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.”
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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.”