The only known pair of pulsars has just revealed a wealth of unique cosmic insights.
Scientists have been observing the pair of pulsars, or pulsating neutron stars, for 16 years. The measurements confirmed Einstein’s theory of gravity, known as general relativity, to a new level of precision, physicists said in a paper published Dec. 13 in Physical Review X, and hinted at the The subtle effects of theory.
Pulsars are spinning dead stars composed of dense neutrons that appear to flicker as their beacon-like beams of radiation regularly sweep across Earth. Variations in the timing of these pulses can reveal the motion of the pulsar and the effects of general relativity. While physicists have discovered many individual pulsars, this is the only pair known to orbit each other. In 2003, the discovery of the double pulsar system J0737-3039 opened up a possible new way to test the general theory of relativity.
One of the pulsars spins about 44 times a second, while the other spins about every 2.8 seconds. The slower pulsar dimmed in 2008 because of a strange phenomenon in general relativity that spun its beam out of view. The researchers will continue to monitor the remaining visible pulsars, then combine the new data with the old observations to improve the precision of the measurements.
Here are five takeaways from the new study:
- Einstein was right in many ways.
With this pair of pulsar binaries, we can simultaneously perform five independent tests of general relativity to check whether various properties of the orbits match the predictions of Einstein’s theory. For example, researchers measure the rate at which the orbit’s ellipse rotates, or precesses, to see if it is as expected. It turns out that all parameters are in accordance with Einstein’s theory.
What’s more, Scott Ransom, an astrophysicist at the National Radio Astronomy Observatory in Charlottesville, Virginia, said, “Each individual test of general relativity has become so precise that … it must include general relativity. The higher-order effects of relativity to match the data.” This means the measurements are so precise that they hint at the subtle properties of gravity.
- Gravitational waves are draining energy.
The observations show that the pulsar’s orbit is shrinking. By measuring the time it took for the pulsars to complete their orbits each time, the researchers determined that the pair of pulsars shortened by about 7 millimeters per day.
That’s because pulsars excite gravitational waves as they orbit, which are ripples in space-time that vibrate outward and carry energy away. This apparent contraction was first detected in a galaxy with a pulsar and a neutron star in the 1970s, providing early evidence for the existence of gravitational waves. But the new results are 25 times more precise than previous measurements.
- Pulsars are losing mass.
There is also a subtle effect that will change this track. The pulsar gradually slows down over time, losing rotational energy. Because energy and mass are two sides of the same coin, that means faster pulsars lose about 8 million tons of mass per second.
“When I first realized this, I was really stunned,” Kramer said. “While it sounds like a lot, the mass loss only affects tiny adjustments to the orbit. Previously, scientists could Ignore this effect, because the change is very small. But orbital measurements are now accurate enough that it makes sense to take it into account.
- We can know the direction of rotation of the pulsar, which hints at its origin.
By studying the timing of a pulsar as the light from a pulsar passes its companion star, scientists can tell which direction the faster pulsar is spinning. The results showed that the pulsar was spinning in the same direction as its orbit, providing clues to the formation of the pulsar binary.
The two pulsars began as neighboring stars exploded one after the other. Normally, when a star explodes, the remnants it leaves behind are washed away, breaking them apart. The faster pulsar spins in the same direction as its orbit, which means the explosion that formed the pulsar didn’t give it too much jolt, which helps explain how the binary system remains intact.
- We have a clue about the pulsar’s radius.
Gravitational effects are known to cause the orbit’s ellipse to precess, or rotate, about 17 degrees per year. But in the new study, a subtle tweak was involved. As the pulsar rotates, it acts like the twisting skirt of a whirling dancer, dragging the fabric of spacetime behind it, changing the precession.
This drag effect means that the faster pulsar must have a radius of less than 22 kilometers, an estimate that, if more precise in future work, could help reveal the origin of the extremely dense neutron star material that makes up the pulsar physical properties.