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Michelson Morley Question

  1. Jan 11, 2004 #1
    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.
  2. jcsd
  3. Jan 11, 2004 #2
    The Michelson/Morley experiment was supposed to detect the existance of "ether", the medium in which light was thought to be traveling. As the Earth moved through this ether we should have detected a difference in the speed of light as measured from two different points at the same time, because one place would be going with the flow of ether and one would be going against it. The experiment found no difference and therefore there was no such thing as ether. In a few years Einstein came along and stated that the speed of light is constant for all observers, which explained this rather offbeat result.
  4. Jan 11, 2004 #3


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    Einstein's Special Relativity: the speed of light is constant for all observers regardless of inertial frame of reference. That means the speed of light is the same regardless of the motion of the objects. How? TIME is not constant. The rate of the passage of time differs for different observers and as a result, the speed of light they observe is the same.
  5. Jan 11, 2004 #4
    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.
  6. Jan 11, 2004 #5

    This guy explains some of what I just said. The graph is from Einstein’s 1911 paper. dt = far away reference time, so that the far away clock won’t be influenced by the sun’s local gravitational field. c’dt is a faster part of a light wavefront, and cdt is a slower part. Thus the light beam experiences a gravitational refraction as Newton predicted in "Optics” in 1704. But atomic clocks resting at the point of c’dt and cdt would measure the local speed of “c”, since the clocks slow an appropriate amount, due to the gravitational potential at those points.
    Last edited by a moderator: Apr 20, 2017
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