Right again, Einstein
ALBERT Einstein (1879-1955) is without doubt one of the greatest names in science, and is best known for his theories of relativity.
I write this in the plural sense, because the first one, announced in 1905, is called the Special Theory of Relativity, and it was in 1916 that his General Theory was published.
The Special Theory relates to the strange effects that arise when objects move at very high speeds; for example, “moving clocks run slow” is a famous result of this. The General Theory, however, is a theory of gravity that is quite different from the way envisaged by Isaac Newton centuries earlier.
Effectively, it describes gravity as a distortion in “space-time”, in which matter causes a bending of space, which in turn results in the way in which matter moves.
So far, everything that we have observed has supported Einstein’s ideas. This has continued recently with a further test involving the motion of a star close to the giant black hole at the centre of our galaxy.
The centre of the galaxy is in the direction of the constellation of Sagittarius, which is currently seen in our eastern evening sky.
If you are able to observe the starry sky from a location well away from the city lights, the view of the Milky Way — formed by the light of countless stars in the nearby spiral arms of the galaxy — is magnificent. The Milky Way is relatively bright in the direction of Sagittarius, because of a large star cloud that happens to be roughly in line with the direction of the centre of the galaxy.
We have known of the existence of this black hole for quite some time. You won’t see the black hole, of course, but astronomers with highly sophisticated equipment operating at the Very Large Telescope — that is its actual name, although it consists of four separate telescopes — in Chile have been studying a star called S2. It is one of many orbiting around the black hole, and it completes each orbit every 16 years in a very elongated, or oval-shaped, path.
At its closest, S2 approaches the black hole to within about 20 billion kilometres, and this most recently happened on May 19. Comparing this distance with the size of our solar system, that’s about four and a half times the distance between the sun and Neptune, the most distant of the sun’s planets. It sounds like a long way to us, but that’s close on astronomical scales!
One prediction astronomers were hoping to verify was that the star’s light would be shifted in wavelength — making it look slightly redder — by being immersed in the strong gravitational field. It turns out that the observations matched the theory very well.
Einstein’s equations also predict a very particular change in the position of the star’s closest point to the black hole.
They were used to explain the same phenomenon happening with the planet Mercury in our solar system, and it is hoped that as the star moves away from the black hole, observation of its motion will verify that prediction, too.
This is the first time that General Relativity has been put to the test in such extreme conditions as a star orbiting a black hole.
It has been another important test of the theory, which has once again come through with flying colours. A few years ago, a different prediction — the existence of gravitational waves — was also found to be correct.
I think it is quite remarkable that Einstein, so long ago, came up with such a fine explanation of effects that were still to be observed, some of which have been verified decades after his death.
It is sad that he never lived to see these wonderful results we are obtaining today, but he did at least receive great satisfaction in 1919, when his prediction that light would be bent around a massive object was shown to be correct during a total eclipse of the sun.