Actions That Can Change The Past

In this quantum experiment, observing a particle can seemingly reach back in time to decide whether it traveled as a wave or a solid object. Deciding how to measure the particle changes its history after it has already passed through a slit.

This experiment uses entangled pairs of photons to explore the strange nature of reality. By choosing whether to erase information about a photon's path, scientists effectively force the photon to retroactively change its behavior. It challenges our linear understanding of time and causality by suggesting that the present can influence past events. The results demonstrate that, at a quantum level, the universe does not finalize its history until it is actively observed.