Scientists have been looking for decades to confirm a weird quantum effect first predicted in 1936. Have they finally found hard evidence for it?
Observations of a neutron star 300 light-years away show evidence for the existence of virtual subatomic particles that pop into and out of existence, argues an international team of scientists. The find would validate a prediction made 80 years ago by a fundamental quantum theory that describes the weird world of very small particles. But not everybody is convinced that the scientists have found a smoking gun.
When Paul Dirac penned the equations of quantum electrodynamics (QED), he was formulating a fundamental theory of physics that underlies our understanding of subatomic particles. The theory mathematically — and very successfully — describes both how light interacts with matter and how charged particles interact with one another.
Two consequences of QED quickly became apparent. The first, in 1928, was the prediction that every particle has an antimatter partner — a particle with the same mass but opposite charge. Physicist Carl Anderson discovered the electron’s antiparticle, called the positron, four years later, a Nobel-worthy accomplishment.
A second implication of QED is that the vacuum of space itself could be teeming with temporary particles. Due to the uncertainty inherent in the quantum world, subatomic particles and antiparticles spontaneously pop into and out of existence; they "borrow" energy via the Heisenberg uncertainty but can only do so for a (very) limited time. In 1936, physicists reached the conclusion that these so-called virtual particles, each only in existence for a tiny fraction of a second, could have real, measurable effects on light, twisting its polarization in the same way that liquid crystals do in LCD displays. This quantum effect is known as vacuum birefringence.
Although the existence of vacuum birefringence has proven difficult to directly confirm, physicists generally accept that it’s real. “Let’s say [vacuum birefringence] isn’t there,” says Jeremy Heyl (University of British Columbia, Canada). “Essentially you’d have to remake everything, because it’s a very basic consequence of QED.”
Neutron Stars as Cosmic Laboratories
Directly measuring vacuum birefringence requires an incredibly strong magnetic field and sensitivity that’s currently impossible in the lab. But Nature has provided its own cosmic laboratory in the form of neutron stars. These crushed stellar remnants carry powerful magnetic fields, enhancing the effect of vacuum birefringence to measurable levels.
So Roberto Mignani (National Institute of Astrophysics in Milan, Italy, and University of Zielona Góra, Poland) and colleagues used the Very Large Telescope in Chile to observe the bright, nearby neutron star RX J1856.5-3754.
This neutron star is essentially a naked ember, its hot surface glowing. The star’s strong magnetic field polarizes this light, but because the star is only a pinprick in our skies, by the time the combined radiation arrives at Earth, those polarizations cancel out. The neutron star’s light ought to be 0% polarized.
Instead, what Mignani and colleagues observed was light polarized somewhere between 11% and 21% — fairly high.
The amount of polarization is high enough, in fact, to suggest vacuum birefringence is responsible, the authors say in the February 11th Monthly Notices of the Royal Astronomical Society. Heyl agrees.
But, as George Pavlov (Penn State) points out, it’s not an open-and-shut case. We don’t know how the neutron star is oriented, relative to Earth — whether it’s rotating edge-on to our line of sight or pointing just one of its poles straight at us*. That orientation will determine if the radiation’s original (non-birefringence) polarization cancels perfectly. If we were gazing directly at the neutron star's magnetic pole, we'd see magnetic field lines radiating out in every direction; all of the polarization imparted to light would cancel out. But gazing along the neutron star's magnetic equator, we could see polarization levels up to 20% even without the additional effects of birefringence, he cautions.
“The degree of polarization measured by the authors . . . is rather high indeed,” Pavlov says, and he agrees that it’s consistent with the QED effect. “But it is not a proof that they’ve discovered vacuum birefringence observationally, because such a high degree of polarization could be reached without vacuum birefringence.”
To Higher Energies
The real smoking gun for vacuum birefringence will likely come from missions looking at the polarization of X-rays rather than visible light. A neutron star’s surface is so hot that, while it does emit some visible light, most of its light is emitted at X-ray energies. So while vacuum birefringence does affect the polarization of visible light, it’ll affect X-rays a whole lot more.
It’s quite possible that an X-ray polarimeter may launch in the next few years. There are three mission concepts currently under study: the European Space Agency’s X-ray Imaging Polarimetry Explorer (XIPE), NASA’s Imaging X-ray Polarimeter Explorer (IXPE, and yes it really is almost the same acronym), and another NASA concept called Polarimetry of Relativistic X-ray Sources (PRAXYS). If any of these missions is selected, it’ll launch early in the next decade.
“We’ll immediately verify this QED result that [Mignani and colleagues] see in the optical,” Heyl predicts, “but on top of that we can use the measured polarizations now to probe the structure of the magnetic field in detail.” The effect could be used not only to explore neutron star surfaces in detail, but also the environments around black holes.
* Fun fact: A neutron star’s strong gravitational field bends light radiating from its surface. So when you look at a neutron star, you see more than a full hemisphere at a time — you also see some light from surfaces pointed away from you.