This section is part of the HSC Physic syllabus Module 7: Nature of Light under Light and Special Relativity.
Einstein conducted a couple of thoughts experiment which led to the principles of relativity.
A thought experiment is an experiment which is imagined.
Measuring Light From a Moving Train
If two beams were emitted from a moving train, the person on the train would see both beams be emitted at the same time.
However, someone not on the train, waiting from the outside, would see the beam at the front of the train move faster than the one at the back of the train.
Lightning Strikes a Moving Train
Imagine someone standing on a platform, and a train comes roaring past.
Lightning strikes the back of the train, right in front of the person on the platform.
The person on the platform and the person on the train would see the lighting hit at the same time since they are both the same distance away from the strike.
But to someone at the front of the train, the lighting would strike slightly later, because the light from the strike would have to travel a distance to reach the person at the front of the train.
So the person at the front of the train would say the lighting strike happened later than the person on the platform thinks it did.
This experiment involves twins. One twin stays on earth while the other travels in a high-speed rocket. When the twin returns, they find that the one that stayed on earth is older than the one who travelled.