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.

10 Responses to “Gravitational Redshift around the Black Hole at the Centre of the Milky Way”

  1. It was even in the normal news on radio here in Germany. And correctly explained in a one-minute executive summary to boot.

  2. Phil Uttley Says:

    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.

  3. Phil Uttley Says:

    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

    • Some questions for the experts (I am not an expert in this area):

      Einstein’s theory of gravity, general relativity, has passed stringent tests in laboratories, elsewhere in the Solar Sytem, and in pulsar binaries.

      Right.

      Nevertheless it is known to be incompatible with quantum mechanics and must differ from the true behaviour of matter in strong fields and at small spatial scales.

      OK.

      A key aspect of general relativity to test is the strong equivalence principle (SEP), which states that all freely falling objects, regardless of how strong their gravity, experience the same acceleration in the same gravitational field.

      Yes, and several such experiments have been performed, though this paper adds a new twist. OK.

      Essentially all alternatives to general relativity violate this principle at some level.

      Really? Does any theory of quantum gravity violate the SEP? I’m actually reading a book about quantum gravity at the moment (see above: I am not an expert) and my guess is that such an obvious test would have been mentioned. Yes, many classical alternatives to GR violate the SEP. But what about quantum gravity?

      • Phil Uttley Says:

        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).

      • Interesting. “We know that GR is not correct because it is not quantum” (which is probably true) is a common trope, and quantum gravity is something really, really interesting, but it looks like the referee and I agree that, in this case, mentioning quantum gravity is at best a red herring.

      • I also spot two former colleagues from Jodrell Bank on the author list.

    • “This is the pre-refereeing version as required by Nature’s rules”

      I think that “Nature” should be in italics here. 😐

    • 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.

  4. Michel C. Says:

    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.

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