It’s the ultimate speed limit — and the key to understanding the universe. But how did scientists determine just how fast light travels?
The ancient Greek mathematician Euclid believed that vision was possible thanks to rays emitted from the eyes. Hero of Alexandria claimed that light must travel at infinite speed, since distant stars appear instantly when we open our eyes.
In the 11th century, a mathematician from Basra, Abu Ali al-Hasan ibn al-Haytham, wrote The Book of Optics — a work as significant as Newton’s Principia. In it, he argued that light travels from the object to the eye at a finite speed, which varies depending on the medium. For example, it moves more slowly through glass or water than through air.
In the 13th century, Roger Bacon drew on al-Haytham’s ideas to argue that light moves faster than sound, at a very high, but still finite, speed. Still, many at the time believed that light might travel instantaneously in a vacuum, slowing only when passing through matter.
As late as the 17th century, scientific giants like Kepler and Descartes insisted that light moved instantaneously. Kepler argued that since space offered no resistance, light could have no speed limit.
Descartes based his reasoning on lunar eclipses: if light had a finite speed, the alignment of the Sun, Earth, and Moon during eclipses would noticeably shift. Since no such shift was observed, he concluded light must travel infinitely fast.
Early Attempts to Measure Light’s Speed
The first experimental attempts to measure the speed of light were made around the same time. In 1629, Dutch philosopher Isaac Beeckman proposed a test in which the flash of a cannon would reflect off a mirror placed a mile away, with the time difference measured between the initial flash and its return.
Galileo later proposed a lantern experiment in 1667, where a cloth-covered lantern would be uncovered, and a distant observer would do the same upon seeing the light. But the test revealed no measurable delay.
In hindsight, we know that the round trip would have taken about one hundred-thousandth of a second, too fast for human reflexes to detect, and the distance was far too short.
But the distances between planets, on the other hand, are vast enough that light takes several minutes to travel from one to another. All that’s needed is a reliable reference point for measuring time intervals.
Rømer’s Breakthrough
In Paris, Giovanni Cassini was studying Jupiter’s moons, which disappear behind the planet and reappear regularly. Cassini noticed inconsistencies in their timing and theorized that light had a finite speed.
His assistant, Ole Rømer, confirmed this in 1676 with careful observations: Jupiter’s moon Io reappeared sooner when Earth was moving closer to Jupiter, and later when Earth was moving away.
The conclusion? Light takes longer to travel a greater distance, and shorter when that distance is reduced.
Rømer’s observations revealed this correlation, and he’s now credited with the first scientific measurement of light’s finite speed.
Huygens & Bradley: Refining the Estimate
In 1690, Dutch mathematician Christiaan Huygens used Rømer’s data to estimate the speed of light at 220,000 km/s — about 70% of today’s accepted value.
The next major milestone came in 1729 with James Bradley, later Royal Astronomer and Director of the Greenwich Observatory.
He discovered the phenomenon of stellar aberration — similar to walking in the rain: if raindrops fall vertically but you’re moving forward, the drops appear to come from an angle, so you tilt your umbrella.
Bradley applied this analogy to starlight: Earth’s motion causes the apparent position of stars to shift during its orbit.
His measurements showed that light travels 10,200 times faster than Earth’s orbital speed, giving a value of 295,000 km/s, just 2% off the modern figure.
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Back to Earth: Fizeau & Foucault
Measuring such high speeds requires either great distances or precise timekeeping. In 1849, French physicist Louis Fizeau developed a terrestrial method:
He passed a light beam through the teeth of a rotating wheel. A mirror 8 km away reflected the beam back. Depending on the wheel’s speed, the returning light would either pass through the next gap or be blocked.
By adjusting the rotation, Fizeau calculated the speed at 313,000 km/s.
In 1862, Léon Foucault refined the method using rotating mirrors, finding a more accurate result: 299,796 km/s, very close to the current accepted value of 299,792.46 km/s.
Maxwell, Michelson & Einstein
In 1864, James Clerk Maxwell revolutionized physics with his theory of electromagnetic waves, describing light as oscillations of electric and magnetic fields that propagate through space.
In a vacuum, these fields interact in such a way that light travels without attenuation. By the late 19th century, physicists calculated light speed from these constants, yielding 299,788 km/s — the most accurate value at the time.
In 1887, Albert Michelson and Edward Morley tried to detect Earth’s motion through the so-called “luminiferous ether”, believed to be the medium through which light traveled. They used interference experiments with split beams of light, expecting shifts in phase due to ether drift.
But the experiment showed no difference in light speed in any direction — a shocking result. This led Albert Einstein to discard the ether concept and develop his Special Theory of Relativity in 1905. One key insight: the speed of light in a vacuum is constant and forms a fundamental limit in nature. No object with mass can reach it, and any massless particle must travel at this speed.
Measuring Light in Modern Times
Light slows down in transparent media like water or glass, but electrons can still travel faster within the medium — though not faster than light in vacuum.
In the 1950s, before lasers, scientists used resonating cavities to measure frequency and wavelength of electromagnetic waves — yielding a speed of 299,792 km/s with ±3 km/s uncertainty.
Want to try a fun version of that experiment at home? Put a chocolate bar in a microwave (remove the spinning plate!) and watch where it melts first. The distance between two melted spots is half a wavelength. Multiply that by the microwave’s frequency (usually 2.45 GHz) — and you’ll get a rough estimate of the speed of light!
Modern space missions bounce radio waves off distant spacecraft, giving light speed values accurate to within 20 trillionths.
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Defining the Meter by the Speed of Light
Advanced laser systems now produce light with extremely stable frequencies. When split and recombined, the resulting interference pattern allows scientists to measure the wavelength with extreme accuracy.
In 1972, this approach achieved precision better than 4 parts per billion. Today, thanks to ultrastable lasers and atomic clocks, we know the speed of light to be 299,792.458 km/s — with an uncertainty of just 1 m/s.
Atomic clocks define time with incredible precision, so today the meter is defined through the speed of light: Since 1983, the meter has officially been defined as the distance light travels in 1/299,792,458 seconds in a vacuum.
Five Key Terms That Describe the Nature of Light
Magnetic Permeability
A measure of how easily a medium can become magnetized or allow magnetic field lines to pass through it.The speed of light in a vacuum is inversely proportional to the square root of the product of the vacuum’s dielectric permittivity and its magnetic permeability.
Light Aberration
An optical phenomenon in which stars appear to shift from their true positions in the sky. This effect results from the finite speed of light combined with the motion of the Earth.
Atomic Clock
The most precise timekeeping device currently available. It relies on the microwave frequency emitted by electrons as they transition between quantized energy levels within an atom.
Cavity Resonator
A hollow conductor sealed at both ends, in which an electromagnetic wave is sustained through continuous reflection off the inner walls. By carefully choosing the dimensions and wavelength, one can generate standing or traveling waves within the cavity.
Dielectric Permittivity
An electric charge generates an electric field. The extent to which an insulating (dielectric) medium resists the formation of that field is known as its dielectric permittivity.The relative dielectric permittivity of a substance is determined by comparing the capacitance of a test capacitor with a dielectric to that of the same capacitor in a vacuum.
