Gravitational Redshift around the Black Hole at the Centre of the Milky Way
I’ve just been catching up on the arXiv, and found this very exciting paper by the GRAVITY collaboration (see herefor background on the relevant instrumentation). The abstract of the new paper reads:
The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A* is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU, ~1400 Schwarzschild radii, the star has an orbital speed of ~7650 km/s, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z ~ 200 km/s / c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f, with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 +/- 0.09 (stat) +\- 0.15 (sys). The S2 data are inconsistent with pure Newtonian dynamics.
Note the sentence beginning `Over the past 26 years…’!. Anyway, this remarkable study seems to have demonstrated that, although the star S2 has a perihelion over a thousand times the Schwarzschild radius of the central black hole, the extremely accurate measurements demonstrate departures from Newtonian gravity.
The European Southern Observatory has called a press conference at 14.00 CEST (13.00 in Ireland and UK) today to discuss this result.
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In the Dark 
July 27, 2018 at 10:57 am
The strange headline was Einstein 1: Newton 0. Only 99 years too late. Of course the reality of this highly impressive experiment was far more interesting than that straw-man comparison with Newton and IMO it was a poor approach to getting across the importance of the result to the public.
July 27, 2018 at 11:00 am
In contrast, I think this is a far better-presented experiment to test GR, which considers the right null hypotheses, not a straw man that is refuted every time you use GPS on your phone:
https://arxiv.org/abs/1807.02059
July 27, 2018 at 12:11 pm
This is changed in the post-referee version to “In contrast to almost all alternative theories of gravity”
(see https://www.nature.com/articles/s41586-018-0265-1)
I know that that author, Anne Archibald, has thought deeply about this stuff, so it might be worth contacting her to ask about implications for quantum gravity (perhaps an issue is that predictions are had to make, but I am not an expert either).
July 29, 2018 at 7:24 am
I agree, the physical effect measured in S2 is orbital time dilation, which is well known in GPS and binary pulsars, whereas the pulsar & pulsar & white-dwarf system is more novel, because it tests for gravity acting differently on different substances.
That said, the Galactic-centre stars could yield more exciting stuff as instruments get better and better. ArXiv:1508.06293 simulates the recovery of the black-hole spin and its orientation from astrometry and redshift measurements.
July 27, 2018 at 8:27 pm
I think the good news is they have the test bed and they are in control. We had similar results from Taylor and Hulse who received the Nobel prize in 1993, but it was for a binary pulsar. Now they are chasing the beast and its treasure (information). The problem with GR is that it is unbound. One of the limit is highly likely to be found in black holes, but Nature could have a deeper level. I used to think that the limit would necessarily break the equivalence principle, but now I think there are possible solutions. The mechanism for inertia and gravity could just be the same up to the limits, though they represent two opposite emrging effects.