If usefulness isnât a guide to whatâs real, what is?
Seems like Iâve been writing a lot about quantum mechanics lately. Apparently so have a lot of other people. One thing that keeps coming up is the reality or non-reality of the quantum wave function. Raoni Arroyo and Jonas R. Becker Arenhart argue for non-reality: Quantum mechanics works, but it doesnât describe reality: Predictive power is not a guide to reality. (Warning: likely paywall.)
Along similar lines, in an article about what he says are quantum myths, Ethan Siegel argues that superpositions are not fundamental to quantum physics:
Superpositions are incredibly useful as intermediate calculational steps to determine what your possible outcomes (and their probabilities) will be, but we can never measure them directly.
Arroyo and Arenhart take a similar line. They argue that it would be more intellectually honest for wave function realists to call their position wave function pragmatism. As they note in the title of their piece, they donât see predictive success as a guide to reality.
The question I want to ask these people is, if predictive power, if usefulness, isnât your guide to what is real, then what is?
Itâs worth thinking about why we care whether something is real or not. Is the sound Iâm hearing from outside rain? Is the rain real? To say it is is to say I need to take an umbrella with me when I go outside, or be prepared to get wet. To say it isnât is to say I can walk outside without worry of getting wet. We get similar considerations when trying to decide if a stock rally is real or illusory, or, from an evolutionary perspective, whether the sound in the bushes is a real predator or just a figment of your imagination. Reality is that which makes a difference, something which thereâs a possible cost to ignoring.
Admittedly, this is a strange point to make when talking about quantum states. It might seem like whether theyâre real has little to no bearing in our daily lives. But they do seem to make a difference for experimenters and quantum computing engineers. They have to take the dynamics implied in these mathematical tools seriously. In the case of quantum computing, itâs the very dynamics that seem to enable what theyâre trying to do. Failure to treat them as real has consequences.
Now, Iâm a structural realist. I think what we can count on being real in successful scientific theories are the structures they describe, at least to some level of approximation. That doesnât mean we can count on them being fundamental, or that we know what they may be structures of. This is particularly important to remember with quantum theory, where the structures are all we currently have.
Does that mean that, rather than being structures of objective reality prior to a measurement, they could actually be structures of subjective expectations as the QBists argue? Or of the way the experimental equipment has been set up, as other antirealists argue? I suppose so. But that seems to imply the possibilities are completely set by these expectations or preparations, that if scientists really wanted to, they could get any result they wanted.
In practice, something seems to constrain the possible results. Of course, if I put on the epistemic hat, I could argue that those constraints are the constraints on their thoughts (QBism) or practical equipment limitations (other epistemics), not anything in the quantum realm. But taking this literally, that seems to imply that quantum physics is a big illusion, a side effect of the way scientists think or construct experiments. If so, how could anyone be sure that any scientific measurements beyond human senses are to be trusted?
All of that is before remembering that if we think anything objective at all is happening in the physics prior to a measurement, that there are mathematical theorems which kick in and demonstrate that quantum states must describe something real. Epistemic interpretations of quantum mechanics, such as Copenhagen, QBism, and RQM avoid this be saying there is no such objective physics prior to measurement (or interaction). Which, to me, makes calling them âepistemicâ misleading. Qbists in particular argue for a âparticipatory reality,â a notion they inherited from John Wheelerâs âit from bitâ idea.
This selective application of antirealism has always felt like gerrymandering to me. Most of the proponents want to resist the idealism label, but they seem to want to take from metaphysical antirealism just what they need to avoid quantum state realism. It all feels forced.
Interestingly enough, that doesnât appear to have been Niels Bohrâs take. Historians often argue that he was more of a neo-Kantian than either an instrumentalist or idealist. His take seemed to be that the quantum realm was real, but inaccessible, the noumena always beyond the phenomena. Of course, this predates the theorems I mentioned above, which is what forces stronger stances from contemporary epistemic proponents.
But my issue with the Kantian view is it pushes reality into something utterly and forever unknowable. Reportedly, Kantâs motivations for doing this were to preserve space for God, the soul, free will, and morality in response to the âCrisis of the Enlightenment,â which seemed to call all of those things into question. I suspect neo-Kantians are trying to preserve different things, but that kind of preservation likely remains part of their motivation.
But the cost of doing so is to remove the practical aspects I noted above when deciding whatâs real or not. In my view, it removes any utility from the concept of reality, except for talking in terms of theology or overall metaphysics.
Which may be why Arroyo and Arenhart want to use the word âpragmaticâ instead. I think a better strategy is to retain our grounded everyday meaning for âreal,â but admit that we never know whether weâve reached ultimate reality. But this is coming from someone who doesnât share the Kantian or neo-Kantian concerns.
Overall, my theory of reality is pragmatic. But I continue to wonder, for the people arguing against that take, what standard are they using?
What do you think? Are there issues with a pragmatic take on reality Iâm overlooking? If so, what would be a better standard?
#antirealism #Philosophy #PhilosophyOfScience #Physics #QuantumMechanics #realism #structuralRealism
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