You may have heard of the double-slit experiment, in which a single particle fired at two small gaps appears to interfere with itself, as if it had passed through both slits at once. That happens because, until it is measured by a detector on the other side, the particle is in a quantum superposition of two states. In some sense it is able to take both paths.
It’s weird, and difficult to wrap your head around, but now a team at the University of Vienna in Austria have performed a different kind of experiment that is even more mind-bending: putting the order of events into a superposition.
Normally, it’s easy for us to say that event A happens before event B, or vice versa. But Giulia Rubino and her colleagues have created a situation in which these seemingly contradictory scenarios are in superposition. “If you put together quantum mechanics and causal relations, a situation arises in which there is no pre-defined causal order,” she says. “It’s counter-intuitive.”