Dear Cecil: What is the opposite of absolute zero? I can accept the idea that there’s a coldest possible temperature, but I like my limits in pairs. Is there a limit to how hot things can get? If so, what is it and why is it? Mark Stewart, Chicago
There is a limit, sort of, but it’s so inconceivably large that nobody but high energy physicists talks about it (although as I think about it absolute zero doesn’t exactly qualify as breakfast table chatter either). The highest possible temperature, called the Planck temperature, is equal to 10^32 degrees Kelvin. For comparison, the center of the sun bubbles along at 15 million degrees K (15 x 10^6); silicon can be created by fusion at 1 billion K (10^9). In short, the Planck temperature is very toasty indeed.
Some scientists believe that we, or at least our universe, have already experienced the Planck temperature, although it went by so quickly you may have missed it. It occurred at 10^-43 of a second after the Big Bang, the great cataclysm in which the universe was born. (10^-43 of a second, in case you’re not hip to the notation, is an incredibly tiny fraction of time. Time enough to create the universe, but not, as a University of Chicago physicist was once at pains to explain, time enough to get off a disputed last-tenth-of-a-second shot against the Chicago Bulls.)
Absolute zero is easier to understand than the Planck temperature. What we perceive as heat is a function of motion. The colder something gets, the less internal motion or vibration its molecules exhibit. At absolute zero —that is, zero degrees Kelvin or -460 degrees Fahrenheit —molecular motion virtually stops. At that point whatever the molecules are a part of is as cold as it’s going to get.
There’s a lot more latitude in the opposite direction. The faster molecules move, the hotter they get. At 10^10 K electrons approach the speed of light, but they also become more massive, so their temperature can continue to rise. At 10^32 K such staggering densities obtain that greater temperature would cause each particle of matter to become its own black hole, and the usual understanding of space and time would collapse. Ergo, the Planck temperature is as hot as things can get. Or at least it’s the highest temp conceivable in present theory. There’s a chance when a quantum theory of gravity is worked out we may find even higher temperatures are possible. The prospect, frankly, leaves me cold.
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