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Ray of light through an elevator

  1. Feb 2, 2016 #1
    The famous thought experiment that says that the path of light curves in a gravitational field uses the equivalence principle to demonstrate how, if you project a light beam through a hole in an elevator being accelerated through deep space, then to an observer inside the elevator, the path of the light beam would appear to be curved in a parabolic shape:


    "In that case, continued Einstein’s Gedankenexperiment, light must be affected by gravity. Imagine that the elevator has a hole bored straight through two opposite walls. When the elevator is at rest, a beam of light entering one hole travels in a straight line parallel to the floor and exits through the other hole. But if the elevator is accelerated upward, by the time the ray reaches the second hole, the opening has moved and is no longer aligned with the ray. As the passenger sees the light miss the second hole, he concludes that the ray has followed a curved path (in fact, a parabola)."

    Take the same scenario and, instead of using an elevator undergoing constant acceleration, just send through an elevator moving at a constant velocity. The beam is still going to end up lower in the elevator on the opposite side. My question is what is the path through the elevator going to look like? Is it going to be curved or is it going took to look like a straight, sharp angle with no curvilinearity to it?

    Edit: I just thought about it and I'm guessing it's going to look like a straight, sharp angle with no curvinearlity to it. But confirmation would be nice.
  2. jcsd
  3. Feb 2, 2016 #2


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    It's just elementary Kinematics. If something is moving in a straight line at constant speed (from left to right, say) and you move towards it (at constant speed), then from your reference frame it will move from left to right at constant speed and towards you at constant speed. And that's a straight line.

    If you accelerate towards it, then (from your reference frame) it will move at constant speed from left to right and acclerate towards you. And that's a curve.

    This applies to anything moving in a straight line at constant speed (light, someone walking, a ball rolling along the ground). If you accelerate, things will change from moving in straight lines to (in your reference frame) moving in curves. Here's a video that shows this for circular motion:

  4. Feb 2, 2016 #3
    Thanks for the clarification. And thanks for the video, too, pretty cool. That Coriolis "fictive" force always gets me dizzy o0)
  5. Feb 6, 2016 #4
    No, the beam is not gonna end up lower. If the elevator is moving at constant speed then it means it is at rest per se.
  6. Feb 6, 2016 #5


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    It depends on the relative motion between the light's source and the elevator. If they are at rest with respect to each other, then the light goes straight across. If the light source and elevator have a relative motion with respect to each other, then the light enters the hole at, and travels across the elevator at an angle with respect to its floor.
  7. Feb 6, 2016 #6
    Yeah..I thought the light is flashed from the elevator itself. But thanks for clarifying
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