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.”
They compensated, but realized they could only could only reliably detect a deviation from Einstein’s predictions as big as 30 meters.
“Fortunately, 30 meters was still a very stringent test of Einstein’s theory,” Archibald said.
While the pulsar was measured with radio observations, the team measured the motion of the inner companion’s orbit based on optical observations, measuring the Doppler shifts of the white dwarf’s spectrum, the same way some exoplanets are found.
The effect of any deviation from Einstein’s gravity would be very distinctive, the team said, and they could see that signature from only the measurements of the pulsar’s motion.
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, Archibald said.
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The team wrote in their paper that previous tests of this principle using objects in our own solar system have been limited by the weak self-gravity of these bodies, and tests using pulsar–white-dwarf binary systems have been limited by the weak gravitational pull of the Milky Way. This new test has improved on the accuracy on any previous test of gravity by a factor of about ten.
One of the most famous tests of universal free fall came in 1971 when astronaut Dave Scott dropped a hammer and a feather on the Moon during Apollo 15. This was a re-creation of a supposed test by Galileo where he dropped two balls made of differing materials off the Leaning Tower of Pisa, and observed them reaching the ground at the same time.
Were the observations made by Archibald and her team comparable to these famous earlier tests?
“Indeed!” Archibald said. “Galileo argued that it didn’t matter how massive a cannonball was, or what it was made of, it would always fall exactly the same way. Of course, on Earth air gets in the way, but Dave Scott demonstrated on the airless moon that it worked even with a feather. We actually asked the same question: Does our pulsar fall the same way as our white dwarf?”
Of course, Archibald and her colleagues 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. But they were testing the same thing: if the two objects fell the same way.
Archibald added that there is one important distinction about the reason the pulsar and white dwarf might fall differently. While it has been shown numerous times that mass and composition don’t affect how an object falls, her team was testing something different.
“In Einstein’s theory, gravity itself has mass, so an object with really strong gravity could behave differently,” Archibald said. “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: it’s incredibly strong gravity is what might make it fail Galileo’s test.”
Instead, this unique star system confirmed both Galileo’s theory of motion and Einstein’s theory of gravity.
As for any future tests, Archibald and her team said the upcoming Square Kilometer Array, located in South Africa, might be able to find other star systems such as unusual binaries, other triple star systems, or a pulsar orbiting a black hole that might test Einstein’s theory with tighter constraints.
But Archibald said all three of the telescopes used in this current test of fundamental physics performed admirably.
“Astronomy is a wonderful way to find out what’s out there in the universe,” she said, “but this sort of observation is the only way to improve our understanding of a force as fundamental as gravity.”