Show HN: What if the speed of light was 5 km/h?
A web-based relativity simulator that slows the speed of light to walking pace prompts people to explore how everyday perception would change at relativistic speeds. Commenters compare it to earlier educational games, debate whether the underlying physics is modeled correctly, and note limitations such as missing acceleration effects and mobile controls. Many also point out that truly reducing the fundamental speed of light would radically alter gravity, chemistry, and even the existence of stars and planets, so the simulation is best seen as a visual thought experiment rather than a physically consistent universe.
Relativity and “speed through time”
- Commenters debate the popular slogan “everything moves through spacetime at speed c.”
- Critiques: “speed through time” lacks consistent units; comparing clock rates gives a dimensionless ratio, not a speed. The triangle picture (space speed vs time speed with hypotenuse c) is called internally inconsistent.
- Others emphasize: despite slogan issues, it roughly captures that motion through space and rate of proper time are linked; time is relative across frames.
What if c ≈ 5 km/h? Physical plausibility
- Several note that simply “slowing light” while holding other constants fixed breaks physics.
- With c reduced by ~10⁸, Schwarzschild radii grow as c⁻²: Earth, Sun, and even small rocks would be black holes unless G or other constants also changed.
- Atomic structure, chemistry, and biochemistry would be radically different; human-scale organisms and current electronics likely couldn’t exist.
- Some argue our units (meter defined via c) make “changing c” a counterfactual; what really changes are other dimensional constants. Others reply you could adjust G or similar while holding dimensionless constants fixed.
Scope and correctness of the demo
- Multiple commenters infer the demo effectively keeps physics the same but scales visual relativistic effects to human speeds.
- Some praise it as more accurate than earlier games, especially in modeling temporal aspects of Doppler shift.
- Others are skeptical because the code was largely LLM-generated (“vibecoded”), worrying about hidden math errors and preferring source prompts over JS output.
- Technical critique: acceleration is implemented via simple velocity addition, so effects like Wigner rotation and proper accelerated-frame time behavior are missing.
Comparisons, related work, and educational value
- Thread references earlier relativity games and visualizations, including ones that gradually lower c or use puzzle mechanics to teach time dilation, length contraction, and aberration.
- Several recommend sci‑fi and popular‑science books that imagine altered light-speed or physics regimes.
- Many see strong pedagogical potential and request in-simulation explanations, mobile controls, and more interactive experiments.
Perception, latency, and scale
- Commenters reflect on light being “slow” for global multiplayer latency but “fast” relative to cosmic scales.
- Analogies (bacteria in humans, nested self‑regulating systems, universe as computation) arise as people contemplate limits of perception and fundamental “compute” of reality.