In recent years noted string theorist and blogger Lubos Motl has increasingly turned his attention to the foundations of quantum theory. Those of us who study quantum foundations for a living have tended to find his commentary mildly annoying, as he consistently calls those of us who disagree with his views “anti-quantum zealots”, crackpots, and worse.

I have recently come to the realization that Lubos’ views on this subject are completely correct. Specifically, I now believe the following:

- The Copenhagen founders of quantum theory—Bohr, Heisenberg, Pauli, Born et. al.—had things essentially right. They were only missing the details of decoherence theory in order to properly understand the classical limit.
- The decoherent histories formalism as proposed by Gell-Mann and Hartle, gives a completely consistent account of these minor details and is the correct way to understand physical properties and probabilities in quantum theory.
- People who work on high energy physics, and especially string theory, are the ultimate arbiters of truth about the nature of quantum theory. Only they have the background needed to make meaningful statements on the subject. This is especially true of theorists who are or ever have been employed at Harvard. Any idea that has not been discussed by these physicists is probably wrong. No insight is to be gained by actually studying the foundations of quantum theory for several years, rather than working on proper fundamental physics.
- And finally, in the face of any other views on quantum theory, the correct response is always, “It’s quantum stupid!”

Having adopted this new credo, I now realize that my previous view that quantum theory should be founded on a realist ontology that gives a clear picture of what is going on in reality independently of the observer was wrong-headed. Lubos’ blog posts on the subject make a compelling argument that my view was guided more by religious zealotry and communist ideology than by empirical data and rational argument. It therefore seems appropriate that I should enter into a period of repentance by adopting a garb of sackcloth and ashes for a while before emerging cleansed of my previous religious views.

Given the impracticality of wearing sackcloth and ashes in modern life, I have instead decided to wear a t-shirt that identifies me as the anti-quantum zealot that I am. You can see a picture of me wearing this t-shirt at the top of this post. Before embarking on a career in string theory, or more likely quitting academia to become an accountant because I do not have the intelligence to understand real physics, I still have several engagements where I shall have to speak about my previous bigoted research. I therefore promise that I will wear my anti-quantum zealot t-shirt at all such speaking engagements for the next year.

At this point, I would like to urge my colleagues who have also been denounced by Lubos’, and those who hold similar views but have so far flown under Lubos’ radar, to reconsider their views and join me in repentance. If each of us wears an anti-quantum zealot t-shirt publicly then we may be able to prevent others from following us down the path of ideologically motivated nonsense.

Fortunately, I have made it easy for you to purchase your own anti-quantum zealot apparel and merchandise, from the Spreadshirt shop at this link. It is available in any colour, so long as it is communist red. I receive a commission of 2CAD for every purchase from this shop (the rest goes to Spreadshirt, so complain to them about their overpriced t-shirts rather than me). I would dearly love to keep that commission money because I will be short of income for a while as I retrain as a string theorist or accountant. However, that would greatly complicate my tax situation, so I have decided to donate it to a charity that will protect future generations of physicists from adopting anti-quantum ideas. For this purpose, my commission will be donated to the Next Einstein Initiative of the African Institute for Mathematical Sciences (AIMS), which seeks to establish centres of excellence in mathematical science across Africa. AIMS does cover fundamental physics, but I note with approval that they do not currently have programmes in quantum foundations, so they will not be teaching wrong-headed ideas to the next generation of African physicists.

You might be tempted to consider your purchase of anti-quantum zealot merchandise as a charitable contribution, but if you really want to support AIMS you should forget about the t-shirt and just donate all of the money your would have spent to them directly. Anti-quantum zealot merchandise is only intended for those who want to seriously repent for their anti-quantum beliefs.

In order to encourage donations, either through merchandise purchases or direct contributions to AIMS, I will be offering a special prize to whoever makes the largest donation in response to this post by the end of this month (April). You simply have to let me know how much you have donated, either by email, or by leaving a comment if you want to boast about how generous you are (I will be asking the winner to verify their donation by sending copies of their receipts).

What is this special prize you ask? Well, it is a collection of schwag that I stole from my absolute favourite academic publisher—Elsevier—at the recent APS March meeting.

As you can see, it consists of two pens advertising the exciting new journal “Reviews in Physics”, which I assume will soon surpass Reviews in Modern Physics as the premier physics review journal. I believe this because of the extremely rigorous editorial oversight that Elsevier applies to all of its journals.

In addition to this, you get a luggage tag advertising Elsevier’s offerings in Optics, which is filled with some mysterious blue liquid, because everything is better with blue stuff in it. If your luggage accidentally ends up at the Elsevier offices because the baggage handlers read the side of the label displayed in the photo rather than the address written on the back, I am assured that Elsevier will apply their open access policy to your bags and charge you $80 for their return.

You forgot to agree also with his misogyny 😀

Nice t-shirt Matt. But I am worried that the graphics seems to be actually anti-realist, since the atom clearly has electrons following definite orbits around the nucleus. I think you need to replace it by a picture of infinite dimensional Hilbert space. Or change the words to “anti-realist zealot” and post it to Motl.

Lovely tee-shirt Matt. Will you be wearing it on other days too?

Excellent loof lirpa. LOL

I’m new to this blog. So, joking aside, what interpretation do you agree with? You know a lot about this stuff so I’d be interested to hear.

The short answer is “none of the above”, i.e. none of the existing options are appealing to me. Since the time of Schrödinger, we have been beset by the problem of quantum jumps. That is, quantum physicists are prone to jump to conclusions. Explaining why I don’t like the existing options and what I am looking for in an interpretation would take far too long for a comment. Maybe a blog post one day, if I ever get back to writing blog posts on a regular basis.

Would you be willing to comment on this paper by Gell-Mann ( http://arxiv.org/abs/1106.0767 ). He argues that QM is consistent with an inutuitive, unique, fine-grained reality. It hasn’t picked up much traction in the quantum foundations world.

Also (and perhaps you have answered this elsewhere in your blog), do you have sharp criteria an interpretation must satisfy?

Many years ago, I wrote a blog post about the criteria I think an interpretation must satisfy. I don’t stand by all of that now and would phrase things in a more subtle way now, but it still gets one or two of the basics right.

Regarding the Gell-Mann-Hartle paper, I don’t have time to go through it again right now. The short answer is that if your interpretation involves negative probabilities then you cannot claim that it is realist in any straightforward way. The argument that this is OK because the fine-grained histories cannot be observed does not pass muster because that is an argument appropriate for an operationalist rather than a realist. If you are a realist then the fine grained history exists and is knowable in principle, even if it is not known to us. Thus, the standard arguments in the foundations of probability would imply that they have to be assigned non-negative probabilities.

I actually think that the difficulty of coming up with a satisfactory interpretation of QM means that we really don’t understand probability (even nonquantum probability) as well as we think we do. Neither a Bayesian, subjective view of probability nor a frequentist, objective view of probability is completely satisfying.

Whilst I agree that employing a consistent notion of probability in the foundations of quantum theory is highly desirable, as people get into all sorts of confusions by employing different notions at different points in their argument, I don’t think that solving the problems of probability theory will help much in the foundations of quantum theory. Arguably, the foundations of probability need to be sorted out independently from quantum theory, as probability and statistics are key facets of the scientific method as a whole, so they need to be sorted out before we even think about their applicability to a particular theory of physics. It would be surprising if, after so doing, a particular theory of physics could then change that picture too much. I also suspect that there is a conflation of two hard problems going on here, i.e. we don’t agree on how to understand probability theory and we don’t agree on how to understand quantum theory, so therefore the two must be related. That’s just a fallacy, similar to the one made when people argue that a theory of consciousness is needed to understand quantum theory. It is also worth noting that many of the conceptually difficult aspects of quantum theory, such as Bell’s theorem, can be formulated in a way that does not depend on probabilities, e.g. the GHZ version, so it is hard to see how getting sophisticated about probability would help here.

Thanks for the blog link. I’ll give it a read.

“If you are a realist then the fine grained history exists and is knowable in principle”

This looks like a controversial statement to me. A realist might postulate the existence of a fine-grained history for a physical system, but reject the idea that it is knowable in principle, as they might reject the idea that the fine-grained history of the system can be exactly correlated with some arbitrarily sophisticated recording apparatus.

I should have stated my argument more carefully, so let me give it another go.

If you are an operationalist, it is sufficient to supply the probabilities for everything that is observable. Clearly, these are going to be ordinary real-valued, positive probabilities, because they are related to relative frequencies, betting odds, or whatever they are in your favourite interpretation of probability, in the usual way. Now, in the course of your calculations you may find it convenient to introduce some unobservable entities. If you can find a calculus that involves assigning exotic probabilities to these unobservable entities, be they negative, complex, or whatever, then that is perfectly fine because the unobservable entities don’t actually exist, and the operationalist can simply view the calculus as a convenient way of summarizing the relations between the probabilities for observable entities. For the operationalist, the whole calculus is just a convenient fiction for getting at the probabilities for observable things.

On the other hand, for the realist, things are not so simple. If you are positing that the unobservable entities actually exist then you have to give some meaning to statements about their probabilities. It is not enough to point out that the probabilities for all observable things are positive. Although that is surely still an important consistency check on the theory, merely demanding this as your definition of what you mean by an exotic probability is an operationalist answer. So, although I can’t say that Gell Mann and Hartle’s proposal is outright inconsistent, I maintain that it is incomplete. They have not done enough to explain what their exotic probabilities mean.

In my view, there is good reason to doubt that exotic probabilities are meaningful in a realist framework. This is because you are positing that the things to which you assign these exotic probabilities actually exist. In Gell-Mann and Hartle, there is a set of fine grained histories, one of which is realized, but the set is assigned an exotic probability measure. What does it mean then if the actual history in one realization of an experiment is assigned a negative or complex probability?

Now, which actual history occurs in any given run of the experiment may be unknowable to me, and it may also be unknowable to anybody who is limited by the same physical theory as me. However, in a realist framework this is not a fundamental limitation on knowledge, but a contingent fact about how the laws of our universe happens to operate. We can certainly imagine a god-like super-observer, not limited by the current laws of physics, who can know all the facts that are true of our universe. No such super-observer may exist in our universe, but the point is that such an entity is logically conceivable within a realist framework. That’s what I meant by “knowable in principle” in my previous comment.

Now, we can imagine how super-observers would reason in the face of uncertainty. Although they can know everything in principle, we can imagine a super-observer who simply hasn’t bothered to look at some part of the universe, and they are interested in making bets about it, perhaps with other super-observers. How should such an entity reason? Well, clearly, they have to use ordinary classical probability theory for exactly the same reasons that we have to use it for observable things, because for them everything is observable in principle. So, in fact, we can see that there must exist an ordinary classical probability measure over all the things that actually exist, observable or not. If you are positing that there are some real unobservable entities that cannot ever be assigned ordinary probabilities then you have a problem with realism. One can still maintain that the exotic probabilities are useful in the operational sense, but it must ultimately be possible to derive them from a theory which does assign ordinary probabilities.

I will admit that there is one big gaping hole in this argument, which is that the foundations of probability are themselves controversial. Many people, myself included, argue that the current foundations are not sufficient to entail that ordinary classical probability is the only rational theory to use for reasoning in the face of uncertainty. However, usually the gaps in these arguments are expressed in operational terms, e.g. we cannot determine whether outcome A of experiment E would have occurred if experiment F was actually performed, because experiments E and F are incompatible, so performing one precludes finding out what the outcome of the other would have been. Therefore, we may say that there is no operational meaning for the joint probability of A and B (where B is an outcome of F), so we don’t have to satisfy an equation like p(A AND B) < = P(A). This is an operational gap in the usual argument for the probability calculus, but if you want to argue from a realist point of view then you have to give me some account of what reality is like that entails nonclassical probabilities and this must be given in terms of the nature of reality itself rather than the nature of my limited access to it. In other words, you have to give an account that would apply to super-observers, rather than just regular ones. This is why I say that Gell Mann and Hartle's account is incomplete. It is not necessarily incompletable, but that looks like a very hard task and, at least to my knowledge, nobody has given a realist account of exotic probabilities ever in any context. Let me just make one more comment. If you want to write quantum theory in terms of exotic probabilities, there are an infinite number of ways to do it. For every basis of the vector space of operators on a Hilbert space, there is a corresponding pseudo-probability function. Why then, should I believe Gell Mann and Hartle's account rather than any of the other natural ones, such as Dirac's complex measure, the Wigner function, the Q function, the P function, etc.? This non-uniqueness ought to be troubling to say the least. To repeat my bad Schrödinger joke, this seems like the sort of quantum jumping to conclusions that we ought to be avoiding. The goal is not to come up with an interpretation of quantum theory that works. At this point we have several of those. The goal is to come up with the correct understanding of quantum theory.

Thanks for the thorough response. It has given me a lot to think about. Particularly, the idea of a super-observer capable of identifying the real history is interesting. I.e. What is not a settleable bet for us is still a settleable bet for super-observers. I see what you mean by “knowable in principle”.

Though trying to describe super-observers who, while not constrained by physical laws, still settle bets about physical systems sounds difficult. For example, Gell-Mann and Hartle make the statement “A set of histories {Cα} is recorded if there are alternatives with projection operators {Rα} at one time that are correlated with the histories.” A super-observer would presumably supernaturally record the one real history while also dismissing the fact that there are is no corresponding projection operator that satisfies equation 3.1 in the paper.

Perhaps the formalism in the paper is limited to demonstrating a consistency between the existence of a real fine-grained history, and the Decoherent Histories procedure used by ordinary, natural, IGUSes trying to learn what they can about the history.

In the absence of any other entries, the winner of the contest to donate the most money to AIMS Next Einstein Initiative is Ding Jia, a student I worked with who donated $70USD. The pens and luggage tag are headed his way.

I have an elementary cource in quantum machanics from the university, and I happened to pass the exam without knowing the Copenhagen Interpretation, which I later learned about, and found fully as ully as the pronounciation of the Danish Language, the native language of Niels Bohr. Danish is essentially the same language as Norwegian and Swedish, but with an incredibly blurred pronounciation. Even though this peron is a total conformist regarding the fundamantals of Quantum mechanics and relativity, I think he is right about certain political issues, like the threat against our freedom from the Islamic movement and about the Global warming Hoax. But i am not so sure about his view of the links between all bad things and comunists, and America as the savior of the western freedom ideals, though.