Science - USA (2021-12-24)

IMAGE: © MICHAEL KRAMER/MPIFR

1548 24 DECEMBER 2021 • VOL 374 ISSUE 6575 science.org SCIENCE

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ince 2005 astronomers have monitored

a pair of spinning neutron stars—two

superdense objects whirling around

each other every 2.5 hours while emit-

ting metronomic radio pulses. That

patient scrutiny has yielded the most

wide-ranging tests yet of Albert Einstein’s

theory of gravity. Unsurprisingly, the general

theory of relativity, as it’s called, passed all

the tests, leaving physicists both elated at the

feat and disappointed that it yielded no clues

to a “theory of everything” that would tran-

scend general relativity.

The unusual astronomical system—the

only known double pulsar—enabled the team

to put general relativity to seven tests, includ-

ing the most precise measurements yet of

how ripples in spacetime called gravitational

waves carry energy away from the stars, re-

ducing their masses and causing their orbits

to shrink. The system also displayed other ef-

fects that had never been measured before,

including how photons from one of the stars

slow down and bend as they pass through the

intense gravitational field of the other.

“This is a really impressive suite of tests for

general relativity—which of course passes,”

says Scott Ransom of the U.S. National Ra-

dio Astronomy Observatory, who was not in-

volved in the study.

When the stellar duo was discovered in

2003, “We didn’t realize what sort of a gem

it was,” says Michael Kramer of the Max

Planck Institute for Radio Astronomy, who

led the study. The stars, collapsed stellar

remnants made of tightly packed neutrons,

have masses greater than the Sun squeezed

into balls the size of a city, generating in-

tense gravitational fields. Their frenetic

spins turn them into pulsars, emitting

radio beams that sweep past Earth like a

lighthouse—one of them every 2.8 seconds,

the other every 23 milliseconds. Together,

they act as two giant clocks whirling in

each other’s intense gravity. By looking

for tiny variations in the pulses’ timing,

which reveal how gravity affects their or-

bits and the propagation of the photons

themselves, researchers can put relativity

to extreme tests.

To measure the pulses, Kramer and his

colleagues used six large radio telescopes

around the globe plus the Very Long Base-

line Array, a set of 10 dishes spanning the

United States. The fact that the team just

kept on observing the pulsars month after

month, for years, without publishing any

results became a running joke among col-

leagues. It’s been “a long time coming!”

says Cherry Ng of the University of To-

ronto’s Dunlap Institute for Astronomy &

Astrophysics, who commends the team for

its mammoth effort. “Combining data from

multiple telescopes is not an easy task.”

When the 56-page paper landed last week

in Physical Review X, researchers felt it was

worth the wait. “It’s an incredible paper

and is incredibly thorough,” Ransom says.

In total, the team says, general relativity

agrees with the observations to a level of at

least 99.99%.

The measurements were so precise that

some couldn’t be explained by general rela-

tivity’s simpler predictions: for example, how

gravity slows time, shifts light toward longer

wavelengths, and ratchets the axis of ellipti-

cal orbits forward slightly on each revolution,

an effect called precession. Theorists had to

factor in other, more esoteric predictions of

Einstein’s theory, such as the way a dense,

spinning mass twists the spacetime around

it, and the way extreme gravity can distort

the shape of an orbit. The latter is “a subtle,

subtle effect that’s never been measured be-

fore,” says Ransom, who was impressed by

just how many of these “higher order” effects

they had to invoke.

Although confirming general relativity is

satisfying, researchers want to find a chink

in its armor one day. General relativity can-

not be a complete description of how the uni-

verse works because, as far as anyone knows,

it cannot be reconciled with that other pillar

of modern physics, quantum mechanics. An

unexplained gravitational aberration might

hint at a path to a more complete theory. But

the double pulsar results only make it harder

to conjure up “other theories consistent

with the data,” says team member Thibault

Damour of the Institute for Advanced Scien-

tific Studies near Paris.

In addition to a testbed for relativity, the

double pulsar may offer a window into the

interiors of pulsars. To understand the behav-

ior of the densely packed neutrons, research-

ers need to know a pulsar’s density. Its mass

can be determined easily from its orbit, but

size—the other factor in density—is harder to

gauge. A subtle coupling between the spin of

each pulsar and its orbit, predicted by general

relativity, could provide a way to calculate its

radius—if the coupling could be measured

accurately enough. Ransom, who calls pulsar

size “a holy grail,” says he had hoped the dou-

ble pulsar researchers might be able to pin

it down, but they were only able to set size

limits. “It proved much trickier to pick out.”

More patience will be needed. Ransom

points out that in pulsar studies, the longer

you observe, the better accuracy you achieve.

In 10 or 20 more years, he says, the team may

be able to bear down even further. “It gets

much better with time.” j

The only known double pulsar system beams out

radio waves that sweep past Earth metronomically.

NEWS | IN DEPTH

ASTROPHYSICS

With double pulsar, theory of

gravity passes toughest test yet

Study 16 years in the making shows how gravity distorts

orbits and slows down photons

By Daniel Clery