Physics 2000 Science Trek Electromagnetic Waves

Measuring the Speed of Light

How has the speed of light been measured?

That's a very good question. In the early 17th century, many scientists believed that there was no such thing as the "speed of light"; they thought light could travel any distance in no time at all. Galileo disagreed, and he came up with an experiment to measure light's velocity: he and his assistant each took a shuttered lantern, and they stood on hilltops one mile apart. Galileo flashed his lantern, and the assistant was supposed to open the shutter to his own lantern as soon as he saw Galileo's light. Galileo would then time how long it took before he saw the light from the other hilltop.

And then he could just divide the distance by the time to get the speed. Did it work? Nope. The problem was that the speed of light is simply too fast to be measured this way; light takes such a short time (about 0.000005 seconds, in fact) to travel one mile that there's no way the interval could have been measured using the tools Galileo had.

So what you'd need is a really long distance for the light to travel, like millions of miles. How could someone set up an experiment like that?

Well...during the 1670's, the Danish astronomer Ole Roemer was making extremely careful observations of Jupiter's moon Io. The black dot is Io's shadow. Io makes one complete orbit around Jupiter every 1.76 days; the time it takes to make each orbit is always the same, so Roemer expected that he could predict its motion quite precisely. To his astonishment, he discovered that the moon didn't always appear where it was supposed to be. At certain times of the year, it seemed to be slightly behind schedule; at other times, it was slightly ahead.

Hubble Space Telescope image of Jupiter, its satellite Io and Io's shadow

That's weird. Why would it orbit more quickly at some times and more slowly at others?

That's exactly what Roemer wondered, and no one could think of any plausible answer. Roemer did notice, however, that Io seemed to be ahead of its predicted orbit when the earth was closer to Jupiter, and behind when it was farther away...

This has got to have something to do with the speed of light, but I don't quite see how it all fits together.

Well, think about this: if light doesn't travel infinitely fast, then it must take some amount of time to get from Jupiter to earth. Let's say it takes an hour. Then when you look at Jupiter through a telescope, what you're actually seeing is light that left an hour ago--so you're seeing what Jupiter and its moons looked like one hour in the past.

Wait a second--I think I see where this is going. When Jupiter was farther away, light would take even longer to get from there to here, so that Roemer was seeing Io as it had been at an even earlier time than usual--maybe an hour and fifteen minutes ago, instead of an hour. And the opposite would happen when Jupiter and the earth were especially close together. So Io wasn't changing its orbit at all; it would just appear to be in different places depending on how long its light had taken to get here.

Very good! Now, knowing how much Io's timing seemed to change and how much the distance from earth to Jupiter varied, Roemer was able to calculate a value for the speed of light. The number he came up with was about 186,000 miles per second, or 300,000 kilometers per second.

In the years that followed, as better equipment and techniques were developed, many other people were able to measure the speed of light more accurately. With the resources of today's technology, we can measure it to an incredibly high precision. For instance, astronauts have attached a mirror to a rock on the moon; scientists on earth can aim a laser at this mirror and measure the travel time of the laser pulse--about two and a half seconds for the round trip. (The idea behind this experiment is not so different from Galileo's, if you think about it...) And anyone who measures the speed of light, at any time, using any method, always gets the same result: just slightly less than 300,000 kilometers per second.

Other kinds of electromagnetic radiation, like radio waves and microwaves, are supposed to travel at the same speed as light. Has their speed been measured also?

Yes; in 1888, more than 200 years after Roemer's observations, Heinrich Hertz generated some electromagnetic waves in his laboratory. He measured their speed and came up with that familiar number, 300,000 kilometers per second--a very strong piece of evidence that light and electromagnetic radiation are the same thing.