What if the speed of light were that of a cyclist?

ALBERT EINSTEIN was well known for his , or “thought experiments”, conducted in imaginary versions of the real world. He used them to test ideas that observation could not confirm. But it is also possible to do imaginary experiments in worlds which are themselves imaginary, and thus to illuminate reality in a novel way. This was a particular skill of another 20th-century physicist, George Gamow, who explained his subject to the laity via a Mr Tompkins and his adventures in alternative wonderlands.In the first of these tales Mr Tompkins dreams of a place where the speed of light is about that of a bicycle. How would a cyclist in such a world look to a watching pedestrian? Gamow’s answer was that the cyclist would shrink from back to front, and the faster he travelled the more slowly his pedalling feet would revolve. Gamow’s cyclist is compressed by an effect called the Lorentz contraction and his pedalling is affected by a slowing of the clock known as time dilation. These things are both consequences of the speed of light being constant no matter how rapidly its source is approaching or receding—an observation that was the starting point for Einstein’s own .