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"exec" writes:

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

Original Submission