Readers of Quantum Enigma sometimes come with a lot of previous thinking, and opinions, about free will. Were we revising Quantum Enigma once more, we’d be more explicit about how, in the two-slit experiment, free will arises in a very narrow, but very fundamental way. And we’d emphasize even more strongly how it is absolutely crucial to the quantum measurement problem. (Since our second edition came out, free will, usually called “free choice” by physicists, has increasingly come up in the physics research literature.)
First, something about free will on a level above quantum mechanics: Probably most of us agree that our choices (or our wills) are heavily influenced by biology, our culture, our past experiences, and evolution, etc. Does this mean that we do not have free will? This is the question about free will, or the lack of it, that religion, moral philosophy, psychology, sociology, and perhaps the judicial system are concerned with. This can lead to interesting discussions. However, this is not the issue arising in the quantum experiment.
The issue of free will that comes up today when physicists treat the foundations of quantum mechanics is more focused. It’s often expressed as the free choice of the experimenter to choose to do a particular experiment, or make a particular observation.
A crucial point for the quantum measurement problem is the assumption that an experimenter can at least make some choices that are not “linked” with an external physical situation. What we mean by “linked” is displayed in the two-slit experiment described in Chapter 7 of Quantum Enigma (2nd edition) and briefly below. We later consider what a denial of this assumption of free choice would imply.
We assume that our box-pairs version of the two-slit experiment described in Chapter 7 of Quantum Enigma (2nd edition) has been read, and we here just emphasize aspects essential to the free choice issue.
The experimenter is presented with a set of box pairs, each pair of boxes containing a single object. By choosing to open each box of a pair in turn (the “sequential experiment”), the experimenter can establish that each object was wholly in a single box, and the other box of that pair was totally empty.
Alternatively, by choosing to open the boxes of each pair at about the same time (the “simultaneous experiment”), the experimenter can establish a contradictory situation: that each object was not wholly in a single box; that each object was distributed over both boxes of its box pair.
Presented with a set of box pairs, the experimenter can freely choose to establish either of two contradictory physical situations.
Put yourself in the position of this experimenter. You assume your choices are freely made. If so, your free choices apparently created what had been the situation of the object in the box pairs.
That’s the creation of reality by observation, the unexplained “collapse” phenomenon of quantum theory. It’s the quantum measurement problem. It’s controversial. In Chapter 15 of Quantum Enigma, we treat different interpretations of what’s going on.
Another possibility is that your choices were not free. They were “linked” to the actual situation in the box pairs. If that’s true of your choices in this simplest of experiments, could it be that none of your choices are free? We would then exist in a totally deterministic world that conspires to make us think that our choices are free. If so, are we all just programmed robots?
In fact, the delayed-choice two-slit experiment we described has never actually been done with human experimenters making the choices. Things happen too fast. Instead, the “free choices” were made by fast random number generators, which are presumably completely unconnected with what’s in the box pairs. Experiments in space with humans separated by huge distances, and therefore longer time-scales, are seriously proposed. It would, of course, be inexplicable if conscious humans making free choices gave different results.