Congruence (general relativity)

Congruence is a big fancy word that means a group of things that are all moving in the same way. Just like when you and your friends are walking together in a line, you are all moving in the same direction and at the same speed, that’s a congruence.

When it comes to general relativity, congruence is used to talk about how things are moving in space and time. For example, we can think of rays of light moving through space as a congruence. Each ray of light is like a friend in your line, all moving in the same way.

Just like how you and your friends can change directions or speed up or slow down, the rays of light in the congruence can also change. This happens when they are affected by other things like gravity.

So, congruence is a way to talk about how things are moving together in space and time in a more complicated way than just walking in a line with your friends. It helps us understand how things move and change in the universe according to the laws of general relativity.