Temperature Scale

Physicists let a cloud of rubidium atoms fall down a drop tower more than a hundred meters tall and, while it fell, slowed its spreading until it matched 38 trillionths of a kelvin. Absolute zero is not at the bottom of this page. Each power of ten colder costs about as much effort as the last, so on this scale it lies infinitely far down.

At 170 billionths of a kelvin, in 1995, a few thousand rubidium atoms stopped behaving as separate particles and merged into a single quantum wave. It was the first Bose-Einstein condensate, seventy years after Einstein predicted it.

The chips inside today’s quantum computers run at about a hundredth of a kelvin, more than two hundred times colder than deep space, at the bottom of a refrigerator that hangs like a golden chandelier. Any warmer and the jiggle that warmth brings scrambles the fragile quantum states they compute with.

The coldest natural place known is the Boomerang Nebula, five thousand light-years away, at about one kelvin. Gas blown off a dying star there has expanded so fast that it has cooled below the afterglow of the Big Bang: it is colder than empty space.

Empty space is not at absolute zero. It is filled with the afterglow of the Big Bang, and anything left alone long enough in the dark between the galaxies settles to 2.7 kelvin. The magnets of the Large Hadron Collider are kept colder still, at 1.9 kelvin.

Helium turns liquid at 4.2 kelvin. In 1911, at that temperature, Heike Kamerlingh Onnes watched the electrical resistance of mercury vanish completely: the discovery of superconductivity. The magnet of every hospital body scanner still sits in a bath of liquid helium.

At the bottom of craters near the Moon’s poles, where sunlight has not reached for billions of years, a spacecraft in orbit measured about 26 kelvin, colder than the surface of Pluto. Frozen water may have lain there, untouched, for most of the Moon’s history.

Nitrogen, almost four fifths of the air you breathe, turns liquid at 77 kelvin and is cheap enough to sell by the liter. In 1987 a ceramic was found that superconducts at 93 kelvin, and superconductivity no longer needed rare and costly helium.

The Earth’s surface averages about 288 kelvin, fifteen degrees Celsius. Life has been found at work from about minus twenty degrees Celsius up to 122. On this page that whole range is a sliver: a fifth of a single power of ten.

You run at 310 kelvin. At rest your body gives off about as much heat as a hundred-watt bulb, and it glows: not in light you can see but in the infrared, which is how a thermal camera finds you in the dark.

Molten basalt pours out at around 1,400 kelvin. Anything this hot glows in colors you can see, and the color gives its temperature away: dull red is cooler, yellow is hotter. Explaining that glow is what led Max Planck to the quantum, in 1900.

Sunlight is the glow of a surface at 5,772 kelvin. Counted by wavelength, its brightest color is a blue-green, but all its colors mixed together are what our eyes evolved to call white.

For a few millionths of a second a lightning channel heats the air to about 30,000 kelvin, five times hotter than the surface of the Sun. The air expands so fast that it sets off a shock wave, and you hear it as thunder.

Above the Sun’s surface lies its corona, at one to three million kelvin, hundreds of times hotter than the surface beneath it. More than eighty years after that was discovered, what heats it is still not settled, and a spacecraft has now flown through it to find out.

The core of the Sun is at about fifteen million kelvin. By the rules of classical physics that is far too cold for two protons to get close enough to fuse. They manage it because quantum mechanics lets them tunnel through the barrier between them.

A fusion reactor on Earth has to run near 150 million kelvin, ten times hotter than the Sun’s core, because it cannot squeeze its fuel the way the Sun’s own weight does. In 2024 a Korean reactor held 100 million kelvin for 48 seconds.

When the core of a massive star collapses, it reaches about a hundred billion kelvin in under a second. Almost all of that energy leaves as neutrinos: in 1987 detectors in Japan, the United States and Russia caught two dozen of them, from a star in a neighboring galaxy.

Lead nuclei smashed together at the Large Hadron Collider made a drop of quark-gluon plasma at about 5.5 trillion kelvin, hot enough to melt protons and neutrons back into quarks and gluons. The whole universe was like this for its first twenty millionths of a second or so.

About ten trillionths of a second after the Big Bang, the universe cooled through about 2 × 10¹⁵ kelvin, and the electromagnetic and weak forces went their separate ways. They parted smoothly, over a range of temperatures, as the Higgs field took on its value. That value gives the W and Z, the top and bottom quarks and the tau their masses, as measured; most of a proton’s mass comes from elsewhere.

At 1.4 × 10³² kelvin a single particle carries so much energy that its own gravity matters as much as its quantum nature, and every theory we have breaks down. Whether anything can be hotter, and what that would mean, is not settled. It is the same moment the Time Scale starts from.