via Astronomy Now.
90 years ago, Kendal born Sir Arthur Stanley Eddington (commemorated in an exhibition at Kendal Museum at the moment) led an expedition to Principe that was to lend vital observational support to the new theory of Relativity. Einstein had postulated that the presence of mass warped space-time in such a way that a lensing effect could be seen around large objects – a distortion in the light from behind. Eddington’s team hoped to measure this by seeing the apparent displacement of stars near to the Sun during a total eclipse (which shielded the Sun’s light from the observers, making other stars easier to see). The 1919 eclipse was particularly useful as the Sun was eclipsed in front of the Hyades star cluster, the brightest cluster of stars in the sky. Eddington took images during the eclipse and found Einstein’s predictions to be born out. Several measurements since have confirmed this.
The most recent measurement, carried out by the Very Long Baseline Array Interferometer, which takes the data from several individual radio telescopes and adds them together to get an image with the resolution of a larger telescope, though without the sensitivity of one, has continued this trend.
Eddington in fact compared his measurements with three theories – one without any deflection (light unaffected), one with half deflection (as would be the case if light had mass and this was what caused it to be affected – as Newton had considered to be the case) and one with full deflection (relativity). This full deflection value is called gamma and observations compare what they see to what Einstein predicts (gamma = 1) by measuring gamma.
The latest measurements, made by a team led by Sergei Kopeikin of the University of Missouri, got a value of 0.9998 +/- 0.0003, as 1.0000 lies within the range of uncertainty (0.9998 + 0.0003 = 1.0001) this means the measurement is in agreement with Einstein’s prediction.
It is important to measure gamma accurately as even a difference from Einstein of one part in a million is enough for quantum theories of gravity to find their way in and take over from the Relativistic view that gravity is a consequence of the curvature of space-time.