Einstein’s universal freefall confirmed

The fundamental principle at the base of Einstein’s Theory of General Relativity – that all objects in freefall accelerate identically – has been verified on a stellar scale.

Einstein predicted that all objects behave identically when falling in an external gravitational field. For his theory to remain unchallenged it has long been thought critical to establish that the principle holds at all scales within the universe.

In particular, it was felt important to test whether the prediction plays out even in systems involving objects with strong “self-gravity” – that is, where the combined gravity of the constituents within an object, which thus hold it together, is extremely powerful. Neutron stars are examples of stellar objects with strong self-gravity.

Interestingly enough, despite their huge size, most objects in the universe – the majority of planets, stars and even galaxies, for instance – do not exert powerful enough self-gravity to able to be used as test subjects for the envelope-pushing extremes of Einstein’s theory.

Calculations arising from NASA’s MESSENGER mission to Mercury, which wound up in 2015, have been used to verify general relativity in relation to that planet’s orbit around the sun. However, on cosmic scales, the self-gravity of the bodies involved was too weak to test the predictions at their upper limits.

Similarly, the weak gravitational pull of the Milky Way limited the precision of attempts to test the theory using measurements obtained from pulsar and white dwarf binary star systems.

Now, however, a team of astronomers led by Anne Archibald from the University of Amsterdam in the Netherlands, has found a way around such limitations.

In a paper published in the journal Nature, she and her colleagues report results arising from the careful measurement of a triple star system, called PSR J0337+1715.

The system was discovered in 2014, and comprises a pulsar, or neutron star – a super-dense object with, thus, extremely strong self-gravity – and two white dwarf stars.

One of the white dwarfs is locked in a tight 1.6 day orbit around the pulsar. The pair then take 327 days to orbit the second white dwarf. The whole system completes its dual orbits in an area smaller than the space described by Earth’s orbit around the sun.

Archibald and colleagues studied the system to detect how the gravitational pull of the outer white dwarf affected the orbits of the inner one and the pulsar. If Einstein’s predictions were incorrect – that is, if they did not describe the behaviour of objects with very strong self-gravity – then the pulsar and the inner white dwarf should have behaved differently.

They didn’t. The scientists found that the measurements for the accelerations of the two bodies had a “fractional difference” of just 2.6 millionths. The result, they write, establishes the universality of freefall to a measurement “almost a thousand [times] smaller than that obtained from other strong-field tests”.

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