[Paleopsych] SW: On Anthropic Reasoning
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Thu Sep 15 01:35:51 UTC 2005
Cosmology: On Anthropic Reasoning
The following points are made by M. Livio and M.J. Rees (Science 2005
1) Does extraterrestrial intelligent life exist? The fact that we can
even ask this question relies on an important truth: The properties of
our Universe have allowed complexity (of the type that characterizes
humans) to emerge. Obviously, the biological details of humans and
their emergence depend on contingent features of Earth and its
history. However, some requirements would seem generic for any form of
life: galaxies, stars, and (probably) planets had to form;
nucleosynthesis in stars had to give rise to atoms such as carbon,
oxygen, and iron; and these atoms had to be in a stable environment
where they could combine to form the molecules of life.
2) We can imagine universes where the constants of physics and
cosmology have different values. Many such "counterfactual" universes
would not have allowed the chain of processes that could have led to
any kind of advanced life. For instance, even a universe with the same
physical laws and the same values of all physical constants but one --
a cosmological constant Lambda (the "pressure" of the physical vacuum)
higher by more than an order of magnitude -- would have expanded so
fast that no galaxies could have formed. Other properties that appear
to have been crucial for the emergence of complexity are (i) the
presence of baryons (particles such as protons and neutrons); (ii) the
fact that the Universe is not infinitely smooth, allowing for the
formation of structure (quantified as the amplitude of the
fluctuations in the cosmic microwave background, Q); and (iii) a
gravitational force that is weaker by a factor of nearly 10^(40) than
the microphysical forces that act within atoms and molecules -- were
gravity not so weak, there would not be such a large difference
between the atomic and the cosmic scales of mass, length, and time.
3) A key challenge confronting 21st-century physics is to decide which
of these dimensionless parameters such as Q and Lambda are truly
fundamental -- in the sense of being explicable within the framework
of an ultimate, unified theory -- and which are merely accidental. The
possibility that some are accidental has certainly become viable in
the context of the "eternal inflation" scenario [1-3], where there are
an infinity of separate "big bangs" within an exponentially expanding
substratum. Some versions of string theory allow a huge variety of
vacua, each characterized by different values of (or even different
dimensionality) . Both these concepts entail the existence of a
vast ensemble of pocket universes -- a "multiverse." If some physical
constants are not fundamental, then they may take different values in
different members of the ensemble. Consequently, some pocket universes
may not allow complexity or intelligent life to evolve within them.
Humans would clearly have to find themselves in a pocket universe that
is "biophilic." Some otherwise puzzling features of our Universe may
then simply be the result of the epoch in which we exist and can
observe. In other words, the values of the accidental constants would
have to be within the ranges that would have allowed intelligent life
to develop. The process of delineating and investigating the
consequences of these biophilic domains is what has become known as
1. P. J. Steinhardt, in The Very Early Universe, G. W. Gibbons, S.
Hawking, S. T. C. Siklos, Eds. (Cambridge Univ. Press, Cambridge,
1983), p. 251
2. A. Vilenkin, Phys. Rev. D 27, 2848 (1983)
3. A. D. Linde, Mod. Phys. Lett. A 1, 81 (1986)
4. S. Kachru, R. Kallosh, A. Linde, S. P. Trivedi, Phys. Rev. D 68,
5. A. G. Riess et al., Astron. J. 116, 1009 (1998)
COSMOLOGY: ON THE ANTHROPIC PRINCIPLE
The following points are made by Lawrence M. Krauss (Nature 2003
1) The recognition, in the light of observational data, that
Einstein's infamous cosmological constant might not be zero has
changed almost everything about the way we think about the Universe,
from reconsidering its origin to re-evaluating its ultimate future.
But perhaps the most significant change in cosmological thinking
involves a new willingness to discuss what used to be an idea that was
not normally mentioned in polite company: the "anthropic principle".
2) This idea suggests that the precise values of various fundamental
parameters describing our Universe might be understood only as a
consequence of the fact that we exist to measure them. To paraphrase
the cosmologist Andrei Linde, "If the Universe were populated
everywhere by intelligent fish, they might wonder why it was full of
water. Well, if it weren't, they wouldn't be around to observe it!".
3) The reason that physicists have been so reluctant to consider the
anthropic principle seriously is that it goes against the grain of
current attitudes. Most physicists have hoped that an ultimate
physical explanation of reality would explain why the Universe must
look precisely the way it does, rather than why it more often than not
would not. Into the fray has entered James Bjorken. In a paper (Phys.
Rev. D 2003 67:043508) entitled "Cosmology and the Standard Model",
Bjorken proposes a new "scaling" approach, based on well-established
notions in particle theory, for exploring how anthropically viable a
small cosmological constant might be.
4) The realization that an extremely small, but non-zero, cosmological
constant might exist has changed the interest of physicists in
anthropic explanations of nature precisely because the value it seems
to take is otherwise so inexplicable. In 1996, physicist Steven
Weinberg and his colleagues Hugo Martel and Paul Shapiro argued that
if the laws of physics allow different universes to exist with a
cosmological constant chosen from an underlying probability
distribution, then galaxies, stars and presumably astronomers might
not ultimately evolve unless the cosmological constant were not much
larger than the one we apparently observe today.
Notes by ScienceWeek:
The "cosmological constant" is a mathematical term introduced by
Einstein into the equations of general relativity, the purpose to
obtain a solution of the equations corresponding to a "static
universe". The term describes a pressure (if positive) or a tension
(if negative) which can cause the Universe to expand or contract even
in the absence of any matter ("vacuum energy"). When the expansion of
the Universe was discovered, Einstein apparently began to regard the
introduction of this term as a mistake, and he described the
cosmological constant as the "greatest mistake of my life". But the
term has reappeared as the proposed source of apparent accelerated
ON QUINTESSENCE AND THE EVOLUTION OF THE COSMOLOGICAL CONSTANT
The following points are made by P.J.E. Peebles (Nature 1999 398:25):
1) Contrary to expectations, the evidence is that the Universe is
expanding at approximately twice the velocity required to overcome the
gravitational pull of all the matter the Universe contains. The
implication of this is that in the past the greater density of mass in
the Universe gravitationally slowed the expansion, while in the future
the expansion rate will be close to constant or perhaps increasing
under the influence of a new type of matter that some call
2) Quintessence began as Einstein's cosmological constant, Lambda. It
has negative gravitational mass: its gravity pushes things apart.
3) Particle physicists later adopted Einstein's Lambda as a good model
for the gravitational effect of the active vacuum of quantum physics,
although the idea is at odds with the small value of Lambda indicated
4) Theoretical cosmologists have noted that as the Universe expands
and cools, Lambda tends to decrease. As the Universe cools, symmetries
among forces are broken, particles acquire masses, and these processes
tend to release an analogue of latent heat. The vacuum energy density
accordingly decreases, and with it the value of Lambda. Perhaps an
enormous Lambda drove an early rapid expansion that smoothed the
primeval chaos to make the near uniform Universe we see today, with a
decrease in Lambda over time to its current value. This is the
cosmological inflation concept.
5) The author suggests that the recent great advances in detectors,
telescopes, and observatories on the ground and in space have given us
a rough picture of what happened as our Universe evolved from a dense,
hot, and perhaps quite simple early state to its present complexity.
Observations in progress are filling in the details, and that in turn
is driving intense debate on how the behavior of our Universe can be
understood within fundamental physics.
Notes by ScienceWeek:
Active vacuum of quantum physics: This refers to the idea that the
vacuum state in quantum mechanics has a zero-point energy (minimum
energy) which gives rise to vacuum fluctuations, so the vacuum state
does not mean a state of nothing, but is instead an active state.
If a theory or process does not change when certain operations are
performed on it, the theory or process is said to possess a symmetry
with respect to those operations. For example, a circle remains
unchanged under rotation or reflection, and a circle therefore has
rotational and reflection symmetry. The term "symmetry breaking"
refers to the deviation from exact symmetry exhibited by many physical
systems, and in general, symmetry breaking encompasses both "explicit"
symmetry breaking and "spontaneous" symmetry breaking. Explicit
symmetry breaking is a phenomenon in which a system is not quite, but
almost, the same for two configurations related by exact symmetry.
Spontaneous symmetry breaking refers to a situation in which the
solution of a set of physical equations fails to exhibit a symmetry
possessed by the equations themselves.
In general, the term "latent heat" refers to the quantity of heat
absorbed or released when a substance changes its physical phase
(e.g., solid to liquid) at constant temperature.
The inflationary model, first proposed by Alan Guth in 1980, proposes
that quantum fluctuations in the time period 10^(-35) to 10^(-32)
seconds after time zero were quickly amplified into large density
variations during the "inflationary" 10^(50) expansion of the Universe
in that time frame.
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