Reflections
of Absolute Zero
By Phil Berardelli
ScienceNOW Daily News
9 April 2007
Super cool.
Researchers have developed a technique to cool this dime-sized
mirror (small circle suspended in the center of metal ring) to
within one degree of absolute zero.
Credit: Christopher Wipf/MIT
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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 (-237 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.