Though the standard model of particle
physics, the mathematical framework
for explaining how stuff is held together,
“The electron EDM
is one of the places
where there should be
a good chance of seeing
some new phenomena
that can’t be explained
in the standard model,
and could in turn help
to explain this matter-
antimatter imbalance
in the universe,” says physicist David
DeMille of Yale University.
Spotting the dipole moment would
mean that the electron has some kind of
internal structure, a bizarre concept for a
particle that is supposed to buzz around
the nuclear hearts of atoms and molecules with its mass concentrated into an
essentially sizeless point. Although no
one has yet measured the electron’s electric dipole moment, researchers think it
should exist and could be within reach of
today’s modern laboratory setups.
“There are good theoretical reasons to think that
it isn’t too far away,”
says physicist Larry “T h
i hi
i
Measuring the
inner shape of the
famous particle
could help solve
a cosmic mystery
By Alexandra Witze
Long thought to be a simple speck of negative charge, the humble lectron may be hiding one more surprise in its depths.
The electron was the first fundamental particle discovered. It was the first to
have its charge measured, and it inspired
the mathematical equation that first
hinted at the existence of antimatter, the
exotic, oppositely charged counterpart
to ordinary matter.
Now the electron is poised to go one step
further, by helping scientists understand
why matter triumphed over antimatter
in the early universe. In theory, the Big
Bang should have created matter and
antimatter in equal amounts, but if so
they would have annihilated each other
and left nothing behind.
Hints in time Reversing time (by
switching the direction of a particle’s
spin) changes the direction of the
magnetic dipole (blue) but not the
electric dipole (green). This means
that possessing an electric dipole
moment would give particles a way
to violate time-reversal symmetry.
Hunter of Amherst College in Mas-
sachusetts, who has been hunting the
electron’s electric dipole moment since
the 1980s. “What has made us all dedicate
our lives to it is the real good chance that
something might emerge soon.”
Researchers are now betting on several
ways they might succeed, from study-
ing ultracold atoms to
lopsided molecules to
magnetized ceramics.
Within the next few
months, scientists at
Imperial College Lon-
don are expected to
report the latest limit
on the size of the elec-
tron electric dipole
moment, the first such
improvement in a
decade.
An actual measurement of that dipole
moment “would be a
big, big discovery,” says Eugene Commins,
a physicist now retired from the University of California, Berkeley. “That would
be a Nobel Prize.”
TIME-REVERSAL DIAGRAM: PH YSICS WORLD DECEMBER 2009; ATOM: © GALINKA86/ DREAMSTIME.COM; RULER: JANGELTUN/ISTOCKPHOTO
What lies within
Physicists suspect that electric dipole
moments exist because they allow
particles to violate what’s known as time-reversal symmetry. Although symmetry
sounds like a good thing, scientists know
that processes involving other particles
(such as B mesons) behave differently
whether running forward or backward,
a violation of time-reversal symmetry.
In order for this to happen, the electron
(and other fundamental particles) must
