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

Black holes and the event horizon

  1. May 9, 2012 #1
    I'm curious - once an object passes the event horizon the image of that object remains on the event horizon only to become more redshifted rather than dissipating. two questions: 1) why does the image remain if the light stops traveling? if the light cannot travel to the observer, than there would be no 'image' in the classical sense. and 2) if the image is seen, why can we not spot black holes merely by looking for clusters of objects that have ceased moving relative to one another?
  2. jcsd
  3. May 9, 2012 #2


    User Avatar
    Science Advisor
    Gold Member

    As an object approaches the event horizon it also becomes [to an external observer] time dilated, hence, appears to take literally forever to actually reach the event horizon. Its light also approaches infinite redshift.
  4. May 9, 2012 #3
    Note: to avoid confusion, I am not the OP. I've been bothered about this for a while. Shouldn't Black Holes fail to form, since they'll take literally forever to collapse to the Event Horizon? (To an external observer, of course.)
  5. May 9, 2012 #4


    User Avatar
    Homework Helper
    Gold Member

    A couple of points in addition to what Chronos said.
    Allow me to explain something from an completely idealized, theoretical case. Consider something falling into a large, non-rotating black hole, and the black hole is far more massive than the object. Assume that we, the observers, are far away (such as here on Earth). From our frame of reference (not moving at relativistic speeds, relative to the black hole), the thing that we observe hanging around near the horizon --approaching the orizon but never quite getting there -- is the object itself. It's not just an 'image'. It really is the object. And this is based on theoretical grounds which can be shown with the mathematics of relativity.

    By the same mathematics of relativity, in the frame of reference of the falling object, it passes through the horizon in a very uneventful way (this assumes that the black hole is very large, so we can ignore tidal forces). If a person fell through the event horizon, they wouldn't notice anything particularly special at the moment they crossed the horizon.

    This might seem like a paradox, but it's really not. "But which scenario is really the 'true' scenario?" isn't the question that's worthwhile asking. Both are correct. It just depends on one's frame of reference.


    Moving on, that incredible red-shift, that we see from our frame of reference, is more important than what it might seem. The effective resolution of an object that can be observed is dependent on the wavelength of light involved. It's not possible to observe any details on/of an object smaller than around 1 wavelength. This incredible red-shift caused by the black hole will eventually cause the wavelength of the light emitted by the falling object to become as large as the black hole itself. At that point, no more detail can be obtained (from our position/frame of reference) about the object than the fact that the object is somewhere near the black hole's horizon. But nothing else; all other details smaller than the black hole itself (that we can "see" e.g. with a radio telescope) are gone. (Edit: rather than say "gone," maybe I should just say "hidden from our detection.") The wavelength of light is just to large to discern details smaller than a black hole.

    That by the way is still described from a theoretical point of view. I'll talk about some practical aspects in a moment.

    Black holes are really pretty small objects. If (hypothetically speaking) our sun formed a black hole, it would only be about 6 km wide. A super-massive black hole at the center of the galaxy is still just a tiny, tiny speck compared to the overall central galactic bulge. So even large black holes are far too small to "see" directly with telescopes (even if they weren't 'black').

    Now to more practical aspects. When matter falls into a black hole, it is likely to get compressed and scattered with other infalling matter well before it approaches the horizon. Together the matter forms what's called an accretion disk, which orbits the black hole. Due to the high pressure and high kinetic energy, such accretion disks can give off x-rays and high energy particles. When the accretion disk is giving off detectable amounts of energy, we call the black hole "active."

    Another way to detect a black hole, particularly the super-massive black hole at the center of our galaxy, it to track the path of the stars that orbit it. We can't see the black hole, but we can see the light from the orbiting stars.
    Last edited: May 9, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook