Physics News Update No. 623

When a neutron star pulls matter from a nearby companion onto itself, powerful x-ray bursts, visible to telescopes in Earth orbit, can result. Some astronomers believe the bursts leave behind an ocean of debris, heavy nuclei mostly, on the neutron star's surface. Occasionally much larger "superbursts," with up to 1000 times the amount of x rays than other bursts, can flare up. Henrik Schatz of Michigan State University (517-333-6397) and his collaborators Lars Bildsten from UCSB and Andrew Cumming of UCSC believe that an energy blitz is generated when high energy photons strike the heavy nuclei in the debris ocean, springing free either a proton, neutron, or alpha particle, which then recombine with the residual nuclei forming lighter, stronger bound nuclei and free energy.

This photodisintegration process is triggered by the thermonuclear explosion of a small amount of carbon, but may then proceed subject to positive feedback: the warmer the surface temperature the more disintegration, which in turn leads to warmer temperatures. The runaway production of energy through the conversion of heavy nuclei into lighter nuclei could be unique in astrophysics: all other thermonuclear energy generation (such as those inside our sun) proceeds by fusing lighter nuclei into heavier nuclei. (Upcoming article in Astrophysical Journal Letters; see also http://groups.nscl.msu.edu/nero/)

Lorentz invariance, the idea that the result of a physics experiment should stay the same whether the apparatus is motionless or traveling at some great constant speed relative to a reference point, is taken for granted in the theory of special relativity. Yet in recent years some scientists have come to question this pillar of physics, and to suggest theoretical models (called "standard model extensions," or SMEs ) incorporating Lorentz violations and experimental ways of settling the matter (see Update 578, ). In these models, the speed of light is not universal but will have extra terms dependent on the speed or orientation of the apparatus (see http://media4.physics.indiana.edu/~kostelec/faq.html ).

Even before the advent of Einstein's relativity, the Michelson-Morley experiment tried to perceive (unsuccessfully) a difference in the speed of light when the Earth was traveling in two different directions in space while on opposite sides of its orbit around the sun. Now scientists have to be more subtle in their approach. In one new laboratory experiment, just completed by Stanford physicists (John Lipa, 650-723-4562) microwaves in two resonant cavities (one oriented east-west, the other pointing vertically) are monitored as the Earth sweeps around the sun. Any orientation- or speed-dependent changes in the speed of light would alter the resonant conditions of the cavities in a measurable way. The geometry of the experiment gives it optimal sensitivity to a number of coefficients in a generalized SME. The Stanford group sees no such anisotropy at the level of 10-13 for velocity-independent terms, and at the 10-9 level for velocity-dependent terms. (Lipa et al., Physical Review Letters, upcoming article)

Ground Temperatures Since the year 1500 can be read back by examining the temperatures in deep boreholes. Temperatures in the Earth's crust are determined by a combination of surface climate effects and internal heat flow. The general trend is a linear rise in temperature with depth, but this is modulated by heat perturbations which act in a nonlinear way; typically perturbations penetrate about 20 meters of depth per year or about 150 m in 100 years. Hugo Beltrami (St. Francis Xavier University in Nova Scotia) has examined temperature-depth profiles from 826 places around the world. Taking into account the known temperature anomalies, he is able to work out the average surface energy flux and temperature for many localities and for the world as a whole back for a period of 500 years. Beltrami (902-867-2326) finds that global average surface temperature has increased by 0.45 K in the last 200 years. During this time, however, some places have experienced more dramatic average temperature swings: for example, parts of Africa show a cooling while northern Canada is warmer (3-4 K) during the same period. (Geophysical Research Letters, vol 29, 23, 2111; also see http://geophysics.stfx.ca/public/index.html)

• Hypography Science Feed
• Science Forums Feed
• Science Blogs Feed