From what I've gathered, Planck length is the smallest measurable length, though we do not know whether it is the smallest length physically possible. The Planck temperature is called the theoretically highest temperature, meaning this theory assumes that the Planck length is the smallest possible length. Given that it is (presumably) a theory, that makes me wonder what reasons we have to think the Planck length is the smallest length. I get speculating that it is the smallest length, given that it is the smallest measurable length; but calling it a theory means there's actually some reasons, or even evidence. So, what are those reasons?
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4There is no reason to think that the Planck 'anything' is the biggest/smallest possible. It is just a number obtained from combining constants. – Jon Custer Sep 02 '22 at 17:12
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So, why do some professionals think so? Or are there only amateurs that think so? – user110391 Sep 02 '22 at 17:14
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1See https://physics.stackexchange.com/questions/28720/how-to-get-planck-length, particularly the bit in an answer: "Many other quantities that have units of length may be much shorter than the Planck length." – Jon Custer Sep 02 '22 at 17:31
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Or are there only amateurs who think so?
Bingo :) The Planck units don’t give sharp limits on the existence of quantities like length, time, etc (or rather, there’s no reason to think that they do). Instead, they provide a scale at which a more complete theory of fundamental physics (which incorporates quantum gravity) will be relevant.
For example, when the mass of a black hole is equal to the Planck mass, then its Compton wavelength will be on the order of its Schwarzschild radius (which is also the order of the Planck length). The weird bits of quantum mechanics and gravity will intersect, and we just don’t know what happens in that scenario.
J. Murray
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I'ts only a reference that came from the universal constants, as far as I know there is no theoretical limit
