But to Maudlin, the mystery of the double slit is even more pronounced when one tries to detect which slit the particle goes through. The interference pattern goes away. But why? It’s because the system being used to detect the particle as it goes through the double slit becomes entangled with the particle. “Schrödinger said that what was really new about quantum mechanics was entanglement,” said Maudlin. “And so from that point of view, the really [surprising] quantum mechanical effect is the disappearance of the interference.”
