Question: how do you know how fast light is?

Edward Guy answered on 20 Mar 2023:

It has been measured by quite a few scientists over the past few centuries using a number of different methods. Galileo did an experiment where he shone light from a lantern to an assistant who, as soon as he saw it, shone his lantern back. Galileo measured the time it took and knew how far away his assistant was, and was planning to calculate the speed of light by dividing the distance between them by time it took to see his assistant’s lantern. It didn’t work because light travels far too fast, plus there’s all the errors in the experiment including reaction time to see the lantern and shine one back.

The first speed of light measurement was by a scientist called Roemer, who used eclipses of Jupiter’s moon Io to calculate how long it would take light to travel to Earth and, with the estimated distance of earth to Jupiter at the time, calculated a value for the speed of light. Up until that time, most scientists believed there was no speed of light and that it was ‘instant’.

Since that first measurement, many others have remeasured it using other techniques. Most modern methods involve using lasers where the light beam is split and recombined, or use astronomy measurements over large distances, like Roemer.