Has vacuum birefringence been seen at long last? – Physics World
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Astronomy and space
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Home »<br>Astronomy and space » Has vacuum birefringence been seen at long last?
Astronomy and space
Research update
Has vacuum birefringence been seen at long last?
17 Aug 2026
Reading Time: 4 minutes
Smoking gun? Artist’s impression of a magnetar. Polarized X-rays from such an object could contain evidence of vacuum birefringence. (Courtesy: ESA) ">
Smoking gun? Artist’s impression of a magnetar. Polarized X-rays from such an object could contain evidence of vacuum birefringence. (Courtesy: ESA)
The first direct glimpse of vacuum birefringence has been claimed by astronomers who have studied X-ray and radio emissions of a neutron star that is both a magnetar and a pulsar. A central, but unconfirmed prediction of quantum electrodynamics (QED), vacuum birefringence involves the polarization of the quantum vacuum by powerful magnetic fields.
Made by a US-led international team, the observation could lead to the use of magnetars as natural laboratories to study other extreme-field phenomena of the quantum vacuum. However, another group headed by researchers in Italy remains unconvinced that the data constitute a “smoking gun” for vacuum birefringence. They believe that alternative explanations remain viable.
Birefringence occurs when the refractive index of a medium depends on the polarization of the radiation passing through it. When refraction occurs at such a medium, there is an angular separation of the two differently-polarized waves. This occurs with certain crystals such as calcite and lithium niobate, where the anisotropic lattice structure interacts differently with oppositely polarized photons. Birefringence can also arise from the interaction of photons with magnetic fields in plasma.
Virtual pairs
A key prediction of QED – first made in 1935 by Werner Heisenberg and Hans Euler – is that sufficiently strong magnetic fields can make a vacuum birefringent by polarizing the virtual electron–positron pairs that fluctuate in and out of existence. Ninety years on, however, vacuum birefringence had not been observed because the huge fields required cannot be generated in the laboratory.
“Having a natural lab already existing that could look into some of these effects is something that would be very beneficial,” says astronomer Rachel Stewart of George Washington University in the US. “That’s something people have probably been looking into for decades.”
Stewart and colleagues’ natural lab is a magnetar. These objects comprise a rare subtype of neutron star that have extreme magnetic fields of up to 1011 T. This makes magnetars the most magnetic objects observed in the universe and bright sources of X-rays. Data from NASA’s Imaging X-ray Polarimetry Explorer (IXPE) telescope...