Can someone please explain to me why or how this statement is true? I don't understand it..

Question

Two events that are simultaneous in one inertial frame of reference will not necessarily be simultaneous in any other inertial frame of reference.

Thank you

Answer 1

This is one of the fundamental postulates of the special theory of relativity. It has been proven experimentally to be true.

The separation of two events in space time can be written
dx^2 + dy^2 + dz^2 - c^2 dt^2

That quantity is invariant when transforming from one intertial coordinate system to another. It is possible for the spatial separation of the two events in four-dimensional spacetime to increase while the time interval decreases or become zero.