Does a Black hole emit anything?

[n4nova]

black hole...

hello everyone...


though i am a student of information technology i love astronomy very much.i have a question for all of u. that is..." does black holes really emit nothing...does really everything going inside a black hole can't come outside of it??"

srijani.

 

[Chris Hillman]

No. (Or maybe yes! But that probably doesn't mean what you might think!)

Hi, srijani,

You asked whether it is really true that nothing which "enters a black hole" (falls past the event horizon) can remerge.

A good short answer might be: yes, according to gtr this is pretty much the defining characteristic of a black hole.

It is probably worth adding that "black hole" means both a precise theoretical notion in a particular theory, gtr (actually also in many related theories), and a class of astrophysical objects which we could very pedantically call "black hole candidates". These are real objects which certainly seem to exhibit the properties expected from gtr models of "black holes". But gtr is a classical theory, whereas Nature adores the quantum, so physicists expect that gtr must break down at very large energy densities (equivalently, very large curvatures). As yet, no quantum theory of gravitation is known, but there are some fairly solid approximations which work near the horizon of theoretical black holes which go beyond gtr and include some elements of quantum field theory. This leads to the prediction of "Hawking radiation", and even suggests the possibility that black holes might (very very very slowly!) "evaporate". This would be nonsense according to gtr, but the approximations in question suggest that a future quantum theory of gravitation might allow such things. So my point is that any discussion of black holes depends upon what theories one is using, and one also needs to distinguish between astrophysical objects called black holes and our theoretical models of these creatures.

Having said this much, I should probably stress that gtr has been very well tested, and is not expected to break down near (inside or out) the event horizon of a solar mass black hole (meaning an astrophysical object). So when I hinted that something might in some sense "emerge" from a black hole, this should be regarded as a very small and/or very long term effect. In particular, if an object such an asteroid or an astronaut falls into an astrophysical black hole, we cannot currently hold out any hope that this object might re-emerge intact, or even that bits of it might reemerge.