3 ms·
> it sounds like the theory of explaining how classical causality falls out isn’t fleshed out yet & is still a work in progress Yes, that's true. One of the co
by pdonis 3y ago
> it sounds like the theory of explaining how classical causality falls out isn’t fleshed out yet & is still a work in progress
Yes, that's true. One of the confusing things is that this "indefinite causal order" thing is very limited--for example, you can't use it to send actual information faster than light. It can produce correlations that seem to defy our usual intuitions about cause and effect, but to actually make use of such things, you have to wait for ordinary classical (i.e., light speed or slower) transmission of the information you need to do so.
For example, in the Alice an Bob scenario at the start of the Quanta magazine article you linked to, while it's possible for Alice's dropping the plate and Bob burning himself to be correlated in ways that defy our classical intuitions, Alice can't make any use of the fact that Bob burned himself until the information from Bob reaches her at light speed or slower, and Bob can't make any use of the fact that Alice dropped the plate until the information reaches him at light speed or slower.
(Btw, the article is misleading on this point, since it describes Alice as dropping the plate when she hears Bob cry out in one version, and Bob burning himself when he hears Alice drop the plate in the other version. If the Alice and Bob events are spacelike separated, which is the scenario that causes all the head scratching, these descriptions are wrong--Alice and Bob can't hear each other faster than light. A proper description would be something like: Alice drops a plate for no apparent reason, she says it just happened out of the blue before she could react; and Bob burns himself for no apparent reason, he says it just happened out of the blue before he could react; but it turns out that every time these things happen, they happen together--Alice never drops a plate without Bob burning himself, and vice versa.)
So it's even possible to interpret all this as not changing our concept of "causality" at all, just putting a little quantum wrinkle in that only has limited effects.
- vlovich123 3y agoI don’t think it’s as simple. The way I’m understanding your framing is that it’s not that there’s no causality, but we have events that are always correlated but causality is impossible to determine due to quantum effects. However, what I read in the link I posted is that not only is it impossible to determine, there are multiple causal orderings and they are all valid due to superposition (ie it’s not just a mathematical artifact of our model but it actually behaves this way in the quantum realm). Of course I think no one actually knows and it’s one of the big mysteries of modern day physics, but the fact that this “model artifact” is able to be experimentally exploited to achieve things not possible classically (eg extracting energy from a thermally neutral environment) makes me think the quantum aspect is actually extremely weird.
- pdonis 3y ago> The way I’m understanding your framing is that it’s not that there’s no causality, but we have events that are always correlated but causality is impossible to determine due to quantum effects. Not just due to quantum effects; due to the fact that the time ordering of the events is not invariant; it's different in different frames. The quantum effects come in because without them the correlations would not be there and there would be nothing counterintuitive to explain. > there are multiple causal orderings In the sense that the time ordering of the events is not invariant, yes, you can say that. But relativity says that no actual physics can depend on something that's not invariant and depends on your choice of reference frame, so really it would be better to say that there is no ordering; the ordering is undefined and can't play a role in anything. > and they are all valid due to superposition No, superposition does not play a role here. We're talking about measurement results; there is no superposition of those. For example, Alice drops the plate and Bob burns himself. There's no superposition there, just two results that always occur together, and classical physics can't explain why. Quantum mechanics can, and its explanation implies that there is some connection between the two events, but if it's a causal connection, it must be one that can "work" in either order, so to speak--or more precisely, that doesn't have to "know" anything about ordering to work.