Quantum correlations do not imply instant causation

August 12, 2016
Credit: Christina Giarmatzi, University of Queensland

A research team led by a Heriot-Watt scientist has shown that the universe is even weirder than had previously been thought.

In 2015 the universe was officially proven to be weird. After many decades of research, a series of experiments showed that distant, entangled objects can seemingly interact with each other through what Albert Einstein famously dismissed as "Spooky action at a distance".

A new experiment by an international team led by Heriot-Watt's Dr Alessandro Fedrizzi has now found that the universe is even weirder than that: entangled objects do not cause each other to behave the way they do.

Distinguishing cause from effect comes naturally to us. Picture yourself in a room where someone is flicking a light switch. Intuition and experience lets you establish a simple causal model: the switch causes the lights to turn on and off. In this case, correlation implies causation.

If we could entangle two lights, you would see them turn on and off at random, regardless of how far apart they are, with no obvious switch and in perfect lockstep. Einstein's preferred explanation of this mysterious effect was that there must be a hidden light switch which acts as a common cause for our entangled lights.

Einstein's world view was finally proven wrong last year by groups in Vienna, Delft and Boulder. However, the search for an answer how entanglement really works continues, and one popular explanation was that entangled objects could cause each other perhaps instantaneously, ignoring the universal speed-of-light limit.

In a new experiment, Dr Fedrizzi and colleagues found that this claim is also wrong. They set up individual photons to act like entangled light bulbs and then subjected them to two tests.

In the first, they essentially flicked the themselves to test their causal hypothesis. In the second, they tested a new inequality by theory collaborator Rafael Chaves, which shows that nonlocal causality can in general not explain quantum entanglement.

'Spooky' action

"We have spent decades to prove Einstein wrong and can now start looking into extended models of his 'spooky' action", says Dr Fedrizzi. "What we found is yet another mechanism that doesn't explain this quintessential feature of entanglement. Our results drastically narrow down the remaining explanations."

The techniques the team developed also have practical applications such as in cyber-security. They can for example be used to increase the level of trust we have into quantum encryption devices.

Their study has been published in Science Advances.

Explore further: Researchers explore quantum entanglement

More information: M. Ringbauer et al. Experimental test of nonlocal causality, Science Advances (2016). DOI: 10.1126/sciadv.1600162

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44 comments

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lincruste
4.6 / 5 (12) Aug 12, 2016
So much words and so little about the experiment itself...
luke_w_bradley
3.9 / 5 (9) Aug 12, 2016
"Recent experiments put strong constraints on realist interpretations of quantum mechanics, ruling out maximally epistemic (47) and local causal (3–5) models. Our results exclude a broad class of nonlocal causal models, thus contributing to a clearer picture of the status of reality and causality in quantum mechanics."

So what's left? I wish they would say something about that, about nonlocal causal models outside the class of models ruled out, and what they must look like.
KultiVator
4.4 / 5 (9) Aug 12, 2016
The quantum world is difficult and perplexing enough already, so it'd would be great to see articles that report the core of the story more thoroughly and coherently than...

"They [..] subjected them to two tests.

In the first, they essentially flicked the light switch themselves to test their causal hypothesis. In the second, they tested a new inequality by theory collaborator Rafael Chaves, which shows that nonlocal causality can in general not explain quantum entanglement."
forumid001
1.6 / 5 (12) Aug 12, 2016
It's sad to see how pathological the fundamental research of QM has become, with so much attention paid to voodoo ideas of "instantaneous action", "super space", "more weird than spooky action"....
antialias_physorg
4 / 5 (12) Aug 12, 2016
So much words and so little about the experiment itself...

They have the link to the full article at the bottom. Just use your mouse and click.

with so much attention paid to voodoo ideas

Got any better ones? Let's hear 'em (with the math to back 'em up...otherwise spare us the brainfarts)
julianpenrod
1.2 / 5 (6) Aug 12, 2016
Among other things, the light bulb "analogy" is flawed, even illegitimate, in that, as described, you are looking at the two bulbs constantly. You see both bulbs when they are off, then see them turn on simultaneously. That's not the case with "entangled" particles. One particle's characteristics is determined, and, then, the other particle, assumed to have been in no particular state, is observed and assumed to have been made to change to that state by the other particle's having been observed. It can be said to be no different at all from, conventionally, a white marble and a black marble being chosen from a box and put in two boxes, then moved far apart, and one marble being observed. Conventionally, they retain their natures all the while. It can be said to be more a sign of the limitations of conventional measuring systems.
shavera
4.1 / 5 (7) Aug 12, 2016
Don't mistake Science Journalism with the state of science research. Journalists just love the phrase "spooky action," but that's practically unheard of in physics.

Luke: What's left is what very many scientists thought it was from the start. Quantum mechanics doesn't have 'realism.' Which is to say, superpositions of states are truly superpositions, without any definite meaningful 'state.' It is only if you require quantum particles or systems to have 'real' states outside of measurements where you must invoke all this craziness about faster-than-light signalling or backwards-in-time influences and such.

It's just one of those weird things where journalists picked up on the cooler sounding idea ("spooky action at a distance"), but science has generally been on the more straightforward one (QM is truly probabilistic, not deterministic).
shavera
3.6 / 5 (5) Aug 12, 2016
julianpenrod presents the 'realist' kinds of interpretation. That there's some underlying 'black or white' bit of information that really exists, even if we can't measure it. If that was true, then there *must* be some process by which that underlying data communicates the measurement process it's gone through, and that must happen faster than light or backwards in time or all sorts of weirdness to make results work.

To borrow from penrod's example, the local "non-realism" scenario is like putting a marble into a box and it truly is both black and white at the same time, and remains that way maybe until it gets measured (though there's a whole different discussion of the 'measurement problem' in QM, and what that really means for these superposed states).
julianpenrod
1.2 / 5 (6) Aug 12, 2016
shavera says there is no one or the other bit of information that would define a particle before it is observed. That there is a kind of mixed state they are in. But that would mean that there literally could be a marble in a box that is both white and black. It would mean that the person standing behind you could be a superposition of your brother and your sister, until you looked. But, it can be asserted that your brother and sister would be self observers. Is a marble also self observant? Is it, then, not the case that a particle "observes" itself, "knows" what it is, even if what could be termed a biological being wasn't watching? They seem to "know" what kind of things to do, as if they "knew" what they were.
julianpenrod
1 / 5 (6) Aug 12, 2016
If a particle is "entangled" but turns out to be negative and is proved negative by impacting a centimeter away from the center of a target in an electric field, it must have moved a tenth a centimeter away, a half a centimeter away, before being observed. Or do they constitute expanding regions as they move outward? Even then, they "know" they are not neutral particles.
RealityCheck
1.6 / 5 (5) Aug 12, 2016
Hi shavera and luke_w_bradley. :)

@shavera: Careful, mate, you are conflating two different concepts. IIRC, certain posters here (maybe even including yourself?) have in the past taken great pains to point out that "superposition" concept is NOT the same as "entanglement" concept; the former being merely undefined states co-located states, while the latter are correlated states across separation distances/positions not communicable at greater than lightspeed.

@luke_w_bradley: Re question of "what's left?". Perhaps only TWO things left that can be happening: EITHER (1) there is faster than lightspeed communication via as yet unobservable 'space connection' that remains between entangled entities until they are disturbed via 'destructive interference' with a similar connection between other particles and/or by an observational construct/device which collapses all local connections passing through it; OR (2) its all chaos/stats/probability etc 'artifacts'.

Cheers all. :)

tinitus
Aug 13, 2016
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theon
1 / 5 (4) Aug 13, 2016
They will not show Einstein wrong, and one day they will become aware of the fact the Santa Bell got it wrong.
tinitus
Aug 13, 2016
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tinitus
Aug 13, 2016
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tinitus
Aug 13, 2016
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tinitus
Aug 13, 2016
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Phys1
4.3 / 5 (6) Aug 13, 2016
The entanglement occurs only between massive particles - and photons, right? The photons provide the same interference pattern in double slit experiment, like the massive particles, OK? This implies, that the photons are also massive particles and their entanglement is mediated with pilot wave around photons, which is spreading with speed of light.


Zeph is back.
Phys1
3.7 / 5 (6) Aug 13, 2016
Tinitus
You told use about the water surface hundreds of times
but it has has nothing to do with quantum mechanics.
luke_w_bradley
3.5 / 5 (4) Aug 13, 2016
Hey Shavera, so the way that works out philosophically is that an entangled particle, when measured, doesn't cause a change in its entangled twin, (no spooky action at a distance) but rather a correlative relationship exists between measurements? Or does disallowing realism still allow us to have a nonlocal correlative "state" the particles share, only weird because its probabilistic in nature not deterministic?

Still trying to wrap my mind around this.
tinitus
Aug 13, 2016
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tinitus
Aug 13, 2016
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tinitus
Aug 13, 2016
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torbjorn_b_g_larsson
3.7 / 5 (3) Aug 13, 2016
I have a hard time understanding physical causality without some form of signal propagating spacetime cone analogous to Einstein's light cones. However, it seems to me that the paper's [ http://arxiv.org/...67v1.pdf ] "causal models" scheme models that for signal speed approaching infinity. I.e. there is still a time process that orders causality within a "signal cone"..

Nice result, I had no idea that one could test for signaling with speeds way above the universal speed limit, but thought we simply had to accept that it works. Not only is (light cone) causality saved, it is - ha - universally saved.
tinitus
Aug 13, 2016
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torbjorn_b_g_larsson
4.2 / 5 (5) Aug 13, 2016
@julian penrod: You are referring to a classical analog. But there is already experiments that show that QM is truly non-local. There the experimenters chose what test to do without giving enough time to change out the classical correlation that would explain the result.

@shavera: QM is not all probabilistic though, anymore than biological evolution is. Shrödinger's differential equation show that QM is non-probabilstic in between interactions (say, observations) of the system. In sum QM propagates states non-probabilistically. [ https://en.wikipe...c_system ]
Eikka
5 / 5 (3) Aug 13, 2016
The effect is not weird - it just follows from the laws of physics that information cannot travel faster than light. There cannot be an instant causal effect between distant objects.

The weirdness comes from the idea that the property they're measuring exists and is real before the measurement itself. In quantum physics you have to reject one of the three: localism, realism, or causality. Rejecting localism or causality leads to breakdown of physics, but rejecting realism works perfectly fine: the property you think exists only becomes real when you interact with the object and the property becomes necessary.

That means the information about the entangled property exists outside of the pair itself, and has plenty of time to propagate because you cannot separate the pair faster than the speed of light. The eventual result of the experiment is carried along, or decided by the rest of the world around the objects and made real by the act of looking what the outcome was.

Eikka
not rated yet Aug 13, 2016
For, if you send an entangled pair of atoms in two different directions, there isn't enough information in the system to begin with to decide which atom has a spin pointing up and which has it down. The only information present is the fact that they are opposite.

That is, neither entangled atom actually has a spin at all until they encounter the device which measures the spin, and by measuring the device essentially imparts a spin property on the atom.

That still doesn't tell us which atom had which spin, but it's not needed: when the two experimenters who measured their part of the pair measure each other, their mutual interaction follows the original information: the pair was opposite.

As to who got what, that's still in superposition, which isn't broken until a third party comes along and adds the information to the system to decide who got which spin. The superposition exists until it becomes necessary for the outcome to be in a particular way.
j3s5y909
1 / 5 (1) Aug 13, 2016
Hello guys! Newbie here

I'm starting with physics and I know that this is way too much for my level but this was too interesting to ignore it. Does this mean that there is a speed faster than light?

Thanks for your kindness.

Jim4321
3 / 5 (2) Aug 13, 2016
OK gurus two questions

Mermin proposed that a consistent interpretation of QM could be made in terms of correlations, and that correlata are not needed. Does that add to this discussion?

Feynman once proposed that the fundamental element of theoretical QM modeling is the probability amplitude. Does taking the idea of probability amplitude as fundamental and deriving the rest from that add anything?

This is all above my understanding.
tinitus
Aug 14, 2016
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tinitus
Aug 14, 2016
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tinitus
Aug 14, 2016
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Reg Mundy
1.7 / 5 (6) Aug 14, 2016
"What we found is yet another mechanism that doesn't explain this quintessential feature of entanglement. Our results drastically narrow down the remaining explanations."
Unfortunately, for establishment scientists the "narrowed down" explanations do not include the obvious one, which is their misunderstanding of what TIME actually is.
tinitus
Aug 14, 2016
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tinitus
Aug 14, 2016
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ProfRaccoon
1 / 5 (1) Aug 14, 2016
Einstein could not be right anyway, because what ever the outcome of the QM "entanglement" experiments, his conflicting ideas could not be correct 100%, so forget Einstein. If not even "non-local causality" is the case, what remains of the idea of entanglement aka quantum correlation? Is it very "strange" that particles or processes that are at a great distance from each other behave independently (weird, isn't it?). And if particle's 'probability wave functions' become ONE probability distribution, then a MUTUAL exchange of superluminal signals (which is a CO-relation) is such that we cannot tell the difference between cause and effect, but that does not mean there is NO cause and effect. Are quantum states like the 'strange attractors' in macroscopic chaotic behavior, such that 'small butterfly' signals from other chaotic systems influences the probability that the primary chaotic system switches to a different 'strange attractor'?
Da Schneib
5 / 5 (2) Aug 14, 2016
This one's gonna make my brain hurt.
Arginx
1 / 5 (1) Aug 15, 2016
@tinitus
For example, I'm increasingly convinced, that the entanglement is directional effect and it applies in similar way, like the entanglement / alignment of galaxies with filaments of dark matter. It means, when we arrange the particles into a single line, they will get entangled more intensively and the superluminal character of entanglement will get more pronounced. It could also explain many aspects of cold fusion, which can be described with massive entanglement of atom nuclei, which are colliding along a single line inside the metal lattice. We can talk about it like about 1D condensate of atom nuclei. This compact line may serve as an absorption environment for neutrons and gamma rays, which are lead with the resulting dense area of vacuum like with waveguide and they get completely absorbed and thermalized there.


For me entanglement is more like vortex and I agree with you about the rest. ;-)
tinitus
Aug 15, 2016
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PhysicsMatter
5 / 5 (2) Aug 15, 2016
Of course correlation is not causation and never was, also if we assume instancy of two events causation cannot be defined.

You could read about it last year at:

https://questforn...-quanta/
tinitus
Aug 15, 2016
This comment has been removed by a moderator.
Hyperfuzzy
1 / 5 (3) Aug 15, 2016
Now we have a new measure in physics, weirdness. Never mind the algebraic error in Einstein's theory, i.e. his calculations using a scalar constant, lambda nu = c, as the speed of the wavelet. He didn't see the wavelet does not change, so in order to understand the constant, he changed space and time and no one caught the error. Funny ..
Hyperfuzzy
1 / 5 (2) Aug 15, 2016
Why we use a non-causal math, QM, to explain causality is beyond reason.

Also I watched the movie, "Einstein's Biggest Blunder" and LMAO.

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