Transcript
MARCUS DU SAUTOY
But what if the suspect car was moving at the speed of light? Well, now things get interesting! If the police are parked, and the suspect goes past at the speed of light, then, not surprisingly, 3x108metres per second – the speed of light – usually referred to as c. But, if the suspect spaceship travels at the speed of light and the police are chasing at half the speed of light, then what would the speed gun show then?
Well, curiously, it wouldn’t be half the speed of light. It would be measured as c, exactly the same as if the police were stationary. And if they were going in the opposite direction? c again. Curious, eh? This was a problem that occupied physicists and astronomers in the late 19th century. They knew that the Earth was moving around the Sun at 30 kilometres per second. But no matter in what direction they measured the speed of light, it always came out the same. Well, how could this be?
The question was answered by Albert Einstein in 1905. In his theory of special relativity, he asserted that the speed of light in empty space is an unvarying, fundamental constant of nature. Everyone will measure the same value, no matter how fast they are moving. There is no ‘cosmic roadside’ where we have to stand to measure the speed of light properly.
On the face of it, this doesn’t make sense. But Einstein showed that the fixed speed of light was actually a consequence of something deeper: that ‘your time’ and ‘my time’ flow differently when we are in relative motion.