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Einstein's understanding of gravity, as described in his general theory of relativity, predicts that all objects fall at the same rate, regardless of their mass or composition.
This theory has passed a test test here on Earth, but scientists have wondered if this remains true for some of the most massive and dense objects in the known universe, an aspect of the known nature under the name of Principle of Strong Equivalence.
The new findings, published in the journal Nature, show that Einstein's ideas on gravity continue to dominate even in one of the most extreme scenarios.
To date, Einstein's equations have passed all tests, from minute laboratory studies to observations of planets in our solar system.
However, alternatives to Einstein's general theory of relativity predict that compact objects with extremely high gravity, like neutron stars, fall a little differently from objects of lesser mass.
This difference, predicted these alternative theories, would be due to what is called the gravitational bonding energy of a compact object, the gravitational energy that holds it together.
In 2011, the National Bank Telescope (GBT) of the National Science Foundation (NSF) discovered a natural laboratory to test this theory under extreme conditions: a triple-star system called PSR J0337 + 1715, located at around 4,200 years -light of the Earth.
This system 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.
"This is a unique star system," said Ryan Lynch of the Green Bank Observatory in the United States.
"We do not know any other similar, which makes it a unique laboratory to test Einstein's theories," he said.
Since its discovery, the triple system has been observed regularly by the GBT, the Westerbork Synthesis Radio Telescope in the Netherlands and the NSF's Arecibo Observatory in Puerto Rico.
If the alternatives to Einstein's gravitational image were correct, then the neutron star and the inner white dwarf would each fall differently to the outer white dwarf.
"The inner white dwarf is not as massive or compact as the neutron star, and therefore has less gravitational binding energy," said Scott Ransom, an astronomer of the National Observatory of Radioastronomy in the United States.
Through meticulous observations and careful calculations, researchers were able to test the severity of the system using neutron star pulses alone.
They found that any difference in acceleration between the neutron star and the inner white dwarf is too small to be detected.
"If there is a difference, it is not more than three parts in a million," said Nina Gusinskaia of the University of Amsterdam in the Netherlands.
This puts severe constraints on all alternative theories to general relativity, the researchers said.
The result is ten times more accurate than the previous best test of gravity, which makes Einstein's proof of the strong equivalence principle much stronger, they said.
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