Astronomers are using the most extreme objects they can find to put Einstein's theory of general relativity to the test.
General relativity, Einstein’s breakthrough theory of the nature of gravity, has withstood every experimental challenge since he published it in 1916. No test has yet found it to be even the tiniest trace off course. But since general relativity doesn’t mesh with quantum mechanics — the other great foundation of modern physics, which governs the other three of the universe’s four fundamental forces — astronomers haven’t stopped trying to push general relativity beyond its limits in hopes of finding hints of something new.
At the 7th Harvard-Smithsonian Conference on Theoretical Astrophysics held May 14-17 in Cambridge, Massachusetts, astronomers described test after test of general relativity using extreme-gravity objects from pulsars to supermassive black holes. Here’s a small sampling.
If you want to push general relativity to its limits, you’ll need the strong gravity near compact, massive objects. Neutron stars fit the bill; they form when a star roughly the mass of our Sun collapses into a sphere with the density of an atomic nucleus and the size of Manhattan. Even better is when a spinning neutron star’s magnetic field whirls jets of radiation that sweep past Earth. The jets look like lighthouse beacons, flashing us as the pulsar rotates. Some pulsars spin ridiculously quickly — with periods down to a millisecond or two — and with fantastic regularity that matches or beats the best artificial atomic clocks.
The theorist’s real treasure, though, is to find two pulsars orbiting each other. One example of this holy grail goes by the unglamorous name PSR J0737-3039. Some 1,800 light-years away in Puppis, a pulsar rotating every 23 milliseconds (pulsar A) orbits another rotating every 2.8 seconds (pulsar B). In a scale model, they would be two marbles 750 feet apart. When pulsar A’s radio blips travel through the strong gravitational field around pulsar B on their way to us, the timing of this otherwise perfect clock is distorted in several ways predicted by general relativity. So far, general relativity has passed five different tests using this one binary, all with flying colors (S&T: August 2010). Read more about the binary pulsar.
Another test requires a more unevenly balanced system, such as the pulsar (PSR J1738+0333) that orbits a white dwarf only one-fifth the mass of the Sun. Any two massive objects orbiting closely will gradually spiral together, speeding up as the system emits energy in the form of gravitational waves. These gravitational waves have a certain pattern to them, one that is magnified in an asymmetric system like J1738+0333. Most alternative theories of gravity predict that the pattern should be dipolar, but the orbit of J1738+0333 is slowing in agreement with Einstein’s theory instead, which predicts quadrupolar gravitational waves. Read more about the pulsar – white dwarf binary.
If the mass asymmetry of J1738+0333 is a boon for theorists, imagine if astronomers found a pulsar orbiting the Milky Way’s central black hole, weighing in at 4 million times the mass of the Sun. Star-formation theories say that anywhere from 10 to 100 pulsars should be doing so. Scott Ransom (National Radio Astronomy Observatory) has been looking for them, but to no avail. His “last, best search” for these perfect clocks in relativistic orbits around the giant black hole will happen later this year. If he doesn’t find any, then not only do astronomers miss out on some fantastic tests of relativity, but they’ll also have to acknowledge that something is wrong with their understanding of how stars form in the center of the Milky Way.
Imaging Supermassive Black Holes
A very direct test of Einstein’s theory would be to actually see a black hole. It’s hard to believe that that once-distant goal is scheduled to become a reality by 2015. Shep Doeleman (Haystack Observatory) presented a status update on the Event Horizon Telescope, an array of millimeter-wave radio antennae that should image the silhouette of the Milky Way’s central black hole (S&T: February issue, page 20).
There are four phases to putting this array together. The Large Millimeter Telescope in Mexico is already operating, but the real excitement will start when the sensitive and powerful Atacama Large Millimeter/submillimeter Array (ALMA) in Chile is incorporated in 2015. The team would also like to add two more arrays, one at the South Pole (South Pole Telescope) and another in Europe (IRAM), to create longer baselines.
“This is not a ‘first-light’ instrument” where the whole telescope goes online at once and starts taking images, Doeleman said at the conference. “We have planned technical improvements that should result in big jumps in performance along the way.”
The black hole’s silhouette should actually be the size and shape of the “last photon orbit.” The hole, which is a bit smaller, will swallow any photon that orbits inside that radius. Once the Event Horizon Telescope is able to resolve the size and shape of the dark silhouette, the proof will be in the pictures — black holes exist. Then the real fun begins.
One test of general relativity is to look for the black hole’s “hair.” General relativity says black holes “have no hair,” which is physicists’ delightful way of saying that a black hole has only three properties that define it completely: its mass, spin, and electrical charge. In other words, no matter what you toss into a black hole — stellar matter, popcorn, the kitchen sink — no outside observer would be able to tell what you added, other than by the effect on the black hole’s mass, spin, and charge. It has no other characteristics whatsoever.
But some alternative theories of gravity suggest that black holes do have a bit more to show, if only a vanishingly small amount. If the Milky Way’s black hole is somehow hairy, that will affect the shape of the silhouette. So in 2015, we may see whether our local supermassive black hole is as bald as predicted.
The Future of Gravity
Personally, I don’t think they’ll soon find proof that general relativity is wrong — Einstein’s theory has passed every test for almost a century already. Proponents of alternative theories, by contrast, have had to modify their theories over and over to cope with observations. (For example, some alternative theories of gravity that were initially formulated to do away with the need for dark matter later had to incorporate a modified form of dark matter in their equations after all.)
But it’s clear that general relativity is not complete. Until gravity can be united with the electromagnetic, weak, and strong forces, astronomers will keep testing general relativity to its limit, hoping for clues to a Theory of Everything.