Re: Genealogical Momentum
From: Tim Tyler (tim_at_tt1lock.org)
Date: 03/14/05
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Date: Mon, 14 Mar 2005 01:07:40 -0500 (EST)
Perplexed in Peoria <jimmenegay@sbcglobal.net> wrote or quoted:
> "Tim Tyler" <tim@tt1lock.org> wrote in message news:d0vta3$2skn$1@darwin.ediacara.org...
> > Perplexed in Peoria <jimmenegay@sbcglobal.net> wrote or quoted:
> > > "Guy Hoelzer" <hoelzer@unr.edu> wrote in message news:d0tfar$23ck$1@darwin.ediacara.org...
> > > > in article d0shbh$1o73$1@darwin.ediacara.org, Tim Tyler at tim@tt1lock.org
> > > > > Guy Hoelzer <hoelzer@unr.edu> wrote or quoted:
Warning: this post is mostly more off-topic physics-discussion :-|
> > > > >> BTW, if you want an example of equations that are not reversible
> > > > >> take a look at reaction diffusion equations, which I think were
> > > > >> originally created by Alan Turing. You can also have a look at
> > > > >> the equations Prigogine used to describe open dissipative systems.
> > > > >
> > > > > Reaction diffusion equations can be reversible in principle.
> > >
> > > Rather obviously not. Or rather, only at equilibrium (where they don't
> > > DO anything!)
> >
> > You are mistaken about that :-(
> >
> > There's nothing about either the reaction or diffusion elements that
> > requires irreversibility.
[...]
> > > I think that what you are suggesting is that these equations
> > > can be reductionistically derived from reversible laws of physics.
> >
> > No, I - meant just what I said.
> >
> > Also, be advised that I know what I'm talking about in this area [...]
>
> I think we both know what we are talking about, and we have no disagreement
> on the facts. What we have a disagreement on is the use of language. I
> think that your use of the word "reversible" has been distorted from the
> norm by your interest in cellular automata.
>
> Irreversibility can manifest itself at either the macro level or the micro
> level. It is well known that micro level reversibility can manifest
> itself as macro level irreversibility. And, it is also well known that
> (with a few exceptions involving the weak force) physical law is time
> reversible at the micro level. That is why most people use the word
> "reversible" to refer to the macro level. At that level, phenomena
> such as N-body celestial mechanics are called reversible, whereas
> phenomena involving the second law such as diffusion or chemical reactions
> far from equilibrium are called irreversible. I think that you already
> know this. Using the word "reversible" at the micro level would be
> rather pointless, since (with a few exceptions involving the weak force)
> everything is always reversible.
Not everything is necessarily reversible. In the past physicists
have entertained the possibility that the laws of physics are
*not* reversible. They used terms such as "wave function collapse"
to describe the idea.
Modern formulations of physical law all describe the universe as being
reversible - but I don't think reversibility is obvious or trivial.
Only last year, Stephen Hawking was claiming temporal symmetry was
violated, in opposition to practically all other physicists. See:
http://google.com/search?q="black+hole+information+loss+paradox"
...for details about that. He has since publicly recognised his mistake.
In principle - reversibility might yet be overturned by someone
finding an irreversible wrinkle in some microscopic system somewhere.
> However, in cellular automata, the micro rules may be either reversible
> or irreversible. So, it is no longer true that the word "reversible" is
> useless at the micro level.
The term "reversible" was never useless at the "micro level" in the first
place. The concept it described was always clear.
What was *not* clear for some time was whether the physics of our universe
was *really* microscopically reversible or not.
> Which level was Guy talking about? Clearly the macro level. Are reaction
> diffusion differential equation systems irreversible at that level?
> Of course they are. The state variables are concentrations of various
> chemical species (as a function of spatial location). The state variables
> in reaction diffusion ARE NOT the velocities of the molecules! Those would
> be appropriate state variables only in a micro-level analysis.
>
> Of course, you know all this already. And upon encountering Guy's
> assertion that reaction diffusion equations are not reversible, you
> decided that you had to "correct" him. Did you tell him the interesting
> but irrelevant fact that these irreversible equations result from an
> underlying micro level reversibility? No, you told him that the
> equations are reversible. I am not going to apologize for suggesting
> that you intention was not to enlighten. It was to create a controversy.
*Some* equations used to describe reaction diffusion systems are
irreversible. Others are not. It depends on which system you
are looking at.
Your claim - quoted above - that non-trivial reaction diffusion
equations could obviously not be reversible in principle
seems to me to fail in the face of equations that completely
describe a reaction-diffusion system while remaining
entirely reversible.
> > > > > Basically, the whole of the laws of physics exhibit time-reversal
> > > > > invariance - or at least all the modern attempts to formulate
> > > > > physical law have this property - so the rule of thumb is if
> > > > > you can do something in the real world, it can be done reversibly.
> > > >
> > > > How would you argue that the second law of thermodynamics exhibits
> > > > time-reversibility? They are inconsistent IMHO.
> > >
> > > To say nothing of kaon decay (violates CP symmetry).
> >
> > ...which does not, of course, violate reversibility either - since
> > it is *T*CP symmetry which is can't be violated:
>
> Hmmm. I thought that we were discussing T invariance, which CAN be
> violated (and is in kaon decay).
What I said was "reversibility".
IMO, reversibility under our universe's laws of physics should mean
much the same as T-invariance. And it /would/ mean that - if
physicists properly embraced the possibility that parity and charge
could be chiral dynamical processes - analogous to spin.
However, regrettably - possibly for reasons of historical legacy -
that is not their conventional usage.
This leads directly to highly misleading ideas - such as the one you
are referring to here - that T-invariance is violated in kaon decay.
That /would/ only be true if you *define* parity and charge to
be time-independent entities.
However, it doesn't look as though parity and charge are remotely
time-independent :-|
As Feynman pointed out, an electron moving backwards in time
becomes a positron. Its parity and charge are flipped. So: definitions
of C and P that don't take that dependency of charge and parity on time
into account are archaic - and that the underlying cause of assertions
about T-invariance being violated is bad terminology.
Spins automatically reverse when time is inverted - and in
Feynman digrams, parity and charge work in just the same way.
In practice, they probably do so for the same reason that spins
reverse: they all have some underlying chiral dynamical representation.
So: if you adopt *sensible* terminology relating to parity and charge,
reversibility in physics would be the same as T invariance, which
would hold.
However, today, only a few physicists use such terminology - and while
physics is still reversible, the operation that inverts the passage of
time is no longer simply inverting time, it's inverting time, parity
and charge; the invariant is not simply T, it is TCP - because of the
history of the definitions of C and P :-(
> You are here suggesting that kaon decay is reversible, if you change
> the matter into anti-matter and also somehow flip left and right.
Not left and right: charge. Positive to negative - and visa versa.
> Well, yes. So we will come up with yet another non-standard definition
> of "reversible" and Tim's claim that the laws of physics are reversible
> is justified. Left is right, matter is anti-matter, and Tim is right
> once again.
The term "reversible" in the context of dynamical systems means that they
can be run backwards - and thus that they don't lose information about
their state during their evolution.
The operation that actually runs the system backwards need *not* be
simply inverting T. It may be that a transition function needs to
be applied to the state of the system at the point at which T is
inverted before the reverse evolution will proceed smoothly.
AFAICS, this usage of "reversible" is the standard usage of the term
in dynamical systems theory.
IMO, our physical universe happens to be one which is
reversible under the transformation T -> -T - *provided*
a sensible description of the state of the universe is employed.
Anyway, it seems like our disageement mostly centred around terminology.
I /hope/ I have been clear enough now about what I mean.
-- __________ |im |yler http://timtyler.org/ tim@tt1lock.org Remove lock to reply.
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