute "going fast".  There is no ether sea that you can measure speed relative to.  That's what relativity is all about.