Astronomers have discovered the first pulsar with two stars circling it. By watching the three objects orbit one another, observers will soon be able to perform the best-ever test of the "strong equivalence principle", which is a key prediction of Albert Einstein's general theory of relativity.

Like the Newtonian theory of gravity that came before it, Einstein's general theory of relativity says that gravity does not discriminate: it accelerates all objects equally, no matter what their size, shape or composition. Apollo 15 astronaut Dave Scott demonstrated this so-called equivalence principle on the Moon in 1971 by dropping a hammer and a falcon's feather, which hit the lunar surface simultaneously.

The strong equivalence principle of general relativity goes further, saying that gravity should accelerate all objects in the same way even if the objects hold themselves together with their own gravity. In other words, the gravitational self-energy that binds a planet or star together should have no effect on how it is accelerated. This is unlike theories that seek to topple general relatively and predict a small deviation related to gravitational self-energy called the Nordtvedt effect.

Three-body test

The most exacting test of the strong equivalence principle performed so far involves tracking the Earth and the Moon. As they orbit the Sun, both are continually falling through the solar gravitational field. Einstein's theory says that the Earth and the Moon should behave the same, even though the Earth has greater self-gravity. Precise laser-ranging measurements of the distance between the two bodies back this up by revealing no evidence of the Nordtvedt effect.

"The problem with tests of the strong equivalence principle here in the solar system is that none of the objects is strongly self-gravitating," says Scott Ransom of the National Radio Astronomy Observatory in Charlottesville, Virginia. In contrast, a pulsar is ideal. It arises when a massive star explodes and collapses; it is typically just 20 km across but about 50% more massive than the Sun, so its gravity strongly binds it together.

Now, Ransom and colleagues have discovered a pulsar named PSR J0337+1715 that will put Einstein to the test, thanks to the two stars that circle it. All pulsars spin fast, but this one, located 4200 light-years away in the constellation Taurus, spins especially quickly. It is a millisecond pulsar and each second it spins 365.953363096 times. Knowing its period to this incredible precision makes the pulsar an outstanding clock that astronomers can exploit.

Extraordinary and very rare

By recording when the pulsar's pulses reach Earth, Ransom's team discovered small delays caused by the gravitational tugs of two companion stars. Many millisecond pulsars have one stellar companion, which has dumped material onto the pulsar and spun it up to high speed. But astronomers have never before found a pulsar with two stellar companions. "It's really in a pretty extraordinary and very rare system," says Ransom.

Both companions are white dwarfs, which have weaker self-gravity than the pulsar. Both are larger than the Earth but less massive than the Sun. One white dwarf is much closer to the pulsar than Mercury is to the Sun and orbits it every 1.629401788 days. The other white dwarf is about as far out as the Earth is from the Sun, circling the pulsar and the inner white dwarf every 327.257541 days.

If I was going to hope, I would hope that we show that [the strong equivalence principle] is wrong
Scott Ransom, National Radio Astronomy Observatory

The pulsar and the inner white dwarf can be thought of as Scott's hammer and the feather: both are falling through the gravitational field of the outer white dwarf. Although the pulsar has much greater self-gravity, Einstein says both it and the inner white dwarf should respond in exactly the same way.

By carefully monitoring delays in the pulsar's pulses, Ransom and colleagues are currently tracking the exact positions of all three objects. "Very, very soon we are going to be able to make tests of the strong equivalence principle that are orders of magnitude better than anyone's been able to do before," says Ransom, who expects a verdict within a year. "If I was going to hope, I would hope that we show that it's wrong. That would be a really great thing to move science forward."

Not the final word in gravity

So far, Einstein's theory has passed every test. "But general relativity is probably not the final word in gravity, since it doesn't mesh well with quantum mechanics," Ransom says. "So eventually, in some deep dark corner of parameter space, it's probably going to fail. This could be it; we just don't know yet."

Clifford Will, a physicist at the University of Florida in Gainesville and author of the book Was Einstein Right?, calls the new pulsar the greatest test the strong equivalence principle has ever faced. "I hope Einstein prevails," he says. "General relativity is so unbelievably simple by comparison [with alternative theories] that to me it just has the feeling that it has to be right, down to the quantum level." Einstein formulated general relativity in 1915, and Will says a confirmation in 2015 "would be a great 100th birthday present for Einstein's theory".

The discovery is described in Nature.