Vacuum Bends Light: 140 Hours of X-ray Observations of a Neutron Star Yield the Strongest Evidence Yet for "Vacuum Birefringence," Predicted in 1936

Explanation

"Vacuum" is not empty space. According to quantum electrodynamics, pairs of virtual particles are constantly being created and annihilated in a vacuum. In 1936, Heisenberg and Euler predicted that in an extremely strong magnetic field, these virtual particles would align with the direction of the field, causing the vacuum itself to split light in two (birefringence), just like a calcite crystal. The magnetic field required is 44 trillion gauss — impossible to create on Earth.

The place where this phenomenon can occur is a type of neutron star called a magnetar. A team led by Rachel Stewart of George Washington University and Hoa Dinh Thi of Rice University observed the magnetar 1E 1547.0-5408, which has a magnetic field of about 220 trillion gauss, for more than 140 hours in March–April 2025 using NASA's X-ray polarization observatory IXPE. The degree of X-ray polarization reached up to 80%, about three times that of similar objects. Furthermore, the direction of polarization was fixed to the direction of the magnetic field, behaving the same way as the radio polarization observed with the Parkes radio telescope. This is exactly what would be expected if vacuum birefringence were occurring.

The team called it "the strongest evidence in 90 years," while stating that "it cannot yet be said that vacuum birefringence has been discovered," since other explanations have not been completely ruled out. IXPE plans to observe the same object further.

The paper was published in the scientific journal Nature on August 5, 2026.