KIDS: How the Speed of light was actually Measured

Credits
It has long been believed that light had an infinite speed. The daily experience would seem to confirm: as soon as you turn on a lamp, the light instantly floods the space. However, as early as the seventeenth century, the Danish astronomer Ole Rømer hypothesized that the light had an enormous speed, but not infinite.
It was Rømer himself who determined the speed of light in 1676 while working at the Royal Paris Observatory, directed at the time by Giovanni Domenico Cassini.

Credits

THE ATTEMPTS OF GALILEO GALILEI

Galileo Galilei had also tried his hand before him, but without success. Galileo's experiment involved placing two lanterns at a distance of one mile and calculating the time the light took to get from one point to another: together with an assistant he took a shielded lantern and went to the top of two hills that a mile away. Galileo discovered his lantern, and the assistant on the other hill, as soon as he saw the light, discovered the lantern in turn.

The Pisan scientist would then have to measure the time necessary to see the light from the other hill and at that point it would be enough to divide the distance for the time to get the speed of light. The experiment did not lead to any result: to travel a mile, the light takes about 0.000005 seconds, an immeasurable value with the tools available to Galileo.

Credits

THE MEASUREMENT OF OLE RØMER

However, if the distances to be made to the light become wider, a measurement is possible even with less sophisticated instruments. It is precisely what Rømer did in 1676 by observing the motion of Io , one of Jupiter's moons.

Io complete an orbit around Jupiter in 1.76 days. Rømer, however, realized that the time taken by the moon was not always the same. At certain times of the year, when the Earth was farther away from Jupiter, it took more time; on the contrary, when Earth and Jupiter were closer, the moon seemed to anticipate its revolution.
Rømer's thesis was simple but brilliant: the difference was due to the speed of light: if this is not infinite, then it must take some time to come from Jupiter to the Earth; when the Earth is farther away, it takes more time.

The Rømer hypothesis was not well seen by the director of the observatory, Giovanni Domenico Cassini . Then Rømer, to convince his boss, announced that the eclipse of Io, scheduled for November 9, 1676, would take place 10 minutes before the time that all other astronomers had deduced from the previous transits of the moon.
The prediction occurred punctually and Cassini had to convince himself. Rømer explained that the speed of light was such that it took 22 minutes to travel the diameter of the earth's orbit. Rømer, which had an inaccurate value of the Earth's orbit, calculated the speed of light in 220,000 km per second, an incorrect measurement (the precise speed is 299,792,458 km / s), but certainly the earliest ever measured until at that time.

THE ANNIVERSARY OF 7 DECEMBER

Rømer communicated his discovery to the Academy of Sciences and the news was then published on December 7, 1676, a date commonly referred to today as the first determination of the speed of light.
In 1790 the Dutch mathematician Christiaan Huygens used the idea of ​​Rømer to calculate the most precise way the speed of light and managed to derive a numerical value very close to that accepted today.
Later the speed of light was measured by physicists with absolute precision: a light ray travels in the vacuum at 299.792.458 meters per second. In a second it could take seven and a half turns of the Earth along the line of the equator.

Credits

RØMER

After his stay in Paris, in 1681 he returned to Denmark, where he began teaching astronomy at the University of Copenhagen. Of the scientific writings produced at that time there was almost nothing left: they were destroyed in the great fire that broke out in the city in 1728. His passion for units of measurement also concerned the everyday: as a real mathematician he was the main person responsible for the introduction of a national system for weights and measures in Denmark in 1683 (initially based on the "foot of the Rhine": but in Rømer's intentions, it would have had to refer to astronomical constants, a result that was reached only after his death).
He also designed a temperature scale that bears his name: today it is no longer in use, but the German physicist Daniel Gabriel Fahrenheit have used it as a basis for elaborating the Fahrenheit scale. In the last years of his life he was appointed head of the Copenhagen police, and he did not lose the opportunity to invent something else: the first street lamps - in oil - for the city.

Credits

FASTER THAN LIGHT?

In reality there is no faster thing in the universe of light. Indeed, there can be nothing faster, even theoretically, as Albert Einstein postulated in his famous theory of special relativity . From his formulas it is deduced that in nature there is a maximum speed limit. This has to do with the mass of things: every object, according to Einstein, increases its mass as it moves faster (that is, beyond a certain limit, the energy that pushes an object turns almost entirely in mass and only for an ever smaller fraction in speed).
This becomes evident only at high speeds: if one could shoot a 55-gram tennis ball in space at a speed of 500 million kilometers per hour, the mass of the object would increase to 62 grams. If the speed reached 1.079 million km / h - corresponding to about 99.98 percent of the speed of light - the mass of the ball would be as much as 2.5 kilograms.
Any further approximation to the speed of light would increase the mass of the ball, and at 99.9999 percent would be 1.2 tons. At that point, however, a huge force would be needed to give an increase in speed. To accelerate a large mass, in fact, it takes more thrust than it takes for a small mass.
In practice: the faster the ball, the greater the mass becomes, and consequently more energy-intensive and further acceleration. Up to the limit situation, where any increase in speed would require an energy greater than that available in the universe: the non plus ultra of the speed that a body can reach.

Gif by - @nitesh9