Speed of Sound & Light

Sound is a mechanical wave that travels by vibrating particles. In solids, particles are packed tightly together with strong intermolecular bonds, so vibrations transfer much faster. In air, molecules are far apart and loosely coupled, making energy transfer slower. The speed of sound in steel (5960 m/s) is about 17 times faster than in air (343 m/s).

According to Einstein's Special Theory of Relativity, no object with mass can reach or exceed the speed of light in vacuum (c ≈ 3×10⁸ m/s). As an object approaches c, its mass increases and the energy required approaches infinity. However, the universe's expansion can cause distant galaxies to recede faster than light — this is not movement through space but the expansion of space itself.

The speed of sound in air increases with temperature. The approximate formula is: v ≈ 331 + 0.6T m/s, where T is temperature in °C. At 0°C: 331 m/s. At 20°C: 343 m/s. At 100°C: 391 m/s. Higher temperature means air molecules move faster, transferring vibrations more quickly.

A light-year is the distance light travels in one year — approximately 9.461 × 10¹⁵ metres (about 9.46 trillion km). It is a unit of distance, not time, despite the word 'year' in the name. When we observe a star 100 light-years away, we see it as it was 100 years ago — we are literally looking back in time.

The speed of light in vacuum is now defined as exactly 299,792,458 m/s — it is a fixed constant that defines the metre. Historically, Ole Rømer first estimated it in 1676 by observing delays in Jupiter's moon eclipses. The metre is now defined as the distance light travels in 1/299,792,458 of a second.