system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - why does time slow down as you approach the speed of light?
gpt
When Einstein was first thinking about relativity, he came up with a thought experiment using a a loop of metal on a train travelling near the speed of light. He wanted to see what Maxwell's equations (which describe all of electromagnetism) would say if you ran a current through that moving loop.

The math behind that experiment showed that if Maxwell's equations were true, then Einstein's thought experiment would create infinite energy. Einstein thought that this couldn't be true - Maxwell's equations must hold for everybody no matter how they are moving.

The only way to resolve the infinite energy problem and keep Maxwell's equations in tact is if the speed of light (i.e. the distance it travels in a certain amount of time) is the same for every observer. Applying more maths, the only way that the speed of light can be the same for every observer (when you change the observer's speed) is for the distance light travels and the time it takes to travel it are DIFFERENT for every observer.

Specifically, the math shows that if an observer is moving close to the speed of light (or is watching someone else moving close to the speed of light) they will see that everyone else is moving slower - time is slowing down.