DARTH VADER wrote:
What exactly is space time ?

The totality of all events; "event" defined as a primitive, the same
way "point" is a primitive in ordinary 3-dimensional geometry,
representing something that has a specific location at a specific
instant.

A "point" of 3-D geometry is actually temporal -- comprising a stream
of events that are considered to be "at the same place". Since "same
place" is relative, the general concept comprises not just "static"
points, but ALL continuous streams of events (i.e., all motions).

Most human languages recognize this equivalence at a fundamental level
in some fashion or another; some go further an explictly equate the
concepts. For instance, in Japanese, the term for to "be at", to "go
to" or to "come from" are all represented by the same word -- whose
meaning is simply that of continuation of existence.

Why cant we travell ...

"Travelling" and "moving" are temporal notions. It doesn't make sense
to talk about travelling through spacetime, itself; since the totality
of all events is timeless. In what "time" would you travel through
spacetime? You don't have yet another time running around for "moving"
in spacetime. It's changeless and timeless. It's 3-D spaces (more
accurately, layerings of spacetime into sequences of 3-D 'snapshots')
that "move" in time; not spacetime, itself.

Even there, the notion is not automatically a given. A 3+1 dimensional
spacetime does not automatically admit any notion of "motion" or
"change" globally in the sense of having a layering into 3-D spaces. A
special property -- called "global hyperbolicity" is required for that
to be the case. If a spacetime is not globally hyperbolic (e.g. one
way for it to fail to be globally hyperbolic is if there exists a
stream of events that ends up where and when it starts -- a 'time
travel' loop), then the spacetime is not globally hyperbolic, there is
no layering of it into 3-D spaces, and there is no notion of "change"
or things "moving" in time -- except locally.

Quantum theory requires a notion of time as change (i.e. a spacetime
where 3-D snapshots "move" in time); so there is fundamental problems
even defining what a quantum theory is supposed to be in a spacetime
that is not globally hyperbolic. In contrast, relativity essentially
rejects any notion of spacetime as "space moving in time", and rejects
the notion of there being any such thing as "change" itself --
particularly since it allows for spacetimes that are not globally
hyperbolic.