Revolutionizing the Universe: String Theory & Quantum Gravity

[marcus]

The Coming Revolutions in Fundamental Physics--as seen by string-master David Gross

http://seminput.aei.mpg.de/more_info.php?which=2478&talk=index

==quote==
Max-Planck-Institut für Gravitationsphysik, Albert-Einstein-Institut

The Coming Revolutions in Fundamental Physics

Wed 7 Oct 2009, 14:00
Prof. David Gross (Kavli Institute, Santa Barbara)

"I review the present state of knowledge in elementary particle physics and the questions that we are currently addressing. I discuss the experimental revolutions that might occur at the Large Hadron Collider, soon to be finished at CERN. I shall also review the state of string theory. The necessity to go beyond the standard model of particle physics and to understand quantum gravity has led to this ambitious attempt to unify all the forces of nature and all forms of matter as different vibrations of a string-like object. But string theory is still in a pre-revolutionary stage. Although remarkable progress has been achieved in the last decade we still lack a fundamental understanding of the theory. Many string theorists suspect that a profound conceptual change in our concept of space and time will be required for the final formulation of string theory."
==endquote==

One could even say "for the initial formulation", let alone a final one.
A profound conceptual change in the treatment of space and time has been a requirement for quantum gravitists from the very start and it is nice to know that many string theorists are beginning to suspect this. It seems clear now that such a profound conceptual change was in some sense dictated by the success of 1915 General Relativity over the past 90 years.

Carlo Rovelli's perspective on this was summed up pretty well in a short essay he wrote in 2006 on the unfinished revolution in our conceptions of space and time. I'll get the link.

 

[marcus]

It's nice to see a kind of convergence of minds. Leaders in several areas of research coming to a common perception.
http://arxiv.org/abs/gr-qc/0604045

==quote from Rovelli "Unfinished Revolution" (2006) page 2==

...Others, on the other hand, and in particular some hard–core particle physicists, do not accept the lesson of GR. They read GR as a field theory that can be consistently formulated in full on a fixed metric background, and treated within conventional QFT methods. They motivate this refusal by insisting than GR’s insight should not be taken too seriously, because GR is just a low–energy limit of a more fundamental theory. In doing so, they confuse the details of the Einstein’s equations (which might well be modified at high energy), with the new understanding of space and time brought by GR. This is coded in the background independence of the fundamental theory and expresses Einstein’s discovery that spacetime is not a fixed background, as it was assumed in special relativistic physics, but rather a dynamical field.

Nowadays this fact is finally being recognized even by those who have long refused to admit that GR forces a revolution in the way to think about space and time, such as some of the leading voices in string theory. In a recent interview [1], for instance, Nobel laureate David Gross says: “ [...] this revolution will likely change the way we think about space and time, maybe even eliminate them completely as a basis for our description of reality”. This is of course something that has been known since the 1930’s [2] by anybody who has taken seriously the problem of the implications of GR and QM. The problem of the conceptual novelty of GR, which the string approach has tried to throw out of the door, comes back by the window.
==endquote==

 

[marcus]

The venue is interesting. It is the Quantum Gravity Division of something with a very long name MPG (AEI): Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut) located on the outskirts of Berlin.

In the AEI organizational chart, the division (which Hermann Nicolai directs) is called The Quantum Gravity Division:
http://www.aei.mpg.de/english/aboutInstitute/ResearchProfile/index.html

but on the other hand if you look in more detail, its research area is called "Quantum Gravity and Unification":
http://www.aei.mpg.de/english/research/teams/quantumGravity/index.html

So that's where David Gross is coming to deliver his talk, which can be seen as a kind of status report for String in the broader context of where fundamental physics is going.

Check out the description of Nicolai's section of the AEI, the "QG & U" research wing. It is a kind of perspective, a thumbnail sketch of a vision:
==quote QG&U overview==
Quantum Gravity & Unified Theories

Director: Prof. Dr. Hermann Nicolai
This division is concerned with the unification of general relativity and quantum mechanics into a theory of quantum gravity, which also provides a consistent framework for incorporating the other fundamental forces in nature.

Despite intense efforts over the last years it is far from clear at this time what a consistent theory of quantum gravity will look like and what its main features will be. In view of these uncertainties, the best strategy appears to be one which is both diversified and "interdisciplinary". For this reason, the division aims to represent the major current approaches to quantum gravity, in particular string theory and loop quantum gravity.

The canonical approaches to quantum gravity emphasize the geometrical aspects and appear well suited to deal with unsolved conceptual issues of quantum gravity, such as e.g. the "problem of time" or the interpretation of the "wave function of the universe". Important new insights have been gained over the past decade in the framework of loop quantum gravity, which is one of the division's main research directions. This approach, which complements and extends the old "geometrodynamics" approach, employs a non-perturbative and background independent framework allowing to describe the fluctuations of geometry itself, and leading to a discrete structure at the Planck scale. On this basis, it is now possible to study the full quantum dynamics of gravity. Most recently, these concepts have been successfully applied to the study of cosmological or black hole singularities, where classical general relativity breaks down, spawning an entirely new field of research, loop quantum cosmology.

String theory, on the other hand, takes a very different point of departure in tackling the problem of quantum gravity. The requirement of mathematical consistency and the non-renormalizability of perturbatively quantized gravity, and the need to incorporate the non-gravitational interactions are likely to force us to go beyond Einstein's theory. This may not only lead to a "geometrization" of the other fundamental forces (as exemplified by Kaluza Klein theories and supergravity) and the unification of matter and gravity, but to an entirely new type of theory, which could explain how space-time is dissolved at very small distances, and in which Einstein's theory emerges only as an "effective low energy theory". Superstring and supermembrane theory, and supersymmetric matrix theory are the most promising ansätze so far in this direction. Major progress in this area has been recently achieved by members of the division, in particular the framework of the so-called AdS/CFT correspondence, and the study of certain infinite dimensional symmetries, which might underlie a unified and non-perturbative description of string theory ("M theory").
==endquote==

I have the sense that a different set of premier venues are emerging. Now if you want to know what's happening in fundamental physics, what are the Meccas?, where do you go?

I think you probably go to Perimeter (in Canada), or Utrecht (where 't Hooft is), or to AEI (in that Berlin suburb), or to Geneva, as well as a few other places possibly better known some time ago like Stanford, Harvard, Princeton, Santa Barbara. But the list has changed. It now contains new names of premier venues that only recently showed up on the radar, that you might not have thought of ten or fifteen years ago. Something causes these new centers to grow up and get on the map---maybe it is partly good direction-philosophy that does it.

Anyway it is nice that David Gross is going to the AEI to tell them about what revolutions to expect in fundamental physics. We must surely all wish him the best of luck with his talk! Too bad that (as far as I know) it will not be on-line. Slides PDF for his opening lecture to Strings 2009 are available online if anyone's curious:
http://strings2009.roma2.infn.it/talks/Gross_Strings09.pdf
To get a sense of how his host at AEI approaches things, this talk from the Planck Scale 2009 conference is helpful:
http://www.ift.uni.wroc.pl/~rdurka/planckscale/index-video.php?plik=http://panoramix.ift.uni.wroc.pl/~planckscale/video/Day1/1-3.flv&tytul=1.3%20Nicolai

 

[Thomas Larsson]

A wise man has said: People on the verge of breakthrough don't talk about it. They break through.

 

[tom.stoer]

String theory, on the other hand, takes a very different point of departure in tackling the problem of quantum gravity.

...

Major progress in this area has been recently achieved by members of the division, in particular ... the study of certain infinite dimensional symmetries, which might underlie a unified and non-perturbative description of string theory ("M theory").

It would be nice to learn more about it. I tried to understand Nicolai' work on E(11), but I failed. Firstly because I was not able to understand the math; E(8) ist OK, but beyond that it becomes quite complex :-) And secondly because I was not able the follow the physical reasoning. It seems that he is hiding U(1)*SU(2)*SU(3) in some infinite-dimensional symmetry. I mean we know since SU(5) and SO(10) that the SM is contained as a subgroup, s that's not a breakthrough. There must be something else that that tells us (or at least Nicolai) that it makes sense to study E(11).

 

[Bob_for_short]

... In a recent interview [1], for instance, Nobel laureate David Gross says: “ [...] this revolution will likely change the way we think about space and time, maybe even eliminate them completely as a basis for our description of reality”.

Doing without space and time sounds indeed cool. Maybe David Gross can also do without money? Not completely but to some extent. It is exactly what I need - some money. Just to finish my calculations. The revolutionary thing about my research direction is that it is based only on known experimental facts and usual theoretical notions and it does not invoke any nightmare constructions.

 

[Bodicea]

http://seminput.aei.mpg.de/more_info.php?which=2478&talk=index

Many string theorists suspect that a profound conceptual change in our concept of space and time will be required for the final formulation of string theory.[/COLOR]"
==endquote==

One could even say "for the initial formulation", let alone a final one.
A profound conceptual change in the treatment of space and time has been a requirement for quantum gravitists from the very start and it is nice to know that many string theorists are beginning to suspect this. It seems clear now that such a profound conceptual change was in some sense dictated by the success of 1915 General Relativity over the past 90 years.

I've been saying this for the last 10-15 years...and its not just the quantum theorists that need a massive re-think. So glad to hear a paradigm shift may be iminent!

 

[RUTA]

I've been saying this for the last 10-15 years...and its not just the quantum theorists that need a massive re-think. So glad to hear a paradigm shift may be iminent!

There's no question that we need something new. I doubt the string or loop people will introduce it, they've been in their mindset for decades and it's gotten them nowhere.

 

[friend]

There's no question that we need something new. I doubt the string or loop people will introduce it, they've been in their mindset for decades and it's gotten them nowhere.

What's needed is to find some underlying theory from which both QM and GR can be derived. From what I've seen of so far, all the candidate quantum gravity theories just assume that QM is correct and the principles of QM can be applied to GR. Just because the least action of the Hilbert-Einstein action leads to GR doesn't justify putting that action in the path integral and coming up with quantum gravity. This doesn't tell us if subtle changes will be needed in GR or not. There's only two ways of coming up with a valid theory. Either you have experimental evidence which rules out contenders. Or you proceed from some reasoning that can not be denied because it is perfectly logical. So which way is more likely to generated a valid theory which accounts for both QM and GR?

 

[marcus]

I've been saying this for the last 10-15 years...and its not just the quantum theorists that need a massive re-think. So glad to hear a paradigm shift may be iminent!

There's no question that we need something new. I doubt the string or loop people will introduce it, they've been in their mindset for decades and it's gotten them nowhere.

Annual strings conferences go back to 1988. Here's a list of links for 20-some strings conferences typically giving details like organizers, speakers, programme, time, place etc.
http://mitchell.physics.tamu.edu/Conference/string2010/past.html
Some of the earlier links don't get you very much but the later ones, like Strings 2001 thru Strings 2009 are pretty extensive, some have video of the talks, or slides+audio.

Annual loops conferences go back to 2005. There was a Loops 2005, then they skipped a year, then a Loops 2007, then it was subsumed in a broader conference called QG²-2008, and then Loops 2009.

Before I'd talk about nonstring QG research being stuck in a mindset, I'd give them a chance, maybe 20-some consecutive annual Loops conferences? Loops 2025?

But if you actually look at the nonstring QG research being done, it's a changing mix of partly competing partly cooperating approaches. The canonical LQG approach began to yield to spinfoam around 1998 or 1999. Then spinfoam LQG was reformulated around 2006-2007. Meanwhile there is a lot of crossing back and forth with Group Field Theory, CDT, Noncommutative Geometry, Asymptotic Safety...

The young researchers (postdocs and others) that I have spotted doing Loop/Foam research hardly behave as if they are stuck in any narrow mindset. I see them ALSO out in neighbor fields as I say like: Regge, dynamical triangulations, noncommutative QFT (whatever that is), asymptotic safety QG.
I would guess that at least 30% have crossed at least one border---done at least one paper in at least one of the neighbor fields. I'm not saying this is good. I'm saying that's how it is. There is no narrow mindset---there's a cluster of approaches and people can cross back and forth across the boundaries.

Lately there's been more explicit rivalry. Like Renate Loll pointedly suggesting at conference that CDT is making better progress than Loop lately, etc. I like to see this kind of vitality. But researchers can still work in several officially-competing areas.

String isn't excluded from the young researcher's menu either. Loop researchers now and then do write string papers. I'll give you some links to recent articles, if you want. So I think the *mindset* business may be overstated or questionable. Of course we are all subject to Zeitgeist and the blinders of our historical era, but these are on the whole rather restless mobile and creative people---whatever their "mindset" is, it doesn't seem especially narrow.

On the other hand, what Bodicea said about *imminent* isn't obvious. I don't think anyone can say that some definite shift is in progress. Maybe one can be identified maybe not. In any case I personally don't have the clearsightedness to say.

It's certainly interesting what's going on. So we onlookers can afford to be patient and enjoy the show.

 

[marcus]

What's needed is to find some underlying theory from which both QM and GR can be derived. From what I've seen of so far, all the candidate quantum gravity theories just assume that QM is correct and the principles of QM can be applied to GR.

I think both 't Hooft and Smolin (and perhaps others) have made tentative stabs in that direction.

I don't want to suggest there is any reason to be hopeful about a big step like that, but just to offer the lukewarm comfort of suffering suspense in good company. Probably Gerard 't Hooft would agree with you--companions in impatience
He's written about this several times, and may have discussed it in his chapter in Oriti's book---the chapter called The Fundamental Nature of Space and Time. Personally I found it hard to read and couldn't much out of it. I find more substance in 't Hooft's recent Erice talk---focused on prospects for incremental progress.

Smolin has a paper from 2006 with a title like "Could QM be an approximation to another theory?" As I recall the idea was uncertainty could result from disordered locality---the formulation was rudimentary and preliminary. Fra started a thread on it:
https://www.physicsforums.com/showthread.php?t=209536
You may know the paper, Friend. If not, FWIW:
http://arxiv.org/abs/quant-ph/0609109There may be a lot of stuff along these lines that I can't recall or am not aware of.

't Hooft's paper is actually a little maddening.
http://www.phys.uu.nl/~thooft/gthpub/QuantumGrav_06.pdf
He wants a deterministic QM, or for QM to embedded in a larger deterministic theory. And he is a respected Nobel laureate. I admire him enormously. How can he be saying things like this? I'll give a sample:
==quote page 7 of 't Hooft==
The topic that we dubbed “deterministic quantum mechanics”[11] is
not a modification of standard quantum mechanics, but must be regarded
as a special case. A short summary, to be explained in more detail below,
is that our conventional Hilbert space is part of a bigger Hilbert space;
conventional Hilbert Space is obtained from the larger space by the ac-
tion of some projection operator. The states that are projected out are
the ones we call “unphysical”, to be compared with the ghosts in local
gauge theories, or the bulk states as opposed to the surface states in a
holographic formulation. In the bigger Hilbert space, a basis can be found
such that basis elements evolve into basis elements, without any quantum
mechanical superposition ever taking place.
One of the simplest examples where one can demonstrate this idea is
the harmonic oscillator,...
==endquote==
[and then he proceeds to work out an example]

 

[atyy]

In fact the revolution has already begun with the emergent spaces of AdS/CFT, BFSS and IKKT known from string theory (Chapter 5 of http://books.google.com/books?id=ZNr0jue-b9cC&dq=solvay+space+tome&source=gbs_navlinks_s)

Another place to look is quantum field theory on noncommutative spaces. Such theories arise from string theory (http://arxiv.org/abs/hep-th/0106048); from spin foams (http://arxiv.org/abs/hep-th/0512113); and from group field theory (http://arxiv.org/abs/0903.3475).

Also consider the emergent photons and electrons of Wen, which shares with AdS/CFT a long line of descent from emergent gauge theories going back to the late 1970s. He speculates his model is a specific case of a vague concept of "long-range entanglement" that may also apply to emergent gravity (http://arxiv.org/abs/gr-qc/0606100).

Mark Van Raamsdonk has written a fascinating speculative essay which touches on all 3 aspects, and mentions previous work by Crane, Rovelli, Smolin and Markopoulou as having a similar ethos (http://arxiv.org/abs/0907.2939).

But who knows, maybe the true revolution shall be no revolution (http://arxiv.org/abs/0805.2909)

 

[marcus]

But who knows, maybe the true revolution shall be no revolution (http://arxiv.org/abs/0805.2909)

That would be the real revolution, wouldn't it?

Gravity renormalizable after all, morally speaking. The paper you cite is maybe the key one that turned things around for Steven Weinberg. The strongest evidence of a UV fixed point with a finite (actually small) number of attractive directions, and if I remember correctly, including matter.

So, as Weinberg said, string etc might not be needed, and might not be how the world is. The world might simply be describable by adapting what we already have: QFT with general relativity.

I think the paper you cited was the main one instrumental in bringing him to the position he stated in his July Cern talk. I'll paste the abstract,

http://arxiv.org/abs/0805.2909
Investigating the Ultraviolet Properties of Gravity with a Wilsonian Renormalization Group Equation
Alessandro Codello, Roberto Percacci, Christoph Rahmede
86 pages, 13 figures
(Submitted on 19 May 2008)
"We review and extend in several directions recent results on the asymptotic safety approach to quantum gravity. The central issue in this approach is the search of a Fixed Point having suitable properties, and the tool that is used is a type of Wilsonian renormalization group equation. We begin by discussing various cutoff schemes, i.e. ways of implementing the Wilsonian cutoff procedure. We compare the beta functions of the gravitational couplings obtained with different schemes, studying first the contribution of matter fields and then the so-called Einstein-Hilbert truncation, where only the cosmological constant and Newton's constant are retained. In this context we make connection with old results, in particular we reproduce the results of the epsilon expansion and the perturbative one loop divergences. We then apply the Renormalization Group to higher derivative gravity. In the case of a general action quadratic in curvature we recover, within certain approximations, the known asymptotic freedom of the four-derivative terms, while Newton's constant and the cosmological constant have a nontrivial fixed point. In the case of actions that are polynomials in the scalar curvature of degree up to eight we find that the theory has a fixed point with three UV-attractive directions, so that the requirement of having a continuum limit constrains the couplings to lie in a three-dimensional subspace, whose equation is explicitly given. We emphasize throughout the difference between scheme-dependent and scheme-independent results, and provide several examples of the fact that only dimensionless couplings can have 'universal' behavior."

 

[heinz]

Also the ideas of Schiller ( http://arxiv.org/abs/0905.3905 and
http://www.motionmountain.net/research ) remain an
interesting approach to unification. His ideas are somewhere in
the middle between string theory and loop quantum gravity,
combining aspects from both approaches. But his prediction of
"no Higgs" is hard to swallow.

hz

 

[marcus]

Heinz, if people like Percacci (see abstract in above post) and Steven Weinberg are pointing in the right direction, then extra machinery like strings and loops might not be needed. They may have no ontological grounds. Some associated mathematical tools may prove valuable, however.

Atyy, since you often hark back to the notion of space time "emergence" I want to urge you to consider to what extent space and time are already emergent in 1915 General Relativity.

You probably remember the famous Einstein saying to the effect that "The principle of general covariance deprives space and time of the last scrap of objective reality."

Points in spacetime have no physical existence. The differential manifold is temporary scratchpaper that is thrown away after the work is done.

In GR the continuum is in a quiet modest way emergent. Not however in a drastic manner as in the fancier mathematics you mentioned---from some abstract algebra or holographic illusion.

In GR the continuum is emergent in a very different sense from emergence from baroque contrivance. It has no physical existence in any simple sense because the gravitational field is not defined on anything definite. It is what in mathematics is commonly called an "equivalence class."

You solve the equations and you get a certain layout of geometry and matter, realized on the smooth manifold or continuum you are using. But this is only one possible representative of the solution. You can obtain a huge number of equally good representatives by morphing the geometry and matter around on the manifold, and by re-mapping it on all sorts of other manifolds. This set of equally good representatives, this "equivalence class" is the real field, the real geometry+matter solution.

The theory is invariant under active diffeomorphisms (not merely under arbitrary smooth change of coordinates). If you morph the geometry and move the matter in a compatible way, covariantly, you end up with an equally good solution that has the same information. So points in the manifold are meaningless. The continuum itself has no physical existence.

The illusion that it does exist simply arises from geometric and material relationships---the kind of thing a spin network tries to diagram. It's an old story. The best known quotes from Einstein, making this point, are from around 1916.

 

[heinz]

Marcus, Percacci and Weinberg may have a point. But assuming that general relativity and quantum field theory is all there is, how can we understand the value of the cosmological constant? And particle masses? Here I prefer Schiller's approach, who at least claims to solve these problems.

hz

 

[Fra]

What's needed is to find some underlying theory from which both QM and GR can be derived. From what I've seen of so far, all the candidate quantum gravity theories just assume that QM is correct and the principles of QM can be applied to GR

It's interesting to note that even that you don't see any possible sense in my view on evolving law, I still share your basic quest here.

Where we differ is mainly the strategy to make progress - and this essentialy the view of science, I would like to clarify because this is relevant to the topica of discussion (IMHO):

There's only two ways of coming up with a valid theory. Either you have experimental evidence which rules out contenders. Or you proceed from some reasoning that can not be denied because it is perfectly logical. So which way is more likely to generated a valid theory which accounts for both QM and GR?

I think you vote for the latter choice?

However for me, that really isn't a clear strategy and both are needed, because how could logical perfection be a strategy? It is more a conception of what we are looking for, rather than an idea of howto get there.

I think if we ask how to find WHICH perfect logic system that describes reality, then we are lead to some new thinking and I think even the development of "inference systems" follow the same feedback driven model as does experimental selection for theories.

I want to suggest that this is different in views analogous to Poppers view of science, he wanted deductive clarity to save us from the "inductive madness in which he say no sense", but didn't want to see that he completely ignored the CHOICE of hypothesis. Ie. we need a "rational strategy" for going from killed hypothesis to NEW hypothesis. This is what I call evolving hypothesis.

So for me, logical inference and experimental feedback are not cleanly separable, since the inference system itself is evolving as a result of feedback. Not all "observations" are results from accelerators. In a sense, human contemplation and consumption of ALL evidence, even from other systems in nature, complex systems such as biology and human brain, are also "experimental" in the sense of feedback.

I think it is very interesting to note that it seems that the different strategies here, seems to be at least partially tracable back to different views of science.

I think that what is sometimes referred to as making progress from pure thought, or logic, is in fact a process that does contain feedback from the environment as well. So even apparent pure reason, does rely on the action-reaction and feedback model for selection.

I'm sure this form of "pure reason" can tell use a lot about unificaiton of GR and QM, but I do not think of this form of "pure reason" as "perfect logic" :) I even think that the key to progress is teh fact that human inference is NOT perfect. This is what distinguishes as from simple computer algorithms. A computer does more perfect inference than a human, but it is also more easily led into a state of halt (read killing a theory by falsification); a human always recovers from such situation; so does nature itself.

So I think that it's not necessarily an unquestionable position that we have two extremet choices, experimental evidence from more and more expensive accelerators, OR "pure logic".

There might be a third way, that acknowledges both. That's where I place my vote.

/Fredrik

 

[marcus]

Marcus, Percacci and Weinberg may have a point. But assuming that general relativity and quantum field theory is all there is, how can we understand the value of the cosmological constant? And particle masses? Here I prefer Schiller's approach, who at least claims to solve these problems.

hz

Indeed they may have a point, and so may Schiller! It is something that a number of people are working on now. The most articulate person pushing ahead with this program, that I know of, is Hermann Nicolai.

He is a former string theorist who is director of a branch of the Max Planck Institute that focuses on quantum gravity and unification theories. You may know his work. His outfit is based near Potsdam.

Nicolai and his co-author Meissner show how to generate the required particle masses. The get two Higgs, and a prediction of about 207 GeV for the lighter one. They get massive neutrinos with reasonable bounds on the mass. They seem to have handles on the axion and the CP problem. I don't know about the cosmological constant. But give them time.

The main thing is they have the absolute minimal, most bare bones, austere economical extension of the SM that is able to work out until Planck scale. At the Planck scale presumably new physics takes over. Their aim is cover the whole range from weak to Planck---from 100 GeV to 10¹⁹ GeV, and do that without introducing anything new.

No new energy scale should appear between weak and Planck: no "GUT" scale, no low energy SUSY, no extra dimensions effects. If SUSY exists, it should not have any effect until Planck scale of 10¹⁹ GeV.

So this is a boldly minimalist program which is directly testable. The testability is what I like so much. Theories are not to believe, they are to test. If you cannot yet test, it is not yet a theory but may develop into one. If you can never test it is crap. Theories are not for picking favorites to dream about. They really are meant for testing. So I like this one a lot.

Did you happen to watch Nicolai's July talk about it? I don't know if I'm telling you what you already know. Maybe there are points of similarity with the Schiller one you are following.
Here's the video
http://www.ift.uni.wroc.pl/~rdurka/planckscale/index-video.php?plik=http://panoramix.ift.uni.wroc.pl/~planckscale/video/Day1/1-3.flv&tytul=1.3%20Nicolai

 

[atyy]

OK, let's suppose Asymptotic Safety is true, and experimentally verified to some very high level of accuracy. Would that mean the end of emergent gravity? I would hope not! If you look at Fig 6 in Niedermeier and Reuter's http://relativity.livingreviews.org/Articles/lrr-2006-5/ , Asymptotic Safety is the hypothesis that we lie exactly on a line such as the red one pointing upwards away from the fixed point. But wouldn't that be experimentally indistinguishable from the black line over a wide range of energies? Well, where does the black line come from? A theory in which geometry is emergent :tongue:

Is this what Magnen et al (Scaling behaviour of three-dimensional group field theory, http://arxiv.org/abs/0906.5477) are saying in their footnote 3: "Such a transition is usually called geometrogenesis. But even a QG₄ ultraviolet topological fixed point might not be the end of physics. It could hide at still higher (transplanckian?) phase transition which we could e.g. nickname topologicogenesis, perhaps as a condensation of a combinatoric phase. Quantum graphity is a candidate catchy name for such a combinatoric phase [40]. But such a highly speculative scenario presumably cannot be explored with QG₄ alone. In our notation, graphity or such meta-theories of gravity could be nicknamed QG_∞."

 

[atyy]

BTW, I believe Asymptotic Safety is supposed to flow to positive cosmological constant at high energies (but of course that doesn't explain why it's positive at low energies) - see figure 2 of http://relativity.livingreviews.org/Articles/lrr-2006-5/ (the arrows point in the direction of low energy).

 

[heinz]

The main thing is they have the absolute minimal, most bare bones, austere economical extension of the SM that is able to work out until Planck scale. At the Planck scale presumably new physics takes over. Their aim is cover the whole range from weak to Planck---from 100 GeV to 10¹⁹ GeV, and do that without introducing anything new.

...

Did you happen to watch Nicolai's July talk about it? I don't know if I'm telling you what you already know. Maybe there are points of similarity with the Schiller one you are following.
Here's the video
http://www.ift.uni.wroc.pl/~rdurka/planckscale/index-video.php?plik=http://panoramix.ift.uni.wroc.pl/~planckscale/video/Day1/1-3.flv&tytul=1.3%20Nicolai

I saw the video; incredibly, the Nicolai/Meissner and the Schiller approach both predict nothing new, no Susy, no GUT, up to the Planck scale! The difference is that Schiller also predicts no Higgs boson. The LHC will decide.

 

[marcus]

I saw the video; incredibly, the Nicolai/Meissner and the Schiller approach both predict nothing new, no Susy, no GUT, up to the Planck scale! The difference is that Schiller also predicts no Higgs boson. The LHC will decide.

I'm glad to hear that Schiller's is another theory with nothing between weak and Planck. That makes it eminently testable. If something is found, it shoots Schiller's down. Indeed since the S. theory has no Higgs, finding one would rule the theory out. It would be great to have several theories---all with no GUT, no extra dimensions, no SUSY, all making testable predictions that would allow them to be distinguished.

BTW I don't see how Schiller's approach would work. With Nicolai-Meissner you have a Higgs and you can generate the masses of the other particles. You have a see-saw scheme for neutrino mass. They attend to the necessary details. I am suspicious that Schiller's theory might not--but I would have to take a look.

 

[friend]

So for me, logical inference and experimental feedback are not cleanly separable, since the inference system itself is evolving as a result of feedback

If a statement or a theory is not ultimately true as opposed to false, then you can never be sure you have a final theory. I suppose that would be good new for those who are always looking for more funding of their research programme.

 

[Fra]

If a statement or a theory is not ultimately true as opposed to false, then you can never be sure you have a final theory.

True. But to me the way theories or axiomatic systems evolve, are more fundamental than any current state of theory. To understand current theory, is to understand how it evolved there.

Paradoxally as it may seem, I think this very thing if taken seriously, will lead us to better effective theories than the idea that there is one unique eternal universal TOE, because then this is not just a human issue, there is a physical basis for this uncertainty of law and they way it's infered, that should also apply to any physical system.

Today, the scientific standard doesn't seem to apply to the physical models themselves. I find this strange. If this was taken seriously, the set of possible "physical" theories should largely be reduced.

A typical ignorance of this is in string theory. Instead they seem to end up with even larger and larger sets of possible theories.

I suppose that would be good new for those who are always looking for more funding of their research programme.

On the contrary, as I see it, this idea should put more constraints on the choice of research program, rather then less. However there should always be a spectrum of diversity, which is healthy.

A program isn't funded based on future results anyway. It's funded based on present expectation on it's potential as a strategy to successful results. And this latter is exactly the focus of evolving effective theories.

If you for example think that my reasoning would suggest flooding string programs with even more programs, then the case is the opposite. I think they have had their chances to do their magic, but at some point one has to start to think wether something is wrong with the strategy that doesn't pay off. Just because a programs makes little or no predictions or have a landscape of possibilities doesn't qualify it as a rationally evolving theory.

The evolving law is rather completely different, it's not a theory evolving in a theory landscape, it's rather a theory where the window of possible future frameworks are implicit in the uncertainty of the theory, constrained and assigned on demand. It's almost the opposite of string theory. It's related to the point of evolveing "configuration spaces" that smolin mentions.

This is well known from biological systems, where it's not possible to make sense out of a configuration space that lists all future possibilities, since the information required for that exceeds the complexity of the university or current state of the systme in the general case.

I have to admit that CURRENTLY these ideas are immature, and currently it's also probably also a bit too earlty for say the commercially constrained researchers to work on this. In this sense, the funders are rational. This leaves no other option but to hope that someone with less commercial constraints are able to take this to the level where it becomes interesting for the commercially driven research. This "activation energy" needs to be supplied by non-mainstream efforts.

I think this will happen, but I don't know when. MAybe the market isn't mature yet, but from the point of view of intellectual quests of understanding our world, we have no reason for wait for the mainstream on this.

/Fredrik

 

[RUTA]

The young researchers (postdocs and others) that I have spotted doing Loop/Foam research hardly behave as if they are stuck in any narrow mindset. I see them ALSO out in neighbor fields as I say like: Regge, dynamical triangulations, noncommutative QFT (whatever that is), asymptotic safety QG.
I would guess that at least 30% have crossed at least one border---done at least one paper in at least one of the neighbor fields. I'm not saying this is good. I'm saying that's how it is. There is no narrow mindset---there's a cluster of approaches and people can cross back and forth across the boundaries.

Lately there's been more explicit rivalry. Like Renate Loll pointedly suggesting at conference that CDT is making better progress than Loop lately, etc. I like to see this kind of vitality. But researchers can still work in several officially-competing areas.

String isn't excluded from the young researcher's menu either. Loop researchers now and then do write string papers. I'll give you some links to recent articles, if you want. So I think the *mindset* business may be overstated or questionable. Of course we are all subject to Zeitgeist and the blinders of our historical era, but these are on the whole rather restless mobile and creative people---whatever their "mindset" is, it doesn't seem especially narrow.

On the other hand, what Bodicea said about *imminent* isn't obvious. I don't think anyone can say that some definite shift is in progress. Maybe one can be identified maybe not. In any case I personally don't have the clearsightedness to say.

It's certainly interesting what's going on. So we onlookers can afford to be patient and enjoy the show.

I like those non-string and non-LQG approaches (Regge, CDT, causets, etc.) and greatly appreciated your posting links to videos and papers (in another thread).