Quantum Physics: cause and effect can be reversed (General)

by David Turell , Thursday, March 11, 2021, 18:57 (1143 days ago) @ David Turell

Weird as it sounds superposition does it:

https://www.quantamagazine.org/quantum-mischief-rewrites-the-laws-of-cause-and-effect-2...

"Over the last decade, quantum physicists have been exploring the implications of a strange realization: In principle, both versions of the story can happen at once. That is, events can occur in an indefinite causal order, where both “A causes B” and “B causes A” are simultaneously true.

***

"The possibility follows from the quantum phenomenon known as superposition, where particles maintain all possible realities simultaneously until the moment they’re measured. In labs in Austria, China, Australia and elsewhere, physicists observe indefinite causal order by putting a particle of light (called a photon) in a superposition of two states. They then subject one branch of the superposition to process A followed by process B, and subject the other branch to B followed by A. In this procedure, known as the quantum switch, A’s outcome influences what happens in B, and vice versa; the photon experiences both causal orders simultaneously.

***

"With the emerging frameworks, “we can make predictions without having well-defined causality,” Brukner said.

***

"The operational question is: In quantum gravity, what can we, in principle, observe? Hardy thought about the fact that quantum mechanics and general relativity each have a radical feature. Quantum mechanics is famously indeterministic; its superpositions allow for simultaneous possibilities. General relativity, meanwhile, suggests that space and time are malleable. In Einstein’s theory, massive objects like Earth stretch the space-time “metric” — essentially the distance between hash marks on a ruler, and the duration between ticks of clocks. The nearer you are to a massive object, for instance, the slower your clock ticks. The metric then determines the “light cone” of a nearby event — the region of space-time that the event can causally influence.

***

"What we normally think of as causal relationships — such as photons traveling from one region of the sky to another, correlating measurements made in the first region with measurements made later in the second region — act, in Hardy’s formalism, like data compression. There’s a reduction in the amount of information needed to describe the whole system, since one set of probabilities determines another.

"Hardy called his new formalism the “causaloid” framework, where the causaloid is the mathematical object used to calculate the probabilities of outcomes of any measurement in any region. He introduced the general framework in a dense 68-page paper in 2005, which showed how to formulate quantum theory in the framework (essentially by reducing its general probability expressions to the specific case of interacting quantum bits).

***

"In “the most beautiful experiment” done so far, according to Rubino, Jian-Wei Pan at the University of Science and Technology of China in Hefei demonstrated in 2019 that two parties can compare long strings of bits exponentially more efficiently when transmitting bits in both directions at once rather than in a fixed causal order — an advantage proposed by Brukner and co-authors in 2016. A different group in Hefei reported in January that, whereas engines normally need a hot and cold reservoir to work, with a quantum switch they could extract heat from reservoirs of equal temperature — a surprising use suggested a year ago by Oxford theorists.

***

"In a key paper in 2019, Magdalena Zych, Brukner and collaborators proved that this situation would allow Alice and Bob to achieve indefinite causal order.

***

"A “quantum equivalence principle” analogous to the equivalence principle that, a century ago, showed Einstein the way to general relativity. One way of stating Einstein’s equivalence principle is that even though space-time can wildly stretch and curve, local patches of it (such as the inside of a falling elevator) look flat and classical, and Newtonian physics applies. “The equivalence principle allowed you to find the old physics inside the new physics,” Hardy said. “That gave Einstein just enough.”

***

"But ultimately quantum gravity must be specific — answering not just the question “What can we observe?” but also “What exists?” That is, what are the quantum building blocks of gravity, space and time?

***

"Hardy thinks his causaloid framework might be compatible with loops or strings, potentially suggesting how to formulate those theories in a way that doesn’t envision objects evolving against a fixed background time. “We’re trying to find different routes up the mountain,” he said. He suspects that the surest route to quantum gravity is the one that “has at its heart this idea of indefinite causal structure.'”

Comment: all of this is at the quantum level of reality, not our level. Only we see causality