American physicist Steven Weinberg’s 1992 positivist Tiny Tim and realist Scrooge, and respect to their dialogue on the quantum mechanical implications of determinism and free will.

“Everyone agrees on how to use quantum mechanics, but there is serious disagreement about how to think about what we are doing when we use it. For some who felt wounded by the reductionism and determinism of Newtonian physics, two aspect of quantum mechanics seemed to offer a welcome balm. Where human beings had no special status in Newtonian physics, in the Copenhagen interpretation of quantum mechanics humans play an essential role in giving meaning to the wave function by the act of measurement. And where the Newtonian physicist spoke of precise predictions the quantum mechanician now offers only calculations of probabilities, thus seeming to make room again for free will or divine intervention.”

“Some scientists and writers like Fritjof Capra welcome what the see as an opportunity for reconciliation between the spirit of science and the gentler parts of our nature. I might, too, if I thought the opportunity was a real one, but I do not think it is. Quantum mechanics has been overwhelmingly important to physics, but I cannot find any messages for human life in quantum mechanics that are different in any important way from those of Newtonian physics.”

Tiny Tim: I think quantum mechanics is just wonderful. I never did like the way that in Newtonian mechanics if you knew the position and velocity of every particle at one moment you could predict everything about the future, with no room for free will and no special role for humans at all. Now in quantum mechanics all you predictions are vague and probabilistic, and nothing has a definite state until human beings observe it. I’m sure that some Eastern mystic must have said something like this.

Scrooge: Bah! I may have changed my mind about Christmas, but I still know humbug when I hear it. It is true enough that the electron does not have definite position and momentum at the same time, but this just means that these are not the appropriate quantities to use in describing the electron. When an electron or any collection of particles does have at any time is a wave function. If there is a human observing the particles, then the state of the whole system including the human is described by a wave function. The evolution of the wave function is just as determinist as the orbits of particles in Newtonian mechanics. In fact, it is more deterministic, because the equations that tell us how the wave function develops over time are too simple to allow chaotic solutions. Where’s your free will now?”

Tiny Tim: I’m really surprised that you should reply in such an unscientific way. The wave function has no objective reality, because it cannot be measured. For instance, if we observe that a particle is here, we cannot conclude from this that the wave function before the observation had a vanishing there value; it might have had an here and there values, with the particle just happening to show up as here rather than there when t was observed. If the wave function is not real, then shy are you making so much of the fact that it evolves deterministically? All that we ever measure are quantities like positions or momenta or spins, and about these we can predict only probabilities. And until some human intervenes to measure these quantities, we cannot say that the particle has any definite state at all.

Scrooge: My dear young person, you seem to have swallowed uncritically the nineteenth-century doctrine called positivism, which says that science should concern itself only with things that can actually be observed. I agree that it is not possible to measure a wave function in any one experiment. So what? By repeating measurements many times for that initial state, you can work out what the wave function in the state must be and used the results to check our theories. What more do you want? You should really bring your thinking up to the twentieth century. Wave functions are real for the same reason that quarks and symmetries are—because it is useful to include them in theories. Any system is in a definite state whether any humans are observing it or not; the state is not described by a position or a momentum but by a wave function.

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See main: Chemistry professor paradox

“Life’s just an extension of atoms, nothing more, it has to be that because atoms and varying forms of energy are all there is … At the time of [the] origin [of evolution theory] it wasn’t yet understood that at the level of photons and electrons and other small particles the laws of cause and effect no longer apply; that electrons and photons simply appear and disappear without individual predictability and without cause. So today we have as a result a theory of evolution in which ‘man’ is ruthlessly controlled by the cause-and-effect laws of the universe while the particles of his body are not. The absurdity of this seems neglected. The problem doesn’t lie in anyone’s department. Physicists can ignore it because they are not concerned with man. Social scientists can ignore it because they are not concerned with subatomic particles.”