One of Albert Einstein's most famous theories just passed its most rigorous test yet thanks to a laboratory more than 4,200 light-years from Earth.

As outlined in his theory of relativity, Einstein's understanding of gravity predicts that all objects will fall at the same rate, independent of both composition and mass. Also called the "equivalence principle," it's a famous concept that was first explored in the early 17th century by Galileo and later expanded on by Johannes Kepler and Isaac Newton. In 1907, Einstein stood on the shoulders of these giants to formulate the math behind why and how the equivalence principle existed, using it to guide his development of general relativity.

As shown in this video from the Apollo 15 mission to the moon, even NASA astronauts tested the equivalence principle on the lunar surface.

Despite Einstein's theory of gravity passing countless tests in experiments on Earth and beyond, alternative theories were proposed arguing that objects with extreme density, such as collapsed neutron stars, would prove the exception.

In an effort to explore if the equivalence principle could pass its most extreme test yet, an international team of astronomers turned their attention to a star system discovered in 2012 and located some 4,200 light-years from Earth. Called PSR J0337+1715, it contains 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.

## The answer lies in density

So what makes this distant laboratory so special? As the researchers explain in a new paper published in the journal Nature, it all comes down to the difference in density between the neutron star and its neighboring white dwarf.

"This is a unique star system," Ryan Lynch of the Green Bank Observatory, and coauthor on the paper, said in a statement. "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."

Neutron stars are the smallest and densest stars, the result of a once-massive star collapsing on itself. How dense? It's estimated a normal matchbox-sized object containing neutron-star material would weigh an astounding 3 billion tonnes. By comparison, white dwarfs, remnants of stars that have exhausted their fuel, are slightly less dense.

If the alternative theories were correct, the difference in densities between these two giant objects should cause one to fall faster towards the outer white dwarf they orbit.

Over the course of six years, researchers used the Green Bank Telescope in West Virginia to record radio waves issuing from the neutron star and track its position in proximity to the inner white dwarf. The work is so precise that, even from a distance of thousands of light-years, they can track the location of the neutron star to within a few hundred meters.

After more than 400 hours of observation, they concluded any difference in acceleration between the two objects was simply too small to detect.

"These observations have limited the difference to be less than 3 parts in a million," study participant Duncan Lorimer, West Virginia University professor of physics and astronomy, said in a release. "This groundbreaking result limits the room for any alternative theories of gravity and has improved upon the best previous tests by a factor of about ten."

So whether it's a hammer and a feather or a neutron star and a white dwarf, objects really do fall at the same rate regardless of composition or mass. Decades after his passing, Albert Einstein's theories continue to hold firm even into the farthest reaches of space. As Lorimer states, however, there's one force yet to explore where things could yet get interesting.

"We know that the theory will ultimately break down when trying to describe the singularities of black holes," he added, talking about the one-dimensional point within a black hole where the rules of physics as we know them don't apply. "However in the regime probed by these experiments, Einstein's theory reigns supreme."

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

Distant star system proves Einstein right again
New research shows that even massive neutron stars fall at the same rate, regardless of their mass or composition.