Speed of light: The cosmic limit that shapes our universe | Technology News

Galileo’s Lantern, Rømer’s estimate

Centuries before Einstein, in the early 1600s, Galileo tried to measure the speed of light with nothing more than shuttered lanterns and a stopwatch. He stationed a colleague on a distant hill, uncovered his lantern, and waited for the return signal. The delay was imperceptible, leading him to conclude that if light did have a speed, it was extraordinarily rapid — too fast for human senses to detect.

The first real proof that light’s speed was finite came in 1676. Danish astronomer Ole Rømer was observing the eclipses of Io, one of Jupiter’s moons. He noticed the timings of these eclipses varied depending on Earth’s position in its orbit: when Earth was moving away from Jupiter, Io seemed to be “late,” and when moving closer, “early.”

Rømer reasoned that the changes weren’t in Io at all — they were in the travel time of light across the vast gulf of space. By comparing the delays, he estimated light’s speed at about 220,000 km/s. Not exact, but revolutionary. For the first time, light had been shown to move not infinitely fast, but at a measurable velocity.

Fizeau’s cogwheel

Two centuries later, French physicist Hippolyte Fizeau made the first precise terrestrial measurement. In 1849, he sent a beam of light through the gaps of a rapidly spinning cogwheel to a mirror eight kilometers away. When the returning beam was blocked by the next tooth, he could calculate the travel time. His result — about 313,000 km/s — was remarkably close to today’s accepted value.

The Michelson–Morley experiment

By the late 19th century, physicists believed light traveled through an invisible medium called the luminiferous ether, much like sound requires air. If Earth was moving through this ether, the speed of light should vary depending on direction, like a boat moving with or against a current.

In 1887, Albert Michelson and Edward Morley put this to the test with a brilliantly precise interferometer. A beam of light was split into two perpendicular paths, bounced off mirrors, and recombined. If one beam traveled “upstream” through the ether, it should arrive slightly later, shifting the interference fringes.

But nothing shifted. The speed of light was the same in all directions. It was a shocking null result — demolishing the ether theory and leaving physicists searching for an explanation.

Einstein’s resolution

In 1905, Einstein gave the answer: the speed of light is constant for all observers, everywhere, always. There was no ether; instead, space and time themselves must flex to preserve this constancy. Moving clocks tick more slowly, moving objects contract, and mass and energy become interchangeable.

Einstein later recalled how as a teenager he imagined chasing a beam of light. That thought experiment — so simple yet profound — became the seed of relativity, a theory that reshaped the very notions of space and time.

Why is the Speed of Light Constant?

Maxwell’s equations give a clue: they predict electromagnetic waves (which include light) propagate at a fixed velocity, determined only by two properties of empty space — its ability to sustain electric and magnetic fields. In other words, the vacuum is not “nothing” but a medium with physical character, and this sets the speed of light.

But why is it always the same, no matter who measures it? Relativity suggests that c is more than the speed of light — it is the ultimate speed at which any influence or information can travel. Massless particles like photons must always move at this pace, while anything with mass can never quite reach it. In that sense, the constancy of light’s speed is not just a fact about light, but about the very geometry of spacetime.

Einstein himself wrestled with this duality, noting: “It seems as though we must sometimes use one theory and sometimes the other, while at times we may use either… separately neither fully explains the phenomena of light, but together they do.”

And as one writer, Robert Brault, wryly put it: “Nature decrees that we do not exceed the speed of light. All other impossibilities are optional.”

This remains one of physics’ deepest mysteries: not just how light moves, but why the universe itself has chosen this one cosmic limit.

Why it matters today

The constancy of light’s speed isn’t just abstract theory — it shapes everyday life. GPS satellites must adjust for relativity, or your phone’s navigation would drift kilometers each day. Astronomers looking at galaxies millions of light-years away are literally seeing the past, since light takes so long to arrive. Every telescope is a time machine.

The speed of light also defines the limits of what we can know. Because the universe has a finite age, we can only see as far as light has traveled since the Big Bang: about 46 billion light-years in any direction. That is the radius of the observable universe. Beyond that lies information forever inaccessible, not because our instruments are weak, but because light itself has not had time to arrive.

The last word

From Galileo’s lanterns to Rømer’s eclipses, from Fizeau’s cogwheel to Michelson’s interferometer, the quest to understand light’s speed has reshaped physics. It revealed that nature has a limit so fundamental that even time and space must bow to it.

The speed of light is more than a number. It is the thread that ties the universe together, the reason cause precedes effect, and the measure by which we glimpse the past. Humanity once tried to chase it — only to discover that in chasing light, we were really learning what it means to exist in this universe at all.

Shravan Hanasoge is an astrophysicist at the Tata Institute of Fundamental Research.