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Vertical light weightless?

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  1. Sep 12, 2015 #1
    Let's say a light beam is sent towards a planet from far away. On the surface of the planet no measurement can ever tell that the light beam is approaching, until the light beam has arrived, because information can not travel faster than the light beam.

    Now at the moment when the light beam is 10 m above the planet surface, let's magically reverse all velocities. Now the light beam is traveling upwards, and nothing below 10 m has any idea that the light beam is there. Particularly instruments that measure gravitational pull do not measure any pull from the light beam.

    So, is the following conclusion correct: No momentum exchange happens between a mass and a light beam that moves towards the mass or away from the mass?

    Note: When all velocities were magically reversed, I was using "time reversal symmetry law", or what ever the correct term for that kind of thing is. I hope I used it correctly. :smile:
     
  2. jcsd
  3. Sep 12, 2015 #2

    DaveC426913

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    To reverse the light beam, it would have to reflect. It won't happen magically. Momentum will be transferred to the mirror, which will be transferred to the Earth.
     
  4. Sep 12, 2015 #3

    Dale

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    Time reverse symmetry does not allow you to magically violate conservation of momentum. It means that if there is a scenario obeying the laws of GR which ends with a beam of light going vertically down there is also a different scenario that starts with a beam of light going up and that scenario also obeys the laws of GR. momentum is conserved in each scenario, and they are separate scenarios.
     
  5. Sep 13, 2015 #4
    Time reversal symmetry just goes over my head. So I'll just ask this question:

    Let's say a light beam is sent towards a planet from far away. On the surface of the planet no measurement can ever tell that the light beam is approaching, until the light beam has arrived, because information can not travel faster than the light beam.

    The light beam is traveling downwards, and nothing below it has any idea that the light beam is there. Particularly instruments that measure gravitational pull do not measure any pull from the light beam.

    So, is the following conclusion correct: No momentum exchange happens between a mass and a light beam that moves towards the mass?
     
  6. Sep 13, 2015 #5

    ZapperZ

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    Why do you have to go through all this gymnastics? Why can't you just ask "If I shoot a photon at you, can you tell that a photon is coming BEFORE it hits you?"

    Isn't this the same thing?

    Zz.
     
  7. Sep 13, 2015 #6
    It might be helpful if you remember that the force of gravity travels at the speed of light.
     
  8. Sep 14, 2015 #7

    PeterDonis

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    If you mean, no momentum exchange happens before the light beam reaches the mass, yes, that's correct. The momentum exchange happens when the light beam reaches the mass. Why would you expect otherwise?
     
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