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Why can't we just keep heating matter past Planck's temperature? Planck's temperate or 10 to the power of 32 Kelvin is the hottest temperature possible. If you're shooting an atom with heat radiation and it reaches Planck's temperate why can't you keep heating it.
I do have a theory though - Lets say you're shooting an electron with a laser and heating it up. Every time an electron goes into a higher quantum state it requires more energy to bring it into the next one. At first the difference of energy required is not noticeable but eventually it requires more and more energy to further excite the electrons of the atom(s) being heated. After a long time some of the electrons are at such high energy levels that they require infinite energy to go into a higher quantum state.