A Brief History of c Change
Galileo described stationing observers on distant hilltops with covered lanterns. The first observer would uncover his lantern, and upon seeing a light the second observer would uncover his. In this way Galileo hoped to detect any time delay. Unfortunately, a good watch had not yet been invented. Galileo could not detect any finite speed.
In 1676 Dutch astronomer Ole Roemer finally measured the speed of light. He did so using another Galileo discovery, the moons of Jupiter. By measuring the times when the moons passed in and out of Jupiter’s shadow, he finally found that light has a measurable speed. Since Roemer’s time, many other experiments have been devised to more accurately measure c. Roemer visited Isaac Newton in 1679; this discovery is mentioned in Newton's Principia.
Isaac Newton could imagine cannonballs fired at orbital speed, and calculate that a satellite in a circular orbit would have a certain velocity at a given altitude. Since Newton knew that Moons and apples are both guided by one law, might he have suspected that light is also affected by gravity? Centuries later, Einstein would show that light is indeed affected by gravitation. Light is far too fast to be drawn into orbit around the Earth, but the mass of a Universe would indeed pull light into a circular path.
There has long been speculation of whether the speed of light has always been constant. In 1874, the year after Maxwell published his equations, William Thomson and P.G. Tait claimed to have found a decrease in the speed of light. (W. Thomson, and P.G. Tait, Natural Philosophy, V. 1, p. 403, 1874) Thomson was the 1st Lord Kelvin and a very prominent scientist of the time. J.Q. Stewart in 1931, H.E. Buc in 1932, and P.I. Wold in 1935 also suggested that c changes.
Change in c has been put forward as a cause of cosmological redshifts. This "tired light" explanation has been disfavoured by experiments, but we will see that c does have an affect on redshifts. Barry Setterfield has written an exhaustive study using statistical analysis to claim change in c. More recently John Moffat, Joao Maguiejo and Andreas Albrecht have independently suggested a change in c as a solution to cosmological puzzles. Many, many scientists have speculated that c changes.
In conclusion, anyone can speculate that the speed of light changes, and many people have. For this to be more than the story of a speculation, we need the theory behind c. Why does light travel at 299.792.5 kilometres per second, not faster or slower? What is the principle behind c? NEXT: A Theory that a child could understand.