Is there an upper limit on temperature? Is the 'Planck temperature' the upper limt?
I don't think so. You can keep piling more and more energy into a system and it will raise its temperature further and further.
There is a theoretical lower limit on temperature though. That is absolute zero, when no particles move at all.
I was going to mention phonons here too, but are they strictly more to do with vibration than temperature, considering vibration and temperature are fairly interchangeable concepts?
Temperature is not a measure of energy in general. It is a measure of the average kinetic energy of particles in a given location.
So you're really asking whether there is an upper limit on kinetic energy, and whether this is called the Planck temperature.
The answer is yes, and yes.
Here is the Wikipedia entry, which covers it pretty neatly:
The Planck temperature, named after German physicist Max Planck, is the natural unit of temperature, denoted by Tp. The Planck units, in general, represent limits of quantum mechanics. Talking about anything being "hotter" than the Planck temperature doesn't make a lot of sense; it is the temperature at which black holes are theorized to evaporate and the temperature at which the Universe "started" and cooled down from there, according to current cosmology. It is difficult to imagine anything being hotter than the Big Bang.
Tp = Mp/k = square root of (hc^5/Gk^2) = 1.41679 × 10^32 K
Mp is the Planck mass
c is the speed of light in a vacuum
h is the Reduced Planck constant (or Dirac's constant)
k is the Boltzmann constant
G is the gravitational constant
Thanks for the reply.
What I now wonder is; how near to this temperature do the 4 forces (gravitation, electro-magnetism, weak and strong nuclear) combine to manifest themselves as one force? Is there any actual theoretical temperature at which this occurs? Is it the Planck temperature? Do you think there could be anywhere so hostile (theoretical or not) in the universe that in that place there is only one force?
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