Sean Carroll: There is no classical world

I’m not a physicist but wanted to direct you to Preposterous Universe, where, in a new post called “There is no classical world,” Sean Carroll mentions an experiment showing definitively that even a macroscopic object—a mirror cooled to near absolute zero—behaves according to quantum rather than classical mechanics.  That is, instead of remaining absolutely still, it vibrates at an amplitude of 0.000000000000001 meter, as predicted by QM.  And they could measure that vibration! There’s a nice animated video that describes it clearly. You will want to see that.

The universe continues to astound me.  I don’t understand quantum mechanics, but that puts me in good company.


  1. Posted June 17, 2013 at 11:52 am | Permalink

    I love being fascinated. Thanks for sharing.

  2. Posted June 17, 2013 at 11:59 am | Permalink

    I don’t understand quantum mechanics, but that puts me in good company.

    “If you think you understand quantum mechanics, you don’t understand quantum mechanics.”

    Widely attributed to Feynman, though a secure source is lacking. Perhaps one of your thousands of erudite readers can supply one?

    • Michael Fisher
      Posted June 17, 2013 at 12:13 pm | Permalink

      I don’t know if he said that exact quote, but he did say THIS on video, which amounts to the same thing:-

      “…if I’m going to explain this theory, the question is, are you going to understand it? Will you understand the theory?

      When I tell you first that the first time we really thoroughly explain it to our own physics students is when they’re in the third year graduate, graduate physics. Then, you think the answer is going to be no. And that’s correct, you will not understand.

      But this business about not understanding is a very serious one that we have between a scientist and an audience. And I want to be at work with you, because I want to tell you something. The students do not understand it either.

      And that’s because the professor doesn’t understand. This is not a joke, but very interesting…”

    • Gregory Kusnick
      Posted June 17, 2013 at 12:40 pm | Permalink

      I consider that Feynman quote (if indeed he ever said it) to be in the same category as Lederman’s “God particle” and Hawking’s “mind of God”. Journalists love to repeat it, but it conveys exactly the wrong message about science.

      In fact there has been considerable progress in understanding quantum mechanics — not just how to do the math, but what it actually says about the deep structure of the world — since Feynman’s heyday. The basic concepts are (or should be) accessible to anyone with a decent science background.

      To imply that not even the experts understand it, or that it’s impossible in principle to understand, is counterproductive and (in my opinion) disrespectful to people like Carroll whose business is understanding and explaining it.

      • Posted June 17, 2013 at 12:45 pm | Permalink

        OK, but does anyone really understand quantum decoherence and the collapse of the wavefunction? Or the apparent faster-than-light information exchange in separated quantum -entangled particles?

        • Michael Fisher
          Posted June 17, 2013 at 12:58 pm | Permalink

          Re your “faster-than-light information exchange” the only thing exchanged is the internal quantum state ~ no external information is passed

          • Posted June 17, 2013 at 1:11 pm | Permalink

            That still means that one component of an entangled system communicates information about the quantum state with another component at FTL rates.

            • Posted June 17, 2013 at 1:17 pm | Permalink

              I don’t think it helps to think of it in terms of communication or instantaneousness or the like.

              Rather, it all happens at the beginning.

              Imagine you’ve got a dollar bill. You tear it in half, and put each half in an envelope. You mail the envelope to different sides of the planet.

              The instant your friend opens the one envelope, she knows which half of the bill is in the other envelope. Oooo! Spooky FTL communication!

              …but, obviously, not really.

              The exact same type of thing happens with quantum entanglement, only you can do more interesting things than just tear pieces of paper in half (and thereby have that much more wiggle room for the types of confusion and misdirection that stage musicians love).


              • Darth Dog
                Posted June 17, 2013 at 1:48 pm | Permalink

                The “torn in half bill in a sealed envelope” is an example of a hidden variable. That is not what is happening with quantum entanglement.

              • Posted June 17, 2013 at 2:04 pm | Permalink

                If you’re seeing a hidden variable in my description, then I probably either didn’t describe it very well or it picked a bad example.

                The point I was trying to make is that everything happens at the moment of entanglement, and all the “spooky” stuff is just a matter of revealing what had already happened at the moment of entanglement. You can just put off looking at the entangled state of affairs until later, and you can figure out the whole state of affairs from just a piece of the puzzle.


              • Posted June 17, 2013 at 2:11 pm | Permalink

                Ben, that is completely wrong. The state is not decided at the beginning. See Bell’s theorem and its experimental disproof, confirming QM.

              • Posted June 17, 2013 at 2:14 pm | Permalink

                There are hidden variables in your example–the halves actually exist before the observer looks at them. QM is fundamentally different. The property being measured does not have a definite value before measurement.

                Entanglement cannot, however, be used to transmit information FTL.

              • Posted June 17, 2013 at 2:20 pm | Permalink

                The point I was trying to make is that everything happens at the moment of entanglement …

                That’s not the case, and way understates the weirdness of quantum mechanics.

                The point is that the measured spin state of a system depends on the measuring apparatus, and the use of that apparatus then determines not only the state of one part of the entangled system, but also the state of the other part, even if it is very different.

                To take your analogy. Someone sends half a dollar each in two envelopes, one to Australia and one to Iceland. If you weigh either one it is found to be 50 cents in coins, but if you open it without weighing it is found to be half a paper dollar.

                Now, if you weigh the one in Iceland, and a nanosecond later the one in Australia is inspected, then the Australian one will *also* be found to be coins, whatever you do to it (even if you open it without weighing).

                And if you open the one in Iceland without weighing, then the Australian one is then found to be the other half of the paper bill.

                Ditto if you weigh or open the Australian once first, doing that then determines what the Icelandic one is found to be.

                How does the person sending the half-dollar to Australia know whether the receiver in Iceland will weigh it or not? There is no way that this can all be set at the time of sending.

              • Diana MacPherson
                Posted June 17, 2013 at 2:35 pm | Permalink

                Ooooo I like how you explained this. Usually analogies are wasted on me but I like this one!

              • Gregory Kusnick
                Posted June 17, 2013 at 2:23 pm | Permalink

                As Lou said, it’s not all decided at the beginning; it matters what measurements you do at the two detectors.

                The best explication of this I’ve found is David Mermin’s “Can you help your team tonight by watching on TV?”.

              • Posted June 17, 2013 at 2:36 pm | Permalink

                Okay, my brain is starting to hurt and I’ve got work to do, so I’ll bow out and ponder this further later….


              • Posted June 17, 2013 at 4:16 pm | Permalink

                Ben and Diana, here is the perfect analogy to explain the problem: “Bertlemann’s Socks and the Nature of Reality” (google it) by the man who discovered the nonlocal nature of reality, JS Bell himself. There is something for every level of expertise in the wonderful article. Another great explanation is the old 1979 Scientific American article by B D’Espagnat, “The quantum theory and reality”, which derives the simple inequality that must hold if things are real and there is not instant action at a distance. A high school student could have proven it, but even Einstein did not think of it. Bell did, and explains it in the “socks” essay.

                Bell’s essay is old (1980) and mentions a critical experimental test about to be performed by Aspect. The test has since been done, and proved QM predictions and disproved local reality.

                There is an interesting connection to free will. If we don”t have the freedom to choose different experimental arrangements than the one we actually did choose, this may provide a way out of the worst of the paradox.

              • Darth Dog
                Posted June 17, 2013 at 4:21 pm | Permalink

                I thought Chad Orzel’s book “How To Explain Physics to Your Dog” had a really good explanation of quantum entanglement and how Bell’s theorem is tested experimentally.

              • Posted June 17, 2013 at 4:32 pm | Permalink

                The more I think about it, the more it looks like Bell’s theorem could be used to disprove the existence of free will. We now know this statement is true: Either free will does not exist OR reality is nonlocal and there is instantaneous action at a distance. We don’t like either one of those choices, but maybe the free will one is the easier of the two for physicists to give up.

            • Michael Fisher
              Posted June 17, 2013 at 1:33 pm | Permalink

              HERE’S A LONG LIST of examples of FTL travel of non-information

              These don’t violate special relativity nor causuality

              • Posted June 17, 2013 at 2:06 pm | Permalink

                You’re right about no violation of relativity, and no use of the thing as an FTL communication device, however it is still the case that there is FTL influence between one part of a quantum-entangled system and another part, and the influence must be traveling at thousands of times the speed of light (or be instantaneous).

                Even if that only concerns “internal” information, inaccessible to an external observer, it is still weird and ill-understood.

            • Torbjörn Larsson, OM
              Posted June 17, 2013 at 4:11 pm | Permalink

              But the point is that no information (state or otherwise) is communicated, or it would break relativity. And quantum field theory tells us quantum theory and relativity is compatible.

              What happens is that the local state shows “local” spacetime is funny in quantum system. To take your analogy, the system is both in Australia _and_ in Iceland since it is entangled, until you weigh or open either envelope. But no information is passed _through_ spacetime, it only appears so because we haven’t figured in entanglement.

              I believe Penrose’s twistor space makes this explicit, i.e. there is an isomorphism between a point in twistor space and two points in spacetime (and vice versa).

              Another way to describe it is to note that the only thing that is communicated is correlation (entangled subsystems). If you weigh the envelope in Australia or Iceland, the only outcome of the operation in the other end is that you are told a weight. And if you open it, the only outcome in the other end is that it will be opened.

              Remember, no one actually choose between weighing or opening. It is the _experiment_ that gives the correlation that makes sense, and what correlation makes sense is only decided (chosen to perform) in one end and not the other.

              Information is famously in relativity not observation of correlation but causality, light cone communicated signals.

            • Torbjörn Larsson, OM
              Posted June 17, 2013 at 4:22 pm | Permalink

              however it is still the case that there is FTL influence between one part of a quantum-entangled system and another part, and the influence must be traveling at thousands of times the speed of light (or be instantaneous).

              Even if that only concerns “internal” information, inaccessible to an external observer, it is still weird and ill-understood.

              I think there are many problems with this description.

              – What is “influence” here? It needs a description.

              Isn’t it just another description of entanglement? Entanglement is not “weird and ill-understood”.

              – To be able to discuss the system behavior vs relativity, which is of interest, we need to distinguish between relativistic information, which this is not, and correlation, which it is.

              We also need to distinguish between “local” events and “internal” information, because it is the former which is in question.

              – Quantum theory has no “internal”, hidden, inaccessible to an external observer, information.

              If you ask Carroll, IIRC what you call “internal” states are not existing before the measurement (in many world theory). He has a blog post on that somewhere, unless my memory fails me.

        • Gregory Kusnick
          Posted June 17, 2013 at 1:02 pm | Permalink

          Disclaimer: I’m not a physicist and this isn’t my specialty.

          That said, my understanding is that decoherence is pretty well understood theoretically and has entered the realm of engineering practice with regard to quantum computing. And the reason nobody understands wavefunction collapse is because it’s a wrong-headed idea; decoherence alone is enough to explain our observations without any additional collapse postulate. I’ve found David Deutsch (The Fabric of Reality) and David Albert (Quantum Mechanics and Experience) to be pretty clear on these issues.

          And it’s well-established that no information is transmitted (faster-than-light or otherwise) by EPR-type entanglement experiments. Again, Deutsch and Albert (among others) cover this pretty well.

          • Torbjörn Larsson, OM
            Posted June 17, 2013 at 4:29 pm | Permalink

            Ironically I just read something that shows classical decoherence theory and the many world theory so beloved by Sean Carroll may have a problem:

            You may not need an environment for a quantum system to behave like a classical system. Just divide a closed system into two large, weakly interacting sub-systems and classical behavior emerges.

            “”What we’ve been able to show is that such quantum-to-classical transitions can emerge even in the dynamics of simple closed quantum systems. By coupling two quantum chaotic subsystems—so-called ‘kicked quantum rotors’—they both go from being localized due to destructive interference to displaying diffusive energy transport in a completely classical fashion.”

            Eg the classical behavior is innate to the quantum system. But I’m not sure they have showed complete isolation as of yet. So decoherence is perhaps not threatened, but our idea of how it works.

            Yes, on the information vs Bell test expereiments (EPR-type entanglement). Carefully defining information, as Einstein did, shows it is “locality” under entanglement that is the (perceptual) problem.

        • Kevin Henderson
          Posted June 17, 2013 at 2:53 pm | Permalink

          We (as physicists) do not know the limit of decoherence from first principles. The final solution will certainly have to include gravitation which is not included in QM yet. A wonderful paper was recently written about the subject (academic paywall):

          though there are several more on the ArXive:

          Note that 30 orders of magnitude separates the coherence time of the mirrors Carroll is referring to and things like a real Schrodinger Cat. In that sense we are hardly close to a formal statement that the world is all QM and no classical.

          • chemicalscum
            Posted June 17, 2013 at 6:01 pm | Permalink

            The preprint of the Stefan Nimmrichter and Klaus Hornberger paper in Phys. Rev. Lett. is here:


            Thanks gotta go read the paper now.

    • logicophilosophicus
      Posted June 17, 2013 at 1:36 pm | Permalink

      “I think I can safely say that nobody understands quantum mechanics.” Feynman, 1965 – hence the shut-up-and-calculate approach.

    • MKray
      Posted June 18, 2013 at 3:31 am | Permalink

      Feynman’s `nobody understands QM’ was spoken by him in the Messenger Lecture on QM (Cornell, 1964) broadcast by the BBC and shown to our QM students. From the context, I have always taken it to mean that we cannot understand QM if we insist on trying to make mental classical models. You can read it for yourself in the QM chapter of the book in which all the lectures were published: “The character of physical law”.

  3. Diana MacPherson
    Posted June 17, 2013 at 12:02 pm | Permalink

    Wow, who knew? Very interesting indeed but I have angst over how Deepak Chopra is going to manipulate this information.

    • gbjames
      Posted June 17, 2013 at 12:07 pm | Permalink


    • Posted June 17, 2013 at 12:34 pm | Permalink

      Whatever new interesting stuff physicist discover, new agers will misrepresent it as supporting their beliefs.

    • Sastra
      Posted June 17, 2013 at 1:41 pm | Permalink

      Sad to say that was my first thought too: the classical “material” world has been replaced by quantum physics, vibrating strings of conscious energy. Everything is Mind.

      It’s possible though that the fact that it’s Sean Carroll saying ‘there’s no classical world’ will stop the woosters from reinterpreting and appropriating it for their own. Carroll’s too well known as a gnu atheist; they’d have to spin it as “Famous Atheist Physicist Changes Mind” and not even the most optimistic of the wide-eyed wonderers would or could expect this particular famous atheist physicist here to keep silent out of respect for faith.

      It would be like announcing that Richard Dawkins renounces evolution because he says something about how Darwin was wrong about genetics. They’d become a laughingstock bigger laughingstock when even average people can notice the deception.

      • darrelle
        Posted June 18, 2013 at 9:17 am | Permalink

        I agree with you, it would make them even appear even more ridiculous. But, that has never stopped them before. And Chopra is perfectly comfortable with lying about what Sean Carroll said, the context and what it implies about reality. He has done so many times to many people, in person, right in front of them and a large audience.

        But of course none of that should give anyone pause to report, comment or explain stuff like this.

  4. Posted June 17, 2013 at 12:47 pm | Permalink

    Quantum mechanics? It’s all done with mirrors! 😉



    • Reginald Selkirk
      Posted June 17, 2013 at 12:50 pm | Permalink

      And smoke?

  5. Posted June 17, 2013 at 12:51 pm | Permalink

    My understanding is that the math of quantum mechanics is perfectly well suited to accurately modeling Newtonian-scale mechanics; it’s just that you wind up carrying a lot of extra precision that’s generally impossible to measure. “Measure with a micrometer, mark with chalk, cut with an axe.”

    My further understanding is that it’s exactly the same case with relativistic mechanics: it works just swimmingly at Newtonian scales, but it’s generally just wasted effort in practice.

    But, again as I understand it, the really interesting bit is that relativistic mechanics doesn’t work so well at the quantum scale and quantum mechanics doesn’t work well at relativistic scales. And that bridging that gap is where physicists today are seeking their fame and fortune.

    Any actual physicists who have a better clue, feel free to jump in here….



    • Ben
      Posted June 17, 2013 at 1:03 pm | Permalink

      Quantum mechanics + special relativity = quantum field theory (QFT), which works just great.

      You’re thinking of trying to meld QM and general relativity (i.e., gravity), which is an unsolved problem.

      This doesn’t have anything to do with the mirror experiment, though.

    • Torbjörn Larsson, OM
      Posted June 17, 2013 at 4:44 pm | Permalink

      I think the problem is actually the reverse.

      When they measured supernova photons over cosmological scales and got a peek into Planck scales (resolving Planck time differences), they saw that relativity played nice – spacetime was perfectly smooth. That was _not_ expected, even in string theory, I think.

      You can say that it is the theories that doesn’t work well, but they are pretty good indicators of the physics. And the physics plays perhaps nicer than expected.

      The problems when they marry general relativity and quantum mechanics to bridge the gap in the simplest possible way (semiclassical theory) is narrow theoretical AFAIU. They haven’t actually _seen_ the problems when working the systems as such.

  6. Posted June 17, 2013 at 12:55 pm | Permalink

    I’m envisioning how Woody Allen would mention that everything is moving and this is partly why investing in things like real estate becomes so problematic.

  7. madscientist
    Posted June 17, 2013 at 4:53 pm | Permalink

    One of the things realized over 70 years ago was that a lot of the phenomena we observe are the bulk product of many other events. Reflection off a mirror? Random absorption and re-emission of light. The color of, say, silver or gold is a bulk property. Once you isolate small enough groups of atoms the color of these metals is black. In the news a few weeks ago a research group even announced that they can make gold appear as any number of colors rather than the usual – well, gold. One other big thing of course is that this absolute determinism which had been prized by Plato through to Newton is not at all necessary and in fact likely nonexistent. Contrary to popular culture, the entire history of the universe was not determined at the Big Bang. It can’t be – the best we can do is make reasonably precise predictions based on the outcome of many events – when we look at single events in isolation there’s no predicting anything other than what we already know to be impossible.

  8. Michael Fisher
    Posted June 17, 2013 at 6:45 pm | Permalink

    Sean has just posted THIS at his place with THIS groovy 27 minute video embedded

    The video is worth a watch ~ enjoyable

    The charming Keith Ward opens for God & the equally charming Sean Carroll comes in to bat for Nature at 17:25 min

    “entertaining and informative talks as part of the the 2nd mini-series (Is “God” Explanatory) from the “Philosophy of Cosmology” project. A University of Oxford and Cambridge Collaboration”


    ** Keith Ward quote:- “God is a problem for cosmology” LOL

    ** Sean Carroll:- Two amusing stories plus rather accommodationist [IMO] closing remarks. Quote:- “…we naturalists have not advanced very far at all in telling people how to live fulfilled, useful, value-filled lives. Even though I think we’re right about the physics & how the world works and even the metaphysics [too]…”

  9. Matt
    Posted June 17, 2013 at 6:54 pm | Permalink

    Archaeologist says – I thought it was going to be about Greeks and Romans when I saw the title

    • Diana MacPherson
      Posted June 17, 2013 at 6:58 pm | Permalink

      Ha ha! buh dum da tish!

    • gbjames
      Posted June 17, 2013 at 7:02 pm | Permalink

      I had the same response.

    • Posted June 18, 2013 at 12:31 am | Permalink

      Surely that would be “Classical” rather than “classical”?


      • Diana MacPherson
        Posted June 18, 2013 at 5:58 am | Permalink

        I thought that too but it was still funny because I didn’t see it – maybe it was the lower case “c” that made me miss the joke.

        • Posted June 18, 2013 at 6:49 am | Permalink


          Anyway, would anyone think that Herbert Goldstein’s Classical Mechanics was about Greek and Roman chariot maintenance? 😀


          • Diana MacPherson
            Posted June 18, 2013 at 6:58 am | Permalink

            Ha ha – you’ve made a whole new category of jokes for Classics students! I wonder what classical ballet is like 😉

          • John Scanlon, FCD
            Posted June 24, 2013 at 8:46 am | Permalink

            What was that novel so trendy back in the 70s – Stoicism and the Art of Chariot Maintenance?

            • Posted June 24, 2013 at 10:18 am | Permalink

              By “70s”, I take it you’re thinking of the first century CE … 


    • BillyJoe
      Posted June 18, 2013 at 3:25 am | Permalink

      What? You don’t know who Sean Carroll is?

      • gbjames
        Posted June 18, 2013 at 5:50 am | Permalink

        Why of course we do. He’s an excellent molecular biologist at the University of Wisconsin-Madison!

  10. marksolock
    Posted June 17, 2013 at 7:42 pm | Permalink

    Reblogged this on Mark Solock Blog.

  11. BillyJoe
    Posted June 18, 2013 at 4:00 am | Permalink

    “a mirror cooled to near absolute zero”

    Actually, it’s cooled to absolute zero.

    Also,quantum physicists have always believed that quantum physics extends to macroscopic objects. It was just impossible to detect…
    …until now!

    • Kevin
      Posted June 18, 2013 at 6:59 am | Permalink

      So, they just violated the Third Law of Thermodynamics?


      I suppose that’s why everyone else thought it was impossible?

  12. John Weiss
    Posted June 18, 2013 at 9:53 am | Permalink

    Trying to understand quantum mechanics is very much like trying to scratch an itch that you cannot quite reach.

    Another delight in a delightful world.


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