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Monday 23 April 2018

# How do you measure the speed of light?

Dr Justin Donnelly Everyone knows that after a flash of lightning, there's a delay before you hear the crack and rumble of thunder.

This is because light travels much faster than sound. But even light doesn't reach us instantly. It travels at about 300,000km per second.

We don't notice it in everyday life, but this lag becomes noticeable when the distances are large. It takes eight minutes for the light from the Sun to reach us, so we're seeing it as it looked eight minutes ago. Some stars are so far away that we're seeing them as they looked hundreds of years ago. The limited speed of light acts like a time machine -- the further away something is, the further back in time we're looking.

Many different ways of measuring the speed of light have been attempted.

In 1676, Danish astronomer Ole Romer noticed that the moons orbiting Jupiter were not where his calculations predicted they would be. What's more, the error got bigger when Jupiter was further away. Actually, the calculations were right -- they just didn't take into account the time it took for light to travel from Jupiter to Earth (about 42/42 minutes on average). Sure enough, when Romer waited, the moon eventually ended up where he predicted.

Around 1849, French physicist Fizeau shone a beam of light through the teeth of a rotating wheel. The beam would pass one side of a tooth, and by the time it bounced off a mirror kilometres away the wheel would have rotated slightly so the beam would return on the other side of the tooth. Knowing the distance the light travelled and how quickly the wheel must spin to allow this to happen gave a value for the speed of light.

Today, there are a variety of methods used to measure the speed of light (lasers have proven particularly useful as stable light sources). In 1983, it was decided to define the speed of light as 299,792.458km per second and more effort is now put into measuring distance and time more accurately.

Irish Independent Supplement