To comprehend the negative temperatures scientists have now devised, one might think of temperature as existing on a scale that is actually a loop, not linear. Positive temperatures make up one part of the loop, while negative temperatures make up the other part. When temperatures go either below zero or above infinity on the positive region of this scale, they end up in negative territory.
With positive temperatures, atoms more likely occupy low-energy states than high-energy states, a pattern known as Boltzmann distribution in physics. When an object is heated, its atoms can reach higher energy levels.
At absolute zero, atoms would occupy the lowest energy state. At an infinite temperature, atoms would occupy all energy states. Negative temperatures then are the opposite of positive temperatures — atoms more likely occupy high-energy states than low-energy states.
“The inverted Boltzmann distribution is the hallmark of negative absolute temperature, and this is what we have achieved,” said researcher Ulrich Schneider, a physicist at the University of Munich in Germany. “Yet the gas is not colder than zero kelvin, but hotter. It is even hotter than at any positive temperature — the temperature scale simply does not end at infinity, but jumps to negative values instead.”
As one might expect, objects with negative temperatures behave in very odd ways. For instance, energy typically flows from objects with a higher positive temperature to ones with a lower positive temperature — that is, hotter objects heat up cooler objects, and colder objects cool down hotter ones, until they reach a common temperature. However, energy will always flow from objects with negative temperature to ones with positive temperatures. In this sense, objects with negative temperatures are always hotter than ones with positive temperatures.
From Good Omens:
“Look. I don’t care if you believe me or not,” he sighed, “all I’m telling you is what I saw. It was an old car, a Rolls, or a Bentley, one of those flash vintage jobs, and it made it over the bridge.”
One of the senior army technicians interrupted. “It can’t have done. According to our instruments the temperature above the M25 is somewhere in excess of seven hundred degrees centigrade.”
“Or a hundred and forty degrees below,” added his assistant.
“… or a hundred and forty degrees below zero,” agreed the senior technician. “There does appear to be some confusion on that score, although I think we can safely attribute it to mechanical error of some kind.”
(This was true. There wasn’t a thermometer on earth that could have been persuaded to register both 700°C and -140°C at the same time, which was the correct temperature.)
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