You may have heard that temperature  has an absolute minimum.

A point where you can go no colder. And it’s true!

Temperatures colder than  negative 273.15 degrees Celsius, which is the same as zero Kelvin, are impossible. That’s why it’s called absolute zero. Exceeeeept…it turns out that there  is a way to get below absolute zero,   into so-called negative absolute temperatures.

And it may be more common than you think. Because the trick isn’t to get colder, but hotter. [♪ INTRO] For everyday things, from your morning  mug of tea, to the air you blow over it, temperature tells you how kinetic energy is  distributed among all the particles involved. The higher the temperature, the higher the  average of that distribution of energies is.

As the temperature drops, there’s  less heat energy to go around, so more of the particles will have  less energy, bringing the average down. So absolute zero is when there’s  zero energy in the system, and the particles have practically zero motion. But there’s a famous law of physics called  the third law of thermodynamics that says that you can never actually reach absolute zero,  since that would require using infinite energy.

You can get as close as you like to  absolute zero, but never quite reach it. And these days, scientists can get down to a few billionths of a degree above absolute zero. But if you can’t even reach absolute zero, how  do you make negative absolute temperatures?

It requires a complete overhaul of  how you think about temperature. Because temperature doesn’t have  to just be about kinetic energy. Every kind of energy in a system combines  to make one grand temperature value.

For normal things like coffee mugs, it’s  really only kinetic energy that matters. But in systems where you can create  negative absolute temperatures, these other sources matter a lot more. For example, particles have a  strange, quantum property called spin, which is sort of like a momentum  they have from spinning around.

If a system of these particles  is put in a magnetic field, the spins react with the field to provide  the system with another source of energy. The exact amount isn’t about how  fast anything is spinning, though. It depends on how aligned  the spins are with the field.

This defines a sort of  ladder of possible energies. And at a given temperature, a certain number  of spinning particles are on each rung. If your stuff’s at a higher temperature, you’ve  got more of your particles on the higher rungs.

In general, the particles want to fall  down the energy ladder to the bottom rung. It’s a more stable position, just  like when standing on a real ladder. But absorbing heat energy  pushes them into a higher rung.

So far, this is nearly identical to  the kinetic energy version of things. But there’s a crucial difference: for spin energies, there’s a  highest possible rung on the ladder. Particles can’t store infinite  energy in their spins.

A spin can only be so aligned  with a magnetic field. So unlike with the kinetic energy  of your coffee mug’s molecules, you can’t keep adding heat energy  to push spins up the ladder forever. And this upper limit drastically changes  how our weird quantum system stores heat.

Now, the forces that are trying to  drag the particles down the ladder to lower rungs are almost irresistible. Even if you could add infinite heat to the  system, once you leave it alone, it’ll settle into a state where the particles are equally  distributed among all the energy levels. This is the infinite temperature limit, and it’s key to understanding  negative absolute temperatures.

But since we have to reach negative  absolute temperatures by heating things hotter than infinity, and that sounds impossible, I need to explain something  even weirder about temperature. Thanks to JMP for supporting this SciShow video! JMP is a statistical analysis software that  makes powerful analytics quick and accessible.

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You can find it at jmp.com/scishow. If you look at the actual equations  physicists use for all this stuff, you’ll notice that temperature  is in the denominator. That means instead of things  depending on temperature, they kinda depend on the inverse of temperature.

One divided by T. But what the heck is the inverse of temperature? For the sake of this episode,  we’ll call it coldness.

Because if you try to plug  in a temperature of zero, one over T slowly becomes infinity. Absolute zero is infinite coldness. Which sounds right to me.

If we pretend it’s physically possible for a  clump of particles to reach exactly zero Kelvin, then the energy distribution of those  particles would look something like this. All of them are on the bottom rung. Now, let’s start increasing the temperature.

As things get “hotter”, the  amount of coldness decreases. The particles on our energy  ladder are starting to spread out, but the vast majority are still  sitting somewhere on the lower rungs. And once we get the system all the  way up to infinite temperature, we’re dividing by such a large number  that one over T is effectively zero.

In other words, infinite  temperature is zero coldness. Which also sounds right to me. And as we’ve already established,  the distribution looks like this.

All particles are equally distributed  across the possible energy levels. So by thinking in terms of coldness,  we’ve gone from infinity down to zero. But if you’ve ever seen a full number  line before, you know we can go further.

For systems like the collection of  spins from before, scientists have some clever experimental tricks that let them  go from some positive amount of coldness past zero coldness, and into negative coldness. If we go back to our visual aid, we can  see that there are now more particles on the higher-energy rungs of the  ladder than in the lower-energy ones. This is the key sign that you have  negative absolute temperatures.

And both the coldness and the  regular temperature are negative now. One divided by a negative  number is a negative number. And if we kept heating things to the  end of the scale, we’d reach negative infinite coldness, where all of our particles  occupy the top rung of the energy ladder.

Because we’re plugging in another  infinity in our denominator, but negative this time, this corresponds  to negative zero temperature. Which for the record, isn’t  the same as positive zero. And I know this may all sound a bit kooky, but  this ‘coldness’ idea isn’t just some metaphor.

And systems with negative temperatures  can exist in the real world! In fact, you probably have one near you right now! This is how lasers work!

You pump energy into a solid to raise  its particles into higher energy levels, getting a distribution that indicates  a negative absolute temperature. Then, as some of those particles naturally  drop in energy, they release light. But with all that dropping, it means  you have to keep pumping energy into maintaining that negative temperature state.

It’s not stable, which is why  you’ll hear some scientists argue that this doesn’t really count  as a true negative temperature. But there’s other technology  out there that scientists can   use to make stable negative temperatures. For example, researchers created a  negative one Kelvin system all the way back in 1951…just two years after negative  temperatures were first predicted.

They did this by quickly flipping  the magnetic field the spins were in, so their energy distribution suddenly turned  upside-down, making negative temperatures. And research into negative-temperature  systems is still ongoing. In 2024, one group claimed  that the material they used had transitioned into a brand new phase of matter.

However, that claim is controversial, and experts disagree on how to  interpret what the team saw. But really, the general concept of negative  absolute temperatures is controversial, with some physicists  proposing that they only exist under a misinterpretation of  the laws of thermodynamics. So there’s still lots to study and  understand about these weird systems.

Whatever’s going on, two things are clear:  that physicists think negative temperatures are hotter than hot right now, and  that studying them is cooler than cool. [♪ OUTRO]