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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 mbad or composition. This theory has pbaded a test after the other here on Earth, but is it still true for some of the most mbadive and dense objects in the known universe, an aspect of nature known as the Principle of Strong Equivalence? An international team of astronomers has given this persistent question its most rigorous test. Their findings, published in the journal Nature show that Einstein's ideas about gravity continue to dominate even in one of the most extreme scenarios that the Universe can offer .
Removes all the air, and a hammer and a feather will fall at the same rate – a concept explored by Galileo in the late 1500s and famously illustrated on the Moon by the astronaut. Apollo 15 David Scott.
Newtonian physics, it took Einstein's gravity theory to express how and why this is so. To date, Einstein's equations have pbaded all tests, from minute laboratory studies to observations of planets in our solar system. But alternatives to Einstein's theory of general relativity predict that compact objects with extremely high gravity, such as neutron stars, fall a little differently from objects of lesser mbad. This difference, predicted these alternative theories, would be due to the so-called gravitational bonding energy of a compact object – the gravitational energy that holds it together.
In 2011, the National Bank Foundation (GBT) National Bank Telescope (GBT) discovered a natural laboratory to test this theory in extreme conditions: a three-star system called PSR J0337 + 1715, located at about 4 200 light years from 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 star system unique, "said Ryan Lynch of the Green Bank Observatory in West Virginia, and co-author on the paper. "We do not know any other alike, making it a unique laboratory for testing Einstein's theories."
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. The GBT has spent more than 400 hours observing this system, taking data and calculating how each object is moving relative to the other.
How did these telescopes study this system? This particular neutron star is actually a pulsar. Many pulsars rotate with a constancy that rivals some of the most accurate atomic clocks on Earth. "As one of the most sensitive radio telescopes in the world, the GBT is ready to capture these weak radio wave pulses to study extreme physics," Lynch said. The neutron star of this system pulsates (rotates) 366 times per second
"We can explain every pulse of the neutron star since we started our observations," said Anne Archibald of the University of Amsterdam and the Dutch Institute of Medicine. radio astronomy and lead author on paper. "We can say a few hundred meters near where the neutron star is and where it is going."
If the alternatives to Einstein's gravity image were correct, then the neutron star and the inner white dwarf would fall differently to the outer white dwarf. "The internal white dwarf is not as mbadive or compact as the neutron star, and therefore has less gravitational binding energy," said Scott Ransom, astronomer at the Observatory. National Radioastronomy Institute in Charlottesville, Virginia.
Through meticulous observation and careful calculations, the team was able to test the severity of the system using the pulses of the neutron star 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 does not exceed three parts in a million," said co-author Nina Gusinskaia of the University of Amsterdam. This imposes severe constraints on all alternative theories to general relativity.
This result is ten times more accurate than the previous best gravity test, which makes Einstein's proof of the Strong Equivalence Principle much stronger. "We are always looking for better metrics in new places, so our quest to learn new frontiers in our Universe will continue," concluded Ransom.
Learn more:
Stronger tests of Einstein's theory of general relativity with binary neutron stars
More information:
Anne M. Archibald et al, Universality of the free fall of the orbital motion of a pulsar in a triple stellar system, Nature (2018). DOI: 10.1038 / s41586-018-0265-1
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