Causality and the Scope of Quantum Theory

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The nature of causality in physics is fraught with complication. The symmetries obeyed by the standard model of particle physics - together with spacetime - conspire against the “noise” of quantum mechanics to precisely define a causal structure.

Part of the structure for how this works seems like a conspiracy baked into the theory. Antimatter essentially exists precisely to cancel out inconsistencies with causality.

Special relativity does some damage to our intuition on causality. Any two events that we can perceive as simultaneous can - in principle - be observed to happen in any order, depending on your motion relative to those events. (For a longer explanation, check out our video on the matter).

But symmetry isn’t always there to save us. Recent experiments in quantum mechanics have managed to create systems which have events that don’t have a well-defined causal order. As Natalie Wolchover puts it

“Alice accidentally drops a plate; the sound startles Bob, who burns himself on the stove and cries out. In another version of events, Bob burns himself and cries out, causing Alice to drop a plate”

Wolchover’s latest piece is a summary of what’s been happening in the field, but summarizing it here won’t do it any favors. So go read it!

But in the off chance you need some motivation: theoretical models of indefinite causal order in quantum systems are shown to cause heat to flow between two objects at equal temperature! A sort of intrinsically quantum mechanical refrigeration.

What’s fun about these ideas is that nothing sinister or fundamentally novel is present. In technical speak, Unitarity is still preserved. We’re just discovering new ideas.