I was reading about the quest for absolute zero temperature in New Scientist (18 March, p 10). Is there an equivalent maximum temperature? If so, what is it? And how could one reach it?
• There is indeed a theoretical maximum temperature – the Planck temperature, or TP. To understand why, we have to understand the relationship between temperature, energy and wavelength. Simply put, higher temperatures correspond to higher energies, and higher energies correspond to shorter wavelengths. (We’re talking electromagnetic radiation here, because no material object, not even any elementary particle of matter, can exist at anywhere close to the Planck temperature. Photons are all you get.)
So, the maximum possible temperature is the temperature beyond which you can’t pack any more energy into a photon. And this is limited by how short a wavelength it is possible for a photon to have. This is limited, in turn, by the smallest possible distance that can be defined in the universe, referred to as the Planck length, which is 10-20 of the diameter of a proton. The reason why nothing can be smaller than the Planck length is because at that point, current physical theory breaks down because it is impossible to describe anything beyond TP without a theoretical understanding of quantum gravity, which we don’t have.
This temperature turns out to be about 1.42×1032 kelvin.
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How could one reach the Planck temperature? It is estimated that the universe was at TP about 10-42 seconds after the big bang. At this time, the entire universe was roughly one-billionth of the diameter of a proton. So, compress the entire universe into a space a billion times smaller than a proton and you’ll be more or less there. I advise eye protection and sunblock. Really, really strong sunblock.
Phil Stracchino, Gilford, New Hampshire, US
• Temperature is a measure of the kinetic energy from the random motion of atoms in a material. The fastest speed atoms can move is the speed of light, so this sets an upper limit on the maximum temperature.
However, it is impossible for matter to be accelerated to light speed, so this limit can never be reached. The practical upper limit is the maximum temperature possible before the container a substance is held in melts/sublimes or is blown apart. For solid vessels, the highest melting point recorded is for tantalum hafnium carbide, which has a potential melting point of 4263 kelvin.
But if we do away with solid vessels and use magnetic fields (such as in fusion reactors) instead, then higher temperatures can be reached. Fusion doesn’t begin until 15 million °C and many of today’s reactors top 100 million °C.
The gravitational fields of the largest stars can constrain materials in their cores estimated to be as hot as 200 million °C and that is probably the highest stable temperature currently in the universe.
During the collapse of the largest stars into supernovae and black holes, the inside temperatures could reach as high as 100 billion °C. However, it’s hard to get inside to check.
Simon Iveson, Chemical Engineering Discipline, The University of Newcastle, New South Wales, Australia
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