In a very recent thread, some of our physicists suggested that the temporal relationship between cause and effect is not clear-cut. I Searched to try to find that thread, but came up only with two old threads: one discussing Huw Price’s ideas and another thread discussing the “pilot wave” interpretation.

So … consider Grover’s algorithm for quantum computing. With *n* qubits, we start with a superposition of *2[sup]n[/sup]* states; do some calculations on these qubits; let “the wave function collapse”; and … Presto! — a solution is found faster than possible with classical computing. It seems magic, almost as though a future event (an oracle’s recognition of search success) reaches backwards in time and causes the qubits to assume that desirable pattern.

My eyes glaze over long before I can properly grasp Grover’s algorithm (and a simple intuitive explanation would be appreciated) but I don’t think full understanding is needed for my question. I just want to ask: Is this algorithm a good example of what physicists mean when they speak of cause-effect relationship being unclear? Is it possible to speak of the future oracle “causing” the qubit behavior?

A follow-up question relates to the alleged quantum tunneling associated with the high efficiency of chlorophyll’s energy conversion. Again, my eyes glaze over before understanding any details. But *intuitively*: What “causes” the electronic excitation to “tunnel” its way to the reaction center? Can the successful reaction be viewed as a “cause” of the exciton’s quantum motion, much as the oracle in Grover’s algorithm?