Determinism, Free Will, and Quantum Physics


You are not the only correspondent who asks me physics questions.

Here’s an interesting question from a young friend of mine.

I’m interested in hearing your thoughts on the following thought experiment:

Let me pose to you this entirely fantastical scenario: Let’s say there was a way to make copies of the universe such that, up until a given point in history, every aspect of it is exactly the same–every drop of rain that ever fell, every beam of light from every star in the history of the universe, every human choice ever made. Now let’s say that, starting at any given point in history, we can interject an event–say, a person asking another person a question–such that the event occurs exactly the same way, right down to the subatomic level, in every identical parallel universe. Let’s say the question is, “Choose any number between 1 and 100.”

Would we expect the person to choose the same number in every parallel universe? If they do, is this sufficient proof for determinism? But if they choose different numbers, is this sufficient proof for the existence of free will and/or quantum indeterminacy (and to what extent are free will and quantum indeterminacy related?)

Good questions. Today’s the first day of my classes at U of Tampa, and these are the types of questions I have in mind for my The Big Questions slide that I tell my students they will be learning about.

The answer to your first question, according to our best knowledge and thought, is No. Certain types of events that take place in the microscopic world are literally unpredictable. I say that even the world itself can’t predict the outcome, but that’s not the way other physicists might say it.

Here’s an example of such a microscopic event. You shine a beam of light on a partly reflecting mirror, designed so that half of the light arriving at its surface passes through and half reflects. Suppose you turn this mirror so it is at 45 degrees to the incoming light. Half the light goes through, and half makes a 90 degree turn. You put two light detectors in these two beams. A physicist might use a doodad known as a photomultiplier tube. Now you start reducing the intensity of your incoming light beam, by adding shaded filters, and you turn up the sensitivity of your detectors to compensate. Eventually, the incoming beam will appear as a flow of individual particles of light, photons, that appear as sharp peaks in the detector output. Light, those photons, travels at the “speed of light” of course, which is 1 foot per billionth of a second. Suppose the distance from the light source to the light detectors is 10 feet, so each photon takes 10 nanoseconds to go from the source to one or the other detector. Suppose you turn down the light intensity so that you see those detector peak signals about once every microsecond. (These days, these are easy things to do in a laboratory, and we could probably find some physics teacher doing a YouTube showing this.) I think you can do the math: About once every microsecond, you see that a photon has arrived at one of the detectors, and it must have left the source, 0.01 microseconds or 10 nanoseconds earlier. Thus, nearly all the time, there are no photons in your equipment. Every now and then one zips from the source to a detector. You always get a full photon in one detector or the other. You never get a fraction of a photon in one and the rest of it in the other at the same time. A photon cannot be split.

Let’s say that if the photon went straight through, you write down a 1, and if it bounced you write down a 0. You will end up with something like this: …10011010001001011001110010….

If you keep this up for a while, like for a second, which will give you a million 1s and 0s, (because we said that you have about one photon in a microsecond) you will see that nearly 50% of your numbers are 1s and nearly 50% are zeros. In the number above, there are 12 1s and 14 0s. But you won’t see …101010101010101010… very often even though that counts as a random string. Indeed, the pattern of 1s and 0s from your detectors will pass every known mathematical test for randomness. Physicists would say that photons have only a few properties, such as energy and spin, and these do not determine whether any single photon goes through or reflects. Indeed, nothing about the photon before it meets the mirror determines it. Transmission or reflection for a photon is fundamentally random.

Thus, suppose your “interjected event” were a 1 if the photon goes through and a 0 if it reflects. Half of the time, entirely at random, your person would choose 1 and half the time a 0. With a few more mirrors and detectors, I think you can see that we could easily set up your random choice of integers from 1 to 100.

We could do the same experiment with any quantum, microscopic particle, such as an electron, proton, or neutron, or hydrogen atom. While all that I’ve said is in accord with quantum theory, I’d like you to understand that my colleagues have done these experiments, so I’m reporting to you observed results.

While questioning, thoughtful normal people, non-physicists that is, might find this interesting for a few moments, what they are mainly interested in are your subsequent questions about determinism and free will for ourselves. The operations of our brain are (I assert [for good reasons I claim]) entirely the result of immense numbers of microscopic, quantum events acting in accord with the laws of nature, which physicists believe have certain random, undetermined, aspects. Yet when immense numbers of these quantum events occur there may be little randomness in whether a given neuron will fire or not. Up here, in the large world things behave more normally, shall I say. But quantum randomness may still decide outcomes.

Suppose it were to happen that one neuron secretes in a second 998,034 signaling molecules, neurotransmitters, across a synapse, which causes the next neuron to fire, but repeating the experiment, it secretes 997,843 and the next neuron doesn’t fire. The small fluctuation in secreted neurotransmitters is the result of the microscopic quantum events that are the chemical reactions forming the neurotransmitters or their release from some vacuole. If the downstream neuron fires, you order a ham on rye sandwich, if not, a tuna melt. Everything follows the laws of nature, yet sometimes you order one and not the other. But where is the “you” that makes the choice of your own free will about lunch? The fact that the underlying quantum phenomena are random doesn’t provide room for “you” to make the choice, but it will feel to you as if you did.

The answer to your last question, therefore, is that the world is, in some ways, undetermined, but our choices are illusions.

These matters of nature’s behavior and of human perception, free will and determinism, are important and profound. If our actions are the result of our present state and our surroundings with no “you” involved, then what does it mean to hold a person responsible for a crime? Yet we try to distinguish between the mentally ill, compelled to criminality, and a mentally healthy person who commits a crime. The one not guilty, the other guilty, for the same act. Both acts the result of microscopic quantum events that follow the laws of nature. In theology, many argue that the Lord allows us freely to choose and holds us responsible for our choice.  Yet, I say, the Lord appears to have designed the laws of nature so that it is not “we” who make our choices.

I’ve tried to answer your questions by appealing to our deepest understanding of nature, quantum theory, but precise determinism runs into trouble even if there weren’t the unalterable randomness in microscopic quantum events. This is the realm of chaos theory by which the tiniest imprecisions may lead to radically different outcomes. Although the result of each tiny movement forward in time is predictable, to some precision, the result of many tiny movements is unpredictable. Consider the butterfly effect. On a warm summer’s day, a butterfly flits to the left over a hot parking lot, and a tiny blip of air happens to move upward. If the butterfly had moved to the right, no upward moving blip of air. The first case leads to a thunderstorm, the second not. The laws of fluid mechanics and thermodynamics are well-known and complete, but we cannot predict whether there will be a thunderstorm next Thursday. This is weather forecasts are for 70% chance of rain. You can easily imagine that the outcome of brain activity in apparently identical circumstances, a sequence of neurons firing or not, might well be different due to small quantum fluctuations, smaller scale butterfly flutters. All in accord with the laws of nature, determined by them. Yet “you” were not the agent making the choice of quantum outcomes.

Thus, everything follows the regularities of nature, determinism, but some outcomes are random. These random outcomes, ham on rye or tuna melt, are not the result of human agency, but the bistro customer feels as if she chose one or the other.


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