First proof of quantum effect in vacuum?

Neutron star could confirm 80 years old theory of vacuum birefringence

Light coming from the surface of the strongly magnetized neutron star (left) becomes linearly polarized on its way to Earth. This suggests that the empty space around the neutron star undergoes a quantum effect known as vacuum birefringence. © ESO / L. Calçada
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First proof after 80 years? Astronomers could have found the first evidence of a postulated quantum effect of the vacuum decades ago. Accordingly, quantum fluctuations in empty, strongly magnetized space cause the polarization of light to change. In the light of a neutron star, researchers could now have detected this vacuum birefringence for the first time.

The vacuum of space is not empty - even if that seems so at first glance. Instead, space is filled with the usual theory of quantum fluctuations: there are always short-lived pairs of virtual particles and antiparticles. According to one hypothesis, this quantum fluctuation could even be behind the enigmatic Dark Energy - and ensure that information can escape from a black hole.

Postulated 80 years ago

As early as 80 years ago, physicists Werner Heisenberg and Hans Heinrich Euler predicted a further effect of quantum fluctuations: Under the influence of very strong magnetic fields, space and vacuum change in such a way that they influence the polarization of the light. "According to quantum electrodynamics (QED), a highly magnetized vacuum behaves like a prism for the propagation of light, " explains Roberto Mignani of the INAF in Milan.

This so-called vacuum birefringence would have to change the polarization of a light beam - theoretically. But in the last 80 years, this vacuum effect has not been proven experimentally. Even the PVAS experiment in Italy, in which a laser beam was sent through a vacuum in a strong magnetic field, could only narrow down the effect, not clearly prove it.

Neutron star as a test object

But now Mignani and his colleagues could have found evidence of vacuum birefringence for the first time - and not in the laboratory, but at its origin, in space. For their study they had observed the 400 light-years distant neutron star RXJ1856.5-3754 with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. This star is one of the isolated neutron stars and is therefore particularly well suited for observations. display

This wide-angle image shows the sky region around the very faint neutron star RXJ1856.5-3754 in the constellation Southern Crown. The neutron star itself is too faint to be recognizable here, but lies in the lower center of the image. ESO / Digitized Sky Survey 2

Neutron stars are the remnants of massive stars that have exploded as supernovae. Only the extremely condensed star of the core remains. However, one more characteristic of the measurement is that neutron stars have strong magnetic fields that are billions of times stronger than those of our sun. Theoretically, therefore, the vacuum in its environment would have to show vacuum birefringence.

"Excessive" polarization

And indeed, when the astronomers analyzed the neutron star's radiation using the FORS2 instrument, they noticed a note: "In the absence of the polarization effects of vacuum birefringence, the radiation from such a star would have to be drastically depolarized, " report Mignani and his colleagues. But with RXJ1856.5-3754 this was not the case: the researchers showed a linear polarization of about 16 percent.

"Our models can hardly explain the high linear polarization we measured with the VLT if the vacuum doubling effects predicted by the QED are not taken into account, " says Mignani. Other possibilities for example a polarization due to scattering on dust grains did not match the observation data.

"First official voucher"

According to the astronomers, the polarization they observe must therefore have been caused by the vacuum birefringence and could thus prove this quantum phenomenon for the first time. "Our measurements provide the first clues for the quantum electrodynamics predicted polarization effects of the vacuum, " the researchers note.

As you explain, subsequent polarization measurements of the neutron star RXJ1856.5-3754 must now confirm and improve their results. Polarization measurements on further isolated neutron stars could also help to make the evidence for vacuum birefringence more robust.

"Polarization measurements with the next generation of telescopes, such as ESO's European Extremely Large Telescope, could play a key role in investigating the effects of vacuum birefringence predicted by QED on many other neutron stars." says Mignani. (Monthly Notices of the Royal Astronomical Society., In press; arXiv: 1610.08323)

(ESO, 01.12.2016 - NPO)