Wednesday, April 11, 2012

Roland Omnès and QM: 20 years later

In April 1992, exactly 20 years ago, Roland Omnès published his 50-page article
Consistent Interpretations of Quantum Mechanics (a Russian FTP server)
in Reviews of Modern Physics. So far, the article has accumulated 472 citations. It was the last month before our high school leaving exams – which I may remember rather well because we will have a 20-year reunion on Saturday. ;-)

Finally, I decided not to embed La Marseillaise as a sign of the author's nationality. Instead, here you have pictures from a movie with a really great music by Michel Magne. Update: Oops, if I had fully appreciated Shannon's words, there would be some Breton/Celtic video above instead of this one. ;-)

At any rate, when I came to the college in Prague in September 1992, just five months later, I got access to the libraries and articles about the interpretation of quantum mechanics had a rather high priority – probably next to the string theory articles – and it just happened that I found Omnès' review rather quickly and it was the final nail in the coffin of my doubts about our complete understanding of the foundations of quantum mechanics.

For a year or so, I may have been kind of convinced that most of the research trying to make the foundations of quantum mechanics "more imaginable" and suggesting that there was something wrong with the modernized Copenhagen interpretation was pseudoscience but it was still very useful to read Omnès' article that had articulated those facts clearly and authoritatively.

To those who have doubts that the logic behind quantum mechanics has been fully understood – and to many others – I recommend to read this clearly written paper. Or they may buy the 300-page book below. Or if they find it too long, just read the summary about objectivism on pages 376-378 of the review above.

Twenty years is a rather long time. Things were very clean 20 years ago and almost clean 85 years ago. Many similar articles that make a complete sense have been written but the school of thought assuming that "it can't be right" and trying to search for an "objective", i.e. classical replacement of quantum mechanics is still alive and kicking.

One of the reasons behind the persistent survival of all this crackpottery is the failure of its proponents to study proper sources that actually make sense. That's why I decided to post a blog entry that contains almost nothing else than the link to the full paper by Omnès. I don't believe that too many people who are obsessed with the anti-Copenhagen zeal will actually read it but if there will be at least someone who will appreciate the clarity of quantum mechanics for the first time, it will be a satisfactory result.

Next linear collider

A very different topic.

At various points, e.g. in 2009, I discussed the plans for the next linear collider that could investigate the Higgs boson and perhaps other things more accurately than the LHC. The Symmetry Breaking Magazine tells us that CLIC, the CERN-based project, and the ILC, the internationally diluted project, are getting a common leadership. They're getting unified.

I must mention that some folks also have a plan for LEP3 (see also a fresh Greek Powerpoint talk), a circular electron-positron collider in the LHC tunnel. The recycling of the tunnel from LEP did work rather well so why not repeat it?


  1. Dear Lubos,
    I was reading with great interest all
    your posts regarding entanglement
    and and the copenhagen interpretaion of Q.M. We all forget that all these
    discussions are based on Non-relativistic Quantum Mechanics. we must not forget that relativistic corrections must be included which may alter any long time behavior of any system. Everything is different in relativistic theories. Below are
    my earlier comments sent to you.
    Dear lubos,
    i am also, in a sense, an entanglement denier. entanglement
    exists in a clear form in non-relativistic quantum mechanics
    and the things become much less clear in relativistic quantum
    mechanics, ie. in quantum field theory.
    let us take an example (on which bell-theorem relies) of a system decaying from
    a singlet state into two 1/2 spin particles.
    in dirac theory the total spin is not a constant of motion
    but the total angular momentum. this makes difference in the
    derivation of the bell's inequalities.

  2. Dear Stefan, thanks for your interest and kind words. But otherwise, concerning your thoughts about the relationships between QM and relativity, I must say: No way!

    Relativistic quantum theories are not "completely different" than non-relativistic ones. They obey exactly the same postulates of quantum mechanics as the non-relativistic ones. Relativistic quantum theories are just a special example of quantum mechanical theories. The special case means that the Hamiltonian \(H\) may be given friends, the Lorentz generators, so that they form the Poincare algebra.

    The discussions about the EPR-style experiments may sometimes assume non-relativistic quantum mechanics, i.e. a non-relativistic Hamiltonian, but it's just for the sake of simplicity. Nothing changes about those matters in relativistic theories and indeed, the founding fathers understood that relativity was compatible with QM. In particular, relativity makes it manifest that there's no superluminal information transfer in QM. The right explanation of EPR-like experiments agrees with the fact that the observed correlation doesn't depend on any superluminal information transfer but relativity makes this point even more strict: such a superluminal propagation would be downright illegal.

    So once again, no. Switching to relativistic theories - quantum field theories or string theory, which are the only two major consistent frameworks for relativistic quantum QM - doesn't imply any "loopholes" or "exceptions" from the rules of QM. In particular, Bell's inequalities hold for relativistic local realist theories as well and relativistic quantum theories such as QFTs are capable of violating Bell's inequalities. There's really no difference; for low-speed, low-energy experiments such as the EPR ones with electrons, relativistic and non-relativistic theories actually agree almost exactly, the differences are tiny.

    There are no differences when it comes to the postulates of QM. Relativistic theories are a subset of theories, quantum or otherwise, not a "completely alien" class.