Wednesday, April 19, 2006


The American Institute of Physics Bulletin of Physics News
Number 774 April 19, 2006 by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi

shown up in comparisons of the spectra of hydrogen gas as recorded
in a lab with spectra of light coming from hydrogen clouds at the
distance of quasars. This is another of those tests of so-called
physical constants that might not be absolutely constant. For
example, the steadiness of the fine structure constant (denoted by
the letter alpha), defined as the square of the electron's charge
divided by the speed of light times Planck's constant, has been in
dispute ( ). Some
tests say it's changing, others say it isn't. This is an
issue since alpha sets the overall strength of the electromagnetic
force, the force that holds atoms together. Similarly, the
proton-to-electron mass ratio (denoted by the letter mu) figures in
setting the scale of the strong nuclear force. There is at present
no explanation why the proton's mass should be 1836 times that of
the electron's. The new search for a varying mu was carried out by
Wim Ubachs of the Vrije Universiteit Amsterdam. He and his
colleagues approach their task by studying hydrogen gas in the lab,
performing ultra-high-resolution spectroscopy in the
difficult-to-access extreme-ultraviolet range. This data is compared
to accurate observations of absorption spectra of distant hydrogen
(which absorbs light from even more distant quasars) as recorded
with the European Southern Observatory (ESO) in Chile. The
astronomical hydrogen is essentially hydrogen as it was 12 billion
years ago, so one can seek hints of a changing value for mu. Why
the comparison? Because the position of a particular spectral line
depends on the value of mu; locate the spectral line accurately
(that is, its wavelength) and you can infer a value for mu. In
this way, the researchers report that they see evidence that mu has
decreased by 0.002% over those 12 billion years. According to Ubachs
(, ), the statistical confidence
of his spectroscopic comparison is at the level of 3.5 standard
deviations. (Reinhold et al., Physical Review Letters, 21 April
2006, laser website at )

NUCLEAR QUANTUM OPTICS. Normally the atomic realm, characterized by
an energy scale of electron volts or less, is very much removed from
the nuclear realm, where energy levels are measured in thousands and
millions of eV. Some laser interactions in nuclei can be achieved
indirectly by using light to create plasmas, whose secondary
particles either interact with nuclei or, in a tertiary step,
produce gamma rays which then influence nuclear states. Scientists
at the Max-Planck-Institut fur Kernphysik have now studied how
present and future x-ray laser facilities will make possible direct
laser intervention in the nucleus and how this will open up a new
branch of quantum optics. X-ray sources such as the TESLA device at
the DESY lab in Hamburg will not only deliver high-intensity,
high-energy beams but will, at least partially, consist of coherent
(laserlike) radiation. One doesn't need coherent light to excite a
nucleus, but coherence can be important in exercising greater
control over optical phenomena analogous to those in atomic
systems. Examples include exciting a complete population inversion
of the target nuclei or even producing some kind of nuclear
"electromagnetically induced transparency." One of the
Thomas Burvenich (, says that an
additional benefit of nuclear quantum optics will be the direct
measurement of specific nuclear facts, such as nuclear dipole
moments and the energy levels of nuclei. (Burvenich et al.,
Physical Review Letters, 14 April 2006; lab website at )

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