The critical rationalists disliked post-1925 quantum mechanics (QM) as much or more than they disliked Marxism. It’s not that QM wasn’t successful when judged against their methodology. It was. Theorists made dramatic predictions that matched later experimental results to an absurd degree of precision. Check. Anomalies were handled by adjusting or expanding the theory in ways that allowed new predictions that, in due time, survived attempts at refutation. Check. So the critical rationalists should have been happy, but they weren’t.
What’s the problem? In a word, it’s that QM is an instrumental theory, and those are the wrong kind of theories. In a draft of the final essay in this series, I used QM to explain instrumentalism, but then I thought of a better (or, at least, cuter and shorter) example.
Still, the QM story is interesting, so I present it as an optional supplement. As Abraham Lincoln didn’t actually say (of course), “People who like this sort of thing will find this the sort of thing they like.” It is in that spirit that I offer these words to you.
The second quarter of the 20th century was an awkward time for theoretical physicists working on teensy entities. I’ll attempt to explain some of the issues, but I caution that I’m woefully unqualified to do so. I enrolled at CalTech intending to be a scientist, specifically: a physicist. I discovered I wasn’t smart enough, so I transferred to the University of Illinois, which at the time had a much better computer science department and was way cheaper. (Around $4000 a semester in 2026 dollars.)
It all has to do with causality.
Since at least Galileo, the assumption in physics has been that there is no action at a distance. An effect on something over there caused by something over here must have that causality carried from here to there by some thing (or things). Consider heat. Galileo wrote:
“Those materials which produce heat in us and make us feel warmth, which we call by the general name fire, would be a multitude of minute particles having certain shapes and moving with certain velocities. Meeting with our bodies, they penetrate by means of their consummate subtlety, and their touch which we feel, made in their passage through our substance, is the sensation which we call heat.”
Causality is contact, you might say. Kepler agreed. Quoting Wikipedia:
Kepler’s laws were developed based on a physical theory of planetary motion in which the Sun emitted magnetic fibrils which pulled the planets into orbits. The fibrils were somewhat elastic allowing non-circular motion driven by the inertia of the planets.
The Newton of the Principia also agreed, though he left undescribed what specifically was moving the planets:
“that one body may act upon another at a distance, through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity, that I believe no man who has in philosophical matters a competent faculty of thinking, can ever fall into it. Gravity must be caused by an Agent acting constantly according to certain laws; but whether this Agent be material or immaterial, I have left to the Consideration of my readers.”
Newton also wrote a book on Opticks. He had to confront the problem of what causes light’s remote effects. His solution was that light is formed of corpuscles (particles) – not dissimilar to Galileo’s minute particles of heat.
Newton’s approach fell out of favor in the early 1800s as experimental evidence came to favor a wave theory of light. But that doesn’t violate “causality is contact” because waves are waves in something (like an ocean). It’s the wave’s “medium” that does the pushing and pulling and contact.
The “something” that light waves are waves in was called the luminiferous aether, which is “an invisible and infinite material with no interaction with physical objects.“ Seems absurd, right? Insert here doubtless ignorant snark about dark matter or dark energy.
It turns out that the experimental evidence pushed against the existence of a luminiferous aether. Efforts to save the theory were put to bed in 1905 by special relativity, after which you could respectably consider light a wave in… um, nothingness? Let’s talk about something else, shall we?
We could talk about the photoelectric effect… except that – in the same year as special relativity – Einstein also explained the photoelectric effect by assuming that light is tiny packets of energy (photons).
So the situation in the early 20th century was that some experiments suggested light is particle-like: a photon has a definite position in space at a given time, and it follows a definite path. At the same time, other experiments pointed to a wave-in-nothing explanation: waves are present simultaneously at many points, with values at those points changing continuously over time. (Particles move; waves spread.)
Particle or wave? The question arguably came to a head around 1926. In September 1925, Heisenberg published a framework theory for quantum mechanics. In January of the next year, Schrödinger published a different theory. The two theories were in competition:
“For the main contestants, Heisenberg and Schrödinger, the issue at stake was which view could claim to provide a single coherent and universal framework for the description of the observational data. The choice was, essentially between a description in terms of continuously evolving waves, or else one of particles undergoing discontinuous quantum jumps.”
Great! We can see which theory survives experiment and thus resolve (at least for now) the particle-or-wave question. Unfortunately, in May of the same year, Heisenberg published another paper showing that the two approaches were equivalent. If I understand that correctly, the two theories would make all the same predictions, so no experiment can decide between them.
Meanwhile, other problems had been cropping up. One had to do with probabilities.
In “classical” physics, if you know the initial state of the system, you can predict its final state exactly. By that, I mean something like limits in calculus: the more precise your measurements of the initial state, the more precise your prediction will be. If a cue ball with a given mass and velocity (momentum) strikes the eight ball at a given angle, you know for sure whether the eight ball will go into the corner pocket (leaving aside friction, rips in the table’s fabric, how level the table is, yada yada yada).
However, the same is not true at the quantum scale. If a photon hits an electron and propels it into a detector’s screen, no matter what you know about the collision, you can’t predict where the electron will hit. The best you can do is calculate probabilities: it’s more likely to hit there than here. But what does it mean for a single cause to have in principle an infinite number of possible effects? (That is, measurement error isn’t the issue.)
To compound things, there are certain properties of the system that cannot be simultaneously known (complementarity). The most famous example is Heisenberg’s uncertainty principle: the more precisely you measure a particle’s position, the less you know about its momentum.
That seems tractable: just measure the position at one moment and the momentum at another, then work your way backward (using a billiard-ball style of reasoning) to the momentum the particle must have had at the moment its position was measured. Einstein proposed ingenious thought experiments for how you might know position and momentum at the same moment, but they never worked. The upshot, per Frescura and Hiley:
By using one particular piece of apparatus only certain features could be made manifest at the expense of others, while with a different piece of apparatus another complementary aspect could be made manifest in such a way that the original set became non-manifest, that is, the original attributes were no longer well defined.
I think “non-manifest” might mean that, once you’ve measured the momentum, the earlier measurement of position is useless in further calculations/predictions. But I don’t know, because I don’t have the math, and I think the upshot of the “quantum revolution” is that trying to use analogies with macroscope entities – even analogies as simple as “objects have at all times both a position and momentum” – to “picture” the quantum world – is a mug’s game. Frescura and Hiley again:
In the traditional view, it is assumed that there exists a reality in space-time and that this reality is a given thing, all of whose aspects can be viewed or articulated at any given moment.
Some people came to believe that view is wrong at the quantum level.
-
One experimental apparatus is intended to measure position, and another is intended to measure momentum. They will, independently, produce results – but that doesn’t mean there’s an entity, down there at the quantum level, that simultaneously possesses all the measurable properties. Konrad Hinsen comments: “Measurement requires interaction and thus changes the quantum state. If you measure the momentum of a particle, it has a new quantum state after the measurement, and in that state, position is undefined. It’s not just impossible to measure position and momentum at the same time. There is no possible quantum state in which position and momentum have well-defined values.”
-
Similarly, it’s meaningless to speak of a photon as being either a particle or a wave. Some apparatus will produce results compatible with a particle, some with a wave, but what’s measured isn’t really either, or both, or neither.
-
Until the electron is measured to have hit the screen, it’s meaningless to talk of its position. If you insist on talking about what the electron “is” before it hits the screen, what it is, is a probability distribution. (And what does that mean?)
Einstein came to accept the results of quantum mechanics, but he still wanted causality:
“[…] quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the ‘old one’. I, at any rate, am convinced that He [God] is not playing at dice.”
Einstein believed that there must be some sort of causal theory underlying QM that didn’t require “spooky action at a distance":
“Without doubt quantum mechanics has grasped an important fragment of the truth and will be a paragon for all future fundamental theories, for the fact that it must be deducible as a limiting case from such foundations, just as electrostatics is deducible from Maxwell’s equations of the electromagnetic field or as thermodynamics is deducible from statistical mechanics.”
But he was never able to find a causal theory “below” quantum mechanics that would generate its equations. I don’t believe anyone has.
In contrast to Einstein, other theorists gave up on causality, mostly adhering to the Copenhagen interpretation (really, the Copenhagen interpretation_s_). Per Wikipedia:
“Features common across versions of the Copenhagen interpretation include the idea that quantum mechanics is intrinsically indeterministic [and] that objects have certain pairs of complementary properties that cannot all be observed or measured simultaneously. Moreover, the act of ‘observing’ or ‘measuring’ an object is irreversible, and no truth can be attributed to an object except according to the results of its measurement.”
When it came to the quantum world, it seems most physicists gave up on the type of explanation that would have pleased Galileo (or Einstein). Some continued occasionally banging away at the metaphysical questions, but others preferred to “Shut up and calculate” (N. David Mermin, usually attributed to Feynman).
It’s possible this was a sociological or cultural thing. Physicists who hit their stride in the early parts of the last century had philosophical training and commitments different from younger physicists. Succeeding generations might have been more comfortable working on the “what” and leaving the “why” (the causality) to others. Konrad Hinsen again: “Action at a distance is perhaps no longer an issue for later physicists because they grew up with an atomic picture of matter. Distance zero doesn’t exist in the (sub-)atomic world. Interaction always happens at some distance, even among the constituents of an atom.” That’s weird to think of: not only is causality not contact, nothing ever comes into any contact with anything.
The critical rationalists think scientists shouldn’t have done that. As someone who benefits every day from applications of quantum mechanics – lasers, integrated circuits, MRI machines, electron microscopes, and whatnot – I’m happy that physicists have been metaphysically incorrect but instrumentally productive. But I have a more expansive view of what science is for than do the critical rationalists. (See the next essay.)