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Impossible scenario?

  1. Nov 12, 2011 #1
    Impossible scenario?!?

    Aargh, pleeaase help me wrap my head around this whole relativity thing.

    We just started the chapter on relativity at uni and our professor is telling us light speed is invariant and wants us to contemplate an impossible hypothetical scenario for next class! Help!!

    Here's the scenario:

    A satellite is launched into deep space with a laser and particle accelerator onboard. The PA accelerates a proton to 97% of the speed of light, opens a hatch, and lets the proton fly out. The laser is situated adjacent to the PA opening and the instant the proton leaves, the laser emits a light pulse in the exact same direction.

    OK, the proton is at .97c but yet it must measure the laser pulse racing ahead at light speed and not .03c? HUH??

    Who's moving at what speed relative to whom? Doesn't the 97% light speed count for anything?
  2. jcsd
  3. Nov 12, 2011 #2


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    Re: Impossible scenario?!?

    From the point of view of the satellite, the light is indeed racing ahead of the proton at .03c. However, in the point of view of the proton itself, the light is racing ahead of it at 1c.
  4. Nov 12, 2011 #3


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    Re: Impossible scenario?!?

    In order to measure the speed of the light pulse, the proton will have to put a mirror out in front of it a measured distance away. Then it needs to start a stopwatch at the moment of common launch and stop it when the reflected light puse gets back to the proton. the proton then takes twice the measured distance (because the light pulse has to travel both directions) and divide it by the measured time interval to get the speed of the light pulse. The proton will get c as the answer.
  5. Nov 12, 2011 #4
    Re: Impossible scenario?!?

    The proton can never achieve c as its mass would have to increase infinitely in order to do so. Even it was accelerated to 99.99% recurring of c it can never achieve c because of its increasing mass. That's because c is the ultimate speed limit in the universe according to GR and is defined by the speed of light in a vacuum. Therefore the light will always travel at c to the proton's point of view because a photon has zero rest mass unlike the proton or any other massive particle so nothing can catch up to it when it's moving through the vacuum.
  6. Nov 12, 2011 #5
    Re: Impossible scenario?!?

    Yes it is very strange and confusing.

    Light moves at the same speed in every reference frame. It is completely unintuitive. You just have to accept it and get used to it.
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