The Epistemology of Microphysics

[A lecture delivered at the 49th Annual Meeting of the American Maritain Association at Loyola Marymount University, Los Angeles, CA on March 21, 2026]

Microphysics is the branch of physics that studies molecules, atoms, and elementary particles. Two facts about it are especially noteworthy from the point of view of the theory of knowledge. The first is the astounding amount we have come to learn about this part of reality, despite our having no direct perceptual access to it. The second is that progress has slowed considerably in recent decades, at least in the opinion of many physicists. Among these is Sabine Hossenfelder, who argues that the source of the problem is that contemporary research in fundamental physics is dominated by mathematical constructs that are nearly impossible to test empirically and embraced instead for largely aesthetic reasons.[1] Similar criticisms have been raised by Roger Penrose, Lee Smolin, Peter Woit and others.[2]

What I will argue is that both the success and the frustrations of microphysics have common epistemological roots, which Thomistic philosophical considerations help to expose and illuminate. While these considerations are not inherently theological, they have implications for theology just as they do for physics. And it will turn out that the scope and limits of what we can know where the micro-world is concerned parallel the scope and limits of what Thomism says we can know by reason alone about the existence and nature of God. In both cases, the human intellect can press well beyond what the senses alone could reveal, but only so far. And in both cases, the intellect’s powers give out as it approaches one end or the other of the ontological spectrum – the divine essence being at the top of that spectrum, and what Thomists call prime matter at the bottom.

From atomos to strings

The place to begin is with a brief overview of how physics has come to know what it knows about the micro-world.[3] The first steps were taken by Democritus and other ancient Greek atomists. Noting that physical objects are divisible into parts, those parts into smaller parts, and those parts into yet smaller parts, it was natural to extrapolate to the existence of even smaller parts below the level of those that can be perceived. Phenomena such as evaporation, density, and permeability also lent support to the idea. Evaporation could be explained by reference to unobserved particles moving apart from one another, and density by reference to such particles being tightly packed together. The movement of sounds and liquids through what look to be solid objects could be explained by way of the thesis that such objects are actually collections of particles separated by empty space, which provides an avenue through which sound and liquid can pass.

However suggestive, such speculations did not yield rigorously testable predictions. But considerable progress was made after interest in atomism and related ideas was revived with the scientific revolution. Studying the compression and expansion of gases, Robert Boyle (1627-1691) found that changes to the volume of a gas did not alter its mass. This was hard to understand unless a gas is not a continuous thing but rather a collection of particles separated by empty space, with compression and expansion involving changes in the distances of the particles from one another. Moreover, on the basis of this assumption, Boyle was able to formulate and support by experimental test his famous law describing the relationship between the volume of a gas and its pressure.

The kinetic theory of gases developed by Daniel Bernoulli (1700-1782) added further detail to the story. Since a gas will spread evenly throughout a container it occupies, the particles that make it up must be in continual random motion. For if they weren’t, they would collect in some part of the container rather than remaining evenly spread out. The pressure of a gas could then be analyzed in terms of the collisions of particles against the inside surface of the container. As the volume of the container increases or decreases, the distance the particles would have to travel to hit its inside surface will correspondingly increase or decrease, which leads to decreases or increases in pressure in conformity with Boyle’s law. Temperature could also be explained in terms of the kinetic theory, which identifies it with the average kinetic energy of particles in motion. James Clerk Maxwell (1831-1879) and Ludwig Boltzmann (1844-1906) would go on to formulate the theory with mathematical rigor.

Such mathematical and predictive precision made the reality of unobserved particles harder to deny, but what really settled the matter in the minds of scientists were developments in modern chemistry. The law of the conservation of mass was established by Antoine Lavoisier (1743-1794), and this opened the way to determining the mass of each element in a compound. By doing so, Joseph Proust (1754-1826) was able to show that the elements are always to be found in compounds in fixed proportions, a principle that would come to be known as Proust’s law. Applying Proust’s law, John Dalton (1766-1844) argued that there must be some smallest unit of an element that cannot be broken down into parts that retain the properties of that element. This he called an atom. A molecule, the smallest unit of a compound, is thus made up of atoms. Using Proust’s law, the relative masses of different elements, and thus of different atoms, could be deduced. For example, from the proportion of hydrogen to oxygen in water, the oxygen atom could be shown to be sixteen times more massive than a hydrogen atom. This in turn allows us to infer the relative masses of yet other atoms. For example, since carbon dioxide also contains oxygen, knowing the mass of oxygen allows us to determine the mass of carbon. From the relative atomic mass of the elements, Dmitri Mendeleev (1834-1907) was able to work out the periodic table, and successfully to predict new elements and their properties from the gaps in the table.

Now, the chemist Amedeo Avogadro (1776-1856) had argued that equal volumes of gases contain equal volumes of molecules given that temperature and pressure are fixed. This is so even if the volumes differ in mass. The mathematical analysis of gases worked out by Maxwell and Boltzmann opened the way to determining exactly how many molecules are in a volume of gas. This allowed, in turn, for inferences concerning the absolute mass of molecules and atoms, and about their sizes as well. The predictive successes of a theory that revealed even the mass and size of atoms as well as the properties of the elements made the reality of the atom appear certain.

The electron was discovered by J. J. Thomson (1856-1940), who showed that cathode rays could be made to curve away from an electrically charged plate with a negative charge and toward one with a positive charge. This showed them to behave like particles rather than waves, and particles of a negatively charged kind, specifically. From the size of the charge of these particles, their mass was worked out, and this turned out to be smaller than that of the smallest atom. Study of the photoelectric effect, in which light causes electrons to be emitted from a metal surface, showed that electrons are already present in the atoms from which they are ejected. Further study showed that electrons had the same properties even when they came from the atoms of different kinds of metal.

Since atoms are electrically neutral, the presence in them of negatively charged electrons suggested that there must be some component with a positive charge to neutralize the negative charge. Experiments by Ernest Rutherford (1871-1937) involved firing positively charged particles at thin gold leaf, behind which was a photographic plate. The resulting patterns showed that most of the particles passed through as if nothing were there, while a few were sign