Proper length

Proper length means the length of an object according to how it looks to you when you're looking at it. It's like when you're playing with your toy car and it looks like it's one size when it's close to you, but a different size when you put it far away. The proper length of the car is the size you see when you look at it from where you are standing.

Sometimes, when objects are moving very fast, their proper lengths can actually change. This is called "relativistic length contraction." Imagine your toy car was going so fast that it looked smaller to you than it did before. That would be like relativistic length contraction. Scientists have to take this into account when studying very fast objects like particles in a particle accelerator or stars in outer space.