We all know: the light It is the fastest physical entity in nature. Albert Einstein He taught us that nature is made so that the light speed (i.e. the propagation speed of an electromagnetic wave) in the void is the same for everyone and represents an insurmountable limit for any physical entity with its 299.792.458 m/s. Enough to perform 7 and a half turns around the equator in just a second, or to get to the moon in 1.28 seconds. Such a speed that is almost wondering: but how the hell did they measure it? The first laboratory measurement of the highest possible speed in the universe took place in 1849 with one of the most beautiful experiments in the whole history of physics: theFizeau experimentin which a beam of light interacted with a rotation toothed wheel. With this simple and brilliant method, Fizeau calculated a value of 314,000 km/s, surprisingly close to the one accepted today.
Before Fizeau: Galileo and Rømer
The first to try it was one of the inventors of the modern scientific method: Galileo Galilei. He took his assistant and sent him at night with a lantern on top of a hill, while he was with another lantern on top of another hill. The plan was simple: Galileo discovers his lantern, allowing the light to spread; As soon as the disciple sees the light, he discovers his lantern and Galileo records the time spent since he discovered the lantern when he saw the light of the assistant. Speed is simply the ratio between double the distance between the two lanterns (i.e. the total path made by light) and the time measured by Galileo.
The experiment was not conclusive, and it is not difficult to imagine why: the distances were too small and the watches of the time Too little precise To measure such a high speed, not to mention the fact that the only ones reaction times Of the two experimenters they were much higher than the time needed to light to make back and forth.
Galileo, however, provided indirectly The foundations for the first quantitative measure of the speed of light, with the discovery of Medici satellites planet Jupiter. What do Jupiter’s moons have to do with the speed of light, you could ask yourself. Well, the Danish astronomer Ole Rømer He discovered in the 70s of the seventeenth century that the timing of the eclipses of the Giorne Lunes with the planet itself changed according to the period of the year in which these eclipses occurred: the more the land during its orbit around the sun was close to Jupiter, the more the eclipses took place “in advance”. Conversely, when the earth was farther from the giant planet, the eclipses took place “late”. In all, twenty minutes danced. Rømer thought, correctly, that the difference was due to the fact that The light takes a certain time to cross the size of the terrestrial orbitthat is, twice the land-sell distance. Well: knowing more or less how much the land-sun distance and how much they “delay” the eclipses of the Medici satellites over six months we can estimate the speed of light. The Danish astronomer obtained a value of approx 225,000 km/sthat is, the 20% less of the correct value.
Fizeau’s experiment for the size of the speed of light
To make the qualitative leap, however, it took an experiment conducted not using the planets of the Solar System but in the laboratory, with precise measures in a controlled environment. But how to do it? The idea came to the French physique Hyppolite FIZEAUand it was as simple as it is brilliant. The absolute protagonist of this experiment – although in reality it was a vast family of experiments – was one Dentata wheel whose rotation speed had to be able to reach high values and be regulated in a very precise way.
Take the light of a powerful torch or another light source and get it merge through a system of lenses to make the light beam Well compact and colliend. At this point the bundle crosses one semi -transparent mirror 45 ° inclined. What does semitransparent mean? It means that the light passes through but does not pass it into the opposite direction. We will soon understand why. The light crosses this mirror, deflects of 90 ° and from there continues in a straight line. Immediately afterwards he meets the edge: if he meets a tooth, the bundle is passed off and no longer continues, otherwise he continues his path up to a mirror that the French physique had arranged to the beauty of 8,633 km.
Initially the wheel is firm and arranged so that the light can pass. The mirror does its job: reflects the light and postpones it back to the semi -praise mirror, which this time lets her pass and leads her to the eye of Fizeau, which otherwise could not have seen anything. In this situation theMaximum light intensitybecause all the light emitted by the light source goes back.
Now, what happens if We turn the wheel at increasingly high speeds? Initially Fizeau will observe one decrease in light intensitybecause not all the light goes back: in fact, a part, during the “return” journey, meets the teeth of the wheel. As the speed of the wheel increases, however, a specific rotation speed arrives so FIZEAU no longer sees any light. That is, no light goes back from the main mirror. This happens because, at that speed, the light crosses the wheel in the voids between two teeth, affects the mirror but systematically when it goes back it affects the tooth following the crack from which it has passed and therefore cannot reach Fizeau’s eye.
The corner speed of the wheel for which light disappears (i.e. how many laps it does in a second, so to speak) is the key to measuring the speed of light. This speed, in fact, allows us to calculate the time that takes a “emptiness” to become a “full” in the edge of the wheel. The calculation is simple: just divide the period of the wheel (i.e. what it takes to complete a lap) for the double of the number of teeth of the wheel (double because the distance from the center of a crack in the center of the next tooth is half of the distance between two following teeth). Fizeau’s wheel had a period of 0.08 seconds And 720 teeththerefore by making the calculations we get:
0.08 seconds / (2 · 720) ≈ 0,000055 seconds.
Well, once this time interval we take the speed formula that we all studied at school:
Speed = distance / time
The distance is known: it is double the distance between the wheel and the mirror (the double because the light must cross this distance on the first leg and return, therefore twice) and the weather is what we have just calculated. So we have:
2 · 8633 meters / 0.000055 seconds ≈ 314,000,000 meters per second.
with a lower waste of the 5% Compared to the accepted value (299,792.48 meters per second). A certainly impressive result Considering the relative simplicity – at least from the conceptual point of view – of the apparatus with which the measurement and simplicity of the mathematics and physics required to arrive at this value was made!
