What is Constancy of the speed of light?

The speed of light is always the same, no matter where you are or how fast you're going.

Imagine you're on a playground, and you have a super-fast friend who can run as fast as light, that’s about 300,000 kilometers every second! Now, imagine your friend runs from one end of the playground to the other. No matter if you’re standing still or running toward them, they always reach the other side in the same amount of time. That's constancy of the speed of light, it doesn’t change just because you're moving.

Think of light as a clock that ticks at the exact same rate everywhere. If you’re on a swing and watching your friend run, or if you're standing still, their clock, the time it takes them to run, stays the same. It's like having a really fast, super-accurate clock in your pocket, and no matter how you move, it always keeps ticking at the same speed.

So even though we’re moving around on Earth, or zooming through space, the speed of light never changes, just like your friend’s run on the playground.