Quantum Field Theory and the Limits of Knowledge

Last week I had the pleasure of giving a seminar to the philosophy department at the University of North Carolina. Ordinarily I would have talked about the only really philosophical work I’ve done recently (or arguably ever), deriving the Born Rule in the Everett approach to quantum mechanics. But in this case I had just talked about that stuff the day before, at a gathering of local philosophers of science.

So instead I decided to use the opportunity to get some feedback on another idea I had been thinking about — our old friend, the claim that The Laws of Physics Underlying Everyday Life Are Completely Understood (also here, here). In particular, given that I was looking for feedback from a group of people that had expertise in philosophical matters, I homed in on the idea that quantum field theory has a unique property among physical theories: any successful QFT tells us very specifically what its domain of applicability is, allowing us to distinguish the regime where it should be accurate from the regime where we can’t make predictions.

The talk wasn’t recorded, but here are the slides. I recycled a couple of ones from previous talks, but mostly these were constructed from scratch.

The punchline of the talk was summarized in this diagram, showing different regimes of phenomena and the arrows indicating what they depend on:


There are really two arguments going on here, indicated by the red arrows with crosses through them. These two arrows, I claim, don’t exist. The physics of everyday life is not affected by dark matter or any new particles or forces, and its only dependence on the deeper level of fundamental physics (whether it be string theory or whatever) is through the intermediary of what Frank Wilczek has dubbed “The Core Theory” — the Standard Model plus general relativity. The first argument (no new important particles or forces) relies on basic features of quantum field theory, like crossing symmetry and the small number of species that go into making up ordinary matter. The second argument is more subtle, relying on the idea of effective field theory.

So how did it go over? I think people were properly skeptical and challenging, but for the most part they got the point, and thought it was interesting. (Anyone who was in the audience is welcome to chime in and correct me if that’s a misimpression.) Mostly, since this was a talk to philosophers rather than physicists, I spent my time doing a pedagogical introduction to quantum field theory, rather than diving directly into any contentious claims about it — and learning something new is always a good thing.

  1. Hi Sean,

    In my opinion you’re making to strong a claim. There are several things we do not know, especially regarding the limits / validity of QFT. For example, what if AdS / CFT is a core feature of physical reality. Are we really sure there are no new effects that could play a role in the energy-scale of your favourite EFT?

    Also, forgetting about AdS / CFT and similar exotic stuff, it seems to me that what you’re saying is something like; assuming QFT as we know it is true, and everything on slide 30 doesn’t happen, then slide 29 is true. In my opinion there are to many ifs here to make any sweeping claims about reality outside the realm of QFT.

  2. By the way, I’m not saying you’re wrong of course, just that to me it feels a bit speculative. As you mentioned in your talk, from a historical perspective sweeping conjectures have the bad habit of ending up wrong…

  3. I don’t understand. Saying “emergent phenomenon of everyday life” is a bit of a moving target. Tomorrow’s “everyday life” may include devices that couple exotic phenomena to the kinds of fields that we observe easily. But I guess you mean something less dependent on the definition of “everyday”. But saying that the physical phenomena at a particular energy scale (that we can access regularly right now) do not depend on the details of the physics at other (higher) scales doesn’t seem particularly profound either now that we understand the renormalization group.

  4. I always compare our present understanding of reality to Mendeleev and “his” periodic table. He had absolutely no possibility of “understanding” atomic physics (remember that subject anyone?) without the forthcoming quantum mechanics and atomic particles. There was no way that pure thought could have explained the periodic table in his day. We are at the same level of understanding the standard model and field theory. Progress will be steady and perhaps slow, but nevertheless it will happen.

  5. Sean,

    I’m having a hard time understanding exactly what you mean by the “laws of physics underlying everyday life”. I take it that you obviously agree that there are observable phenomena that happen at our everyday energy scale which could be caused by physics we don’t understand yet (as a trivial example, consider a physicist who drinks a beer because he discovered a new particle at the LHC. Drinking a beer is a pretty everyday life-like activity, and that activity wouldn’t have happened if the new particle didn’t exist. This is a contrived example, but presumably much less obvious ripple effects from weakly interacting particles happen all the time). So your claim can’t be that “nothing that we observe in everyday life is the result of physics that we don’t understand”. On the other hand, though, if you’re merely claiming that approximate laws of physics in everyday life such as Newtonian gravity and the behavior of materials can (in principle) be derived from physics that we understand, then you probably don’t need to go as far down as QFT (non-relativistic quantum mechanics is just fine). So could you provide some clarification about your claim? Thanks.

  6. So Sean, did you get any useful feedback regarding your claim, from the philosophical perspective? Would be interesting to hear if there was anything substantial.

  7. Nice but your final slide does not account for mind. At least it cannot be shown to account for mind, that is consciousness. That is you may be wrong about the deleted arrow between “underlying reality/theory of everything” and “every day life”.

  8. Travis,

    I am not Sean, but here is my understanding of his claim: yes, indeed, a physicist may have a beer after a discovery of, say, dark matter friction (http://mnras.oxfordjournals.org/content/448/1/814.abstract), but any analysis of this action, does not require involving any dark matter model: human cognition, society, biology, chemistry, atomic physics, QM/QFT are enough to trace the consequences of a stream of photons from the researcher’s screen showing the result of a simulation to their reaching for a cold one. Or their decision to engage in the relevant research in the first place.

    The only way dark matter affects human actions is through the “core theory,” not directly.

  9. Shmi Nux,

    But you would not come to the correct conclusion about whether or not the physicist chooses to drink a beer unless you take into account the interaction of the particle with the detectors at the LHC. If the particle does not interact with the detectors in a way that is noticeable by the physicist, then he will not drink a beer, and if it does, then he will. So you cannot say that unknown physics will never have any effect on human actions. I might be saying something here that is a little too obvious, but my purpose is to point out that if unknown physics can have obvious effects on the everyday world, then it almost certainly can also have some nonobvious effects.

  10. The more interesting question is whether we will need them in the future and on that this has less to say than the limitations of our current knowledge.

  11. Travis,

    I suspect that Sean’s point is that there is no direct effect of dark matter on the top levels, so one cannot invoke any known “non-core” physics to justify any of the mystical stuff, like chakras, reiki, bio-energy healing, homeopathy, astral bodies, afterlife, etc. If any of that got experimentally confirmed one day, it would have to be grounded either in the core theory (very unlikely, given what we currently know), or would require some new currently unknown non-core theory (say, bugs in the underlying simulator running our Universe, or something).

    Not sure if this statement is too strong, hopefully Sean chimes in eventually.

  12. Thank you for the great slides and commentary, and for the great discussion you started in the comment section, Sean.

    I think that’s one hell of a claim to make when at this point in time. While it’s possibly true, it’s unjustified. We haven’t even began to understand biology properly, which is arguably as “everyday *life*” as it gets.

    Very recently people have began to see real evidence for entanglement being used in photosynthesis. http://arxiv.org/abs/0905.3787
    Would we have been able to see that “something is missing” had we not already known about entanglement? I’d argue against it.

    And yes, there is the matter of the C-word that must not be named (consciousness).
    Maybe it’ll turn out to be nothing. But can you, as a scientist, so quickly rush to this conclusion while knowing so little about the way the brain functions?
    We don’t even have a clue how simple anesthesia works, which is administered regularly in dentists’ offices around the world (and all known anesthetics have no apparent chemical similarity). Maybe this too will turn out to be explainable, but until an explanation is found it is very suspect.

    If we restrict ourselves outside the realm of biology then *perhaps* there is a point to be made here, but that’s one serious asterisk*.

    Every major discovery in physics began as “we can understand everything there is to understand, except this little funny thing here, and we can probably explain that too”. Naturally now these funny things are hidden behind much greater complexity, but they may very well prove as unexplainable as the photoelectric effect without quantization of light.

  13. Dr. Carroll,

    Thank you for making Physics cool again
    I was getting board of the series The Universe and Through the Wormhole

  14. 0) Thanks, a layperson can learn something from these slides.
    1) Maybe another obvious caveat, but your claim seems time-dependent: If we survive and expand, the short red arrow will *eventually* affect everyday life, right? (Via dark energy constraining how much of the universe we can access, then making life increasingly lonely, then let’s change the subject:-)
    2) slide 13: p2 | q looks like a typo
    3) slide 26: what are the white lines?

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  16. Frankly, the laws of physics are NOT understood, and may never be. Godel’s Incompleteness Theorem, also misnamed “Godel’s Proof”, shows this cannot be the case using recursive, reiterative logic and maths, of which physics is replete with examples. Further, as your post so imaginatively states, some approaches can tell us where completeness is. Frankly, that’s impossible to demonstrate. Godel’s Theorem cannot tell us WHERE the incompleteness is, nor what to look for, nor give examples of it.

    That physics is complete is simply not the case, as witness Neutrino mass and transforms; the dark matter/energy problem; the finding of black holes; the pulsar Neutron star, magnetar group, the latter of which is not at all understood; the recent finding of the EMdrive, where high power microwaves create thrust, again, not understood nor predicted by physics; and the recent finding that radioisotopes have different decays rates at different places in earth’s orbit. NONE of these were predicted by physics, and this list is not at all complete.

    Even Feynman weighed in, as has an astute post above, that NONE of biology could be understood or developed from Quantum mechanics. Biological events are simply ignored by QM. Nor can QM, relativity, and thermodynamics be unified, as the latter gives “Time’s arrow” and relativity and QM do not. QM is a probabilistic model, which is probably the most likely to be true; the other two are classical, deterministic models, which are unlikely to be true given the universe’s underlying uncertainties. Those uncertainties are highly likely to make complete knowledge, BTW, impossible.

    Most all our models are incomplete. Kuhn’s “Structure of Scientific Revolutions” explored this over 50 years ago. & his observations are still guiding us today. Not much has changed.

    Limits to knowledge is a particular interest of mine, particularly since have created/discovered a major new insight into how mind can be created from the neurophysiology of the cortex, as an emergent phenomenon. Ever more interesting is the potential ability of fMRI and the use of cortical evoked potentials to detect and image the moral conscience at work in the frontal lobes. Interestingly enough thinking about thinking (introspection) has been fMRI imaged in the frontal lobes toa unique area. Structure —–> Function in the brain. Crick’s “Astonishing Hypothesis” has been de rigeur in the clinical neurosciences for over 120 years.

    Many might consider/include these facts/findings into their own model of incompleteness, too.

  17. Sean, to wind the clock back a bit. Do you consider general relativity necessary to explain the physics of everyday life? Neutrinos? Quarks?

    Dark matter and dark energy don’t seem more esoteric to me than those three things. We just stumbled across them earlier.

  18. I also would be curious about Sean’s answer to Latverian Diplomat’s question. What is the minimal set of physics that is needed to fully understand the high-level phenomena of every day life? And when was that full understanding reached?

  19. I’m also confused by the issue raised by cmt and Travis. As human beings, we only have direct access to the macro phenomena of everyday life. So the only way it is possible to learn anything about the underlying theory, or about the dark matter etc., is if it has some effects on macro phenomena that are not captured by the standard model. It seems like if what you wrote were literally true, it would be impossible to learn anything further about those two boxes.

  20. I don’t agree with your 31/31 slide: (1) Astrophysics and cosmology belong to the phenomena of our everyday life; (2) The “Core” theory is what points towards the existence of dark matter and dark energy; (3) a theory of everything does not need to be linked to a underlying reality (in an ERP sense).
    Anyhow Sean, good effort. It was nice to read your slides.

  21. For those in the comments above wanting more clarification of Dr.Carroll’s claim, see the three previous posts on this subject which he linked to at the end of the current post.

    In my interpretation, the effects of quantum mechanics, including entanglement, are included in “everyday life”. Dr.Feynman, in “QED” states that the human eye was the most photo-sensitive equipment available in early QM experiments, capable “of detecting a single photon”. And of course many of us have heard geiger-counters click. I do not think Dr. Carroll excludes QM effects in his definition. The point is, the mechanisms relevant to our daily lives, including QM, do not provide for ESP, psychic clairvoyance, miraculous healing, communication by prayer, uploading of our memories and personalities to some cosmic cloud, and so on. If such mechanisms exist, they have somehow been hidden from us in the trillions of experimental data points which physics experiments have generated. Is this a Godel-level proof that they don’t exist? No, only as much evidence as anyone could reasonably ask for, at this point.

    P.S. Dr. Feynman may have said that biology (billions of years of random contingencies) cannot be (practically) derived from physics, but I don’t think he meant that there is anything in biology which is inconsistent with current physics.

  22. JimV,

    I don’t know whether your allusion to entanglement is in response to my previous comment.
    If so, I would like to clarify:
    What I meant was that suppose there is a continuation of QM, called XM, that allows for a new phenomenon not predicted by QM (just as entanglement was not predicted by pre-QM theories). Let’s call this new phenomenon Xentanglemet.
    I argued that there’s a reasonable chance that every cell in our, say, digestive system, makes use of Xentanglemet without our knowledge. I’m that case, you can’t explain the simple behavior of eating a burger without resorting to XM.

    I argue that this scenario is conceivable because biology is so very complex that we can’t see the physics at play unless we are specifically looking for it, if that.
    I argued that in a hypothetical universe where physicists did not discover QM (but instead thought of chemistry as fundamental , for example), the physicists looking at photosynthesis, which runs their whole darn planet, would not notice anything amiss. It would be very easy for them to claim that photosynthesis is perfectly explainable using their available tools, and it’s just that it’s so complex that an explanation has not been provided *yet*.

    The point I’m trying to make is that until we *are* capable of deriving biology from physics, we can’t be certain we’re not missing physics.

    It’s too bad that people in the hard sciences tend to scoff at the problem of consciousness to the point of associating it with ESR, miraculous healing, praying, etc. In my mind this view is anti-intellectual.

    Until we are at a point where we can make a computer simulation that takes nothing but the laws of physics and some boundary conditions as input, and produces the actions of a living organism as output, all of biology is suspect.

    We’ve only been going at this for some 3 centuries. Is it that inconceivable we’re not done yet?

  23. Sean, I’m not sure why you’re representing the two top rectangles “Higher-level macro-phenomena of everyday life” and “Astrophysics/Cosmology” as disconnected. My brother (Joel Kastner) is an astronomer at RIT. and he routinely makes observations of the distant heavens part of his ‘everyday life’. Thus there is clearly empirical contact between these 2 domains. We can certainly distinguish two different scales of phenomena perhaps, but if we can make empirical contact between them, then surely they are not disconnected as shown?

    Postscript edit: I see that Antonio has raised this issue as well.