Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Black Hole; Gravity & Speed Within Questions

  1. Jul 20, 2009 #1
    2 questions;

    Is there a point, either at or just beyond the event horizon where a photon would "float" due to an equal amount of gravity vs the energy of the photon trying to "avoid" said gravity? Does this affect any relativity laws if there is a brief moment where a 3rd frame of reference could possibly see a "motionless" particle of light?

    For an object that crosses the event horizon, does it's velocity increase at any kind of fixed rate, thus smashing into the singularity? Or does the effect of gravity on the objects frame of time cause it to slowly "blend" into the singularity?

    If the 2nd question is hard to answer because of a lack of knowledge within black holes, what are some of the larger theories regarding what is happening at the center, if there are any?
  2. jcsd
  3. Jul 20, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    not fish-like

    Hi KevinMWHM! :smile:

    No, a black hole is not like a waterfall: the speed of light cannot "match the speed of space" in the way a fish could match the speed of a waterfall.

    Photons always move, they can't float. :wink:
    I can't remember the exact formula, but a photon will hit the singularity within a very few wavelengths.
  4. Jul 20, 2009 #3
    Hi Tim, Thanx for the link,

    So in regards to my first question, would I be correct in relating it to the radius of photon sphere in that a photon, always being in motion, could never be in the scenerio I depicted?

    thanks also for the answer

  5. Jul 20, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper

    ah, that's an autolink … a unique PF feature! :biggrin:
    Yes, a photon can't hover, it can only keep the same "height" by moving "sideways". :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook