Specifically, relativity forbids you from measuring the time it takes light to travel from point A to point B. It's a radical idea, but it's supported by every measurement of light's constant speed.īut several physicists have pointed out that while relativity assumes the vacuum speed of light is a universal constant, it also shows the speed can never be measured. If instead, you assumed the speed of light was absolute, space and time must be affected by relative motion. The speed of light appeared to be constant.Įinstein found that the problem was in assuming that space and time were absolute and the speed of light could vary. But measurements to observe aether drift came up null. But if the aether exists, then the observed speed of light must change as the Earth moves through the aether. In the late 1800s, it was thought that since light travels as a wave, it must be carried by some kind of invisible material known as the 'luminiferous aether.' The reasoning was that waves require a medium, such as sound in air or water waves in water.

When Einstein proposed the theory of relativity, it was to explain why light always had the same speed.

What is the speed of light