“If the astronaut found that light slowed down relative to his motion, he would have detected the very ether wind that Michelson failed to detect”?

The following extract is from Relativity Simply Explained by Martin Gardner (Amazon link):

Consider an astronaut in a spaceship that is racing alongside a light beam. The ship is traveling with half the speed of light. The astronaut will find, if he makes the proper measurements, that the beam is still passing him at its usual velocity of 299,800 kilometers per second! Think about this for a moment and you will soon realize that this must indeed be the case if the notion of an ether wind is discarded. If the astronaut found that light slowed down relative to his motion, he would have detected the very ether wind that Michelson and Morley failed to detect. Similarly, if his spaceship travels directly toward a source of light, moving with half the speed of light, will he find the beam approaching him twice as fast? No, it is still moving toward him at 299,800 kilometers per second. Regardless of how he moves relative to the beam, his measurements will always give the beam the same speed

I cannot follow the reasoning of this statement. The Michelson and Morley experiment proved that there was no ether wind, but I cannot see how it proves anything else.

• Neither can I see how or why Gardner made the connection. Is there more about it on previous pages that would give it context, there must be. – user81619 Jul 28 '15 at 17:55
• What else is it supposed to prove? Do they assert it proves anything else? – paparazzo Jul 28 '15 at 17:55
• I don't understand what the issue is: Gardner says that if the astronaut detected a difference in $c$, then there is an aether wind, which would contradict the MM experiment. – Kyle Kanos Jul 28 '15 at 18:01
• Is Martin Gardner saying if there is no ether wind, then the speed of light must have the same value c in all inertial frames of reference? @ KyleKanos @ Frisbee – Asim Jul 30 '15 at 23:31

Sorry, but the Michelson-Morely did not prove that there is no ether wind. It proved that, if there is an ether wind, it is at rest with respect to the surface of the earth, regardless of the motion of the earth with respect to the rest of the universe.

While it is extremely difficult to imagine how this could possibly be, since it requires that the ether be entrained by the surface without affecting the momentum of the earth, this has not stopped various folk from trying, and some of them have been extremely ingenious.

Gardner's statement simply reflects this hypothesis, and suggests that, far from the surface of a large body, any ether should not be entrained, and the MM experiment would give different results - assuming that the ether wind exists. He rejects the idea that this is going to happen, since he is in fact explaining SR.

Since Gardner wrote his book, it's worth pointing out that various aspects of space travel, including the behavior of the GPS system and communications with probes to other planets, make the entrained ether wind very, very hard to support. GPS, for instance, requires that the ether wind be entrained at considerable distances from the earth, and it's hard to see how this can be.

Michelson and Morley compare the speed detected when moving at two different speeds (going with the rotating earth and perpendicular to the direction of the rotating earth).

It observed no difference.

If an astronaut moved in two different ways and compared the speed detected when moving at those two different speeds they should also get no difference.

Why?

Because different people and/or different times and/or different places should get the same results if doing the same experiments. Because the kinds of things we theorize about are reproducible results. Of course logically it is possible they get different results.

But it's possible that any experiment could come out differently of done again. If it did we'd try to find out what factors affect the results and make a new theory when we figure out what factors affect the results.

Since Michelson and Morley type experiments have been done multiple times in many places by many people no one is really expecting them to come our differently. So we predict there will be no difference.

That's all.