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Comment by dvh | Hacker News Reader

I don't understand. Isn't action at a distance this: you take red and green marble and randomly put them in two bags. You take one bag with you to Pluto. Then you open the bag and see the red one, now you instantly know what marble is in the bag that stayed on Earth. This doesn't break any law so what's the problem?

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The problem is that Bell's theorem (which is supported by all of our experimental data so far, afaik) states that "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics."

Basically, hidden variables models can't explain the results of some of our experiments and no-one's found a way to make the two match up.

Reference: https://en.wikipedia.org/wiki/Bell's_theorem

> "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics."

It's more accurately phrased as, "No physical theory of local hidden variables with observations that are statistically independent can ever reproduce all of the predictions of quantum mechanics."

This is actually a new loophole that 't Hooft is exploring in his cellular automata interpretation of quantum mechanics, ie. it has local hidden variables, but because all matter shares a common history from the Big Bang, no observations we can possibly make can be statistically independent of each other.

It also changes things from "local hidden variable" to "universal hidden variable," it's just a different implementation of universal hidden variables than the Many Worlds Interpretation.

Do you have any resources where I can read more about this aimed towards a motivated layman with some working knowledge of QM terminology?

https://physicstoday.scitation.org/do/10.1063/PT.6.4.2017071...

https://arxiv.org/abs/1405.1548

The free e-book linked from that article is probably the most up to date description:

http://www.springer.com/us/book/9783319412849

> no observations we can possibly make can be statistically independent of each other.

Yep. When I'm reading these articles, "Everything Connects" is a pretty useful heuristic for flowing with apparent discrepancies in theories...although of course nailing down concrete theories and finding precise explanations for phenomena is important.

Great point, more people need to know about this "small" detail.

That resolution of the problem is generically called "superdeterminism". The OP mention that multiple times...

"Thus, it is conceivable that freedom of choice has been restricted since the beginning of the universe in the Big Bang, with every future measurement predetermined by correlations established at the Big Bang"

https://en.wikipedia.org/wiki/Superdeterminism

Usually you don't need to go that far back. The fact that the photon emmiter always emmits pairs with opossite spin is good enough.

The phrasing here reminds me of Godel's Incompleteness Theorem...probably not coincidental.

> "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics."

GIT paraphrased to match: "No axiomatic system can produce all truths within that axiomatic system."

In the hidden variables case (yours), it was predetermined which marble was in the bag before you opened it. In the retrocausal case, the information about which marble is in the bag simply isn't part of reality until you've checked. With macroscopic objects like bags and marbles, this is ridiculous, because someone had to put the marble in the bag (a "shared cause"). However, Bell's Theorem shows that entanglement isn't a sufficient "shared cause" to explain his experiment. Thus, we either need superdeterminism (the "shared cause" is the origin of the universe!), spooky action at a distance, or retrocausality.

>>" the information about which marble is in the bag simply isn't part of reality until you've checked"

It seems to me that the universe uses lazy evaluation (1).

(1) https://en.wikipedia.org/wiki/Lazy_evaluation

It's even weirder than that, though, because with lazy evaluation you still have a complete description of the state involved; it's just a matter of unpacking it by executing the description. That's more analogous with a hidden variables theory. With retrocausality, it seems that performing an observation fundamentally adds information to that which has been observed.

If retrocausality can be likened to a computational mechanism, I'd be really interested to know what it is! Like I said in another comment, it seems like there's some element of monotonic computation involved, but the brand of monotonicity is quite unusual.

>>"it seems that performing an observation fundamentally adds information to that which has been observed."

disclaimer: I have not idea what I'm talking about.

Is it not that the "adds information" that you mention, it's like sampling a probability distribution? Of course, then your program, the universe, it's not deterministic anymore (but I think that it's already the case when quantum physics is involved).

So, your lazy function, don't do anything until is required and then it returns a sample from a probability mass function (not a pdf because we are talking quantum here).

No. See the Kochen-Specker theorem. You can choose which way to measure in ways that sort of interact with one another statistically even after two particles are far separated.

Can anything be "forgotten" once it's been determined?

For example--can two entangled particles be created, then forced to interact in a way that collapses their wave functions, then sucked into a black hole, without having been observed by anything that isn't sucked into that black hole--does their collapse matter? Does the universe forget it?

If so, then could the whole universe be a reversible computation, once it's summed up from big bang to big ... whatever happens at the end?

This makes me think of dangling pointers and memory leaks.

That can also be called "retro causality".

Imagine instead of Green and Red, it could be either Green or Red, or Blue or Yellow. If you measure it's Red/Green-ness, one will be red, and the other will be green. If you measure it's Blue/Yellow-ness, one will be blue and the other will be yellow. You can't measure the blue/yellow-ness of one and the red/green-ness of the other. As soon as one is measured, the other collapses into a particular color.

The problem is that the state of the your particle, in Pluto, changes when somebody on Earth observes it. The marbles were neither exactly green nor red before somebody looked.

It is not as simple as you'll get with macroscopic things, but yes it is not as crazy as some people imply either.

I always though the problem is with the "collapse" thingy. You don't observe Green marble. You just entangle yourself with the marble pair. And so does the guy with other marble.

In the end two marbles, you, and the other observer become a single entangled system, who has two states: you've seen green marble (and the other guy red), and you've seen red marble (...).

And now if I "observe" that system, I also become entangled, and also will have two states: "I saw you claiming you saw green", and "I saw you claiming you saw red", and so on.

From a physical point of view, in that description there's no mystery. What might be interesting is how you (as in your mind) seemingly experience only one outcome. I even suspect we don't.

> From a physical point of view, in that description there's no mystery.

There is no explanation either.

What do you need explained? This is the nature of things according to the Standard Model.

how you seemingly experience only one outcome

(the Standard Model? of particle physics?)

The correlation in the quantum case is stronger https://youtu.be/ZcpwnozMh2U?t=18m10s

There's no problem. The copenhagen interpretation is a joke, same as the "spooky action at a distance". The formula to reproduce this mess is too many PhDs with zero intuition, a hierarcical system based on seniority where status=correctness, and jokes gone wrong (ie copenhagen interpretation). The "collapsing universe" is simply the realization that the other marble is blue, and so is the "instant" information transfer.

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