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Skriven 2004-10-21 13:22:00 av Michael Ragland (1:278/230)
Ärende: (Part2) Re: Alien Life
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***** Continued From Previous Message *****
 confirmed unambiguously by scientific
observations. Science has both a revolutionary and a conservative side,
revolutionary in the proposing of dramatic new possibilities and
conservative in the requirement of demanding experimental evidence
before they are accepted.
As with past notions, the idea of a megaverse will require experimental
confirmation before it is accepted. Superstring theory and the existence
of extra dimensions will likewise have to clear the same hurdle.
Megaverse may be the right path and it may not— the existence of a
cosmological constant has caught us all by surprise and some genius may
yet calculate its value in a way we cannot even imagine right now,
showing us a new road to follow.
Whatever happens, we are all grateful that some very exciting
experiments in both particle physics and cosmology will be taking place
in the coming years. Hopefully they will help us sort it out. 
GINO SEGRE, a professor of Physics and Astronomy at the University of
Pennsylvania, is the author of A Matter of Degrees.

Lenny Susskind
A year or two ago most theoretical high energy physicists would have
dismissed any talk of the anthropic principle as anti-science. However,
as I said in the interview; "because of unprecedented new developments
in physics, astronomy and cosmology these same physicists are being
forced to reevaluate their prejudices about anthropic reasoning." The
attitude among the more thoughtful physicists has softened to "hmmm,
maybe we better think about this." The messages of Steve Giddings and
Gino Segre reflect this less biased mindset. Segre correctly emphasizes
the importance of experimental tests of theoretical ideas. In this
connection I want to point out that Weinberg predicted that if the AP is
correct, the cosmological constant would turn out to be non-zero.
Moreover he predicted the correct order of magnitude. This was more than
a decade ago. Finally I want to re-emphasize that it's not just the
cosmological constant that is pushing us in the "anthropic landscape"
direction. The success of inflation strongly suggests that we live in a
very big universe. The other clear fact is that string theory gives rise
to a stupendously rich landscape with perhaps 10(500) vacua with no
reason to prefer one over the other. Sure it's possible that some genius
will come along and explain the cosmological constant by some
mathematical magic but things sure don't seem to be going in that
direction. 

Gerard 't Hooft
During the '80s, a number of physicists became more and more excited
about what was called "super string theory". The rather bizarre
mathematical equations that emerge if one attempts to subject
"relativistic strings" to the laws of Quantum Mechanics, had previously
appeared to be inconsistent, but are now recognized as possibly
describing fundamental elementary particles together with gravitational
forces quite similar to those of Einstein's general theory of
relativity. Even so, inconsistencies continued unless one postulated
very special kinds of projection schemes and symmetries, such as
supersymmetry. 
Supersymmetry of the type needed has not yet been detected among the
real particles of Nature, and also other predictions of the theory could
not yet be checked against experiment. These are by themselves no
reasons to dismiss the theory; supersymmetry is also predicted by other
arguments, and the domain of physics where the theory should apply
directly, the so-called Planck domain, is so far separated from what can
be observed under controlled circumstances, that one should really
admire these deep and stimulating ideas than try to ridicule them, as
some other physicists are sometimes seen doing. 
However, when I hear Lenny say that "this theory is going to win, and
physicists who are trying to deny what is going on are going to lose",
then to my opinion he is going too far. I have several reasons for
advising my friends to practice caution, modesty and restraints when
they air their suspicion that this theory "is" the everlasting and
complete theory of the Universe. If this theory indeed allows for 10^500
distinct solutions out of which we somehow have to choose—some say it
is 10^1000 solutions, nobody really seems to know—then this must be
seen as an enormous setback. Less than a decade ago we still hoped that
some stability argument could be used to single out the single, "
correct" solution; apparently this hope has been abandoned. Now, they
are invoking the "anthropic principle", which really means: try all of
these solutions until you find a Universe that looks like the world we
live in. This is not the way physics has worked for us in the past, and
it is not too late to hope that we will be able to find better arguments
in the future.
On top of this, there are even more serious objections against
"superstring theory". It has already been recognized now that
superstring theory itself only describes a tiny corner of our world, the
corner where these strings happen to interact only weakly, because as
soon as they interact more strongly, nobody can follow the equations
anymore, let alone solve them. In the past, whenever I complained about
this, my voice was hardly heard, but now all string theorists say: "O,
yes, but then the theory can be reformulated in terms of another theory
that is related to the previous version by what is called 'duality'."
And, for convenience, it is then forgotten that this new theory, called
'M-theory', again only exists in a few tiny little corners of the world.
How do we plan to formulate and understand the complete picture? Can one
obtain a complete picture along such lines at all? String theorists are
so confident of their expectations that such questions are usually
ignored. 
This is because the duality schemes that have been discovered are
extremely suggestive. Indeed the mathematical equations repeatedly turn
out to show a magnificent degree of perfection. But what does all of
this really mean? String theorists say: " this can only mean that our
theories are true, and this is the scheme used by God to create our
Universe." 
It is hard to argue with that, since such arguments have some religious
overtones. My own "religion" tells me that theories of this sort can
never be more than approximations. Perhaps the approximations contain
some truth, but the ultimate laws of Nature must contain a fundamental
and simple, concise relation between 'cause and effect', between past
and future, between close-by and far-away. Such principles could not be
built in whatever formulation of 'M-theory' people could give. This is
because the duality arguments that are being used do not refer to the
local equations, but to their symmetry properties instead. This should
be recognised as a weakness of the theory. Take the proud boasts
concerning black holes; the resulting picture leaves no shred of
locality or causality in the laws controlling these mysterious objects.
But this is what I am waiting for. Such a simple demand is unfortunately
far too much to ask from what is now called superstring theory or
M-theory, and as long as I don't see any progress in this respect I
treat the claims with caution and restraint. 
GERARD 'T HOOFT, Nobel Laureate, is Professor of Theoretical Physics at
University of Untrecht. 

Leonard Susskind
Gerard advises caution and restraint. That's hard to argue with. I
consider myself to be a cautious, rather conservative physicist. I
really don't like new ideas. But I also find wisdom in a quote from
Sherlock Holmes; "When you have eliminated all that is impossible,
whatever remains must be the truth, no matter how improbable it is." A
couple of times I have reached the point where I felt forced to a very
unconventional idea, because I could see no way out of it. One case that
particularly comes to mind is the "Holographic Principle." This was a
crazy idea but I would guess that Gerard felt the same way as I did; all
conventional alternatives led to paradox or inconsistency. That is
exactly the way I feel about the cosmological constant. 
I've watched for 40 years as people tried this scheme, and that scheme,
to explain the absence of vacuum energy, but they all failed. I've also
seen string theorists fail over and over in trying to find a "vacuum
selection" principle that would pick out a particular version of the
theory. Add to this the fact that astronomers find that the cosmological
constant is non-zero but just barely small enough for galaxies to form,
I personally feel that we have come to a point where "whatever remains
must be the truth, no matter how improbable it is." Here's what we know:
The cosmological constant is probably not zero but falls in the narrow
range of values that allows galaxies, stars and planets to form. The
evidence for this is empirical.
There is growing empirical evidence confirming the inflationary theory
of cosmology. It follows that the universe is much larger than what we
can observe. 
Theories of inflation tend to produce domains of space with varying
vacuum properties such as the vacuum energy (cosmological constant).
This is from theoretical studies.
String theory has a very large number of vacuum solutions. Some are
supersymmetric but these do not support ordinary chemistry. In addition
there appear to be a huge number of non-supersymmetric vacua with non
zero cosmological constant. As Gerard says, the numbers could be as
large as 10 to the 500 power or bigger. The evidence for this is
mathematical but not rigorous. 
Gerard may not find a pattern here but I do. It's a matter of taste and
judgement.
My comments about the "theory winning" and "theorists in denial" was
mainly aimed at those string theorists want to avoid the facts. Their
own theory is pointing in a very different direction than what they
hoped. I did not have in mind people like 't Hooft who remain skeptical
of string theory. However I do take exception to his claim "the
resulting picture leaves no shred of locality or causality in the laws
controlling these mysterious objects." Here I can only say that I
believe Gerard is wrong.
Finally, I would ask Gerard; do you have a better idea?_____ I want to
add one technical comment to the above response. In Gerard's message he
says "the ultimate laws of Nature must contain a fundamental and simple,
concise relation between 'cause and effect', between past and future,
between close-by and far-away. Such principles could not be built in
whatever formulation of 'M-theory' people could give." I completely
agree with the first sentence in quotes. I don't agree with the second.
The present formulation of (uncompactified) M-theory is called M(atrix)
theory. It is a conventional quantum mechanical theory with a
Hamiltonian and a Shroedinger equation. The relation between past and
future, cause and effect are exactly the same as in any other quantum
mechanical system. While I certainly agree that there is a lot missing,
I think it is too much to say, "the resulting picture leaves no shred of
locality or causality."

Maria Spiropulu
I don't know how else to understand the anthropic principle other than
the "simplistic" way. Does anybody have a scientifically precise
definition of this principle and how to apply it?
In the physics I have learned there were many examples of where the
mathematics was giving infinite degenerate solutions to a certain
problem (classical mechanics problems e.g.). There the problem was
always a mistake in the physics assumptions. Infinity is mathematical
not physical, as far as I know.
There lies the difference between math and physics. In math you have the
equation and you look for the solution—the solution can be a set of
solutions-infinite solutions. In physics you start from the answer—the
real world (scale by scale as I learned from Polchinski) and you seek
the equation. There are measurements (well there are many measurements,
many experiments, resulting in one arithmetic value for this or that),
and you look for the equation. If the equation gives you nonsense, then
it is not the measurement that it is wrong but the equation.
In other words one should not expect to derive the uniqueness of the
universe starting from an infinite set of solutions to a beautiful
equation. One should start from the universe, which is the one universe
that we measure, and try to find a theory that describes it.
I don't understand anthropic remarks like the sun-earth distance is just
right to allow the appropriate chemistry for humans to be. Of course it
does. But before the chemistry was there, the distance was the same. It
is more interesting to research the thermonuclear reactions in the sun,
discover something about the neutrinos, understand the radioactive
warming of the earth's core, study the earth's atmosphere, and in
general find why the temperature and chemistry is what it is—not for
us to be here but for the phenomena to be what they are. And I find it
rather absurd to believe that if we were not here the sun-earth distance
would be different and the universe would be upsidedown.
The whole anthropic thinking seems to me intellectually decadent. It
takes obviously true positive statements, then negates them to makes a
conditional negative argument, which is then regarded as profound or
scientific.
The argument "The environment has to be right for us to exist" is
obviously right. But scientifically I find it is a redundant statement.
Of course I cannot be in an environment that I cannot survive in, and
study that environment at large. But I can study the enviroment I live
in and this is what I do. The life-centric view of the works of the
cosmos seems to me too mystical to be able to deal with scientifically.
MARIA SPIROPULU, a physicist, is currently at CERN. She has been working
at the Tevatron with UCSB and was an Enrico Fermi Fellow at the
EFI/University of Chicago.  
Back to THE LANDSCAPE: A Talk with Leonard Susskind
John Brockman,  Editor and Publisher  
Russell Weinberger, Associate Publisher  
contact: editor@edge.org 
Copyright © 2003 by Edge Foundation, Inc 
All Rights Reserved.


"It's uncertain whether intelligence has any long term survival value.
Bacteria do quite well with it."

Stephen Hawking
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