MIT physicists have carried out the most refined version yet of the famous double-slit experiment, offering striking confirmation of quantum theory while disproving a long-standing idea proposed by Albert Einstein. Their results reveal, with exceptional atomic precision, the elusive dual nature of light – behaving as both a wave and a particle, but never fully as both at the same time.

First performed in 1801 by Thomas Young, the original double-slit experiment showed that light passing through two narrow openings produces an interference pattern of bright and dark stripes, demonstrating wave-like behavior. However, quantum mechanics later revealed a deeper mystery: light also consists of particles called photons. When scientists attempt to measure which slit a photon travels through, the interference pattern disappears, and light behaves strictly as a particle. Observing one nature hides the other.

In 1927, Einstein challenged this interpretation during a debate with Niels Bohr. He argued that a photon passing through one slit should slightly disturb it, and that this disturbance could be measured without destroying the interference pattern – potentially revealing both particle and wave properties simultaneously. Bohr countered using the uncertainty principle, insisting that measuring the path would inevitably erase the wave pattern.

Now, MIT researchers have tested this question under highly controlled conditions. They cooled more than 10,000 atoms to ultracold temperatures and arranged them in a crystal-like lattice, using individual atoms as slits. By carefully adjusting how tightly the atoms were held in place, they controlled how much information could be gained about a photon’s path. The results were clear: the more path information obtained, the weaker the interference pattern became. Even when removing the “spring-like” confinement Einstein envisioned, the outcome remained unchanged. Light’s wave and particle aspects could not be observed simultaneously – confirming Bohr’s view and reinforcing the foundations of quantum mechanics.

For more details, read the full article by MIT.