Originally posted by Vosh
I thought the idea was that light should move faster coming towards the Earth than moving away but that it was found that the two speeds were the same; but isn't the Earth moving towards something adding to its rate just as the Earth moving away from something adds to the rate at which it is moving away? Clearly I am missing something. Many thanks.
You are basically correct, but there are some other details you need to know. You are talking about relative light speed of c + v and c – v. You can see that in Chapter 9 of Einstein’s book, also in Section 2 of his 1905 paper.
What he deduced in 1911 is that atomic clocks slow down in a gravitational field just as the local speed of light does, so an atomic clock resting inside a gravitational field will always measure “c” as the local speed of light at that clock.
This often confused with the rumor that “the speed of light is always ‘c’ relative to all observers”. This is not quite true. When using atomic clocks resting inside a local gravitational field, they will measure ‘c’ for the local speed of light, at the clock, but, as he explained in the 1911 theory, if we use an outside clock that is not affected by the local gravitational field, then we can tell that light is speeding up a little and slowing down. All this information is contained in his 1911 theory.
Michelson and Morley thought the Earth was moving through a universe-stationary ether at about 18.6 mps. However, in the 1911 theory, Einstein deduced that the gravitational field of every astronomical body acts like a speed regulator. So, Michelson and Morley got a “null” result because their machine was resting inside the earth’s gravitational field, so the speed of light was the same, locally, no matter how they turned their machine.
There is no large universe-stationary ether. As far as I can tell from the Einstein theories, local gravitational fields seem to control the local speed of light.
So, when the Earth moves toward a distant star, we are moving toward its on-coming light beam at c + v, however, according to Einstein, that light beam slows down near the earth, when it reaches the influence of the earth’s gravitational field, and if we measure its speed with a local atomic clock, we will measure and calculate the local speed of “c”. So, once the c + v light photons get near the earth, they slow down to “c”, as measured by a local atomic clock at the place where their local speed is measured.
