# Please pay! 299,792,458 m / s as the maximum limit for the speed of light

In our series “Pay, thank you!” we present spectacular, amazing or groundbreaking numbers and the stories behind them. In our “Number, please! Classics” we again highlight the special articles in our column. Today’s episode is from 2018 and is about the speed of light – We have republished one of the most talked about episodes for you as a revised and updated podcast episode. It is finally available to listen to. Have fun with it!

Light moves 299,792,458 meters per second in a vacuum. It has been known for centuries that there is a limit to the speed of light. However, there was a long way to go to determine the exact speed of light.

Even in ancient times, philosophers and scientists debated whether light propagated at infinite speed or at a predictable speed. In 450 BC. considered the speed of Empedocles to be limited. About 100 years later, Aristotle postulated that the spread was infinitely fast and shaped the vision at that time with his statement.

The Heron of Alexandria, who probably lived around 100 AD, introduced a rather unusual argument for the infinity of light: For him, the light came from his eyes, and when these visibly illuminated the distant stars, it had to be scattered infinitely. velocity.

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In the early 17th century, Galileo Galilei suspected that the speed of light could be measured and would find out with a trial arrangement: At night, he let two people stand high up with a line of sight. These each held a lamp as a light source, but the lights were covered. One must then make the glow visible, the other person, as soon as he perceives the light from the other lamp, let his lamp shine in shifts.

The time must be estimated. In order to calculate the reaction time, several tests were performed over different distances. All tests ended with the result that – of course due to the too fast propagation of the light – no delay could be detected. Unfortunately, the formula Pi times thumbs had no measurable effect.

The Danish astronomer Ole Rømer, on the other hand, made an astonishing discovery in 1676 during a long-term observation of Jupiter and its four known moons at that time: According to his observations, the moon Io needed almost 42.5 hours to orbit Jupiter once. As the moon moved uniformly, the position could be calculated with it. Rømer noted, however, that Io eventually entered Jupiter’s shadow later and later. This delay increased to several minutes after six months.

Over the next six months, the delay steadily decreased until the start matched again exactly one year after the first observation. But since nothing had changed during the lunar cycle itself, the difference in distance had to be responsible for it. The delay could even be calculated in advance. The farther Earth’s position was from Jupiter, the later Io entered Jupiter’s shadow. The light therefore took longer to reach the earth.

This observation proved for the first time that the speed of light is limited. According to Rømer, the light had to take 22 minutes to cross the diameter of the Earth’s orbit, from which his colleague Christian Huygens later calculated a speed of light of 212,000 km / s using observational data from Giovanni Domenico Cassini.

In 1849, the French physicist Armand Hippolyte Fizeau designed an apparatus for terrestrial determination of the speed of light. The so-called gear method: He placed a light source behind a semi-transparent mirror that was rotated 45 °, so that flashes of light were cast on a mirror 8.6 km away, which reflected the rays. A rotating gear was fixed between the two mirrors: its teeth disconnected the light beam at a certain speed. With this device, the speed of light could be determined with a deviation of about 7 percent. Using the rotating mirror apparatus developed by his colleague Léon Foucault, who used a parabolic mirror in addition to a rotating mirror, it was later possible to determine the speed of light with an accuracy of one percent.

The rotating mirror method works like this: the light emitted from a specific point hits a rotating mirror. From there, it is thrown up on a parabolic mirror, which reflects the light beam. The light signal is directed to the target point by the mirror, which now rotates further.

Over time, new methods and new technologies such as laser technology have been used to continuously refine the provision. In 1973, the Boulder Group, commissioned by the National Bureau of Standards, developed the most accurate method ever developed. At the 17th General Assembly of Weights and Measures in 1983, the definition of the speed of light as c = 299,792,458 m / s and the redefinition of the meter were announced:

Out of “One meter is 1,650,763.73 times the wavelength of the vacuum-propagating radiation emitted by atoms of the nuclide krypton-86 as they go from the 5d5 state to the 2p10 state” became a little less cryptic: “One meter is the distance the light travels in a vacuum of 1 / 299,792,458 seconds”

As Rømer has already noticed, due to the enormous distances in space, light travels a long time even at the speed of light. While a radio signal needs about 1.3 seconds to travel to the moon, 384,000 kilometers away, it currently takes almost 22 hours to travel to the US space probe Voyager 1.

According to current knowledge, the speed of light can not be exceeded. The existence of so-called faster than light or superluminar particles, called tachyons, can be largely ruled out. However, they always play a special role, at least in science fiction stories.

Milestones in measuring the speed of light:

about 1638 Galileo, lantern experiment, without result
1675, Rømer and Huygens, Jupiter lunar observation, result: 212000 km / s
1849, Hippolyte Fizeau, gear test, result: 315000 km / s
1862, Léon Foucault, rotating mirror, result: 298000 km / s ± 500
1926, Albert A. Michelson, rotating mirrors, result: 299796 km / s ± 4
1972, Evenson et al., Laser interferometry, result: 299792.4562 km / s ± 0.0011
1983, 17th general conference on weights and measurements, definition of the meter, result: 299792.458 km / s

Distance units based on the speed of light:

1 light-second (1 Ls) = 299,792,458 kilometers ≈ 300,000 kilometers
1 light minute (1 Lm) = 17,987,547.48 kilometers ≈ 18,000,000 kilometers
1 light hour (1 Lh) = 1,079,252,848.8 kilometers ≈ 1,080,000,000 kilometers
1 light day (1 Ld) = 25,902,068,371.2 kilometers ≈ 26,000,000,000 kilometers
1 light-year (1 Ly) = 9,460,730,472,580.8 kilometers ≈ 9,500,000,000,000,000 kilometers

In contrast to its rigid top speed, light moves more slowly in media other than vacuum. An everyday effect: If you put a stick in the water, it seems to bend, as the light moves more slowly in water at about 225,000 km / s than in air or in a vacuum. the refractive index n = cvakuum / cvand> 1.

Meanwhile, it has even been possible to brake the light to a walking speed of 1.6 km / h. A Bose-Einstein condensate, which has an extremely high refractive index, was used for this purpose. The condensate was cooled down so that the temperature is only slightly above absolute zero. This changes the properties of the atoms, the refractive index is over 100 trillion times higher than glass. This can sometimes slow down the light or even freeze it in a way.

Researchers hope that this will open up new opportunities for information processing. So a kind of light speed limit for faster computer technology.

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