Q: Will black holes ever release their energy and will we be able to tell what had gone into them?

Physicist: In any reasonable sense the answer to both of these questions is a dull “nope”.  In theory however, the answer is an excitable “yup”!

Blackholes lose energy through “Hawking Radiation”, which is a surprising convergence of general relativity, quantum mechanics, and thermodynamics.  Hawking (and later others) predicted that a blackhole will have a blackbody spectrum.  That is, it will radiate like people, the sun, or anything that radiates by virtue of having heat.  Hawking also calculated what temperature a blackhole will appear to be radiating at.  He found that for a blackhole of mass M: , where everything other than M is a physical constant (even the 8, depending on who you talk to).  A more useful way to write this is to plug in all the constants to get:

, where M is in kilograms and T is in degrees Kelvin.  That “” makes it seem like blackholes should be really hot, and in fact small ones (like those we hope to see at CERN) are crazy hot.  However, if the Sun ( kg) were a blackhole its temperature would be about 60 nK (nano Kelvin).  …!  You wouldn’t want to lick it, or your tongue would stick.

Here’s the point.  Deep space glows.  It has a temperature of about 2.7K, which means that any blackhole that could reasonably form ( kg, or several Suns) is going to be way colder than that.  Since the blackhole is colder it will actually absorb more energy than it emits.  In order for a blackhole in the universe today to actually shrink it must have a temperature above 2.7K, and so it must have a mass less than , or around half the mass of the Moon.  Alternatively, you could wait several trillion years for the universe to cool down, and then the blackholes would start to evaporate.

As for the second half of the question: General relativity would suggest that when things fall into a blackhole they are erased.  Once they fall in, there’s no way to tell the difference between a ton of Soylent Green and a ton of Pogs (metric tonnes of course).  This makes quantum physicists really uncomfortable, because in addition to all the usual conservative laws (energy, momentum, drug policy) quantum physicists have “conservation of information”.  Lucky for them they also get to play with entanglement.  So if you chuck in a copy of War and Peace the blackhole will radiate thermally (which is the most randomized way to radiate) and will seem to scramble everything about Tolstoy’s pivotal work.  If you look at one outgoing photon at a time you’ll gain almost zero information.  If however, you can gather every outgoing photon, interfere them with each other and analyze how they are entangled you could (in theory) reconstruct what fell in.  However, you’d need to catch at least half of the photons before you could demonstrate that they hold any information at all.

This view of blackholes, that they hide information in the “quantum entanglement” between all of their radiated photons, makes them suddenly far more interesting.  Without going into to much detail, if you have N non-entangled 2-state particles you can have N bits of information, but if you have N entangled 2-state particles you can have 2^N bits of information.  Allowing for entanglement frees up a lot of “extra room” to put information.

Suddenly, you’ll find that most (as in “almost all”) of the entropy in the universe is tied up in blackholes.  Also (again in theory), a carefully constructed blackhole can be the fastest and most powerful computer that it will ever be possible to create.

So, yes, blackholes will release all their energy, but you have to wait for the universe to cool down almost completely.  And, yes, we can tell what went into them, but we’ll have to wait for them to evaporate completely (after the universe has cooled down) and catch, without disturbing, almost every single particle that comes out of them.