── 構文的仮定としての光速度不変原理

Why No One Called It a Syntactic Assumption

On the Constancy of the Speed of Light

Abstract

The constancy of the speed of light has long been treated as a foundational principle of modern physics. This paper argues that such treatment reflects not a logical necessity nor an empirical inevitability, but a syntactic choice: a decision about how physical descriptions are organized. Rather than disputing the validity or utility of the assumption, this work asks a different question—why it was never named as such. By examining historical, conceptual, and linguistic factors, we show that the absence of a syntactic vocabulary prevented the reassignment of its epistemic status. The goal is not to revise relativity, but to clarify the grammatical role played by its most famous postulate.

1. Introduction

The statement that the speed of light in vacuum is invariant for all inertial observers is commonly introduced as a principle.
It is not derived.
It is not explained.
It is assumed.

This paper does not challenge the assumption’s empirical adequacy, predictive success, or historical importance. Instead, it addresses a narrower and more structural question:

Why has this assumption never been described as syntactic?

The issue at stake is not whether the assumption is true, but what kind of statement it is. The distinction matters because misclassification at the level of syntax propagates downstream—into interpretation, explanation, and ontology.

2. Proximity Without Naming

2.1 Einstein

The constancy of the speed of light was introduced as a principle, motivated by the failure of classical kinematics to accommodate electrodynamics. Its role was primarily organizational: to stabilize the transformation rules between observers.

In later reflections, Einstein expressed dissatisfaction with principle-based formulations, suggesting a preference for constructive explanations. Yet even there, the invariant speed of light was not demoted in status; it remained a foundational axiom. The discomfort was acknowledged, but the category itself remained unquestioned.

2.2 Poincaré

Poincaré explicitly characterized simultaneity as a convention and understood measurement protocols as coordinated choices rather than discoveries. He came close to recognizing the invariant speed of light as a rule embedded in measurement syntax.

However, the language of convention stopped short of a more general grammatical classification. Convention still implies choice within a theory, not the syntactic scaffolding that enables the theory to be written at all.

2.3 Constructive Interpretations

Lorentz-type approaches treated light-speed invariance as an emergent phenomenon rather than a principle. Yet the explanatory effort remained causal and physical: searching for mechanisms beneath the invariance.

The syntactic question—why this invariance must be placed at the level of transformation grammar—was never asked.

2.4 Contemporary Philosophy

Modern discussions distinguish between principle theories and constructive theories, or between explanation and redescription. These debates sharpened conceptual clarity but retained the same categories: principle, law, convention, geometry.

The term syntax—as a classification of descriptive rules independent of ontological commitment—was never introduced.

3. Why the Name Never Appeared

3.1 Mathematical Overdetermination

The formal success and aesthetic power of Minkowski spacetime made the underlying assumption appear explained. Geometry seemed to replace grammar. Once the equations worked, the syntactic decision that enabled them disappeared from view.

3.2 Fear of Structural Collapse

Reclassifying the constancy of light speed from principle to assumption appeared to threaten the entire theoretical edifice. Time dilation, length contraction, relativistic causality—all seemed to depend on it.

As a result, discussions rarely stopped at the level of epistemic status. Critique jumped directly to replacement or rejection, which discouraged careful reclassification.

3.3 Absence of Syntactic Vocabulary

Physics possessed words for laws, principles, constants, and hypotheses—but not for the rules governing how descriptions are written. Without a grammatical category for such rules, the invariant speed of light had nowhere else to go.

The result was conceptual inertia.

4. Reclassification Without Revision

Calling the constancy of the speed of light a syntactic assumption does not deny its effectiveness. It restores proportionality.

This reclassification does not modify predictions. It modifies interpretation. It clarifies that the assumption operates prior to dynamics and independently of ontology.

In this sense, the statement “the speed of light is invariant” functions grammatically, not physically.

5. Conclusion

The invariant speed of light was never false.
It was never misguided.
It was never even questioned in the right way.

It was simply never named for what it is.

By restoring it to the status of a syntactic assumption, we gain a clearer view of what relativity does—and what it does not claim to explain. The task is not to overthrow a theory, but to correct its grammar.

It was not an error of physics.
It was an absence of vocabulary.

Short Letter Version

Why the Constancy of the Speed of Light Is a Syntactic Assumption

Abstract (Letter)
The constancy of the speed of light is traditionally presented as a foundational principle of special relativity. This note argues that its role is better understood as a syntactic assumption: a rule governing how physical descriptions are coordinated between observers, rather than a physical law or empirical invariant. Reclassifying its epistemic status clarifies long-standing conceptual confusions without altering empirical predictions.

Main Text

The invariance of the speed of light is usually introduced as a principle, not derived from deeper dynamics. While empirically successful, its conceptual role has often been overstated.

This letter proposes a modest reclassification: the constancy of the speed of light functions as a syntactic assumption, not as a physical principle in the causal or ontological sense.

By syntactic assumption, we mean a rule that constrains how descriptions are written and transformed, rather than a statement about what physically exists or dynamically causes phenomena. In special relativity, the invariant light speed fixes the grammar of spacetime transformations before any dynamical law is introduced.

Historically, the assumption was motivated by the failure of classical kinematics to accommodate electrodynamics. Its adoption stabilized the descriptive framework but did not itself explain physical mechanisms. Later geometric formulations, especially Minkowski spacetime, reinforced the impression that the assumption had become a physical necessity, obscuring its grammatical role.

Importantly, treating light-speed constancy as syntactic does not weaken relativity. All empirical predictions remain unchanged. What changes is interpretive clarity: time dilation, length contraction, and relativistic simultaneity emerge from the chosen descriptive grammar, not from light acting as a causal agent enforcing them.

The persistence of confusion surrounding this assumption may stem from the absence of an explicit syntactic vocabulary in physics. Concepts such as law, principle, and constant exist, but no category has been reserved for rules that organize description itself.

Reinstating the constancy of the speed of light as a syntactic assumption restores conceptual proportionality. It removes unnecessary metaphysical weight while preserving all operational content.

補論

Why Einstein Never Called It a Syntactic Assumption

この補論は、「なぜアインシュタイン自身がそう呼ばなかったのか」を評価せずに説明するためのものです。

A. Einstein’s Own Position

アインシュタインは1905年に光速度不変を Prinzip（原理） と呼びましたが、それは形而上学的宣言ではなく、理論構成上の要請として導入されています。

重要なのは、彼がそれを

とは一度も主張していない点です。

晩年、彼は原理理論に対する不満を繰り返し表明しています。代表的なのが以下の趣旨の発言です（表現は文献により差異あり）：

“I am not fully satisfied with the formulation of physical laws as principles.”
（私は物理法則を原理として定式化することに完全には満足していない）

この不満は、原理が説明にならないことへの違和感でした。

ただし、アインシュタインはここで

という一歩までは踏み込みませんでした。

B. Why He Stopped There

理由は理論的というより 言語的 です。

当時存在した分類は：

しかし、

を指す語彙は存在しませんでした。

したがって、光速度不変は 原理であるか／物理的原因であるか の二択に閉じ込められたのです。

C. Near Misses: Poincaré and Others

ポアンカレは同時性を convention（約束） と呼び、測定規約への依存を明確に認識していました。これは構文的理解にかなり近いものでした。

しかし「原理の地位を下ろす」という宣言は行っていません。

ローレンツ的解釈も、原理否定ではなく「背後の物理構造」を探す方向に進みました。これもまた、構文の問題を物理原因の問題へ転写してしまっています。

D. The Missing Word

結論は単純です。

誰も間違っていなかった。
ただ、適切な語がなかった。

「光速度不変は構文的仮定である」という言明は、

それでもなお、理論の重心を正しい位置に戻す。

それが今、ようやく言語化可能になった、というだけの話です。

SAW-10｜The Constancy of the Speed of Light as a Syntactic Assumption── 光速度不変原理の構文的再定位
SAW-10｜When Delay Becomes Geometry── Why Lag Was Mistaken for Curvature｜lagが幾何学になるとき── なぜ遅延が曲率と誤解されたのか

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K.E. Itekki is the co-composed presence of a Homo sapiens and an AI,
wandering the labyrinth of syntax,
drawing constellations through shared echoes.

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| Drafted Jan 21, 2026 · Web Jan 22, 2026 |