What Is the Speed of Light?

Why Is the Speed of Light So Important?

• Universal speed limit: Nothing with mass can accelerate to c. As objects approach it, their energy requirement grows without bound (special relativity).
• Defines the metre: Since 1983, the metre is defined as the distance light travels in vacuum in 1/299,792,458 of a second.
• E = mc²: The speed of light links mass and energy. A small amount of mass contains an enormous amount of energy because c² is so large.
• Causality: The finite speed of light means information has a maximum travel speed, which preserves cause-and-effect ordering in the universe.
• Astronomy: We measure cosmic distances in light-years (the distance light travels in one year ≈ 9.46 × 10¹² km).

How Was the Speed of Light Measured?

• 1676 — Ole Rømer: First quantitative estimate (~220,000 km/s) from timing eclipses of Jupiter's moon Io.
• 1849 — Hippolyte Fizeau: Toothed-wheel method gave ~315,000 km/s.
• 1862 — Léon Foucault: Rotating-mirror method refined the value to ~298,000 km/s.
• 1879 — Albert Michelson: Improved rotating-mirror experiments gave 299,910 km/s — remarkably close to the modern value. He won the 1907 Nobel Prize.
• 1972 — Laser interferometry: The most precise measurement: 299,792,458 ± 1.2 m/s, which became the defined value in 1983.

Speed of Light in Different Media

Light slows down when passing through transparent materials. The ratio c/v is the refractive index (n) of the medium:

• Vacuum: n = 1.000 (299,792 km/s)
• Air: n = 1.0003 (299,702 km/s)
• Water: n = 1.33 (≈225,000 km/s)
• Glass: n ≈ 1.5 (≈200,000 km/s)
• Diamond: n = 2.42 (≈124,000 km/s)

Light Travel Times

• Moon → Earth: ~1.3 seconds
• Sun → Earth: ~8 min 20 sec
• Sun → Jupiter: ~43 minutes
• Nearest star (Proxima Centauri): ~4.24 years
• Across the Milky Way: ~100,000 years
• Nearest large galaxy (Andromeda): ~2.5 million years

People Also Ask

Why can nothing travel faster than light?

Einstein's special relativity shows that the energy required to accelerate a massive object grows toward infinity as it approaches c. At exactly c, infinite energy would be needed. Only massless particles (photons, gluons) travel at c — and they can never go slower.

Does light slow down in water or glass?

Yes. The effective speed of light decreases in a medium due to absorption and re-emission by atoms. The refractive index n = c/v tells you the slowdown factor. Individual photon-atom interactions still happen at c between absorptions.

Is the speed of light the same for all observers?

Yes — this is Einstein's second postulate. No matter how fast you move, you will always measure light in vacuum at exactly c. This counterintuitive fact leads to time dilation and length contraction.

Can space expand faster than light?

Yes. The expansion of space itself is not limited by c. Galaxies beyond the observable universe are receding from us faster than light due to cosmic expansion. No information or matter is travelling through space faster than c — space itself is stretching.

What is a light-year?

A light-year is the distance light travels in one year: approximately 9.461 × 10¹² kilometres (5.879 × 10¹² miles). It is a unit of distance, not time. The nearest star system, Alpha Centauri, is about 4.37 light-years away.