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The fastest thing

Space and time · level 3, lesson 2 of 6

A question to start

If you could ride on a beam of light, how many times could you go round the Earth in one second?

The idea

Light in empty space travels at exactly 299,792,458 metres each second. That is almost 300,000 km each second. Nothing that carries information goes faster.

To find how long light takes to go somewhere:

\[\text{time (seconds)} = \frac{\text{distance (km)}}{300{,}000}.\]

The Earth is about 40,000 km round, so in one second light could go round it about 7 and a half times.

Radio waves, the signals that carry messages to spacecraft, are a kind of light. They travel at the same speed.

in one second:7 and a half timesthe Earth

Worked examples

1. Round the Earth. \(300{,}000 \div 40{,}000 = 7.5\) times in a second.

2. Canada, coast to coast. About 5,000 km: \(5{,}000 \div 300{,}000 \approx 0.017\) seconds.

3. A light-second. In one second light goes 300,000 km. That distance is called a light-second.

Try it

  1. About how fast does light travel?
  2. Do radio waves go faster or slower than light?
  3. How long does light take to go 600,000 km?
  4. How long does light take to go 3,000 km?
Answers
  1. About 300,000 km each second.
  2. The same speed: radio waves are a kind of light.
  3. \(600{,}000 \div 300{,}000 = 2\) seconds.
  4. \(3{,}000 \div 300{,}000 = 0.01\) seconds.

Common mistake

"Light is instant." For a room or a town, it might as well be. But across space, the time light takes becomes seconds, minutes and even years.

Where it leads