Astronomers find more evidence that Einstein got gravity right. One of his predictions was that all objects fall the same way despite their mass or composition.

Until now Einstein’s equations have passed all tests. Even his theory of falling objects has passed tests here on Earth. But scientists weren’t sure if the equivalence principle applies in different situations, like with the most massive and dense objects in the known universe, for example.

However, an international team of astronomers proved Einstein’s theory. All objects fall at the same rate, regardless of their mass or composition.

The research shows that Einstein’s predictions work even in the most extreme scenarios in the Universe. Their findings were published in the journal Nature.

Astronomers tested the equivalence principle in a unique system. A system composed of two superdense stellar corpses known as white dwarfs and an even denser neutron star. Neutron stars are fast-spinning types known as pulsars. These objects are collapsed cores of stars that have undergone supernova explosions and are the densest stars in the Universe.

This triple star system called PSR J0337+1715 lies about 4,200 light-years from Earth. The pulsar, which rotates 366 times per second, co-orbits on the interior with one of the white dwarfs; the pair circles a common center of mass every 1.6 Earth days. This duo is in a 327-day orbit with the other white dwarf, which lies much farther away.

“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.”

Since its discovery, the GBT, the Westerbork Synthesis Radio Telescope in the Netherlands, and the NSF’s Arecibo Observatory in Puerto Rico have constantly observed the triple system. They have calculated how each object moves in relation to the other.

Many spinning neutron stars are pulsars. They send regular lighthouse-like electromagnetic signals out through space. Radio telescopes here on Earth can capture them.

“We can account for every single pulse of the neutron star since we began our observations,” study leader Anne Archibald, a postdoctoral researcher at 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.”

However, a violation of the equivalence principle would manifest as a distortion in the pulsar’s orbit. The neutron star and the inner white dwarf in PSR J0337+1715 would each fall differently toward the outer white dwarf.

“The inner white dwarf is not as massive or compact as the neutron star, and thus has less gravitational binding energy,” said team member Dr. Scott Ransom, an astronomer with the National Radio Astronomy Observatory.

So, through careful observations and calculations, the team was able to test the system’s gravity using the pulses of the neutron star alone. They found that any acceleration difference between the neutron star and inner white dwarf is too small to detect.

“If there is a difference, it is no more than three parts in a million,” said co-author Nina Gusinskaia of the University of Amsterdam. This places severe constraints on any alternative theories to general relativity.

So, even compact objects with extremely strong gravity, like neutron stars, fall the same way as objects of lesser mass.

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Thumbnail image: An artist’s impression of the triple star system PSR J0337+1715. Credit: NRAO / AUI / NSF / S. Dagnello.