Re: Symmetry

Barry <Sirdry@xxxxxxxxxxx> wrote:

I would appreciate it if you could find my errors in the following
observations. They seem to cause much dissent:

[...]
(i) There is nothing in GR, per se, that predicts that "singularities
will not be observed" or that forbids closed timelike curves. Cosmic
censorship has to be invoked in both cases.

This is a misunderstanding of cosmic censorship. The cosmic censorship
conjecture is not something outside of GR that needs to be independently
"invoked." It is a hypothesis that *within GR*, given certain positivity
conditions for energy, generic initial data never evolves to produce
naked singularities. We don't know whether this hypothesis is correct;
it can be proven in certain special circumstances, and there are no
known counterexamples. If it is correct, it is "in GR, per se."

In short, the response to your claim that "There is nothing in GR, per se,
that predicts that `singularities will not be observed'" is that it is
highly premature -- we don't know whether this is correct or not.

(ii) I very much doubt that singularities exist. Most theoreticians
suggest that GR "breaks down" at "singularities".

For "typical" singularities -- those at which some curvature invariant
diverges -- it is almost certainly true that *classical* GR "breaks down,"
in the sense that quantum gravitational effects become important.

(iii) At singularities, timelike trajectories seem to "stop" or "leave
the Universe".

At singularities, at least *some* timelike or null trajectories seem to
"leave the Universe." This is, again, a classical statement; no one knows
how classical singularities would manifest themselves in quantum gravity.

(iv) Having two conflicting uses for the word "time" ("coordinate time"
and "proper time") has led to unresolved paradoxes.

No. In quantum gravity, there are a variety of problems collectively
called the "problem of time," but none of these has much to do with your
statement here.

(v) Nature has a special space in her Heart for symmetries.

Maybe.

(vi) Nature shows herself to us in a 3-dimensional guise.

It depends how we look.

I really don't know why those statements cause such dissent.

Well, some of them are wrong, and some are, at least, highly ambiguous.

Steve Carlip
.

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