Neutron star observations support 80-year-old prediction
ESO, Digitized Sky Survey 2
Observations of the dense remnant of an exploded star have provided the first sign of a quantum effect on light passing through empty space.
Light from the stellar remnant, a neutron star located about 400 light-years away, is polarized, meaning that its electromagnetic waves are oriented preferentially in a particular direction like light that reflects off the surface of water (SN: 7/8/06, p. 24). That polarization is evidence of “vacuum birefringence,” a quantum effect first predicted 80 years ago caused by light interacting with the vacuum of space in a strong magnetic field. Scientists report the result in a paper to be published in the Feb. 11, 2017 issue of Monthly Notices of the Royal Astronomical Society.
“It’s the most natural explanation,” says astrophysicist Jeremy Heyl of the University of British Columbia in Vancouver, who was not involved with the new result. But he cautions, other sources of polarization could mimic the effect, and additional observations are necessary.
According to quantum electrodynamics, the theory describing how light interacts with charged particles such as electrons, empty space isn’t really empty. It is filled with a roiling soup of ethereal particles, constantly blipping into and out of existence (SN: 11/26/16, p. 28). As light passes through the void, its wiggling electromagnetic waves interact with those particles. Under strong magnetic fields, light waves that wiggle along the direction of the magnetic field will travel slightly slower than light oscillating perpendicular to the direction of the magnetic field, which rotates the overall polarization of light coming from the star.
A similar effect commonly occurs in a more familiar situation, in what are known as birefringent materials. The liquid crystals in computer monitors similarly rotate the polarization of light. Horizontally polarized light, for example, is sent to each pixel, but a filter lets only vertically polarized light escape. To switch on a pixel, the liquid crystals twist the light waves 90 degrees so the waves will pass through.
But evidence for the quantum version of the effect was not easy to come by. Observing it requires a magnetic field stronger than those that can be produced in the laboratory, says astrophysicist Roberto Mignani of the National Institute for Astrophysics in Milan, coauthor of the new study. The magnetic field around the neutron star that Mignani and colleagues studied is about 10 trillion times the strength of Earth’s. But the star is incredibly faint, making measurements of its polarization difficult. “A neutron star of this kind is about as bright as a candle halfway between the Earth and the moon,” Mignani says.
Using the Very Large Telescope in Chile, the scientists found that visible light from the neutron star was about 16 percent polarized, a result consistent with scientists’ theories of vacuum birefringence. But, says Heyl, the polarization could also occur as a result of an unexpectedly large amount of plasma surrounding the star.
For airtight evidence of the effect, scientists could study X-rays from neutron stars, where the polarization effect should be even stronger. Although no telescope currently exists that can make such measurements, there are several proposed X-ray satellites that may soon be able to clinch the case for vacuum birefringence.
Scientists might want to keep their fingers crossed. If future measurements overturned the evidence for vacuum birefringence, the effect’s absence would be difficult to reconcile with the theory of quantum electrodynamics, Heyl says. “It’s essentially one of the basic predictions of the theory, so to fix it you'd really have to rip the theory all the way back down to the foundations and rebuild it.”
R. P. Mignani et al. Evidence for vacuum birefringence from the first optical-polarimetry measurement of the isolated neutron star RX J1856.5−3754. Monthly Notices of the Royal Astronomical Society. Vol. 465, February 11, 2017, p. 492. doi: 10.1093/mnras/stw2798.