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# Why can't we get down to absolute zero?

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What is Absolute Zero, and does it really exist anywhere in the universe? Could we ever reach Absolute Zero in real life?

There are all sorts of reasons to be curious about the limits of cold. Maybe you're an incredibly lame supervillain who uses the power of freezing, and you want to understand the extent of your powers. Or you're wondering if it would be possible to outrun a wave of cold. Either way, in this week's "Ask a Physicist" we'll explore the farthest limits of cold.

Top image: Herkie on Flickr.

Every now and again, I get a quickie question like one from a reader named Luomana who asks, simply:

Does all motion really stop when we hit absolute zero? Can it ever actually get that cold?

Since a one line answer ("Sort of." and "No,") probably wouldn't be very satisfying for the rest of you, I put a call out on twitter and facebook asking for your ultimate questions in coldness.

Today, for your edificiation, we're going to do an absolute zero roundup. Let's begin with the obvious, just so we're all on the same page.

What is absolute zero?

Even if you're not a physicist, you presumably wandered over to io9 today with at least a passing familiarity with the concept of temperature. But in case you didn't — and you simply cannot understand why it is that you keep losing limbs to frostbite — here's a quick tutorial.

Temperature is a measure of the amount of internal, randomized energy in a material. This "internal" part is pretty important. Throw a snowball, and even though the bulk motion is pretty fast, the snowball itself is still quite cold. On the other hand, if you look at the air molecules flying around the room right now, a typical oxygen molecule is hauling ass at about a thousand miles per hour.

I usually get called out when I gloss over details, so for the experts, I need to point out technically, temperature is a little more complicated than that. The true definition of temperature really tells you how much energy you can put in for every unit of entropy (disorder, for want of a better word) that you pump in. But ignoring that (and for our purposes it doesn't make much of a difference), the idea is that the random motions of air molecules or vibrations of water molecules within ice will get slower and slower and slower as you lower the temperature.

Absolute zero, -273.15 Celsius, or -459.67 Fahrenheit, or simply 0 Kelvin, is ostensibly the point at which these thermal motions stop entirely.

Does everything really stop?

Classically, everything stops at absolute zero, but in reality, quantum mechanics once again rears its ugly head. One of the predictions of quantum mechanics is that you can't ever measure the exact position or momentum of a particle with perfect certainty. This is known as the Heisenberg Uncertainty Principle.

If you could chill a sealed room down to absolute zero, some very strange things would happen (though more on that in a bit). The pressure in the air would drop to essentially zero, and since it's air pressure that normally resists gravity, the air would collapse to a very thin layer on the floor.

But even so, if you were able to measure the individual molecules, you'd find something surprising: they're still vibrating and rotating, but just by a tiny, tiny amount — quantum uncertainty at work. To make things concrete, if you were to measure the rotation of a Carbon Dioxide molecule at absolute zero, for instance, you'd still find that the oxygen atoms are flying around the carbon at a few miles per hour — probably faster than you would have guessed.

Again, I have to make with the lawyer-talk. When talking about the quantum world, it becomes a bit nonsensical to even talk about motion. After all, everything on those scales is governed by uncertainty, so it's not so much that the particles have motion, it's more that you can't ever measure them as not having motion.

How low can you go?

Getting down to absolute zero has essentially the identical problem with getting up to the speed of light. Getting to the speed of light requires an infinite amount of work, while getting down to absolute zero requires extracting an infinite amount of heat. Just to make it clear, both of these are impossible.

That said, there's a lot of running room between "witch's teat" and "fires of hell," and while we can't actually ever reach absolute zero, we can still get surprisingly close. The lowest temperature ever recorded on earth was recorded in Antarctica in 1983 at a relatively balmy 184K.

Of course, if you want to get really cold, you'll want to go into the depths of space. The entire universe is flooded with remnant radiation from the Big Bang, putting emptiest recesses of the universe at approximately 2.73K, just a wee bit cooler than you get with liquid helium, the sort of temperatures we've been able to reach on earth for over a century.

But low-temperature physicists take freeze rays to a whole new level, and surprisingly, these freeze rays take the form of lasers. Wha? I know what you're thinking. How can this possibly be right? Lasers are supposed to blow things up.

True, but lasers have a very special — one might even say, defining — property; all of the light is emitted at exact the same frequency. Now, normally, neutral atoms don't interact with light at all unless the frequency is exactly right. But if the atom is flying toward the light source, the light will get Doppler-shifted, and the light will appear to have a higher frequency than it would otherwise. The atom absorbs a lower energy photon than it would otherwise. So if I tune my laser slightly low, fast moving atoms will absorb those, and when they subsequently emit a photon in a random direction, they will, on average, lose some energy. Do this again and again and again (and keep tuning your laser in the process), and you can cool down your gas to temperatures of less than a nanoKelvin, a billionth of a degree.

It gets even more extreme. The world record lowest temperature is less than one ten-billionth of a degree above absolute zero. These devices typically involve trapping individual atoms in magnetic fields. The "temperature" isn't so much based on the motion of the atoms themselves, but on the spin of the atomic nuclei.

Now, to be fair, you might consider some of these records to be cheating. When we normally think about cooling something down to less than a billionth of a degree, you probably have a picture in your mind of all of the air molecules essentially frozen in place. It's hard to imagine building a doomsday device out of something that just freezes atomic spins.

Ultimately, though, if you really want to experience cold temperatures, all you have to do is wait. In about 17 billion years, the background radiation in the universe will cool down to about 1K. In about 95 billion years, it'll be approximately 0.01K. In 400 billion years, deep space will be roughly the temperature of the coldest experiments here on earth, and it just gets colder from there. Incidentally, if you're wondering why the universe gets so cold so fast, you can thank our old friend dark energy. The universe is in an accelerating phase, entering a period of exponential growth that could last forever. Things get cold and lonely very fast.

But, why do we care?

It's all very well and good to try to break records, but what's the point, really? Oh, there are lots of good reasons, beyond just the bragging rights.

The good folks at the National Institute of Standards and Technology, for instance, would really just like to build a better clock. Time standards are ultimately based on things like the frequency of a cesium atom. If the cesium atom is moving around too much, that creates uncertainties in the measurements, which ultimately create uncertainties in time.

But more importantly, from the mad science perspective, materials behave insanely at extremely low temperatures. For example, just as a laser consists of photons which are all in sync with one another — at the same frequency and phase — bulk materials known as Bose-Einstein condensates can be made in which all of the atoms essentially converge to the same state. Basically, you end up borg-like amalgam where each atom loses its individuality and the entire mass reacts as one zero-super-atom.

At very low temperatures, many materials become superfluids, which mean that they can do things like flow without viscosity, form into ultrathin layers, and even seemingly defy gravity in an effect to reach minimum energy. Also, at low temperatures, many materials become superconducting, which means that they have literally no electrical resistance. Superconductors are able to respond to external magnetic fields in such a way as to completely cancel them inside the material. As a result, you can combine cold temperatures and magnets and get yourself some levitation.

Why is there a lowest temperature and not a hottest temperature?

Let's go to the other extreme. If temperature is just a measure of energy, then you could simply imagine atoms going closer and closer and closer to the speed of light. Couldn't this just continue indefinitely?