Don't hit me if the question is too lame but here it goes

1) We all know that to see something light must get reflected from it's surface and then reach our eye's

2) Any thing which goes in black holes get incinerated and destroyed completely

3) For a observer outside the event horizon he will see that the falling object slows down and eventually stops moving and seems like it is stuck there.

So my question is if to see something light must get reflected from it's surface and then reach our eyes, and something going in black hole gets completely destroyed then how a person outside the even horizon is able to see it -- I mean it is completely destroyed then what is reflecting the light so that we can see it (sorry if not clear in language)

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    $\begingroup$ Point 2 isn't necessarily true. $\endgroup$ – Kyle Kanos Feb 15 '14 at 4:27
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    $\begingroup$ You can't really destroy a photon.. Mass-energy whatever isn't get destroyed inside Black Hole. $\endgroup$ – Earth is Donut Feb 15 '14 at 12:00

The answer is that we can't see a black hole, which is I would guess what you were leading up to. There are only two ways we can detect a black hole using light (of various wavelengths):

  1. if it occludes something behind it

  2. if it's surrounded by an accretion disk

Point (1) is actually quite complicated because a black hole doesn't simply mask whatever is behind it, but instead it acts as a gravitational lens. This can cause various optical effects such as microlensing. I mention microlensing because it has been used a way to search for small compact objects. We haven't found any black holes with this technique, but we have found several extrasolar planets.

Point (2) is how we find candidate black holes such as Cygnus X-1. When the black hole is part of a binary system it can accrete matter from its companion. That matter is heated by tidal forces and radiates X-rays. Likewise we believe that quasars are supermassive black holes accreting matter from the galaxy around them. The intense radiation of a quasar comes from tidal heating of this matter.

Response to comment:

In our coordinate system the matter faling into the black hole never crosses the event horizon. As the matter approaches the event horizon it appears to move more and more slowly, and the light from it is increasing red shifted. However the matter will never fade completely to black. That's why we can still see it.

When I say in our coordinate system this sounds like a technical point, but in our coordinate system means as timed by our clocks and as measured by our rulers, so it's a real effect. As we measure time it would take an infinite time for anything to reach the event horizon let alone cross it.

  • $\begingroup$ no i am not asking why we can't see black holes i am asking how we can see a object stuck on the event horizon if it is destroyed by the gravity of the black hole already $\endgroup$ – Deiknymi Feb 16 '14 at 3:20

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