Extracts from the Internet

Testing the general theory of relativity

Lense-Thirring effect

I. Ciuffolini of the University of Lecce, Italy, and E Pavlis of the University of Michigan, USA, have measured the magnitude of the Lense-Thirring effect. This effect consists in the fact that spacetime near a massive body is additionally curved - and hence test body trajectories are additionally altered - if the body rotates. Over a span of 11 years, laser pulse round trip times have been measured between the Earth and the mirrors mounted on the LAGEOS and LAGEOS II satellites. In this way, the distances to and the orbit shape of the satellites were determined with high precision. The Lense-Thirring effect gives a correction of several meters per year to the orbit's parameters. The orbit as a whole plays a role of a giant, slowly precessing gyroscope. Although the measured Lense-Thirring effect was 99% of the general relativity prediction, there was a 10% error to this result - due primarily to nonuniformities in the gravitational field of the Earth. To account for the nonuniformities, data from the GRACE satellite were used. With data from the Gravity Probe B satellite that are forthcoming in the very near future, the error in the measurement of the effect will be reduced to 1%. Source: http://physicsweb.org/articles/news/8/10/12/1

Equivalence principle for isotopes

The equivalence principle for macroscopic bodies was tested with an accuracy of 10^-13. The most accurate test on individual atoms has been performed by S.Fray and his colleagues from Germany. The researchers used atomic spectroscopy to study the relative acceleration of the isotopes ⁸⁵Rb and ⁸⁷Rb falling in a gravitational field. The relative acceleration - and hence the accuracy of the equivalence principle test - was (1.2 + - 1.7)x10^-7. A similar test with the same accuracy was performed for isotopes of one type in different substates of hyperfine-split energy levels. The measurement data have about three time the accuracy of the previous experiments with atoms. Source: physics/0411052

A new type of superconductivity

Electron Cooper pairs in superconductors that have been known so far have an even value of orbital angular momentum, L=0 or L=2. L=1 superconductivity was predicted theoretically some 40 years ago, but a decisive experimental confirmation of its existence has been lacking - until now. A team of physicists from the US and Japan have obtained the first strong evidence for the existence of `odd' superconductivity in strontium ruthenate Sr₂RuO₄. In their experiment, a sample of strontium ruthenate was connected by two Josephson junctions to a normal superconductor. Cooper pairs could tunnel through the junctions and interfere. What was measured was the electric current through the junctions as a function of the external magnetic field. It was found that the interference was accompanied by the destruction of the pairs - something which is only possible if L=1. Source: Science 306 1151 2004

`Signal velocity' of a light pulse

Recent experiments have shown that for a light pulse in a dispersive medium both the phase and group velocity can exceed the speed of light in vacuum, c. This does not violate the principles of relativity theory, however, because the speed of information transfer is always less than c. Researchers at the University of Geneva have for the first time confirmed this in a direct experimental way by studying how polarized laser pulses propagate in an optical fiber. As expected, the light pulse propagation speed - the so-called `signal velocity' at which information is transmitted - did not exceed # even though the pulse's group velocity was 1.76c. Source: Phys.Rev.Lett. 93 203902

Unusual dark matter halo

NGC 4555, an elliptic galaxy which is single in the sense of not belonging to any cluster or group, has a very extended and massive halo of dark matter, according to the Chandra space X-ray observatory. The X-ray-emitting halo of the galaxy extends to a distance of about 60 kpc. The dynamic structure modelling of NGC 4555 showed that the surrounding dark matter halo is 300 times more massive than the gaseous halo and so much larger than its counterparts around other optically similar galaxies that have been studied. The formation mechanism of the galaxy NGC is not yet known. Source: astro-ph/0407552