Casimir force learns to swim
http://physicsweb.org/articles/news/11/6/4/1
6 June 2007
Physicists in the US have shown that the Casimir force -- a
mysterious quantum phenomenon that draws nearby mirrors together --
can exist in a fluid. The researchers found that two gold-plated
surfaces submerged in ethanol experienced the attraction when brought
within 200 nm of each other, albeit two times weaker than the force
that would be found in a vacuum. This could, they say, lead to a new
"quantum floatation" effect, which could be used to design better
sensors (arxiv.org/abs/0705.3793v1).
First predicted by Hendrik Casimir in 1948, the Casmir force arises
when two facing mirrors are brought towards each other in a vacuum.
According to quantum mechanics, any electromagnetic fields bouncing
back and forth between the mirrors should constantly fluctuate in
strength. At very small mirror separations these fields exert a
radiation pressure on the surfaces that is, on average, stronger on
the outer than the inner surfaces. This causes an overall Casimir
force that draws the surfaces together.
Now, Jeremy Munday and Federico Capasso of Harvard University in the
US have shown that the Casmir force can still exist when the vacuum
is replaced by a fluid. They attached a 40-µm-diameter polystyrene
ball covered in a thin layer of gold to a cantilever, which they then
suspended inside a bath of ethanol with a flat gold surface at the
bottom. By reflecting a laser off the cantilever onto a
position-sensitive photodetector, they could then determine the
strength of the Casimir force attracting the ball from the size of
its displacement.
They found that the force became detectable when the ball was brought
within 200 nm of the flat surface, and when the separation was just
50 nm the force rose to 120 pN \u2013 about two times weaker than the
force that would be found in a vacuum. This shows, according to
Munday, that the ethanol screens the Casimir force by altering the
allowed modes of the electromagnetic fluctuations between the
surfaces.
Recently physicists have realized that the Casimir force must be
taken into account when designing micromachined devices, and
theorists have even considered exploiting it to test the validity of
Newton's laws of gravity on sub-millimetre distances. Munday told
Physics Web that he is now proposing a form of "quantum levitation",
which would use a different fluid medium that causes the mirrors to
repel rather than attract. "In that case, one could levitate one
object above another while in a fluid," he explained. "Because the
objects are not in physical contact, static friction is virtually
eliminated, and the levitated object would respond to even the
slightest force, allowing for very sensitive sensors and
accelerometers."
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