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Confusion with Michelson-Morley Experiment

  1. Jun 20, 2013 #1
    Please can someone help me as I really don't understand the Michelson-Morley experiment - there seem to be (from what I have read) two different versions on it. Both versions assume the same apparatus where two light beams are sent out perpendicular to one another. One version says that if a light beam is sent in the same direction as the earth is travelling then it will have a speed c-v (v being the velocity of the earth) and then after it reflects from the mirror its return journey will be c+v. The other beam is sent perpendicular to this. It puts emphasis on the fact that everything is stationary with respect to the so called "ether". In the second version, it describes the motion of the earth through the ether as creating an "ether wind" which is OPPOSITE to the direction of the earth moving through it (like a car creating a backward wind despite the air being still outside). This version says that if you send out a beam of light in the same direction as the wind (and therefore opposite direction to the earth) you will have a speed c+v and in the opposite direction a speed c-v. These versions seem completely different to me! The first one makes no mention at all about an ether wind. Both these versions come from reliable books; first version - Great Scientific Experiments (Rom Harre), second version - Why does E=mc2 (Brian Cox). Please clarify.
  2. jcsd
  3. Jun 20, 2013 #2


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    Staff: Mentor

    (Let's say that the earth is moving from left to right, so the forwards-moving direction is towards the right).

    They're describing the same situation with different words. Imagine a car driving down the road through still air at a speed of 100 km/hr... I could say that the air is at rest and the car is punching through the air at 100 km/hr, or I could say that the car is at rest while experiencing a 100 km/hr wind from the front. Either way, the car is moving at 100 km/hr relative to the air and the ground, and the air is at rest relative to the ground.


    In version 1, we're saying that the earth is moving to the right through the ether at speed v, so the right -moving light beam moving at c through the ether might be expected to move at c-v relative to the earth.

    In version 2, we're saying that the earth is at rest and the ether is moving to the left at speed v. If the light is moving to the right at speed c relative to the ether, it's still moving at c-v relative to the earth.
    Last edited: Jun 20, 2013
  4. Jun 20, 2013 #3
    Yeh that sounds much more logical when you put it like that, thanks!!
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