- An object in motion tends to remain in motion, and an object at rest tends to remain at rest;
- Force equals mass times acceleration;
- For every action there is an equal and opposite reaction.
These rules work for most situations, both on Earth and in space. But we have to make special rules when we are thinking about the speed of light. First, the theory of relativity says that space and time are essentially the same thing, so that things have to move in space and time at the same time, or it’s the same thing for something to move in time and in space. It’s called relativity because the way rules seem to act will seem different to observers in different places. It may seem like we are standing still, here on Earth, but really the Earth is only one thing in space, and it’s always moving.
Second, Albert Einstein’s theory of special relativity says that when things are moving very fast, Newton’s laws of motion will not predict how they will move. That’s because nothing can move faster than the speed of light. Instead, things will act according to different rules.