Einstein’s theory of gravity, general relativity, predicts that all objects fall in the same way, regardless of their mass or composition. But does this principle also hold for objects with extreme gravity? An international team of astronomers have tested this using three stars orbiting each other: a neutron star and two white dwarfs. Their findings, published in Nature on 5 July 2018, prove that Einstein’s theory still passes the test in such extreme conditions.
A hammer and a feather fall with the same acceleration on the Moon. And a light cannon ball hits the ground at the same time as a heavy cannon ball when dropped off the leaning tower of Pisa. Even the Earth and the Moon fall in the same way towards the Sun. Einstein’s theory of gravity has passed all tests in laboratories and elsewhere in our Solar System. But most alternative theories predict that objects with extreme gravity, like neutron stars, fall a little differently than weak gravity objects.
Luckily, astronomers have found a natural laboratory to test this theory in extreme conditions: the triple star system called PSR J0337+1715, located 4200 light years from the Earth. In this unique system, discovered in 2012, a neutron star is in a 1.6-day orbit with a white dwarf, and this pair is in a 327-day orbit with another white dwarf further away. If alternative theories of gravity are correct, then the neutron star and the inner white dwarf will fall differently towards the outer white dwarf.
'We were able to measure this by looking at the neutron star alone,' explains first author Anne Archibald, a postdoctoral researcher of the University of Amsterdam and ASTRON, the Netherlands Institute for Radio Astronomy. 'The neutron star, a millisecond pulsar, behaves like a clock: it rotates 366 times per second, and beams of radio waves rotate along with it. They sweep over the earth at regular intervals, producing pulses like a cosmic lighthouse. We have used these radio pulses to track the motion of the neutron star.'
The team of astronomers followed the neutron star for six years using the Westerbork Synthesis Radio Telescope in the Netherlands, the Green Bank Telescope in West Virginia, US, and the Arecibo Observatory in Puerto Rico, US. 'We can account for every single pulse of the neutron star since we began our observations,' says Archibald. 'And we can tell its location to within a few hundred meters. That is a really precise track of where the neutron star has been, and where it is going.' If the neutron star fell differently from the white dwarf, the pulses would arrive at a different time than expected.
Archibald and her colleagues found that any difference between the accelerations of the neutron star and white dwarf is too small to detect. 'If there is a difference, it is no more than three parts in a million,' says Nina Gusinskaia, PhD student at the University of Amsterdam. 'Now, anyone with an alternative theory of gravity has an even narrower range of possibilities that their theory has to fit into, in order to match what we have seen. Also, we have improved on the accuracy of the best previous test of gravity, both within the Solar System and with other pulsars, by a factor of about ten.'
'This much more stringent test of gravity was made possible by our discovery of this wonderful triple star system,' says Jason Hessels, Associate Professor at ASTRON and the University of Amsterdam. 'We currently don't know of any other natural laboratories that are as promising, but the upcoming biggest radio telescope in the world, the Square Kilometre Array, is expected to find most of the detectable pulsars in our galaxy, including ten times as many millisecond pulsars as are now known. Among these yet undiscovered systems may lurk even more powerful tools for understanding the Universe: unusual binaries, other triple star systems or a pulsar orbiting a black hole. Perhaps one of these may provide our first peek at a theory beyond Einstein's.'
Universality of free fall from the orbital motion of a pulsar in a stellar triple system. Anne M. Archibald, Nina V. Gusinskaia, Jason W. T. Hessels, Adam T. Deller, David L. Kaplan, Duncan R. Lorimer, Ryan S. Lynch, Scott M. Ransom & Ingrid H. Stairs