Ontology

Do You Exist?§

At first glance, you might answer "of course I exist — I can feel myself." But wait: who, what, and where are you? We need to pin that down before we can even say you exist, because the answer depends entirely on what you mean by "you."

Working with Epistemology§

Imagine you ask a theologian

As established last section, feel free to believe what you want — but souls and gods are not falsifiable, and so are of no interest to physics. That immediately narrows the options for what could count as "you" under science. Here are a few:

• You are your DNA
• You are every single cell that makes you up
• You are your brain structure and neural networks
• You are your life experience, history and memory
• etc...

Each of these is a candidate answer science can take seriously — but notice they compete. They cannot all be the whole story at once: identical twins share the same DNA yet are two different people; nearly every cell in your body is replaced over the years, yet you still feel like you. Which layer counts as "you" is itself an open question.

And here's the connection: physics faces the exact same kind of puzzle about nature itself. Just as we cannot agree which layer of you is the real "you," physicists cannot agree which layer of the world is the thing that truly exists. So they ask:

• What are time and space?
• Do time and space exist?
• Does gravity exist — how does it work, and why?
• Do fields exist? (a field being, loosely, a quantity — like temperature, or a pull — that takes a value at every point in space)

These are all ontological questions.

Definition of Ontology§

Components of the word§

• Ontos: "that which exists" or "being"
• Logia: "the study of, or science of"

Together it means: the study of being and existence.

In other words, the questions it asks are: what exists, why does it exist, and how does it exist?

Outside of Science§

Just to be clear: outside science, ontology need not answer to epistemology at all. A theologian asking whether the soul exists is doing ontology too — they simply are not bound by the demand that the claim be falsifiable. Inside physics, we are.

Is Ontology Mandatory?§

Throughout the history of physics, ontological questions come up constantly — yet, as the examples below show, physics gets its work done without ever finally settling them.

Examples§

1. When Newton declared space and time absolute, Leibniz pushed back on purely logical grounds: if you shifted the entire universe five feet to the left, nothing you could ever observe would change — so there is no reason to call the "shifted" version a genuinely different arrangement. Space, he argued, is not a fixed stage sitting underneath things; it is nothing more than the relations between those things. (This is the relational view of space — connected to, but distinct from, Einstein's relative-spacetime view; a later chapter takes up how the two fit together.)

2. Physics had long treated space, time, matter, and energy as separate kinds of thing. Einstein’s relativity collapsed those divisions: it merged space and time into a single entity (spacetime), and merged mass and energy into one. That is exactly what $E=mc^2$ says: a mass m is equivalent to an amount of energy E, the two tied together by the speed of light c squared. What counted as a fundamental category of existence had changed.

3. When Newton worked out gravity, he was famously uneasy about his own theory: it had one mass pull on another straight across empty space, with nothing in between to carry the force. He called such unmediated action "so great an absurdity" that no competent thinker could accept it — and wished for a deeper account of what gravity actually is.

4. Einstein took the operationalist stance we met in the last chapter — a concept is only meaningful if you can state how to measure it — and defined spacetime as what our rulers and clocks actually read, instead of Newton's unmeasurable absolute lurking beneath.

5. When energy was first identified, it was treated as a bookkeeping quantity — something that happened to stay conserved in the math, and nothing more. Only later did physicists begin to ask whether energy is a real thing in its own right, or just a number that always balances.

So: Important, but Not Mandatory§

Notice that none of these ontological questions are falsifiable: whether we treat our measurements as the real thing, or as merely a shadow of something deeper, is not a claim any experiment could ever refute. (The physics in those examples — mass–energy equivalence, say — is of course falsifiable and well confirmed; it is only the ontological reading laid on top that no experiment can touch.) The definition of existence is philosophical, not physical. This is why ontology, though it constantly drives physics — supplying the questions and hunches that lead to new theories — is not mandatory to it. We do not need to settle whether energy truly exists, or how, or why, in order to use its equations, make falsifiable predictions, and get the physics right.

Scientific Realism vs. Anti-Realism§

Even if we never settle whether the things physics talks about are "really there," we still have to decide how seriously to take them — and here people fall into two camps.

Scientific Realism§

Scientific Realism is the belief that our physics equations really do describe what exists — and that we are right to believe in the things they invoke, even the ones we can never see. If the equation says "electron," then electrons literally exist out there in objective reality, whether or not anyone is measuring them.

Scientific Anti-realism§

The opposite view. When the equation says "electron," the anti-realist takes it at face value — it really is a claim about electrons — but believes only the part about what we can observe: that the theory reliably predicts our measurements. Whether an electron is literally out there causing them, they leave open. Crucially, anti-realism does not claim electrons don't exist; it claims physics simply cannot tell us either way, so we withhold belief about the unobservable rest and stay agnostic. The philosopher Bas van Fraassen puts it like this:

"Science aims to give us theories which are empirically adequate; and acceptance of a theory involves as belief only that it is empirically adequate."

"A theory is empirically adequate exactly if what it says about the observable things and events in the world, is true—exactly if it 'saves the phenomena'."

In plain terms: a theory is empirically adequate if everything it says about what we can actually observe turns out true — never mind whether the unobservable machinery it invokes (electrons, fields) is really there. The anti-realist accepts the theory exactly that far, and no further. (This is van Fraassen's constructive empiricism — one influential, and fairly moderate, version of anti-realism: it never claims the unobservables are fake, it only declines to assert they are real.)

Ontological Crisis: Quantum Mechanics§

This is where the "not mandatory" rule gets pushed to its breaking point. Here we are not even sure whether the question below can be answered by science at all, or whether it is pure metaphysics — reasoning about what is real that lies beyond anything an experiment could test — and depending on which answer you pick, you end up believing in wildly different pictures of the reality we live in.

The Wave Function ($\Psi$)§

First, some orientation: quantum mechanics is the physics of the very small — atoms, electrons, light — where the tidy rules of everyday objects break down and outcomes turn irreducibly probabilistic. It sits right at the edge of the classical world this book has been building, and it pushes the ontology question to its limit.

Quantum mechanics is built around a mathematical object called the wave function, written $\Psi$. Think of it as a recipe that, for every place a particle could turn up, hands you a number; squaring that number gives the probability of finding the particle there. The recipe predicts the observable results beautifully. But here is the ontological question: is $\Psi$ a real, physical thing — an actual wave rippling through the universe (the psi-ontic view) — or is it merely a map of our own ignorance, a bookkeeping of probabilities in our heads (the psi-epistemic view)? Notice this is just the realism-vs-anti-realism question from a moment ago, now aimed at the wave function itself: psi-ontic is realism about $\Psi$, psi-epistemic is anti-realism about it.

Why it matters: the math works perfectly either way, but the ontology you adopt changes what you think is actually happening. Lean one way and you may conclude the universe genuinely splits into countless branches, each carrying a different outcome (roughly, the Many-Worlds picture). Lean the other and you may hold that a particle has no definite value until it interacts with a measuring device, at which point the possibilities collapse to one (roughly, the Copenhagen picture). Note that Copenhagen turns on measurement — an interaction with a large, classical apparatus — not on a conscious human "looking," which is a popular myth worth resisting. (A later chapter of its own picks the quantum thread back up.)

Underdetermination of Theory by Data§

Definition§

It is a well-recognized possibility in the philosophy of science that two completely different ontological pictures of the universe can yield the exact same observable predictions — leaving the data powerless to decide between them. We just met a live case: the psi-ontic and psi-epistemic readings of quantum mechanics predict identical laboratory results while painting utterly different pictures of what is real.

Example§

Take special relativity. Einstein explained why moving clocks tick slow and moving objects shrink — a later chapter builds the intuition for why — by throwing out absolute space and time altogether. But you can reproduce every single one of his predictions while keeping Newton's absolute space — so long as you also add a hidden "ether": a privileged frame of rest, undetectable by any experiment, in which those clocks and rulers really do change as they move through it. (This was essentially Lorentz's route — the "hidden, undetectable frame" we saw in the last chapter.) The two pictures flatly disagree about what is real — is there a privileged frame of rest, or not? — yet no measurement can ever separate them, because the ether was built to be invisible. Physicists went with Einstein's version, but they chose it for its simplicity and operational honesty, not because the data forced their hand.

So — Do You Exist?§

Back to the question we opened with. Physics can describe you down to the finest detail — it can predict how you move, what you are made of, how your neurons fire. But whether there is a fact of the matter about what you ultimately are, underneath all those successful predictions — whether the real "you" is your DNA, your cells, your memories, or no separate fact at all — is exactly the kind of ontological question we have just watched physics run up against. One it may be unable, and does not need, to answer. You are free to hold your own view. Just keep it honest about which part is physics, and which part is a choice.