# What is the temperature inside a Black Hole?

1. Is it insanely hot? Cause of so much gravity?

2. Is it close to absolute zero? Cause the matter is so closely packed, there is hardly any space for particles to move?

3. Is it room temperature? Cause Cooper didn't die in Interstellar when he went into a Black Hole?

• 1) We don't know anything about what's in a black hole. 2) Don't think that everything in Interstellar is scientifically correct or even plausible. – HDE 226868 Jan 5 '16 at 0:58
• Interstellar reference was a joke!! – Dumbledore Jan 5 '16 at 1:32
• Inside ... we don't know. But we might be able to assign a temp to the event horizon using "black hole thermodynamics" en.wikipedia.org/wiki/Black_hole_thermodynamics – Eubie Drew Jan 5 '16 at 3:23
• It might depend where inside the black hole. It's possible that after falling inside a supermassive black hole you wouldn't even notice the event horizon and the inside might be similar to empty space. It's also possible that the massive gravity stretches and tears space time so much that there's some exotic things going on. It's also possible that inside the black hole is an infinitely dense singularity. Densely packed particles tend to be hotter not colder, but packed into a singularity, temperature doesn't make sense. I think, "we don't know" is the best answer. Fun question. – userLTK Jan 7 '16 at 5:28
• @userLTK Thank you for the explanation, sir/madam. – Dumbledore Jan 7 '16 at 13:28

The temperature of a black hole as seen from the outside is the temperature of the Hawking radiation, which @jyoti has discussed.

The temperature you would experience as you fell freely through the event horizon, is a different question. I think the answer depends on what else is falling into the black hole at the same time. From its own viewpoint, everything that falls into a black hole reaches the singularity in a fixed maximum time (a few days for the very largest supermassive black holes) so you could only exchange energy with things that fell in around the same time as you did. If there was very little such stuff, then I don't think you'd notice anything out of the ordinary -- you'd be exposed to the cold of space, just as you were before (assuming the black hole is big enough that you haven't been heated up the tides of the black hole distorting you. On the other hand if lots of stuff was falling into the black hole with you then the whole lot would get churned up, and you'd be falling in as part of a mass of very hot plasma. Eventually, at least in a non-spinning black hole, you'd approach the singularity and be ripped apart by tides.

Temperature of a black hole is determined by 'black body radiation temperature' of radiation which comes from it (if something is hot enough to give off bright blue light,it is hotter than something that is merely a dim red hot.) For black holes the mass of Sun, radiation emitted from it is so weak and so cool that temperature is only one-millionth of a degree above absolute zero. some black holes are thought to weigh a billion times as much as the Sun and they would be a billion times colder, far colder than what scientists have achieved on Earth. Even though these things are very cold, they can be surrounded by very hot material. As they pull gas and stars down into their gravity wells, material rubs against itself at a good fraction of speed of light. this heats it up to hundreds of millions of degrees. Radiation from this hot, in-falling material is what high-energy astronomers study.
A black hole behaves as though its horizon has a temperature and that temperature is inversely proportional to the hole's mass: T = (6 x 10^-8)M. Here M is in units of solar mass (2 x 1033 grams). Temperature is in degrees Kelvin. This means that a hole recently formed by gravitational collapse of a star (which has to have a mass larger than about 2 suns) has a temperature less than 3 x 10^-8 centigrade above absolute zero which is very cold. Mass with a finite temperature radiated energy. Anything which radiates energy is also losing mass.As black hole loses mass, emission of energy from black hole increases and its temperature increases and thus rate of mass loss increases.As mass of black hole gets small,one has unstable 'runaway effect'. Black hole gets hotter and hotter which causes M to decrease rapidly.When the hole is reduced to a fraction of the size of an atomic nucleus, it will be trillions of degrees. Hole will burn up and disappear. Lifetime of a black hole is greater than age of the universe.

• Googling for "For black holes the mass of Sun, radiation emitted from it is so weak and so cool that temperature is only one-millionth of a degree above absolute zero" yields a boatload of other hits. Care to name your source for proper attribution? – Christian.K Jan 5 '16 at 11:28
• I agree with Christian. Could you please cite your sources? Thanks. – Dumbledore Jan 6 '16 at 13:46
• The question was: What is the temperature inside a Black Hole? – John Duffield Apr 9 '18 at 11:00
• The nucleus of an atom is not a black hole, which suggests that a black hole that evaporates to the mass of a nucleus of an atom would cease to be a black hole. – Acccumulation May 30 '18 at 21:23

The temperature of radiation emitted by a static black hole (without an accretion disc) is from pair production. The black hole loses mass when a virtual (negative-energy) particle-antiparticle pair forms exactly on the event horizon. One of the virtual particles falls into the black hole, separating the pair. The other particle becomes real and is emitted as positive mass-energy, and the particle that falls into the singularity remains virtual as negative mass-energy. The net result is that mass-energy conservation is preserved, and the black hole loses mass-energy by emitting a real particle. Since the escape velocity below the event horizon is beyond the speed of light, any particles created inside the black hole (real or otherwise) would always immediately fall into the singularity. So if you were able to safely cross the event horizon (which is speculative—some recent studies have hypothesized that there many be "wall of fire" at the event horizon), I would think that you should only be able to register absolute zero, with the exception of any incoming radiation raining down from above you. The virtual negative-mass component of Hawking radiation should presumably be undetectable, and even if it was, it would register as being below absolute zero, in violation of thermodynamics (and energy conservation). Note though that the "temperature" of the singularity itself can be thought of as being measured in Planck temperatures (10^32 kelvins), since spacetime breaks down at that point (the Planck scale of M-theory, where one Planck temperature is the equivalent of one Planck energy, or the Planck mass).

What is the temperature inside a Black Hole?

Absolute zero. That's because of the infinite gravitational time dilation. Temperature is a measure of motion of sorts. Gravitational time dilation means things are moving slower. When the gravitational time dilation goes infinite there is no motion, and so no temperature. This is why the black hole was originally known as the frozen star. See the 1939 paper by Robert Oppenheimer and Hartland Snyder on continued gravitational contraction. People often say this is outdated, see for example Brown’s mathpages article the formation and growth of black holes, but I don't think it is. I say that because I've read the Einstein digital papers, and I like to think I know how gravity works.

Regardless of that, in 1972 Stephen Hawking, Brandon Carter, and Jim Bardeen wrote a paper and said "the effective temperature of a black hole is absolute zero”. Robert Wald said much the same in black hole physics: “κ has nothing whatever to do with the physical temperature of a black hole, which is absolute zero by any reasonable criteria”. Note that Hawking radiation is nothing to do with the temperature inside a black hole.

Is it insanely hot? Cause of so much gravity?

No. Or should I say I don't think so. I don't actually know for certain.

Is it close to absolute zero? Cause the matter is so closely packed, there is hardly any space for particles to move?

Yes AFAIK it's close to absolute zero. Or it's actually absolute zero. But not because matter is so closely packed. In fact there might not even be any particles inside a black hole.

Is it room temperature? Cause Cooper didn't die in Interstellar when he went into a Black Hole?

No, it isn't room temperature. Interstellar is just science fiction I'm afraid, even though it was portrayed as science fact.

• Time dilation is relative. I'll admit that the time dilation inside a black hole makes my head hurt when thinking about it, but time feels normal from any individual's perspective, so time dilation won't change local temperature. There's always going to be motion at the local level, whether that motion is at 100% time dilation relative to outside doesn't make the temperature absolute zero from the local perspective. – userLTK May 29 '18 at 16:44
• @userLTK : If there was motion at the local level your gedanken observer could point his torch upwards and turn it on. But the black hole is black, so there's clearly an issue with that. IMHO the best way to think about it is that there isn't any local perspective when time dilation goes infinite. – John Duffield May 29 '18 at 17:55
• Sure, from the outside, but this question is about theoretical temperature on the inside. But that's not really the point. Are we discussing temperature and shouldn't temperature be measured locally? Locally the "infinite time dilation" is irrelevant. – userLTK May 30 '18 at 7:22