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Tim13
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from a layperson's perspective - if a supermassive black hole is more massive than a million suns then why is it cold? An answer in plain english is truly appreciated for the layperson like me.
Chronos said:Electromagnetic radiation cannot escape the event horizon of a black hole.
Tim13 said:from a layperson's perspective - if a supermassive black hole is more massive than a million suns then why is it cold?
twofish-quant said:You have to distinguish between the black hole itself and the stuff around the black hole.
But you can see why the black hole is cold by thinking about what "cold" is. If I have a glass of ice water, what does it mean to say that it's cold? Well, it means that if I put something next to it, the ice water will absorb energy from something next to it.
Now if I have something sitting right outside the black hole, the energy will flow from that thing to the black hole, but almost nothing will come back. I.e. the black hole is cold.
Tim13 said:I get the analogy of a glass of ice water but it is a non-sequitor. The BH absorbs energy due to gravity not temp or is that wrong?
twofish-quant said:This is the topic that made Stephen Hawking famous, since he was one of the first people to point out that temperature and gravity are related.
It turns out that it doesn't matter *why* the BH sucks up energy. As long as it sucks up energy, you can associate a temperature with it.
Hawking also pointed out that one consequence of this is that black holes must have some radiation. If black holes sucked up all radiation and energy then they would have a temperature of absolute zero and you could create a perpetual motion machine. Since this would be impossible, black holes must emit a tiny amount of radiation and Hawking showed how to calculate that amount of radiation.
Tim13 said:I don't accept this explanation by Hawkins. Any household appliance sucks up energy. All household appliances radiate energy.
Tim13 said:I don't accept this explanation by Hawkins. Any household appliance sucks up energy. All household appliances radiate energy.
Temperature of a BH shouldn't be "associated" or "assumed" to be based on energy absorbed without any reference to energy radiated. Am I missing something??
clamtrox said:and how many household appliances have temperature zero?
See, if you have something that can absorb energy while still maintaining zero temperature, then the laws of physics (and thermodynamics in particular) don't really work anymore. That would correspond with the black hole having infinite degrees of freedom (or entropy, if you prefer)
kbar1 said:That is what Hawking thought, I think. A black hole (if we assume that it doesn't have a temperature, i.e. doesn't emit energy) violates the second law of thermodynamics, by sucking up everything that falls on it (reducing the entropy of the universe), but doesn't compensate for it (by emitting energy). Therefore, it must emit particles/energy in some way, and have an temperature, which is predicted by the second law.
Tim13 said:If it really emits no energy (in this universe) then isn't it like a one way street to a different universe?
And if we could measure something that massive why wouldn't it be very hot? Aren't very massive objects (not beyond an event horizon) really very hot?
twofish-quant said:No particular reason why massive objects have to be hot.
Tim13 said:I suppose you are right. There is no particular reason why the mass of millions of suns (equal to a supermassive black hole) when compressed to a tiny volume would get hot. It is probably cold enough to make ice cream.
Tim13 said:I suppose you are right. There is no particular reason why the mass of millions of suns (equal to a supermassive black hole) when compressed to a tiny volume would get hot.
twofish-quant said:There's a difference between the black hole and the region right outside the black hole. The region right outside the black hole gets heated up to hundreds of millions of degrees, and the heat comes because when something collapses, it releases a massive amount of gravitational energy.
So the region right outside the black hole is enormously hot.
Now the black hole itself is very cold for the reasons that have been mentioned.
Tim13 said:I understand (perhaps mistakenly) the phrase to mean only the actual supermassive body beyond the event horizon (and also the event horizon it creates). Please correct me if that understanding is wrong.
Therefore it is not possible to measure the heat of the body called a black hole.
If you can't measure its temperature how do some postulate that black holes are cold?
Intuitively I would suspect the body is radiating heat well above the average temperature of the universe.
twofish-quant said:You can measure the temperature of the black hole. You put different objects near the black hole and see if heat flows to the black hole or away from it. Something that is hotter than the black hole will have heat move toward the black hole. Something that is colder than the black hole will absorb heat from the black hole.
Since the black hole is black, it's going to be very cold.
If you were standing near a black hole, it would start to *feel* cold because the black hole will be sucking up energy from it's surroundings and you would feel the coldness of the black hole in the same way that you feel the heat of the sun.
Tim13 said:I understand and agree with you at a certain level. I guess it is somewhat irrelevant how the apparent coldness is caused.
Tim13 said:But is it possible that the black hole draws something hot towards the black hole not because the black hole is colder but because it warps space/time? And isn't it also possible that something (apparently) colder than the black hole might still be pulled into the black hole because of the warped space/time? If these things are theoretically possible then would you admit that it just isn't possible to measure the temperature of a black hole?
Of course, sound is just vibrations of air molecules and anyone who thinks otherwise is a mischievous hippie.Tim13 said:Last question - If a tree falls in a forest and no one is around to hear it, does it make a sound?
Tim13 said:But is it possible that the black hole draws something hot towards the black hole not because the black hole is colder but because it warps space/time?
And isn't it also possible that something (apparently) colder than the black hole might still be pulled into the black hole because of the warped space/time?
If these things are theoretically possible then would you admit that it just isn't possible to measure the temperature of a black hole?
kbar1 said:What's the current hypothesis as to what happens when the black holes become too small to exist?
twofish-quant said:Doesn't matter. Temperature is temperature. The black holes gravity sucks up heat and the event horizon keeps it from reradiating. That makes it cold.
Doesn't matter.
There is a precise definition of temperature in the zeroth law of thermodynamics. You can take the definition of temperature and they show that black holes have a temperature that can be measured.
clamtrox said:So the temperature of Hawking radiation is proportional to 1/M where M is the mass of the black hole. The power thermal objects radiate is P ~ AT4 where A is the surface area. For a black hole, A~M2 so all in all you have P~M2 M-4 ~ M-2. If you also remember that mass is energy, we can write the power as P ~ dM/dt ~ M-2 and integrating this expression, you will find that it will take a finite time to radiate all the energy away for a black hole of any size. You can also see that when M is very small, the radiation energy increases sharply. Basically when the black hole gets small enough, it explodes releasing the rest of its energy in a very short time period.
As for what happens with the singularities and all that, no one knows.
lazypast said:I find that very interesting and understandable. How long would it take for a black hole to come to the end of its life? I can only assume it's longer than the age of the universe.
Chronos said:Radiation cannot escape a black hole unless emitted outside its event horizon.
justsomeguy said:Not only can nothing escape out from inside the event horizon, nothing can even move in any spatial direction except towards it.
That's another way of looking at it. It's surfaces, all the way down!ImaLooser said:I would think that every "surface" inside the outermost event horizon is also an event horizon
ImaLooser said:, but spacetime is very weird inside of black holes. IF GR applies -- a big if -- then I'm told that time and space reverse roles, something I do not understand at all. There is a professor at UC Boulder who has done considerable work on this. He has a web sites and some videos of his simulations. He thinks that the black hole acts as an extreme accelerator and the energy inside goes toward some sort of infinity.
A search onjustsomeguy said:That's another way of looking at it. It's surfaces, all the way down!
Do you have a name or some links? I've heard the turn of phrase in the sense that "the singularity doesn't occupy a point in space, but a point in your future" once you've crossed the event horizon, but I just took that to mean that no matter what you do, you're going to intersect it.
It's just intuitive (to me anyway) that if you can't escape the event horizon once you're inside, you can't even move towards it -- only away from it, and towards the singularity. Same goes for light. If light emitted from something 1mm inside the event horizon can't get out, then it seems obvious that anything 2mm (or 200km) inside can't move outward either.
I blame the confusion in this regard on all the poor little cartoon illustrations showing a spaceman standing on the surface (of what) holding a flashlight pointed up, with the beam going up and then just curving back down into the surface.
Given that events occurring inside the event horizon are entirely unobservable, is there a point to this line of discussion?enosis_ said:Yes, as your first post clarified. Unless I misunderstood the OP - we are concerned with the internal temperature of the black hole - rather than the area surrounding it?