Measuring the Speed of Light
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.
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?
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
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