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Behavior of light accelerating

  1. Aug 13, 2008 #1
    "Behavior of light accelerating"

    Imagine a spaceship which has got a small lightbulb standing on the floor. You turn on the lightbulb and the light shoots towards the ceiling, but before the light reaches the ceiling the spaceship accelerates a bit and stay at some velocity.

    My question is,, what would this look like for an observer onboard the ship. Would the light suddenly slow down durring its flight and then pick up the pace again?

    (thx in advance, this is an awesome forum) ^^
  2. jcsd
  3. Aug 13, 2008 #2
    Re: "Behavior of light accelerating"

    The spaceship and everything attached to it accelerates however the light beam does not accelerate.
  4. Aug 13, 2008 #3
    Re: "Behavior of light accelerating"

    Jennifer is absolutely correct in what she says.
    There are some interesting side effects though.

    Rather than trying to view the photon "in flight" it is better to try and time the interval between emmision and reception. Imagine the light bulb sends signals from the floor every second and when the rocket is stationary, they arrive at the ceiling at regular one second intervals. When the rocket has a sudden surge of acceleration, a recorder at the top of the rocket will notice that there is a blip on the chart where the interval between photon arrivals was slightly longer than one second. For as long as the rocket maintains constant acceleration the intervals will remain longer than one second and when the rocket stops accelerating and cruises at a new higher velocity, the intervals will return to regular once per second just as when the rocket was at rest.

    When the rocket is accelerating it is progressively length contracting and the while this tends to reduce the delay in the arrival of the photons it does not exactly cancel out the effect, partly because the clock on the floor is more time dilated than the clock at the ceiling during the acceleration. A similar delay would be recorded if the rocket was simply parked vertically on a masssive gravitational body.

    If the rocket was suddenly accelerated sideways from left to right while the photon was going up, the photon would miss its usual target at the ceiling. To the occupants of the rocket it would look like the photon must have followed a curved path to the left. When the rocket settles down to a new (still sideways) velocity the photons will once again hit the ceiling target normally and appear to travel in a straight line from the point of view of the occupants.

    In other words at any constant velocity, everything inside the rocket behaves as if the rocket is at rest. During acceleration things are slightly different.
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