A weird quantum effect may have just been observed for the first time in the polarised light of empty space around neutron star. The effect, known as vacuum birefringence, was first predicted by astronomers over 80 years ago and says that a highly magnetised vacuum should act as a prism for the propagation of light.

Scientists from Italy and Poland used the ESO's Very Large Telescope to observe the extremely dense and strongly magnetised neutron star RX J1856.5-3754, which is around 400 light years from Earth.

Neutron stars are what is left when massive stars (at least 10 times the size of our Sun) explode as supernovas. All that is left is the dense core with extreme magnetic fields, billions of times stronger than the Sun.

However, neutron stars are dim, meaning they are difficult to observe with normal telescopes. However, one of the instruments on the VLT allowed astronomers to study the light emitted from RX J1856.5-3754.

Vacuums in space

Normally, vacuums are considered to be completely empty regions of space. This means light can travel through without being altered.

However quantum electrodynamics (QED), which is the quantum theory relating to the interaction of photons and charged particles, says space is full of virtual particles. Extremely strong magnetic fields – like those around RX J1856.5-3754 – could (theoretically) modify empty space to effect the light passing through it. But this effect – vacuum birefringence – has never been observed.

Now, the researchers believe they have seen the light around the neutron star being altered. Their findings are published on the pre-print server arXiv.org.

After analysing the data, the team found evidence of linear polarisation, with the light being altered by about 16%. This, they say, is the result of vacuum birefringence taking place in the empty space around the star.

neutron star
Colour composite photo of the sky field around the lonely neutron star RX J1856.5-3754 ESO

Roberto Mignani, from INAF Milan, said: "The high linear polarisation that we measured with the VLT can't be easily explained by our models unless the vacuum birefringence effects predicted by QED are included."

Silvia Zane, from UCL, added: "This VLT study is the very first observational support for predictions of these kinds of QED effects arising in extremely strong magnetic fields."

Roberto Turolla, from the University of Padua in Italy, said: "This effect can be detected only in the presence of enormously strong magnetic fields, such as those around neutron stars. This shows, once more, that neutron stars are invaluable laboratories in which to study the fundamental laws of nature."