Einstein Right Again: Even the Heaviest Objects Fall the Same Way

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Einstein’s general relativity assumes not just the regular equivalence principle, but a specific version of it called the strong equivalence principle. According to relativity, gravity works by warping space-time itself, and anything with mass would exude gravity. So dense enough objects, such as pulsars, aren’t just passive passengers riding on space-times curves — they’re actively warping it themselves, too. The strong equivalence principle states that even these objects would fall at the same rates as others, too.

By looking at PSR J0337+1715 for about six years, the paper’s authors can see whether or not that’s the case: If Einstein, and the strong equivalence principle, are right, then the inner pulsar-white dwarf pair should be stable, and the outer white dwarf’s gravity would affect them both the same. If Einstein’s wrong, then the scientists would see some kind of wobble as the neutron’s own gravity affects its orbit around the white dwarf.

Gravity Gains
Spoiler: Einstein won! He pretty much always wins.

The team saw exactly what they’d expect to see if the strong equivalence principle is right, no anomalies or weirdness, with a margin of error of only 30 meters, according to a university video explaining the findings.

This finding rules out several alternative theories, including some versions of string theory. It also confirms that our current understanding of gravity, the general theory of relativity, remains a reasonable way to understand the universe.

“Tests of this principle have a long heritage,” says Clifford M. Will in a Nature article accompanying the paper. “It is quite extraordinary that the responses of different materials to gravity should be so similar. In Einstein’s unique imagination, there was a reason: gravity is not a force that acts on all of these particles in some fantastically fine-tuned manner, but is simply an effect of space-time geometry. The constituents of matter follow universal paths in a space-time that is curved by massive bodies.”

Maybe one day we’ll have to discard Einstein’s theories for something better, but for now they’re still here to stay.

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