Astronomers May Have Spotted 'Vacuum Birefringence' From a Magnetar's Magnetic Field — a Quantum Effect Heisenberg Predicted 90 Years Ago

A team led by Dr. Marcus Lower of Swinburne University of Technology reports in Nature on August 18 what may be the first detection of vacuum birefringence — light refracted by the 「virtual particles」 that briefly appear and disappear in apparently empty space — using NASA's Imaging X-ray Polarimetry Explorer (IXPE) and CSIRO's Parkes radio telescope on magnetar 1E 1547.0-5408. If confirmed, the result would give physicists a new way to test quantum electrodynamics in conditions that cannot be reproduced on Earth, and would vindicate a prediction Werner Heisenberg first sketched in the 1930s.

What Vacuum Birefringence Actually Is

Quantum electrodynamics predicts that a perfect vacuum is not really empty: it is filled with virtual particles that briefly come in and out of existence. In an extremely strong magnetic field, those virtual particles should refract light in a specific way — a phenomenon called vacuum birefringence, because light from the same source appears to bend at different angles depending on its polarization, much as light refracts differently through calcite or quartz. Magnetars, the rare neutron stars with the strongest magnetic fields known in the universe, are the only natural laboratories where the effect is large enough to be detected from Earth.

How the Team Saw It

The team studied magnetar 1E 1547.0-5408 with NASA's IXPE X-ray polarimeter, supported by NICER on the International Space Station and radio observations taken with CSIRO's Parkes telescope (Murriyang). By carefully tracking how the polarization of X-rays and radio waves changed direction as the magnetar rotated, they determined that 1E 1547's magnetic and rotational axes are almost aligned and that the magnetar is observed nearly pole-on. X-rays generated by the magnetar and detected by IXPE showed extremely high levels of polarization, and the direction of that polarization remained tied to the magnetic field in the same way as the radio measurements — a signature consistent with vacuum birefringence acting on the photons as they leave the magnetar's extreme magnetic field.

Why It Matters

Vacuum birefringence was first predicted in the 1930s and has been sought ever since; until this week no observation had crossed the threshold for a definitive detection. Confirmation would give physicists a new way to investigate the quantum vacuum under some of the most extreme magnetic fields in the universe — magnetic field strengths that cannot be produced in any Earth-based laboratory — and would constrain extensions of the Standard Model that predict different vacuum behavior. The paper also adds magnetars to the short list of cosmic objects (alongside black holes and neutron-star mergers) where extreme-conditions physics can be tested directly.

What Comes Next

Dr. Lower and the international team say additional observations and more advanced computer simulations will be needed to distinguish the vacuum-birefringence signal from other physical processes taking place around magnetars, including surface emission and magnetosphere plasma effects. 「With these future data on hand and our updated simulations, we may finally be able to complete the quest started by Heisenberg nearly 90 years ago,」 Lower said. The paper, 「Vacuum birefringence and the polarized X-ray emission from a radio magnetar,」 is published in Nature with DOI 10.1038/s41586-026-10859-z.