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New Theory Explains How Objective Reality Emerges from the Strange Underlying Quantum World

The views expressed are those of the author and are not necessarily those of Scientific American.

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Quantum theory is one of the most profound discoveries of humanity. In my view, it’s on a par with Cuban cigars and single malt whiskey. The theory has been hugely successful in showing us the inner workings of the universe. However, it is plagued by serious problems and provokes fiery debates, such as the giant argument that erupted on the Cosmic Variance blog a couple of weeks ago. One of these problems is the unanswered question of how the everyday macroscopic world emerges from its strangely behaving microscopic constituents.

Let me put it this way. Nobody doubts that the moon is circling the Earth even when we’re not looking. In the microworld, however, quantum objects can behave as if they were created by the mere act of observation. It goes like this: you open a box and you see an atom there. Can you infer that it was there the moment before you looked inside? The mind-boggling answer is NO! This was proven theoretically in 1964 by John Bell and later confirmed experimentally, beginning with the work of John Clauser and others in the early 1970s (described in a classic Scientific American article by Bernard d’Espagnat). If the same were true for the moon, it would be like saying that the act of looking into the night sky made the moon appear.

So where does this leave us? Physicists believe that quantum physics should describe all objects big and small, yet the microworld is very different from the macroworld. We have to explain how the latter is the result of the former.

With some colleagues I recently proposed and then generalized a new solution. Interestingly, our explanation for how the classical world arises from the quantum world does not invoke “decoherence” or any of the other mechanisms proposed before.

Decoherence is probably the best-known model introduced so far to explain how the microscopic world’s quantum weirdness disappears. Decoherence is the loss of information about a quantum state to the surrounding environment to the point where the state has lost many of its quantum properties. It had been believed that decoherence is ubiquitous in large systems and happens extremely fast. However, recent research done by my colleague at the Centre for Quantum Technologies in Singapore, Vlatko Vedral, indicates that decoherence can be much slower in large systems than was expected (more about this in Scientific American’s June cover story “Living in a quantum world”).

In our recent paper, we take a different approach. We consider how measurements work in the macroworld, finding that some quantum features are simply unobservable. Most remarkably, this approach shows that something called quantum nonlocality disappears for objects big enough to contain roughly the Avogadro number of atoms—the number of atoms you’d expect in a few grams of matter.

The idea of quantum nonlocality goes back to a seminal paper by Albert Einstein, Boris Podolsky and Nathan Rosen (EPR) published in 1935. They described a simple thought experiment which showed that quantum theory cannot be reconciled with two common-sense and very reasonable assumptions about the fabric of reality, namely, locality (there is no “spooky action at a distance”) and realism (measurements reveal objective properties of physical systems).

Their ingenious test exploits the notion of “correlations,” something that we encounter in everyday life. For example, if a clock in New York strikes midnight, then we know that at the same time a clock in Singapore strikes noon. The EPR experiment tests what kind of correlations one can observe between measurements performed on quantum particles, highlighting that quantum correlations can be stronger than the logic of locality and realism dictates. Since then, experiments have confirmed quantum theory’s predictions, forcing physicists to accept that the fabric of reality is nonlocal, nonrealistic or both!

The problem then becomes how to reconcile the local realism of the macroscopic world—the moon and other big objects really do follow these two common-sense assumptions—with quantum theory. We solve this problem: we show that macroscopic observables are always local realistic even when the underlying states are quantum.

When one measures large systems, it is impossible to do it precisely. For instance, if you have a gas in a container, you can measure its pressure, temperature, etc., but you cannot measure the velocities of all the particles of the gas. This translates technically to a limited set of observables that are average properties of the system.

Our result derives from this concept of macroscopic observables being a kind of average. There is a limit to the number of quantum correlations each particle can have with another, which is referred to as the “monogamy” of quantum correlations. The concept is simple: if particles A and B exhibit correlations of the kind predicted in the EPR experiment then A and B can only have local and realistic correlations with other particles.

This monogamous behaviour extends to correlations between larger groups of quantum particles, which is the main idea behind our result. Imagine you are making a macroscopic measurement between two regions in space, A, containing quantum particles A1, A2, A3, etc., and B, containing B1, B2, B3, etc. The measurement samples all possible pairs. Due to monogamy, as you increase the number of particles, the overall strength of the correlations measured dilutes. For instance, AiBj may be strongly correlated but then Ai and any other B-particle exhibit only local realistic correlations (see figure). Analysing the statistics, we find that local realism emerges for macroscopic correlations without us needing to invoke any other mechanism.

This doesn’t mean there are no macroscopic quantum effects, just that any such effects will be too weak to demonstrate themselves in as bizarre a way as they do on the microscopic level. Whatever quantumness is present can never be strong enough to contradict local realism. It is, after all, reassuring to know that the moon is there when you don’t look at it. What the remaining quantumness in the macroscopic world means is unclear at this stage. The debates will surely rumble on.

Dagomir Kaszlikowski About the Author: Dagomir Kaszlikowski is a Principal Investigator and Associate Professor at the Centre for Quantum Technologies at the National University of Singapore. His research group tackles fundamental questions in quantum theory and contributed to the discovery of “information causality” as a possible underlying principle of quantum mechanics. Dagomir was born and studied in Poland before moving to Singapore, where he enjoys spending some of his free time underwater, exploring deep wrecks of the South China Sea.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. denysYeo 10:20 pm 12/5/2011

    I am currently reading “Quantum Enigma” – by Bruce Rosenblum and Fred Kuttner, which seems to me (being a psychologist rather than a physicist) to explain the issues associated with quantum physics in a very readable manner. Your article adds to this discussion and suggests, as I understand it, that there is some form of reality to be observed in the macro world (ie., the moon is there whether we are observing it or not), but we still cannot be sure of any form of reality in the micro world until we carry out an observation – but using your method we can get a better idea of the boundary between these two “states”. Or maybe it is a continuum rather than a boundary?

    Anyway, I am thankful to people like you, and Rosenblum and Kutter for at least trying to help people like me understand a little more what this quantum world is all about.

    Denys Yeo
    New Zealand

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  2. 2. Dr. Strangelove 3:32 am 12/6/2011

    Most physicists don’t believe quantum mechanics should describe all objects, small and big. Some physicists with philosophical inclinations desperately want to believe the macro world also exhibits “quantum weirdness.” But this is not supported by experiments.

    Physicists can invent theories as to why big objects are not weird like subatomic particles. This is one theory based on some experiments I read years ago. The act of observation requires colliding subatomic particles or photons to the particle you are observing. This collapses the state of superposition of the particle being observed.

    Similarly, a large object is composed of many atoms that bump into each other. This collapses the state of superposition of the atoms without need of observation. So large objects behave the same way whether you are looking at it or not.

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  3. 3. joenn 3:29 pm 12/6/2011

    My take on the difference between the Quantum world and the world we can see is that there isn’t one. I remember in Seattle’s Science center there was a display that dropped small balls at the top of an array of pegs such that when a ball hit a peg, it would either bounce to the right or left and down to another peg where it did the same thing. 20 or more layers later it would fall into an array of long columns where you could see how many balls fell into each one. They would run hundreds of balls at a time and each time it would form a predictable curve. You couldn’t predict where each ball individually would go but you could predict with great accuracy where all of them would go. In my way of looking at it the balls bouncing around in the pegs are what’s going on in the quantum world and we can’t see that directly. What we can see is the curve or the results of all of the interactions. You are looking at another example in your computer screen. There are only 3 colors on a standard color screen so how can we see so many colors? At the dot by dot level the 3 colors work by one set of rules but the screen as a whole apparently works by another.

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  4. 4. heyman29323 3:56 pm 12/6/2011

    We do have quantum processes in the macro-material world all around us. Photosynthesis is a prime example. The properties of green tea attacking free radicals using quantum tunneling is another example. Any nero-surgeon you ask might have an example or two of it. We wouldn’t have TV or Radio without it. We change our reality with it whenever we put our minds to it. What we call the “animal survival instinct” is just “quantum weirdness” in a way. The sea turtle knows to make a run for the sea the very moment it is hatched. How does it know to do this without reasoning. It is more connected to quantum energy than we are. We lost it somewhere along the line. But it is all around us. We just don’t recognize it sometimes.

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  5. 5. Dr. Strangelove 9:48 pm 12/6/2011

    In the macro world, it is possible in principle to predict where each ball will go using Newtonian mechanics. But in practice we do not know all the initial conditions to do the computation so it appears random. In the micro world, the Uncertainty Principle states it is impossible to determine both momentum and position of subatomic particles simultaneously even in principle. That’s the difference between macro and micro.

    The fact that TV has electronics that obey quantum mechanics doesn’t mean that it exhibits quantum properties. The TV does not appear and disappear out of nowhere like virtual particles.

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  6. 6. Pariah 10:16 pm 12/6/2011

    In my opinion the quantum presence in the macro-world is “consciousness”.

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  7. 7. heyman29323 11:53 am 12/7/2011

    True the TV only obeys quantum processes and the TV itself doesn’t appear out of nowhere, but the image on the TV certainly does. But yes I agree that wasn’t the best example. But I still find it ironic that this so-called archaic technology (compared to the computer anyway) still has better picture quality than a computer’s image. And that is probably because of the quantum mechanics formulas it follows.

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  8. 8. mikej77 1:17 am 12/8/2011

    We are in the box with the Moon in it and not in the box with the Moon not in it. We might think about how macro and micro humans are.
    Technically we are unable to receive the light from the Solar System that does not have the moon orbiting the earth.

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  9. 9. Wayne Williamson 6:16 pm 12/8/2011

    Interesting article…just wondering if an atomic clock would become useless if the amount matter is reduce below the “Avogadro number”

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  10. 10. Kel Feind 9:25 pm 12/12/2011

    The relationship between the quantum world and the macro world exists in so far as we compare our predictions with what we come to observe. I don’t really know what I am going to do tomorrow. I can imagine many possibilities about what might happen and each thought could be associated with a probability. Tomorrow night, I’ll know what actually happened, the possibilities will “collapse” into actual events. We don’t consider this quantum weirdness, because it is just how things exist for us but it seems to me to be the link between sentient beings and quantum mechanics

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  11. 11. skeam 5:51 am 02/21/2012

    What is this “monogamy” of quantum correlations, which this new proposal is based on? Tell me more.

    What about all the recent news about ever higher numbers (more than 2) of particles having been entangled?

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