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A study published in Nature on 5 August 2026 reported the strongest evidence so far for vacuum birefringence, a prediction of quantum electrodynamics that empty space behaves like a crystal when an extreme magnetic field passes through it.
The evidence comes from the radio-emitting magnetar 1E 1547.0-5408, observed for more than 140 hours between March and April 2025 by NASA's Imaging X-ray Polarimetry Explorer (IXPE), together with NASA's NICER telescope and Murriyang, CSIRO's Parkes radio telescope in Australia.
The X-ray emission showed a polarisation degree of about 65 per cent at 2 keV, rising to nearly 80 per cent at certain rotation phases - almost three times higher than in comparable sources and higher than standard emission models can produce.
The polarisation direction was found locked to the magnetar's magnetic field in the same way as its radio waves, the second signature the team looked for, and this was the first coordinated radio and X-ray polarisation measurement of a magnetar.
The prediction being tested dates to 1936, when Werner Heisenberg and Hans Heinrich Euler derived it from quantum electrodynamics; the magnetic fields needed are far beyond anything a laboratory can produce, which is why magnetars serve as the natural laboratory.
Quantum electrodynamics holds that a vacuum is not truly empty: pairs of virtual electrons and positrons continually appear and vanish within it. Ordinarily this makes no observable difference to light passing through. But place the vacuum in a magnetic field of extreme strength and those virtual pairs align with it, and the vacuum stops being the same in every direction. Light polarised parallel to the field then travels at a slightly different speed from light polarised perpendicular to it - two refractive indices in what should be nothing at all. That is exactly what birefringent crystals such as calcite do to light, which is where the name comes from. The consequence an instrument can measure is that X-rays leaving the star are forced into a preferred polarisation direction and stay locked to the field geometry as they travel outward, producing a much higher degree of polarisation than the emission process alone could explain, and one that varies with photon energy.
Simple Analogy: Empty space near a magnetar behaves like a sheet of polarising film that the magnetic field has switched on - light comes out combed into one orientation rather than jumbled.
A magnetar is a type of neutron star; pulsars are neutron stars whose beamed emission sweeps past the Earth. Both are remnants of core-collapse supernovae of massive stars.
The orientation of the electric field of a light wave. Birefringence - two refractive indices depending on polarisation - is familiar in calcite crystals; the result here is that a vacuum in an extreme magnetic field does the same thing.
The quantum field theory of the interaction of light and matter, from which the 1936 Heisenberg-Euler prediction follows and whose picture of a vacuum full of virtual particle pairs this observation supports.
X-rays do not penetrate the atmosphere, so instruments such as IXPE and NICER must be flown above it; IXPE is dedicated specifically to measuring the polarisation of cosmic X-rays.
GS Paper 3 > Science and Technology, Awareness in the Field of Space; Developments in Physics
General Awareness > General Science and Current Affairs in Science
A neutron star with an exceptionally strong magnetic field, the strongest known in the universe, which powers bursts of X-rays and gamma rays and in some cases radio pulses.
The property of a medium of having two different refractive indices depending on the polarisation of the light passing through it, as in a calcite crystal.
The orientation of the oscillating electric field of a light wave; a high degree of polarisation means the photons arriving are strongly aligned with one another.
Short-lived electron-positron and other pairs that quantum field theory holds are continually created and annihilated in the vacuum, giving it measurable physical properties.
The quantum field theory describing how light and charged matter interact; one of the most precisely tested theories in physics.