Neutron stars are the very dense remnant cores of massive stars that have exploded as supernovae at the ends of their lives. They have extreme magnetic fields that are billions of times stronger than that of the Sun. By studying the light emitted from an extraordinarily dense and strongly magnetized neutron star using ESO's Very Large Telescope (VLT), astronomers may have found the first observational indications of a strange quantum effect first predicted by Werner Heisenberg and Hans Heinrich Euler in the 1930s. The polarization of the observed light suggests that the empty space around the neutron star is subject to a quantum effect known as "Vacuum Birefringence," which results from the interaction between photons and virtual charged particles that appear and vanish all the time due to quantum mechanics.