When Galileo first set out to measure light’s motion, it wasn’t even clear if light did involve motion. And despite his experiments pointing the other way, the genius of Galileo rounded up to conclude that light travels with a finite speed. Here’s the saga of how Galileo tried his best to unveil the true nature of light.
Everyone knows that nothing can travel faster than or even as fast as light. The detail-oriented troublemaker might stop you right there and point out that you must correct your statement and instead say: no material body can possibly move with a speed that is greater than or equal to the speed of light in empty space (which is flat and static as well). As most of us usually do, let’s shrug off the troublemaker for a moment, and take a step backwards and wonder what the phrase travelling of light even means. How do we even know that light is something that travels? Shouldn’t your everyday experiences with light, such as turning your room lights on, lead you more naturally to the conclusion that light is an entity that simply exists everywhere and manifests before us whenever we turn the bulb on; rather than imagine the more bizarre alternative that light shoots out from the bulb and begins to “travel” everywhere around it? No one has really seen light travel. Not from everyday experience at least.
Let us see in this series of articles, wherefrom the idea of motion of light cropped up and how ingeniously the thinkers of the past put forth ideas and set up simple experiments to not just prove that light indeed travels, but also to write down a specific number for the speed with which it travels.
The three Galileos argue
In his famous book “Dialogue Concerning Two New Sciences” that Galileo published in 1638, he presents a number of scientific facts and ideas in an entertaining fashion, in the form of dialogues between three characters – one, the naïve; the other, the less naïve; and the third, the smarty pants (all three of which were really Galileo himself at different stages of his thought processes).
At one instance, the less naïve expresses his fascination for the ability of light from the sun, to burn and melt metals, with the help of mirrors. Speaking of light itself, he says, “…with regard to the surprising effect of solar rays in melting metals, must we believe that such a furious action is devoid of motion or that it is accomplished with the most rapid of motion?” The smart one here jumps in and tries to resolve the problem by comparing the action of light burning metals, with the other known burning processes. For example, lightening in the skies, or the explosions with powders used in mines, are all quick burning processes that involve rapid motions. So, by this analogy, he feels it is not at all unnatural to assume that light accompanies motion, a very rapid motion.
Having made this assumption, there arises the need to answer the next question of how rapid light’s motion is. Does it travel with infinite speed, taking no time at all to cover any distance; or is there a limit to how fast light can travel? There’s only one way to answer this – experiment.
Shining lamps atop hills
The smartest of the three, presents an experiment he has thought up to find out if light travels instantaneously. His plan is to pick two volunteers with very sharp reflexes, and hand each of them a lamp (call them A and B). They are to be placed a few meters apart, facing each other. Both A and B initially cover their lamps with their hands to block off any light reaching the other. One of them makes the first move, say A, and uncovers his lamp. As soon as B sees the light from A’s lamp, he uncovers his lamp. And this, they do over and over again, until B has sharpened his reflexes to uncover his lamp as soon as he sees light from lamp A, without fumbling.
Once the training is done, A and B are to be each placed on two different hilltops separated by a distance of 2-3 miles. A uncovers his lamp and at the same time notes the time at which he does so. B, with all the practice at hand, responds promptly by uncovering his lamp the moment he sees light from A. And finally A notes down the time at which he sees light from B. Knowing the distance light had to travel from A to B and back to A, and having noted the time it took to travel, the speed of light would be all ready.
Galileo (as the legend has) actually performed this experiment with one of his associates, as he narrates via one of the characters in the book. He chose two hilltops about a mile apart. But the only conclusion Galileo could arrive at, despite all the pains he took, was that light, if not travels at infinite speed, at least travels at a speed much greater than the speed of sound. The speed is simply too great to be measured with such a crude set up that involved direct dependence on the slacky human reflexes to record time periods using the pathetic water clocks (available in those times), in order to measure light zoom through small distances.
So, what’s the speed?
This was an extremely clever approach of Galileo in unlocking the secrets of light travel. He first assumed that light indeed involves rapid motion, and then even devised and performed an experiment to know if the motion was instantaneous. And to top it all off he concluded that light travels with a finite speed (too large, though), despite his effortful experiment seeming to indicate otherwise.
We’ll see in the coming articles in this series, what prompted Galileo to assume a finite speed of light, and how much more ingenious thinking was necessary to break the mold and reveal the precise speed with which light travels.
Cover image by Rob Taylor