Australasian Science: Australia's authority on science since 1938

Stellar Triplet Weakens Rival Relativity Theories

Astronomers observing the complicated orbital dance of three compact stars have concluded that Einstein’s theory of general relativity still can’t be proven wrong.

Einstein’s strong equivalence principle says that all objects should fall the same way in a gravitational field, regardless of their composition or how dense they are. The triple-stellar system discovered in 2012 was considered the perfect system to test the theory due to the substantial differences in the make-up of the three stars. “This particular system consists of one ultra-dense neutron star and two less-dense white dwarf stars, which makes these stars the dream team for testing relativity,” said Dr Adam Deller of Swinburne University.

Making the whole test possible was the fact that the neutron star can be observed as a radio pulsar, meaning the bright beam of electromagnetic radiation that it emits can be detected from Earth. “The radio pulsar star acts like a clock on the sky. It spins in a very predictable way, and each time it sweeps past the Earth we see a little blip of radio emission, which we can treat like the ticks of a clock.”

After 5 years of intensive observation of the system, the international research team concluded in Nature ( that the theory of general relativity remains relevant.

According to Dr Anne Archibald, the principal author of the paper from the University of Amsterdam and The Netherlands Institute for Radio Astronomy, the researchers can account for every single pulse of the neutron star since they began the observations. “We can tell its location to within a few hundred metres. That is a really precise track of where the neutron star has been and where it is going,” Archibald says.

This precise measurement of the pulsar’s location is the tool used to test the strong equivalence principle. “By tracking the motion of the pulsar via pulsar timing, we can tell whether it, and its nearby less-dense companion, are both falling towards the third and more distant star in the same way as general relativity predicts, and we couldn’t detect any difference,” Deller says.

Since Einstein came up with the theory of general relativity, scientists have tried to come up with a better, alternative explanation for how gravity works. A whole range of these competing theories have been severely constrained by the findings of this test.

“The theory of general relativity has fixed parameters that can’t be changed, whereas other theories of gravity have parameters whose values can change to keep them consistent with this result,” Deller says. “The results of this research shrink the amount of wiggle room available to alternative theories of gravity. We still don’t know if general relativity is correct, but now we know that the right gravitational theory must look even more similar to general relativity that we previously thought.”