# Can Something Be So Hot That Its Temperature Flips Upside-Down?

Winter is almost here in the northern hemisphere, and depending on where you live, that could mean dealing with a lot of negative temperatures in the near future. But when physicists talk about “negative temperature,” they don’t just mean that it’s 10 degrees below zero. They’re talking about a kind of negative you can only reach by turning the heat up.

## That’s a Spicy Heatball

Maybe you’ve heard that the hotter a substance gets, the more its particles jostle around. Heat up water to steam and the H2O molecules are all abuzz with energy. Lower the temperature enough to freeze and they’ll slow to a standstill. But it’s not quite that simple. Heat isn’t just the average energy of the particles in a system; it’s also how that energy is distributed among those particles. We’re about to get a little conceptual, but please bear with us — it gets really cool at the end.

Imagine that each of the particles in a substance is actually a brick, and you’re building a pyramid out of those bricks based on how much energy they have. The lowest level of the pyramid is made up of the bricks with the lowest level of energy, the particle at the tippy-top is the one with the highest level of energy. If the system is pretty cold, then the pyramid will be wide and short; if the system is very hot, then the pyramid grows taller and skinnier.

In this model, at some point, you get to what physicists call the “absolute high”: a column of bricks one brick in width reaching to infinity. But things get really weird if you pump the temperature up a little bit more. Suddenly, the pyramid reemerges, but upside down. Now the lowest level of the pyramid is its point — and the temperature that describes its shape becomes negative.

## Negative Temperature in the Real World

Now picture a real pyramid, one made of stone. Flip it upside-down. Not very stable, is it? The same is true for the pyramid of particles. The high-energy particles will inevitably “fall” to the lower levels as their energy levels fall. Doing so causes those particles to emit particles of light, and the whole process is called “population inversion.” You’ve almost certainly seen that phenomenon in action. Every time you click the button on a laser pointer, the device pumps atoms up from a lower level to a higher level. As those high-energy particles fall back to a low-energy state, they produce photons and you get that steady beam of laser light.

Generally, those inverted populations will eventually reach an equilibrium, like water being poured into two containers connected by a tube. But one new paper found that sometimes, that wasn’t necessarily true. Usually, if you’ve got two containers — in this case, quantum systems — where one is at its absolute high and the other isn’t, then both will even out to about the same. But that’s only if they’re the same size to begin with. If the high-energy system is larger than the low-energy system, the energy of the smaller system will actually rocket upward, creating that bizarre inversion. Science isn’t always intuitive, but that’s where things start getting cool. Negative cool.

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