Why is C the symbol for the speed of light? (2004)

Original Article ↗ Hacker News Discussion ↗

Origin of the symbol c

  • Several comments restate the article’s main answer: c comes from Latin celeritas (“speed”), surviving in English as “celerity.”
  • A quote from Asimov is cited saying explicitly that c is for celeritas.
  • Others note that this notation predates Einstein and that c was originally used as the “Lorenz constant,” not specifically “speed of light.”
  • Some mention related usages like (c_s) for speed of sound and shared Latin roots in “acceleration” and “celebrity.”

Competing folk explanations and myths

  • Some people were taught that c stands for “constant” (because the speed of light is constant in all reference frames); the thread notes this is a good story but historically incorrect.
  • Others propose “speed of causality” as a backronym; this is acknowledged as neat but historically false.
  • There’s recognition that many letter choices in physics/math are arbitrary conventions that stuck once influential authors used them.

Causality, relativity, and what c really represents

  • Several comments stress that c is more fundamental than “speed of light”: it’s the maximum propagation speed of changes in fundamental fields (“speed of causality” in a loose sense).
  • Discussion touches on photons having zero proper time and the idea of “trading” motion through time for motion through space as speed approaches c.
  • Others correct over-simplifications: special relativity is undefined at (v = c) for massive objects, photons don’t have a well-defined rest frame or 4‑velocity, and “photon’s POV” is not strictly meaningful.
  • Quantum entanglement is raised; replies invoke the no‑communication theorem to reconcile it with finite c.
  • A more technical sidebar debates whether “causality” is a fundamental axiom (Einstein causality in QFT) or just a consequence/label for constraints already in the equations.

Numeric value and units

  • One subthread asks why c has its particular numerical value; responses emphasize that:
    • In natural units, the speed of light is just 1; meters and seconds are human conventions.
    • Questions about “why this value” rapidly connect to fine‑tuning and anthropic arguments, with some disagreement over how tightly constrained it really is.

Notation, typography, and off-topic jokes

  • Multiple comments insist it should be lowercase c, not C; some mention the ideal italic Unicode symbol.
  • Side discussion about title case in English headlines and its annoyances.
  • Off-topic but related notation: why “m” for slope, alternative forms like (y = ax + b), and language-specific coincidences (e.g., Basque word for slope).
  • Many jokes play on C the programming language, Roman numeral C = 100, “C is for cookie,” simulation jokes, and “wrong answers only,” reflecting a generally playful tone alongside the serious physics discussion.