Time is immutable, unchanging and constant, no matter where you are in the Universe, right?
Actually no, time is not the same anywhere, and we can turn to Albert Einstein – as is so often the case – for a simple explanation of why this is true.
More thought experiments
Einstein’s theory of relativity started with a simple fact: light always travels at the same speed, no matter how fast the light source is moving.
This might not seem very odd, but it leads to a startling conclusion: the passage of time differs depending on your motion relative to others.
Einstein's train experiment
Let’s try one of Einstein’s thought experiments to show that time isn't the same everywhere, and for everyone.
You’re in a moving train carriage and have placed torches at each end facing towards the centre.
If you turned the torches on simultaneously, you would not be surprised to see the two light beams arrive at the centre at the same time.
If you had a friend who was watching from the station as the train passes by, they would see the two light beams travel at the same speed relative to them, but before the beams arrived at the centre, the train has moved.
They would see the beam coming from the front of the carriage arrive first. To them, the beams do not arrive at the centre simultaneously.
How do two observers not agree on whether two events occurred simultaneously? Is our experiment wrong?
No. It demonstrates that the passage of time is relative, not absolute. Time is not the same everywhere.
A ticking clock isn’t the reliable constant you might think. Turns out it’s all relative.
