Does physics truly have anything to say about consciousness?

Every once in a while, scientists will bite off more than they can chew. Just as we normally use that phrase to mean “taking on a task that’s beyond your means to accomplish with the resources you currently have,” that same limitation applies to a wide variety of scientific problems. Whereas the fundamental laws, particles, and interactions of the Universe are exquisitely well known (up to a point), the vast array of complex, composite structures that emerge from those basic building blocks of reality often attain properties that arise in a non-obvious way from their constituent parts.

Sometimes, by simulating many-body systems and imposing the proper boundary conditions, we can indeed derive large, macroscopically observable properties from those fundamental rules; the color of a sodium lamp is one such example, the success of a coaxial cable in transmitting radio-frequency signals is another.

At other times, however, the rules are a lot more complex, and we can only state that something happens (or must happen), lacking a full understanding of how it happens. Perhaps no puzzle that falls into this category is more mysterious than the nature of consciousness: something that humans definitively possess, and yet can only describe subjectively.

What does it truly mean to be conscious? Where does consciousness come from? Are humans the only conscious species, or do other animals, non-animal forms of life, or even non-living things possess some form of consciousness? While many have opined and put forth hypotheses on the matter, it remains a mystery. Here’s what physics — the most fundamental of all the sciences — has to say about consciousness.

This rough sketch shows an interconnected network of neurons, similar to the ones present in the human brain. Note that the substrate for this network, including glial cells and the blood vessels that feed them, are present, but not shown here.

At the very core of the matter are two basic ideas:

• the idea that we live in a material reality, and that everything that exists in our material reality can be described in terms of, well, the constituent parts of reality that exist in space and time,
• and the idea that any phenomenon — including consciousness — can be rigorously defined and put to experimental, observational, and/or measurable tests.

To a physicist’s way of thinking, these are non-negotiable starting points for attempting to gain a physical understanding of any phenomenon in the Universe.

However, when it comes to consciousness, many who opine on the matter find themselves unconstrained by these concerns. For example, there are those who posit that instead of the “material reality” assumption — an assumption that has held true over and over again whenever we’ve been able to put reality itself to the critical test — that either the mind, a mind-like aspect, or some ill-defined form of consciousness is what’s truly a fundamental and omnipresent feature of reality. This idea, known as panpsychism, is a very old notion in the circles of philosophy, but comes along with the dual problem of being untestable and unfalsifiable. As all of our tests of reality rely on testing objects that are measurable within reality itself, panpsychism is forever locked away from the realm of scientific testability, and hence, holds no interest to physicists who adhere to an evidence-based worldview.

This chart of particles and interactions details how the particles of the Standard Model interact according to the three fundamental forces that quantum field theory describes. When gravity is added into the mix, we obtain the observable Universe that we see, with the laws, parameters, and constants that we know of governing it. However, many of the parameters that nature obeys cannot be predicted by theory, they must be measured to be known, and those are “constants” that our Universe requires, to the best of our knowledge.

A materialist view of reality, importantly, doesn’t simply state that “reality is nothing more than the sum of its parts.” Instead, it’s important to remember that, from a physical standpoint, even:

• a very simple set of fundamental ingredients,
• adhering to a simple set of just a few rules,
• can very swiftly wind up creating large numbers of extremely complex outcomes,
• many of which display emergent properties that are not “obviously” encoded, in a trivial way, by the underlying rules and ingredients.

For example, if you take all of the quarks in the Standard Model of particle physics and just leave them in a confined space, they will swiftly bind together into a huge array of composite structures (baryons) that will then swiftly decay away into other, less massive particles. After only about a microsecond, the only quark-containing particles remaining will be protons and neutrons.

Similarly, if you take only protons and neutrons and attempt to combine them together into any imaginable configuration, you will find that there are hundreds of stable (or quasi-stable, i.e., stable over cosmically long time intervals) configurations: the elements and isotopes of the periodic table. It is from these combinations that all of chemistry and biology fundamentally arises, all from just a few fundamental rules and types of raw ingredients.

The elements of the periodic table, and where they originate, are detailed in this image above. Alongside, at right, is a color-coded representation of where the elements composing the human body arise from. Despite being made only of protons, neutrons, and electrons, there are more than 90 naturally occurring elements (and over 200 total isotopes) in the periodic table, each with their own unique physical and chemical properties.

For human beings, the material reality of our composition has been well-studied for centuries. We know that, atomically, we are made of approximately ~10²⁸ atoms. The most abundant atomic species are oxygen, carbon, and hydrogen by mass, with a substantial amount of nitrogen, calcium, and phosphorus, followed by smaller amounts of potassium, sulfur, sodium, chlorine, and magnesium. Other elements, present in smaller quantities, also play a major biological roles, for example: iron, fluorine, zinc, copper, lithium, and even vanadium, which is the least abundant element in the body (with just 110 nanograms worth in a typical human) that has a known biological function.

Those atoms are configured together into a variety of molecules, which are distributed across trillions of cells within the body, which are further organized into organs — large collections of cells that possess specific structures that perform certain essential biological functions — and those organs sum up to make a complete human being. Within the body, of specific relevance to consciousness, is the body’s nervous system, including the human brain. For most of us, there’s an assumption that seems so obvious that “of course consciousness arises in the brain” that it’s rarely challenged. It would mean, if true, that if we want to study human consciousness, we have no choice but to study the human brain.

The human mind is one of the great mysteries of modern science, as we cannot sufficiently explain how the brain in general, or consciousness in particular, works. However, it’s a reasonable “null hypothesis” to presume that electricity, i.e., the flow of electrons, is the primary driver behind our perceptions that we are conscious. Although quantum effects may play a role, it’s an unnecessary complication to presume that consciousness is anything other than the flow of electricity.

You then might wonder just how the brain produces consciousness, and what mechanisms are at play. To begin, we can talk about the structure of the brain with some confidence. The human brain, primarily, is composed of two classes of cells:

• neurons, which transmit electrical and chemical signals,
• and glial cells, which are defined by the fact that they do not produce electrical impulses, and are instead thought to form a substrate that supports neurons.

Nearly all discussions of consciousness focus on the neurons and ignore the glial cells. This makes sense on the surface, as one can easily argue that the only difference between a living human (which possesses consciousness) and a deceased human (which no longer does) is the presence of those electrical neural impulses. Take them away, and consciousness ceases to be.

But glial cells may yet play a vital, if only poorly understood, role in the presence of consciousness. Glial cells are known to come in four different types: ependymal cells, astrocytes, microglial cells, and oligodendrocytes. Each type of glial cell performs a series of functions: producing cerebrospinal fluid and aiding in neuroregeneration for the ependyma, biochemically controlling the cells of the blood-brain barrier and providing nutrients to neurons for the astroglia, performing immune functions and maintaining and sustaining normal brain functions for the microglia, and supporting and insulating the axons of neurons for the oligodendroglia.

Microglia (colored green), the smallest of the four main classes of glial cells, play several essential roles in maintaining brain health and function. They are thought to provide support to neurons, but their role in the phenomenon of consciousness has yet to be quantified.

In addition, the brain contains blood vessels, salts, a differentiated composition (gray matter and white matter), and is covered in three different types of meninges, or protective coverings: dura mater, arachnoid, and pia mater.

Most often, when people discuss consciousness, they assume that:

• it arises from the brain,
• it is driven by neuronal activity and that all other cells serve only as “support,”
• it appears, definitively, in humans (but not necessarily in any other living creature),
• and that it’s associated with our most advanced, highest-level abstract thoughts.

It is in this framework that examinations of consciousness often take place. We typically conduct MRI studies while humans are in various states — sober or intoxicated, calm or stimulated, awake or asleep, in REM sleep versus in non-REM sleep, in a state where long-term memories are formed versus one in which they are not, etc. — measuring the various types of brain activity that are and aren’t present, in our attempt to study how the firing of neurons in the brain corresponds to a variety of conditions experienced by a human subject.

But these are experiments that, although they are an important part of research into the workings of the human brain, assume that consciousness is simply driven by classical, electrical activity in the human brain. That’s a possibility, but far from the only one.

A fruit fly brain as viewed through a confocal microscope. The workings of the brain of any animal are not fully understood, but it’s eminently plausible that electrical activity in the brain and throughout the body is responsible for what we know as “consciousness,” and furthermore, that human beings are not so unique among animals or even other living creatures in possessing it.

There are a variety of functions that take place inside living organisms, including in brains, that rely not only on signals (electric and chemical) that invoke classical physics alone, but that either suggest or even require some sort of quantum interaction. Some animals can orient themselves with Earth’s magnetic field by taking advantage of inherently quantum processes like magnetoreception. A mathematical equivalence has been shown between the classical physics of brain responses and the probabilistic wave equations of quantum mechanics. Quantum mechanics plays an essential role in photosynthesis, and large networks of tryptophan, found in sub-elements of neurons (as well as elsewhere), exhibit the phenomenon of quantum superradiance.

One hypothesis about consciousness is that it doesn’t arise from electro-chemical impulses and neural connections, but rather that quantum entanglement between microscopic cellular structures known as microtubules is the underlying culprit. Because neurons contain these microtubules, the idea goes, and these microtubules control a number of functions — controlling the movement, growth, and shape of the cells — perhaps they are the site of quantum processing that’s fundamental to consciousness. An experiment performed on microtubules showed that laser-induced excitations propagated within them to great distances in awake patients, but not within patients under anesthesia. However, almost nobody defines “consciousness” as the opposite of “being unconscious” (except, perhaps, for anesthesiologists), and so this hypothesis remains on the fringes.

Natural neurons are connected to one another across various synapses, and as synaptic connections are strengthened, neurons become more likely to fire together: something that occurs when the brain learns. An artificial neural network models these neurons as nodes that are encoded with a specific value, and the connectedness of the nodes can strengthen or weaken dependent on whether they take on identical or different values from one another.

But one must wonder: how can we even define what “consciousness” is? Many give it a definition akin to U.S. Supreme Court Justice Potter Stewart’s threshold test for pornographic content: I know it when I see it. This, however, is an arbitrary definition in many ways, and there is no widely agreed-upon definition for what consciousness actually is.

• Are all humans conscious? Does this include newborn babies? Sleeping humans? Humans still developing in the womb?
• Are animals other than humans conscious? Many brain-containing animals, from dogs to cats to horses to birds, exhibit strong individualistic preferences and behavioral oddities — what many would call personalities — and observations such as these have been validated through scientific studies. Is a brain a sufficient and necessary ingredient for consciousness to be achieved?
• Do living organisms without brains exhibit consciousness? Simple subjective awareness, or the ability for an organism as a whole to act as a unified structure that engages in acts of self-protection and self-preservation, particularly in response to various stimuli in their environments, may be enough, as many have suggested.

It’s a very challenging problem: one of a universally agreed-upon definition for what consciousness even is. Before we can move on to questions such as, “Is consciousness quantum in nature?” we should at least be able to answer the yet-unanswered question of “What even is consciousness?“

A fascinating class of organisms known as siphonophores is itself a collection of small animals working together to form a larger colonial organism. These lifeforms straddle the boundary between a multicellular organism and a colonial organism. Because it responds to stimuli in its environment all as one unified unit, it could arguably be construed to be conscious as a whole, beyond the mere behavior of its constituent parts.

It might seem like these are scientific questions, as there are certainly scientific ideas and hypotheses out there about consciousness, and a number of scientific experiments that have been performed and documented that touch upon many of these (and other surrounding) issues.

But consciousness, without an agreed-upon, robust definition for what it is, can hardly be said to have advanced to a point where we can study it scientifically. Much like chemistry 400 years ago, physics 1000 years ago, or astronomy 5000 years ago, consciousness research today is an example of the very beginnings of science: science in its infancy, or science that has not yet moved beyond the realm of speculation or philosophy.

In fact, the most compelling definition of consciousness that I’ve ever heard didn’t come from a scientist of any variety, but rather from the recently-deceased philosopher Daniel Dennett, who simply posed that consciousness was the ability to understand, “I am me,” or to otherwise possess an internal conception of what we call “one’s self.” Humans have clearly crossed this threshold and are conscious; dogs have as well, as if you have two dogs and call one of their names, the dog whose name you called will respond differently from the dog whose name you didn’t call. Rather than being a property that’s special to humans and to human brains, consciousness may simply be a physical manifestation of an emergent property associated with any form of life itself.

This drawing shows a variety of human, monkey, and ape skulls from a variety of extant species. The older apes have smaller cranial capacities and smaller brains than humans, but all such examples of the specimens shown here are assumed to have achieved what we would call “consciousness.” Many less-evolved creatures, and perhaps even all living things, may justifiably be considered conscious at some level.

The big takeaway from all of this is that if you hear a claim that purports to explain consciousness, there are a few critical things you should be asking yourself.

• What is the definition of consciousness that they’re using, and how can it be tested for, at least qualitatively?
• In terms of explanatory power, any theory of consciousness should be able to make testable predictions that, if they are shown not to be borne out by experiment, measurement, and observation, will falsify that theory. What, therefore, are this theory’s testable predictions?
• And, perhaps most importantly, can this explanation detail how what we perceive of as consciousness arises from purely physical entities, without invoking some sort of mystic quality that exists outside of our physical reality?

If the claim does not clearly answer any of these three types of questions, then what you have encountered is not an explanation of consciousness; it is merely a not-fully-baked germ of an idea. To be sure, there are a lot of non-physicalist theories of consciousness out there, but none of them are “theories” in a scientific sense; only in an informal, idea-esque sense. If we want an understanding of how something we can observe within our physical reality behaves, there must be a physical underpinning of it: whether that’s fundamental, emergent, or a combination of the two. There are a great many things that remained unexplained at the present time, and consciousness is one of them. However, that doesn’t give me cause for any despair; it simply reminds me of what my differential equations professor told my class back in college:

“Most of the differential equations that exist cannot be solved. And most of the differential equations that can be solved cannot be solved by you.”

Consciousness is a very difficult puzzle: one that is difficult to even define, much less to solve. But it is just as much a part of our physical reality as anything else we interact with, and any approach that asserts otherwise has a fatal flaw from the outset: it’s already abandoned science.