Indications of a change in the proton-to-electron mass ratio
Date: Thursday, April 20, 2006 @ 20:34:20 GMT
Topic: Science

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

INDICATIONS OF A CHANGE IN THE PROTON-TO-ELECTRON MASS RATIO have 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 important 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 researchers, 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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