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Reflections of Absolute Zero
9 April 2007 (All day)
If you want to really see quantum mechanics in action, you've got to turn the temperature down so low that even atoms stop moving. Physicists have come close to achieving this "absolute zero" state by using precision-tuned lasers, but the technique has only allowed researchers to freeze small groups of atoms at a time. Now members of an international team say they have managed to cool a dime-sized mirror to within one degree of absolute zero, the lowest laser-induced freeze yet achieved with a visible object.
One of the greatest enigmas in physics is how matter can be governed by the four basic forces of nature--electromagnetism, which governs light, heat and electricity; the strong and weak nuclear forces, which bind atoms together; and gravity--and still follow the rules of quantum mechanics, which operate only at the subatomic level. In other words, scientists want to know how solid objects keep from flying apart when their atoms are also influenced by the chaotic nature of quantum physics. The major research obstacle has been that natural forces overwhelm quantum effects. The only way to cancel those forces entirely is to cool an atom down to absolute zero (-273 degrees Celsius), where quantum forces apply exclusively.
Scientists have been able to get within a billionth of a degree or so of this state at the atomic scale using several techniques, including laser cooling. Likened to controlling a bowling ball by hitting it with ping-pong balls, laser cooling involves firing pulses of light at a specific frequency that exactly matches an atom's motions. The tuned pulses dampen those motions, and eventually the atom loses all of its energy that isn't generated by quantum effects. The problem has been that scientists have not been able to use lasers to supercool anything bigger than a few atoms.
In an upcoming issue of Physical Review Letters, a team of physicists involved in the Laser Interferometer Gravitational-Wave Observatory (LIGO)--which is located in facilities in Washington State and Louisiana--reports that they have cooled a 1-gram mirror to about 0.8 degrees above absolute zero by combining two laser-cooling techniques. The first, called optical trapping, maintains the mirror in a precise position, while the second, called optical damping, cools it. There's still a long way to go before quantum effects can be observed, cautions lead researcher Nergis Mavalvala of the Massachusetts Institute of Technology in Cambridge, but "the most important thing is that we have found a technique that could allow us to get large objects to ultimately show their quantum behavior for the first time."
Other scientists have achieved temperatures much closer to absolute zero by laser-cooling atoms, and have gotten closer to that target with solid objects using other methods, says physicist Christopher Monroe of the University of Michigan in Ann Arbor. The difference here, he says, is that the laser-cooling used by the LIGO team has the potential of hitting "much lower limits" than anything else so far. He notes, however, that the team's specialized mirror resembles an atom more than a solid object in some respects because of its precise interaction with light.
If the effort is successful, Mavalvala says, it will also lead to much more sensitive instruments for LIGO, which is attempting to detect elusive phenomena called gravity waves. Predicted by Einstein but not yet observed, the waves are thought to be emitted by the most violent events in the universe, such as black hole collisions.