Pulsar System Confirms Core Principle Of General Relativity To Its Most Accurate Level Yet

Artist's impression (not to scale) of the pulsar and its closest white-dwarf companion with their orbits and the second white dwarf in the background. Guillaume Voisin CC BY-SA 4.0

The Theory of General Relativity was published by Albert Einstein in 1915 and it is still the best understanding of gravity we have. It's not perfect, or complete, so scientists love to put it to the most stringent tests to look for ways to expand it, but the theory has stubbornly overcome all of them so far.

Reported in Astronomy & Astrophysics, the latest in a long list tested a cornerstone of general relativity, the strong equivalence principle, or the "universality of freefall". This is the idea that the gravitational motion of a body depends only on its initial position and velocity, not by what it is made of. You may be more familiar with the example of if you drop a feather and a hammer in a vacuum, they reach the ground at the same time.

The first version of this principle was proposed by Italian physicist Galileo Galilei, who famously dropped two cannonballs of different weights from the leaning tower of Pisa. A team of European researchers used something a bit more extreme for this new test: PSR J0337+1715, a pulsar that is orbited by two white dwarfs. We are yet to find another system like this.

In 1971, astronaut David Scott conducted Galileo's famous hammer/feather drop experiment on the Moon, during the Apollo 15 mission. 

Pulsars are neutron stars, the collapsed core of a star that has gone supernova. These pulsars can rotate very fast emitting pulses that can be recorded as if they were extremely precise clocks. They are extremely stable making them an ideal testbed for relativity. PSR J0337+1715 spins on its axis 366 times per second.

Researchers can record these pulses with a precision of nanoseconds, which allows them to determine its motion. Despite the system being 4,200 light-years away, the position of the pulsar is known to a matter of a few hundred meters. Researchers can compare this motion to predictions from general relativity to test its robustness.

“[I]t is the unique configuration of that system, akin to the Earth-Moon-Sun system with the presence of a second companion (playing the role of the Sun) towards which the two other stars ‘fall’ (orbit) that has allowed to perform a stellar version of Galileo's famous experiment from Pisa's tower. Two bodies of different compositions fall with the same acceleration in the gravitational field of a third one,” lead author Dr Guillaume Voisin, from the University of Manchester, said in a statement.

"The pulsar emits a beam of radio waves which sweeps across space. At each turn, this creates a flash of radio light which is recorded with high accuracy by Nançay's radio telescope. As the pulsar moves on its orbit, the light arrival time at Earth is shifted. It is the accurate measurement and mathematical modeling, down to a nanosecond accuracy, of these times of arrival that allows scientists to infer with exquisite precision the motion of the star."

The new test is based on six years of observations and follows in the footstep of previous work while almost doubling its accuracy. According to the study, the observation of the extreme gravity field of the pulsar cannot be more than 1.8 part per million than general relativity’s prediction. This is the most accurate confirmation of the universality of free fall yet.

Testing a theory time and time again is the cornerstone of the scientific method. The hope is that by pushing at the seams of relativity and quantum mechanics we could one day achieve a theory of gravity that can encompass both successfully.

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