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Beyond the Causal Veil

by Bill Meacham on October 24th, 2011

Quantum indeterminacy operates inside your brain. What does that say about the nature of human will and decision-making?(1)

We’ve taken a look at the world of quantum physics before, but a little recap is in order in case you missed it. The quantum level is where thing are quite tiny, less than about 100 nanometers long. Here things behave very strangely. We can describe their properties and behavior mathematically by a formula called the “wave function,” and under certain circumstances the wave function divides into two or more pairs or branches, each with its own consequences. Each of these branches represents a potential future or a potential version of reality. When observed, only one of these branches is perceived; that is, only one of the potential futures becomes the actual perceived present.

Double-Slit experiment resultsA famous experiment, widely replicated, called the Double-Slit experiment reveals the strangeness of this level of reality. It consists of sending light through two side-by-side vertical slits to a recording medium, such as film; and it shows, among other things, that light can behave both as a stream of particles and as a wave. When light is sent through one slit at a time, a vertical band appears. In this case light acts like a series of particles that go through the slit, hit the recording medium and make an impression. If you open the slit on the right, the band appears on the right, and if you open the slit on the left, the band appears on the left. You would expect that if both slits were opened, the result would be two side-by-side bands. In fact, however, the result is a strong band in the middle, the expected bands on the left and right, and then dimmer bands extending outward in each direction. Light in this case acts like waves that cause interference patterns. That is, when a crest meets a crest, a more intense crest results; and when a crest meets a trough they cancel out. The bands of light are from the crests reinforcing each other, and the darkness in between is the from crests and troughs canceling each other out.

Even more interesting, when light is emitted one photon at a time and aimed at the two slits, it shows the same interference pattern. You would expect that a photon would go through one slit or the other. In fact it appears to act like a wave that goes through both slits, interferes with itself, and results in an impression in one and only one of the bands.

And you cannot predict in advance where the photon will make an impression.

You can predict that given a great number of photons, they will result in bands. That is, they won’t all end up in the same place, but rather in various places according to their probability distribution. But there is only a probability, not an absolute certainty, that any single photon will end up in one place or another.

We might well ask what causes the wave, which is mathematically described as a collection of probabilities of being detected in various places, to be in fact detected at only one place. I’ll return to this question shortly. For now, note the quantum indeterminacy, our inability to predict the final location of any single photon. The sequence in which the singly-emitted photons will arrive is completely unpredictable. We have a radical discontinuity of causality.


In ordinary life and in classical (non-quantum) physics, we have a clear concept of causality: a cause is something that reliably produces an effect. Given the same or a similar set of circumstances, we expect the same results to appear. Hitting a billiard ball at a certain angle and with a certain force will always cause it to move in a certain direction and at a certain speed. This conception of causality has three parts:

  • Regularity – A cause always produces its effect according to physical laws that can be discovered by observation and experiment.
  • Temporal sequence – The cause always precedes its effect in time. The cause never follows the effect.
  • Spatial contiguity – There is always some physical connection or spatial contact between the cause and its effect, or a chain of such connections.

At the quantum level, the regularity is missing. There is no set of circumstances that causes the photon always to be detected in a specific place. (And, as we have seen, sometimes spatial contiguity is missing as well.)

Once the photon has been detected then the ordinary chain of causality takes over. The beginning of a macroscopic event can certainly be dependent on a microscopic event. In that case, each microscopic possibility at the beginning can lead to a different chain of macroscopic events at the end.

This becomes important when we consider that some events in the brain happen at the quantum level.


The human brain is a mass of electrochemical activity. It contains approximately 100 billion nerve cells, or neurons, and up to five quadrillion connection points between them. Neurons are the fundamental elements of the brain; they transmit electrochemical impulses to and from other neurons, sense organs or muscles. Some impulses are triggered by sense organs, and some by the excitation of neighboring neurons. Some impulses excite or inhibit neighboring neurons and some cause muscle contractions that move the body.

Parts of a neuronA neuron consists of several parts: numerous dendrites, which look vaguely like trees with many branches, a cell body, and a single axon, a tube that divides at the end to many terminals. Dendrites are the incoming channels; they receive electrochemical impulses from other cells, which then pass through the body and out the axon terminals. Between the axon terminals and the dendrites of the neighboring neurons are gaps, called synapses, only twenty nanometers wide. On the other side of the synaptic gap from the axon is a receptor area on a dendrite of a neighboring cell. An axon can have many terminals, and each dendrite can have many receptor areas. Thus each neuron transmits impulses to and receives impulses from a great many neighboring neurons. Some neurons receive impulses from up to 10,000 neighbors. Some in the cerebellum receive up to 100,000. Clearly the brain is an organ of almost unimaginable complexity.

The impulse traveling through the neuron is an electrical charge. A neuron either transmits the impulse (we say it fires) or it does not; it is a binary element, either on (firing) or off (not firing). When the electrical charge reaches the synaptic gap, it triggers the release of chemicals, neurotransmitters, which is why we call brain activity electrochemical. A single release of a neurotransmitter might be too weak to trigger the receiving neuron, but since each neuron forms outgoing synapses with many others and likewise receives synaptic inputs from many others, the combination of several inputs at once can be enough to trigger it. Or the receipt of an inhibitory neurotransmitter can prevent an impulse that otherwise would have fired. The output of a neuron thus depends on the inputs from many others, each of which may have a different degree of influence depending on the strength of its synapse with that neuron.

What is interesting for the present discussion is what happens to cause the neurotransmitters to travel across the synapse. The chemistry is a bit complex, but basically neurotransmitter chemicals sit docked in little pockets, called vesicles, waiting for something to release them. When the electrical impulse arrives at the terminal, it opens up channels that let calcium ions in. The calcium makes the vesicle fuse with the cell wall and open up so the neurotransmitters go out into the synaptic gap and then hit the receiving neuron.

The channels through which calcium ions enter the nerve terminal from outside the neuron are tiny, only about a nanometer at their narrowest, not much bigger than a calcium ion itself. The calcium ions migrate from their entry channels to sites within the nerve terminal where they trigger the release of the contents of a vesicle. At this submicroscopic level of reality, quantum indeterminacy is in play. A given calcium ion might or might not hit a given triggering site; hence, a given neurotransmitter might or might not be released; hence the receiving neuron might or might not get excited (or inhibited).

In other words, at the most fundamental level, brain functioning is not causally determined.

And since the ordinary chain of causality takes over after the quantum event happens, quantum uncertainty at the synaptic level can lead to causal uncertainty at the level of the whole brain. And that means – since the state of the brain at least heavily influences, if not causally determines, our perceptions, thoughts, feelings and actions – that human conduct is not fully causally determined in the physical world.


What causes a quantum event – in this case the impact of a calcium ion on a triggering site – to cease being merely a probability and start being something that happens at a certain place? Not anything in the physical world. There is a causal discontinuity in nature. Events at the quantum level of reality have no physical cause, but are themselves causes of subsequent events. What is on the other side of the causal discontinuity?

At this point we move beyond what physics can tell us, but clearly it leaves open the possibility that human will is free and even that something that transcends our ordinary notion of the physical – a soul, perhaps, or a god or a plethora of deities – intervenes in the physical world.

Some protest that the causal uncertainty at the quantum level of reality is merely statistical. Events happen randomly; hence, we can draw no conclusions about nonphysical causality, free will, the existence of a soul or of God, or any such thing. In particular, they say, a decision that is initiated by a random occurrence is no more free than one initiated by physical causality. But random as they may be individually, quantum events considered as a group certainly do exhibit regularities. Light passed through double slits exhibits distinct patterns, not random noise.

Consider a pointillist painting, which consists of distinct dots of pigment. If you look at it up close, all you see is random dots. When you view it from afar, you see identifiable forms and shapes, recognizable objects, patterns. So what are the patterns that we find in the behavior that issues from the firing of our brain cells? Does what is outside the bounds of physical causality have any regularity or structure of its own that we can use to understand and predict what it will do? Are there any categories of causal explanation that might be applicable?

The answer is, yes, of course there are: the concepts that pertain to agents. We explain the behavior of agents not in terms of physics and chemistry but in terms of their perceptions, beliefs, desires and goals.

By “agent” I mean the usual: something with will and intention, something that initiates movement without an external nudge, something that acts or has the power to act on its own rather than merely reacting to events. Agency is a different category of causation from physical causation. What agents do is not uncaused, but what causes agents to act is their beliefs and desires, not mechanical or chemical forces. And what agents do is not completely predictable. We try to influence people by persuasion, but we can only influence them, we cannot completely control them. Rather like a single photon, we can never be sure what somebody will do until they have done it. Nor can we be sure what we ourselves will do until we have done it. And afterwards we recognize that we could have done differently.

We are agents not automata. In other words, we have free will. Now the question is, what shall we do with it?



(1) What follows is summarized from my paper “The Quantum Level of Reality,” located here: That paper contains more detail and all the footnotes and references. See also “Do Humans Have Free Will?” here:

From → Philosophy

  1. Eric Mercer permalink

    OK, I’ll let the cognitive scientist me deepen what you’ve written. And btw, I was subscribed to Philosophy Now before your contest entry was in it! 🙂

    Your listing of classical philosophy’s criteria for causality is more problematic than you describe. As you point out, the probabalistic nature of quantum physics breaks the traditional regularity requirement. On the bright side (as you also point out), events in our quantum physical world are not random. They are more correctly described as “stochastic” — having an element of randomness, but shaped by causes and conditions.

    However, it’s not just the regularity requirement that is creaky. The temporal sequence requirement was badly damaged by Einstein’s demonstration that events in space-time can appear in different orders to different observers. So what is the “real” sequence? Events with inertial or very close observers can be ordered relatively consistently, but the farther away in space-time the observers are and the bigger differences in their speed, the more room for alternative perception of ordering exists. Fortunately for us, we share a relatively small space (planet Earth) where everything is fairly local relative to the speed of light. But that’s just circumstances, and not a firm foundation for defining causality. In physics this is patched over by saying “cause must precede its effect according to all *inertial* observers.” Which admits that Your Mileage May Vary.

    Lastly, the spatial contiguity requirement was badly damaged by the Bell inequality test experiments. The common phrase among physicists for this class of phenomenon is “spooky action at a distance.” But the general conclusion from these sorts of demonstrations is that the notion that local realism holds true for all physics is seriously in doubt, and a wide variety of better conceptions of what reality might be like are actively being debated and explored. All we really can be sure of is that our common conceptions are simplistic at best, bogus at worst.

    But for the physical entities that we commonly perceive ourselves to be, and the circumstances in which appear to exist, the unrealistic but roughly correct classical concept of “causality” is OK. But merely OK. Good enough to get you to the corner store to buy milk.

    Turning to the deeper intent of the article… the current physical understanding of how human decision-making happens in the brain is that it is stochastic. A result of causes and conditions, but with a strong dose of randomness thrown in. Is that enough to make people feel good about themselves as “free” beings? Mostly not, but then perhaps many people are eager to delude themselves about how free they are in any case.

    I tend to subscribe to Daniel Dennett’s arguments and conclusions that people have all the “free will” that they could sensibly desire, but not those they non-sensibly desire (e.g. freedom from causes and conditions).

  2. Eric Mercer permalink

    Oh! I just did another re-reading of your article and see that you mentioned the problems with the spatial contiguity criterion. So you were already onto that.

    I sometimes get the feeling that modern physics is telling us that our best ideas about both causality and free will are still kindergarten-level in the grand scheme of things. The grand scheme is pretty grand, though. Humanity is still growing.

  3. steve permalink

    Well done, Bill. Really one of your best pieces to date. Unfortunately, though, it leaves me as unconvinced of the existence of free will as ever, save as a a hypothetical construct we must adopt to order our daily lives.

  4. Amina permalink

    I agree. One of your best pieces. Thank you. Reminds me of what Ken Wilber said once (not really a big Ken Wilber expert or fan, nor do I understand 99% of what he says but this stuck with me)..”Unless spirituality can pass through the gate of science, then of liberalism, it will never be a significant force in the modern world, but will remain merely as the organizing power for the prerational levels of development around the world.”

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