Why can't we see objects fall into a black hole?

[spizma]

I read somewhere a while ago that if you were to look at an object falling into a black hole you would never see it actually fall past the event horizon, only continually approach it. That sounds very strange, is it true? If it is why so?

 

[Riogho]

The intense gravity causes time from your frame of refenerence to appear to stop.

 

[CPL.Luke]

however this is only if you are not falling into the black hole as well, if you are then you won't see it stop.

 

[Riogho]

Yes, if you are not initially falling 'into' the black hole, you will never. You will most likely orbit it :P

 

[Denton]

So then what happens if 2 objects fall into a black hole at separate time periods, by the objects view they would enter the black hole at separate times, but by an outside observer the first one would slow down and stop before the event horizon and then the second one would do the same.

So doesn't that mean we see 2 object occupying the same amount of space?

 

[Jorrie]

So then what happens if 2 objects fall into a black hole at separate time periods, by the objects view they would enter the black hole at separate times, but by an outside observer the first one would slow down and stop before the event horizon and then the second one would do the same.

I think it's not quite correct to say that radially infalling objects appear to "stop" before they reach the event horizon, as viewed by an observer either orbiting a black hole (or being held stationary at some fix radial coordinate outside the hole). In the coordinates of such an observer, the time for an infalling object to reach the horizon is infinite and hence its radial coordinate velocity approaches zero asymptotically. One that was "in front" would however always stay marginally in front.

This may be loosely described as 'stopping' or 'freezing' in place, but I think that super-sensitive observations can always show relative movement.

 

[pervect]

I read somewhere a while ago that if you were to look at an object falling into a black hole you would never see it actually fall past the event horizon, only continually approach it. That sounds very strange, is it true? If it is why so?

Yes, that is correct.

I'm not sure why you think it sounds strange. Since light cannont escape a black hole, you couldn't possibly see the object after it went inside the event horizon. In fact, you can't see the event horizon either - which you can think of as light that is "trapped" in place. You can think of the light in the black hole as running the "Red Queen's race" from Alice in Wonderland - it is running as fast as it can, but it doesn't actually get anywhere.

As an object falls into a black hole and approaches the event horizon, it takes longer and longer for the light to escape. When the object reaches the event horizon, escape is impossible.

For more reading, you might try Ted Bunn's black hole faq:

http://cosmology.berkeley.edu/Education/BHfaq.html#q4

there are also some good sci.physics.faq entries on black holes.