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.
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. Another odd consequence of negative temperatures has to do with entropy , which is a measure of how disorderly a system is. When objects with positive temperature release energy, they increase the entropy of things around them, making them behave more chaotically.
However, when objects with negative temperatures release energy, they can actually absorb entropy. Last week, scientists reported that molecules in an ultra-cold gas can chemically react at distances up to times greater than they can at room temperature. In experiments closer to room temperature, chemical reactions tend to slow down as the temperature decreases. Practically, the work needed to remove heat from a gas increases the colder you get, and an infinite amount of work would be needed to cool something to absolute zero.
If you know your atoms are inside your experiment, there must be some uncertainty in their momentum keeping them above absolute zero — unless your experiment is the size of the whole universe.
The lowest temperature ever measured in the solar system was on the Moon. The coldest known place in the universe is the Boomerang Nebula , 5, light years away from us in the constellation Centaurus.
Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. In doing so, the excess heat from the gaseous state dissipated and the system achieved a temperature merely six kelvins above absolute zero—the closest attempt of its time.
Get smart. Sign up for our email newsletter. Sign Up. Read More Previous. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Create Account See Subscription Options. Continue reading with a Scientific American subscription. Not quite. And, of course, the activity within each atom continues no matter how cold it gets. Electrons keep moving, as do protons and neutrons. Guillaume Amontons , a French inventor who lost his hearing in childhood and never went to college, figured out the basic concept in His experiments showed that air pressure is proportional to temperature, and he deduced that there was a minimum temperature at which pressure would drop to nothing.
He even made an estimate of that temperature, minus degrees C — remarkably close to the actual value. He set absolute zero as 0 on his scale, getting rid of the unwieldy negative numbers. Physicists now rely on the Kelvin K scale for temperature measurements. The energy left over from the Big Bang warms the whole universe, keeping it well above absolute zero.
The average temperature of space is 2. Surprisingly, some celestial objects are colder than empty space.
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