Index Key:  PHY056
Author:     Jan
Subject:    It is about time
Text:       Once, when I studied math, I read about the diverse versions of 
the GUT.  One thing that puzzled me was, that the theories seemed to talk 
about space-time being some kind of 'quantum foam', that as a consequence time 
would be 'broken up'; a state that should presumably have existed at a very 
early time, when the energy density was high enough.  But how can one talk 
about time, when time is not well defined.  Or did I just not understand it?

Response #:  1 of 1
Author:      A. Smith
Text:        Well, I do not think anybody understands it yet - That is why 
there really is no theory right now.  The problem basically is how to quantize 
gravity.  Gravity is associated with the shape and actual fabric of space-
time.  From experience with the quantum versions of other forces, it is 
expected that at low energies, for long times and large distances (the realm 
of our normal experience) the laws of quantum gravity will reduce to the 
ordinary laws we know describe the universe pretty well - the "classical" 
version of gravity due to Einstein.  It is also known from previous experience 
roughly where energies become "low", times become "long" and distances "large" 
for quantum gravity - these are the so-called Planck scales arrived at by 
combing Planck's constant h with the gravitational constant G and the speed of 
light.  The energies at which the quantum nature of gravity become important 
are something like 10^16 times what particle accelerators can achieve, and the 
length scales and time scales are correspondingly shorter than anything we 
have investigated up to now.  This puts it fairly well beyond experimental 
investigation for the foreseeable future.  But theoretical speculation 
persists ence the quantum foam and other ideas.  Basically we do not have a 
clue what space-time looks like at extremely short times and lengths, but we 
are pretty sure it is very strange.  However, because regular laws are obeyed 
for longer times and lengths, the regular definition of time works fine for 
this.