
#91
Aug1705, 10:18 AM

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Telos, you are definitely on to something about time spent drawing testable conclusions from a theory so that it may be shown false. 



#92
Aug1705, 11:11 AM

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http://www.physicsforums.com/showthread.php?t=80760 In this thread, which Ratzinger started in June 2005, some knowledgeable PF people like Haelfix, selfAdjoint, Ohwilleke, concisely reported some reasons why there has been a decline, over the past 3 years or so, in interest and popularity of string research. Increasing worry and pessimism among string researchers was reflected in the Toronto discussion video. http://www.physicsforums.com/showthread.php?t=84585 One encounters expressions of disillusion among graduate studentssome of whom are changing fieldsand symptomatic efforts being made by stringloyal bloggers (such as Motl and Distler) to shore up morale among the graduate students. We also see some statistical indications of the decline in string interest, popularity, optimism. One can always argue about how to interpret the various statistical measures, however. I must say that observing this string "downsizing" going on has increased my respect for the honesty and courage of certain researchers such as Andy Strominger. He made an excellent two minute statement in the Toronto discussion at time 1:28:20 It put me in mind of a story in the Bible where a general tells his soldiers that anyone who wants to can go home, and about half of them leave (this is called "downsizing"), then the remaining ones go on to win the battle. Anyway Juan, I will try to say what I think about your message that string theory is a waste of time. 



#93
Aug1705, 11:30 AM

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2. increasing numbers of people seem to be deciding that FOR THEM it is a waste of time, and so they are getting out of the field, or they are not writing so many research papers as they did. (However on arxiv I see a growing number of string papers by people at Beijing Normal and other large Chinese universities. These people do not think string is a waste of time for them and they are responsible for an increasing fraction of the research postings.) 3. your statement does not have a clear meaning unless you specify a waste of time AS WHAT. I think a lot of people would agree that string theory can lead to ideas and results that are interesting AS MATHEMATICS. 4. your statement would not apply to a mathematically gifted young person who goes into string and discovers something interesting and valuable AS MATHEMATICS. You cannot say that such a person is wasting their time! What gives mathematics intrinsic WORTH is the interest it evokes from other mathematicians. It does not need to be a fundamental testable model of nature. 5. however your message, suitably qualified and restricted, is a very helpful one to have expressedand voicing it actually DOES STRING THEORISTS A FAVOR by increasing the pressure on them to arrive at a nonperturbative background independent formulation that makes falsifiable predictions. This is the only way to be sure that string theorizing is not a waste of time AS PHYSICS. 



#94
Aug1705, 01:01 PM

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#95
Aug1705, 01:46 PM

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#96
Aug2005, 08:49 AM

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As said, I do not think that the decline of string research program is just temporary one. In the past, there were difficulties but now people is seeing that each year original objectives of string theory are far, and far, and farther. String theory history looks like a divergent asymptotic series.
Now, let me reply your very interesting comments. As said by Nobel laureate P. Anderson this year, string theory is a futile exercise as physics. I substituted “futile exercise” by “waste of time” but my evaluation of string theory continues being correct. I would state that string theorists provide none serious argument why we would believe on string theory, only bold statements like "it is the most promising way" or wrong claims like "is the only was to quantum gravity". I see an injustice here with people that are not string believers. Has string theory been interesting on mathematical topics? Of course, but that does not justify the hype around it and its study on physics dept. Moreover, let me say that the impact of string theory in the whole of mathematics is not so huge, at least, it is not more important (by orders of magnitude) that impact of some field theoretical techniques. For example, contrary to popular belief, Fields Medal awarded to Witten was not by the application of pure string theory methods to math, most of mathematical work of Witten was from field theory. Atiyah, who is many times more smart and versed that i in these topics, affirms that string theory has had an impact on mathematics which has been really quite extraordinary. Well, he said that in a popular interview. However, far from popular claims, I see not radical advances on mathematics as provided by the own Atiyah on "index theorems" (theory of quantum operators in quantum field theory). Do not forget that LQG is claimed background independent whereas continue to be an “inefficient” approach to quantum gravity. In fact, there is no possibility for obtaining a consistent classical limit converging to GR after of 40 years from Hamiltonina gravity: geometrodynamics, Astherkar QGR, LQG, etc. Smolin, as others loop theoreticians, assumes that relationism is correct, but it is not as already said. The idea of that causality becomes a fuzzy notion because of fluctuation of light cones is completely wrong. 



#97
Aug2005, 12:36 PM

P: 4

i am sure strings are going to disappear and first sign of this is the idea os landscape of vacuua. it is such a comic trash that during seminar you feel like quiting physics because such are the things promoted as future directions.
but stringers cannot fool th world much long and more so because some of them are serious researchers with a conscience still alive. plase read freeman dyson: disturbing the universe and you will know hy string theory qualifies as a failure. The problem of delay in this being branded a failure is obvious, INERTIA. there are too many researchers persuing it who are trained as stringers unlike the masters who were all high energy physicists. S now we have this young generation of ignorant people who doesnt even know where to find mistake to stop doing it since they simply do not know physics. It is just poor quality mathematics. 



#98
Aug2005, 01:04 PM

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"...serious researchers with a conscience..." Public support for physics ultimately depends on the trust that nonspecialists have in the selfcritical, "selfpolicing" ability of theorists to remain engaged with empirical reality. So your perception that there are some who are not indulging in a mathematical escapade is very important. 



#99
Aug2005, 01:15 PM

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Among tested, wellestablished theories, General Relativity is the most background independent model we have. When quantizing Gen Rel, it is obvious to try to preserve the B.I. feature if one can. The comparative success or failure of various attempts to do this is not relevant to the validity of the effort. With both String and canonical Loop experiencing difficulties, one sees that it is actually the most background independent approach that is currently making the most progress. 



#100
Aug2005, 02:48 PM

P: 1,545

Marcus
On Strings I feel it’s future may well becoming a waste land. But I believe it has been very valuable in identifying the 11 dimensions issue. Lack of progress indicates that this idea is likely just wrong. But ANY future theory that proves 11 D as wrong, should also be able to explain why the 11 D issue appeared to be viable at all. Just this additional ‘test’ of future theories, I think that can be worth quite a bit. Also, You mentioned something else I could use a little help on “how I think” Your comment tells me I need to Fine Tune my thinking a bit. Does the following make sense: SR Special Relativity  background dependent Works on a ‘dependant’ background of space and time in a classical manner. Just the Newton formulas were inadequate and the measures of space or distance over time need to be understood by the better formulas provided by relativity. GR General Relativity  background independent The use of a warping of time and space into “space/time’ to understand gravity, releases us from a background dependent measure. That is the physics we see, relativity included, is not dependent on any background traceable measure in either distance. But rather only dependent on the “relationships” between physics events that cannot be tied down to a measurable background reference of space and time. A fine point but seems an important one I’d not fully recognized. In a similar fashion : Quantum Theory  background dependent Quantized the minimum amount of energy to be found in light “packets” now photons. And set minimum size of change in measure we could expect to ever make in both time and distance (space). Natural limitations associated with this made near impossible to make significant progress until. Quantum Mechanics  background independent Instead of “warping” the relationship of time and space, used the uncertainty principal to allow measure and predictions at the quantum level to become understandable. Thus one way to explain the inability of combining the physics of QM and GR even though they are both “background independent” is that they arrived at their independence in dramatically different forms (warping vs., probabilities) that we so far have been unable to interrelate. (I'd previously considered not being able to combine the two as a dependent vs. independent issue) Is this a reasonable tune up to my thinking? Let me know if I’ve gone off track on the “background” issue as it is a bit new to me. Also are there any other “wellestablished theories” that arrive at their background independence though some other manner than GR or QM? I’m assuming that most all, like MStrings, have their foundation in QM. Thanks RB 



#101
Aug2005, 03:27 PM

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does anyone have an Intro to D.G. or Intro to Manifolds link? Randall there are two abstr. math. ideas you need that are actually very simple and easy to getManifold and Metriconthemanifold. For 150 years the fundamental paradigm for a continuum that everyone uses is a Manifold (defined by Riemann around 1850). the most common meaning of B.I. is you start with a Manifold without a metric. in a B.D. theory you start with a manifold and give yourself a metric on it to start with as well have to go back later 



#102
Aug2005, 04:03 PM

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Randall it is soooooo simple. I wish you would take a moment and think it over and come back and say honestly that you understand perfectly clear as day.
the reason we accumulate math concepts over the decades is ultimately MENTAL ECONOMY. they make thinking more efficient. and this is a case the fundamental object in D.G. is the manifold which corresponds to the idea of a continuum without geometry. it is a blob that has coordinate functions defined on it (local charts that are smooth and compat where they overlap) but does not have any dingus or appliance that can tell you the distance between two points because it has coordinates, at any point on the manifold you can explore all the possible directions in which you can take the derivative!!!!!! All the possible DEEEXES, and when you think calmly and patiently about this for a while you realize that this collection of all possible deeexes IS the tangent space. it captures the essence of what we want the tangentspace at any give point to do for us. and it is intrinsic (defined without reference to anything surrounding the manifold) this is a fundamental Idea of Western Civilization, like the freedom of the individual and the rule of law etc. this is the Idea of the Continuum which has been standard for 150 years It is INTRINSIC. it doesnt have to be embedded in any larger space for you to know its tangent space at each point and be able to do calculus etc, and it STILL HAS NO IDEA OF GEOMETRY built in. to do geometry you introduce a "metric" gizmo which is a bilinear dingus defined on the tangent space at every point blah blah and once you have a metric g(m) defined at every point m of the manifold then you can compute distances, angles, areas, volumes etc. The most common meaning of B.I. is that you start with a manifold without a metric. In Gen Rel you start with a 4D spacetime manifold and some matter and you set up this equation and Presto! you PULL THE METRIC OUT OF THE HAT! (EDIT: selfAdjoint objects to the wording. I mean that you solve for the gravitational field, which is the metric. more discussion of details of this in later posts...) the metric, or geometry, can be totally freeform and it is determined dynamically by interaction with matter through the equation of the model. This is VERY DIFFERENT FROM perturbative STRING THEORY where they start with a manifold that already has a priorchosen metric defined on it. Having a prior chosen metric lets you define the twangy equation by which the little thangs be vibratin'. Without that prior metric you got nothing to start with, stringywise. 



#103
Aug2005, 04:23 PM

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#104
Aug2005, 04:33 PM

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the fundamental string stumbling block is that in over 20 years nobody has succeeded in doing string without priorchoosing a metric
but to truly do Gen Rel you cannot choose the metric because geometry is a dynamic thing that comes out of the modelthe geometry of the manifold is gravity and you do not stipulate it in advance in standard vintage Gen Rel the gravitational field IS the metric g(m) and it is what you solve for it is the unknown distance function that the Einstein equation is about. this is the basic obstacle that string research has always been up against  an amusing behavior of string theorists, which you can see recently over at the Coffee Table blog, is that whenever anybody reminds them of this major roadblock they immediately start talking defensively. they dont stop, take a breath, and say "Yes that is right, we really need to put some effort into a nonperturbative, background independent formulation!" Instead, they start making excuses and talking about the INADEQUACIES OF LOOP which is really irrelevant. They get into this complicated distracting discussion about how String "really" doesnt need to be B.I. and how it "really" is B.I. (if you define B.I. right) and how Loop is not "really" B.I. (if you define B.I. a certain way), and sometimes they start complaining that it isnt NICE of anyone to point out this defect because it might give nonexperts the idea there was something wrong. We just had an example of this at a couple of stringy blogs this month when folks were reacting to Smolin's paper "The Case for Background Independence". That paper was friendly advice, and the reaction was defensive, as if it were an attack. Check this out: http://golem.ph.utexas.edu/string/archives/000621.html I am waiting for someone like Andy Stromiger (who I suspect has guts) to come out with a clear statement on this and say "yes we need a B.I. formulation that we can really calculate with, and we ought to give his high priority and work on it, and we can do it" 



#105
Aug2005, 04:43 PM

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all the terms in the equation are defined based on the metric, so the metric comes before all that other stuffcurvature tensor like you say. the idea I am trying to get across is somewhat simpler and more basic: the gravitational field IS the metric you don't start off knowing the metric you SOLVE for it exactly as you say, part of solving for the metric is going through stuff like the RiemannChristophel tensor whose definition is based on the metric thanks for adding some clarification, selfAdjoint. would be great to have some standard links to basic D.G. and a standard exposition of what B.I. is about I appreciate your help 



#106
Aug2005, 04:54 PM

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my idea of a really background independent nonperturbative approach to QG is Loll triangulations path integral. I will get some links
http://arxiv.org/hepth/0404156 Emergence of a 4D World from Causal Quantum Gravity http://arxiv.org/hepth/0505154 Reconstructing the Universe http://arxiv.org/hepth/0505113 Spectral Dimension of the Universe http://arxiv.org/hepth/0411152 Semiclassical Universe from First Principles more here http://arxiv.org/find/grp_physics/1/.../0/1/0/all/0/1 Eventually I hope to see some string theorists implement a version of string theory on the Loll CDT spacetime as a foundation. (rather than on their usual kind of manifold) The original work in Dynamical Triangulations in early 1990s (which led to Loll CDT path integral in 1998) was actually an attempt by Ambjorn to find a NONPERTURBATIVE FORMULATION OF STRINGhe thought he was doing matrix theory and he ended up with the CDT path integral. One of Smolin's points is that a headon effort to make string nonperturbative, or background independent, is likely to be fruitful (as it has been in the past) whether or not one finds a passage to the original goal Another is that by throwing out assumptions one makes a theory more predictivethe less you assume the harder it is to build and the more restrictive it isso the more falsifiable. So he proposes making the theory less dependent on comfortable background assumptions as a way out of the landscape confusion. 



#107
Aug2005, 04:57 PM

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Some manifolds are just so miserable that there is no way to equip them with a differentiable structure. Thus, no derivatives for you! Some other manifolds are too accomodating: there are many fundamentally different ways to equip them with a differentiable structure! So you have to select which one you like before using derivatives! 


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