| Topic: |
Science > Philosophy |
| User: |
"Sir Frederick" |
| Date: |
04 Apr 2004 12:07:18 PM |
| Object: |
On the Space-Time Vacuum |
THEORETICAL PHYSICS: ON THE SPACE-TIME VACUUM
ScienceWeek http://scienceweek.com
The following points are made by R. B. Laughlin (Science 2004
303:1475):
1) In discussing cosmic matters it is impossible not to draw
analogies with science fiction from time to time, for the issues
are as large as those depicted in science fiction and equally
mysterious, despite being experimentally constrained.(1) Our
knowledge of the cosmos is still very primitive, and much of our
thinking about it correspondingly speculative, more along the
lines of what might plausibly have been than what is so.
Plausibility is an interesting concept in theoretical physics,
usually amounting to either a physical analogy with something
observed to occur elsewhere in nature or a mathematical
extrapolation of microscopic law. The latter, however, is
actually a shibboleth, for the things that matter are nearly
always collective organizational phenomena that cannot be
reliably predicted from microscopics. The shapes of galaxies and
the behavior of cosmic jets are simple cases in point, but the
observation also applies to the grandest issues of modern
cosmology: inflationary expansion and the hierarchical
consolidation of matter after the big bang (2-4). The absence of
predictive power is actually self-evident, because there would be
no point in measuring these things if one could calculate them.
As a practical matter, all plausibility arguments that count are
analogies.
2) It may seem shocking to speak of the vacuum of space-time as
an organizational phenomenon, but this is actually just a matter
of semantics. The idea behind the words is mainstream and fully
consistent with the facts. It has been known since the 1950s, and
routinely verified by accelerator experiments since then, that
empty space is a kind of matter quantum-mechanically similar to a
rock (5). The standard model of elementary particles is grounded
firmly on the idea of space as a phase. A multiplicity of such
phases and a complex sequence of transitions among them in the
early universe are corner-stones of modern particle cosmology.
The existence of such phases is implicated in the structure one
sees on intergalactic scales, and the heat released in the
transition between two of them is the ostensible power source of
inflation. Inflation itself is partly motivated by these phases,
because they make the observed uniformity of the universe
unnatural and something requiring explanation.
3) The semantic incongruity, however, like the sublimated worries
about modern life that give us science fiction nightmares, belies
something important -- unfinished business of the 1970s that has
been slowly and systematically tearing physics apart. Stripped of
their confusing mathematical descriptions, the phases of the
vacuum boil down to physical analogies with phases of ordinary
matter, natural phenomena observed to exhibit universality. That
means that their properties at long length and time scales, where
we normally do experiments, do not depend on microscopic details
at all, and thus do not constrain them when measured. A simple
example of emergent universality would be sound propagation in
fluids and solids, an effect perfectly well accounted for as the
motion of atoms, but also a generic property of the phases not
requiring atoms to make sense. Sound is an especially pertinent
example because it has a second identity at low temperatures as
an emergent elementary particle with properties identical to
those of particles of light. Insensitivity to microscopic detail
thus turns the concept of fundamental on its head, in that it
makes principles of self-organization the truly important thing,
rendering the quantum underpinnings of the Universe, whatever
they are, unknowable in the absence of experiments that reach
shorter scales and irrelevant to behavior we presently see.
Little wonder that physicists remain bitterly divided over full
acceptance of the vacuum as a phase.
References (abridged):
1. Akira, 124 min, directed by Katsuhiro Otomo (Kodansha Ltd.,
Japan, 1988)
2. S. Weinberg, The First Three Minutes: A Modern View of the
Origin of the Universe (Basic Books, New York, 1994)
3. M. Rees, New Perspectives in Astrophysical Cosmology
(Cambridge Univ. Press, Cambridge, 2000)
4. A. H. Guth, A. P. Lightman, The Inflationary Universe: The
Quest for a New Theory of Cosmic Origins (Perseus, New York,
1998)
5. M. E. Peskin, D. E Schroeder, An Introduction to Quantum Field
Theory (Westview, Boulder, CO, 1995)
Science http://www.sciencemag.org
--------------------------------
THEORETICAL PHYSICS: ON THE THEORY OF EVERYTHING
The following points are made by R.B. Laughlin and D. Pines
(Proc. Natl. Acad. Sci. US 2000 97:28):
1) The term "Theory of Everything" refers to the ultimate theory
of the Universe, a set of equations capable of describing all
phenomena that have been observed, or that will ever be observed.
It is the modern incarnation of the reductionist idea of the
ancient Greeks, an approach to the natural world that has been
extremely successful and which for many people is the central
paradigm of physics. A special case of this idea is the general
wavefunction equation of nonrelativistic quantum mechanics, which
describes the everyday world in terms of known quantities: the
charge and mass of the electron, the charges and masses of the
atomic nuclei, and Planck's constant. Less immediate things in
the Universe, such as the planet Jupiter, nuclear fission, the
Sun, or the isotopic abundances of elements in space are not
described by this equation, because important variables such as
gravity and nuclear interactions are missing. But concerning
everyday people-scale phenomena, this equation is for all
practical purposes the Theory of Everything for our everyday
world.
2) The authors suggest, however, that examining the Theory of
Everything, it becomes obvious that it is not even remotely a
theory of every thing. We know the equation is correct because it
has been solved accurately for small numbers of particles
(isolated atoms and small molecules) and found to agree in minute
detail with experiment. But it cannot be solved accurately when
the number of particles exceeds approximately 10. The authors
suggest that no computer existing, or that will ever exist, can
break this barrier -- it is a "catastrophe of dimension": If the
amount of computer memory required to represent the quantum
wavefunction of one particle is N, then the amount of computer
memory required to represent the wavefunction of k particles is
N^(k). Although it is possible to perform approximate
calculations for larger systems, and such calculations have in
many cases been valuable, the schemes for approximating are not
first-principles deductions, they are rather art keyed to
experiment. These approximate approaches thus tend to be the
least reliable precisely when reliability is most needed, i.e.,
when experimental information is scarce, the physical behavior
has no precedent, and the key questions have not yet been
identified.
3) A variety of physical phenomena easily observed in the
laboratory (e.g., the *quantum Hall effect, *superfluid helium,
*Josephson effect) permit measurements of exact quantities that
cannot be deduced by direct calculation from the present Theory
of Everything, for exact results cannot be predicted by
approximate calculations. The authors suggest this point is still
not understood by many professional physicists, who find it
easier to believe that a deductive link exists, and has only to
be discovered, than to face the truth that there is no link. But
the absence of a link is true nonetheless, and not denied by the
reliability of such experiments: The important consideration is
that experiments concerning these physical phenomena work because
there are higher organizing principles in nature that make them
work.
4) Concerning Big Bang cosmology and attempts to develop
fundamental theory from considerations and observations of the
early Universe, the authors suggest that no one familiar with
violent high-temperature phenomena would dare to infer anything
about the equations of quantum mechanics by studying explosions,
for explosions are unstable and quite unpredictable from one
experiment to the next. The authors suggest that the assumption
that the early Universe should be exempt from this problem, and
that a Theory of Everything can be inferred from observations of
the early Universe, is not justified by anything except wishful
thinking. It could very well turn out that the Big Bang is the
ultimate emergent phenomenon, "for it is impossible to miss the
similarity between the large-scale structure recently discovered
in the density of galaxies and the structure of styrofoam,
popcorn, or puffed cereals."
5) The authors suggest that the fact that the essential role
played by higher organizing principles in determining emergent
behavior continues to be disavowed by so many physical scientists
is a poignant comment on the nature of modern science. To solid-
state physicists and chemists, who are schooled in quantum
mechanics and deal with it every day in the context of
unpredictable electronic phenomena such as that exhibited by
*Kondo insulators or *cuprate superconductivity, the existence of
these organizing principles in emergent behavior is so obvious
that it is a commonplace not discussed in polite company. But to
other scientists, the idea is considered dangerous and ludicrous,
since it is fundamentally at odds with the reductionist beliefs
central to much of physics. But, the authors suggest, the safety
that comes from acknowledging only the facts one likes is
fundamentally incompatible with science, and sooner or later this
attitude "must be swept away by the forces of history".
6) The authors point out that for the biologist, evolution and
emergence are part of daily life. For many physicists, on the
other hand, the transition from a reductionist approach may not
be easy, but should, in the long run, prove highly satisfying.
Living with emergence means, among other things, focusing on what
experiment tells us about candidate scenarios for the way a given
system might behave before attempting to explore the consequences
of any specific model. This contrasts sharply with the imperative
of reductionism, which requires us never to use experimental
observations in the formulation of theory, as the objective of
reductionism is to construct a deductive path from the ultimate
equations to the experiment without "cheating". But this is
unreasonable when the behavior in question is emergent, for the
higher organizing principles -- the core physical ideas on which
the model is based -- would have to be deduced from the
underlying equations, and in general this is impossible.
Repudiation of this physically unreasonable constraint is the
first step down the road to fundamental discovery.
7) The authors conclude: "The central task of theoretical physics
in our time is no longer to write down the ultimate equations but
rather to catalogue and understand emergent behavior in its many
guises, including potentially life itself. We call this physics
of the next century the study of complex adaptive matter. For
better or worse we are now witnessing a transition from the
science of the past, so intimately linked to reductionism, to the
study of complex adaptive matter, firmly based in experiment,
with its hope for providing a jumping-off point for new
discoveries, new concepts, and new wisdom."
Proc. Nat. Acad. Sci. http://www.pnas.org
--------------------------------
Notes by ScienceWeek:
quantum Hall effect: In classical physics, the Hall effect is the
development of a transverse voltage across a current-carrying
conductor in a magnetic field, the voltage being perpendicular to
both the direction of the current and the direction of the
magnetic field. In quantum physics, there are two other Hall
effects, an integer charge quantum Hall effect, and a fractional
charge quantum Hall effect, these quantum Hall effects being
observed at extremely low temperatures (a few kelvins) and
extremely intense magnetic fields (at least several tesla). Both
quantum Hall effects were first noted in the 1980s, and the
fractional quantum Hall effect, although experimentally observed,
has not been theoretically resolved. In 1982, R.B. Laughlin (one
of the authors of the paper reviewed in the present report)
postulated the theoretical existence of quasi-particle
excitations with fractional charge e/3, where e is the
conventional electronic charge, the quasi-particle being the
statistical result of the collective motion of many electrons. R.
de-Picciotto et al (1997) apparently demonstrated unambiguously
the existence of quasi-particles with fractional charge as
predicted by Laughlin's theory.
superfluid helium: In general, a "superfluid" is a fluid that
flows without any resistance. "Superconductivity" is sometimes
considered as a special case of superfluidity in which the
"fluid" components (electrons) are charged. But more
conventionally, superfluidity is considered a property of liquid
helium at extremely low temperatures, a property that enables
liquid helium to flow without friction. Both helium isotopes
(sup4)He (the common isotope, often denoted as helium-4) and
(sup3)He (the rare isotope, often denoted as helium-3) possess
superfluidity under special circumstances.
Josephson effect: In general, any of the phenomena that occur at
sufficiently low temperatures when a current flows through a thin
insulating layer between two superconducting substances. Such
phenomena were predicted theoretically by B.D. Josephson in 1962.
Josephson was 22 years of age when he made his theoretical
discovery; he received the Nobel Prize in Physics in 1973.
Kondo insulators: An insulator exhibiting the Kondo effect. The
Kondo effect is a large anomalous increase in the resistance of
certain dilute alloys of magnetic materials in nonmagnetic hosts
as the temperature is lowered. In general, the Kondo effect
occurs when an impurity atom with an unpaired electron is placed
in a metal, producing an interaction of localized electrons with
delocalized electrons.
cuprate superconductivity: In general, high-temperature
superconductivity exhibited by certain copper alloys. The
accepted theory of ordinary low-temperature superconductivity is
the Bardeen-Cooper-Schrieffer theory (1957). At the present time,
a successful theory of high-temperature superconductivity has not
been developed, in spite of a great deal of effort. J.G. Bednorz
and K.A. Mueller shared the Nobel Prize in Physics in 1987 for
their discovery of high-temperature superconductivity in a
ceramic oxide (lanthanum-barium-copper) alloy at 30 kelvins, at
that time the highest superconductivity temperature ever
observed.
ScienceWeek http://scienceweek.com
--
Best,
Frederick Martin McNeill
Poway, California, United States of America
mmcneill@fuzzysys.com
http://www.fuzzysys.com
http://members.cox.net/fmmcneill/
*************************
Phrase of the week :
We have entered the cell, the mansion of our birth,
and have started the inventory of our acquired wealth.
-- Albert Claude (1899-1983)
:-))))Snort!)
*************************
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