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I Does light move, or does space and time move past light?

  1. Oct 28, 2018 #1
    Could it be possible for light to not move at all but remain still while space and time moves past it? The light would just exists as the continuum of space time moves past light.
     
  2. jcsd
  3. Oct 28, 2018 #2

    PeroK

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    The definition of motion is a change in position over time. By this definition, light moves.
     
  4. Oct 28, 2018 #3
    Impossible.

    By your definition we would have collided with the Sun or any Star which emits Light a very very long time ago.
     
  5. Oct 28, 2018 #4
    Said sun and stars would be moving with space time as well
     
  6. Oct 28, 2018 #5
    But could it be that position is what is really moving.
     
  7. Oct 28, 2018 #6

    PeroK

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    Spacetime does not move. Spacetime is the continuum within which motion can be defined.
     
  8. Oct 28, 2018 #7
    A star emits light in all directions.
    Which way would your poor star move then? Every direction?
     
  9. Oct 28, 2018 #8

    Dale

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    There is no inertial reference frame where this is correct. You could construct a non-inertial coordinate system where this is correct, but the laws of physics would not be what we are used to at all.
     
  10. Oct 28, 2018 #9
    You have heard that the universe is expanding
     
  11. Oct 28, 2018 #10

    Nugatory

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    Mentors' note: This post had been derailed by a more-heat-then-light argument involving personalities rather than physics. The offending posts have been removed and the primary rabble-rousers have been banned from the thread. Everyone is reminded to please focus on the physics of the matter, and when you see posts that you consider to be personal attacks, violations of the PF rules, or rude report them instead of replying.
     
  12. Oct 28, 2018 #11

    Nugatory

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    We have, and in our cosmology section you will find many threads discussing what this means.

    However, the effects of that expansion are utterly negligible (and you should not dispute that assertion unless and until you have tried calculating their approximate magnitude) at the scale of a solar system, so @Rada Demorn point is valid. Even if it weren't, you should also consider the point @Dale makes in post 8 above, in the context of two light bulbs one meter apart and shining in all directions.

    Asking questions in a physics forum can be an effective way of learning, but you have to listen to the answers. Even more effective is to spend some time reading about what is already known, and for that we highly recommend the book "Spacetime Physics" by Taylor and Wheeler; that will get you caught up to what physicists knew about 100 years ago.
     
  13. Oct 28, 2018 #12

    bhobba

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    I do not know what you mean by space-time moving? For simplicity we will discuss non curved space-time - what is called inertial or flat space-time. That may be what is 'confusing' the situation for you. As conceived by physicists space-time also involves a coordinate system - do you mean that is moving? If so one of the strange properties of light is it does not matter how fast a space-time coordinate system moves light always travels away from it at the speed on light. In fact from relativity and coulombs law you can prove all of electromagnetism and that light moves at the speed it does relative to an inertial coordinate system:
    http://www.cse.secs.oakland.edu/haskell/Special Relativity and Maxwells Equations.pdf

    As Nugatory correctly says cosmological 'effects' such as space-time expansion are negligible here on earth - for most practical purposes the earth can be considered an inertial frame. Strangely though although true for virtually all practical purposes the curvature of space time (general relativity) needs to be taken into account for the GPS to work - but such exceptions are rare.

    Thanks
    Bill
     
    Last edited: Oct 29, 2018
  14. Oct 29, 2018 #13

    Vanadium 50

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    Imagine two light beams moving past each other. Are you arguing both are stationary and space is moving past them in two different directions?
     
    Last edited: Oct 29, 2018
  15. Oct 29, 2018 #14

    CWatters

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    A mirror or worse an infinity mirror would also appear to cause problems.
     
  16. Oct 29, 2018 #15

    Dale

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    You can make coordinate systems with four null vectors. So you can have each null ray “standing still” regardless of the direction.

    Edit: actually, I am not sure if null vectors off axis are “standing still” in these coordinates
     
    Last edited: Oct 29, 2018
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