Neutron stars have magnetic fields so strong they turn empty space into a prism.

This phenomenon is known as vacuum birefringence, a quantum mechanical effect first predicted by Werner Heisenberg and Hans Heinrich Euler in 1936. According to quantum electrodynamics (QED), the vacuum is not truly empty but is filled with virtual particles that constantly pop in and out of existence. Under normal conditions, these particles do not affect light, but the magnetic fields of neutron stars are trillions of times stronger than Earth's.In 2016, a team led by Roberto Mignani from INAF Milan used the European Southern Observatory's Very Large Telescope (VLT) to observe the neutron star RX J1856.5-3754. This star is located about 400 light-years from Earth. The team detected a linear polarization of about 16 percent in the visible light coming from the star's surface. This level of polarization is too high to be explained by anything other than the vacuum itself acting as a prism.The intense magnetism aligns the virtual particles in the vacuum, creating a physical directionality in space. Light passing through this 'polarized' vacuum splits into two components, much like light passing through a calcite crystal. This discovery provided the first observational evidence for a QED prediction that had remained unproven for 80 years. It demonstrates that extreme gravity and magnetism can fundamentally alter the properties of the universe's fabric.