Neutron stars are one of the densest objects in the universe. The magnetic poles of some neutron stars emit beams of electromagnetic radiation, and the high-speed spin causes the beams of radiation to pass across the Earth at certain intervals like light emitted by lighthouses. These fast-spinning neutron stars are also known as pulsars.
Since Jocelyn Bell Burnell first discovered pulsars in 1967, a large number of pulsars have been confirmed. But in 2003, scientists first discovered a binary system of pulsars orbiting each other, named J0737-3039.
This dual pulsar system consists of two pulsars, one of which rotates very fast, about 44 times per second; the other young companion has a rotation period of about 2.8 seconds. The powerful gravitational field of this pulsar binary system provides a near-perfect gravitational laboratory for scientists to launch the most rigorous test of Einstein's theory of gravity, general relativity.
Since its discovery, astrophysicists have been observing the system continuously. Now, an international team of researchers from 10 countries has published observations of J0737-3039 collected over a period of 16 years by seven radio telescopes around the world, showing that they have tested general relativity with an unprecedented level of precision and revealed some new relativistic effects that have never been observed (but are in line with theoretical expectations).
On December 13, the researchers published their latest results in the journal Physical Review X.
The beams of radiation emitted by two pulsars (blue circles) orbiting each other (yellow beams) are guided by a strong magnetic field (light blue donut packs). These two pulsars revealed the space-time warp effect (blue grid) caused by pulsars, validating Einstein's theory of general relativity. | Image source: M. Kramer / Max Planck Institute for Radio Astronomy
In the new study, physicists measured a range of features of double pulsars and got many unique insights.
First, by measuring the time it takes for the two pulsars to complete each orbital run, the researchers found that the pulsars' orbits were contracting, and they were moving about 7 millimeters closer to each other each day. The reason why this orbital contraction phenomenon occurs is because when two pulsars are orbiting each other, they will stir up space-time, release gravitational waves, and carry away energy.
In the 1970s, astronomers first observed this contraction effect in a binary system of pulsars and neutron stars, providing early evidence for the existence of gravitational waves. This time, the new results confirm this again, and the measurement accuracy is 25 times that of the previous measurements, and 1000 times that of the gravitational wave detectors currently available.
In addition to gravitational waves, in analyzing the change in orbit of a binary pulsar due to the effects of electromagnetic radiation, the researchers also considered another subtle effect related to Einstein's famous mass-energy equation E=mc. In past studies, scientists have ignored this effect in their calculations because it is too weak. But now, since the measurement of the orbit is already accurate enough, it is necessary to take it into account.
Through the mass-energy equivalence relationship, the researchers found that over time, the pulsar gradually slowed down and lost its rotational energy, and this energy loss meant that the faster pulsar would lose about 8 million tons of mass per second! However, while this number may seem large, it will only have a minor effect on the track.
In addition, the new study also observed for the first time that light is delayed not only by the strong space-time curvature around the companion star, but also by detecting radio photons emitted from a pulsar and tracking their movement in the strong gravitational field of the companion star, but also the phenomenon of a small angle deflection of 0.04°. Never before had an experiment made such a measurement at such a high curvature of space-time.
In addition to the deflection angle of the light, the researchers also obtained clues related to the formation of this double pulsar by measuring the time it takes for the light to pass through the companion star. They found that the faster pulsar spins in the same direction as its orbit, a message that suggests that the two pulsars started out as two adjacent stars that exploded one after the other. Under normal circumstances, when a star explodes, the remnants of it are kicked away, causing such a binary star system to be "forced apart." If the spin of the faster the pulse is more consistent with the direction of its orbit, it means that the explosion that formed the pulsar did not cause it much vibration, which explains why the two pulsars are so closely combined.
Finally, the researchers also got clues related to the radius of the pulsar. Based on the gravitational effect, they calculated that the ellipsoid of the orbit would precede about 17° per year. But in the new study, there is a subtle adjustment, that is, the pulsar will drag the space-time structure behind it as it rotates, like a spinning dancer twisting the hem of the skirt, changing the precession. This drag effect means that faster pulsars must have a radius of less than 22 kilometers. If future work makes this estimate more precise, it may help reveal the physical properties of the extremely dense neutron star material that make up pulsars.
Why test general relativity under increasingly stringent conditions?
We know that general relativity is currently the best theory for describing gravity. However, general relativity is incompatible with quantum mechanics, which describes three other fundamental forces, prompting scientists to find any clues in the laboratory and astronomical observations that may be biased in general relativity.
However, more than a century later, general relativity has undergone an inexhaustible number of rigorous tests, but it has still not been shaken. The latest findings show that all the observations match Einstein's theoretical predictions, which proves once again that Einstein was right. Now, researchers are thinking about what other subtle effects should be taken into account next. They look forward to someday in the future when they will be able to measure deviations from general relativity.
Next, this two-pulsar system will continue to be monitored, and scientists expect that the next time its pulse time changes, it will bring us other new revelations.
#创作团队:
Text: No two Beidou
#参考来源:
https://www.sciencenews.org/article/pulsar-dead-stars-general-relativity-einstein
https://www.uea.ac.uk/news/-/article/challenging-einsteins-greatest-theory-with-extreme-stars
https://physics.aps.org/articles/v14/173
https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.041050
#图片来源:
封面图：M. Kramer / Max Planck Institute for Radio Astronomy