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The Speed of Light

For a long time the speed of light was too fast to measure.

Opinion was divided amongst early scientists regarding the speed of light. Some like Aristotle thought that light travelled instantaneously, while others like Al-Haytham thought it was fast, but finite.

Galileo tried to measure how fast the light from a lantern travelled from one hilltop to another and back again but failed because light was too fast—and human reaction time too slow. All Galileo could report was that light was faster than sound.

The speed of light was first measured by noticing that the moons of Jupiter took longer to appear in their predicted places when Jupiter was far from the Earth than they did when Jupiter was close. Olaf Roemer suggested this was because light took extra time to reach Earth when Jupiter was further away.

Another astronomical measurement, called stellar aberration, confirmed the finite speed of light. The speed of light was not measured on Earth until Armand Fizeau and Leon Foucault developed their experiment—essentially a refinement of Galileo's—in the early nineteenth century.

The speed of light measured by these experiments agreed closely with the speed that James Clerk Maxwell calculated for electromagnetic waves—nearly 300 thousand kilometres per second.

However the speed of light given by Maxwell's theory could not easily be reconciled with the laws of motion given by Newton without modifying one theory or the other. Eventually Albert Einstein solved this problem with his Theory of Relativity.

Relativity suggests that the speed of light in a vacuum is the fastest possible speed in the universe—a kind of cosmic speed limit.

magnifyGalileo Galilei


Wave of electromagnetic energy
magnifyWave of electromagnetic energy

magnifyAlbert Einstein

Traffic sign in Vinci
magnifyTraffic sign in Vinci
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