By Adrian Cho, Sciencemag.org
A simple observation of an extremely dim star may point to, literally, the biggest manifestation of weird quantum phenomena yet. Light from a lonely neutron star 400 light-years away is polarized, just like light reflecting off a pond, a team of astronomers reports. This suggests that, as predicted, the neutron star's ultraintense magnetic field is distorting empty space through a quantum mechanical effect involving ghostly “virtual” particles lurking in the vacuum—the sort of thing usually seen only on the atomic scale.
“It’s really cool,” says Nir Shaviv, an astrophysicist at the Hebrew University of Jerusalem, who predicted the astrophysical effect in 2000 but was not involved in the current work. “This is a macroscopic manifestation of quantum field,” adds Jeremy Heyl, an astrophysicist at the University of British Columbia in Vancouver, Canada, who, along with Shaviv, made the prediction. “It’s manifest on the scale of a neutron star.”
The photons that make up light are electromagnetic waves rippling through space. When light is polarized, the photons oscillate back and forth in the same direction—say, up and down for vertically polarized light or side to side for horizontally polarized light. Now, new observations show that light from a nearby neutron star is significantly polarized, reports a team led by Roberto Mignani, an astronomer at the Institute for Spatial Astrophysics in Milan, Italy, reports. The researchers studied the neutron star, known as RX J1856.5-3754 with the European Southern Observatory’s Very Large Telescope array on Cerro Paranal in Chile, in May and June of 2015. They found that the light from the neutron star is polarized to about 16.4%, as they report today in the Monthly Notices of the Royal Astronomical Society.
Simply making that measurement was a substantial feat. The neutron star shines much brighter in x-rays than in visible light. But the scientists wanted to search for the polarization effect, and there currently are no x-ray instruments with sufficient polarization sensitivity. So, instead, the researchers had to study its faint optical glow, Mignani explains. That task was similar to spotting a candle place halfway between Earth and the moon, he says.
Though challenging, measuring that polarization would be a validation of an important quantum mechanical effect. Since the invention of quantum theory in the 1920s, physicists have known that the vacuum of empty space is not a sterile, static thing. Thanks to quantum uncertainty, the vacuum roils with particle-antiparticle pairs that pop in and out of existence too quickly to be seen. Although these virtual particles cannot be captured directly, they can affect the properties of the vacuum. For example, by interacting with those pairs a strong electric field can change the vacuum and, hence, the inner workings of atoms.
In the 1930s physicists realized that a very strong magnetic field would affect the virtual particles in the vacuum and make light travel at different speeds depending on the direction of polarizations. The two-speed effect is known as birefringence and it is used in many optical devices. It gives the crystal calcite its famous ability to produce double images of objects. But the effect arises through quantum effects. In the vacuum, the virtual particle pairs can move more easily along the magnetic field than perpendicular to it, Heyl explains. So light polarized along the magnetic field interacts more strongly with the virtual particles and is slowed ever so slightly compared with light polarized perpendicular to the field, Heyl says.
In 2000, Heyl and Shaviv predicted that the magnetic field of a neutron star—which is 10 trillion times stronger than Earth’s—would be strong enough to induce birefringence in the neighborhood of a neutron star. The birefringence would then lead to an overall polarization of the light from a neutron star, they argued. Now, Mignani and his colleagues say they have spotted the effect.
To draw that conclusion, the researchers had to rule out other effects, such as polarization that could arise from dust particles along the line of sight. The star is somewhat close to a molecular cloud, Mignani notes. But the cloud appears behind the star and not in front of it, he says. Even if it were in front of the neutron star, calculations suggest it would produce only a polarization of 1%. “We are confident that there is no effect of the interstellar medium,” Mignani says. “The physics we see is intrinsic to the neutron star.”
Shaviv agrees that it looks like Mignani and colleagues have spotted the effect. “It definitely looks like it, smells like it,” he says. However, Heyl says the result is not definitive, as it might be possible to explain away the polarization by assuming an unexpectedly thick haze of plasma around the neutron star. Researchers should repeat the observation at other wavelengths, he says, as the quantum effect would get stronger at shorter wavelengths, but the plasma effect would get weaker.
The real payoff could come with a space-based telescope that could measure polarization for x-ray light. In that case, the polarization from the quantum effect should be essentially 100%, Heyl says. Studying the polarization would then enable astrophysicists to infer properties such a neutron star’s size and the strength of its gravity at its surface. U.S. and European researchers are proposing to launch in the next decade x-ray telescopes capable of making such polarization measurements, Heyl says. “Hopefully in the next 10 years it will go from discovery—of which we have the first hint—to using this as a tool.”