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

Confused about the event horizon

  1. Dec 26, 2009 #1
    I have recently finished "Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity".

    Great book, but I am confused about the behavior of time at the event horizon of a black hole. I spent some time looking at existing threads on this site, but was unable to find clarification.

    I read repeatedly, that from the perspective of an outside observer, time will stop at the event horizon. So if I am watching something fall into a black hole, it will seem that it stops at the brink of the precipice and I will not actually see it cross into the abyss.

    I understand that this statement is not fully true; I would not see the something frozen in time. Instead, I would see the red shift of any light to longer and longer wavelengths.

    My question: Time will stop at the event horizon, so from the perspective of the outside observer, nothing actually ever falls into the black hole. Does it all just accrete at the event horizon?
  2. jcsd
  3. Dec 26, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Paul! Welcome to PF! :smile:
    ah, http://books.google.com/books?id=P_...avity+from+the+Ground+Up"&client=safari&cd=1".

    Time doesn't stop at the event horizon, it's just that our coordinate system breaks down there.

    Yes, in our coordinate system, the speed of an infalling object gets slower and slower.

    (btw, this has nothing to do with the accretion disc of a black hole , which is at radius 6M, three times as far out as the event horizon).
    Last edited by a moderator: Apr 24, 2017
  4. Dec 27, 2009 #3
    Great. Thanks for the quick response.

    If "our coordinate system breaks down" at the event horizon, what coordinate system does apply?
    Last edited: Dec 27, 2009
  5. Dec 27, 2009 #4

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The Eddington-Finkelstein, Painleve-Gullstrand, and Kruskal-Szekeres coordinate systems all cover the event horizon of a Schwarzschild black hole.
  6. Dec 27, 2009 #5


    User Avatar
    Science Advisor

    I'm going to go out on a limb here and suggest perhaps tiny-tim meant that the coordinate time (time measured by an observer at infinity) is what gives this whole "time stopping at the horizon" business. For an infalling observer, he will experience nothing of the sort.
  7. Dec 27, 2009 #6
    Great. That is what I was afraid of. I am not so good at calculus, and tensor calculus is way beyond my meager skills.

    Thanks for the responses.
  8. Dec 28, 2009 #7


    User Avatar
    Science Advisor
    Homework Helper

    Last edited by a moderator: Apr 24, 2017
  9. Dec 30, 2009 #8
    The second book looks great. I will buy a copy from Barnes and Noble. It looks like it will give a basic introduction of why General Relativity was needed to give gravity a consistant behavior across multiple frames of reference.

    Thank you for the guidance.
    Last edited by a moderator: Apr 24, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook