Quantum physics says goodbye to reality


An article published this month in Nature reports on a new set of tests on Bell’s Thm.

Link: Quantum physics says goodbye to reality (April 2007) – News – PhysicsWeb.

Markus Aspelmeyer, Anton Zeilinger and colleagues from the University of Vienna, however, have now shown that realism is more of a problem than locality in the quantum world. They devised an experiment that violates a different inequality proposed by physicist Anthony Leggett in 2003 that relies only on realism, and relaxes the reliance on locality. To do this, rather than taking measurements along just one plane of polarization, the Austrian team took measurements in additional, perpendicular planes to check for elliptical polarization.

They found that, just as in the realizations of Bell’s thought experiment, Leggett’s inequality is violated – thus stressing the quantum-mechanical assertion that reality does not exist when we’re not observing it. “Our study shows that ‘just’ giving up the concept of locality would not be enough to obtain a more complete description of quantum mechanics,” Aspelmeyer told Physics Web. “You would also have to give up certain intuitive features of realism.”

It’s a good article and worth the read. The point is that reality is fundamentally subjective – and determined by the observer doing the act of observation.

The really interesting question of course (and the point of Schrödinger’s cat paradox) is “How does the Universe know we’re looking?”

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  1. I’ve read this article twice and I still don;t understand it. They appear to use “reality” as a technical term here which they don’t bother to define. So, can we keep locality and give up “realism”? Personally, I’d be ready to chuck locality first, but that’s me. “Spooky action at a distance” isn’t so spooky in quantum realms.

  2. Would we say reality is fundamentally subjective or fundamentally interpretive from the human side of matters? Or both?

  3. Ruidh – the problem with action at a distance is that we have to invent some sort of mechanism that tells an object it’s being acted upon.
    When I taught the concept, I would usually ask the students to think about a ball that was dropped. How does the ball know which way to fall? Einstein got rid of Newtonian action at a distance by saying that the ball fell in the direction that time moved the slowest – toward the greatest gravitational well. In a quantum description (assuming you can quantize gravity) we imagine that the ball is interacting with a hail of gravitons being emitted from the gravitational source, and that is what causes it to move. In action at a distance – neither of these occur. (Newton and Kepler wondered if it might have been angels passing along the info.)
    Christopher – If I understand how you’re using the terms, I think we’d have to say it’s fundamentally subjective. (At least as best we understand the act of observation right now – which is to say hardly at all…) The subjectiveness is what really bothers most physicists who want someway to say that reality exists in the absence of being observed. Feynman however argued that we’re just going to have to get over our hangups about how reality works and get our minds wrapped around what is instead of what we think ought to be.

  4. Paul Martin says

    Nick, does any of this relate to the real world as we know it, or is this strictly a problem for theoretical physicists?
    Let me explain the question: quantum mechanics is a description of matter at very small dimensions. The correspondence principle demands that the theory reduces to classical mechanics for large quantum numbers. In other words, it at least cannot contradict the large body of data consistent with classical mechanics. Because of that, I am skeptical about making sweeping philosophical statements about the nature of reality based on quantum mechanics. Quantum mechanics has its uses within its own realm of applicability, but outside of the physics lab, most of us live in worlds well described by Newtonian physics, where realism and locality work quite well. Or, am I missing something? (I have to admit that Bell’s theorem was always very opaque to me.)

  5. Paul – that’s the $64K question isn’t it? Newtonian physics works really well, and lets us do useful things easily. Quantum Physics and Relativity theory are correct, but they’re mathematically difficult to massage.
    The Correspondence Principle is really just a hand-waving sort of thing. There’s no obvious physical reason that it should be so – it’s just makes a pragmatic sense.
    Quantum effects are important on the macroscopic scale when we use things like transistors or nuclear devices… and you can’t explain such behavior in terms of classical physics. Even simple things like the instantaneous transitions from one energy level to another in a Hydrogen atom are disallowed by classical thinking – and since that’s were most of our light comes from, it’s not a negligible thing.
    I do think it’s fair though to ask sharp questions of any quantum theory – since I don’t know that anyone is happy with our lack of understanding the phenomenology that is at work.

  6. ruidh says

    Was it Feynman who suggested that all electrons and positrons are really the same ur-electron bouncing backwards and forwards in time wherever it needs to be observed? I have no problem considering the possibility that photons scatter backward in time from an interaction and interfere with the other half of an entangled pair and influence it’s polarization– non-locality, if you will.
    I’ve always had a problem envisioning what’s going on with photons mediating both attraction and repulsion in electromagnetism. An electron and a positron exchange virtual particles to accomplish an attraction. But, how does the photon which is emitted “know” if it is destined to attract a positron or repel an electron? What is different about that photon from one what will mediate a repulsion?

  7. I think it was Fermi (or perhaps Dirac) who suggested that the reason all electrons were exactly alike was because there was only one in the Universe and we were just seeing it over and over again.
    The virtual photons that are emitted by a moving electron or a proton carry the “information” about the charge… If an electron is being told there’s a negative charge nearby, it’s repelled, etc. The information is passed along via the momentum transfer (though clearly I’m over-simplifying…)

  8. Where is the information carried? A photon is massless, spin 0. It only has a few degrees of freedom: frequency (or momentum) and polarization. What else is there?

  9. The information is really in the wave function of the photon. The problem is that to work out how the “info” is transmitted to the election is not something obvious to understand (at least not for me anyhow.) You have to work out the path integrals for the interaction.
    Feynman did a pretty incredible job of finding a way to describe all this by drawing pictures of the interactions – but the pictures aren’t “real” per-se, they help us make sure we’re listing all the possible ways that particles encounter each other – and each diagram is equivalent to calculating the associated path integral.
    (I think I’ve got this right – it’s been over 20 years since I’ve solved these sorts of problems.)

  10. ruidh says

    I remember his little “clock” or “timer” explanation from QED. But that discussion never did it for me.
    I think that appeals to wave functions are appeals to non-locality.

  11. ruidh: pretty much yes. But Quantum Mechanics is by it’s nature a non-local theory. Or am I misunderstanding your point?

  12. ruidh says

    Ummm, but isn’t that the point of the article above — to what extent are non-locality and “reality” necessarily violated by QM?

  13. Ah. I see what you’re asking ruidh. Ok.
    QM is by its very nature non-local. The issue has been that lots of folks have felt that the non-locality was a mathematical artifact and not something inherently true. Most folks believed that QM functioned like Statistical Mechanics – as a very useful mathematical tool, but not something that describes the local reality. People came to think as Einstein did that once we figured out what the “hidden” variables where, we could drop QM and describe a microscopic phenomenon in a absolute and deterministic way.
    Bell’s Thm (or the EPR paradox) are a way to test whether or not there are hidden variables in a relationship or not. (Bell’s Thm works in lots of situations – QM is just one of them.) The fact that Bell’s Thm verifies again and again and again seems to be saying that in fact QM non-locality is inherent to reality and not simply a mathematical artifact. The EPR paradox is just true in other words – even if it seems paradoxical to us.
    The issue then is how to understand it means to say that “the act of observing a system collapses its wave function”… There are a number of ways to express what’s happening there, from the many worlds interpretation to the Princeton and/or the Berkeley ones.

  14. Steven says

    Would I be wrong in suggesting that most of the “controversies” that exist in QM stem from a stubborn need to find classical explanations for quantum phenomena? For example, this need to find a mechanism to explain action at a distance. QM says there is no distance and there are not two separate objects between which action takes place. Likewise, the statement that observation creates reality only makes sense if the oberserver and the observed are distinct entities, which is a classical notion. Are we just constantly trying to pound the round pegs of QM into the square holes of classical physics?

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