In the previous post, we saw how “because Y” can be misread as an ontological explanation when it is really a theory-internal statement. Physics offers a particularly vivid illustration of this misread: gauge symmetry and the existence of light.
This post explores how constraints in physics operate as enabling conditions, not causal agents, and what lessons we can draw for responsible explanation.
Gauge Symmetry and Physical Necessity
Gauge symmetry is a central principle in modern physics. It specifies how certain fields may vary while preserving internal consistency. Within this framework, the existence of photons — the quanta of light — follows necessarily. In other words, light is unavoidable given the constraints imposed by gauge symmetry and the structure of electric charge.
Notice the level of explanation: it is entirely internal to the theory. It says nothing about why the universe has gauge symmetry or why charges exist. It tells us only that once these structural features are adopted, certain phenomena must appear.
This is a textbook case of theory-internal necessity.
Constraints Enable, They Do Not Cause
It is tempting to translate this into causal language:
“Gauge symmetry produces photons.”
“Charges cause light to exist.”
Yet this is a category mistake. Constraints in physics are not agents. They do not act. They enable intelligible patterns within a theoretical framework. They specify the space of possibilities rather than pushing the world in one direction.
A more accurate framing is:
Given the structure of possibility defined by gauge symmetry, the behavior we observe is required.
This subtle distinction preserves the explanatory power of physics without stepping into unwarranted ontological claims.
Why This Distinction Matters
Physicists often slide into the misread “why” effortlessly. It is easy to hear an internal dependency as a metaphysical declaration:
The necessity within the theory feels deep.
It appears to answer the ultimate question: “Why is there light?”
Yet the answer is conditional. It is true only relative to the adopted constraints. Mistaking it for ontological necessity obscures the real boundaries of explanation and risks the allure of metaphysical overreach.
Recognizing the distinction allows us to:
Appreciate the success of theoretical physics.
Keep track of the level at which explanations operate.
Maintain lucidity about what has actually been answered.
General Lesson
Gauge symmetry is a perfect illustration of a general principle: constraints explain intelligibility, not existence.
In physics, constraints specify which phenomena are possible or necessary within a theoretical framework.
In other domains — biology, society, symbolic systems — the same pattern emerges: constraints shape what can occur, without themselves being ultimate causes.
The discipline is to notice when a dependency claim is being heard as a metaphysical explanation, and to own the cut being made.
Looking Ahead
In the next post, we will see this pattern in biology. Natural selection provides another instance where theory-internal necessity is often mistaken for ontological causation. Just as gauge symmetry explains the inevitability of photons within a framework, so evolutionary principles explain the distribution of traits within a population and its environment.
By tracing the pattern across domains, we can cultivate clarity and responsibility in how we hear and make explanations.
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