Seen a number of news articles recently that black holes have an end of life and turn into white holes that will spew out matter.
So I assume that all black holes suck in matter up to the element iron from the stars and dust that it consumes. What happens to this matter once inside and in what form is it spewed out ?
In as iron, spit out as iron ?
Or
Is everything discombobulated inside and spit at as something else, perhaps hydrogen ?
Black holes evaporate, this is not the same as becoming a white hole, indeed the actual maths works from the assumption that there is not an associated white hole which is a necessary assumption to make unambiguous predictions.
In terms of what comes out, the answer would be a thermal bath of various fundamental particles such as photons. This is independent of what goes into the black hole, though some have argued that other considerations may mean that the what goes out might not be quite as independent from what goes in as originally believed.
Stupid quantum physics question time:
I thought we get a black hole when an object becomes so massive (and dense) that even photons cannot escape gravity’s pull.
But if Hawking radiation causes the black hole to lose mass, why wouldn’t it start letting light out again, at least briefly, before vanishing?
Well photons coming out is light … whether or not is visible probable depends on the rate of release, yes?
Asympotically fat, I have heard this evaporation bit discussed in terms of information and never quite got what they were talking about. Care to take a stab at explaining that to the rest of us?
The superficial answer to your question is quantum pair production happening in the vicinity of the event horizon causes “negative mass” virtual particles to sink into the BH, while the “positive mass” partner gets away, thus reducing the BH mass by that much. It’s not that the BH actually ejects particles or photons at any time in its existence as a BH. Hopefully the experts will weigh in and set me straight.
Still, though, the BH mass is reduced, yes?
So it seems to my simple and no doubt confused mind that at some point, the mass is decreased to the level that it no longer is a black hole, even if we’re talking something .01 millimeter wide and still very dense. At that point, wouldn’t it radiate again?
For any mass, there is an associated radius called the Schwarzschild radius, directly proportional to the mass. If you have that much mass packed into that small a radius, you have a black hole. A black hole’s event horizon is always located right at its Schwarzschild radius. If the mass of the black hole changes (up or down), so does its Schwarzschild radius, and thus the location of its event horizon. So if a black hole loses some mass due to Hawking radiation, what you have left is a lighter and smaller black hole-- It never ceases to be a black hole.
At least, as far as we know. The math behind Hawking radiation was developed for holes with mass much greater than the Planck mass, and we know that it contains approximations which must break down by the time we get that small. The physical behavior of a black hole must therefore change somehow, some time before it evaporates entirely. How? We don’t know. Ask again when we have a theory of quantum gravity. But it’s certainly not outside the realm of plausibility that, at some point before (but very close to) the end, a black hole might cease to be a black hole.
In a nutshell: One of the fundamental foundations upon which quantum mechanics is built is the notion that information can never be entirely lost: It can become so hopelessly scrambled that it would be utterly impractical to ever recover it, but it cannot be lost. On the other hand, our best theories concerning black holes suggest that when anything falls into a black hole, almost all information about it is lost completely, and that the Hawking radiation which eventually comes out is entirely random. Obviously, something’s got to give, here. The best guess anyone has is that the information isn’t actually completely lost, and that the Hawking radiation is nonrandom in an extremely subtle and complicated way, such that it carries off all of the information, but nobody’s really sure how this would work.
Oh, and it should also be mentioned that Hawking radiation is an extremely slow process, at least for black holes of astronomical size (i.e., the only sort we know to exist). For a hole the size of a star, you’d have to wait until the Universe is expanded by about a factor of a million relative to now, before the hole would even start shrinking: Before that time, it’ll eat more just from the cosmic microwave background than it’s emitting. The larger the hole, the slower the process, and for the largest holes we believe to exist, it’ll literally take over a googol seconds for them to evaporate completely.
A black in the late stages of evaporation is reckoned to release a lot of energy in the form of Hawking radiation, but this radiation doesn’t originate from inside the black hole.
If the question is, for an evaporating black hole,“Can something be inside horizon*, but escape to infinity as the horizon shrinks?” or additionally “Can anything even cross the horizon of an evaporating black hole as it recedes?” those seem actually very sensible questions. One problem with answering these questions though is that process of the black hole evaporating hasn’t been directly modeled, even semiclassically; but assumptions obtained from the semiclassical model of Hawking radiation can be fed back into the classical physics to create classical models of black hole evaporation. I believe in the classical model the answer is that objects may still cross the event horizon and when they do the inevitably meet the singularity, though the answer may be a bit more complicated if you add rotation into the mix.
*in this case I mean the apparent horizon, rather than an event horizon as the event horizon is defined by the fact nothing can escape from it.
As the radiation is black body, the amount being released of a given frequency depends on the temperature of the black hole.
The overarching idea of the information paradox is fairly simple: in QM the normal evolution of a system is such that you can always evolve the current state back in time to find out its state at any previous time, but if Hawking radiation isn’t dependent on all the properties of what goes into a black hole then once something falls into a black hole you cannot evolve the state after the object has fallen in to find the state of the system before the object falls in. This is called information loss.
Thanks. I think I’ve got the basic idea now.