# Negative Temperatures That Are Hotter Than The Sun

Jan 4, 2013
Originally published on January 7, 2013 12:23 pm
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IRA FLATOW, HOST:

This is SCIENCE FRIDAY. I'm Ira Flatow. Think it's cold outside right now? Imagine being in outer space, where the temperature is a steady 455 degrees below zero Fahrenheit. That's about three degrees Kelvin, the heat leftover from the Big Bang. Go just a little lower, and you approach zero Kelvin, absolute zero.

And I know you've been taught in school, like I was taught, that you can't get any lower than absolute zero, right? Well, maybe you can because this week, scientists announced that they've been able to take potassium gas and bring it to negative temperatures, that is, that's right, below zero Kelvin. They published the results in the journal Science.

So how do you get below absolute zero? Could this have any practical application? I want to warn you that this concept of negative temperature may have you scratching your head. It made my hair hurt the first time I heard about it and thought about it.

Well, joining me now to walk us through this tricky physics is Vladan Vuletic. He is a quantum physicist at the Massachusetts Institute of Technology in Cambridge, and he joins us from there. Welcome to SCIENCE FRIDAY, Vladan.

VLADAN VULETIC: Welcome, and thank you for having me.

FLATOW: Is it possible? Is this gas colder than absolute zero?

VULETIC: It has a negative temperature, but actually negative temperatures are in some sense hotter than positive temperatures.

FLATOW: Let me get that straight.

(LAUGHTER)

FLATOW: Let me get that straight. This is the hair-hurting part. Negative temperatures, you go below zero, and you start getting warmer again?

VULETIC: Maybe the easiest way to think about it is as follows: In measuring a gas at room temperature, that means atoms, molecules are racing around, some at slow velocities, some at faster velocities. But there's more atoms at slow velocities than at fast velocities. As you heat the gas up, many more atoms go to fast velocities. At very, very high temperatures, like on the sun, there are more atoms - there are almost equal numbers of atoms at different speeds.

Now if you give more energy to the gas into a system where there are more atoms that are moving at high velocities than at low velocities, this corresponds to a negative temperature, something that would not easily naturally occur.

FLATOW: So sort of negative - the word negative is a misnomer here.

VULETIC: Well, it's a historical accident. When Lord Kelvin first introduced the concept of absolute temperature, he introduced absolute zero as the point where atoms don't, particles don't move. With the more modern definition, we would probably prefer this to be negative infinity.

FLATOW: Negative infinity?

VULETIC: Well, you can think of the temperature as a loop. So if you start at zero degrees, then most atoms are standing still. As we add energy, the energy, the temperature becomes positive and becomes plus infinity, and when we add more energy so that there are more fast atoms than slow atoms, then we start - we come around to negative infinity, and then we approach zero from below.

Small negative temperatures correspond to a situation where most of the atoms are in a high-energy state, and there are very few atoms in a low-energy state.

FLATOW: As opposed to the normal state of affairs, where most of them are just the opposite.

VULETIC: Exactly, and in positive temperatures, most of the atoms are in low-energy states.

FLATOW: Can you pump up the atoms and get them into a higher state to get more negative temperature?

VULETIC: It turns out that it's possible under certain circumstances. Maybe the best known example is actually the laser. For the laser to emit the light as it does, it needs to have more atoms in an up energy state than a low-energy state. So in some sense you can think of the laser as a negative temperature system.

FLATOW: So actually lasing happens when the atoms drop down from a higher energy state to a lower and then release the light.

VULETIC: Exactly. The light ignition happens then. For that you need to have more atoms in the higher energy state.

FLATOW: Is there any practical application to making a gas that's so hot/cold like that?

VULETIC: We don't know. This is mostly fundamental research. There may be unknown properties. There are, as I said, very few examples of negative temperature systems. The laser is one. One possible practical application is so-called heat engines. They are systems that convert heat to, say, electrical energy, for instance, you know, some - a gas, (unintelligible), or something else. And there are limits on the efficiency of such machines that were established by the French engineer Carnot in the 18th century.

It turns out that negative temperatures can actually beat those limits.

FLATOW: Wow. Could these negative temperatures exist somewhere in outer space? Or is it just a lab creation?

VULETIC: We believe that they exist in some systems in outer space. So astronomers have observed strange emission lines from atoms that they couldn't understand except with the assumption that there are more atoms sitting in some high-energy states than in some low-energy states, so again negative temperatures.

So there are some stellar gases that emit almost laser light, and we believe these are - they are at negative temperatures.

FLATOW: If we were to put our finger in some gas that had these high negative temperatures, would we be able to feel it? Would we know it's hot?

VULETIC: We would actually heat up, yes. So if you brought some object in contact with the negative temperature system, the object would actually heat up, not cool down. That's because the negative temperature system has actually more energy stored in it.

FLATOW: Now let's just think outside the box for a minute because I know you just can't do basic research and not have some idea where it might possibly lead someday. Where might this be useful? You mentioned lasers normally do this now, making a gas that is hot when it's in that negative energy state. What kind of other use could you put it to?

VULETIC: Well, people are trying to understand some fundamental quantum effects that have practical applications like superconductivity. This is an effect where basically wires can conduct resistant - can conduct without resistance, without energy loss. These things usually happen at low temperatures, and we don't understand them fully and it's believed that these systems can be used to model and better understand for instance these superconductors.

FLATOW: Dr. Vuletic, thank you for taking time to be with us today.

VULETIC: Thank you for having me.

FLATOW: Good luck to you. Vladan Vuletic is a quantum physicist at MIT in Cambridge. Transcript provided by NPR, Copyright NPR.