🚀 go-pugleaf

jbaez@riesz.mit.edu (John C. Baez) writes:
: 
: I am awed by your certainty.  Many of the best researchers in quantum
: gravity would dearly like to know whether spacetime is a
: semi-riemannian manifold.  What info do you have that they don't? 
: Barring seances with Einstein's ghost, I would prefer to say
: "spacetime is well modelled by a semi-riemannian manifold on
: sufficiently large distance scales."  

Two points. First, nobody in that discussion mentioned quantum effects.
So naturally what I meant was that CLASSICAL spacetime IS a
semi-riemannian manifold. The kind of thing I am criticising is the
speculation that big-bang type singularities are meaningless because no
clock could endure arbitrarily large tides. To that I reply that
CLASSICALLY time is well defined mathematically [as the length of a
timelike worldline] and that I am not interested in clocks. In short, I
was repudiating operationalism, not quantum gravity!
ON THE OTHER HAND I am extremely sceptical of statements such as "at
sufficiently small length [huh? what's that?] scales, spacetime is
replaced by [wormholes, worms, knots ,loops, strings, etc]. Spacetime is
the set of all events. What does it mean to say that events are
"replaced" by anything? : 
: On the other hand, I agree that the idea of time being "defined" by
: clocks is essentially a cop-out unless one has a good theory of what
: counts as a clock and why cesium clocks are regarded as "better"
: clocks than baloney sandwiches.  (In other words, although I am of an
: operationalist bent, I don't think one can weasel out of the issue of
: time simply by defining time to be that which is measured by that
: which we build to measure time.)
: 
: As for whether time "exists," until someone lays out their precise
: definition of "exists" I will use my everyday sloppy one, according to
: which time, numbers, baloney sandwiches and societies all exist, even
: though they are very different sorts of things.   
: 
Sorry about the title, I am just learning how to play with this thing.: