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B Two ways of detecting absolute motion

  1. Aug 17, 2016 #1
    There are two ways by which absolute motion might be detected: by measuring motion with respect to a beam of light, and by making use of inertial effects that arise when an object is accelerated. So why MichelsonMorley experiment showed the first method to be unworkable? Einstein's special theory of relativity explained why.

    Correct me if I am wrong. Special relativity proved that no matter the position or speed with which a person is moving, the speed of light will be the same. Is this correct? otherwise how does SR prove that the first method is unworkable?
  2. jcsd
  3. Aug 17, 2016 #2


    Staff: Mentor

    Absolute motion relative to what? That's not a joke; all motion is relative.
  4. Aug 17, 2016 #3


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    Staff: Mentor

    Special relativity didn't prove that the speed of light is the same, it started with the assumption (Einstein's second postulate) that it was.

    We choose to make this assumption because:
    - It agrees with all the experimental evidence we have (including the Michelson-Morley experiment).
    - It is consistent with the laws of electricity and magnetism, which were solidly established more than a half-century before the discovery of relativity.
    - The conclusions that follow from that assumption can be tested, and when we test them they are confirmed.

    That doesn't mean that it's right, but it does mean that any alternative that doesn't do all of the above has to be wrong.
  5. Aug 17, 2016 #4


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    Staff: Mentor

    You can't use acceleration detect absolute motion because per Newton's first and second Laws of Motion, acceleration works the same regardless of your current state of motion.
  6. Aug 18, 2016 #5
    Since the speed of light is the same in any reference frame measuring your speed using it as a reference will always produce: ta-da, the speed of light.
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