EVERYTHING falls the same, from the lightest feather to the heaviest star. That’s part of the foundation of Albert Einstein’s theory of general relativity. Now, observations of three stars orbiting one another have confirmed that it holds up even for some of the heaviest objects in the universe.
An object is in free fall when the only force acting on it is gravity. The strong equivalence principle, a pillar of the theory of general relativity, has it that all objects subject to the same gravitational forces should fall at the same rate, even if their masses differ.
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In space, two objects orbiting a third one at the same distance should have the same acceleration, even if one of the pair is much more massive than the other – after all, orbiting is just continuously falling.
We have tested this using the orbits of Earth and the moon around the sun. They both fall at the same rate, despite Earth being more massive, but our planet is not heavy enough to rule out the possibility that relativity may break in areas of extreme gravity.
at the University of Amsterdam in the Netherlands and her colleagues have performed a more robust test using observations of three stars about 4200 light years away (, ). The stellar system consists of a pulsar and a white dwarf star orbiting one another in a tight binary that orbits a second, distant white dwarf.
The pulsar is both more massive and more compact than the white dwarf in the binary, so the gravitational energy holding it together is about 10,000 times stronger. That makes it the most extreme test of the strong equivalence principle ever carried out.
The team found that the pulsar and white dwarf were accelerating in orbit around their distant companion at essentially the same rate. If they differ, it is by a factor of less than one-ten-millionth, satisfying the strong equivalence principle and proving Einstein right – again.
This article appeared in print under the headline “Einstein’s theory passes triple-star test”
