The sequence of causes and effects doesn’t capture the fundamental nature of things

We usually imagine reality as a chain of events: A causes B, which then causes C. Causes come first and effects follow. At the quantum level, however, this order may not always be fixed.

Experiments involving the quantum switch suggest that two processes can occur in an indefinite causal order. Instead of either A causing B or B causing A, a quantum system can behave as though both possibilities are present simultaneously.

This does not mean that causality disappears from everyday life. Dropped plates still fall, fire still burns, and actions still have consequences. But the familiar sequence of causes and effects may emerge only at the level of ordinary experience rather than describe the deepest structure of reality.

The question becomes especially important in attempts to combine quantum mechanics with gravity. In Einstein’s theory, gravity affects space and time. If matter can exist in a superposition of different states, the structure of spacetime—and therefore the order of events—may also become indefinite.

Physicists are now developing ways of describing reality without assuming a fixed causal order. Such theories remain incomplete, but they challenge one of our most basic intuitions: that the universe must always know which event happened first.

Perhaps cause and effect are not the foundation of reality.

Perhaps they are simply how reality appears to creatures like us.

Sources:

Natalie Wolchover, “Quantum Mischief Rewrites the Laws of Cause and Effect”, Quanta Magazine, 11 March 2021.