Astronomers may have found some of the strongest evidence yet for one of the strangest predictions of quantum mechanics: even apparently empty space can influence the way light travels. The phenomenon, known as "vacuum birefringence", was predicted nearly 90 years ago by Werner Heisenberg, one of the pioneers of quantum mechanics, whose work suggested that a perfect vacuum is not really empty but contains "virtual particles" that briefly appear and disappear.
Researchers, including Dr Marcus Lower from Swinburne University of Technology, investigated this long-standing quantum mystery by studying a magnetar, a rare type of neutron star that possesses the strongest magnetic fields known in the universe. Their observations may represent the first detection of vacuum birefringence occurring within the extraordinarily powerful magnetic field of a magnetar. If confirmed, the result could give scientists a new way to investigate the quantum universe. The findings were recently published in Nature.
According to theory, an exceptionally strong magnetic field can affect the sea of virtual particles associated with the vacuum, under whose conditions the particles are expected to influence how light travels, refracting it in a specific way and producing vacuum birefringence. Magnetars provide a rare opportunity to search for this effect because their magnetic fields are strong enough to make the predicted quantum behaviour potentially observable.
Dr Lower was part of an international research team that studied the magnetar 1E 1547.0-5408 using NASA's Imaging X-ray Polarimetry Explorer (IXPE). The observations were supported by the NICER X-ray telescope aboard the International Space Station and by Murriyang, CSIRO's Parkes radio telescope, which is owned and operated by Australia's national science agency. Radio observations collected by Dr Lower using Murriyang, followed by analysis on Swinburne's Ngarrgu Tindebeek supercomputer, helped researchers investigate what could be the first direct detection of this previously theoretical quantum phenomenon.
Dr Lower noted that although vacuum birefringence was predicted in the 1930s, scientists have yet to obtain a definitive detection. "Detecting vacuum birefringence requires a magnetic field more than 100 million times stronger than any we have created on Earth. Fortunately, nature has provided us with magnetars, which are the perfect cosmic laboratories to search for this effect", he said.
The researchers closely followed how radio waves from the magnetar changed direction as the star rotated. From those measurements, they determined that the magnetic and rotational axes of 1E 1547 are nearly aligned, and that the magnetar is observed from an almost face-on perspective. Together, these features give scientists an unusually favourable view for searching for vacuum birefringence around 1E 1547.
The team found two important clues pointing towards the quantum effect: X-rays generated by the magnetar and detected by IXPE showed extremely high levels of polarisation, and the direction of that polarisation remained linked to the magnetic field of 1E 1547 in the same way observed in radio observations. "Because of the strength of the magnetic field, Heisenberg's virtual particles align with the direction the field points", said Dr Lower.
If the interpretation is confirmed, the result could help physicists test how established theories of quantum physics behave under some of the most extreme conditions found anywhere in the universe. Dr Lower noted that additional observations and more advanced computational simulations could help establish whether the signal really comes from vacuum birefringence. Those improvements should make it easier for researchers to distinguish the predicted quantum signature from other physical processes occurring around magnetars. "With this future data in hand and our updated simulations, we could finally complete the search initiated by Heisenberg nearly 90 years ago", he concluded.
