Speed of Light

“Twelve million miles a minute, and that’s the fastest speed there is” – or so says this song from Monty Python’s the Meaning of Life. In fact, it’s closer to 299,792.458 km/s (186,000 miles/s), or more conveniently, 3 x 108 m/s. If you flew around the world at the speed of light (in your spaceship), you would circle the globe about 7.5 times per second. And yet, the nearest star to Earth (other than our sun), Alpha Centauri, is 4.37 light years away, meaning it takes light 4.37 years to cross that distance. In fact, although the speed of light is incomprehensibly fast, it’s slow on the scale of the known universe, which is about 93 billion light-years across!

Sunlight takes about 8 minutes and 17 seconds to travel the average distance from the surface of the Sun to the Earth.

But the speed of light is not just about interstellar distances. It matters, practically speaking, since it’s light that holds together all matter as electromagnetic force. And it’s good that it’s so fast because otherwise, our atoms wouldn’t bind together so closely, and life would be impossible.

Now, the speed of light has further significance for several reasons:

  1. It’s also the speed of every other fundamental force – the speed of all massless particles, which include not only photons, but also the W and Z bosons of the electroweak force, the gluons of the strong force, and hypothetically, the gravitons of gravity.

  2. It is the constant that interrelates space and time into Einstein’s space-time. In Einstein’s theory, the speed of light is the same for all observers, while space and time change as you accelerate. If you are traveling 99% the speed of light and you turn on your headlights, they work just fine from your point of view. From an outside observer’s point of view, they just crawl out of your headlamps.

  3. It’s the number that interrelates mass with energy – E=mc2, where c is the speed of light, m is mass, and E is energy. So, the speed of light is involved in the conversion of mass into energy which makes nuclear weapons possible.

Before the seventeenth century, most people believed that light is transmitted instantaneously. Galileo doubted this and planned to measure it by signaling with lanterns a few miles apart. But light is too fast for that to work. The first person to measure “c” was Ole Rømer in 1676. He had noticed a difference of up to 1000 seconds between the times predicted for eclipses of Jupiter's moons and the times the eclipses were observed. He realized this could be due to the varying distance between Earth and Jupiter and calculated that “c” was 214,000 km/s, which wasn’t bad considering that the distances between planets were uncertain at that time.

Ole Rømer, portrait by Jacob Coning from c. 1700

In 1728, James Bradley measured “c” using “stellar aberration” – changes in the apparent positions of stars over the year due to the way Earth’s orbit around the sun makes it intercept their light rays at different angles. He calculated 301,000 km/s.

In 1849, Armand Fizeau measured it by passing light through a spinning toothed wheel to a mirror 8km away and reflecting it back through the wheel. He got 315,000 km/s. Leon Foucault improved on this result a year later using rotating mirrors, which gave 298,000 km/s.

Measurements improved in the next century using atomic clocks to time laser beams, giving us the number quoted at the beginning of this article, which was fixed as a definition during the 1970s.

But perhaps it’s more important to understand the remarkable role “c” plays in physical law as the speed at which time stops and mass cannot exist – the conversion factor between mass and energy and time and space.


“Today, we commit to this next great leap into the cosmos because we're human, and our nature is to fly.” – Stephen Hawking

“We sometimes hear of things that can travel faster than light. Something called 'the speed of thought' is occasionally proffered. This is an exceptionally silly notion especially since the speed of impulses through the neutrons in our brain is about the same as the speed of a donkey cart.” – Carl Sagan, Cosmos

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