=?ISO-8859-1?Q?Gy=F6rgy_Szondy?=:
(Bilge) wrote in message ...
Wendy Yamamoto:
Physicists and astronomers use conventional units of time in speaking
of the Big Bang ("one second after the Big Bang", etc.). But how
meaningful are such statements?

That depends. Unless something very surprising pops up, what happened
on the order of a second is pretty sound. By looking at the current
temperature and size of the universe and then extrapolating backwards,
it's possible to get much more detailed than one second. We know the
strength of the weak and strong interactions and since unification
of those forces happens at specific temperatures, it's fairly straight
forward to provide a picture of the universe beginning around the
first 10^-40 seconds or so. Steven Weinberg's book, "The First Three
Minutes" gives a very good and easy to read account of exactly what
the title describes.

If time and space are local and depend on the relative velocity of the
observer, how can our own local definitions of time (seconds, minutes)
be used to describe an interval of "time" during Big Bang conditions?

Measurements made by different observers can be reconciled because
relativity describes the relationship. Essentially, all observers
should agree that the universe started at the same time.

Big Bang assumes that the worldline of all particles started at the
same point. Threfore you can observe the univerese from the
perspective of any particle (or object) and use its time.

You missed the point. What she was asking was in regard to observers
who have relative velocities with respect to each other and therfore
must reconcile their measurements. Those observers will see the same
phenomena differently. That is why I qualified that the way I did.
Had I simply left it as you've stated it, I wouldn't have answered the
question being asked.

That again depends. If there are no surprises, then presumably a second
is a second is a second, at least until you get below 10^-40 (or perhaps
10^-50) seconds or so. At that point, I'm not sure it would be meaningful
to think of the universe in those terms. That would be the era of quantum
gravity and neither E&M or nuclear forces would have existed.

Second is measured by clock. Time in General Relativity is also
defined by clock. Therefore 3 second is allways the same length of
time by definition.

What do you define as a clock when there is nothing to use as a
standard clock? We use the speed of light, but since light did not
exist before electroweak symmetry breaking, this is obviously not
a universal definition. One could presume the constant c is more
fundamental that the phenomena to which it applies, but I think that
can only be justified to a point and I am not sure it can be justified
at all prior to the planck time. If general relativity breaks down
at the planck time, then I don't think that you have any real means
of defining time and distance in any way that resembles the definitions
used in general relativity.