some of the largest detectors in the
world: the gravitational wave detectors
of the LIGO project, built to search for
gentle ripples in spacetime thought to
be produced by (among other cosmic
events) colliding black holes.
Chasing ever greater sensitivities,
these researchers use lasers to still
the vibrations of their detectors’ giant
mirrors — the behemoths of the optomechanical world, weighing in at more
than 10 kilograms. Despite their immense
size, these mirrors have now been cooled
to 234 quanta, MIT quantum physicist
Nergis Mavalvala and LIGO colleagues
reported in 2009 in the New Journal of
Physics. “Our challenges are really the
same as everyone else’s, but we need
to somehow cool our gram and kilo-gram-sized objects to nanokelvins,”
Working on another gravitational wave detector called AURIGA,
researchers in Italy set the record for
largest object effectively cooled via optomechanics. An aluminum bar weighing
more than 1 ton reached a mere 4,000
quanta, the team reported in Physical
Review Letters in 2008.
Whether such large mirrors and
bars could ever demonstrate quantum
effects, though, is an open question. In
principle, some physicists say, quantum
mechanics should hold for objects of any
size. “We don’t know of any fundamental
limit,” Harris says.
Practical considerations may ultimately limit the size of quantum
objects, though. Any observation, be it
by a pair of eyes or a stray, colliding air
molecule, can destroy a quantum state.
The larger an object is, the harder it is
to keep isolated. But that isn’t stopping
researchers with bigger objects from
lining up behind Cleland and the NIST
team to stretch the bounds on quantum
“If we can prove that quantum
mechanics holds for larger and larger
objects, that would be quite spectacular,”
says Dirk Bouwmeester of UC Santa Barbara. “But it would also be spectacular if
we can prove that it doesn’t. New theo-ries would be needed.”
1619 Johannes kepler suggests
that the pressure of sunlight explains
why comets’ tails (above) always
appear to point away from the sun.
suggesting that the mill spins
because of the pressure of light.
scientists now understand that
the heat transferred by light is
responsible for the mill’s spinning.
interest in the pressure exerted by light goes back centuries.
1746 leonhard euler shows theoreti
cally that the motion of a longitudinal
wave might produce pressure in the
direction it is propagating.
1873 James clerk Maxwell (above)
uses electromagnetic theory to show
that light reflecting off a surface or
absorbing into it would create pres
sure. bright sunlight, he calculates,
would press on the earth with a force
of about 4 pounds per square mile.
1873 that same year, sir william
crookes invents the radiometer,
or light mill (above), incorrectly
1876 adolfo bartoli, unaware of
Maxwell’s work, infers radiation
pressure’s existence from the
second law of thermodynamics.
1900 russian physicist pyotr
lebedev announces at a meeting in
paris that he had measured the
pressure of light on a solid body.
1903 ernest fox nichols and
gordon ferrie hull measure the pres
sure to an accuracy within less than
1 percent, publishing the work in the
One of the slowest tortoises in the
race, Bouwmeester’s pace is deliberate.
His mirrors, tens of micrometers across,
vibrate a mere 10,000 or so times per
second and promise an extended quantum lifetime. This durability, he says, is
needed to test a controversial idea that
gravity and quantum weirdness can’t
coexist for long at everyday scales.
More than three-quarters of a century
of research has made scientists more
comfortable with quantum mechanics
at small scales, but supersizing it can
seem as bizarre today as it did to Erwin
Schrödinger. In 1935, he poked fun at the
idea in his famous thought experiment:
a cat in a box that could be both alive
and dead at the same time, as long as no
one peeked inside the box and forced a
choice, killing with curiosity.
s f. Marquardt and s. girvin. “opto
mechanics.” Physics. May 2009.
s to read darpa’s call for proposals
in the area of optomechanics, visit