As a physicist, I've noticed gaps between what can be inferred about our world from physics, and what is common knowledge and belief. While part of this is just the usual gap in knowledge between experts and laypeople, common to all fields, some of these facts are not necessarily internalised and accepted even by all physicists, let alone the general public (though physicists do believe these much more often than the general public do). This suggests to me that there is compartmentalisation going on here, that physics knowledge is not being taken to its logical conclusions, not simply that this knowledge is lacking. In this post, I'll show you what you learn when you don't compartmentalise physics, when you instead take it seriously.

The past and future are as real and existent as the present (Eternalism)

There's a common notion that time consists of a universal, objective present, on either side of which is the objective past and future. Thus, it's easy to think that the present, and only the present, is fundamentally "real", while the past merely "used to exist" and the future "doesn't yet exist", a notion dubbed presentism by philosophers.

Einstein's theory of relativity, however, showed us that this cannot be the case, that time and space must "bend" into each other to keep the speed of light the same for everyone (hence the notion of "spacetime" rather than just space and time). The root of this contradiction is a bit obscure, since time dilation and length contraction, the most well-known results of Einsteinian relativity, do not contradict presentism per se, only the stricter notion that time flows at the same rate for everyone. However, another effect of Einsteinian relativity, lesser known among non-physicists, is that simultaneity is relative. That is to say, that you can't necessarily define a notion of "same time, different place" that would let you say whether or not two events happened at the same time, or which order they happened in.

In special relativity, the subset of Einsteinian relativity that deals only with non-curved spacetime and therefore does not include gravity, there is at least a notion of simultaneity for a given observer, that is different between observers. Given this, when trying to rescue presentism, it might be tempting to declare that one particular frame of reference is the "true" or "correct" one for determining the objective present. In the curved spacetimes of general relativity, however, even this breaks down. While you can separate a curved spacetime into "layers" of space that look like moments in time, there are many ways to do this for any given spacetime, and the mathematical machinery of general relativity doesn't say anything about which is the "right" one. In many cases, the "obvious" way to do this gives you plain nonsense, like that nothing can ever fall into a black hole because time "freezes" at the event horizon.

Despite the fact that it's incompatible with known physics (century-old physics at that), presentism is largely still treated by philosophy as a viable model of time. In this sense, philosophy has failed to absorb the lessons of physics in the way that it did when it abandoned Aristotelian pseudo-physics, and when it ceded the question of atomism to science, hundreds of years ago.

The brain is made of stuff

One major subject of this "physics compartmentalisation" phenomenon is the human mind. Knowing that the mind is a function of the brain (neuroscience) and that the brain is made of atoms (biochemistry), it stands to reason that physics, which tells us about the existence and behaviours of the fundamental particles that atoms are made of, has plenty to tell us about the human mind (or any other mind that could exist). The implications of this, however, are not widely known and accepted, sometimes denied even by physicists. I will go over a few of these implications now.

Souls, ghosts, and afterlives don't exist

A soul is a hypothetical thing that human minds are made of, at least partially, that is something other than the physical particles that make up the brain. Often, this notion includes the idea that a soul can persist after the body and brain are dead, either staying in this world as a ghost or travelling to some other world for an afterlife.

Looking outside of physics for a moment, neuroscience has plenty of evidence linking the mind to the brain and peripheral nerves, and none linking the mind to anything else. For example, psychedelic drugs and physical trauma that affect the brain affect subjective experiences, and destroying or inhibiting the peripheral nerves inhibits sensations.

Physics, however, provides a more fundamental challenge to the idea of souls. First of all, a person's behaviour is ultimately manifested in, and their perceptions ultimately originate from, the physical matter of their bodies. Sights originate from photoreceptors in the eyes, speech is motion of the vocal anatomy, and so on. Anything that can affect these, any candidate for a soul, must therefore physically influence and be influenced by this matter. This means that souls must have interactions with matter that can be measured, that would cause the behaviour of fundamental particles to deviate from the known laws of physics, but no such deviations are measured.

The ultimate conclusion, then, is that souls don't exist, and therefore ghosts (disembodied souls) and afterlives don't exist either. From this, we must also reject large, load-bearing subsets of the claims made by, for example, Christianity and Islam (Heaven and Hell) and Buddhism and Hinduism (reincarnation).

(Libertarian) free will doesn't exist

Intuitively, free will is the idea that there are multiple possible futures, and that our decisions affect which of those futures ultimately materialise. In philosophy, this is called the libertarian view of free will, and the belief in its existence is called libertarianism (not to be confused with political libertarianism).

Right away, eternalism (as discussed in the previous section) puts a dent in this notion; there can't be an indeterminate future that becomes determined if the future already always existed. The more fundamental problem, however, is that the brain is made of particles whose behaviours follow laws of physics, laws that are described by equations. The behaviour of a person, then, obeys a mathematical equation, albeit an extremely large and complicated one.

Some attempts to rescue libertarianism look to quantum mechanics for a solution. First of all, there's the idea that quantum mechanics is mathematically intractable and fundamentally mysterious. This is plainly false: there are equations that describe quantum mechanics, like the Schrödinger equation. Second, there's the fact that quantum mechanics predicts either fundamental randomness (for interpretations like Copenhagen) or many "parallel" worlds that branch into multiple futures (the Many Worlds Interpretation). These do not solve the problem, however. In the former case, the randomness applies to fundamental particles, has well-defined statistics, and doesn't care about whether the particles are part of a brain or not, and in the latter case, all future branches are always equally real, and there is no "determination" of one over the other.

Putting aside libertarian free will, there's the idea of compatibilism, that free will is compatible with the fact that the mind obeys mathematical laws. This is ultimately just a semantic argument about the definition of "free will", lacking any substantive factual claim about the world, so I won't (and can't, really) address it substantively. I will say, though, that I find the idea nonsensical, that if free will is not libertarian, it is meaningless.

Mental illness and brain injury can have any effect on behaviour

Sometimes people claim that mental illnesses (or, for example, traumatic brain injuries) never cause unethical behaviour, and that the claim that this is possible is either prejudice against people with such conditions or an abdication of responsibility by those people. Knowing that the brain is made of matter, and that human behaviours arise from physical processes in that matter, however, it stands to reason that all human behaviours we observe, including the most heinous ones, can be induced by various states of, or changes to, that matter. In that sense, "behaving ethically" is a bodily function in the same way that "producing insulin" is, and must be similarly mutable.

(This, and the earlier point about free will, are very good reasons to favour restorative and rehabilitative justice over retributive and punitive justice. We often make a distinction between people who can't help themselves and people who are fundamentally evil, such as with the insanity defense in criminal law, but this distinction is arbitrary. The right (or rather wrong) brain injury, or electrical stimulation by a mad scientist, could make anyone commit any terrible act, including yourself.)

Machines are, in principle, capable of anything that human minds are (Strong AI)

It's common for people to say that there are some things humans are and can do that machines/computers will never be capable of. These tend to include things like sentience, "genuine" intelligence, and creativity, a position that philosophers call weak AI. This position is incompatible with the fact that the human brain follows the laws of physics, which are equations that are computable. This means that there is at least one computer program capable of everything humans are capable of (a simulation of a human brain), and that we therefore can't rule out the existence of other such programs. Thus, physics affirms the strong AI position, that machines/computers are capable of anything that a human (or any other) mind is capable of.

This notion that physics is computable (at least that of the human brain) is a subtle one. One possible point of contention is that we don't actually have a "theory of everything", a set of equations that consistently describe all physical phenomena. Instead, we have two fundamental physical theories, general relativity and the Standard Model of particle physics, which contradict each other and can only be used in specific situations. This is not a problem for simulating a human brain; general relativity is not necessary for this, only the Standard Model. First of all, the self-gravitation of the human brain is much smaller than most perturbing forces that the brain is subjected to (e.g. an external electromagnetic field), so neglecting it has a much smaller effect than other perturbations that evidently don't affect the brain's capabilities. Second, ambient gravity is unnecessary, not only because the human brain demonstrably functions in zero-g/freefall, but also because the effect of ambient gravity can be captured by acceleration, which the special-relativistic Standard Model can handle.

Another point of contention regarding the computability of the human brain is that the laws of physics are differential equations, the solutions of which are continuous functions which are made of infinite amounts of information. The reason this isn't a problem is that these equations can be approximated, discretely and finitely, to arbitrary precision. An approximation is definitely good enough because, again, the human brain can withstand arbitrarily small perturbations. You wouldn't stop being a person, capable of intelligence and creativity, if I nudged one of your neurotransmitter molecules one angstrom to the side, after all.

Finally, there's the point that quantum mechanics might, depending on interpretation, introduce fundamental randomness to the physics of the human brain. This isn't a problem because, by definition, the brain being random would mean that any possible outcome of the randomness is a valid, possible way for a human brain to behave. Thus, any source of randomness, even a pseudo-random number generator, suffices for a simulation of the human brain.

Separately, there's the objection that simulating a human brain might simply be too computationally intensive in practice. In particular, a quantum-level simulation under the Many Worlds Interpretation would take a truly gargantuan amount of processing power (though quantum computing might be able to help with this). Even if this is true, it doesn't refute the core argument, which is that there's no fundamental gap between human and computer capabilities. The existence of a practical gap would need to be proven separately, and such a proof would need to show that all computer programs with human-like capabilities, not just simulations of human brains, are intractable in practice.

The body is made of stuff

Just as the brain is a common subject of "physics compartmentalisation", so is the body, mainly in the form of myths and pseudoscience related to weight loss and gain.

Excess body fat is always caused by overeating

Here, I'm addressing a family of claims related to the idea that it's possible to be overweight with excess body fat without consuming an excess amount of food, that "it's more complicated than calories in, calories out". For example, that certain drugs or medical conditions can cause weight gain directly, not merely by increasing appetite or reducing energy expenditure, or that it's possible to be overweight while undereating because the body enters a "starvation mode" that causes it to "hold on to fat".

The problem with this is the law of conservation of energy, which "calories in, calories out" is ultimately an expression of. When the body "burns" a "fuel" (food, body fat), this is a chemical process that takes that fuel together with oxygen and produces carbon dioxide (which is exhaled) and water (which is exhaled and urinated). Any given chemical substance has a specific amount of chemical energy associated with it, which is ultimately the electric potential energy associated with the electrostatic forces holding its atoms together as molecules. Thus, any chemical process that converts one set of substances into another set of substances has a specific, fixed change in energy, the difference in chemical energy between the reactants and products, regardless of whether the process is direct or indirect, of which "path" is taken from the reactants to the products. This is called Hess's law.

Thus, because body fat contains a specific amount of chemical energy, producing it requires the body to have spare energy left over after the energy that it uses to keep itself alive and perform physical activity. This energy must come from the food we eat (plus the oxygen we inhale), which itself also contains a specific amount of chemical energy. Because the reactants and products of metabolism, food and oxygen to carbon dioxide and water, are the same as those of combustion, the energy that food contains can, in principle, be measured by combusting food and measuring the energy given off as heating of some surrounding substance (calorimetry). This is true even though human metabolism takes a very different and much more complicated path to carbon dioxide and water than a fire does, because of Hess's law. (In practice, calorimetry may overestimate the amount of energy the human body is able to extract from a food, since indigestible components like dietary fibre are still combustible. It can never underestimate, though; the human body can never extract more energy from food than what a calorimeter measures.)

(Another way of thinking about this, less abstract than the notion of energy, is in terms of "carbon in, carbon out". Body fat contains carbon, which we are constantly exhaling as carbon dioxide, and which only enters the body in meaningful quantities via food, not water (hydrogen and oxygen) or air (nitrogen and oxygen).)

Now, just because "calories in, calories out" is factually true, it doesn't necessarily mean that it's helpful as a mantra for weight loss. The latter is less of a matter of physics and more of a matter of psychology, as well as systemic factors like access to high-quality food that has a good ratio of nutrients to calories. Saying that "losing weight is a matter of consuming fewer calories than you burn" is like saying "personal finance is a matter of spending less money than you earn" in that regard.

Metabolic rate does not differ drastically between individuals with similar builds

It's common to hear people say that they can't lose or gain weight because their metabolism is too slow or too fast, respectively. While energy expenditure does vary greatly between individuals based on many factors, these differences must ultimately manifest as differences in how one's body exchanges heat with their environment. This is because the energy expended by a person's body ultimately becomes heat, which must flow out of the body at just the right rate to maintain a narrow range of safe internal temperatures (308 Kelvin to 311 Kelvin). Thus, differences in individual metabolic rate must necessarily be balanced by differences in the thermal properties of people's bodies. For example, physically smaller people expend less energy and have less surface area to expel heat through, and people who are more physically active sweat more.

A great illustration of these principles is the effect of 2,4-Dinitrophenol (DNP) on the human body. When consumed, DNP increases the body's energy expenditure by essentially making it less energy efficient. Accordingly, it is sometimes used for weight loss, though no doctor would ever prescribe it for this purpose, and DNP is largely illegal, because it causes hyperthermia that can easily become fatal with a marginal overdose. If you're expending more energy, you're necessarily producing more heat. This is what it would really mean to "have a fast metabolism".

Conclusion

In conclusion, there are a lot of facts about the world that physics tells us, when you take it seriously, that are not widely known, or believed when they are known. I have shown and explained examples of these from the subjects of time, the mind, spirituality and religion, AI, and diet and nutrition. Many of these subjects are treated as inherently subjective, fuzzy, or mysterious, when physics has actually turned them into simple matters of fact.