General Relativity as a Tool

By Mark Trodden | January 25, 2006 6:58 pm

Less than a century after Einstein’s development of General Relativity (GR), physicists still marvel at its geometrical beauty and the myriad observational tests it has passed. One of the best tested theories in the history of science (that’s right IDiots – it’s “just” a theory), General Relativity is mature enough that these days it is increasingly being used as a tool through which to make other scientific discoveries. One of the most successful examples of this is the use of gravitational microlensing to tell us about the presence of small mass distributions – particularly planets.

The bending of light around massive bodies, as predicted by Einstein and later confirmed by Eddington’s observations during the 1919 solar eclipse, is a basic result of GR. Taken to extremes, light from a distant object, passing precisely around a closer, massive object leads to the Einstein Ring phenomenon. More typically, what is observed is a brightening of the light from a distant object as a massive object passes across the line of sight. In the eclipse observations, the presence of a known object was used to measure the way in which light bent, and thus to test GR. But these days, GR is so well understood on these scales, that one may turn this technique on its head and use GR as a tool, interpreting the brightening of such a distant object as evidence for an intervening mass distribution.

For planet detection, the increase in light from a far away star as a closer one passes can happen in a subtle way, revealing the presence of another massive object orbiting the nearer star. This is what is being reported for the smallest object so far – a roughly Earth-sized one. As the BBC article puts it

If the foreground star has a planet orbiting it, it will distort the light even more, and will make the star behind it look even brighter. But this effect lasts for a much shorter period, giving astronomers just hours or days to detect it.

Dr Martin Dominik from the University of St Andrews is a co-leader of the PLANET collaboration, one of the microlensing networks used to detect the new planet.

“We first saw the usual brightening reaching a peak magnification on 31 July 2005. On 10 August, however, there was a small ‘flash’ lasting about half a day,” he said.

“By succeeding in catching this anomaly with two of the telescopes of our network and with careful monitoring, we were able to conclude that the lens star is accompanied by a low-mass planet.”

The planet they discovered is unusual compared to other recently discovered extrasolar planets

The planet, which goes by the name OGLE-2005-BLG-390Lb, takes about 10 years to orbit its parent star, a red dwarf which is similar to the Sun but cooler and smaller.

It is in the same galaxy as Earth, the Milky Way, but is found closer to the galactic centre.


Like Earth, it has a rocky core and probably a thin atmosphere, but its large orbit and cool parent star mean it is a very cold world.

Predicted surface temperatures are minus 220 degrees Celcius (-364F), meaning that its surface is likely to be layer of frozen liquid. It may therefore resemble a more massive version of Pluto.

Since I spend a great deal of my time thinking about and investigating the role that GR plays in the early universe, it is nice to be reminded by stories such as this of the important role that GR plays in relatively late-universe science; in this case as a tool.

You can always rely on Albert.


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About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.


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