How did we measure the speed of light?
Is light infinitely fast?
You switch on a lamp and the whole room is lit at once. For a long time people thought light didn’t travel at all, that it was simply everywhere at the same moment. How can you measure whether it travels at all?
The lantern experiment
Galileo tried it with lanterns: two people on hills about a kilometre apart. One uncovers a lantern, and as soon as the other sees it, they uncover theirs. But all he measured was how long it takes people to react, because light covers those two kilometres there and back in seven millionths of a second.
A clock in the sky
The sky offered both a long distance and a clock. Jupiter’s moon Io goes round the planet every 42.5 hours and each time disappears for a while into its shadow. These eclipses are regular and can be predicted.
The clock runs late
In 1676 the Danish astronomer Ole Rømer noticed that the eclipses of Io weren’t always on time. When the Earth is closest to Jupiter, they come as predicted, but as the Earth moves away, they are later and later. On the far side of the Earth’s orbit the delay is about 17 minutes.
Light crosses the whole of the Earth’s orbit
Rømer understood why: light takes some time to travel from Io to us. On the far side of the orbit it has extra distance to cover: the orbit’s whole diameter, about 300 million kilometres. Rømer estimated that this takes 22 minutes; today we know it’s about 17 minutes, roughly 1,000 seconds, so light travels about 300,000 kilometres a second.
A toothed wheel
The first to measure it on Earth was Hippolyte Fizeau, in 1849. He sent light from a lamp through a gap between the teeth of a toothed wheel to a mirror 8.6 kilometres away. The mirror reflected it back, and the light returned through the same gap to his eye. But how does that tell you how fast it travels?
When the wheel spins
Then he spun the wheel. While it turned slowly, it moved only a little before the light came back, and the light passed back through the same gap. As he sped it up, the returning light hit the edge of a tooth more and more and grew fainter. At 12 and a half turns a second it vanished completely: by then the wheel had moved on by a whole tooth, and the light hit a tooth instead of a gap. Moving one tooth took the wheel 1/18,000 of a second, and in that time light travelled 17 kilometres there and back, so it travels about 310,000 kilometres a second.
Try it yourself
Microwaves are light too, just light we can’t see. Take the turntable out of a microwave oven and heat a bar of chocolate in it for 20 seconds. Microwaves bounce off the oven walls and heat most at spots exactly half a wave apart: the chocolate melts about every 6 centimetres, so a whole wave is 12 cm. A wave’s speed is its length times the number of vibrations per second, and that’s on the oven’s label: 2,450 MHz, or 2.45 billion. 12 cm × 2.45 billion is about 300,000 kilometres a second.
And today?
In the 1970s the speed of light was measured with a laser, using the same formula as with the chocolate: wavelength times the number of vibrations. It came out at 299,792,458 metres a second, and since 1983 that number has been fixed; the length of a metre is even defined by it. So today we don’t measure the speed of light, we measure with it: a laser bounced off the mirrors that astronauts left on the Moon takes 2.5 seconds there and back, and from that we know how far away the Moon is, to the centimetre.
So how did we measure it?
Light is so fast that to measure it we need a huge distance or a very precise clock. Rømer used the Earth’s orbit, with the eclipses of Io as his clock; Fizeau used 17 kilometres, with a fast-spinning wheel as his clock. Both showed that light travels at a tremendous but finite speed.