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According to IFLScience, above the Planck Temperature (absolute hot) conventional physics breaks 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, 2015 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 $\endgroup$ Jun 3, 2015 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, 2015 at 13:52
  • $\begingroup$ Related: physics.stackexchange.com/q/1775/2451 , physics.stackexchange.com/q/46397/2451 and links therein. $\endgroup$
    – Qmechanic
    Jun 3, 2015 at 14:26

2 Answers 2

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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. $\endgroup$ Jan 21, 2017 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. $\endgroup$ Jan 21, 2017 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. $\endgroup$ Jan 21, 2017 at 22:09
  • $\begingroup$ Isn't one man's "not entirely speculative", another man's "speculative" ? :-) $\endgroup$ Jan 21, 2017 at 22:37
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    $\begingroup$ @StephenG: That's why I said "I expect" in my answer. $\endgroup$ Jan 23, 2017 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 kinetic temperature, thus particles could smash together and pass their Schwarzschild radius and create a black hole. We would need a better understanding of quantum gravity to understand Planck temperature. Since only mass-less particles can 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. $\endgroup$ May 8, 2019 at 10:36

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