## K1    Early Experiments to Measure the Speed of Light

The speculative-philosophical phase of discussion on the speed of light lasted until 1676. Only Galileo attempted, shortly after 1600, an experimental clarification of the issue, but with his lanterns and helpers stationed on opposing hills, he had no real chance of getting a result. He also correctly interpreted that the speed of light is much larger than the speed of sound. René Descartes on the other hand, put all of his prestige on the line with a (weak) argument for an infinitely large speed of light.

Presentations on the topic ‘speed of light’ might comprise one of the following areas:

• The speculative phase before 1676: Empedocles, Aristotle, Heron of Alexandria, the ancient Indians, Avicenna and Alhazen, Kepler, Francis Bacon and Descartes
• The experiments to measure the speed of light by Galileo and his pupils
• Ole Römer's explanation of the annual ‘late’ and ‘early’ arrival of the eclipse of Jupiter's moon Io (1676). His declaration to the speed of light: 22 minutes for the diameter of the earth’s orbit. For the latter there were only rough estimates.
• James Bradley and the aberration of light (1728). His acknowledgment of the value of Römer's based on a very different measurement provided the breakthrough for the finiteness of the speed of light. Bradley’s value for c was very close to the present value.
• Armand H.L. Fizeau was the first to measure in 1849 the speed of light for a distance of a few kilometers (Gear wheel method)
• Léon Foucault needed onlya light path of a few meters in 1850 for his revolving mirror method and thus was able to measure the speed of light in different media
• The importance of the almost blind physicist François Arago as a supplier of ideas for Fizeau and Foucault
• P. Newcomb and A.A. Michelson improved Foucault's method in 1926 and measured the speed of light to about 0.002% accuracy.
• Definition of the speed of light (and thus in particular the length of a meter) in 1983 by the members of the International Bureau of Weights and Measures (BIMP) to 299,797,458 m/s

Also interesting is the experiment of Fizeau concerning the speed of light in flowing water (1851). He could not understand his results because he assumed the classical addition of velocities. Applying the addition of velocities according to STR yields his results immediately (see e.g., [25-103ff],
[19-80f]  or  [14-120]
).

The definition of the speed of light from 1983 opens the way to the ideas outlined in next section: For c = 1 (by definition) we obtain a new system of units, which takes into account the very core of the STR!