Right Again, Einstein

3 July 2008 (All day)

Daniel Cantin/McGill University

It's relative. Astronomers have been measuring spin precession in an eclipsing pair of pulsars.

As if his reputation needed cementing, astronomers have confirmed Albert Einstein's status as a supergenius once more. Studying a unique pair of pulsars--small and extremely dense leftovers from supernova explosions--researchers have measured an effect that was predicted by Einstein's 92-year-old general theory of relativity. The result, they report tomorrow in Science, is almost exactly what the famous physicist had foreseen.

In Einstein's relativistic universe, matter curves space and slows down time, and the speed of light remains the only constant. But those are the big effects. The theory of relativity also includes some more esoteric details, one of which is called spin precession. The idea goes like this: Two massive bodies orbiting near each other will warp space enough to disturb the central axis around which both are moving, causing them to begin wobbling just like spinning tops. Strong gravity creates this so-called precession, and the more massive the objects, the easier the precession is to observe.

It's not an easy theory to test. Researchers need two very dense objects orbiting very close together, and they have to be able to detect what is going on between them. Black holes are dense, but their event horizons preclude observations. The lack of candidates and telescopic power had frustrated astronomers for years, until the discovery in 2003 of a particular pair of pulsars. These asteroid-sized objects pack sunlike masses, extremely small orbits, and incredibly fast spins. They also emit powerful and ultraregular radio signals that are easily detectable with Earth-based dishes. Most important in this case, one pulsar eclipses the other briefly every couple of hours. That's key to detecting precession, because during each eclipse astronomers can determine the precise angle of the radio signal and therefore the pulsar's wobble over time.

For the past 4 years, an international team has been carefully tracking the signals of one of the pulsars and monitoring the signals' direction during eclipses--a observational technique "that has never been employed before," says astrophysicist and co-author Rene Breton of McGill University in Montreal, Canada. The researchers determined that the precession of the pulsar's orbital axis advances by 4.77 degrees per year, plus or minus 0.66 degrees. Calculations based on Einstein's theory predicted it should advance by 5.07 degrees per year, well within the margin of error.

"It's bang-on," says astrophysicist and Nobel laureate Joseph Taylor of Princeton University. "Einstein's theory passed the test this time," agrees astrophysicist Fotis Gavriil of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who praises the study's "amazing high-precision measurement." So is Einstein's reputation secure? Says Gavriil, "Only with experiments like this will we know for sure."

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