UCL Mathematical & Physical Sciences


First signs of weird quantum property of empty space?

30 November 2016

The polarisation of light emitted by a neutron star

By studying the light emitted from an extraordinarily dense and strongly magnetised neutron star using ESO's Very Large Telescope, astronomers may have found the first observational indications of a strange quantum effect, first predicted in the 1930s. The polarisation of the observed light suggests that the empty space around the neutron star is subject to a quantum effect known as vacuum birefringence.

A team composed of UK scientists from UCL Mullard Space Science Laboratory, University of Zielona Gora (Poland), University of Padova and Osservatorio Astronomico di Roma (Italy), used ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile to observe the neutron star RX J1856.5-3754, about 400 light-years from Earth [1].

Despite being amongst the closest neutron stars, its extreme dimness meant the astronomers could only observe the star with visible light using the FORS2 instrument on the VLT, at the limits of current telescope technology.

Neutron stars are the very dense remnant cores of massive stars - at least 10 times more massive than our Sun - that have exploded as supernovae at the ends of their lives. They also have extreme magnetic fields, billions of times stronger than that of the Sun, which permeate their outer surface and surroundings.

These fields are so strong that they even affect the properties of the empty space around the star. Normally a vacuum is thought of as completely empty, and light can travel through it without being changed. However, in quantum electrodynamics (QED), the quantum theory describing the interaction between photons and charged particles such as electrons, space is full of virtual particles that appear and vanish all the time. Very strong magnetic fields can modify this space so that it affects the polarisation of light passing through it.

Zooming in on the very faint neutron star RX J1856.5-3754

This VLT study is the very first observational support for predictions of these kinds of QED effects arising in extremely strong magnetic fields," remarks Silvia Zane (UCL/MSSL, UK).

Zane explains: "For instance, it predicts that energetic photons can interact with a huge magnetic field and constantly generate pairs of electrons and positrons. These particles and antiparticles have a short life and almost immediately annihilate. This means that a population of "virtual" pairs is almost constantly present in the strongly magnetized vacuum. These "virtual" pairs can deflect the light, therefore   the vacuum becomes birefringent and polarized. Many of these exotic QED effects have not been proved yet."

Among the many predictions of QED vacuum birefringence so far lacked a direct experimental demonstration. Attempts to detect it in the laboratory have not yet succeeded in the 80 years since it was predicted in a paper by Werner Heisenberg (of uncertainty principle fame) and Hans Heinrich Euler.

"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." says Roberto Turolla (University of Padua, Italy).

After careful analysis of the VLT data, the team detected linear polarisation - at a significant degree of around 16% - that they say is likely due to the boosting effect of vacuum birefringence occurring in the area of empty space surrounding RX J1856.5-3754 [2].

Vincenzo Testa (INAF, Rome, Italy) comments: "This is the faintest object for which polarisation has ever been measured. It required one of the largest and most efficient telescopes in the world, the VLT, and accurate data analysis techniques to enhance the signal from such a faint star."

"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," adds Roberto Mignani (University of Zielona Gora).

Denis Gonzalez, a PHD student at MSSL, and Roberto Taverna, a post doc researcher at University of Padova, explain: "There is still a large uncertainty regarding the physical conditions of the matter near the surface of a neutron star. We still do not know if these stars are covered by an atmosphere or not, in the latter case they would emit radiation directly from their solid crust. We developed several models, simulating all possible scenarios.  It was such a surprise when we realised that none of the physical models can explain the observed polarisation signal unless QED is turned on".

Mignani is excited about further improvements to this area of study that could come about with more advanced telescopes: "Polarisation measurements with the next generation of telescopes, such as ESO's European Extremely Large Telescope, could play a crucial role in testing QED predictions of vacuum birefringence effects around many more neutron stars."

"This measurement, made for the first time now in visible light, also paves the way to similar measurements to be carried out at X-ray wavelengths. Future X-ray polarimeters as those currently studied at ESA (XIPE) and NASA (IXPE) may be able to observe systematically this effect in a large number of hotter and more luminous neutron stars, nailing down the observational probe of QED. UCL MSSL scientists and the UKSA are currently largely involved in the assessment phase study of the ESA XIPE mission," adds Silvia Zane and Kinwah Wu (UCL/MSSL, UK).



The polarisation of light emitted by a neutron star (credit: ESO)

More information

This research was presented in the paper entitled "Evidence for vacuum birefringence from the first optical polarimetry measurement of the isolated neutron star RX J1856.5−3754", by R. Mignani et al., to appear in Monthly Notices of the Royal Astronomical Society.
The team is composed of R.P. Mignani (INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Milano, Italy; Janusz Gil Institute of Astronomy, University of Zielona Góra, Zielona Góra, Poland), V. Testa (INAF - Osservatorio Astronomico di Roma, Monteporzio, Italy), D. González Caniulef (Mullard Space Science Laboratory, University College London, UK), R. Taverna (Dipartimento di Fisica e Astronomia, Università di Padova, Padova, Italy), R. Turolla (Dipartimento di Fisica e Astronomia, Università di Padova, Padova, Italy; Mullard Space Science Laboratory, University College London, UK), S. Zane (Mullard Space Science Laboratory, University College London, UK) and K. Wu (Mullard Space Science Laboratory, University College London, UK).