Unique quantum effect spotted in space 80 years after theoretical prediction
Astronomers analyzing data from the Very Large Telescope (VLT) at the European Southern Observatory (ESO) believe they might have found the first indications of the quantum effect of vacuum birefringence. Whilst predicted by quantum electrodynamics (QED) already in the 1930s, the phenomenon had never been observed before.
The observation was made by a team of scientists from Italy and Poland at the Paranal Observatory in Chile while observing the neutron star RX J1856.5-3754, roughly 400 light-years from Earth. Neutron stars are the very dense remnant cores of massive stars (10-29 solar masses) after they have burst into supernovae, combined with gravitational collapse, at the end of their lives. Their surface temperature is about 6 × 105 K and they have extremely high density (1017 kg/m3, i.e. 1014 times greater than the Earth’s density) and huge magnetic fields (104–1011 T, i.e. at least 108 times stronger than the Earth’s one).
What the team of astronomers observed was that the light passing through the space around the star became linearly polarized (i.e. the electric and magnetic fields, whilst orthogonal to one another and to the direction of propagation of the light, oscillate in only one rather than in different directions) by about 16%. The vacuum is, theoretically, ‘an empty region of space’, so light should travel through it unperturbed. However, quantum electrodynamics (QED) – which describes the light-matter interactions within the framework of quantum field theory – predicts that very strong magnetic fields (such as those generated by a neutron star) can modify space and induce polarization onto the light passing through it.
“According to QED, a highly magnetized vacuum behaves as a prism for the propagation of light, an effect known as vacuum birefringence,” says team leader Roberto Mignani from INAF (Milan, Italy). “This effect can be detected only in the presence of enormously strong magnetic fields, such as those around neutron stars,” Roberto Turolla (University of Padua, Italy) added.
Despite having being predicted 80 years ago in a paper by Werner Heisenberg and Hans Heinrich Euler, vacuum birefringence had never been verified experimentally. “The high linear polarization that we measured with the VLT can’t be easily explained by our models unless the vacuum birefringence effects predicted by QED are included,” concluded Mignani. “This shows, once more, that neutron stars are invaluable laboratories in which to study the fundamental laws of nature,” added Turolla.
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