Trio of dead stars upholds a key part of Einstein’s theory of gravity

OXON HILL, Md. — 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.

To test this theory, scientists measured the timing of signals from a pulsar — a spinning, ultradense collapsed star that emits beams of electromagnetic radiation that sweep past Earth at regular intervals. The pulsar in question, PSR J0337+1715, isn’t just any pulsar: It has two companions (SN: 2/22/14, p.8). The pulsar orbits with a type of burnt-out star called a white dwarf. That pair is accompanied by another white dwarf, farther away.

If the strong equivalence principle holds, the paired-up pulsar and white dwarf should both fall at the same rate in the gravitational field of the second white dwarf. But if the pulsar, with its intense gravitational field, fell faster toward the outermost white dwarf than its nearby companion, the pulsar’s orbit would be pulled toward the outermost white dwarf, tracing a path in the shape of a rotating ellipse.

Scientists can use the timing of a pulsar’s signals to deduce its orbit. As a pulsar moves away from Earth, for example, its pulses fall a little bit behind its regular beat. So if J0337+1715’s orbit were rotating, signals received on Earth would undergo regular changes in their timing as a result. Archibald and colleagues saw no such variation. That means the pulsar and the white dwarf must have had matching accelerations, to within 0.16 thousandths of a percent.

Many physicists expect the strong equivalence principle to be violated on some level. General relativity doesn’t mesh well with quantum mechanics, the theory that reigns on very small scales. Adjustments to general relativity that attempt to combine these theories tend to result in a violation of the strong equivalence principle, says physicist Clifford Will of the University of Florida in Gainesville, who was not involved with the research.

The strong equivalence principle might still fail at levels too tiny for this test to catch. So the door remains open for adjustments to general relativity. But the new measurement constrains many such theories better than any previous test.  The result is “really tremendous,” says Will. It’s “a great improvement in this class of theories … which is why this triple system is so beautiful.”