Number 242, September 28, 1995 by Phillip F. Schewe and Ben Stein
NEW TESTS OF THE MOST SUCCESSFUL THEORY IN PHYSICS show the need for
improved information on the proton. Stringently verified for five decades
now, quantum electrodynamics (QED) is the modern theory of the atom. QED
was initially developed to explain the "Lamb shift" in the hydrogen
atom, in which an electron orbiting a proton experiences a slight energy
shift due to the interaction between the electron and the teeming virtual
particles residing in the surrounding vacuum. Modern experimental measurements
have become so precise that theoretical calculations of the Lamb shift
can no longer consider the proton as a pointlike charge but must instead
view it as a sphere in which positive charge is spread out. In the energy
states commonly studied in the Lamb shift, the electron actually has a
probability of spending some time inside the proton, whose size, unfortunately,
is not known to better than about 8%. Using high-resolution laser spectroscopy,
Yale researchers (contact Malcolm Boshier, 203-432-3828) have measured
the hydrogen Lamb shift to a new record accuracy of six parts per million.
Their value (8,172,827 kHz, expressed in units of frequency) agrees with
QED providing they incorporate a relatively large proton radius, such as
the one measured in Mainz (Germany), 0.862 x 10**-15 m. However, a significant
discrepancy between QED and the new Lamb shift result cannot be ruled out
until the proton size is pinpointed to within 1 or 2 percent. According
to Boshier, "It is...far from clear that everything is okay with QED,"
particularly when one considers that experimental measurements of another
Lamb shift, that for the He+ ion, violently disagree with the theory. "The
motivation for Lamb shift measurements is the search for new physics,"
he adds. (D.J. Berkeland, E.A. Hinds, and M.G. Boshier, Physical Review
Letters, 25 September 1995, see also Physical Review Letters, 18 December
1995.)
SOME MATERIALS MIGHT EXHIBIT TWO KINDS OF SUPERCONDUCTIVITY. At low
temperatures electrons in certain materials pair up. The pairs, condensing
into a single macroscopic quantum state, constitute a supercurrent which
does not lose energy, through interactions with the lattice of atoms forming
the material. In low-temperature superconductors the electron pairs are
said to be in an "s wave" state, one in which the electrons have
no angular momentum relative to each other. Some theorists believe that
the pairing mechanism is different in high-temperature cuprate superconductivity
and that the pairs exist in a different state called a "d wave."
Indeed, recent experiments trying to settle the issue lean toward the d-wave
explanation, but the results have not been definitive. Now, K. Alex Muller
of IBM proposes that in some materials both kinds of superconductivity
may be at work. That is, two types of superconductivity may occur at the
same transition temperature in the same material. (K. Alex Muller, Nature,
14 September 1995.)
WHERE DOES THE SOLAR CORONA GET ITS ENERGY? Scientists have long been
puzzled by the fact that the corona is much hotter than the sun's visible
surface. Astronomers viewing new observations of individual coronal loops
recorded with the orbiting Yokhoh x-ray telescope deduce that the rate
of loop heating scales inversely with the square of the loop length. Establishing
this scaling law sets the stage for the next step in arriving at an understanding
of coronal heating. This will be the direct measurement of the strengths
of arching magnetic fields which rise out of the sun and twist the coronal
loops into their barber's pole shapes. (James Klimchuk and Lisa Porter,
Nature, 14 Sept.)
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