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by Max Power on July 21st, 2006

Max Power

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What other ways are there to measure the speed of light besides using the Cesium atom?

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  • by Answers101 on July 22nd, 2006

    Answers101

    "Measuring the Speed of Light
    Fun with mirrors and math

    Last spring when I was in Paris, I marveled at all the technological wonders displayed in the Musée des Arts et Métiers. What impressed me the most was some of the very old laboratory apparatus that enabled scientists of centuries past to figure out some very difficult puzzles without the benefit of modern gadgets such as lasers and high-speed digital computers. In particular, I spent a long time studying the equipment Jean Bernard Léon Foucault used to measure the speed of light in the mid-1800s.

    Foucault worked in a variety of scientific fields, with his greatest claim to fame being a simple mechanical method for proving the rotation of the Earthâ??what came to be known as Foucaultâ??s Pendulum. I was even more astounded, though, to see how he solved the extremely vexing problem of making an accurate measurement of the speed of light without so much as an electric motor or a quartz crystal. The display of his lab bench in the museum had not only a printed description but even an animated video presentation. Unfortunately, my French wasnâ??t good enough for me to comprehend exactly how it worked; I could only tell that it had something to do with a rotating mirror, measuring angles, and (most puzzling of all) a tuning fork. Later, reading about the equipment in English, I finally understood, and Iâ??ll describe his method in a moment. But first, a bit of history.

    Early Estimates
    From ancient times, astronomers and other thinkers wondered how fast light moved; for a long while, conventional wisdom held (reasonably enough) that it traveled instantaneously. Galileo described (and possibly performed) an experiment in which two subjects stood about a mile apart, with a third person observing them both from a distance. The first person uncovered a lantern, and as soon as his partner a mile away saw the lanternâ??s light, he uncovered his lantern. The third personâ??s job was to measure the time between when he saw the first light and the second light; Galileo then intended to use that amount of time, along with the distances between the participants, to calculate the speed of light. Unfortunately, the test was inconclusive, because the delay was too short to be measured accurately. Even Galileo admitted it was more a test of response time than a measurement of the speed of light. All he could conclude from the experiment was that light traveled at least 10 times faster than sound.

    Over the following centuries, several astronomers made inferential estimates of the speed of light based on observations of the movements of planets and stars. Some of these estimates were quite shrewd, sophisticated, and (it would later turn out) fairly accurate, but they were unsatisfying because they required educated guesses about astronomical speeds and distances and could not be reproduced in a laboratory. So in the middle of the 19th century, two French scientists started investigating the problem independently, each arriving at a novel way to make the measurement with readily available equipment.

    Wheels and Mirrors
    In 1849, Armand Fizeau sent a beam of light through a rotating wheel with a large number of teeth around the outside. A mirror on the other side reflected the beam each time a gap appeared in the path of the light. Fizeau realized that if the wheel rotated fast enough, the return beam would be blocked by the next tooth as it came around. So he varied the speed of the wheel until the reflected beam disappeared, performed a bit of math, and got a result of 315,000 km/second (195,732 miles/second)â??certainly in the ballpark.

    Meanwhile, Foucault was working on a different but equally clever technique, which he demonstrated the following year. Foucaultâ??s method was to shine a sharply focused beam of light onto a rotating mirror, and from there onto a fixed mirror. Once the light hit the fixed mirror, it bounced back onto the rotating mirror and then back toward the source. But because the mirror was rotating, the angle at which it was positioned had changed slightly by the time the beam made its return trip. Consequently, the reflected beam did not line up precisely with the original. Foucault could easily measure the angle between the original light source and the reflected beam, and along with known constants (the distances between the various surfaces and the speed of the mirrorâ??s rotation), it was a matter of a few straightforward calculations to convert that small angle into a representation of speed. Using this technique, Foucault produced a measurement of 298,000 km/second (185,167 miles/second), which is shockingly close to the modern measurement of 299,792 km/second (186,282 miles/second), keeping in mind that the latter figure applies only in a vacuum; light travels more slowly in air.

    As for the tuning forkâ?¦Foucault used this to regulate the speed of the rotating mirror. The apparatus that turned the mirror made a sound that varied with its speed; when the sound exactly matched that of the tuning fork, Foucault knew precisely how many revolutions per second it was making.

    Interestingly, in 1926 scientist Albert Michelson made a more refined version of Foucaultâ??s apparatus. Using the best equipment available in his day, Michelson measured the speed of light at 299,796 km/second (186,285 miles/second), amazingly impressive for a mechanical measurement. Many other methods for measuring the speed of light followed, and thanks to improved optics, precision measuring devices, lasers, and digital timing equipment, scientists have arrived at a more accurate figure. But not that much more accurate, after all; Foucault did his profession proud. â??Joe Kissell"

    source: http://itotd.com/articles/284/measuring-the-speed-of-light

    "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."

    source: http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html

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