thought-experiments

Two sisters. One stays on Earth. One boards a rocket, travels to a distant star at close to the speed of light, turns around, and comes home. When they meet again, the travelling sister is younger. Not metaphorically younger. Not "she feels younger." Her cells have divided fewer times. Her clock has ticked fewer seconds. She has genuinely lived less time than her sister who stayed behind.

Two bolts of lightning strike the ends of a moving train at the same moment. At least — that is what an observer standing on the embankment sees. The bolts hit the front and rear of the train simultaneously, and she is standing at the exact midpoint between the two strike marks on the ground. The light from both flashes reaches her at the same instant. She concludes, correctly by her own reasoning:

Here is a room with no windows. You are inside it. You feel a steady pull toward the floor — your feet press against it, objects fall when you drop them, everything behaves exactly as it does when you are standing on the surface of the Earth. Question: Are you on the surface of the Earth, or are you in a rocket accelerating at 9.8 m/s² somewhere in empty space?

This is my favourite room. A thought experiment costs nothing. No lab, no funding, no equipment. Just a mind willing to follow an idea wherever it goes — even somewhere uncomfortable. Some of mine: