
Can retrocausality solve the puzzle of action at a distance? - abhishekjha
https://aeon.co/essays/can-retrocausality-solve-the-puzzle-of-action-at-a-distance
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simonh
What confuses me about these experiments is that, per special relativity, from
the point of view of the photon the moment of it's emission and detection are
the same. It's traveling at the speed of light, so no time passes between
those events. So talking about it's state at this or that time is not talking
about it's intrinsic state, only our external view of it from our frame of
reference. But in terms of a description of the photon in it's frame of
reference, that's meaningless.

EDIT: Or is this what the Paris Zigzag does - treat the state of the photon
and it's twin as a single state, regardless of our time-based view of it,
incorporating the moment of detection, emission and detection of the entangled
twin within the same description?

~~~
alex_young
> from the point of view of the photon the moment of it's emission and
> detection are the same. It's traveling at the speed of light...

From the perspective of the photon (really kind of silly since the photon
can't have any mass, and therefore no observer), there is no time or distance
traveled. It takes 0 time to travel 0 distance.

~~~
jcims
I've been wondering this lately. Does the photon actually exist in the space
between where it is emitted and where it is absorbed? In other words, if
nothing interacts with it along the way, is it really there?

~~~
btilly
The answer is a curious no.

More precisely, the double slit experiment demonstrates that the possibility
that it existed in one place at a particular point in time interacts with the
possibility that it existed elsewhere, and the interaction affects where it
can be possibly found.

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Twisol
This is a really weird, but really neat concept. Where hidden variable
theories assert that the state of a particle is fixed at any given point in
time (but that quantum descriptions don't fully capture this state),
retrocausality seems to say that the state of a particle is _under-determined_
at any given point in time, and when the undetermined fragment of state is
needed to determine the outcome of some future experiment, the result of the
experiment _adds information_ to the past in a consistent manner. But this has
the side-effect of correlating all experiments that depend on this state!

I think this also clarifies why retrocausality doesn't allow information to be
transmitted back in time. Every observation on the under-determined state in
the past by definition doesn't rely on the undetermined fragment, and hence is
consistent with whatever choice occurs in the future. There's some flavor of
monotonicity here, but it's pretty bizzare!

~~~
nabla9
It seems to be similar to Murray Gell-Mann's decoherent histories approach
(aka consistent histories) where the state of a system can be described as
additive sum of all possible histories.

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dvh
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?

~~~
taneq
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](https://en.wikipedia.org/wiki/Bell's_theorem)

~~~
naasking
> "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.

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

~~~
T-A
[https://physicstoday.scitation.org/do/10.1063/PT.6.4.2017071...](https://physicstoday.scitation.org/do/10.1063/PT.6.4.20170711a/full/)

[https://arxiv.org/abs/1405.1548](https://arxiv.org/abs/1405.1548)

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

[http://www.springer.com/us/book/9783319412849](http://www.springer.com/us/book/9783319412849)

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gUXqR8m58CIS
This kind of reminds me of something I thought of a while ago when playing
around with cellular automata in Golly;

With many cellular automata rules, you get chaotic universes, which slowly
stabilize into some final state. Frequently there are small active regions
which take longer to stabilize, and which may travel across the entire state
before they eventually do.

So could our 4-dimensional universe be the final stable state in a
5-dimensional cellular automaton (or some similar structure)? This would allow
every part of the universe to influence every other part without regard for
space/time separation, but the rules could still be set up such that the
particles in the final state adhere to causality, don't exceed the speed of
light, etc...

I think it might only work if the final state represents a fully collapsed
universe without superpositions. I'm not sure how realistic that is.

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comnetxr
They say that action at a distance is "not compatible with special relativity"
but retrocausality is because it's not full retrocausality (can't send signals
back in time) but just a weaker kind. I think they've missed the point on that
as they've just copied the argument for why action at a distance is compatible
with special relativity.

~~~
simonh
That's not what they are saying at all. Your pre-judging the outcome by
calling the end result 'action at a distance', but action at a distance is a
proposed explanation of the outcome, not the outcome itself. That's what they
are trying to explain. Retrocausality is an alternative explanation of the
outcome to action at a distance.

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keithnoizu
My pet theory is when you make an observation you're narrowing down which
subset of the many universes you might reside in. So the particles simply are
going to behave the same even though there is no way to know what reality
you'll get when you check on the first one. However once you do check on the
first one it reduces the subset of possible realities you are in which
includes possible outcomes for observation of the paired particle.

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ericb
We are in a branching simulation. It is lazily loaded and immutable. For
undetermined variables, copy on read causes a branch filled in with each
possible result. So many-worlds, but within a simulation.

~~~
leggomylibro
Have you ever read the Discworld series? There's a side plot where a group of
wizards uses a computer to explore that sort of idea:

\---

...The hypothesis behind invisible writings was laughably complicated. All
books are tenuously connected through L-space and, therefore, the content of
any book ever written _or yet to be written_ may, in the right circumstances,
be deduced from a sufficiently close study of books already in existence.
Future books exist _in potentia_ , as it were, in the same way that a
sufficiently detailed study of a handful of primal ooze will eventually hint
at the future existence of prawn crackers.

But the primitive techniques used hitherto, based on ancient spells like
Weezencake's Unreliable Algorithm, had meant that it took years to put
together even the ghost of a page for an unwritten book.

It was Ponder's particular genius that he had found a way around this by
considering the phrase, "How do you know it's not possible until you've
tried?" And experiments with Hex, the University's thinking engine, had found
that, indeed, many things are not impossible _until_ they have been tried.

Like a busy government which only passes expensive laws prohibiting some new
and interesting thing when people have actually found a way of doing it, the
universe relied a great deal on things _not_ being tried at all.

When something _is_ tried, Ponder found, it often does turn out to be
impossible very quickly, but it takes a little while for this really to be the
case–in effect, for the overworked laws of causality to hurry to the scene and
pretend it has been impossible all along. Using Hex to remake the attempt in
minutely different ways at very high speed had resulted in a high success
rate, and he was now assembling whole paragraphs in a matter of hours.

"It's like a conjurin' trick, then," Ridcully had said. "You're pullin' the
tablecloth away before all the crockery has time to remember to fall over."

And Ponder had winced and said, "Yes, exactly like that, Archchancellor. Well
done."

And that had led to all the trouble with _How to Dynamically Manage People for
Dynamic Results in a Caring Empowering Way in Quite a Short Time
Dynamically_...

\- Terry Pratchett, _The Last Continent_

~~~
ericb
I need to read that...

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UncleSlacky
Isn't this just Cramer's transactional interpretation of quantum mechanics?

[https://en.wikipedia.org/wiki/Transactional_interpretation](https://en.wikipedia.org/wiki/Transactional_interpretation)

~~~
spindle
It's more general than that, and also Price and Wharton don't entirely agree
with Cramer about what needs to be proved, but it's similar, yes. It's
inferior to the Transactional theory in one way, which is that Cramer's
exposition of his theory comes with pictures of elephants.

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AceyMan
I've had this puzzle solved in my own head for quite some time thusly:
Distance is a fiction /aka: arbitrary manifold created by our (conscious?)
selves interacting with "the world," which is really just a big array of
ordered values (N-tuples).

To say that "X is N distance from Y" is no more of a valid proposition than
"Blue is Louder than Yellow".

We attach lots of significance to distance (e.g., 3D space) owing to its
utility for "doing science." (Insert "All models are wrong; some are useful",
but at a grand, multiversal scale.)

My 2¢: YMMV.

(me: Philosophy major with a good bit of sci/math as well)

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tunesmith
So is this saying that if retrocausality is possible, then it's akin to the
kind of time travel where you may have altered something in the past, but the
effects would not manifest until "after" the commencement of the time travel?

Like in a person, if you feel some sort of uncomfortableness that leads you to
make a choice, maybe it's because a minute ago, some time traveler traveled
back a couple of weeks and planted a seed that made you start feeling
uncomfortable!

Maybe all of those, "You know, I never realized until now, but it turns out
I've disliked broccoli for years, I don't know why I eat it every day!"
statements are a result of retro causal time travel. (In this case, George HW
Bush on an anti-broccoli time-traveling crusade.)

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EtDybNuvCu
All of our reasoning would be much easier if free will were an illusion.
Perhaps we should stop fighting it.

~~~
roywiggins
Superdeterminism is even weirder than that though. Why would the universe
conspire to make sure that I picked my observation angle just right to
maintain Bell's inequality? If you use the outcome of a celestial event to
determine the angle, then you've got an absolute enormous conspiracy just to
make sure that Bell's inequality isn't violated. It's deeply weird.

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ggggtez
"we can see that it couldn’t be used to signal, for the same reason that
entanglement itself can’t be used to signal"

Nope, you lost me. This seems too hand wavy.

Also "This isn’t true of many everyday processes – eggs turn into omelettes,
but not the reverse!"... sigh.

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SteveJS
How does this interact with pilot wave theory?

Is the article saying that the causal direction of time is undefined within a
quantum entangled system? Or is that more than what it’s claiming?

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crystaln
Also quantum erasure.

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mikhailfranco
Perhaps retrocausality is just non-locality in time.

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analogic
Yes

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Simon_says
No

~~~
ghusbands
Or yes, but only if you accept time travel.

