| Topic: |
Science > Physics |
| User: |
"sol" |
| Date: |
05 Dec 2003 12:49:30 PM |
| Object: |
Dead Ends |
This was taken from another forum and I thought it relevant here.
Dead ends
Hi!
Strictly speaking, we never know for sure that a theory is a dead end
- until it gives precise and strict enough predictions that are ruled
out by the experiment. This has almost happened with loop quantum
gravity many times, but not quite because every time this theory is
ruled out, its defenders construct a not-quite-working comment that it
might be fixed sometime in future. Most high energy physicists today
are convinced that loop quantum gravity is *probably* a dead end, and
therefore they don't work on it. Of course, they can be wrong.
However, I don't think that they are wrong.
Someone has said that string theory smells like God, but loop quantum
gravity smells like Man. What do we mean? String theory is full of
nice surprises. We often do a calculation, and the mathematics is so
complex and contains so many possibilities to give meaningless
answers, that normally you would expect that a meaningless, anomalous,
divergent result can be the only outcome.
Nevertheless, in string theory all the nonsensical things always
cancel out, and a unique answer follows. String theory is a structure
that is able to teach us; we are not inserting the answers as input.
There is one string theory only; no variations exist. The answers are
derived from nontrivial calculations. We are often forced to think
about subtle details to avoid fast conclusions that would look like
contradictions.
String theory seems to contain all good ideas in physics. It is a very
vast field, and it has the necessary capacity to describe everything
in the real world: quarks and leptons with the correct charges,
interacting with the correct forces, and most importantly these forces
include gravity. We did not create string theory in order to describe
gravity: gravity is an inevitable consequence of string theory that
appeared surprisingly.
Strings in a certain vibrational pattern interact exactly in the same
way as if they were curving the space around. This is true at long
distances - long distances is where we know that gravity must follow
these Einstein's laws (which are approximated well by Newton's laws
for weak gravitational fields) - we know it from observations. At very
short distances, string theory predicts much richer physical phenomena
than ordinary general relativity, and these rich effects don't
contradict observations.
String theory seems to be a totally unique structure: all of its
different versions have been shown to be connected, especially in the
second (duality) superstring revolution in the middle 1990s. We are
permanently learning new things about string theory: we are learning
that our idea about the number of dimensions of space was probably
wrong. We are learning that our geometrical intuition is wrong at very
short distances. We are learning many facts about the previous
theories of reality - quantum field theories; string theory
illuminates so many miraculous and unexpected interrelations between
quantum field theories (as well as gravitational theories based on
string theory) that even people who are not interested in fundamental
physics and quantum gravity simply must observe the developments in
string theory; these developments seem to be essential for many other
subfields of physics (and mathematics).
String theory is responsible for solving many problems in pure
mathematics - problems that were unsolvable previously. Mirror
symmetry is a good example. It is string theory that links two
superficially very different six-dimensional shapes (physics on them
is equivalent!), and allows us to answer difficult questions about one
of them using simple properties of the other. And there are hundreds
of such miraculous mathematical properties.
Loop quantum gravity is a different type of theory. Its goal is not to
explain the origin of quarks; leptons; electromagnetic force; weak
force; strong force. Its only goal is to claim that Einstein's gravity
can be relatively easily reconciled with quantum mechanics. Most
leaders of theoretical physics believe that this opinion has been
proved wrong beyond any doubts. General relativity treated as quantum
theory has a lot of technical as well as conceptual problems - and the
divergences in Feynman's diagrams are the most obvious problem.
The loop quantum gravity people argue that these calculations that
clearly show the inconsistency of quantized general relativity should
not be taken seriously. We should not trust our perturbative
calculations, they say, and they believe that if we just
reparameterize the variables in a relatively obscure way, the problem
will be solved.
There is a lot of evidence that it is not true, and there is no
evidence that it is true. Loop quantum gravity essentially plays with
a sort of randomly chosen simple discrete system that is, in some
formal sense, equivalent to Einsteinian gravity at very short
distances. Nevertheless we know that if we discretize a system, we are
not guaranteed that the physics will follow the original laws at the
long distance scale. Just the opposite: there is a principle that
everything, every term, every correction, every possible interaction
whose existence is not forbidden by a symmetry, WILL occur.
This principle has been tested on hundreds of discrete systems, and it
has always been true. For example, if we put quantum chromodynamics on
lattice, the long-distance physics will NOT follow the naive equations
that we discretized. There will be a lot of new terms generated. If we
apply this to loop quantum gravity, it turns out that more or less
smooth space will NOT exist at all. There is no evidence whatsoever
that smooth space can emerge from loop quantum gravity. Most likely,
the spin networks want to be crumpled to a very chaotic web whose
radius is comparable to the Planck length.
Imagine. It is a theory based on all possible - and very unlikely -
prejudices that people had about the real world decades ago, and it is
very likely that it does not allow the smooth space to exist. This
simply means that most likely, it contradicts *all* observations that
have ever been made. Is there something in the real physics Universe
that can exist without the space?
Loop quantum gravity contains no successful self-consistency checks -
unlike thousands of mathematical checks that just work out in string
theory. Every attempt to calculate something in loop quantum gravity
always leads to a result that seems wrong. The black hole entropy was
calculated with a wrong coefficient by a factor of ln(2)/sqrt(3).pi.
Ridiculous numbers. Attempts have been made to remove this
discrepancy, and they have led to other predictions - for example
about the quasinormal modes of the black holes - and these predictions
have been shown incorrect again. Nothing seems to work, and loop
quantum gravity seems to be a perfect example of the disasters and
inconsistencies that one encounters if she wants to describe quantum
gravity without string theory.
String theory really seems to be the only game in town.
Loop quantum gravity also contains a lot of prejudices that have been
inserted and that are believed to be incorrect. It assumes that the
dimension of space is always four; it assumes that the metric is a
good variable at all distances scales; it assumes that the form of
Einstein's equations essentially does not depend on the scale on which
we study physics; it assumes that non-gravitational physics can be
safely ignored when we study gravitational physics; it assumes that
our intuitive idea about the independence of things separated in space
(for example inside and outside black holes) are correct; it assumes
that there is qualitative difference between momenta and other
charges, between geometry and other fields. Moreover, it seems clear
that loop quantum gravity predicts that Einstein's special relativity
must be heavily wrong.
We have accumulated overwhelming indirect evidence that most of these
assumptions - and probably all of them - are just plain wrong. Loop
quantum gravity is one of trillions of possible wrong theories derived
from about 30 incorrect prejudices that dominated physics in the
1920s, and it predicts no result whatsoever that seems to agree with
another fact that we know about Nature.
All "successes" of loop quantum gravity seem to be artificial parodies
on true successes of string theory. When Strominger and Vafa
calculated the correct black hole entropy from string theory in 1995,
loop quantum gravity people wanted to do the same. It did not quite
work. Because today we are expecting the experiments (not only
accelerators) to confirm many possible cosmic scenarios that emerged
from string theory and that are very exciting and reasonable. Loop
quantum gravity people wanted to be connected with experiments, too,
and therefore they propose totally undefendable experiments to check
their prediction that special relativity is wrong (which we know is
probably not true). It's just their inability to construct a theory
that agrees with the well-tested principles of Nature, not a serious
scientific proposal.
If someone says that the reasons not to study loop quantum gravity -
and focus on string theory - are not rational, he or she is simply
wrong (or a liar, in the worse case). There might be some young people
who really don't understand string theory well and who work on it only
because of some random pressures, but be sure that all senior and
famous physicists know what they're doing - and most of the active
ones are doing string theory and they know very well why they don't
study loop quantum gravity!
I know it is difficult to understand the different situation of string
theory and loop quantum gravity without actually going into the
technical details, but I also know that loop quantum gravity simply
can't compare with strings once you *do* study the details.
Best wishes
Lubos
__________________
Superstring/M-theory is the language in which God wrote the world.
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| User: "Uncle Al" |
|
| Title: Re: Dead Ends |
05 Dec 2003 02:17:41 PM |
|
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sol wrote:
This was taken from another forum and I thought it relevant here.
Dead ends
Hi!
Strictly speaking, we never know for sure that a theory is a dead end
[snip]
One reproducible empirical counterdemonstration of prediction is
wholly sufficient to disprove any theory. Contemporary theoretical
physics therefore carefully constructs theories that have no divergent
empirical predictions. This need not necessarily cover their asses.
Empirically falsifying a postulate falsifies its associated
self-consistent axiomatic construct,
http://www.mazepath.com/uncleal/qz.pdf
http://www.mazepath.com/uncleal/eotvos.htm
The Standard Theory and its extensions predicted a very long but
finite half-life to decay of protons. Super Kamiokande, containing 50
megatonnes of ultrapure water H_2_O, defintively counterdemonstrated
that half-life claim to everybody's satisfaction. SuperSymmetry then
altered a parameter or two, now predicting a proton half-life that is
sufficiently long that it cannot be detected against background
noise. Said theory is now "correct."
Superstring/M-theory is the language in which God wrote the world.
That's what the One True Church (pick One, and One) says about its
bible(s). "If English was good enough for Jesus Christ, it is good
enough for me."
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
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| User: "Jim Roberts" |
|
| Title: Re: Dead Ends |
05 Dec 2003 11:11:27 PM |
|
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"Uncle Al" <UncleAl0@hate.spam.net> wrote in message
news:3FD0E7E5.7726814C@hate.spam.net...
Superstring/M-theory is the language in which God wrote the world.
That's what the One True Church (pick One, and One) says about its
bible(s). "If English was good enough for Jesus Christ, it is good
enough for me."
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
I actually heard a minister say that if the King James Version was good
enough for Paul, then it's good enough for me... Fucking Dumbasses...
Jim
.
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| User: "Igor" |
|
| Title: Re: Dead Ends |
07 Dec 2003 01:39:16 PM |
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"Jim Roberts" <jrzycrim@msn.com> wrote in message news:<bqrodv$266vph$1@ID-184466.news.uni-berlin.de>...
"Uncle Al" <UncleAl0@hate.spam.net> wrote in message
news:3FD0E7E5.7726814C@hate.spam.net...
Superstring/M-theory is the language in which God wrote the world.
That's what the One True Church (pick One, and One) says about its
bible(s). "If English was good enough for Jesus Christ, it is good
enough for me."
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
I actually heard a minister say that if the King James Version was good
enough for Paul, then it's good enough for me... Fucking Dumbasses...
Jim
But didn't all those bible characters speak in perfect King James
English? I saw it on TV so it must be true!
.
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| User: "Y.Porat" |
|
| Title: Re: Dead Ends |
08 Dec 2003 04:27:49 AM |
|
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Uncle Al <UncleAl0@hate.spam.net> wrote in message news:<3FD0E7E5.7726814C@hate.spam.net>...
sol wrote:
This was taken from another forum and I thought it relevant here.
Dead ends
Hi!
Strictly speaking, we never know for sure that a theory is a dead end
[snip]
One reproducible empirical counterdemonstration of prediction is
wholly sufficient to disprove any theory. Contemporary theoretical
physics therefore carefully constructs theories that have no divergent
empirical predictions. This need not necessarily cover their asses.
Empirically falsifying a postulate falsifies its associated
self-consistent axiomatic construct,
http://www.mazepath.com/uncleal/qz.pdf
http://www.mazepath.com/uncleal/eotvos.htm
The Standard Theory and its extensions predicted a very long but
finite half-life to decay of protons. Super Kamiokande, containing 50
megatonnes of ultrapure water H_2_O, defintively counterdemonstrated
that half-life claim to everybody's satisfaction. SuperSymmetry then
altered a parameter or two,
--------------
i would like to enter a comment just at this poit:
'than altered a papameter or two'...........
people tend to relate too much importance to 'predicting rates'
imho predicting rate is one of the weakest proves to
make a theory 'very impressive' think about it
what does it mean and how it is done that 'predicting of rates'
(i would add to it even the probabiliy predictions)
once you analyse it - predicting rates is just ...(in most cases)
*extrapolations of data*
ie you gater experimental data, you fit it into a formula with
free parameters that are determined my the past data
and there you are, to predict additional data!
but please note:
the distance between predicting data or observations
is not yet *undestanding the physical process*!
(in many cases many explanations and interpretations
might work- yet those many is just a demonstartion
of their weakness)!.
it is not much than gatering data
and the way from that untill good undestanding of the
physical process is still a long way.
ps : Uncle ,you see how brave i am ,to comment on any of your
articles, and 'endangering my neck' to get involved with a tiger?(:-)
all the best
Y.Porat
--------------
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