[1] Galilean/Newtonian spacetime assumes that space is Euclidean (i.e. "flat"), and that time has a constant rate of passage that is independent of the state of motion of an observer, or indeed of anything external.
Conceptual issues present an even greater difficulty, since general relativity states that gravity is a consequence of the geometry of spacetime.
To produce a quantum theory of gravity would therefore require quantizing the basic units of measurement themselves: space and time.
[4] A completed theory of quantum gravity would undoubtedly present a visualization of the Universe unlike any that has hitherto been imagined.
In particular, general relativity in reduced dimensions (2+1) retains the same basic structure of the full (3+1) theory, but is technically far simpler.
[7] Space and time may not be fundamental properties, but rather may represent emergent phenomena whose origins lie in quantum entanglement.
Early attempts at constructing spacetimes with extra timelike dimensions inevitably met with issues such as causality violation and so could be immediately rejected,[7] but it is now known that viable frameworks exist of such spacetimes that can be correlated with general relativity and the Standard Model, and which make predictions of new phenomena that are within the range of experimental access.