The main topic this week was how simultaneity is relative to your *frame of reference*. How there are (virtual) *lines of simultaneity* where all points on some line — at all distances from you — share the same moment in time. For any instant you pick, that instant — that snapshot — includes all points in your space.

A line of simultaneity freezes the relative positions of objects at a given moment — which enables distance measurements. Simple example: When their watches both read 12 noon, Al and Em were 30 miles apart. A more mathematical example uses ** x**,

**, &**

*y***(&**

*z***), but it amounts to the same thing: a**

*t**coordinate system*.

The gotcha is that simultaneity and coordinate systems *are relative* when motion is involved!