There Is No Classical World

Caltech’s Institute for Quantum Information and Matter is a fun place. It’s led by people like John Preskill, Jeff Kimble, and Alexei Kitaev — some of the world’s great scientists — so you know the physics is going to be top-notch. But it’s the youngsters, such as postdoc Spiros Michalakis, who are bringing the fun. Suff like the IQIM blog (where you should read John’s recent post on the Maldadcena/Susskind wormhole proposal) and a successful Kickstarter campaign for science-inspired fashion.

The fun is now being ratcheted up even higher, as IQIM is teaming with Jorge Cham of PhD Comics fame to make a series of animated web videos about quantum mechanics. I ask you, who doesn’t love some good videos about quantum mechanics??

Sensibly, they’ve kicked off by spotlighting an interesting experimental result, rather than diving right into the realms of esoteric theoretical speculation. Of course, this is quantum mechanics we’re talking about, so even the experiments get pretty wild in their implications. The work is by Amir Safavi-Naeini and Oskar Painter, who take a small mirror and put it into a quantum state where its center of mass is as cold as it is possible to be. Classically, of course, the mirror can be perfectly still; quantum-mechanically, there is a ground state wave function that still shows “fluctuations” (i.e. the fact that observations won’t always show zero motion).

Now, the mirror is tiny — microscopic, it’s fair to say — but it’s not that tiny. It’s a piece of metal, non just an atom or two. (I didn’t catch what the actual size was.) So the implication here is that things don’t miraculously “become classical” when they are made of many atoms rather than just a few. We don’t notice the quantum-ness of the universe in our everyday lives, but that’s because the systems we encounter are noisy and constantly jostled by their environments, leading to rapid decoherence; not because there is a magical transition to classicalness once you get above a certain number of atoms, or a truly distinct “classical realm.”

Of course, no right-minded person really believes that there is a hard and fast transition to a classical realm once objects get big; rather, there is a sense in which the classical approximation becomes more and more accurate, but it’s always just an approximation. The experimental results here are simply affirming the truth of quantum mechanics. Nevertheless, you can still meet people (the wrong-minded ones) who are willing to believe that electrons and photons are governed by quantum mechanics, but not that they are governed by quantum mechanics. Have them watch this video, and hope that the implications sink in.

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30 Responses to There Is No Classical World

  1. Ted J. Vlamis says:

    I notice your careful choice of words that “there is no classical world”, as opposed to “the entire universe is quantum mechanical”. Any thoughts on reconciling the contradictions between Quantum Mechanics and Relativity.

  2. Sean Carroll says:

    There are no contradictions between quantum mechanics and relativity; quantum field theory reconciles them beautifully. We’re still looking for a complete quantum theory of gravity (whose classical theory is general relativity), but that’s another issue.

  3. joe says:

    The video shows how there is only a red shift of the reflected laser light when the mirror is in its ground state, because no energy can be extracted from zero-point energy of the ground state. At first I thought this was amazing but then I wondered how this differed from the classical prediction. In a classical picture, the mirror in its ground state would be completely motionless. The laser still cannot be blue shifted because the mirror is motionless but red shift is possible because the laser can impart energy to the mirror. Can someone explain how this experiment demonstrates a quantum phenomenon? (BTW, I’m not a quantum denier, I just can’t wrap my head around the experimental results.)

  4. Ted J. Vlamis says:

    Thanks for the clarification.

  5. Doc C says:

    That’s like saying a grain of salt is sodium and chloride. It’s not, it’s sodium chloride. Quantum behavior describes the foundations of our world, but our world is not its quantum foundations. Perhaps there are constraints on the classical manifestations of the quantum foundations, but the classical world emerges from its quantum foundations. The quantum foundations are not isomorphic with our classical world.

  6. Bob says:

    How does the ground state survive the laser? And, like Joe above, why isnt the energy lost by the red-shifted light enough to allow the blue-shift in both the quantum and classical cases?

  7. mlilom says:

    “Nevertheless, you can still meet people (the wrong-minded ones) who are willing to believe that electrons and photons are governed by quantum mechanics, but not that they are governed by quantum mechanics.”

    Hopefully, their temperature is not around 0 Kelvin.

    Just saying.

  8. Brett says:

    Good videos. Cool experiment. I don’t want to be banished, but maybe Laurent Nottale deserves a little more attention. I’m honestly all-in on the idea that everything can be described by an increasingly chaotic wave function with increasing scale and vice verse. That’s how every system in nature seems to work.,

    Doc C,
    I would be so pleased to find out you are a professional physicist. My ego would go through the roof.

  9. Brett says:

    By that, I mean I would be pleased that I understood the video and you didn’t. aw snap grrrl.

  10. Bill Bunting says:

    I was wondering about the electron, which in my imaginings is an energy surplus consequence of the overlap of the proton and the neutron which pops out upon union and rotates around the cleavage of the nucleus. The fact that it stays in proximity of the PN pair suggests that it is being both repelled and attracted, and the fact that it is rotating around the nucleus (if indeed it does) suggests further that the forces working on the electron are changing position at a speed sufficient to cause the electron to reposition very rapidly as it moves through the Higgs field (which will act to limit that speed).

    I plan to collect every PhDComic that is put out and I am hoping that some of them will work backwards to explain how all of this serves to drive chemistry.

    I am making sure that our local high schools are aware of this wonderful adventure.

  11. John Duffield says:

    Nice video. Good physics. I liked the “fresh eyes” theme. The only thing I didn’t like was the bit which suggested particles are points. They aren’t, they’re waves. For example in atomic orbitals electrons “exist as standing waves”. If an electron gets kicked out of an orbital it doesn’t suddenly start existing as a point particle. You can diffract an electron. It still exists as a standing wave. You make electrons (and positrons) out of electromagnetic waves in gamma-gamma pair production. You go from waves propagating linearly at c to waves that aren’t. And it’s quantum field theory. The electron’s field is part of what it is. It is what it is. Standing wave, standing field. Superposition is just a wave thing, and everything is waves.

    I guess people think they’re point particles because they see points on a detector screen, and they don’t know about the optical Fourier transform. Or that those scattering experiments are like probing a whirlpool with a bargepole. When they can’t feel anything hard they say “whatever’s down there must be really small”. Duh!

  12. James Cross says:

    And there is this which has a mention of you, Sean.

    Origin of probabilities and their application to the multiverse byAndreas Albrecht and Daniel Phillips

    Isn’t probability a mathematical concept itself? Could there be a problem with it?

  13. Justin Glick says:


    “There are no contradictions between quantum mechanics and relativity; quantum field theory reconciles them beautifully”

    Quantum field theory has absolutely nothing at all to do with solving the problems between special relativity and quantum mechanics. Have you ever heard of Bell’s theorem? That’s the theorem that states that information travels between particles instantaneously in the rest frame, and thus has problems with causality when viewed from other frames. I highly suggest you read the essay “Bertlemann’s Socks and the Nature of Reality” by Bell himself.

  14. Bob Iles says:


    It’s obvious that you have absolutely no idea what the hell you’re talking about. I recommend that you listen more to Prof. Carroll.

  15. Gizelle Janine says:

    @Glick: Or David Albert. :\

  16. Tom Clark says:

    “Nevertheless, you can still meet people (the wrong-minded ones) who are willing to believe that electrons and photons are governed by quantum mechanics, but not that they are governed by quantum mechanics. Have them watch this video, and hope that the implications sink in.”

    Not sure what those implications are. Realizing that we’re governed by QM, what does this change for us?

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  18. Brain Molecule Marketing says:

    Just curious, if randomness rules, how to bacteria find food – or my dog?

  19. AI says:

    The statement “there is no classical world” makes zero sense.

    There is one world we know exists and there are many models/descriptions of it. Quantum mechanics and classical physics are just two examples of such descriptions, but there are many more like chemistry, biology, sociology, economy, climatology, cosmology and so on. Neither model/description can be equated with the world itself. The map is not the territory.

    Also classical physics is much more useful and relevant to most humans then quantum physics.

  20. Jack Pollock says:

    This was fascinating and well presented. How do you account for possible or perhaps probable vibrations like tremors in the lab from various sources. Generators, plumbing, machinery in other parts of the building or small tremors due to natural ongoing seismic activity.

    I take it some kind of baseline measure of such motions is taken and taken into account?

    I too am largely ignorant of such complex physics, but am fascinated by it all!

    Thank you!

  21. roo bookaroo says:

    How about speaking a little more slowly and distinctly for those of us who are trying to connect with the concepts but are not conversant enough to follow a rapid mumbling of new ideas.
    This is video, not conversation, or private thinking. This is talking to an audience, and trying to communicate clear understandable ideas and convince, not just following the superb speed of your mind.
    Clear speech at a normal flow is a must, as used by all TV presenters.

  22. mgary says:

    Does it bother anyone else that every time they draw a ground-state wavefunction, they draw it with nodes? Actually, it looks like they’re drawing the probability density (the square of the wavefunction), but it still shouldn’t have any nodes if it’s really the ground-state.

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  24. Peter Donis says:

    Is there a paper available on the web ( on this experiment? Videos are nice but a written description would allow for a more thorough understanding of what’s going on.

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