From: Daniel E. Azzopardi (desva@slac.stanford.edu)
Date: Sun Nov 26 2000 - 15:40:11 MST
On Sun, 26 Nov 2000, Eliezer S. Yudkowsky wrote:
> surfaces. Sigh... I either need to actually read my set of the Feynman
> lectures, find a good physics encyclopedia, or give up physical science.
My recommendation would be to stick with Feynman, but don't use physical
science arguments unless they are quantitative and/or predictive.
> Paul Davies sez: "It is easy to dream up scenarios that produce
> unphysical or paradoxical consequences--building perpetual motion
> machines, or even travelling backwards in time. To physicists, these are
> alarming notions."
> Who knows, Davies may turn out to be right; but even so, this strikes me
> as the sort of thing that future historians call a 'blind spot', like the
> unbreakability of the sound barrier or the impossibility of
> heavier-than-air flight. "Despite numerous demonstrations that their own
I think Physicists of our time are generally are much less willing to
dismiss things out of hand (alarming notion!=physical impossibility).
However, I must take issue with the analogies you make here, especially as
you are either glossing over, or overlooking things which make these
different kinds of "barriers", rather than different magnitudes. The sound
barrier/heavier than air arguement is oft quoted w.r.t. travel at speeds
greater than c (tantamount to time travel, which I'll come to). No
physical theory said faster than sound flight was *physically* impossible.
There is a theory which says faster than light travel is physically
impossible though (Relativity, both general and special). Though there are
some (theoretical, not observed) metrics which appear to violate this
locally, globally flat space time is a pretty good approximation. Flat
space time is special relativity(SR), and whatever way you write it, and
however you do it, globally SR has never been violated and there is no
theoretical way to violate it either - QM obeys it, even including EPR
paradoxes and mesoscopic systems.
So - in contrast to the vague reasons given for the limits to
human technology as regards supersonic flight, reliable(*) physical theory
has given us concrete physical limits to our universe, as we understand it
now. There is no loophole without an effect that has yet to be discovered.
Let me say that again another way: though we may construct theories that
don't disallow certain things (eg: time travel) these theories do not
belong to the minimal consistent set of physical theories.
< (*) aside: how reliable is SR? How many tests of SR do we do each day
that CERN / SLAC / FermiLab run? Millions? How many times has SR been
wrong? Never. Would we see it if it was wrong just once? Oh yes!>
Time travel: We need accept only one of: universal flat space time, upper
speed limit for information transfer=c, or causality, to run into immense
difficulties constructing a theory which allows time travel. In fact, with
out accepting causality we run into immense problems constructing any
logical theory. You are confusing technology and science - technology
enables us to do things which are physically possible, but new technology
has never directly shown new physics - only (human) ingenuity at applying
current technologies has (so far) done that. I accept that beyond our
singularity we will be effectively reversing this situation (that's the
whole point!) but you should not construct a logical discussion on points
outside current theories unless you derive a concrete result.
> theories permitted the possibility of time travel / negative energy /
> naked singularities etc., twentieth-century physicists still refused to
> believe, calling them 'alarming notions'." This rant is more about time
> travel then about thermodynamics, but real physical laws don't look like
> they're about to be violated every second Tuesday. Sometimes physics
Thermodynamics is something you should be concerned with. Locally, at
scales <10^-10 m (say) physics is CPT invariant, yet the emergent
properties of matter are anything but (as you have stated). If you hold
that intelligence is an emergent property of correctly /near optimally
arranged matter, than one would have thought we're going to need to be
able to understand the transistion phase from quantum to statistical
mechanics as the best modelled physical analogue we have.
> "permissible fluctuations in entropy", but did you ever hear of a
> permissible fluctuation in mass-energy?
Vacuum pair creation, on short time scales.
> If current physical theory offends a physicist, so what? God does play
> dice with the Universe, Einstein, get over it. Even the more imaginative
> physicists, like David Deutsch, suffer from the same disease, confining
Physicists must work in science fact, not fiction. For example, we have
several candidates for a GUT. Which one do we choose? We can choose our
favorite, we can choose the one that says every particle is really an n-th
dimensional donut because it lets us do something we'd really like to do
(eat donuts), but all that means nothing if we don't have physical
evidence for the theory. We had a theory that predicted the Tau lepton
but we waited until it was observed before we attached that theory to
"reality". Imagination is essential for physics, and there are a lot of
imaginative theories out there. Keep your head in the clouds but your feet
on the ground - until there is evidence for an effect (causality
violation, time travel, whatever) assume it may be possible, but don't
skew your objective judgement of theory and fact.
> closed timelike curves to quantum universes and so on. Time travel,
> *including* global causality violation, is explicitly permitted by General
> Relativity.
No, it isn't. GR must obey SR over large enough distances (and SR is
prerequisite for GR). The Universe obeys SR (flat space time metric). In
SR you can not violate causality, full stop.
> Physicists will just need to learn to deal with it.
Er, no. The Universe is the Universe, and that's what we deal with. You've
taken a model of the universe (GR), together with one small group of
possible interpretations of the model (some fancy metric which requires
negative mass, gravitational potentials larger than those at an event
horizon, or some other such - admittedly theoretically interesting
-nonsense) and remapped that interpretation back on to the Universe,
ignoring the fact that the original interpretation relied on something
unphysical in the first place. Sorry, but that is faulty reasoning. If you
did understand GR completely (which you don't), you still would have no
right to say "Ah, but we can do this in our model (if we push all our
paremeters to one corner of the phase space) therefore we can do this in
reality". In addition, you are attempting to use a technology
(mathematical constructs around GR) to fault the apparent narrow
mindedness of physicists who use and develop that technology as their own
tool. To do that, you better be on a very firm foundation - and it's clear
you're not.
> Oh, never mind. I'm not a physicist. You can get personally annoyed with
> physical theories, but you're supposed to buy the right to do so with a
> physics doctorate. This system works so well generally that I would have
> no real objection to being slapped down by it personally. Not my job.
> </RANT>
I like this though ;-)
> This is an interesting way of looking at it. It looks to me like
> particles being created and destroyed - in particular, negative-energy and
> positive-energy particles annihilating, as opposed to matter-antimatter
> interactions which give rise to gamma rays or other new particles - are
I think you're talking about the same process here: negative energy states
are one way of looking at anti-matter states, at least in basic
formulations of Quantum Field Theories.
> Universe, where a negative and positive particle pair will come into
> existence; thus the current phase-space point of the Universe can give
> rise to multiple points at the next instant - one possible alternate point
> for each place a virtual particle pair can pop up. Similarly, a virtual
> pair ("virtual pair" in the negative-positive, not matter-antimatter,
> sense) can annihilate either over *here* or over *there*, and the state of
> the Universe will be same either way, since neither particle exists any
> more - a many-to-one interaction. It's an interesting question whether
> this same objection applies to Feynman diagrams; that is, whether a
> positron and electron can collide in two different ways and give rise to
> precisely the same gamma particle. I myself don't know.
I'm not quite sure what physics you're trying to describe here; Feynman
diagrams describe point-like interactions; there is only one diagram for
e+ e- annihilation to a photon (one vertex, three legs). Remember Feynman
diagrams are only tools to represent rather complex integrations - the
physics is the physics we get from the Universe, the math is the modelling
(QM, perturbation theory, Hamiltonian, integration), the Feynmen diagram
is the mnemonic.
> (As Hal Finney from "Extropians" put it:
> > There is a 'quantum thermodynamics', it's called statistical
> > mechanics, and entropy still increases there. There's interesting
> > recent work in quantum information theory which suggests that
> > entropy and entanglement are related, and maybe there will be
> > distinctively quantum ways to *locally* decrease entropy ... but
> > there's no inkling of a global violation of the second law.
What is the link between simple symmetry, which allow us to write down
elegantly the Lagrangian for the Standard Model of Particle Physics, and
Statistical mechanics? Where do "simple" CPT invariance and symmetry,
and deterministic particle interactions become the stocastic chaos of
statistical mechanics? At what point is T (Time reflection, interchange
of forward and backward lightcones) no longer a symmetry of a physical
system because of statistics? (Actually, T is violated for weak nuclear
interactions at the quantum level as well...). Once we know the answers to
these questions we can start playing around with the bridge between the
QM world and the "real" macroscopic one.
> The problem is, of course, that the many-worlds model just isn't true. If
You can't just say that, since there are no observational consequences of
the many worlds model. You are ruling something out in almost the same way
as I'd rule out the class of GR metrics that allow time travel, except one
senario has testable experimental consequences, the other doesn't ;-)
Interpretation of QM is philosophy.
> quantum collapse were an observational-effect illusion, it would make no
> difference what the probability amplitudes were; we would have the same
> observed chance of winding up in a .2 probability Universe as a .8
No, the probabilities would still be different.
> crystal in which quantum effects turned heat directly into electricity,
> although it might not be direct current, or at least not direct current of
> a predictable direction, as that would violate CPT.
Point of note: if you're going to allow time travel, you can violate CPT
at the quantum level.
> But your objection is probably correct; e.g., performing a discrimination,
> or continually checking for one, on an entire physical system, could
> easily turn out to have entropy costs far in excess of entropy gained.
This is calculable.
> A more interesting idea is this: Perform the Maxwell's Demon
> discrimination, extract the information, send the information backwards in
> time, and then don't perform the costly process that got you the
> information in the first place. This throws away entropy into a future
> that gets wiped out of existence by tampering with the past.
This is where you need to remember that the theories you're using are a
consequence of causality in the first place. Entropy is a consequence of
the arrow of time. If it were easy for the Universe to dump things in
futures that don't happen then that is what would happen, and we wouldn't
be talking about entropy.
Hmmm.. I've gone on rather longer than I'd hoped, but the points I've
attempted to make are these:
If one is going to use physical theories for some thing concrete
(ie: predictive) then use the theory correctly - don't use a theory for an
analogy, use it to calculate, that is what it is there for. Entropy is a
precisely defined quantity for example (but be aware of the tacit
assumptions made in defining a quantity).
If one is going to speculate about science fiction (a possible
effect which hasn't been observed yet, that would have "scientific"
reprecussions) then speculate, but don't do it within the framework of the
general consensus, since those theories were formulated to explain only
observed effects. Certainly one shouldn't use the fact that an
interpretation of an individual theory allows something to conclude that
that something exists... minimise your assumptions.
Cheers,
Dan
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| Daniel E. Azzopardi | http://www.desva.co.uk | QMW: +20 7882 |
| Back in the UK ! | mail:desva@slac.stanford.edu | 3767 |
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