Stephen M. Barr

Not in any direct way. That is, it doesn’t provide an argument for the existence of God.  But it does so indirectly, by providing an argument against the philosophy called materialism (or “physicalism”), which is the main intellectual opponent of belief in God in today’s world.

Materialism is an atheistic philosophy that says that all of reality is reducible to matter and its interactions. It has gained ground because many people think that it’s supported by science. They think that physics has shown the material world to be a closed system of cause and effect, sealed off from the influence of any non-physical realities — if any there be. Since our minds and thoughts obviously do affect the physical world, it would follow that they are themselves merely physical phenomena. No room for a spiritual soul or free will: for materialists we are just “machines made of meat.”

Quantum mechanics, however, throws a monkey wrench into this simple mechanical view of things.  No less a figure than Eugene Wigner, a Nobel Prize winner in physics, claimed that materialism — at least with regard to the human mind — is not “logically consistent with present quantum mechanics.” And on the basis of quantum mechanics, Sir Rudolf Peierls, another great 20th-century physicist, said, “the premise that you can describe in terms of physics the whole function of a human being … including [his] knowledge, and [his] consciousness, is untenable. There is still something missing.”

How, one might ask, can quantum mechanics have anything to say about the human mind?  Isn’t it about things that can be physically measured, such as particles and forces?  It is; but while minds cannot be measured, it is ultimately minds that do the measuring. And that, as we shall see, is a fact that cannot be ignored in trying to make sense of quantum mechanics.  If one claims that it is possible (in principle) to give a complete physical description of what goes on during a measurement — including the mind of the person who is doing the measuring — one is led into severe difficulties. This was pointed out in the 1930s by the great mathematician John von Neumann.  Though I cannot go into technicalities in an essay such as this, I will try to sketch the argument.

It all begins with the fact that quantum mechanics is inherently probabilistic. Of course, even in “classical physics” (i.e. the physics that preceded quantum mechanics and that still is adequate for many purposes) one sometimes uses probabilities; but one wouldn’t have to if one had enough information.  Quantum mechanics is radically different: it says that even if one had complete information about the state of a physical system, the laws of physics would typically only predict probabilities of future outcomes. These probabilities are encoded in something called the “wavefunction” of the system.

A familiar example of this is the idea of “half-life.”  Radioactive nuclei are liable to “decay” into smaller nuclei and other particles.  If a certain type of nucleus has a half-life of, say, an hour, it means that a nucleus of that type has a 50% chance of decaying within 1 hour, a 75% chance within two hours, and so on. The quantum mechanical equations do not (and cannot) tell you when a particular nucleus will decay, only the probability of it doing so as a function of time. This is not something peculiar to nuclei. The principles of quantum mechanics apply to all physical systems, and those principles are inherently and inescapably probabilistic.

This is where the problem begins. It is a paradoxical (but entirely logical) fact that a probability only makes sense if it is the probability of something definite. For example, to say that Jane has a 70% chance of passing the French exam only means something if at some point she takes the exam and gets a definite grade.  At that point, the probability of her passing no longer remains 70%, but suddenly jumps to 100% (if she passes) or 0% (if she fails). In other words, probabilities of events that lie in between 0 and 100% must at some point jump to 0 or 100% or else they meant nothing in the first place.

This raises a thorny issue for quantum mechanics. The master equation that governs how wavefunctions change with time (the “Schrödinger equation”) does not yield probabilities that suddenly jump to 0 or 100%, but rather ones that vary smoothly and that generally remain greater than 0 and less than 100%.  Radioactive nuclei are a good example. The Schrödinger equation says that the “survival probability” of a nucleus (i.e. the probability of its not having decayed) starts off at 100%, and then falls continuously, reaching 50% after one half-life, 25% after two half-lives, and so on — but never reaching zero. In other words, the Schrödinger equation only gives probabilities of decaying, never an actual decay! (If there were an actual decay, the survival probability should jump to 0 at that point.)

To recap: (a) Probabilities in quantum mechanics must be the probabilities of definite events. (b) When definite events happen, some probabilities should jump to 0 or 100%. However, (c) the mathematics that describes all physical processes (the Schrödinger equation) does not describe such jumps.  One begins to see how one might reach the conclusion that not everything that happens is a physical process describable by the equations of physics.

So how do minds enter the picture?  The traditional understanding is that the “definite events” whose probabilities one calculates in quantum mechanics are the outcomes of “measurements” or “observations” (the words are used interchangeably).  If someone (traditionally called “the observer”) checks to see if, say, a nucleus has decayed (perhaps using a Geiger counter), he or she must get a definite answer: yes or no.  Obviously, at that point the probability of the nucleus having decayed (or survived) should jump to 0 or 100%, because the observer then knows the result with certainty.  This is just common sense. The probabilities assigned to events refer to someone’s state of knowledge: before I know the outcome of Jane’s exam I can only say that she has a 70% chance of passing; whereas after I know I must say either 0 or 100%.

Thus, the traditional view is that the probabilities in quantum mechanics — and hence the “wavefunction” that encodes them — refer to the state of knowledge of some “observer”.  (In the words of the famous physicist Sir James Jeans, wavefunctions are “knowledge waves.”)  An observer’s knowledge — and hence the wavefunction that encodes it — makes a discontinuous jump when he/she comes to know the outcome of a measurement (the famous “quantum jump”, traditionally called the “collapse of the wave function”). But the Schrödinger equations that describe any physical process do not give such jumps!  So something must be involved when knowledge changes besides physical processes.

An obvious question is why one needs to talk about knowledge and minds at all. Couldn’t an inanimate physical device (say, a Geiger counter) carry out a “measurement”?  That would run into the very problem pointed out by von Neumann: If the “observer” were just a purely physical entity, such as a Geiger counter, one could in principle write down a bigger wavefunction that described not only the thing being measured but also the observer. And, when calculated with the Schrödinger equation, that bigger wave function would not jump! Again: as long as only purely physical entities are involved, they are governed by an equation that says that the probabilities don’t jump.

That’s why, when Peierls was asked whether a machine could be an “observer,” he said no, explaining that “the quantum mechanical description is in terms of knowledge, and knowledge requires somebody who knows.” Not a purely physical thing, but a mind.

But what if one refuses to accept this conclusion, and maintains that only physical entities exist and that all observers and their minds are entirely describable by the equations of physics? Then the quantum probabilities remain in limbo, not 0 and 100% (in general) but hovering somewhere in between. They never get resolved into unique and definite outcomes, but somehow all possibilities remain always in play. One would thus be forced into what is called the “Many Worlds Interpretation” (MWI) of quantum mechanics.

In MWI, reality is divided into many branches corresponding to all the possible outcomes of all physical situations. If a probability was 70% before a measurement, it doesn’t jump to 0 or 100%; it stays 70% after the measurement, because in 70% of the branches there’s one result and in 30% there’s the other result! For example, in some branches of reality a particular nucleus has decayed — and “you” observe that it has, while in other branches it has not decayed — and “you” observe that it has not. (There are versions of “you” in every branch.) In the Many Worlds picture, you exist in a virtually infinite number of versions: in some branches of reality you are reading this article, in others you are asleep in bed, in others you have never been born. Even proponents of the Many Worlds idea admit that it sounds crazy and strains credulity.

The upshot is this: If the mathematics of quantum mechanics is right (as most fundamental physicists believe), and if materialism is right, one is forced to accept the Many Worlds Interpretation of quantum mechanics. And that is awfully heavy baggage for materialism to carry.

If, on the other hand, we accept the more traditional understanding of quantum mechanics that goes back to von Neumann, one is led by its logic (as Wigner and Peierls were) to the conclusion that not everything is just matter in motion, and that in particular there is something about the human mind that transcends matter and its laws.  It then becomes possible to take seriously certain questions that materialism had ruled out of court: If the human mind transcends matter to some extent, could there not exist minds that transcend the physical universe altogether? And might there not even exist an ultimate Mind?

Discussion Summary

In my BQO essay I sought to explain in non-technical language the main issues that lead to different “interpretations” of quantum mechanics, and why they present a choice between the anti-materialist implications of the traditional Copenhagen interpretation and the bizarre and (for most people) incredible implications of the Many Worlds Interpretation (MWI).

Another way to frame the argument is in terms of the “ontological status” of wavefunctions. The most obvious thing is to think of a wavefunction as simply a straightforward description of “the world as it is”. But that is equivalent to the MWI, because generally speaking the wavefunction of a system contains a large number of branches in which the system behaves in different ways. The alternative view (adopted in the Copenhagen interpretation) is that a wavefunction is not an account of the world as it is, but of some observer’s state of knowledge of the world. That interpretation brings knowledge (and therefore mind) into the discussion as something that is as fundamental as matter, because wavefunctions themselves are fundamental to our understanding of the world.

These seem to be the only viable choices if one accepts the present formalism of quantum mechanics.  The third possibility, then, is to say that this formalism needs to be changed somehow.  Several commentators on my article either mentioned or proposed approaches to modifying quantum mechanics. Two well-known approaches that they mentioned were “hidden variables theories” and “objective collapse theories”.  Certainly, science can never be absolutely certain that it has arrivedata final and complete description of the physical world; so it will always remain a possibility that present quantum mechanics is incomplete and that a modification of its formalism will resolve all its puzzles, paradoxes, and conundrums.  However, as I explained in my responses, the present formalism of quantum mechanics has been spectacularly successful since the 1920’s in describing with astonishing precision a vast range of phenomena.  It seems less likely than it once did that it will have to be modified.  (Many, including one commentator on my essay, have seen the difficulty of combining present quantum mechanics with Einstein’s theory of gravity as evidence that the former will have to be modified.  Superstring theory shows, however, that Einstein’s theory can be consistently “quantized”.)

Not only does the traditional interpretation of quantum mechanics have anti-materialist implications, but, as noted by more than one commentator, it is compatible with philosophical idealism (which makes mind the only fundamental reality) and even with solipsism (which makes one’s own mind the only reality).  If we reject such extremes and take the view that both matter and mind are fundamental and equally real, then a question arises: how are they related to each other?

Connected with this question is a common objection made to the Copenhagen interpretation, which is that it seems to say that the minds of “observers” have a spooky influence on the physical world.  In particular, the idea that “wavefunction collapse” is caused by “observations” seems to suggest that human knowers actually cause events to happen by knowing them. As one commentator pointed out, this would be a radical reversal of traditional ideas about the relation of knower and known.  Indeed, the traditional interpretation of quantum mechanics strikes many people as “subjectivist”.  It is questionable whether the Copenhagen Interpretation really has these “anti-realist” implications. At the very least, however, it makes problematic the question of what is “really going on when no one is looking”.  In other words, if the wavefunction of a system is not a straightforward description of “the world as it is,” but just of some observer’s knowledge of it — then what WOULD a description of the world as it is look like?  This is, indeed, an awkward question for the traditional interpretation to deal with — though various attempts have been made to answer it.  A second problem connected with the relation between mind and matter, raised by another commentator to my essay, is how the relation between mind and brain is to be thought of.  This is quite mysterious at present.  A third question is what kinds of minds qualify as “observers” in the Copenhagen interpretation of quantum mechanics. One commentator asked why it would have to be only human minds.  The answer is that it wouldn’t, but it seems it would have to be a mind capable of knowing the results of measurements and one not entirely describable by physics. Purely physical devices or inanimate things such as atoms and bacteria would not seem to qualify, whereas humans certainly do. Somewhere in between there is a line, but where it should be drawn is debatable.

It has long been suggested that the special role of mind in quantum mechanics as traditionally interpreted, together with the indeterminacy of the theory, may provide an opening for free will to operate in the world.  Though I did not discuss this in my essay, several commentators did.  Again, this is a murky and much debated subject.

Altogether, it is clear that quantum mechanics raises more philosophical questions that it solves.

Here are two “Big Questions” that it raises.

Two New Big Questions:

1.     Does quantum indeterminacy provide an opening for free will?

2.     In the context of the traditional Copenhagen interpretation of quantum mechanics, what would a complete description of the world through time, apart from what any “observer” knows about it, look like?