Quantum Physics: ambiguous causality (General)

by David Turell , Wednesday, June 28, 2017, 20:46 (2703 days ago) @ David Turell

Cause and effect is all tangled up in quantum mechanics. Which came first is never clear:

https://www.nature.com/news/how-quantum-trickery-can-scramble-cause-and-effect-1.22208

" Walther's group has shown that it is impossible to say in which order these photons pass through a pair of gates as they zip around the lab. It's not that this information gets lost or jumbled — it simply doesn't exist. In Walther's experiments, there is no well-defined order of events.

"This finding1 in 2015 made the quantum world seem even stranger than scientists had thought. Walther's experiments mash up causality: the idea that one thing leads to another. It is as if the physicists have scrambled the concept of time itself, so that it seems to run in two directions at once.

"In everyday language, that sounds nonsensical. But within the mathematical formalism of quantum theory, ambiguity about causation emerges in a perfectly logical and consistent way.

***

"What's more, thinking about the 'causal structure' of quantum mechanics — which events precede or succeed others — might prove to be more productive, and ultimately more intuitive, than couching it in the typical mind-bending language that describes photons as being both waves and particles, or events as blurred by a haze of uncertainty.

"And because causation is really about how objects influence one another across time and space, this new approach could provide the first steps towards uniting the two cornerstone theories of physics and resolving one of the most profound scientific challenges today. “Causality lies at the interface between quantum mechanics and general relativity,” says Walther's collaborator Časlav Brukner, a theorist at the Institute for Quantum Optics and Quantum Information in Vienna, “and so it could help us to think about how one could merge the two conceptually.”

***

"what the Copenhagen interpretation does at least seem to retain is a time-ordering logic: a measurement can't induce an effect until after it has been made. For event A to have any effect on event B, A has to happen first. The trouble is that this logic has unravelled over the past decade, as researchers have realized that it is possible to imagine quantum scenarios in which one simply can't say which of two related events happens first.

"Classically, this situation sounds impossible. True, we might not actually know whether A or B happened first — but one of them surely did. Quantum indeterminacy, however, isn't a lack of knowledge; it's a fundamental prohibition on pronouncing on any 'true state of affairs' before a measurement is made.

***

"Last year, Walther and his colleagues devised a way to measure the photon as it passes through the two gates without immediately changing what they know about it6. They encode the result of the measurement in the photon itself, but do not read it out at the time. Because the photon goes through the whole circuit before it is detected and the measurement is revealed, that information can't be used to reconstruct the gate order. It's as if you asked someone to keep a record of how they feel during a trip and then relay the information to you later — so that you can't deduce exactly when and where they were when they wrote it down.

"As the Vienna researchers showed, this ignorance preserves the causal superposition. “We don't extract any information about the measurement result until the very end of the entire process, when the final readout takes place,” says Walther. “So the outcome of the measurement process, and the time when it was made, are hidden but still affect the final result.”

***

"Quantum causality might supply a point of entry to some of the hardest questions in physics — such as where quantum mechanics comes from.

Quantum theory has always looked a little ad hoc. The Schrödinger equation works marvellously to predict the outcomes of quantum experiments, but researchers are still arguing about what it means, because it's not clear what the physics behind it is.

***

“"If quantum theory is a theory about how nature processes and distributes information, then asking in which ways events can influence each other may reveal the rules of this processing.” (my bold)

"And quantum causality might go even further by showing how one can start to fit quantum theory into the framework of general relativity, which accounts for gravitation. “The fact that causal structure plays such a central role in general relativity motivates us to investigate in which ways it can 'behave quantumly',” says Ried."

Comment: As confusing as usual, but note my bold. God has the universe and life running on quantum information processes