
Physicists provide support for retrocausal quantum theory - metafunctor
https://phys.org/news/2017-07-physicists-retrocausal-quantum-theory-future.html
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woliveirajr
> First, to clarify (...) It does not mean that signals can be communicated
> from the future to the past(...) instead, retrocausality means (... it) 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.

So, hard to explain in layman words: future can't transmit information to the
past, but an action that will be taken in the present can influence some
properties of a particle in the past.

Take two particles, separate them enough, do some "A" thing to one of them and
the other instantaneously will have the only possible state that doesn't break
the universe.

And one viable explanation is that when we've done the "A" thing, this
information was transmitted to the particle in the past. We just couldn't
measure this before (in the past) because measuring interferes with it, and
that property can only be revealed at the same time that we've done the "A"
thing.

Is that it? Kinda of information is transmitted to the past, just can't be
read before some time ?

~~~
petters
That explanation still seems to allow faster than light communication.

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ridewinter
The many-worlds interpretation of quantum mechanics is the only one that's
easy to understand. All others seem to bend over backwards to realistically
explain quantum effects. It's so rare to find clear explanations, but they do
exist.

My favorite from David Deutsch, a main creator of the idea of quantum
computing:

To those who still cling to a single-universe world-view, I issue this
challenge: explain how Shor’s algorithm works. I do not merely mean predict
that it will work, which is merely a matter of solving a few uncontroversial
equations. I mean provide an explanation. When Shor’s algorithm has factorized
a number, using 10^500 or so times the computational resources than can be
seen to be present, where was the number factorized? There are only about
10^80 atoms in the entire visible universe, an utterly minuscule number
compared with 10^500. So if the visible universe were the extent of physical
reality, physical reality would not even remotely contain the resources
required to factorize such a large number. Who did factorize it, then? How,
and where, was the computation performed?

~~~
JadeNB
> My favorite from David Deutsch, a main creator of the idea of quantum
> computing:

It's interesting, but this "I challenge you to answer a philosophical (not
scientific) question in a way that I find acceptable" approach surely skirts
uncomfortably close to dogmatically creationist arguments.

(For the record, my answer, which certainly wouldn't satisfy Deutsch, is: it
works because the math proves it works; there's no more "how" about it than
there is in "how does the computation 10^(250)*10^(250) = 10^(500) work?")

~~~
opportune
I agree with you. I think the need for philosophical justification stems from
our desire to reconcile quantum computing with our classical computing/physics
models. But just because we desire such a thing doesn't mean there exists a
reasonable one.

What I think the truth really is, is that we don't encounter quantum
superpositions in an obvious way in our day to day lives. So it's inherently
"weird" for us, especially since it seems to violate our notion of object
permanence. But that doesn't mean it's wrong, it just means that one of our
evolved instincts isn't applicable on a quantum scale.

~~~
JadeNB
> But that doesn't mean it's wrong, it just means that one of our evolved
> instincts isn't applicable on a quantum scale.

I think that this puts it very well. If a theory doesn't fit with our
intuition, then it might mean that the theory is wrong; but it's at least
plausible that our intuition is wrong, and goes from plausible to
overwhelmingly likely once the theory is backed up by experimental data.
(Anyone, even or especially a scientist, who dogmatically trusts his or her
intuition over science probably could do with a quick refresher in cognitive
biases.)

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amoruso
John Cramer has been pointing this out for years (since the 1980s I think):

"The Transactional Interpretation of Quantum Mechanics and Quantum
Nonlocality"

[https://arxiv.org/abs/1503.00039](https://arxiv.org/abs/1503.00039)

Also, Bell's inequality shows the state vector is nonlocal, and and that
implies causality violation in relativity.

Third, quantum information theory has retrocausality in the form of negative
entropy:

"Negative entropy in quantum information theory"

[https://arxiv.org/abs/quant-ph/9610005](https://arxiv.org/abs/quant-
ph/9610005)

Too many things pointing in the same direction. This is the right way to look
at it.

~~~
BoiledCabbage
What happened to John Cramer?

He hit the popular science media as he was leading up to his modified delayed-
choice quantum eraser for retro-causality. According to him that was a
supposed paradox meaning the result of the experiment was going to have to
give a result that conflicted with our theories and thus give an avenue for
new discover (ie allow us to use a test that doesn't match our model).

It's been a decade and no results were ever discussed. That seems bizarre to
me.

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rthomas6
This reminds me a lot of the sci-fi novel Anathem by Neal Stephenson. Trying
to be spoiler-free as possible, in the book, the many-worlds interpretation of
quantum mechanics is the correct one, and the worlds lie on a directed acyclic
graph, meaning that the flow of information can only move in one direction
along the universes. So in the same way information can only travel in one
direction in time (into the future), information can only travel laterally
into other dimensions in one direction. So, in the book, nobody can directly
influence the past, but the question of existing in multiple worlds and
sending information between them, and thereby potentially exploring many
possible outcomes of a situation simulatenously, is much more open.

In other words, retrocausality WITH information transfer is possible when the
information is flowing from one universe to another, in a certain direction.

~~~
Tossrock
_spoilers_ I think he implies pretty heavily that the Rhetors actually CAN
influence the past, through a mechanism very similar to that described in the
article - "choosing" which way an event happened in the past. Their powers
work at Narrative scale though, not just experimental observation.

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ajuc
There's also the idea of timeless physics, as argued in
[https://en.wikipedia.org/wiki/Julian_Barbour#Timeless_physic...](https://en.wikipedia.org/wiki/Julian_Barbour#Timeless_physics)

or in slightly different form in
[http://lesswrong.com/lw/qp/timeless_physics](http://lesswrong.com/lw/qp/timeless_physics)

It's apparently possible to rewrite all physical equations without time
coordinates and they still work.

I'm not competent enough to understand all of it and to check if it's not
contradicting any experiments, but I'd love to hear if it does.

It seems particularly elegant (because if the state of the universe can't
repeat, then why do we need time?).

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Lxr
I never fully understood the Bell experiments. How exactly do they discount
hidden variables?

~~~
miltondts
They don't here is what Bell said:

"There is a way to escape the inference of superluminal speeds and spooky
action at a distance. But it involves absolute determinism in the universe,
the complete absence of free will. Suppose the world is super-deterministic,
with not just inanimate nature running on behind-the-scenes clockwork, but
with our behavior, including our belief that we are free to choose to do one
experiment rather than another, absolutely predetermined, including the
‘decision’ by the experimenter to carry out one set of measurements rather
than another, the difficulty disappears. There is no need for a faster-than-
light signal to tell particle A what measurement has been carried out on
particle B, because the universe, including particle A, already ‘knows’ what
that measurement, and its outcome, will be."

~~~
euyyn
Ok, and barred that?

~~~
BoiledCabbage
I could be mixing things here, but aren't the three options losing one of

locality, definitiveness and non-conspiracy?

[https://en.wikipedia.org/wiki/Counterfactual_definiteness#Ov...](https://en.wikipedia.org/wiki/Counterfactual_definiteness#Overview)

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naasking
> But by allowing for the possibility that the measurement setting for one
> particle can retrocausally influence the behavior of the other particle,
> there is no need for action-at-a-distance—only retrocausal influence.

I don't quite get it. According to GR, non-locality and retrocausality are
equivalent. So how is retrocausal QM different from Bohmian mechanics/pilot
wave theories?

~~~
lucozade
> So how is retrocausal QM different from Bohmian mechanics

From what I can tell from this, there's no difference in the sense that it's
experimentally indistinguishable.

It's different in the sense that Bohmian mechanics relies on a non-local pilot
wave to match QM results. This frees the other assumption i.e. causality to
get to the same answer.

TBH I can't see either get much attention until someone comes up with a way to
make them produce different results compared to standard QM approaches. Either
that or someone finds a way to make QM calculations substantially easier using
these approaches. In other words, if they produce new physics or have
practical value.

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mirekrusin
// WARN: because past is static, to support retrocausal influence, we're
spawning alternative universes which are joined on retrocausal action choice -
this slows down The Simulation and, if abused, can exhaust resources and
crash.

~~~
rf15
don't remind me that people actually believe that you can infer the existence
of a simulation only with data points that are created and controlled by it.

~~~
alasdair_
There doesn't seem to be anything wrong with this basic idea. As a (bad)
analogy, think about how computer code can be written to detect whether or not
it is running inside a virtual machine or on a "real" device. It's usually
almost impossible to tell but by probing at the extremes and forcing the
system to do things that never happen under common usage, it's possible. (It's
also possible to break out of the system entirely and gain access to the host
OS sometimes, but perhaps that's taking things too far :)

~~~
BoiledCabbage
> It's usually almost impossible to tell but by probing at the extremes and
> forcing the system to do things that never happen under common usage, it's
> possible

Yes, but it's only possible by comparing the results to those same results
when that action is performed outside of a VM.

If you can never baseline your data outside the VM, then you'll never know
you're seeing odd results. You'll just think "that's the way the world works".

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keyle
Isn't this another blanket theory that magically explains a ton of 'spooky'
stuff without actually proving it? Just asking from a common's people point of
view. I'm a practical person, I need tangible facts...

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stupejr
Time to start scanning for tachyon radiation

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rf15
This would allow us to predict the future and potentially allow for time
travel as well. Seems unlikely from the implications alone, but let's see what
peer review of the details will tell us.

~~~
naasking
> This would allow us to predict the future and potentially allow for time
> travel as well.

It doesn't actually, they discuss this in the article. The laws of
thermodynamics impose an arrow of time via boundary conditions, and something
similar would apply here.

~~~
FeepingCreature
Purely as an amateur, it seems unelegant to have something that violates
causality as a mechanism, but doesn't give you a way to violate cause and
effect at a higher level. It's like the universe is conspiring to keep its
awesome tricks from you.

~~~
naasking
> It's like the universe is conspiring to keep its awesome tricks from you.

Well that's just inevitable. You're never going to see a naked singularity,
for instance.

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nilson
sounds like ad-hoc hypothesis

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code5fun
Cmon guys really, first we come up with all this misterious dark stuff and now
this...sigh

