Can the future influence the past? The scientific case for quantum retrocausality

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If it can, it did.


So . . . there’s still time to fix all my fuck ups? Awesome.


Or the past determined how you would make that choice in the future. There is no choice.


I would say that it is important to recognize that just when you measure a system and get the wave function to collapse, it does NOT imply that the system had that value before you measured it. When you force a particle to go through one slit, it will go through one slit. That does not imply that if you don’t force it that it doesn’t go through both slits. There is a lot of non-locality implicit in QM. The analogy that I like is a supersaturated solution. If you precipitate the solute into a crystal, it does not imply that is where the solute was before you precipitated it out.


We’ll never know.

Our attitudes in the future will affect our perceptions of the past.


Calling QM non-local is a little dodgy though. If you make the notion of locality precise, [A(x),B(y)] = 0 if x and y are space-like separated, then quantum mechanics is completely local. The “non-locality” is due to quantum states being defined on Hilbert space rather than R^3, where we and our notions of locality come from. Observable operators can act locally and do, but the states aren’t even candidates for being local or not. It makes things a little weird, but in a way that is quite natural mathematically and not so surprising once you catch up.

This retrocausality… I suppose I’ll take a look. But on the surface it sounds very silly, and I think the main lesson of quantum mechanics is that there is no such thing as causality the way Aristotle would have liked. Look at radioactive decay (a much, much simpler scenario than EPR situations) - at a certain time, a heavy nucleus will decay. There is no causal narrative for why it happens at one time or another. “Cause” is just not an ingredient in that story.


It’s much simpler than that. The past does not exist. It is only a story we tell ourselves. We are free to tell it differently.


Retrocausality comes up in classical electrodynamics as well [1], although it’s possible to work around it with some clever math. The result is still far too much like a causality violation for anyone’s comfort.

[1] The acceleration drag on a charged particle kicks in before the force accelerating the particle.


Yes it can! You just need a sufficiently high power level.


There is no causality. There is no free will. The universe is fully deterministic.

Yes, there is self-awareness, and the self-aware experience of decision making.

But all the decisions are made in advance, from the beginning, for eternity: they’re in the script.

No, the script does not have an “author”. The script simply is, like physics.

This, anyway, is the premise of Robert Silverberg’s excellent novel The Stochastic Man (1975).



First, to clarify what retrocausality is and isn’t: It does not mean that signals can be communicated from the future to the past—such signaling would be forbidden even in a retrocausal theory due to thermodynamic reasons. Instead, retrocausality means that, when an experimenter chooses the measurement setting with which to measure a particle, that decision can influence the properties of that particle (or another particle) in the past, even before the experimenter made their choice. In other words, a decision made in the present can influence something in the past.

Don’t those two things contradict each other? If possible why wouldn’t an experimenter change the properties of a particle on purpose as a way to “signal” to their past. It might not be terribly efficient but it’s doable isn’t it?

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Because most likely, he can’t.
It should be similar to the impossibility of using quantum entanglement (which is faster than the light)
to send information through space faster than the light (which in relativity terms implies also back in time).
You can measure a particle and then know the state of the other one, but you can’t “set” the particle to a particular value (without breaking entanglement, as far as i can understand), hence you can’t communicate FTL.
At the same time when performing, say, the quantum eraser experiment, that has some serious retrocausality aftertaste, you can’t send information back in time.

However i must admit that i don’t understand the experiment if not very superficially (the simplified descriptions given to the layman), and i don’t understand why the part that makes impossible to communicate with the past (the fact that the pattern of “measured” particles is
indistinguishable from the one formed by the combination of the “unmeasured” particles from the two detectors) is essential and unavoidable.

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Also i found the video underwhelming.

Meh. Nothing to see here. Sounds like yet another attempt to “explain” apparent-non-locality via some sort of hidden “mechanism” when QM works just fine in the absence of any such mechanism(s).

“Shut up and calculate” is by far the best interpretation of QM out there.


No, for the same reason you can’t use quantum effects to send information faster than light. You can’t choose which superposed state to observe, you just know that, whichever one you observe, the other observer observes the other one. You can use this to construct an encryption key (quantum encryption) but not to send information faster than light, or into the past.

There is another way to approach this,too, in that information has associated entropy, which has associated energy, so sending information faster than light is a special relativity violation.


It can wind up sounding really unsatisfying, “Well you can have an influence superluminally or retrocausally, just no sending information…” Maybe you can’t send a Morse code signal back in time or FTL with these influences, but surely, the influences must carry information of some kind? It sounds like someone is being weaselly. What exactly is the nature of these “influences,” if they carry no information whatsoever?

And they are being weaselly. The attempts to retain more classical narratives of causality (even in a retro version) and reality are at the cost of great weaselness.

One thing which I think will end up being key to figuring out retro-causality: ketamine


Also, if it did, it can.

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