Einstein’s Theory Tested To Its Limit Using “Falling” Stars

Artist's impression of PSR J0337+1715. NRAO/AUI/NSF; S. Dagnello

One of the principles of gravity that has remained unchanged since the beginning of modern science is that two objects dropped at the same time from the same height will reach the ground together, even if they have different masses. Galileo allegedly demonstrated it by dropping two different cannonballs from the Tower of Pisa. Astronaut David Scott did it on the Moon using a hammer and a feather. Now researchers have used two white dwarfs and a neutron star. Their results are published in Nature.

The system in question is PSR J0337+1715. It is located 4,200 light-years away and is composed of a neutron star orbited by a white dwarf every 1.6 days. The pair is orbited by a second white dwarf that goes around them every 327 days. The neutron star emits pulses 366 times per second, making it an ideal clock for astronomers.

“We can account for every single pulse of the neutron star since we began our observations,” lead author Dr Anne Archibald, from the University of Amsterdam and the Netherlands Institute for Radio Astronomy, said in a statement. “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.”

PSR J0337+1715 is a perfect testbed for Einstein’s general relativity. According to the theory, extremely massive and dense objects like a neutron star should “fall” just like less massive objects, in this case the white dwarfs. But alternatives to Einstein’s theory of relativity suggest a different scenario: heavier objects behave differently than lighter ones.

These observations show that your mass doesn’t matter. Gravity always works the same way, at least to three parts in a million. This result is 10 times more precise than the previous test of the theory. The principle tested here is called the Strong Equivalence Principle.

The precise observations were possible thanks to the Green Bank Telescope (GBT) in Virginia. With this powerful instrument, the researchers were able to conduct the meticulous calculation needed to get this result.  

“As one of the most sensitive radio telescopes in the world, the GBT is primed to pick up these faint pulses of radio waves to study extreme physics,” co-author Dr Ryan Lynch from the GBT added. “This is a unique star system. 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 team will look for similar or more extreme systems to continue to test general relativity to its limits.

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