Physics: Principles and Problems


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How Cold Can We Go?
January 2004

What's the coldest spot on Earth? It isn't the north pole, the south pole, or the windy tip of Mount Everest. While incredibly cold by human standards, these places can't hold an ice cube to the kind of coldness that has recently been produced in an MIT laboratory. There, a team of physicists--including Nobel laureate Wolfgang Ketterle, the team's co-leader--have succeeded in cooling sodium atoms to the lowest temperature ever recorded. These chilly bits of matter are just one half of one billionth of one degree above absolute zero.

Atoms are always jostling around, even in objects that seem to be completely still. By adding energy--say by holding a match under a stick of butter--you cause the atoms to jostle faster, which begins to break apart their hold on one another. This is why the butter melts.

Conversely, by removing energy from a system, you can cause atoms to slow down. When liquid water is cooled enough, its molecules can link together to form ice. This is by no means the end of the story, though; even though the molecules in a frozen substance are all nicely lined up, each individual atom is still jumping and jostling. Continued cooling can slow those atomic motions down more, more, more--in principle, all the way to the point where they stop.

We have to say “in principle,” though, because no real-world cooling process can actually go that far. “Absolute zero” is the term used for the point where molecular motion ceases altogether; it corresponds to -273 degrees C, or -460 degrees F. To achieve this point would violate one of the laws of thermodynamics, so most physicists believe it can’t be done.

A New Record . . . In Being Cold!

Be that as it may, the MIT team has come closer than anyone else in history. Their sodium atoms, which are being suspended using magnets to keep them from colliding with anything and heating up, have broken the record by going below one nanokelvin. A “nanokelvin” is a billionth of a degree. That makes these atoms about a million times slower than room temperature atoms--such as the ones making up the chair where you are sitting right now. Room temperature atoms, if they moved in a straight line instead of crashing into other atoms, would go about as fast as a jet in flight. By contrast, after a full minute, these super-cooled sodium atoms would only have moved two inches.

Why So Chilly?

Why is it important to see how low we can go in the world of super-cooled substances? One answer is that discovery is fascinating for its own sake; part of the challenge in cooling gases to ever-lower temperatures lies simply in finding out whether it can be done. On the practical side, however, physicists learn a great deal about atoms by seeing how they behave under super-cooled conditions. New and unexpected properties emerge at these times, giving us a window into the fundamental workings of matter. It was super-cooling that first produced the "Bose-Einstein condensate," for example, a new form of matter predicted by Einstein but never seen in a lab until the 1990's.

The Future is Super-Cool

These first steps will also allow us in time to begin tapping into the strange behavior of super-cooled substances in order to use them in new technology. Slow atoms of various types will no doubt play a part in everything from atomic clocks to gravity sensors . . . to super-cool devices of which even physicists have yet to dream.

Use the resources below to find out how cold things can get. Then ask yourself this question: how hot can things get? Is there such a thing as "super-heated" matter as well as "super-cooled" matter? If absolute zero is the bottom of the temperature scale, what's the top? How can you find out?



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Physics: Principles and Problems