# When is something Lorentz invariant.

1. May 9, 2013

### Dreak

Hello there,

I'm having a real problem understanding when a certain 'something' (for example Eddington-Finkelstein coordinates) is Lorentz invariant or how you can 'calculate' it.

Heck, I'm not even sure if a coordinate system must be lorentz invariant, or if the metric in the equations has to be lorentz invariant, or both?

2. May 9, 2013

### WannabeNewton

A scalar that is formed out of tensors (trivially or non-trivially) is invariant under lorentz transformations and is said to be a lorentz invariant. Examples include the action, the space-time interval, and the rest mass.