Researchers engaged in the causality of physics’ discussion divide themselves into three main groups: causal fundamentalists, causal non-fundamentalists, and causal eliminativists. Researchers and philosophers assert that some sort of causal ordering is present at the bottom of quantum theories and that causal ordering increases efficiency in quantum information technology, quantum computation, or quantum communication. In this talk, Professor Elise Crull explained topics including why causal ordering in quantum systems would be indefinite, a quantum switch experiment, and a relativistic example. Indefinite causal ordering can be explained by three reasons: the superposition principle, the uncertainty principle, and the experimental reason that it contradicts Bell’s inequality.
A quantum switch occurs when particles propagate from one space-time trajectory to another simultaneously, which enables carriers to transmit data over several channels asynchronously in a quantum superposition of different causal orders, thereby making the relative causal order of communication channels indefinite. When we consider the non-relativistic case, indirect validation of indefinite causal ordering in quantum switches does not rely on causal witnesses to verify causal entanglement but is more efficient for computation in indefinite causal ordering quantum circuits compared with causally ordered quantum circuits. As a result of direct validation of indefinite causal ordering, causal witnesses usually yield results in close agreement with theoretical values for nonseparable causal processes.
The philosophical analysis of the indefinite causal ordering implies that causality might be another aspect of the uncertainty relations because causality is ultimately redefined in terms of variables at the lower level and those variables are susceptible to uncertainty because of quantum mechanics. However, there are limits to the interpretation of uncertainty relations that are specified in the Kockin Specker Theorem on the assignment of well-defined systematic values to systems before measurements are made. Professor Elise Crull points out that much of the conversation about relativistic causality in physics and philosophy is dominated by signaling and the bounds placed on signaling by the no-signaling theorem. However, these indefinite causality ordering processes and superpositions can be used to approach the problem of relativistic causality.
Causality, like space-time, emerges dynamically at the macro level in the form of clear causal connections, but it lacks a discernible structure at the micro-level. As the background time or definite causal structure is inherent these sorts of causal structures emerge from dynamical processes that allow quantum decoherence processes to dampen the superposition, which therefore is only observable via some other probe or under some other environmental condition. At the lower level, causality either doesn’t exist or, if it does, it is indefinite. We need to be clear about what kind of indeterminacy it is, and whether there are bounds to how indefinite it can be. The causal fundamentalists have the hardest job of the three major camps when it comes to metaphysics of causation because of the indefinite causal ordering and Bohemian world that require a primitive ontology. Contrary to fundamentalists, non-fundamentalists may be safe in claiming that causality does not exist in the fundamental make-up of the universe.
–Gaurav Lohkna