Strange quantum experiment shows “negative time” is more than an illusion

Photons traveling through a cloud of atoms can emerge so early that they appear to have spent a negative amount of time inside. Researchers tested whether this was merely a misleading feature of the light pulse by making extremely weak measurements of the atoms. Surprisingly, the atoms confirmed the same negative dwell time. The finding does not break standard physics, but it reveals that one of quantum mechanics’ strangest effects is physically measurable.