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Time dilation confusion

  1. Apr 11, 2013 #1
    I've seen an experiment with a light clock that explains time dilation. A photon is fired and gets reflected off of 2 parallel mirrors. It keeps bouncing back and forth like that, with each bounce qualifying as one tick of the clock. And it looks like that from a stationary reference frame. Now if the clock is moving to the right at some highs speed relative to us, the photon would need to travel in a triangular path. Therefor it takes longer for the clock to tick considering that the speed of light or a photon is constant and equal "c" for all reference frames. I'm sure you all know what the diagram of this experiment looks like: http://spiff.rit.edu/classes/phys200/lectures/dilation/red_time_a.gif

    Now here's what I don't understand. Imagine the same scenario, now with a person in the same reference frame as the clock inside a space ship. So now we have that person and our light clock moving to the right at some high speed. The person looks at the clock, a photon is fired straight up from the bottom mirror to the top mirror so it's just bouncing back and forth infinitely. When we look at the photon being fired, wouldn't the photon just travel straight up and down a few times (non triangular path) and then just slam into the back of the spaceship (because the space ship is moving towards it). That's what I don't understand. Because from what I've read, the photon is moving to the right with the entire space ship. How does it get that velocity to the right when the speed of electromagnetic waves is not dependant of the speed of the source. Why wouldn't the photon slam into the back of the spaceship?

    Thanks in advance.
  2. jcsd
  3. Apr 11, 2013 #2
    Imagine you are in an almost empty 747 airplane, seats removed, flying at a high and constant velocity and without any turbulence.

    Could you bounce a basketball?
    Or play table tennis?

    Would the ball slam into the back of the plane or would it simply come straight up?
    Or when you use the water faucet does the water crash against the wall instead of going straight down?
  4. Apr 11, 2013 #3
    So does that mean that inertia applies to photons also?
  5. Apr 11, 2013 #4
    What would happen when you shine a laser pointer to the roof of the plane, does the light beam curve or does it go straight up?
  6. Apr 12, 2013 #5
    There is a recurring confusion about this, usually from reading Einstein translated from the German, or from other sources that don't clear the distinction carefully. The translations use the word "velocity" for the German word meaning simply "speed". But velocity includes direction...

    So, when you read that the "velocity" of light is independent from the source motion, one may think this includes both speed and direction, and then it does seem to suggest that the light shouldn't "carry long" with a moving source because of the light's direction (the direction being at right angle for the local observer with the clock, but if also at right angle from the perspective of an observer at rest, that observer would expect the light to be left behind the advancing mirrors and hit the space craft wall).

    But only light's speed is independent of the source motion, not its velocity (not the direction component).
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