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About how light clocks would work

  1. Aug 9, 2009 #1
    Hi. I've been reading about relativity and time dilation. Also watched some visualisations on youtube. The question I have is about the light clocks which are always used to demonstrate the theories. Explanations always show the light beam on the moving clock moving in a diagonal motion, zig-zaging up/right and down/right. As the diagonal path is longer, the clock runs slower.

    This implies that light emitted normally from from a source inherits the momentum of it's source, thereby providing the left-->right component of it's diagonal path.

    Is this correct? It seems counter intuitive. I always assumed that a moving (left to right) light source would emit its beam, which would go straight up and be left behind by it's source, which is continuing on it's way along the x-axis.

    Just to make sure i'm clear, I'd expect that light emitted (pointing upwards along the y-axis) at time=0, x=0, velocity=v, would at time t be located at x=0, y=ct. These light clocks make it look like at time t it would be located at x=vt, y=ct.

    Can anyone help? Thanks : )
  2. jcsd
  3. Aug 9, 2009 #2


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    Remember, there is no absolute motion, so as far as the light clock itself is concerned, it is not moving, so the light just goes back a forth between the mirrors. It is only according to the frame in which the clock is moving that the light follows a diagonal path. So for instance if you have two light clocks moving relative to each other, a nest to each would see the light of his clock bouncing up and down in a straight line, and the light of the other clock following a diagonal.

    What you are describing is what would happen if there was an aether or absolute rest frame, SR says there is neither.
  4. Aug 9, 2009 #3
    The light clock you're talking about emits the photon vertically in its own rest frame. The photon travels along the y-axis and strikes the upper plate. All observers must agree that the photon strikes the plate. One observer can't observe the photon to hit it while another sees it whizz by. Either it strikes the plate or not. (This is a fundamental axiom of science: that an external objective universe exists in which events occur that are mutually agreed upon by all observers). Therefore, in a reference frame in which the plate is moving, the photon must still be taking a trajectory that connects the point of emission with the point of reception, i.e., a diagonal path.
  5. Aug 10, 2009 #4
    Ah, thanks guys. I hadn't thought about it like that. I suppose if I were sat on a train, throwing a tennis ball up and down, I'd take it that the tennis ball had inherited the same x-axis velocity as me, by virtue of the train moving, and that what looks like up and down to me, would look diagonal to someone outside the train stood still. Infact, i'm just making that assumption because I always take the ground to be at rest. The tennis ball could just as well have a x-axis velocity of zero, while the ground outside moves backwards.

    Things are seeming much clearer now, and even weirder at the same time :D

  6. Aug 10, 2009 #5


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    That means you're getting it :biggrin:
  7. Mar 28, 2010 #6
    Hi. Was just thinking about this again and found my old post.
    I've got a little confused recently, but i think i might have figured it out. My confusion was from seing the light clocks moving diagonally. They look as though they have inherited the sideways motion of their source. Then, on the wikipedia page about relativity of simultaneity, it has the diagram showing a light source in the middle of a moving train. The light hits the rear of the train before it hits the front because the back is catching up and the front is running away (This is all from an external observers frame).

    Now, is the inconsitency i thought i'd seen due to the fact that in the wikipedia diagram the light movement is on the same axis as the train movement, whereas for the light clocks it's in the other axis?

    So, in the light clock scenario, to the observer the train is moving say 1 quarter speed of light. THerefore when the light is emmitted upwards it then uses the remaining 3 quarters speed of light for the upwards direction, resulting in a diagonal speed of 1 times speed of light?

    Then, in the wikipedia scenario, because the light is emitted on the same axis, the light going forward is simply at the speed of light, and the light going backwards is also simply at the speed of light, so the train catches up the backwards light and runs away from the front light?

    In this way, it looks as though the first scenario light is following it's source sideways, while the second scenario appears like a contradiction to the first?

    Thanks for any help : )
  8. Mar 28, 2010 #7
    ooh...and that just made me think:

    If the light source in the middle emmitted a sphere of light from a single point (i'll imagine a 2 dimensional circle instead) and we superimposed the previously mentioned examples on top of that, then the light clock photon would follow the edge of the circle at something like 10 past midnight, while the rearward beam from the wikipedia scenario would follow the 9 oclock edge and the front facing beam would follow the 3 oclock edge of the superimposed circle?

    I hope that's right because it all seems clear to me now :D

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