Observations of a trio of dead stars have confirmed that a foundation of Einstein’s gravitational theory holds even for ultradense objects with strong gravitational fields.
The complex orbital dance of the three former stars conforms to a rule known as the strong equivalence principle, researchers reported January 10 at a meeting of the American Astronomical Society. That agreement limits theories that predict Einstein’s theory, general relativity, should fail at some level.
According to general relativity, an object’s composition has no impact on how gravity pulls on it: Earth’s gravity accelerates a sphere of iron at the same rate as a sphere of lead. That’s what’s known as the weak equivalence principle. A slew of experiments have confirmed that principle — beginning with Galileo’s purported test of dropping balls from the Leaning Tower of Pisa (SN: 1/20/18, p. 9).
But the strong equivalence principle is more stringent and difficult to test than the weak version. According to the strong equivalence principle, not only do different materials fall at the same rate, but so does the energy bound up in gravitational fields. That means that an incredibly dense, massive object with a correspondingly strong gravitational field, should fall with the same acceleration as other objects.
“We’re asking, ‘How does gravity fall?’” says astronomer Anne Archibald of the University of Amsterdam, who presented the preliminary result at the meeting. “That sounds weird, but Einstein says energy and mass are the same.” That means that the energy bound up in a gravitational field can fall just as mass can. If the strong equivalence principle were violated, an object with an intense gravitational field would fall with a different acceleration than one with a weaker field.
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