clipka wrote:
> Maybe you'd first need to define "point": Are you talking about a point 
> in 3D space which extends along time - i.e. a line in 4D space - or an 
> actual 4D point that only exists at a certain time T?

In all cases I'm referring to a point as a 4D point in spacetime since 
there's no intrinsically unique way to extend a 3D point along time.


> (Note that of course this leaves the definition of distance subject to 
> your frame of reference, as it defines the orientation of the spacelike 
> components of spacetime, but that's a known effect in relativistic 
> physics.)

If you fix a reference frame there's certainly plenty you can do to 
define spacetime distances in a consistent way.  If you want to 
consistently measure distances independent of the reference frame, then 
you're pretty much stuck with what the Minkowski metric tells you.  Of 
course this metric gives you a single value for the distance (not 
independent time and space distances) and thus can't really be used to 
measure pure-spatial distances.

Not that this is necessarily inconsistent with what you said, I just 
thought I'd point out (in case you weren't already fully aware) that if 
you're going to measure spacetime distances independent of the reference 
frame then the way to do it is to use the Minkowski distance and forget 
about trying to determine what the "spatial" distance is at all.

Post a reply to this message