Space & Innovation Unique Triple Star System Tests Einstein’s Theory of Relativity A pulsar orbited by a white dwarf star, which are both orbited by another white dwarf, provide confirmation of the principle of universality of free fall.

Observations from the Westerbork Synthesis Radio Telescope in the Netherlands, along with observations from several other telescopes around the world, helped scientists confirm a principle of Albert Einstein's theory of relativity. | ASTRON Observations from the Westerbork Synthesis Radio Telescope in the Netherlands, along with observations from several other telescopes around the world, helped scientists confirm a principle of Albert Einstein's theory of relativity. | ASTRON

A crucial part of Albert Einstein's theory of relativity is based on a principle called the universality of free fall, which means that all falling objects accelerate identically, regardless of their mass or composition. But does the presence of extreme gravity change how objects move? Some alternative theories of gravity have suggested this might be so. Until now, however, scientists have never been able to fully test this question. Thanks to a unique triple star system, this key prediction of Einstein’s theory has passed one of the most rigorous tests ever, showing that all objects do accelerate the same, no matter how strong the external gravitational field. An international team of astronomers conducted the test by combining 818 observations over six years from three different observatories, making approximately 27,000 measurements of a star system named PSR J0337+1715, located about 4,200 light-years from Earth. Their findings were published today in the journal Nature. This triple star system contains three end-of-life stars: A pulsar orbited closely by a white dwarf star, which are, in turn, both orbited by another white dwarf that is about 1 AU away, which is the same distance between Earth and the sun. This system allows for an investigation of how the pull of the outer white dwarf influences both the inner dwarf and the companion pulsar, which has strong self-gravity. Lead author Anne Archibald, a postdoctoral researcher of the University of Amsterdam and ASTRON, the Netherlands Institute for Radio Astronomy, told Seeker that this is the only pulsar known to be in a system with two other stars. Triple systems are very delicate, she said, and very few survive the supernova explosion that creates the pulsar. And it was the discovery of this unique system that spurred this test of Einstein’s theory. “To do this test, we needed a pulsar, with its regular radio pulses and its incredible density," as well as other objects in the system, Archibald explained. “The pulsar — a rapidly rotating neutron star — rotates 366 times per second, and beams of radio waves produce pulses at regular intervals, and we can use these pulses to track the pulsar.”

If the pulsar and the inner white dwarf fall differently towards the outer white dwarf, then the pulses would arrive at a different time than expected. Archibald and her colleagues used three kinds of observations to make very sensitive measurements to determine if the pulsar moved the same way as the inner white dwarf. They made frequent observations taken with the Westerbork Synthesis Radio Telescope in the Netherlands, less frequent but long (10-hour) observations with the Robert C. Byrd Telescope at Green Bank, West Virginia, and short monthly observations with the very sensitive William E. Gordon Telescope at Arecibo, Puerto Rico. “Having all three of these telescopes allowed us to check them against each other,” Archibald said via email. “These cross-checks were essential to confirming that our test was giving correct results.” Their measurements were so sensitive that the team was hoping to be able to detect a deviation from Einstein's prediction as small as two meters. But they ran into challenges due to a number of complicated effects. “For example, every March our line of sight to the pulsar passes within 2.1 degrees of the sun,” Archibald said. “The solar wind at that point introduces delays in the radio signals we observe. Unfortunately, the solar wind flows out in different directions and different amounts on different days, so compensating for these delays was difficult.”

Anne Archibald is lead author of research confirming that Albert Einstein's principle of universality of free fall holds even under extreme gravity. | Cees Bassa