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# The 2001 Nobel Prize in Physics: Very, Very Cool Stuff

What exactly do we mean when we say something is cold? In fact, what do we actually mean by the word "temperature?" These concepts seem obvious to our human senses, but in reality, there are some much deeper issues involved. This year, the Nobel Prize in physics was given to a team of people who investigated how matter behaves at extremely cold temperatures. How cold is cold? Try 20 billionths of a degree above absolute zero. There's a reasonably firm guarantee that nothing in the universe, ever, has been that cold before.

I remember being surprised as a physics student to learn that there is such a thing as absolute zero. There's a temperature, -459.69 F to be exact, that is the absolute lowest limit of cold. It is impossible for anything to be even a fraction of a degree colder. Why? That's where the question of what "temperature" is comes in.

What we perceive of as heat is actually something a bit abstract: the random motion of atoms or molecules. All matter is made up of atoms, or collections of atoms bonded together called molecules. The air in your room, for example, is made up of molecules of Oxygen, Nitrogen, and Carbon Dioxide, among other things.

Right now, in the air around you, molecules are bouncing off you and the furniture, colliding with other molecules, and just generally wandering around the room. The relative speed at which they do this depends on the temperature of the air. When you go outside on a hot day, the gasses in the air have a higher thermal energy, meaning the molecules are moving around fairly rapidly. On a winter's day, that crisp, cool feeling in the air is caused by the fact that the molecules have less energy, and are moving around more slowly.

The idea that temperature is really just the relative speed of molecules may seem a bit counter-intuitive, but it does explain why there is a bottom limit. If colder and colder temperatures mean that the molecules are moving more and more slowly, what happens when they stop altogether? You guessed it: that's what happens at -459 degrees. Nothing can possibly be colder than that, as the molecules are stopped dead in their tracks.

That all makes reasonably good sense, but as I said before, there are some very deep issues lurking around when you get close to absolute zero, and it all has to do with quantum mechanics. Quantum mechanics is the study of our universe at very small scales. And it does not make intuitive sense.

Quantum mechanics is our best, most rigorously verified theory of what matter is like, but it's impossible for the human mind to really, truly understand. Take the Heisenberg Uncertainly Principle, one of the basic tenets of quantum mechanics.

It turns out that it is impossible to measure the location of an atom and its speed at the same time. Impossible. It's not that we don't have accurate enough instruments, or that the darn things are just too small to see. The universe changes things, spreads things out, so that no one can ever know the exact speed and position of an atom at the same time. If you manage to measure the exact speed (or temperature) of an atom, the atom will sort of disappear, spread out to fill a larger volume so that you can't measure where it is. As strange and perplexing as that sounds, this is a basic characteristic of matter. It doesn't make sense to us, but it really happens. In our common experience, we never try to measure anything that accurately, so we don't notice this effect. But something very weird happens when you get close to absolute zero.

If temperature is truly just a measurement of how fast atoms are moving around, then as the atoms get colder and colder, we should know their speeds more and more accurately because they're getting slower and slower, right? When things get close to absolute zero, the atoms have effectively stopped moving altogether. But we just said that you can't know where something is and how fast it's going at the same time. So, when temperatures get very cold, matter actually does "spread out." Atoms spread out so much, in fact, that matter stops acting like something solid, and instead becomes more of a wave. That solves the problem of the Uncertainty Principle, as instead of measuring the location of a particle, you now have a wave that exists over a range of locations. Atoms, which are usually incredibly small (on the scale of billionths of meters) can spread out enough to be visible under a microscope, and can even be caught on film using a video camera with some magnification.