🚀 go-pugleaf

In article <1992Oct5.091234.29484@nuscc.nus.sg> matmcinn@nuscc.nus.sg (Mcinnes B T (Dr)) writes:
>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. 

In other words, in a certain model of physics spacetime is represented
by a semi-Riemannian manifold.  That sounds compatible with the sentence
you were lambasting, namely "spacetime is modelled by a semi-Riemannian
manifold".  But anyway, it sounds like the model/reality distinction was
not the main thing on your mind...

>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!

I think that all sorts of semi-bogus philosophizing is acceptable as a
warm-up to doing honest physics and mathematics, but it is certainly no
substitute.   If anyone seriously makes the argument you are
criticizing, I suppose I would have to take your side (much as it pains
me to ever agree with you).  Nature is certainly under no obligation to
provide us with clocks that function at arbitrarily high g-forces.  

>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? : 

What's getting replaced is not events but the notion that they are in 
one might replace this notion WITH, but it's definitely worth keeping in

mind as a possibility.  It would have to be some structure which is good
at looking like a manifold at larger distance scales.