First experimental verification of GR

[Nucleonics]

The perihilion of Mercury and the eclipse of 1919 (Eddington expedition) are given as the first experimental verifications of General relativity. However, feeling pedantic, I was curious just which one these is accepted as the vital first prediction of GR in agreement with the real world? If neither, what really was the first real world observation that agreed with GR?

 

[selfAdjoint]

Einstein actually obtained Mercury's perihelion advance a couple of years before he developed his field equations, using partial results and especially the equivalence principle. The 1919 observations confirmed results of the full theory (and I believe the Schwartzschild metric) and thus I would consider it the breakthrough confirmation.

 

[Smilecdl]

I was under the imprression that the Eddington observations of 1919 actually proved nothing and the story is some kind of urban myth.

 

[selfAdjoint]

The Eddington observations were thought to have demonstrated the curvature of light rays near a gravitating body (the Sun). Much much later these observations were criticised, but that was after other observations, done correctly, had confirmed their conclusions. I don't know the detailed history of these other observations, but perhaps Garth or someone else can enlighten you.

 

[Garth]

The Eddington observations were thought to have demonstrated the curvature of light rays near a gravitating body (the Sun). Much much later these observations were criticised, but that was after other observations, done correctly, had confirmed their conclusions. I don't know the detailed history of these other observations, but perhaps Garth or someone else can enlighten you.

Correct, it was just that Eddington's results were not that accurate. Relativity and the 1919 eclipse.

The historical reality of the eclipse experiment is more complicated than the textbook account, and highlights some of the problems involved in retrospectively applying a notion such as predictive power. The results of the eclipse observations were far from clear – they were taken at two remote locations (Sobral in Brazil and the Atlantic island of Príncipe) and thus the telescopes used required accuracy to be sacrificed in favour of portability. In addition, there were mitigating factors such as the Earth's slight rotation during the eclipse, and the temperature differences between day (when the eclipse pictures were taken) and night (when the control pictures were taken), which caused optical anomalies. From the start, the experiments were far from clear-cut, and relied on a series of assumptions and human judgements.

Moreover, the data from the observations were not as conclusive as was professed. Two telescopes were used at Sobral; one produced 8 photographic plates which recorded a mean deviation from the norm of 1.98″ of arc (1 ″ = 1/3600th of a degree), and the other 18 plates with a mean deviation of 0.86″; the two plates from a single telescope at Principe, though of a poor quality, suggested a mean of 1.62″. Einstein's theory suggested a deviation of approximately 1.75″, while Newton's suggested 0.8″. If all the data had been included, the results would have been inconclusive at best, but Eddington discounted the results obtained from the second Sobral telescope, claiming "systematic error", and gave extra weight to the results from Principe (which he had personally recorded), with little justification or supporting evidence. The Astronomer Royal, Sir Frank Dyson, and the president of the Royal Society, J. J. Thomson, sided with Eddington, and on November 6 declared the evidence was decisively in favour of Einstein's theory; much of the scientific community fell in line and agreed.

Later observations, particularly using radio transmissions from spacecraft on the far side of the Sun, have verified the prediction to within 1% accuracy.

Garth

 

[yogi]

If have read several places where the Sun's oblateness will produce about the same degree of perihilon advance as GR - has that been discounted by better experimental data?

 

[Garth]

This is the perihelion advance of the orbit of Mercury, not the deflection of light by the Sun.

Yes, an oblateness of the Sun will also produce a small perihelion advance, this was important to the Brans Dicke (BD) theory that predicted a slightly less advance than the GR value of 43" arc/century, the actual amount depending on the value of the BD coupling constant.

Dicke (1966) thought he had detected an oblateness (5.0 +/ 0.7 parts in 10⁵) that would produce a small advance (3.4"/centry) so that could make up the BD prediction to the observed 43"/century.

However this would mean the observed value coincidentally happens to be that of GR, that the solar interior would have to be rotating once every 2 days, that the oblate precession would affect the other planets (not observed) and that earlier measurements of the Sun's oblateness, which gave a null result, would have to be discarded. Dicke's result, if real, also assumes that the observed solar surface (which he was measuring) coincides with the gravitational equipotential surface (which would produce any r^-3 term in the Newtonian potential to cause the extra precession) but this assumption is theory dependent and could be mistaken, especially if the Sun had a rapidly rotating core.

Garth