Notes on Quantum Indeterminacy

It is important that we understand what modern physicists say about quantum indeterminacy. It is a very unintuitive notion, and I sympathize completely with people who distrust it. My own reaction to it was this: "This can't be right. They must be confusing not knowing the event's cause with the event not having a cause."

If physicists were confused about this, then we wouldn't need to take them seriously. But they are not confused about this. They know the distinction, and they continue to believe in quantum indeterminacy (QI).

In the early days of QI, the physicists seemed to be making that mistake. Some of them claimed that QI was an implication of the Heisenberg Uncertainty Principle. Heisenberg pointed out that we can measure an electron's position, but in doing so we destroy any possibility of measuring it's momentum, and vice versa. So the combination of position and momentum of an electron is uncertain.

Notice that "uncertainty" is a characteristic that is an epistemological concept -- it is defined in terms of knowability. The argument seemed to be Uncertainty, therefore Quantum Indeterminacy:

We can't know both an electron's position and momentum, therefore an electron does not have a determinate position and momentum. (If we can't know it, it doesn't exist.)

But that was a mistake, and physicists now think more like this:

We can't know both an electron's position and momentum because electrons do not have simultaneous determinate positions and momentums.

In other words, QI, therefore Uncertainty. (You can't know something that doesn't have a determinate truth.)

All modern science is falliblist in epistemology. That means that any scientific theory might be wrong. We all acknowledge this. But scientific theories vary a lot in how likely they are to turn out to be wrong. The present state of physics makes quantum indeterminism a very well-established theory. It may still turn out to be wrong, in the sense that any theory may turn out to be wrong. Just like the "fact" that the earth orbits the sun might turn out to be wrong.

But QI is very well established. Very few scientific theories are as well verified by experience. And remember, the evidence is not just that we don't know what the quantum causes are -- the evidence is that there are no quantum causes. Again, it might turn out to be wrong, but we still ought to take quantum indeterminacy seriously.

Quantum indeterminism asserts that certain kinds of events, call them "Q events" are indeterministic. Really really really indeterministic, not just "as far as we know" indeterministic. Q events are (approximately) events that take place at a sub-atomic level. An example is the radioactive decay of a radioactive element. (There are lots of other examples, but this is an easy one to think of.)

Radioactive elements have half-lives. The half-life of an element is the length of time during which one atom of the element has a 50% chance of undergoing radioactive decay. That probabability is a real, objective probability, even though there is no real, objective cause for an individual case of radioactive decay.

The half-life of Uranium-238 is 4.5 million years. It decays into thorium-234, which has a half-life of 24.5 days. There are tons of radioactive elements with various half-lives, some very short and some very long. And it's ALL probability (objective probability) when each event will occur. According to QI, of course.

(I don't really care whether you believe in QI. But I do really want you to understand it.)

Let's call non-Q events "M events" for "macroscopic events". Now, you might think that QI is not a problem for Causal Determinism, as long as we restrict CD to non-Q events. Is that possible? We live in the M-world after all -- the world of macroscopic events, larger than atoms. Maybe CD is true of all M events even though it is false of all Q events.

Nope. First of all, physicists will claim that all M events are merely the additive effects of a lot of little Q events. But even if that's false, there's a bigger problem. There are certain Q events that have a huge influence on events in the M world. One example is sunshine. According to the best physics of today, sunshine comes from nuclear fusion, which is a Q phenomenon. Secondly, nuclear explosions. Thirdly, the clicking of a Geiger counter. We'll discuss in class this penetration of Q events into the M world that we live in (or like to think we live in).

Just to give you an example of how seriously this is taken by physicists, the following is quoted from Abner Shimony, "The Reality of the Quantum World", Scientific American, January 1988.

Shimony describes "indefiniteness" and the "superposition principle" (you donít really need to know what they mean) then continues:

"From these two basic ideas alone -- indefiniteness and the superposition principle -- it should be clear already that quantum mechanics conflicts sharply with common sense. If the quantum state of a system is a complete description of the system, then a quantity that has an indefinite value in that quantum state is objectively indefinite; its value is not merely unknown by the scientist who seeks to describe the system. Furthermore, since the outcome of a measurement of an objectively indefinite quantity is not determined by the quantum state, and yet the quantum state is the complete bearer of information about the system, the outcome is strictly a matter of objective chance -- not just a matter of chance in the sense of unpredictability by the scientist. Finally, the probability of each possible outcome of the measurement is an objective probability. Classical physics did not conflict with common sense in these fundamental ways."

"A number of theorists have maintained however, that [quantum-theory-described physical systems] ... differ from one another in ways not mentioned by the quantum state, and this is the reason the outcomes of the individual experiments are different. The properties of individual systems that are not specified by the quantum state are known as hidden variables. If hidden variables theorists are correct, there is no objective indefiniteness. There is only ignorance on the part of the scientist about the values of the hidden variables that characterize an individual system of interest. Moreover, there is no objective chance and there are no objective probabilities."

Shimony then goes on to report on recent experiments which very strongly indicate that hidden-variables theories are wrong. Indeterminacy is an objective fact and not just a matter of scientists' lack of knowledge.