Scientists don’t always agree with each other. Yes, I know; shocking but true. In cases of collegial disagreement, it’s often fun to quantify the extent of opinion by gathering a collection of experts and taking a poll. Inevitably some killjoy will loudly grumble that “scientific questions aren’t decided by voting!”, but that misses the point. A poll of scientists isn’t meant to decide questions, it’s meant to collect data — mapping out the territory of opinion among people who have spent time and effort thinking carefully about the relevant questions.

There’s been a bit of attention given recently to one such poll, carried out by Maximilian Schlosshauer, Johannes Kofler, and Anton Zeilinger at a quantum foundations meeting (see John Preskill at Quantum Frontiers, Swans on Tea). The pollsters asked a variety of questions, many frustratingly vague, which were patiently answered by the 33 participants.

This plot gives the money shot, as they say in Hollywood:

It’s a histogram of the audience’s “favorite” interpretation of quantum mechanics. As we see, among this expert collection of physicists, philosophers, and mathematicians, there is not much of a consensus. A 42% percent plurality votes for the “Copenhagen” interpretation, while the others are scattered over a handful of alternatives.

I’ll go out on a limb to suggest that the results of this poll should be very embarrassing to physicists. Not, I hasten to add, because Copenhagen came in first, although that’s also a perspective I might want to defend (I think Copenhagen is completely ill-defined, and shouldn’t be the favorite anything of any thoughtful person). The embarrassing thing is that we don’t have agreement.

Think about it — quantum mechanics has been around since the 1920’s at least, in a fairly settled form. John von Neumann laid out the mathematical structure in 1932. Subsequently, quantum mechanics has become the most important and best-tested part of modern physics. Without it, nothing makes sense. Every student who gets a degree in physics is supposed to learn QM above all else. There are a variety of experimental probes, all of which confirm the theory to spectacular precision.

And yet — we don’t understand it. Embarrassing. To all of us, as a field (not excepting myself).

I’m sitting in a bistro at the University of Nottingham, where I gave a talk yesterday about quantum mechanics. I put it this way: here in 2013, we don’t really know whether objective “wave function collapse” is part of reality (as the poll above demonstrates). We also don’t know whether, for example, supersymmetry is part of reality. Wave function collapse has been a looming problem for much longer, and is of much wider applicability, than the existence of supersymmetry. Yet the effort that is put into investigating the two questions is extremely disproportionate.

Not that we should be spending as much money trying to pinpoint a correct understanding of quantum mechanics as we do looking for supersymmetry, of course. The appropriate tools are very different. We won’t know whether supersymmetry is real without performing very costly experiments. For quantum mechanics, by contrast, all we really have to do (most people believe) is *think about it in the right way*. No elaborate experiments necessarily required (although they could help nudge us in the right direction, no doubt about that). But if anything, that makes the embarrassment more acute. All we have to do is wrap our brains around the issue, and yet we’ve failed to do so.

I’m optimistic that we will, however. And I suspect it will take a lot fewer than another eighty years. The advance of experimental techniques that push the quantum/classical boundary is forcing people to take these issues more seriously. I’d like to believe that in the 21st century we’ll finally develop a convincing and believable understanding of the greatest triumph of 20th-century physics.

I don’t find the results of this graph embarrassing at all. I’ve studied quantum mechanics and, without any intention to criticize people such as Zeilinger, I find it puzzling that people are arguing about “interpretations” of QM.

I feel that these interpretations are about trying to make sense out of phenomena that we cannot grasp because they’re outside of the logic and representations of our daily lives. In other words, it seems to me these interpretations are reformulations/projections of QM laws in common sense terms and thus necessarily reductions. A beginning of proof is you would actually compute the exact same things with all these interpretations.

Therefore, the variety of interpretations seems normal. The embarrassing part is that we are actually trying to find a correct interpretation. If I were a bit sarcastic (and I can’t because these people are way smarter than I am), I would say that understanding QM by splitting it in classical logic arguments is the same as trying to understand the laws of universe with good and bad. You just can’t.

I don’t understand why you think this is embarrassing.

Until there is some experiment (or theory) that can distinguish between these interpretations, the question is one purely of taste, and not of scientific merit.

Would you expect consensus on physicists favourite colour?

With no real-world effect, the pole question is just as relevant. And work trying to find a difference should clearly not be as high a priority as work on dealing with actual outstanding scientific problems.

Scientific questions aren’t decided by voting!

Seriously though, what I object to is not that a poll was conducted. It could give us useful information about what people are currently thinking. What I object to is the frequency with which polls on the foundations of quantum theory are conducted in comparison with other fundamental areas of physics. We do not see frequent polls about whether supersymmetry is true, even though that question is arguably just as open. What this suggests to me is that there are more researchers who regard quantum foundations as a matter of politics, choice, or religion as opposed to being a legitimate area of science than there are who regard supersymmetry in this way. If we have learned nothing else from John Bell then it ought to be that questions in quantum foundations can be settled by argument, rigorous theorems, and experiment, just like any other area of physics.

I also disagree with you that it is just a matter of “thinking about it in the right way”. Experiments should play a major role as much as they do in any other area of physics. In fact, I would argue that far more experiments have been done testing the fundamentals of quantum theory than have been done looking for supersymmetry, it is just that, so far, the experiments involved have been far cheaper. Every time we do a Bell inequality experiment, a double slit with larger and larger molecules, or create macroscopic superpositions in SQUIDs we are testing the fundamentals of quantum theory. There have also been a number of other quirky experiments testing things like the Pauli exclusion principle and the absence of third order interference. Direct tests of Penrose’s ideas are on the table in the next few years as well.

It is not that I think any of these experiments will disprove quantum theory, but I do think that experiments will be relevant to the ultimate solution. What will happen, I would guess, is that the correct view of quantum theory will lead to a way of generalizing the theory that will allow us to solve some other longstanding problems in physics. This will make experimental predictions and, upon confirming those, we will also confirm the correct view of quantum theory, since the relevant generalization will not look natural from the point of view of other interpretations of quantum theory. The evidence may come from quantum gravity, cosmology, the physics of black holes, or even non-gravitational high energy physics. The latter is, I admit, a longshot, but my thinking is that if supersymmetry is not confirmed then we will need another way of solving the unification problem and that could conceivably come from replacing quantum theory with something else.

The moral of this is that, if you believe what I am saying, then you shouldn’t stop at coming up with ideas of how to think about quantum theory. Once you have had those ideas you should see what changes to quantum theory are natural from that point of view and what predictions you can make. This is the only way we can hope to separate interpretations. For example, in many-worlds theory we now have fairly well established derivations of the set of observables and the Born rule from the Schroedinger dynamics. Many-worlds views the wavefunction of the universe, evolving unitarily, as a literal description of what is going on. Therefore, a natural thing to do from this point of view is to change the equation of motion. Many people have tried to modify the Schroedinger equation and there are no-go results about superluminal signaling and so forth. However, when they do change the equation they usually assume that all the rest of quantum theory will work in just the same way, with the Born rule, projection postulate and so forth. However, on a many-worlds point of view this is simply not correct. The measurement axioms are supposed to be derived from the equation of motion and not postulated independently. Adding things to the theory ad hoc that are not really independent of the rest of the physics could be the source of the problem. Therefore, many-worlds advocates should look at this again and figure out what current ideas about observables and probabilities in many-worlds theory tell us about what should happen in these modified theories. A good start would be to look at PT-symmetric quantum theory, because it has trouble with negative probabilities when you try to couple a PT-symmetric system to a regular one. After that, one could look at nonlinear Schroedinger equations again.

I have picked many-worlds as an example here because Sean likes it, but I could just as easily outline a program of generalization starting from any other interpretation of quantum theory and they would all be very different from one another.

Wow, I did not expect that my hypothesis that people don’t regard the foundations of quantum theory as a legitimate scientific question would be confirmed by other commenters whilst I was in the course of writing my comment.

I actually find the results of this poll exciting, and heartening. Things would be much more depressing in my view if physicists weren’t thinking as much about this issue, or if a majority of physicists had one favorite interpretation of quantum mechanics without being able to adequately back up any interpretation at this time. The fact that the proper interpretation of quantum mechanics is still unconfirmed, after so many great minds have been considering this issue for a century, suggests to me that there are still richly thrilling times ahead for the fundamentals of quantum theory.

As they say: ‘shut up and calculate!’ — does the physicality of the wavefunction really matter, from a fundamental physics perspective?

As an example, let’s take the equation at the top of the blog: the famous entropy equation from Boltzman’s grave. I would propose that entropy ‘does not exist’ — it is not measurable in any physical sense, but rather, can only be inferred from the properties of the system. It rather succinctly describes the macroscopic properties of an object, but it has no fundamental physicality. Various physical principles that ‘do not exist’ (in that they are never directly observable) are very useful in physics, such as potential energy, entropy, the wavefunction, etc.

(Full disclosure: my vote would be in the information theoretic camp. )

Matt L., what exactly is the question about the foundations of quantum mechanics that you think should be investigated?

I actually like the way the question was phrased. With so many “interpretations” that all agree with the same mathematical and experimental results, preferred interpretations do seem to be a matter of comfort for the most part. As I’ve often said, the interpretation one likes is the one whose philosophical baggage one

dislikes least. (Okay, in individual cases, some actually like the specific philosophical baggage.)So Sean, do you have a “favorite interpretation”?

I have a problem with your phrasing regarding what’s required, but this comment section is too small to contain it. Thus, a ranty post on my own blog.

Very, very cool.

@Anthony Well, the basic question is just “Quantum theory, WTF?” but I realize that needs to be explained a bit. One way of stating the question is “What must the world be like in reality in order for quantum theory to be true?”. As stated, the question looks rather philosophical, but as indicated in my previous comment I believe that different answers to this question lead to different possibilities for generalizing quantum theory. Also, this is the question that John Bell, Kochen-Speker, PBR, etc. were asking when they derived their theorems, so there is no doubt in my mind that it can lead to progress.

This post goes to the heart of the conflict between science and religion. QM is well established as a computational representation of how the universe operates at extremely small scales as those operations manifest themselves at larger scales. However, our experience of reality only occurs at larger scales, so when we try to match the operations on a small scale to the experiences of the large scale, we can’t do it without the math. Keep scaling up and you see what I’m getting at. We can’t directly experience the largest scales of all (infinite) without math, and so our interpretation of infinity will always be diverse.

Neither of these issues has stopped people from living productive, happy lives, or stopped human social and cultural groups from adapting to the universe as it presents itself to them. The mistake is to believe that we will be able to understand the universe the way many believe that God can, or control the universe the way they believe guardian angels can. It just ain’t gonna happen, so let’s move on.

I took Sean’s article to mean that he’s embarrassed that not many possible interpretations have been ruled out in around a century, i.e. that the right experiments haven’t been devised.

I take back my last comment: Cool. But I do understand Sean’s concern, so to speak. I’d say something like concern over problem. But everyone look at it this way, they only asked 33 people. That’s gotta mean something in itself considering what came in first. Plus, graphs suck. So I’m double upset. Believe me, I’ve got a problem with everything.

Additionally: How do people get away with NOT believing in the wave function?!?! I hate to use the word believe, but its like it’s the tooth fairy in the Copenhagen interpretation. The way I see it, which seems adolescent, I’m an artist, it’s what I do. I draw all kind of lines, not just straight ones. Sometimes, when I’m feeling really creative (sarcasm), I draw curved ones. Explain that. 😀

How does decoherence enter into the definition of these interpretations? Is not the fundamental interpretative issue about the reality, or not, of the wave function collapse?

Fair enough Matt. I agree that some effort should be expended in that direction. But I am not worried that problems with more obvious experimental implications are receiving more attention. And I am not worried that 13 functionally identical interpretations exist.

Of course, when we find something that fundamentally challenges our understanding of QM, this kind of work is great. I just don’t think that that is the case at the moment. There are open problems, of course, but nothing that says ‘I just don’t see how we could ever reconcile this experiment with that theory’.

strange normalization! sum of fractions exceeds 1 !

What about David Deutsch’s claim that Quantum Computers prove the Everett’s interpretation?

Well that puts me in the same group as just about every scientist polled, of which I’m not, in that no one understands QM. Holy cow I’m not crazy after all.

i adore the many different interpretations of qm — it shows just how powerful Neils Bohr was and all the permutations that scientists went through to figure out what he meant by that one utterance about what we can know and if we can make meaningful statements aboot universe…

it shows a healthy, non-dogmatic mindset — they are all true in some sense, false in some sense, meaningless in some sense, etc…

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#1. “I think Copenhagen is completely ill-defined, and shouldn’t be the favorite anything of any thoughtful person”. two words; chewin’ tabaccee.

#2 I’m a fan of information based

Sean –

You propose that “the advance of experimental techniques that push the quantum/classical boundary’ will likely settle these questions.” (And I think you’re right to assume that the “shut up and calculate” school is sort of copping out.)

Those advancing experimental techniques most definitely include weak measurement, yes? And in recent years we’ve seen many papers making ambitious claims for and relying on weak measurement, haven’t we?

Like this one from 2009 proposing that, contra Heisenberg, simultaneous measurement of both position and velocity of particles is possible —

“On the Weak Measurement of Velocity in Bohmian Mechanics”

http://xxx.lanl.gov/abs/0808.3324

As you’re doubtless better aware of than I am, there have been more recent papers making more ambitious claims. Given that I’m a mere science spectator, Aharanov’s two-state vector formalism seems an elegant and congenial explanation; I’m prepared to buy the claims for weak measurement. But mere elegance and congeniality, along with my non-scientist status, don’t stop me from noticing that there are questions about weak measurement.

For instance, in these experiments, they aren’t measuring, say, the trajectory of any given photon and they’re still doing *averages,* which are the bread and butter of any quantum mechanic. They’re able to reconstruct the *average* trajectory of a photon, which is cool. Nevertheless, is weak measurement really measuring anything real?

Here’s a paper articulating some of these questions better than I can —

“What do quantum “weak” measurements actually measure?_

– Stephen Parrott, 2009

http://lanl.arxiv.org/abs/0908.0035

Abstract: ‘A precise definition of “weak [quantum] measurements” and “weak value” (of a quantum observable) is offered, and simple finite dimensional examples are given showing that weak values are not unique and therefore probably do not correspond to any physical attribute of the system being “weakly” measured, contrary to impressions given by most of the literature on weak measurements….’

All this is a little off your usual cosmology beat, I know. On the other hand, via the Aharanov/retrocausality connection, it does have to do with your Arrow of Time concerns. Most of all, though, whether it’s here or later, I’d deeply appreciate an intelligent discussion of the pros and cons of weak measurement that’s geared at a level even mere science spectators like myself can appreciate.

If you’re not able to understand the results of a mathematical equation, something has gone wrong.