Einstein’s theory of General Relativity is one of the great success stories of modern science. Few theories have made such extraordinary claims while still shrugging off all challenges with ease. It seems strange, then, that one of the very first predictions made by the theory should be one of the last to be successfully tested. The prediction is this: A gravitational field will distort spacetime in such a way that light passing to a region of lower gravitational potential should have its frequency decreased, resulting in the spectrum shifting to the red. Therefore, if we look at sunlight with a sensitive spectroscope, we should detect this redshift and so find yet another confirmation of General Relativity. Almost a hundred years later, this experiment has finally been performed, according to this paper (arXiv:1207.0177v1 [astro-ph.SR]) by Yoichi Takeda and Satoru Ueno.
It seems a simple test, so why did it take so long? Well, the redshift predicted for light leaving the Sun’s photosphere isn’t very great – it’s equivalent to that caused by the doppler effect from a surface moving away from us at about 600 meters per second. That’s well within the range of modern spectroscopes, but the Sun’s surface is violently turbulent – great cells of superheated plasma roiling and bubbling as the heat from the interior gets transported to the surface. The vertical movement of this plasma also affects the frequency of light as seen from Earth, and it turns out to be extremely hard to pick out the gravitational redshift from the more regular doppler kind. Add to this the effect of intense heat, where the variable motions of individual atoms add their own doppler shifts, smearing out the spectral lines, and the differential rotation of different parts of the Sun’s surface, and you end up with a real mess.
So how exactly did Takeda and Uedo overcome these difficulties? By using the Domeless Solar Telescope at the Hida Observatory of Kyoto University to record spectra through an iodine-cell filter at over 3000 points on the solar surface. The iodine gas within the filter has a unique spectral signature, characterised by many thin, well-defined lines. This gives astronomers a very precise scale to compare with the solar spectrum, letting them measure the local redshifts (or blueshifts) very accurately. And the ability to record so many points of data meant that they could apply statistical methods to average out the various sources of noise we mentioned earlier.
The result of all this work was a figure of 698 +/- 100 meters per second. The authors of the paper admit that the figure seems rather imprecise, but considering the nature of their subject, it’s still an impressive piece of work. And best of all: The theoretically predicted value of 633 meters per second falls well within that range.
General Relativity remains undefeated and stands ready to take on the next challenger!