A neatly aligned row of dominoes stands before you. Satisfyingly, you gently tap the first tile with your finger to topple it. The domino falls and crashes into its neighbor, which falls in the same way, and creates a ripple effect that continues until all the dominoes have toppled.

Falling dominoes illustrate a deeply rooted concept of science and of everyday life: causation. Event B (the last domino falls) occurs because of event A (the first domino falls). B occurs only if A occurs, and the occurrence of A is independent of that of B.

But that simple causal structure of everyday life can break down in the quantum realm. Recent research reveals that causal relationships can be placed in quantum superposition states in which A influences B and B influences A. In other words, one cannot say if the toppling of the last quantum domino is either the result of the first domino’s fall or its cause. The emerging subject of indefinite causality in a quantum world may provide new insights into the theoretical foundations of quantum physics and general relativity.

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