The mystery of the speed of light

Now the surprising thing about the Galilei–Newton theory that we have been discussing so far is that it is wrong. It is not wrong in the sense that it is completely wrong, but wrong in the sense that there is a limit to its applicability and in certain cases it does not work. And that case involves the speed of light.

The speed of light in a vacuum is very very fast. It is 299 792 458 meters per second, or roughly 3 × 108 meters per second. Since we do not want to end up writing this big number repeatedly, we will just represent it with the letter c. To give you an idea just how fast this is, it is fast enough to circumnavigate the Earth seven and a half times per second. The time it takes for light to travel 30 centimeters (about a foot) is only 1 nano-second, which is 0.000 000 001 seconds.

Because c is so large, it was very difficult to measure what it was for a long time. Galileo himself tried it but did not succeed. But by the end of the nineteenth century, the technology had advanced to the point that very accurate measurements of c were possible.