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According to IFLScience, above the Planck Temperature (absolute hot) conventional physics break down.

My question is what happens as you approach this temperature, and, if it is possible, what happens when you cross it?

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  • $\begingroup$ When the article said conventional physics breaks down at that temperature, what it meant was that we don't have a theory that is expected to work at that temperature. We would all like to know what happens. $\endgroup$ – mmesser314 Jun 3 '15 at 13:13
  • $\begingroup$ Ahh right, that makes sense, I was wondering if it was known to be like the speed of light, ie, it would take an infinite amount of energy to hit it so it can't be done, or something along those lines. Hopefully we'll find out what happens sooner or later, sounds like it could be interesting $\endgroup$ – Persistence Jun 3 '15 at 13:16
  • $\begingroup$ When we approach the Planck temp (1 $T_p$), quantum gravitational effect become more significant. When we cross it, we know nothing up to now since there is no complete theory of quantum gravity up to now. $\endgroup$ – Lê Dũng Jun 3 '15 at 13:52
  • $\begingroup$ Related: physics.stackexchange.com/q/1775/2451 , physics.stackexchange.com/q/46397/2451 and links therein. $\endgroup$ – Qmechanic Jun 3 '15 at 14:26
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I expect it's impossible to cross the Planck temperature, just like it's impossible to cross absolute zero or the speed of light.

At the Planck temperature, you start producing miniature Planck-mass black holes, which are the hottest black holes that can exist. If you try to put more energy in the system, you would get larger black holes, which are cooler, and they would start absorbing stuff and cooling things down.

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  • $\begingroup$ This answer seems rather speculative. Wouldn't it be more accurate to say that current theories aren't considered reliable at these temperatures and energies ? We'd need a quantum theory of gravity to come close to saying what you do and we really don't have that in Jan 2017. $\endgroup$ – StephenG Jan 21 '17 at 20:32
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    $\begingroup$ @StephenG: My answer is not completely speculative. (1) If the formula for Hawking radiation extends to Planck-scale black holes, then Planck-mass black holes are indeed the hottest black holes that can exist. (2) If you add energy to a system with black holes, and if this makes the black holes grow, and if it stays in thermal equilibrium, then you do indeed cool the system. $\endgroup$ – Peter Shor Jan 21 '17 at 22:06
  • $\begingroup$ A system's temperature cannot really be defined if the system is not in thermal equilibrium. Thus, if the formula for Hawking radiation extends to near-Planck-scale black holes, I don't see how you can get any temperature larger than the Planck temperature. $\endgroup$ – Peter Shor Jan 21 '17 at 22:09
  • $\begingroup$ Isn't one man's "not entirely speculative", another man's "speculative" ? :-) $\endgroup$ – StephenG Jan 21 '17 at 22:37
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    $\begingroup$ @StephenG: That's why I said "I expect" in my answer. $\endgroup$ – Peter Shor Jan 23 '17 at 2:58
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As you cross the planck temperature the particles in matter travel at the speed of light according to the current model for kenetic temperature, thus particles could smash together and pass their schwartschild radius and create a black hole. We would need a better understanding of quantum gravity to understand planck temperature. Since only mass less particles cab travel at the speed of light only light can be at the planck temperature.

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  • $\begingroup$ The Planck temperature doesn't occur when all particles travel at the speed of light, at least in thermodynamics as it's understood today. As the particles' speed approaches light speed, the temperature approaches infinity. $\endgroup$ – Peter Shor May 8 '19 at 10:36

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