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Photons and gravity

  1. Dec 5, 2007 #1
    If you throw a ball gravity pulls it down. The harder you throw it the farther it will go but it is still constantly being pulled down by gravity. If you look at a photon even it will get pulled down by gravity but the photon will go a lot farther before it would hit the ground. Now if you were to view this photon on a very short interval wouldn't the photon be going in a straight line and then kind of step down towards the ground due to planck length. Just think of the floor and ceiling functions in math. I don't understand how the planck length could even make sense with this because the photon can't be moving slightly down on anything smaller then the planck length. As usual, I probably just don't know everything about the subject and have a lack of understanding. Thanks in advanced.
  2. jcsd
  3. Dec 6, 2007 #2


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    the fact that, in our Euclidian coordinate system, we observe the bending of a ray of light toward a massive object (due to gravity, but in GR they would say that both the photon and ball are travelling in a "straight" line and it is the space-time around it that is curved, due to the massive object). but i don't think that Planck units have anything to do with this. what's the connection?
  4. Dec 6, 2007 #3


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    I think what he's asking is, supposing that in the same way that pixels make up a monitor, planck lengths of space make up our world, wouldn't a photon seem to suddenly skip down into the next planck length instead of moving in a smooth curve a la macroscopic physics?

    Similar to aliasing:

    Last edited: Dec 6, 2007
  5. Dec 6, 2007 #4
    That is exactly what I mean. Basically you explained it better then i could. But I guess the curved spacetime does make sense of it anyway. Thanks for the answer, I've been thinking about this all day.
  6. Dec 6, 2007 #5


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    A photon whose wavelength was about the Planck length would have enormous energy - probably enough to start another big bang. The kinds of photons we deal with ( even gamma rays) have wavelengths which are gazillions* of times bigger that the Planck length.

    * I don't know the actual number but about 10^20 should cover it ( with tip ).

    A gravitating body needs to be very dense before photons are stopped from leaving by the field. On earth, photons escape easily but instead of slowing down they appear to stretch while escaping.
    Last edited: Dec 6, 2007
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