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posted by Fnord666 on Monday June 05 2017, @07:41PM   Printer-friendly
from the better-luck-next-time dept.

Arthur T Knackerbracket has found the following story:

Testing general relativity is a fraught business. The theory has proven to be so robust that anyone who thinks it's wrong gets slapped around by reality in a pretty serious way. The tests that we apply are also limited by our environment, in that we can only look at gravity with precision where it's rather weak: in the lab, or by tracking the motion of planets. That's a  whole range of scales and forces, but it doesn't cover where it might truly matter, which is right next to a black hole.

Observing orbits around a black hole would take a career's worth of measurements and, frankly, who has the time? It is also a rare benefactor who will fund a couple of decades worth of telescope time. Luckily, telescopes have been collecting data for a while, and some of that happens to include the vicinity of some black holes. Recently, some scientists decided to dig up the data and test general relativity in the vicinity of a supermassive black hole.

At the center of our galaxy, there lies a black hole, which like the Rabbit of Caerbannog, fiercely devours unwary wanderers. Nevertheless, there are a few foolhardy stars that orbit close to the rabbit black hole. These stars have orbits of just a couple of decades, and they experience rather large gravitational forces. So, astronomers expect that accurate observations of these stars might pick out deviations from general relativity.

Luckily, the Keck telescopes have been gathering data from the heavens for about 25 years, and over that time, they have turned their unblinking eyes towards the galactic center on numerous occasions. Each time, the observations were performed a bit differently. For instance, the telescopes were upgraded with adaptive optics in 2005, and some of the observations focused on obtaining spectral data rather than imaging. These latter data contain orbital velocity data, because the motion of the star causes a doppler shift in the observed colors of light.

All of this data was combined in a consistent way to map out the orbital positions and velocities of two stars. This is quite an achievement, because for each observation, the telescope is pointing in a slightly different direction, using different exposure times, and accounting for other slight differences. Although other telescopes also have data available, the public records were not detailed enough to allow the scientists to process the data in a consistent way. This is a pity, because, the data set consists of about 100 observations from just these two telescopes. Imagine what might have been obtained if more telescopes had accessible data?

After all of this, what have we learned? General relativity is still right, and it predicted the stellar motion accurately. These measurements tested general relativity in a way that was distinct from all previous ones—in high gravitational fields over long periods of time. In particular, the new measurements helped to put boundaries on extensions to general relativity that follow a kind of modified Newtonian dynamics model. In these models, there is a distance at which a new force becomes apparent, and that force has some unknown characteristic strength. So, astronomers are looking for a consistent distance at which there is a noticeable deviation from predictions. However, the measurements tell us that for any distance that is relevant to the orbit of these stars, a new force would have zero strength.

Or, more precisely, a new force would have a strength that is so small that we cannot yet measure it. Conclusion: general relativity wins again.

-- submitted from IRC


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  • (Score: 2, Insightful) by Anonymous Coward on Monday June 05 2017, @08:06PM (6 children)

    by Anonymous Coward on Monday June 05 2017, @08:06PM (#520947)

    The theory has proven to be so robust that anyone who thinks it's wrong gets slapped around by reality in a pretty serious way.

    This is such a stupid thing to say when the current GR-based models require 95% invisible stuff that has only been detected as deviations from GR predictions. I mean it really makes me doubt the whole cosmology effort is an honest one.

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  • (Score: 3, Interesting) by bob_super on Monday June 05 2017, @08:58PM (5 children)

    by bob_super (1357) on Monday June 05 2017, @08:58PM (#520979)

    The fact that it doesn't explain everything is a known limitation (like most of science).
    But everyone who's tried to prove it wrong has hit a wall...

    • (Score: 1, Interesting) by Anonymous Coward on Monday June 05 2017, @09:03PM (4 children)

      by Anonymous Coward on Monday June 05 2017, @09:03PM (#520982)

      But everyone who's tried to prove it wrong has hit a wall...

      Thank you for your inbox but it predicted a very specific thing about rotation curves, and this was false.

      Then a new model GR + DM was created. So far after much time and energy spent there is no evidence for DM except that GR predicts the wrong thing. Further, if anyone actually relied on the rotation curve estimates for anything they would use other models like MOND since GR + DM predicts nothing while MOND is perfect at predicting this (since the amount of "DM" can be 100% predicted from the amount of visible matter).

      • (Score: 2) by stormwyrm on Tuesday June 06 2017, @01:10AM (3 children)

        by stormwyrm (717) on Tuesday June 06 2017, @01:10AM (#521094) Journal

        Yeah, MOND fits galaxy rotation curves. It was designed to do so. But what happens when you try to apply it to stuff bigger than a single galaxy? Galaxy clusters, large scale structure formation, CMB anisotropies, for all of these, MOND theorists have been forced to resort to some fudge factors that essentially amount to the postulation of some form of dark matter! Galactic cluster collisions show gravitational lensing where there is no visible matter [wikipedia.org]. Large scale structure formation theory informed by dark matter also predicted that galaxy clusters ought to start appearing at redshifts z=2.5, about 11 billion light years away, and that has been borne out by observations [forbes.com]. Dark matter also fits the peaks in the CMB power spectrum [discovermagazine.com]. There is plenty of indirect evidence for dark matter, and with so many mutually supporting lines of evidence pointing to its reality and the continuing failure of any alternative theories to explain all the available data we have today any better or even equally well is why Lambda-Cold Dark Matter is at present considered the standard cosmological model.

        --
        Numquam ponenda est pluralitas sine necessitate.
        • (Score: 2, Interesting) by FatPhil on Tuesday June 06 2017, @01:27PM (2 children)

          by FatPhil (863) <pc-soylentNO@SPAMasdf.fi> on Tuesday June 06 2017, @01:27PM (#521306) Homepage
          MOND makes the model fit measured rotation curves by letting you chose a single parameter that makes the curves seem to fit the model. There's one fudge-factor.

          DM makes the model fit measured rotation curves by letting you chose where you drop dollops of invisible stuff in order to make the curves seem to fit the model. That's an infinitude of fudge-factors.

          Dark Matter is at present considered the standard cosmological model because there is a scientific cartel which is rejecting (quite literally) studies into alternative theories. How many quantised inertia papers have you encountered in the last year, for example? How many DM proponents could even finish this sentence: "DM can be disproved by ______"? How many of them who can finish that sentence are actually attempting to disprove their theory, and how many are simply riding the confirmation-bias train? The parameter space for what DM can be is ever-shrinking, rejecting possible candidates with every new result. And Nima Arkani-Hamed still hasn't eaten his hat, like he promised, when the experiment which he was sure would confirm his N-th model of Dark Matter (which had to be right, as it was so beautiful), actually refuted it - instead, he came up with model N+1. For "science"!
          --
          Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
          • (Score: 2) by stormwyrm on Wednesday June 07 2017, @02:03AM (1 child)

            by stormwyrm (717) on Wednesday June 07 2017, @02:03AM (#521707) Journal

            DM can be disproved in any number of ways. There was a chance with Lambda-CDM structure formation theory, which predicted the formation of galaxy clusters at around z=2.5. If someone found galaxy clusters coming together at a much greater or smaller distance, that would have been a big blow to dark matter theory. But they saw [soylentnews.org] exactly what the theory predicted. Stacy McGaugh had a chance when he discovered the SPARC acceleration law [soylentnews.org], but then it was seen [soylentnews.org] that simulated galaxies assuming Lambda-CDM cosmology were already exhibiting the same acceleration law too. There was also a big prediction from dark matter theory that McGaugh’s SPARC acceleration law would fail to hold at great distances. If someone managed to measure the rotation curves for very distant galaxies and showed that they still obeyed the acceleration law, that would have gone a very long way towards falsifying dark matter and making MOND look a lot more plausible. However, when someone managed to do that, they found that yes, indeed, these young, distant galaxies do not seem to fit the acceleration law. [soylentnews.org] It’s only a three-sigma effect so far so the jury’s still out and we’ll probably need better telescopes like the JWST or Magellan to get really robust results to settle this one way or the other.

            If someone does manage to make an observation or experiment that contradicts current theory, they’ll try to revise the old theory, but it may get to the point that a better explanation can be made, and a new and better theory comes forward and becomes dominant. That’s generally how science progresses. That’s how we got from Ptolemaic astronomy to modern celestial mechanics. That doesn’t yet seem to be what is happening with dark matter and current theories of modified gravity though.

            But I suppose there’s no convincing you if you actually believe that there’s a huge conspiracy of scientists out to suppress these alternative theories. The only thing that’s suppressing these alternative theories is their lack of agreement with observation and experiment. I don’t think Stacy McGaugh, Mordehai Milgrom, or Erik Verlinde have had any problems getting their papers on alternatives to dark matter published. I think it’s a good thing that people are looking at alternatives, and just maybe they might be able to explain things better than the current theory, and then they’ll become the dominant theory. But that day hasn’t come, and so for now dark matter remains part of the standard cosmological model.

            --
            Numquam ponenda est pluralitas sine necessitate.
            • (Score: 2) by FatPhil on Wednesday June 07 2017, @08:13AM

              by FatPhil (863) <pc-soylentNO@SPAMasdf.fi> on Wednesday June 07 2017, @08:13AM (#521806) Homepage
              Independent of whether you think the guy's theory is loonie-tunes, do you think this is appropriate behaviour for a scientific journal *group*:
              http://physicsfromtheedge.blogspot.com.ee/2017/01/a-taste-of-kafka.html
              --
              Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves