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Friction science
gets an update
By Devin Powell
By playing with plastic blocks that stick
and slip much like rock, physicists are
challenging centuries-old ideas about the
nature of friction. Seemingly unimportant differences at small scales can have
big consequences, an Israeli team reports
in an upcoming Physical Review Letters.
“If you want to know how hard you have
to push a specific object [to overcome
friction], and you want to know to high pre-
cision, right now we don’t know what the
answer is,” says Jay Fineberg, a physicist
at the Hebrew University of Jerusalem.
A dose of reality for quantum math
Canadian team devises direct way to measure wave function
By Devin Powell
The fuzzy quantum shape that describes
the physical state of a single particle, its
wave function, has been directly measured in the laboratory, giving this mathematical concept a small dose of reality.
Like a bubble on the breeze, the wave
function usually disappears when poked
or prodded for information. But scientists
in Canada have worked out a gentler way
to touch it, they report June 9 in Nature.
“Measuring the wave function itself is
not ... thought to be a possible thing,” says
Stanford physicist Onur Hosten. “It’s not
really thought to be something physical.”
This interpretation dates to the 1920s,
when physicist Max Born argued that the
wave function, represented by the Greek
letter psi, is a useful mathematical tool
for calculating probabilities for a particle’s location or speed. The equation for
the wave function is the starting point, for
instance, for drawing the colorful shapes
in chemistry textbooks that show the
probability of an electron being in a certain spatial region.
To calculate a wave function, scientists
usually collect lots of indirect measurements using quantum state tomography.
“It’s like working out the shape of a
water wave by moving a light around and
measuring the wave’s shadow on the bottom of the pool,” says Jeff Lundeen, a
physicist at Canada’s National Research
Council in Ottawa. His team devised a
direct interrogation by combining weak
measurements, which provide uncertain
information but do little damage, with
strong measurements, which provide
certainty but destroy the wave function.
“This doesn’t provide any more information than other methods,” says Lundeen.
“It just gives it to you in a different way.”
To demonstrate how this works in the
lab, the team measured the wave function
for the position of a single particle of light,
or photon. The team polarized photons so
that the angle of each particle gave a rough
idea of its location, leaving enough uncertainty to not disturb the wave function.
Eliminating all photons that were moving
in one specific direction — a strong measurement of momentum — allowed mapping the wave function using the particles
that remained.
Lundeen and colleagues aren’t challenging Heisenberg’s uncertainty principle, which says that the location and
momentum of a particle can’t be simultaneously measured. The team had to
weakly measure many photons to work
out the position information. And all these
particles had to be identical, which could
limit the usefulness of the technique.
FROM TOP: © SCIENCE/AAAS; J. LUNDEEN, CHARLES BAMBER
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