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    Hidden Influence Inequality - Bell's Inequality With Less Drama
    By News Staff | October 28th 2012 03:40 PM | 13 comments | Print | E-mail | Track Comments
    Outside the very large and the very small, the universe is rather easy to understand in modern times.  At the very large, we have to try and make dark matter and dark energy work.  At the very small,  quantum predictions challenge our best understanding about the nature of space and time, what we know as Einstein's theory of relativity.

    The implications of quantum theory have been troubling since it was invented in the early 20th Century. The problem is that quantum theory predicts bizarre behavior for particles, such as two 'entangled' particles behaving as one even when far apart. This seems to violate our sense of cause and effect in space and time. Physicists call such behavior 'nonlocal'.

    Einstein first drew attention to the worrying implications of what he termed the 'spooky action at a distance' predicted by quantum mechanics. Measure one in a pair of entangled atoms to have its magnetic 'spin' pointing up, for example, and quantum physics says the other can immediately be found pointing in the opposite direction, wherever it is and even when one could not predict beforehand which particle would do what. Common sense tells us that any such coordinated behavior must result from one of two arrangements: First, it could be arranged in advance; second, it could be synchronized by some signal sent between the particles.


    In the 1960s, John Bell came up with the first test to see whether entangled particles followed common sense. Specifically, a test of a 'Bell inequality' checks whether two particles' behavior could have been based on prior arrangements. If measurements violate the inequality, pairs of particles are doing what quantum theory says: acting without any 'local hidden variables' directing their fate. Starting in the 1980s, experiments found violations of Bell inequalities. And quantum theory was the winner, it seemed. However, conventional tests of Bell inequalities can never completely kill hope of a common sense story involving signals that don't flout the principles of relativity. That's why researchers have sought to devise a new inequality that would probe the role of signals directly.


    Trying to explain quantum “spooky action at a distance” using any kind of signal pits Einstein’s relativity against our concept of a smooth spacetime. Credit: Timothy Yeo / CQT, National University of Singapore

    A team of physicists have proposed an experiment they say could force us to make a choice between those two extremes to describe the behavior of the Universe. Their experiment is based on what the researchers call a 'hidden influence inequality'.

    "We are interested in whether we can explain the funky phenomena we observe without sacrificing our sense of things happening smoothly in space and time," says Jean-Daniel Bancal, from the Centre for Quantum Technologies at the National University of Singapore.

    Experiments have already shown that if you want to invoke signals to explain things, the signals would have to be traveling faster than light – more than 10,000 times the speed of light, in fact. To those who know that Einstein's relativity sets the speed of light as a universal speed limit, the idea of signals traveling 10,000 times as fast as light already sets alarm bells ringing. However, physicists have a ' get out of jail free card' for that: such signals might stay as 'hidden influences' – usable for nothing, and thus not violating relativity. Only if the signals can be harnessed for faster-than-light communication do they openly contradict relativity.

    The new hidden influence inequality shows that the 'get out' won't work when it comes to quantum predictions. To derive their inequality, which sets up a measurement of entanglement between four particles, the researchers considered what behaviors are possible for four particles that are connected by influences that stay hidden and that travel at some arbitrary finite speed.

    Mathematically (and mind-bogglingly), these constraints define an 80-dimensional object. The testable hidden influence inequality is the boundary of the shadow this 80-dimensional shape casts in 44 dimensions. The researchers showed that quantum predictions can lie outside this boundary, which means they are going against one of the assumptions. Outside the boundary, either the influences can't stay hidden, or they must have infinite speed.

    Experimental groups can already entangle four particles, so a test is feasible in the near future (though the precision of experiments will need to improve to make the difference measurable). Such a test will boil down to measuring a single number. In a Universe following the standard relativistic laws we are used to, 7 is the limit. If nature behaves as quantum physics predicts, the result can go up to 7.3.

    So if the result is greater than 7 – in other words, if the quantum nature of the world is confirmed – what will it mean?

    There are still two choices. There is the option to defy relativity and 'unhide' the influences, which means accepting faster-than-light communication. Relativity is a successful theory that researchers would not call into question lightly, so for many physicists this is seen as the most extreme possibility.  The other option is to accept that influences must be infinitely fast – or that there exists some process that has an equivalent effect when viewed in our spacetime. The current test couldn't distinguish. Either way, it would mean that the Universe is fundamentally nonlocal, in the sense that every bit of the Universe can be connected to any other bit anywhere, instantly. That such connections are possible defies our everyday intuition and represents another extreme solution, but arguably preferable to faster-than-light communication.

    "Our result gives weight to the idea that quantum correlations somehow arise from outside spacetime, in the sense that no story in space and time can describe them," says Nicolas Gisin, Professor at the University of Geneva, Switzerland, and member of the team.


    Published in Nature Physics

    Comments

    Bonny Bonobo alias Brat
    Reference: J.-D. Bancal et al, Quantum nonlocality based on finite-speed causal influences leads to superluminal signalling", Nature Physics, DOI:10.1038/NPHYS2460 (2012).

    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Johannes Koelman
    Free pre-print of the article (which happens to be one year old) can be found at: http://arxiv.org/abs/1110.3795 What this article shows is that: if the violation of the Bell inequalities is due to hidden influences propagating at any finite speed (including any speed in excess of the light speed), these hidden influences can be exploited for communication purposes. A bit of a non-issue really. We already know (thanks to the work by Kochen and Specker) that violations of the Bell inequalities as displayed by the quantum laws of nature have nothing to do with propagating hidden influences. The violations of Bell inequalities simply signify a breakdown of naive realism. That's all. Relativity theory is safe. Quantum mechanics is safe. We just have to let go of the reality ether: http://www.science20.com/hammock_physicist/letting_go_reality_ether-85672
    Bonny Bonobo alias Brat
    Experimental groups can already entangle four particles, so a test is feasible in the near future (though the precision of experiments will need to improve to make the difference measurable). Such a test will boil down to measuring a single number. In a Universe following the standard relativistic laws we are used to, 7 is the limit. If nature behaves as quantum physics predicts, the result can go up to 7.3. So if the result is greater than 7 – in other words, if the quantum nature of the world is confirmed – what will it mean?
    Johannes, thanks for the link and your explanation about this article and the linked paper. Any chance you can explain what this article is saying and asking about this magic number 7 and 7.3? The linked paper says that :-
    (A)ccording to the standard textbook description, quantum correlations are achieved through the collapse of the wavefunction, a process that is instantaneous and independent of the spatial separation between particles. Any explanation of quantum correlations via hypothetical influences would therefore require that they "propagate" at speed v = infinity. Clearly, one may ask whether infi nite speed is a necessary ingredient to account for the correlations observed in Nature or whether an infi nite speed v, recovering a principle of continuity, is sufficient. At fi rst, this question seems unanswerable.
    The only place in the linked paper that then specifically mentions the number 7 is the following text :-
    Therefore, any quantum v-causal model for the above quantum correlations will also yield S = 7:2014 > 7 in the con figuration of Figure 3. Inequality (2) is thus violated and therefore one of the two conditions of the Lemma must be violated. But since the model satisfi es condition i), it must necessarily violate condition ii), i.e., the correlations PR must violate the non-signaling conditions (1).This implies that the correlations PR can be used for superluminal communications.
    I realise it is all probably a bit above my head but is it possible to explain the number 7 part in both the article and the paper in laywoman terms and are they even discussing the same concept? If not then don't worry :) 









    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    Johannes Koelman
    Any chance you can explain what this article is saying and asking about this magic number 7 and 7.3?


    Actually, there is not really a 'magic number'. Depending on the set-up (number of particles, degrees of freedom per particle, etc.) different threshold numbers will appear. So don't read too much into the particular value 7 presented here. In another measurement set-up, the threshold value might be 11.
    Bonny Bonobo alias Brat
    Really? Sorry but I can't help wondering why? Is there a simple analogy maybe?
    My article about researchers identifying a potential blue green algae cause & L-Serine treatment for Lou Gehrig's ALS, MND, Parkinsons & Alzheimers is at http://www.science20.com/forums/medicine
    We just have to let go of the reality ether.
    And have you?
    Johannes Koelman
    The most honest answer: (1/sqrt(2)) ( |yes> + |no> )
    Entangled particles don't have a classical location before they're measured (by the obvious generalization of the Kochen-Specker theorem). Therefore they don't exist within space before they're measured. So there is no spooky non-locality. They are neither local nor non-local, in space, before measurement.
    How the hell do a pair of photons that do not exist know that it is time to spring into existence?
    I mean, either they have their noses "in space" to sniff out when they need to materialize and thus we are back to square one, or else they are magicked across by those dratted fairy-winged qualia which get everywhere.
    If I understand mleifer, "It is true that we can assign some definite properties, often misleadingly called “quantum numbers”, to quantum systems whenever there is a (fundamental or effective) superselection rule forbidding superpositions of different values of that property. In fact, such properties are often used to identify the system."
    MikeCrow
    They get magicked out of "nothing", maybe like a water bubble is something from nothing (well water and air).
    Never is a long time.
    Does a value less than 7 (in this particular experiment) actually rule out the quantum version and require relativity to win out?