Einstein’s general theory of relativity predicts that all objects fall at the same rate, regardless of their mass or composition. For instance, if we drop a marble and a cannon ball off the Leaning Tower of Pisa at the same time, both objects will hit the ground at the same time, despite the fact that the cannon is a heavier object.
As far as we know, gravity works according to Einstein’s general theory of relativity because it has been proven correct on Earth over and over again. However, the theory has not been well-tested on very great distances and at large astronomical scales. Scientists did not know whether Einstein’s theory also holds true for objects with extreme gravity.
Alternate theories predict that objects with extreme gravitational environment, like neutron stars, fall a little differently than objects of lighter mass. When researchers tested the theory in extreme conditions in a triple star system, they found that gravity still works the same way as it does on Earth.
The test subject, called PSR J0337+1715, is located about 4,200 light-years from Earth. The rare trio was discovered in 2011 and consists of a neutron star accompanied by a white dwarf. The pair orbits a common center of mass every 1.6 Earth days. It is also in a 327-day orbit with another white dwarf, which lies much farther away.
“This is a unique star system,” said co-author Ryan Lynch from Green Bank Observatory in West Virginia. “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.”
The triple system has been observed regularly by ground-based telescopes. Their focus is the neutron star. This particular neutron star is actually a pulsar which spins rapidly, 366 times per second and emits X-rays and radio waves. These radio pulses were used to track the position of the neutron star.
“We can account for every single pulse of the neutron star since we began our observations,” said principal author Anne Archibald of the University of Amsterdam. “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 alternatives to Einstein’s theory of gravity were correct, the neutron star fell differently from the white dwarf and the pulses would arrive at a different time than expected. But such kind of difference has not been observed so far.