Quote of the Day

By Sean Carroll | April 14, 2012 11:32 am

David Deutsch:

“Despite the unrivaled empirical success of quantum theory, the very suggestion that it may be literally true as a description of nature is still greeted with cynicism, incomprehension, and even anger.”

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  • Doug

    Re: greeted with cynicism etc.

    Is it? Among educated folks?

    Given that this quote is from Deutsch, I can only assume he means many worlds interpretation by “literally true description of nature.” What an utter mischaracterization of other interpretations. Of course all physicists take quantum mechanics to be literally true, but that doesn’t mean we have to take all the mathematical structures to be representative of physical objects. What a dull point of view to have!

  • Peter Morgan

    Given any finite set of experimental results that are caused by a given set of experimental configurations of preparation and measurement devices, it is open to us to introduce an arbitrary number of DoFs to model the results. Indeed, it is routine to introduce infinite dimensional Hilbert spaces, and the corresponding spaces of density operators and measurement operators, to model finite, albeit large datasets. For a finite dataset, we can always solve the equation mu_{ij}=Tr[hatrho_ihat O_j] for the density operator and measurement operators for given experimental statistics mu_{ij}, if we make the Hilbert space large enough.

    There is, in other words, no circumstance that quantum theory cannot model. The only question is how simple the models are. That quantum theoretical models are simpler than contextual and/or nonlocal models is very nice. There are other questions, such as how simply we can model dynamics in complex situations, which is not so problematic in quantum optics but is problematic to some extent for interacting quantum fields, although that may be a matter of either formalism or of something deeper.

  • Barry

    ‘Literally true’ is a tricky one for any scientific theory. The goal is to asymptotically approach ‘the truth’ & push the predictive powers of the theory to the breaking point.

    As far as QM goes, we know it can’t be the complete picture, since Nature can handle quantum & gravity at the same time, but our theories can’t. So QM is a very good model of nature on the small scale, but ‘literally true’ is pushing it. There will be regions of parameter space where QM predictions will diverge
    from observations (although it may be a long time before we can hit those distance scales).

    I understand using short-hand like ‘literally true’ in dealing with the public, but the framing could be a lot better. It’d be better to be more honest about how science progresses & where its limits are. Otherwise we’re just arguing from authority & that’s not the point (even though people respond to it).

  • http://mattleifer.info Matt Leifer

    I think Deutsch is referring to what a philosopher would call “internal realism” when he uses the words “literally true”. This recognises that we are far away from having a final theory, but instead asks, “if this were the ultimate theory of physics, what would it say about the world?”, i.e. it is about the story that the theory tells about the world rather than being about the world itself. Whenever we discuss realism in the context of interpretations of quantum theory, it is this question we are addressing, and future discoveries in say quantum gravity will not necessarily make this question go away.

    That said, the intended “quantum mechanics is literally true = many worlds” raises my hackles as well.

  • PL Hayes

    The “mind projection fallacy” strikes again. It is rather exasperating and incomprehensible.

  • AI

    Seeing as there are multiple different interpretations of quantum mechanics what does it even mean to say that quantum mechanics is “literally true as a description of nature?”

    It makes as much sense as saying that a story of which there are multiple contradictory accounts is “literally true,” without specifying which particular version one has in mind.

    From the context it is clear that what he really means is that “many worlds interpretation of quantum mechanics is literally true as a description of nature,” and i do object to this version.

  • Argos

    Any scientific theory with supporting empirical evidence is true until we find where it is false.

  • Chip Brock

    He is referring to others’ views of quantum mechanics. This is from a review of _Many Minds. Interpretations of Quantum Mechanics_ by Michael Lockwood, whom he defends. (Source: The British Journal for the Philosophy of Science, Vol. 47, No. 2 (Jun., 1996), pp. 222-228) In his review, Deutsch says:

    “However, I must stress that whatever position one takes on the objec-
    tivity of the layering structure, quantum theory leaves no room for any doubt that multiple universes exist. It is as if there were a debate about whether our partitioning of the surface of the earth into latitudes has an objective basis or is merely a human convention. Whichever view one takes, the physical fact remains: the Earth is not confined to a single latitude but really does extend over many parallel latitudes.”


    “Moreover, in so far as it is accurate to speak of one universe as existing in reality, quantum theory implies that it is necessarily equally accurate to speak of other, different universes as existing in reality too. So other universes exist in exactly the same sense that the single universe that we see exists. This is not a matter of interpretation. It is a logical conse- quence of quantum theory, albeit one that somehow, seventy years after the theory was discovered, is still in dispute.”


    “The formalism of quantum theory did not come out of nowhere. It is the solution of a scientificproblem and, as always in science, the problem was not primarily what mathematical formula best predicts the outcomes of experiments. It was what mathematical structures corre- spond best to reality. If we alter the ‘interpretation’ of the theory without regard to the second question, we can conjure up virtually any world we like. But it will not be the real world. The real world is the multiverse, and it does contain many universes.”

    and at the end:

    “A few physicists are likewise beginning to
    realize that the sheer philosophical naivete that still prevails in the profes-
    sion has prevented our most important theories from being properly
    understood and has seriously impeded progress. The twentieth century has been a veritable dark age for metaphysics-indeed it has been char-
    acterized by the explicit repudiation of metaphysics both by philosophers and by physicists. Now that the lights seem to be coming on again, we are in a position to enjoy the one beneficial side-effect of the long darkness: a
    backing of wonderful, urgent philosophical problems, raised by scientific advances in the intervening period.”

    So, wow! Deutsch does not deride quantum mechanics, indeed he seems to go further than many of us might be comfortable doing!

  • Marcel Blattner

    Hmmm… The comments support the quote. Literally.

  • Peter Morgan

    I mostly take it that it’s best to shut up and calculate, albeit one’s choice of what to calculate reveals a lot. I’m mostly seeing a little cynicism in the comments, not overmuch incomprehension is on display, and, seconding Doug, one could be angry at Deutsch’s comment only if one believed that more than a vociferous clique of Physicists would hold fast with such a statement. QM is a perfectly good mathematical structure for constructing empirically effective models of nature.

  • KenF

    What could it possibly mean to say that a scientific theory is “literally true”? A scientific theory is something that people make up to explain and predict certain aspects of our experience of nature. It may be useful for us, it may be enlightening for us, it may be beautiful to us. But to suggest that it is “literally true” is only accurate if you go to the original meaning of “literally”. It is a story that works for us; in that way it it is “literally true”.

    But if you are saying that it is a description of how nature actually is (viewed from some true and objective perspective), then you are taking science way beyond its limits. We only experience nature from a human perspective, and our science fits human needs and is constrained by our human limitations.

    People made this mistake in the time of Newton. They believed in the “literal truth” of absolute space. How could a theory so accurate and useful not be “literally true”?

  • Moshe

    Sounds pretty shocking until you realize what Deutch means for something to be “literally true”, which goes well beyond what most people find self-evident. It is that part that is treated with varying degrees of skepticism.

  • OMF

    Hmmm… The comments support the quote. Literally.

    It’s an inflammatory comment. What did you expect?

    Deutsch, and indeed Sean Carrol, are obviously both trolling for major physicist nerd rage. This rarest of substances is hard enough to come by at the best of times, but right now their cups runneth over with the delicious tears of thousand scientists whose own personal quantum interpretation has been offended. I imagine the pair, sitting by the fireside in a mountaintop chateau, toasting their success as they quaff their shameful spoils.

    There’s a world in which it’s happening right now after all.

  • http://www.darkbuzz.com Roger

    Deutsch is talking about many-worlds. Yes, the idea that many-worlds is literally true is greeted with various objections. With good reason. There is no empirical evidence that many-worlds is any better than any other interpretaion. Where Deutsch goes wrong is to imply that there is empirical evidence for many-worlds.

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  • http://jbg.f2s.com/quantum2.txt James Gallagher

    He just means the universal wave-function evolving according to Schrödinger’s equation is literally true – ie describes everything

    But I still don’t think that implies many-worlds ‘literally’ (!), since you can instead explain superpositions by fundamental randomness at the level of individual states – there is no need for the enitire universe to split.

    Also, as I have recently shown, if you look at the (discrete) ‘flow’ of the universal wave-function according to U(t+dt) = e(hL).U(t) – U(t) – ie the ‘change’ in the wave-function, then 3D space falls out trivially as 3 attractive periodic points in C^n.

  • Torbjörn Larsson, OM


    there are multiple different interpretations of quantum mechanics

    Is that a fruitful description? It doesn’t appear fruitful elsewhere, where people try to eke out predictions to test.

    Say, if decoherence is an actual process, wouldn’t it kill some “interpretations” that, at least in the armchair interpretation, has claimed “instantaneous collapse”? I believe I have read here that instead, for whatever reason, the decoherence process obeys relativity.

    And elsewhere experiments on photons in cavities where decoherence is a gradual process that one can back out of as long as it is partial.

    Likewise, people have claimed, for whatever reason, the wavefunction can be observed (is real) and is not a calculating device. If so, surely that would reject #11 “shut up and calculate”?

    I don’t see why these are not (tried to be) put as theories as elsewhere.

  • Baby Bones

    Personally, I greet QM with incomprehension as a explanation of reality. I don’t think there is a wave function or an infinite-dimensional Hilbert space lurking behind any experiment or version of this world. The formalism of QM is a tool that gets good answers, but it is not an actual set of universal rules. We stumbled upon QM for lack of a better idea. QM is essentially about how to conduct measurements and it rigorously denies that things actually exist for that reason.

    I think the set of all probabilities of instances in the universe is finite and discrete. In particular, instances of especially low frequency cannot be proven to have existed (or have a cause) and must be treated as noise. Also, each instance creates a new set of all probabilities, and instances have varying degrees of effect depending on when they occur. I think that idea is still within the formalism but it would add a cut off to the numbers of eigenvalues and eigenvectors.

    Any measurement humans make now cannot noticeably affect the energy levels of, say, the hydrogen atom, but instances corresponding to hypothetical high-energy measurements made just after the Big Bang by a powerful sentient observer could have changed the energy levels of the hydrogen atom and the subsequent evolution of the universe. This means I am less certain that any Higgs Boson created at CERN is in fact a real Higgs Boson. Maybe it is a close facsimile from what is essentially a simulation of the conditions of the Big Bang. Conversely, our existence, despite being miniscule now, might nonetheless have an odd effect on the laws of the future universe as it expands.

    I think a certain amount of noise is necessary for making a measurement and is not a consequence of making a measurement but reflects an intrinsic aspect of something. This means we can’t know everything about an electron because it doesn’t actually exist in the ways that we can think of it, but we can reasonably be sure that there is something that we cannot change that corresponds very closely to our idea of it. The higher up we go on the energy scale at a certain time scale, the less certain we are of our measurements. I think this is a reasonable way of looking at things, but it isn’t especially enlightening.

  • Jimbo

    “This is often the way it is in physics – our mistake is not that we take our theories too seriously, but that we do not take them seriously enough.” – Steven Weinberg.
    Indeed, Weinberg is Still seeking a QM that HE can understand, even if most of us mortals cannot understand Weinberg, who just cannot accept the ManyWorlds Interp. of QM as Reality.
    I do agree w/Deutsch in the sense I think he’s coming from:
    Eternal Inflation, Cosmological Constant, & String Theory Landscape all suggest a Multiverse, & the MWI is its QM manifestation.

  • Low Math, Meekly Interacting

    Honest-to-the-FSM non-snarky sincere question: What, if any, value is there to “literal truth” in theoretical/scientific models of nature? I should think we would want to avoid making positive statements about literal truth like an all-mighty plague. Literal truth is for religious fundamentalists, not scientists, right? Right??? How is the lack of literal truth even a relevant critique? Why wouldn’t the mere mention of of a requirement for literal truth evoke cynicism and/or anger when applied to ANY theory, however successful?

  • Third Planet

    @literal truth
    I come from an aerospace engineering(BSME) background so I am partial to explanations consistent with experiments. When multiple slit experiments are conducted(see ch 2 of Deutsch’s Fabric of Reality) it appears(with only 1 caveat) that photons are being interfered with by other photons. While we can’t detect the other photons(that’s the caveat), they appear to behave just like the one we can. We can accept this as being literally true or we can devise mathematically consistent abstractions like “wave collapse” for which there appears to be no experimental evidence to account for what constitutes an observer, or when and how collapse occur. If you’re an engineer tasked with explaining why the vehicle works(or not) it really is that simple – a choice between some indirect experimental evidence and well, none.

    “literal” does not mean “ultimate” – most would accept that it is literally true that I am sitting on a solid chair typing this message but that says nothing about what consciousness is or how subatomic forces operate on the surfaces of my skin, clothes and chair to give the appearance of solidity.

  • Doug

    @22, you are taking an analogy made for a popular level exposition much farther than you should (either that or Deutsch is misleading his readers). It seems to me that to be talking about photons interfering with other photons in the manner you imply one has to have already accepted the many-worlds interpretation and be viewing things from that perspective.

    And talk about things for which there is no experimental evidence… many worlds is at the top of that list!

  • Jeff

    Is accepting quantum theory literally any harder than accepting relativity literally? Both theories explain existence on scales far removed from our sense-abilities here in our Newtonion middle. We can no more feel the effects of time dilatation (if that’s the correct term. I’m not a scientist) when walking down the street than we can any quantum level phenomena.


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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