Einstein’s general relativity passes extreme test in pulsar binary system

A pulsar-white dwarf pair has given scientists insight into the nature of gravity.
By | Published: April 25, 2013 | Last updated on May 18, 2023
pulsar white dwarf
A superdense neutron star, emitting beams of radio waves as a pulsar, orbits with a closely paired white dwarf. The grid background illustrates the gravitational distortions of space-time. // ESO/L. Calçada
A strange stellar pair nearly 7,000 light-years from Earth has provided physicists with a unique cosmic laboratory for studying the nature of gravity. The extremely strong gravity of a massive neutron star in orbit with a companion white dwarf star puts competing theories of gravity to a test more stringent than any available before.

Once again, Albert Einstein’s general theory of relativity, published in 1915, comes out on top.

At some point, however, scientists expect Einstein’s model to be invalid under extreme conditions. General relativity, for example, is incompatible with quantum theory. Physicists hope to find an alternate description of gravity that will eliminate that incompatibility.

A newly discovered pulsar — a spinning neutron star with twice the mass of the Sun — and its white-dwarf companion, orbiting each other once every 2.5 hours, has put gravitational theories to the most extreme test yet. Observations of the system, dubbed PSR J0348+0432, produced results consistent with the predictions of general relativity.

Astronomers discovered the tightly orbiting pair with the National Science Foundation’s Green Bank Telescope (GBT) and subsequently studied its visible light with the Apache Point telescope in New Mexico, the Very Large Telescope in Chile, and the William Herschel Telescope in the Canary Islands. Extensive radio observations with the Arecibo telescope in Puerto Rico and the Effelsberg telescope in Germany yielded vital data on subtle changes in the pair’s orbit.

In such a system, the orbits decay, causing gravitational waves that carry energy from the system. By precisely measuring the time of arrival of the pulsar’s radio pulses over a long period of time, astronomers can determine the rate of decay and the amount of gravitational radiation emitted. The large mass of the neutron star in PSR J0348+0432, the closeness of its orbit with its companion, and the fact that the companion white dwarf is compact but not another neutron star all make the system an unprecedented opportunity for testing alternative theories of gravity.

Under the extreme conditions of this system, some scientists thought that the equations of general relativity might not accurately predict the amount of gravitational radiation. Competing gravitational theories, they thought, might prove more accurate in this system.

“We thought this system might be extreme enough to show a breakdown in general relativity, but instead, Einstein’s predictions held up quite well,” said Paulo Freire of the Max Planck Institute for Radio Astronomy in Germany.

That’s good news, the scientists say, for researchers hoping to make the first direct detection of gravitational waves with advanced instruments. Researchers using such instruments hope to detect the gravitational waves that result when neutron stars and black holes spiral inward toward violent collisions.

Gravitational waves are extremely difficult to detect, and even with the best instruments, physicists expect they will need to know the characteristics of the waves they seek, which will be buried in “noise” from their detectors. Knowing the wave’s properties will allow them to extract the signal they seek from that noise.

“Our results indicate that the filtering techniques planned for these advanced instruments remain valid,” said Ryan Lynch of McGill University.