The Speed of Light — What It Is and Why Nothing Can Go Faster

The speed of light — approximately 299,792,458 metres per second, or roughly 300,000 km/s — is one of the most fundamental constants in physics. It's the speed at which light travels through a vacuum, and according to Einstein's theory of relativity, it's the absolute speed limit of the universe.

Putting 300,000 km/s in Context

At the speed of light, you could:

• Circle the Earth 7.5 times in one second
• Travel from Earth to the Moon in 1.3 seconds
• Reach the Sun in 8 minutes 20 seconds
• Travel one light-year (the distance light covers in a year) in — one year
• Reach the nearest star system, Alpha Centauri, in 4.2 years

The nearest large galaxy, Andromeda, is 2.5 million light-years away. Even at the speed of light, getting there would take 2.5 million years. The observable universe is about 93 billion light-years in diameter. These distances are why interstellar travel remains largely in the realm of science fiction.

Why Can't Anything Go Faster?

Einstein's special relativity explains this. As an object with mass accelerates, its relativistic mass increases, requiring more and more energy to accelerate further. To reach the speed of light, an object with mass would require infinite energy — which is physically impossible.

Light itself can travel at c because photons — the particles of light — have zero rest mass. Anything with mass can only approach c asymptotically, never reaching it.

Time Dilation — The Stranger Consequence

As speed increases toward c, time passes more slowly for the moving object relative to a stationary observer. A hypothetical astronaut travelling at 99% the speed of light for what feels like 10 years would return to find that roughly 70 years had passed on Earth. This is not science fiction — GPS satellites, moving fast enough relative to Earth, experience measurable time dilation and their clocks are corrected to account for it.

The Speed of Light in Other Media

Light only travels at c in a vacuum. In other materials, it slows down. The ratio of c to the speed of light in a medium is that medium's refractive index. In water, light travels at about 75% of c; in glass, about 67%. This slowing and bending of light is what produces refraction — the reason a straw appears bent in a glass of water.