This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is just the text of the original post, from February 2018, without the images that appeared with it (which were mostly fairly generic photos ).
A couple of weeks ago, fellow Forbes blogger Ethan Siegel took to his keyboard with the goal of making me sigh heavily, writing a post about interpretations of quantum physics calling the idea that you need an interpretation “the biggest myth in quantum physics.” Ethan’s argument boils down to noting that all of the viable interpretations known at present make identical predictions about the probability of getting particular outcomes for any experiment we might do. Therefore, according to Ethan, there’s no need for any interpretation, because it doesn’t really matter which of them you choose.
As an experimentalist by training and inclination, I am not without sympathy for this point of view. In fact, when I give talks about quantum mechanics and get the inevitable questions about interpretations, I tend to say something in that general vein– that at present, nobody knows how to do an experiment that would distinguish between any of the viable interpretations. Given that, I say, the choice between interpretations is essentially an aesthetic one.
When, then, the heavy sigh on reading Ethan’s post? There are two reasons why I had that reaction, and am writing this post in (belated) response.
The first reason is best explained via a historical analogy. For this, I would point to the infamous arguments between Albert Einstein and Niels Bohr that culminated in the famous paper by Einstein and his colleagues Boris Podolsky and Nathan Rosen that introduced the world to the physics of quantum entanglement. Einstein, Podolsky, and Rosen proposed a thought experiment involving a pair of particles prepared in such a way that their individual states were indeterminate but correlated– most modern treatments make it a two-state system, so that the measurement of an individual particle has a 50/50 chance of coming up with either outcome, but when you make the same measurement on both particles, you’re guaranteed to get the same answer.
This phenomenon turns out to be a rich source of interesting physics to explore, as you can tell from the fact that I’ve written at least ten posts about it (this one, and the nine links in the second paragraph). There’s a thriving subfield of physics that has grown out of the ideas expressed in that one paper, generating both fascinating theoretical approaches and useful experimental technology.
That said, though, for the first three decades after it was published in 1935, the “EPR paper” was pretty much a footnote to physics. As late as the early 1980’s, Abraham Pais’s magisterial scientific biography of Einstein dispenses of it very quickly, as a sort of brief mis-step during the declining years of Einstein’s scientific career.
Why was such an important paper so lightly regarded? Because it was, initially, an argument about interpretations of quantum physics. That is, Einstein, Podolsky, and Rosen used the correlation between separated measurements to argue that quantum mechanics must be incomplete– that the measurement outcomes that orthodox quantum theory says are indeterminate are in fact determined in advance, and just hidden from us. They didn’t disagree with the prediction of the quantum theory, only its interpretation.
The EPR paper has the title “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”, and in keeping with Betteridge’s Law of Headlines, they answered “No,” saying that a deeper theory was needed. Niels Bohr wrote a hasty response, also with the title “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” arguing the opposite. The specifics of the counter-argument aren’t all that important (it’s also not a very well-written paper…); the important thing is that most practicing physicists viewed the whole thing as pretty much an “Is not!” “Is too!” exchange, a purely philosophical matter with no practical importance.
How did the EPR paper get lifted out of obscurity, then? Because an Irish physicist named John Stewart Bell looked into the problem, and thought seriously about what the claims being made really meant, and realized that the sort of model Einstein, Podolsky, and Rosen favored would place hard limits on certain types of measurements. Those limits would not be the same in quantum theory, so a clever experiment can distinguish between them. Bell pointed this out, and not long afterward a small number of physicists began working on those experimental tests. From that seed, much of the modern edifice of quantum information physics has sprung.
So, my first response to Ethan’s line of argument is a reminder of Bell’s work. Just because we don’t currently know of an experiment that can distinguish between interpretations of quantum physics does not mean that we will never know how to do such an experiment. It may be that some future physicist, successor to Bell, will think long enough and deeply enough about quantum foundations to come up with an experiment for which different interpretations genuinely make different predictions. That would be pretty amazing, and for that reason I think it’s worth having some people put some effort into thinking about this stuff.
I would say, though, that even if nobody ever comes up with an experimental test of quantum interpretations– or, to take an extreme case, somebody manages to prove that no such experiment can ever be done– it’s still worth thinking about them, and picking a favorite. The choice between them is currently an aesthetic choice, and may remain so forever, but as anyone who has ever decorated a house can tell you, aesthetics are not nothing.
That is, the way you choose to think about “what’s really going on” in quantum physics may not make any difference in the outcomes you predict for a given experiment, but it will shape the way you think about what experiments to do. If you favor an ontological theory involving a physical collapse of the wavefunction, that may lead you to explore certain lines of inquiry– looking at the speed of that collapse, say– that are most easily conceptualized in that sort of picture. If, on the other hand, you’re more of an epistemic interpretation type, you might instead choose to pursue different lines, ones more easily interpretable in terms of information. If you’re a Bohmian, you’ll think about particle trajectories, and if you like one of the retrocausal approaches you’ll think about future measurements affect past conditions, and so on.
The argument that you should care about quantum interpretations, even in a world with no measurable distinction between them, is the same as any other argument in favor of diversity. Different aesthetic choices influence the way you think about the world, and that change in thinking can lead to new and different approaches to… everything, really. It may not be possible to distinguish old-school Copenhagenism from the latest Many-Worlds hotness in any given experiment, but someone who favors Many-Worlds may be led to do experiments that would never happen in a world where only Copenhagenists exist, depriving us of a different angle on the quantum world.
So, while I don’t think anything Ethan said is wrong in the sense of being incorrect, I sighed heavily on reading it because it’s the wrong take in the sense of being far too narrow and constrained a way to look at quantum phenomenon. Quantum physics operates in a way that runs very counter to everyday experience, and that makes it a theory of unparalled richness. So rich and weird a set of phenomena deserves as diverse a set of approaches as we can bring to bear on it, and that’s ultimately why quantum interpretations matter.
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(If you’re not happy with that “Let a thousand interpretations bloom” take, you should read Scott Aaronson’s post on quantum interpretations, which predates Ethan’s post, and makes an affirmative argument for Many-Worlds. Actually, you should read that even if you like the above…)