First experimental verification of GR

1. Feb 1, 2006

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?

2. Feb 1, 2006

Staff Emeritus
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.

3. Feb 1, 2006

Smilecdl

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

4. Feb 1, 2006

Staff Emeritus
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.

5. Feb 2, 2006

Garth

Correct, it was just that Eddington's results were not that accurate. Relativity and the 1919 eclipse.
Later observations, particularly using radio transmissions from spacecraft on the far side of the Sun, have verified the prediction to within 1% accuracy.

Garth

6. Feb 2, 2006

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?

7. Feb 2, 2006

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

Last edited: Feb 2, 2006