Text 795, 102 rader
Skriven 2006-06-30 18:08:22 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 783
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 783 June 30, 2006  by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi        www.aip.org/pnu

PLUMBING THE ELECTRON'S DEPTHS.  Careful observation of a single
electron in an atom trap over a period of several months has
resulted in the best measurement yet of the electron's magnetic
moment and an improved value for alpha, the fine structure constant,
the parameter which sets the overall strength of the electromagnetic
force. Electrons are of course a part of every atom and as such are
a basic building block of the universe.  And alpha is an important
member of the system of fundamental constants used to describe
nature.
The electron, much lighter than a proton and generally thought to be
a pointlike particle, is about as fundamental an object for study as
one can have in physics.  Nevertheless, the electron's interaction
with the vacuum is anything but simple.  The theory of quantum
electrodynamics (QED) predicts that an electron is perpetually
grappling with virtual particles---such as photons and
electron-positron pairs---emerging briefly from the surrounding
vacuum.  In the absence of these interactions, the magnetic moment
of the electron (referred to by the letter g), which relates the
size of the electron's magnetism to its intrinsic spin, would have a
value of 2.  But direct measurements of g show that it is slightly
different from 2.  The finer these measurements become, the better
one can probe the quantum nature of electrons and QED itself.
Furthermore, if the electron had structure (in the way that protons,
for instance, are made of quarks), this too would show up in
measurements of g.
To gain the greatest possible control over the electron and its
environment, Gerald Gabrielse and his students Brian Odom and David
Hanneke at Harvard create a macroscopic artificial atom consisting
of a single electron executing an endless looping trajectory within
a trap made of charged electrodes---a central, positively-charged
electrode and two negatively-charged electrodes above and below---
supplemented by coils producing a magnetic field.  The combined
electric and magnetic forces keep the electron in its circular
"cyclotron"orbit.  In addition to this planar motion, the electron
wobbles up and down in the vertical direction, in the direction of
the magnetic field.  The heart of the Harvard experiment is to
explore these two motions---the circular motion, which conforms to
quantum rules, and the vertical motion, which conforms to classical
physics---in a new way.
First the quantum part. Like any real atom, this artificial atom is
under the sway of quantum rules, and the captive electron can only
possess certain permitted energies. Electrons have been bound in
traps like this before, but this new experiment is the first to be
laid out so that the electron can reside in its very lowest
quantum-allowed cyclotron states.  The apparatus does this by
controlling stray energy, such as by inhibiting blackbody heating of
the electron by cooling the central enclosure to a temperature of
100 mK and by inhibiting emission by the electron itself through
clever design of the atom trap cavity.  The whole setup is acting as
a one-electron quantum cyclotron.
Second, the classical part. The Harvard experiment is the first to
induce a microscopic object to adjust its own oscillations based on
interactions with its environment (see their publication of a year
ago: D'Urso et al., Physical Review Letters, 25 March 2005).  The
electron, as it moves vertically, induces a very tiny voltage change
in the external electrical circuit supplying the electrodes.
Sensing this change, the circuit can adjust the electrode voltage to
enhance or depress the electron's up-and-down excursions.  This
feedback-actuated self-excitation, if it's not too big or too small,
allows the researchers to measure an oscillation frequency which in
turn is related to the electron's quantum state.
It is this masterful control over the electron's motions and the
ability to measure the energy levels of the electron's artificial
quantum environment that  allows the Harvard group to improve the
measurement of g by a factor of 6 over previous work.  The new
uncertainty in the value, set forth in an upcoming article in
Physical Review Letters, is now at the level of 0.76 parts per
trillion.  No less important than g is alpha. By inserting the new
value of g into QED equations, and thanks to improved QED
calculations of very high accuracy, the experimenters and theorists
together determined a new value for alpha, one with an accuracy ten
times better than available from any other method.  This is the
first time a more precise value of alpha has been reported since
1987.  The new alpha, published in a companion article in Physical
Review Letters, has an uncertainty of 0.7 parts per billion.
The measured value of g can also be used to address the issue of
hypothetical electron constituents.  Such subcomponents, the new g
measurement shows, could be no lighter than 130 GeV.  On the basis
of this experiment one can also place a corresponding limit on the
size of the electron; it must be no larger than 10^-18 m across.
These are not necessarily the best experimental limits on the size
or structure of the electron, but this is, after all, work that is
patently in the realm of low-temperature atomic physics and not the
realm of high-energy particle accelerators, where fundamental
particle properties are normally measured.
The Harvard atom trap effort has spanned twenty years and has
yielded more than half a dozen PhDs.  According to Gabrielse
(gabrielse@physics.harvard, 617-495-4381), an improved value for
alpha should (among other things) contribute to the pending
adjustment of fundamental constants aimed at redefining the kilogram
in a way that avoids the use of an actual weight kept under glass in
Paris.  (Odom et al., and Gabrielse et al., two upcoming articles in
Physical Review Letters, lab website at
http://hussle.harvard.edu/~gabrielse/ )

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 * Origin: Big Bang (1:106/2000.7)