Russell McMahon wrote:
>There is nothing in nature that DEMANDS the <= 100% that we are used to.
>The
>laws of thermodynamics are simply statements of (invariably) observed
>"fact"

This is essentially correct, except that the law of conservation of
energy (which is related, via Noether's theorem, to symmetry with
respect to time translation) has been so successful in every
experiment ever done, that it is as close to a "true fact" as you
are likely to find anywhere in physics. Similar comments apply to
conservation of linear momentum (related to symmetry with respect
to spatial translation) and angular momentum (related to symmetry
with respect to rotations). You may have heard the story of the
"invention" of the neutrino. In the late 1920's, it appeared that
beta decay violated conservation of momentum. Rather than throw
the conservation law out the window, most physicists accepted
Pauli's suggestion that there must be another, unseen particle
that would account for the apparently missing momentum. The belief
in the conservation laws was so strong that physicists were willing
to accept the existence of a particle for which there was *no*
other evidence! 25 years later, the neutrino was observed directly,
and that belief was thus shown to have been justified.

The connection between conservation laws and symmetries of spacetime,
embodied in Noether's theorem, is very deep and very important.

>(up until recently). Some of the "facts" that people seem to be observing
>at
>stellar distances suggest processes that do not fit the laws as we know
>them. [...]

None of this has anything to do with conservation of energy. The
existence of dark matter or a "fifth force" or a non-zero
cosmological constant, none of these violate the conservation
laws. They challenge some aspects of the Standard Model, but they
do not suggest violation of the conservation laws (although the
recent indications that the fine-structure constant might change
slightly over time could challenge global time symmetry, which
could, in turn, indicate a small change in global energy
conservation).

>You can, theoretically, using Einsteinian rules, build a time machine
>[...]

This is correct insofar as there are valid spacetime topologies that
exhibit closed time-like loops. But it remains to be seen how the
universe would deal with violations of causality. Just because
something does not violate classical general relativity does not
mean it is physically possible. For one thing, classical GR can't
be the complete theory of gravity because it is incompatible with
quantum mechanics.

The main reason for this posting is to point out that recent
astrophysical indications that there is still a lot to learn
about our universe do not necessarily challenge the conservation
laws. Indeed, the conservation laws have been so widely, wildly
successful that it will be a long, long time, and only in the
face of very convincing evidence, before physics would abandon
them.

Michael

Thank you for reading my little posting.



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