Testing the theory again

General Relativity passes another test of its predictions.  Why bother?

Our astronomer has been perusing a recent paper concerning the motion of stars in the center of our galaxy.  The work demonstrates a number of things about how astronomy operates nowadays, as well as how science in general works.

First, some background for those who haven’t been studying the subject.  The Milky Way galaxy, a vast flattened collection of stars to which our own Sun belongs, rotates around a center about twenty-five thousand light-years away.  At the very center is an object called Sgr A*, pronounced “Sagittarius A-star,” which is very probably a black hole with a mass some four million times that of the Sun.  Around Sgr A* there are many stars in orbit, some closer and some farther away.  We know this not by looking with telescopes in visible light, because the dust and gas around the Galactic center hide details from our sight.  Any observations have to be conducted in mid-infrared or longer wavelengths.

One of the stars around Sgr A* goes by the unassuming name of S2.  It has a highly eccentric orbit, taking it very close and outward again.  Now, normally the orbits of stars are a matter for observation over many decades, because if they’re far enough from each other to be seen separately, they take a long time to go through their motions.  But since Sgr A* is so massive S2 has to move very quickly, and it goes through an orbit large enough to be seen from Earth in some sixteen years.

Seen, but only by means possible in recent years.  The authors observed in the infrared, as I mentioned.  They also made use of adaptive optics, a technique (or a series of techniques) that removes much of the twinkling effect of our atmosphere.  And they used interferometry, taking advantage of the wave nature of light to see very small details.  Interferometry is routine among radio astronomers, but it’s much harder in the infrared and visible.  Indeed, it turned out to be so difficult that interferometers built into the generation of large telescopes of the 1990s were thought to have been largely failures.  (Perhaps that’s changing, as shown by this result.)

Another sign of the nature of modern astronomy is the list of authors, some 98 of them (listed in strict alphabetical order).  The collaboration collected data using three different instruments, and their results required all of them.

What are their results?  Briefly, that the orbit of S2 is accurately predicted by General Relativity, and entirely inconsistent with Newtonian mechanics.  This is a great success.

But why?  GR has already been proven to be accurate in many different places, in many different ways.  In much stronger gravitational fields, in smaller and larger scales, it has passed all the tests set for it.  (With the possible exception of dark matter, something we may write about in the future.)  Why do it again?

Well, in part because tests of GR are hard.  Newtonian physics is excellent for almost everywhere, under almost every set of conditions; it was not problems with experimental results that led Einstein to his equations a century ago.  New ways to test them are always interesting.  And this one uses a mass much larger than any previous test.  It was possible that some deviation from GR would show up here more plainly than elsewhere.

Theories are never actually proven.  They’re accepted until someone finds something better.