--- title: "What would happen if the speed of light were slower?" description: "Chemistry, nuclear fusion and the mass-energy relationship would all change — stars would not work as they do. The mechanism, what follows in what order, and how con" url: https://atoneplace.net/what-happens-if/speed-of-light-slower/ site: "At One Place" --- # What would happen if the speed of light were slower? **What would happen if the speed of light were slower?: Chemistry, nuclear fusion and the mass-energy relationship would all change — stars would not work as they do.** Speculative The speed of light is not just a traffic limit; it appears throughout physics as a conversion factor between space and time and between mass and energy. Lower it and the energy released by nuclear fusion falls with the square of the change, since E = mc², so the Sun would produce far less energy from the same fuel and stellar structure would be entirely different. The fine-structure constant, which governs the strength of electromagnetic interaction and therefore the size and energy levels of atoms, is inversely proportional to c, so chemistry itself would change: bond energies, molecular structure and the colours of everything. Relativistic effects would become part of ordinary life, with noticeable time dilation and length contraction at everyday speeds if c were low enough. This is marked speculative because c is entangled with the constants that the reasoning uses, so changing it changes the rules mid-argument — physicists generally regard only dimensionless constants such as the fine-structure constant as meaningfully variable. ## How confident this is This is speculation. The scenario is far enough outside anything observed that the answer depends heavily on which assumptions you start from — treat it as a thought experiment, not a prediction. ## Physics - [What would happen if an asteroid hit the Earth?](https://atoneplace.net/what-happens-if/asteroid-hit/) — It depends enormously on size — most burn up, a 20-metre one broke windows across a city, and a 10-kilometre one ended the dinosaurs. - [What would happen if gravity disappeared?](https://atoneplace.net/what-happens-if/gravity-disappeared/) — The Earth itself would come apart — gravity is the only thing holding a planet together. - [What would happen if the Sun became a black hole?](https://atoneplace.net/what-happens-if/sun-became-a-black-hole/) — Earth's orbit would not change at all — the mass would be the same. It would simply go dark and cold. - [What would happen if you fell into a black hole?](https://atoneplace.net/what-happens-if/fall-into-a-black-hole/) — Into a small one, tidal forces would stretch you apart before you arrived; into a supermassive one, you would cross the horizon without noticing. - [What would happen if you fell into a hole through the Earth?](https://atoneplace.net/what-happens-if/hole-through-the-earth/) — You would oscillate from one side to the other and back, with a one-way trip of about 38 minutes. - [What would happen if you travelled at the speed of light?](https://atoneplace.net/what-happens-if/travelled-at-light-speed/) — You cannot — accelerating any mass to light speed requires infinite energy. Approaching it, time dilates dramatically. - [What would happen if gravity doubled?](https://atoneplace.net/what-happens-if/gravity-doubled/) — You would weigh twice as much, most tall structures would need rebuilding, and no bird larger than a sparrow could fly. - [What would happen if you were shrunk to the size of an ant?](https://atoneplace.net/what-happens-if/shrunk-to-ant-size/) — The square-cube law would break you: you would freeze, suffocate, and be unable to drink from a surface of water. - [What would happen if a gamma-ray burst hit the Earth?](https://atoneplace.net/what-happens-if/gamma-ray-burst/) — A nearby one aimed at us would strip a large fraction of the ozone layer within seconds, with the damage arriving as ultraviolet over the following years. ## Common questions ### What would happen if the speed of light were slower? Chemistry, nuclear fusion and the mass-energy relationship would all change — stars would not work as they do. ### How certain is that? Speculative. This is speculation. The scenario is far enough outside anything observed that the answer depends heavily on which assumptions you start from — treat it as a thought experiment, not a prediction. ## Related pages - [What would happen if an asteroid hit the Earth?](https://atoneplace.net/what-happens-if/asteroid-hit/) - [What would happen if gravity disappeared?](https://atoneplace.net/what-happens-if/gravity-disappeared/) - [What would happen if the Sun became a black hole?](https://atoneplace.net/what-happens-if/sun-became-a-black-hole/) - [What would happen if you fell into a black hole?](https://atoneplace.net/what-happens-if/fall-into-a-black-hole/) - [What would happen if you fell into a hole through the Earth?](https://atoneplace.net/what-happens-if/hole-through-the-earth/) - [What would happen if you travelled at the speed of light?](https://atoneplace.net/what-happens-if/travelled-at-light-speed/) - [What would happen if gravity doubled?](https://atoneplace.net/what-happens-if/gravity-doubled/) - [What would happen if you were shrunk to the size of an ant?](https://atoneplace.net/what-happens-if/shrunk-to-ant-size/) - [What happens if you stop eating sugar?](https://atoneplace.net/what-happens-if/stop-eating-sugar/) - [What would happen if the Earth stopped spinning?](https://atoneplace.net/what-happens-if/earth-stopped-spinning/) - [What happens if you stop drinking alcohol?](https://atoneplace.net/what-happens-if/stop-drinking-alcohol/) - [What happens if you stop smoking?](https://atoneplace.net/what-happens-if/stop-smoking/) - [What would happen if the Sun went out?](https://atoneplace.net/what-happens-if/sun-went-out/) - [What happens if you stop eating meat?](https://atoneplace.net/what-happens-if/stop-eating-meat/) ## Sources - Guide for the Use of the International System of Units (SP 811) — NIST (https://www.nist.gov/pml/special-publication-811) [Public domain (US Government)]