Our intuition about concepts such as time, energy and speed is formed through experiences of everyday life. But in some circumstances our intuition misleads us about the true nature of reality.
According to our colloquial language, and to our instinctive characterisation of some everyday experiences, the state of motion of a body is an objective property of the body. But, as was understood by Galileo Galilei some 400 years ago, motion must be viewed as a relative property. When we say 鈥渢he train is moving鈥�, as if it were an objective fact, we are implicitly adopting the station as our reference frame. This is a legitimate convention, but we could equally choose the train as reference frame 鈥� in which case, one should state that the station is moving. As long as there are no accelerating forces present, a game of pool played on the train develops just like a game of pool played in the station.
In 1905 Einstein realised that this Galileian relativity principle does not apply only to mechanical processes; it is also valid for electromagnetic processes. In accomplishing this generalisation, Einstein made successful predictions of some striking new effects that show the universe to have properties that defy common sense.
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These counter-intuitive properties are well verified experimentally but are in conflict with our everyday intuition. They are the source of much of our fascination with relativity. When we use a catapult to throw a stone, the speed of the stone depends crucially on the speed of the catapult at the time of release. But when we turn on a light bulb, the speed of the light turns out to be independent of whatever emits it. In all cases this universal speed takes the value of roughly 300,000 kilometres per second.
This idea leads us to the realisation that, contrary to our innate intuition, time is also relative, rather than absolute. Two twin-sister astronauts on different space ships travelling with some constant relative velocity would have a surprising picture of the ageing process: they would each perceive the other twin as ageing more slowly than themselves. And if one of them manoeuvred her spaceship (by imparting the needed accelerations) in such a way as to eventually rejoin the other twin, then at the meeting point they would both agree that the twin who had remained on the same constant-speed trajectory would have aged more than the twin who had chosen to combine periods of constant-speed travel and periods of acceleration. Since the absolute concept of time is ingrained so deeply into our instinctive world view we colloquially speak of this as a 鈥渢win paradox鈥�. But really it is not a paradox at all. The two twins have not lived the same life: one has experienced a series of accelerations.
Herein is the triumph of Einstein鈥檚 work. Of course, it redefined the universe, giving us such insights as the fact that mass can be converted to energy and that the speed of electromagnetic radiation 鈥� that is, the speed of light 鈥� is a special scale of nature, setting an absolute maximum limit for speeds. But it does more than that. While Galileo鈥檚 first relativistic theory formulates in scientific language some facts that are not in conflict with our everyday intuition, Einstein鈥檚 relativistic theories teach us to be diffident about our own intuition when we are thinking about things beyond the everyday.
鈥�One more time
The absolute concept of time is ingrained so deeply in our world view, that we see paradoxes where none exist鈥�
