Responses to Anil Ananthaswamy and Luboš Motl
|Facing the storm.|
Photo by Selene's Art, used with permission.
A new blog post recently published on Scientific American seeks to rebut another essay I and colleagues published earlier this year on that same venue. The new post is by science journalist Anil Ananthaswamy. Since Scientific American doesn't publish rebuttals of previously published material, Ananthaswamy dresses his post up as a general commentary on the implications of quantum mechanics. But make no mistake: after a trite and verbose introduction explaining—as if it hadn't been explained a kazillion times before—the double-slit experiment, he cites our essay explicitly and then tries to show that its conclusions are not borne out by quantum physics. His attempt, however, is so clumsy that, if anything, it seems to further reinforce the points we've made in our essay. Allow me to elaborate.
There is more to quantum mechanics than just the double-slit experimentAnanthaswamy denies that "the double-slit experiment ... constitute[s] empirical evidence" for our original claim that, as paraphrased by Ananthaswamy, "a conscious observer may be indispensable to make sense of the quantum realm and that a transpersonal mind underlies the material world."
The problem is that, although we did briefly discuss the double-slit experiment to refute assertions made about it by others, our claim was mostly based on von Neumman's chains, the repeated experimental verification of Bell's and Leggett's inequalities, quantum eraser experiments, etc. (see our essay again for the appropriate arguments and links). As a science journalist, one hopes that Ananthaswamy is well versed with how these things differ from the double-slit experiment, and with the fact that there is a lot more to quantum mechanics than the double-slit experiment alone. So why make it look like our entire argument was based on the latter?
Bohmian mechanics as the savior of realism... No, really?Be that as it may, Ananthaswamy then proceeds to claim that our reasoning against realism—which underpins our argument for a transpersonal mind—does not refute the "de Broglie-Bohm theory" (or, more simply, 'Bohmian mechanics').
Yes, that's correct! In a recent academic paper, I even wrote this:
Some nonlocal hidden variables theories that preserve non-intuitive forms of realism—such as perhaps Bohm’s—may still be reconcilable with contextuality. However, these theories postulate—often at the cost of mathematical acrobatics—extra theoretical entities that are both empirically ungrounded and unnecessary for predictive purposes.By granting that, given the experimental results we cite in our original essay, Bohmian mechanics is the surviving hope for realism, Ananthaswamy is implicitly acknowledging that other more popular alternatives indeed aren't viable. And because there are many reasons why Bohmian mechanics has traditionally been rejected by physicists (more on this shortly), Ananthaswamy is ultimately helping my case.
A fellow anti-realistEnter physicist Luboš Motl. Back in February of this year, Motl published a blog post castigating me for yet another, earlier essay on quantum mechanics that I published on Scientific American. Motl opens his attack quite colorfully for a scientist. I can't resist quoting some of it here:
Lots of people whose skulls are confined in a spherical bubble are imagining that they're creative geniuses who are thinking outside the box. But the reality is inside out. One needs to perform the spherical inversion to see it. They're narrow-minded, intellectually limited losers confined into a bubble while the proper solutions require the realm outside the bubble.Although I presume that these lively words were meant for me, I confess they drew an appreciative giggle. You see, a common prejudice people have about physicists is that they are boring. Well, Motl clearly is anything but! And because I knew that Motl is a fairly respected physicist—alumnus, like me, of CERN—I kept on reading so to understand what Motl saw in my essay that he considered so wrong and repulsive.
Lo and behold, practically every point of substance that Motl proceeds to make is in agreement not only with my views, but with the very essay he was supposedly objecting to. Like the essay, Motl points out that theoretical extensions to quantum theory meant to salvage some form of realism are artificial and pointless; that quantum theory, as we know it, is the best theory we have to describe reality; that no extensions are needed; etc. These are precisely the points I was making in my essay. Even my reference to Carlo Rovelli's relational 'interpretation' of quantum mechanics doesn't contradict Motl's argument, for Rovelli doesn't extend quantum mechanics in any way; he simply acknowledges the implications of quantum mechanics.
To this day I remain puzzled by Motl's 'attack' on me, which seems more like an endorsement when one looks at its actual substance.
The problems of Bohmian mechanicsBe that as it may, Motl has been one of the most prominent critics of Bohmian mechanics, particularly in this post in his blog. In yet another post, he probably summarized best the problems of Bohmian theory. According to him, the claim that Bohmian mechanics
ends up with the same predictions as proper quantum mechanics ... is a complete lie. The dart always has a clear position which is guided by the pilot wave. Except that it's easy to see that a particle can't have well-defined other quantum numbers, like the spin, because it would pick an objectively preferred 𝑧-axis in space and that would break the rotational symmetry. Also, in quantum field theory, the number of particles is variable – they may be pair-created and pair-annihilated – so it's clearly impossible that there exist specific classical positions of 𝑁 particles. The number 𝑁 isn't even well-defined. Moreover, two particles could never exactly hit each other and annihilate – the probability in classical physics for an exact hit is zero (which is still true even if there is some extra pilot wave affecting the classical particles' motion). Bohmists also fail to explain what happens with the "objectively real" pilot waves when the particle is measured or absorbed and how the initial state of the pilot wave is prepared. Their theory always inevitable contradicts the Lorentz invariance, prohibits one from choosing situation-dependent i.e. Hamiltonian-dependent bases that are relevant for different observations in different systems, and it just doesn't work at all. The Bohmian mechanics is just a sleight-of-hand meant to convince sloppy people that one doesn't need to abandon the pillars of classical physics – even though they have been clearly falsified. ... Schack correctly points out that the Bohmian mechanics is almost certainly wrong because its basic classical object – the guiding wave – is in principle unobservable because a change of it should in principle impact things at a distance but it never does. So one really needs at least to fine-tune infinitely many things to make these a priori observable aspects of the pilot wave unobservable, to match the reality and avoid the contradictions with relativity, and even an infinite amount of fine-tuning isn't really enough to achieve this goal.This is quite technical, but the underlying idea is this: according to Bohmian mechanics, it's not the case that an elementary physical entity is both a particle and a wave; what is the case is that there is a particle and there is a wave, concurrently but separately. While moving, the particle simply 'rides' the wave. It is this, in turn, that allegedly creates the illusion of wave-particle duality in the double-slit experiment. Clearly, this is an attempt to preserve—as Motl points out—a classical view of reality all the way down to the quantum level. But when one makes the detailed implications of the theory explicit, all kinds of contradictions pop out. Motl attempted to make these contradictions clear in the quote above.
So my critics seem to 'cancel each other out': Ananthaswamy acknowledges that Bohmian mechanics is one of the only realist possibilities that survive the argument we made in our essay, whereas Motl kills Bohmian mechanics for entirely different reasons. So if my critics are both correct, there is no reason to think that I am wrong; much to the contrary. Thanks Anil and Luboš!
Open-ended, promissory arguments are unfalsifiableTo be fair, there is more to Ananthaswamy's case. He points out that there is another surviving theoretical possibility to save realism: so-called objective collapse theories. When I read this, I immediately thought: "Aha, but experiments have now shown that quantum effects persist even for macroscopic objects, which seems to refute objective collapse theories." I immediately knew how I was going to refute Ananthaswamy's argument on empirical grounds. However, as it turns out, Ananthaswamy didn't wait for me and refuted himself! He wrote:
Collapse theories predict that when particles of matter become more massive than some threshold, they cannot remain in a quantum superposition of going through both slits at once, and this will destroy the interference pattern. Arndt’s team has sent a molecule with more than 800 atoms through the double slit, and they still see interference. The search for the threshold continues.Alright! So Arndt has ruled out objective collapse theories for objects as massive as molecules with more than 800 atoms—each comprising dozens, even hundreds of fundamental subatomic particles such as quarks and electrons—but we have to keep on searching! Perhaps we will find the threshold after we shoot a few stars through a solar-system-sized double slit, huh?
If one is going to argue for realism in such an open-ended, promissory manner, then realism becomes practically unfalsifiable.
Before closing this section, allow me to highlight two other experiments that demonstrate quantum effects for "massive" objects, ruling out objective collapse theories all the way to the macroscopic level:
Stabilized entanglement of massive mechanical oscillators
Quantum ground state and single-phonon control of a mechanical resonator
These experiments are mentioned in the essay Ananthaswamy is attempting to refute. Why, then, does he write as if we hadn't addressed the issue and preempted his argument?
Oh, the ironyIronically, years ago Ananthaswamy himself wrote and published an article making a case precisely against the realism he now seems to be defending. He wrote:
A series of painstakingly designed experiments has since shown that Einstein was wrong: entanglement is real and no hidden-variable theories can explain its weird effects.This was pretty much our point. Ananthaswamy even titled his article "Quantum magic trick shows reality is what you make it." Did he since forget that, if "reality is what you make it," you need an observer and objective collapse theories don't hold?
It's about the implicationsThere is an argument made by Ananthaswamy that I find particularly obnoxious and regrettable. Referring again to double-slit experiments—the only thing Ananthaswamy seems to consider salient in the vast world of quantum mechanics—he claims that
these experiments don’t constitute empirical evidence for [claims of a conscious observer's role in measurement]. In the double-slit experiment done with single photons, all one can do is verify the probabilistic predictions of the mathematics. If the probabilities are borne out over the course of sending tens of thousands of identical photons through the double slit, the theory claims that each photon’s wave function collapsed—thanks to an ill-defined process called measurement. That’s all.But is that really all? You see, Ananthaswamy's claim above is as strictly accurate as it seems to willfully miss the point, in the interest of building a rather silly straw man. Yes, quantum theory only predicts the statistical behavior of physical systems, passing no judgment about what these systems essentially are. This is, in fact, the difference between physics and ontology. I acknowledge all this, alright.
But the point, of course, is the (ontological) implications of these predicted behaviors. What Ananthaswamy seems to be suggesting is that we should close our eyes to these implications; pretend that we aren't intelligent or observant enough to notice that they refute our mainstream intuitions regarding what reality should be. Here is nature telling us that she isn't what we take her to be. But, hey, we pretend we don't hear her because all we should do is "verify the probabilistic predictions of the mathematics." What a dimwitted, narrow-minded attitude!
I and my colleagues, co-authors of the original Scientific American essay, refuse to adopt this kind of attitude. We aren't blind. We are capable of critical interpretation and we gladly exercise this capability.
We encourage you to exercise it too.