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Triple Star System Provide Confirmation Of The Principle Of Universality Of Free Fall

A pulsar orbited by a white dwarf star, which are both orbited by another white dwarf, provide confirmation of Einstein’s Theory of Relativity. 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 it was never tested before until now.

Scientists have never been able to fully test this Principle. Because showing that all objects do accelerate the same, no matter how strong the external gravitational field has some special requirements. Thanks to a unique triple star system, this key prediction of Einstein’s theory has passed one of the most rigorous tests ever.

An international team of astronomers conducted the test by combining 818 observations over six years from 3 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 in the journal Nature. 

This triple star system contains three stars: A pulsar orbited closely by a white dwarf star, which is orbited by another white dwarf that is about 1 AU away, which is the same distance between Earth and the sun. Scientist investigated this Triple Star System to measure the influences of the pull of the outer white dwarf over the pull of the outer white dwarf.

Lead author Anne Archibald, a postdoctoral researcher at the University of Amsterdam, said that this is the only pulsar known to be in a system with two other stars. Triple systems are very delicate and very few survive the supernova explosion that creates the pulsar. It was the discovery of this unique system that spurred this test of Einstein’s theory. To do this test, they needed a pulsar, which has regular radio pulses with 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 delicate measurements.  They were measuring if the pulsar moved the same way as the inner white dwarf or not. They made frequent observations taken with the Westerbork Synthesis Radio Telescope in the Netherlands. The less frequent but long (10-hour) observations were made 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 these three telescopes allowed them to cross-check them against each other Which was very essential to confirm that test was giving correct results.

During their observation they ran into many changes, for example, every March line of sight to the pulsar passes within 2.1 degrees of the sun. The solar wind at that point introduces delays in the radio signals they observe. Unfortunately, the solar wind flows out in different directions and different amounts on different days, so compensating for these delays was difficult.

The Pulsar was observed with radio telescope but the observations of inner white dwarf were made with the optical telescope. By there optical observation they measured the motion of the inner companion’s orbit by measuring the Doppler shifts of the white dwarf’s spectrum.

Archibald said, they did not detect any difference between the accelerations of the neutron star and inner white dwarf, and if there is a difference, it would be no more than three parts in a million.

They couldn't drop the stars off a tower, but as the two inner objects move around their orbit with the outer companion, they are continually falling toward it. If the pulsar experienced a different acceleration from the white dwarf, its orbit would be shifted in a way they could detect.

In Einstein's theory, gravity itself has mass, so an object with really strong gravity could behave differently, In fact, once you have an object with strong gravity, Einstein's theory is almost the only one where objects with strong gravity fall the same way as normal objects. So, this is why we needed to use a pulsar

Astronomy is a wonderful way to find out what's out there in the universe, but this sort of observation is the only way to improve our understanding of a force as fundamental as gravity.

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