Physicists from Oxford and Lisbon have run a full 3D, time-resolved simulation showing that empty space can act like a nonlinear medium. Their model finds that three intense laser pulses make photons rebound and forge a fourth beam, echoing a long-standing prediction from quantum electrodynamics.

Classical physics treats vacuum as an absence. Quantum theory disagrees. The vacuum teems with flickering pairs of virtual electrons and positrons that borrow energy briefly and vanish. Strong electromagnetic fields can polarize those pairs. That tiny response turns “nothing” into a medium with a faint optical nonlinearity.

When three high-power laser pulses cross at the right angles and frequencies, quantum electrodynamics (QED) predicts four-wave mixing in vacuum. The combined fields nudge virtual pairs, which then mediate photon‑photon scattering. A new, phase‑matched beam should appear with a frequency and direction dictated by the input pulses.