Einstein's theory of relativity proven right in huge space experiment

Gravity works as expected, study proves – even at the most vast of scales

Gravity works as expected, even at the biggest scales, a new study has found.

The research proves Einstein right at an almost unimaginably huge scale and in the most stringent test yet.

Einstein's theory of general relativity says that all objects fall the same way, despite their mass or their composition. That means for instance that an apple and a cannonball, falling from a tower, will reach the ground at the same time.

But some worried that might not always be true. In cases of extreme gravity, it was thought, alternative kinds of gravitational behaviour might be observed.

By testing the theory in perhaps the most extreme environment possible – in a massive three-star system – the theory still applies.

"This research shows how routine and careful observation of distant stars can give us a high-precision test of one of the fundamental theories of physics," said Ingrid Stairs, professor in the department of physics and astronomy at UBC and a co-author of the study.

The research was done in what scientists referred to as a "natural laboratory" – a triple star system called PSR J0337+1715, located about 4,200 light-years from Earth. The extreme conditions there serve as a useful way for scientists to perform tests in such situations.

"This is a unique star system," said Ryan Lynch of the Green Bank Observatory in West Virginia, and coauthor on the paper. "We don't know of any others quite like it. That makes it a one-of-a-kind laboratory for putting Einstein's theories to the test."

In that system is a neutron star in a 1.6-day orbit with a white dwarf star, and the pair in a 327-day orbit with another white dwarf further away. Scientists have been watching the system for years, and the neutron star sends out an incredibly reliable pulse that means it can be tracked with extreme precision.

If other theories of gravity were correct, then the neutron star and the inner white dwarf would fall towards the outer white dwarf differently to each other. Since the inner white dwarf is not as massive or compact as the neutron star, if the theories gravitational binding energy had an effect then it would be possible to see it from Earth.

The researchers didn't see any effect, suggesting that Einstein's theory of relativity holds true even at such vast scales. "If there is a difference, it is no more than three parts in a million," said coauthor Nina Gusinskaia of the University of Amsterdam.