Guide The Speed of Light

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Early scientists, unable to perceive light's motion, thought it must travel instantaneously. Over time, however, measurements of the motion of these wave-like particles became more and more precise. Thanks to the work of Albert Einstein and others, we now understand light speed to be a theoretical limit: That doesn't stop sci-fi writers, and even some very serious scientists, from imagining alternative theories that would allow for some awfully fast trips around the universe.

The first known discourse on the speed of light comes from the ancient Greek philosopher Aristotle, who penned his disagreement with another Greek scientist, Empedocles. Empedocles argued that because light moved, it must take time to travel. Aristotle, believing light to travel instantaneously, disagreed. In , the Italian astronomer Galileo Galilei stood two people on hills less than a mile apart, each holding a shielded lantern.


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One uncovered his lantern; when the second saw the flash, he uncovered his, as well. By observing how long it took for the light to be seen by the first lantern-holder and factoring out reaction times , he thought he could calculate the speed of light. Unfortunately, Galileo's experimental distance of less than a mile was too small to see a difference, so he could only determine that light traveled at least 10 times faster than sound. He determined that light took time to travel from Io to Earth. The eclipses lagged the most when Jupiter and Earth were farthest apart, and were on schedule as they were closer.

He concluded that light took 10 to 11 minutes to travel from the sun to Earth, an overestimate since it in fact takes eight minutes and 19 seconds. In , English physicist James Bradley based his calculations on the change in the apparent position of the stars due Earth's travels around the sun. Two attempts in the mids brought the problem back to Earth.

French physicist Hippolyte Fizeau set a beam of light on a rapidly rotating toothed wheel, with a mirror set up 5 miles away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the adjacent mirror, and back through the gap.

Another French physicist, Leon Foucault, used a rotating mirror rather than a wheel.

How Fast Does Light Travel? | The Speed of Light

The two independent methods each came within about 1, miles per second of the speed of light measured today. Prussian-born Albert Michelson, who grew up in the United States, attempted to replicate Foucault's method in , but used a longer distance, as well as extremely high-quality mirrors and lenses.

An interesting footnote to Michelson's experiment was that he was trying to detect the medium that light traveled through, referred to as luminiferous aether. Instead, his experiment revealed the aether didn't exist. The pulse may look like a straight beam but actually behaves like waves of light particles. The light can leave the chamber before it has finished entering because the cesium atoms trade energy with the leading edge of the waves as they pass through.

This produces an almost identical light pulse that exits the chamber and travels about 60 feet before the main part of the laser pulse finishes entering the chamber, Wang said. Wang said the effect is possible only because light has no mass; the same thing cannot be done with physical objects. The Princeton experiment and others like it test the limits of the theory of relativity that Albert Einstein developed nearly a century ago.

According to the special theory of relativity, the speed of particles of light in a vacuum, such as outer space, is the only absolute measurement in the universe.

Measuring the Speed of Light with Jupiter’s Moons

The speed of everything else — rockets or inchworms — is relative to the observer, Einstein and others explained. Only in the Lab In everyday circumstances, an object cannot travel faster than light. The Princeton experiment and others change these circumstances by using devices such as the cesium chamber rather than a vacuum. Ultimately, the work may contribute to the development of faster computers that carry information in light particles.

Einstein and special relativity

Aephraim Steinberg, a physicist at the University of Toronto, said the light particles coming out of the cesium chamber may not have been the same ones that entered, so he questions whether the speed of light was broken. Still, the work is important, he said: Not everyone is convinced the NEC scientists did what they claim. Speed of Light No Longer the Limit. Apple CEO defends pricing of new iPhones. Suppose that I measure a particular light signal's speed, and find the usual value of , Not so, according to special relativity! Simply subtracting speeds would only give the correct answer if the observer on that space-ship measured space and time, distance and duration in the same way that I do.

Early Ideas about Light Propagation

As we have seen on the previous page, that's not the case. From my point of view, for instance, the measuring rods on the speeding spaceship are shorter than my own, and its clocks run more slowly than mine.

The Speed of Light

Taken together, all of these relativistic effects combine in precisely the right way to result in a surprising phenomenon: This phenomenon is far from coincidental. In fact, it is a systematic feature of special relativity. All the different relativistic effects combine, quite generally, so that the following postulate holds true: The speed of light is the only speed that is, in this sense, independent of the observer and thus absolute. It also plays the lead role in all of special relativity.

First of all, it defines the absolute speed limit for the transfer of energy, matter and information.