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 (offsite link). 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 (offsite link) 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.
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?
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.
Yes; in 1888, more than 200 years after Roemer's observations,
