Why I am REALLY disappointed about string theory

[tom.stoer]

Not all statements are worth to be remembered :-)

 

[atyy]

In fact, Haelfix's claim is self-evidently correct.

 

[tom.stoer]

In fact, Haelfix's claim is self-evidently correct.

which one? regarding the landscape issue?

 

[atyy]

which one? regarding the landscape issue?

Yes, the one about a theory of gravity which can couple to any form of matter.

 

[tom.stoer]

I don't care in this context. We have a candidate theory (string theory) which has the landscape problem. Regardless if it's right or wrong, it is obvious that nobody will find a new candidate theory w/o any connection to strings but as deeply investigated as strings within the next couple of weeks. But as soon as this new theory is published (I am checking arxiv daily), I will open a new thread.

 

[BenTheMan]

I guess what worries me is two things. One is the presumption and tone of omniscience. You present yourself as someone who can imagine every approach to quantum gravity and matter and who is able to foresee a landscape for every possible approach.

It's very possible that some, as of yet uninvented, approach to quantum gravity may evade these problems. Of course, the burden of proof is on that community to actually come up with a workable theory first. Surely it is possible that such a theory exists, but it is also possible that the LHC will produce fire-breathing dragons which will ravage Europe. It's also possible that some bright graduate student will find a selection principle on the landscape and predict the electron's mass from string theory. It's just not likely.

The point is, you have to follow your nose. When you build a model or a theory, you have to first start with what has worked for you in the past, and then build from there. And at every stage you have to make sure that you effective field theory matches the standard model or the MSSM.

So Haelfix's comment is right, given what we currently know about the theory of quantum gravity, based on 100 years of research of some very smart people. Is it possible that 4 generations of work has led to the wrong conclusions? Sure---it's happened before.

So you can wring your hands and say ``We just don't KNOW!'', or you can shut up and calculate something, and try to advance the state of knowledge to the best of your abilities. Naval gazing works for some, but not for people who are interested in science.

 

[BenTheMan]

A quick provocative claim: what we have constructed so far is nothing but a portion of the space of the consistent theories that include gravity.
(This could be refuted by presenting a theory that is consistent but is not contained in this framework; in a sense this is the question whether the "swampland is hospitable" or not.)

See Wati Taylor's recent work regarding the swampland in 10 and 6 dimensions.

 

[MTd2]

So Haelfix's comment is right, given what we currently know about the theory of quantum gravity, based on 100 years of research of some very smart people.

What is *the* theory of quantum gravity?

Are you single?

 

[marcus]

In fact, Haelfix's claim is self-evidently correct.

https://www.physicsforums.com/showthread.php?p=2826555#post2826555

 

[petergreat]

Regardless if it's right or wrong, it is obvious that nobody will find a new candidate theory w/o any connection to strings but as deeply investigated as strings within the next couple of weeks. But as soon as this new theory is published (I am checking arxiv daily), I will open a new thread.

In the 19th century, no amount of ingenuity from theorists could've explained Mendeleev's periodic table before J J Thompson actually discovered the electron and before the development of quantum mechanics! The SM is better than the periodic table but still has 20+ free parameters. After so many years of efforts in vain, is there any reason for believing that such an absolute mess can be explained from theory alone, without some revolutionary experimental discovery? I don't believe yet another speculative theory posted to arxiv (which will reach you on the same day) will magically solve the world.

P.S. Maybe people should simply give up before seeing new experimental hints?

 

[tom.stoer]

Thanks Marcus!

So let's come back to Gross' question - and to my last two topics (again slightly modified) -

• what string theory really is
• what the fundamental principles are and how the final theory will look like (in terms of strings or other fundamental degrees of freedom)
• what the major obstacles (inherent to string theory) are preventing us from identifying these underlying principles and constructing this unique framework or theory

 

[MTd2]

After so many years of efforts in vain, is there any reason for believing that such an absolute mess can be explained from theory alone, without some revolutionary experimental discovery? I don't believe yet another speculative theory posted to arxiv (which will reach you on the same day) will magically solve the world.

P.S. Maybe people should simply give up before seeing new experimental hints?

Very good point. There is a new preprint today on arxiv.org, about History of Physics, that gives a very insightful view on how we are clueless even with experiments:

http://arxiv.org/abs/1008.0447

Failed theories of superconductivity

Joerg Schmalian
(Submitted on 3 Aug 2010)
Almost half a century passed between the discovery of superconductivity by Kammerlingh Onnes and the theoretical explanation of the phenomenon by Bardeen, Cooper and Schrieffer. During the intervening years the brightest minds in theoretical physics tried and failed to develop a microscopic understanding of the effect. A summary of some of those unsuccessful attempts to understand superconductivity not only demonstrates the extraordinary achievement made by formulating the BCS theory, but also illustrates that mistakes are a natural and healthy part of the scientific discourse, and that inapplicable, even incorrect theories can turn out to be interesting and inspiring.
Comments: 14 pages, 3 figures, to appear in: Bardeen Cooper and Schrieffer: 50 YEARS, edited by Leon N Cooper & Dmitri Feldman

****

It is worthless to just shut up and calculate if you don't turn screws and vice versa.

 

[tom.stoer]

Yes and No.

Of course theoretical physics lacks new fundamental experimental insights from HEP since 2-3 decades (all these experiments either confirmed the SM - W- and Z-bosons, tau, ... or disproved simple GUTs - proton decay).

But there are other experimental data available (for a rather long time) waiting for an explanation (# of dimensions, global and local symmetries, particle spectrum, cosmological constant). The problem is that all these data may be consistent with some version (or vacuum) of string theory, so they do not provide new insight how to proceed.

Look at high-temperature superconductors. It does not help to find a new material with an even higher T_c; we do not understand how it works.

If we agree that string theory is a "sketch of a theory" which could be consistent with all these observations (but that these observations do not help to understand the fundamental principles, "only" the vacua) then we must look for a different modus operandi how to identify or construct the fundamental theory.

That's why I am asking these questions.

 

[MTd2]

Proton decay was not disproved, unless for GUTs with the shortest proton half-life.

 

[tom.stoer]

Proton decay was not disproved, unless for GUTs with the shortest proton half-life.

Agreed

 

[atyy]

Thanks Marcus!

So let's come back to Gross' question - and to my last two topics (again slightly modified) -

• what string theory really is
• what the fundamental principles are and how the final theory will look like (in terms of strings or other fundamental degrees of freedom)
• what the major obstacles (inherent to string theory) are preventing us from identifying these underlying principles and constructing this unique framework or theory

I think there are people looking for these answers, but it's hard.

http://motls.blogspot.com/2009/09/murray-gell-mann-80th-birthday-and.html reporting on Gell-Mann's comments:
"I am puzzled by what seems to me the paucity of effort to find the underlying principle of superstring theory-based unified theory. Einstein didn’t just cobble together his general relativistic theory of gravitation. Instead he found the principle, which was general relativity, general invariance under change of coordinate system. Very deep result. And all that was necessary then to write down the equation was to contact Einstein’s classmate Marcel Grossmann, who knew about Riemannian geometry and ask him what was the equation, and he gave Einstein the formula. Once you find the principle, the theory is not that far behind. And that principle is in some sense a symmetry principle always.

Well, why isn’t there more effort on the part of theorists in this field to uncover that principle? Also, back in the days when the superstring theory was thought to be connected with hadrons rather than all the particles and all the forces, back in that day the underlying theory for hadrons was thought to be capable of being formulated as a bootstrap theory, where all the hadrons were made up of one another in a self-consistent bootstrap scheme. And that’s where superstring theory originated, in that bootstrap situation. Well, why not investigate that further? Why not look further into the notion of the bootstrap and see if there is some sort of modern symmetry principle that would underlie the superstring-based theory of all the forces and all the particles. Some modern equivalent of the bootstrap idea, perhaps related to something that they call modular invariance. Whenever I talk with wonderful brilliant people who work on this stuff, I ask what don’t you look more at the bootstrap and why don’t you look more at the underlying principle..."

http://physics.aps.org/viewpoint-for/10.1103/PhysRevLett.103.081301 "In a wider perspective, what do these results mean for superstring theory and its claim to be the sole pretender to the throne of a perturbatively consistent extension of Einstein’s theory? String theory differs from field theory in that, on top of its pointlike excitations, it has an infinite tower of massive states corresponding to the quantized vibrational modes of the string. However, a closer look reveals that its (still conjectural) finiteness hinges not so much on the presence of these extra states, but rather on a new type of symmetry (called modular invariance), which has no field theory analog. This suggests that the new symmetry that may ultimately explain finiteness must act in a way very different from known realizations of spacetime and internal symmetries. Accordingly, we should view the coexistence of several possibly finite candidate theories only as a first step towards the future construction of an underlying theory of quantum gravity, where classical space and time are only emergent concepts, and which would also be viable nonperturbatively."

Nicolai and collaborators have a very interesting line of work - actually, it's not clear to me if this symmetry is just some there in some special limit, or a more general principle - I find it interesting for its relation to quantum chaos - and the Riemann hypothesis:
http://arxiv.org/abs/hep-th/0207267
http://arxiv.org/abs/hep-th/0212256
http://arxiv.org/abs/0912.0854

 

[marcus]

I promised to get a couple of links.
Atyy jogged my memory about one of them.

But there seems to be a lot of free-floating defensiveness. I'd like to understand that better. Who is supposed to be the enemy?

One has to distinguish between criticisms of the mathematics itself, and criticisms of the program (direction, emphasis...)

The most trenchant criticisms I can remember from recent times were from Nima Arkani-Hamed (November 2008) and from Murray Gell-Mann (I will try to find the links).

Gell-Mann was talking about the direction of the program (avoiding hard fundamental questions of principle in favor of increasing elaboration) and Nima was talking about what he suspects are mathematical limitations (not to expect it to say anything new about high energy physics, but maybe about gravity). ...

But we are told repeatedly about imagined bogeymen. "Armchair experts" who apparently were calling for a complete halt ...

Here is the ScienceNews link to that Gell-Mann interview:
http://www.sciencenews.org/view/feature/id/47280/title/Interview_Murray_Gell-Mann

It is not about the creative potential of the mathematics, but is rather critical of the direction of the program---the (inadequate) vision and priorities of senior people who guide the research effort by the projects within the string community which they support.

This is the ScienceNews Murray birthday interview Atyy just quoted where he refers to the "paucity of effort to find the underlying principle."

 

[BenTheMan]

What is *the* theory of quantum gravity?

Are you single?

THE theory of quantum gravity is string theory, of course.

All kidding aside, though:

THE theory of quantum gravity is that microscopic description of Nature whose low energy limit is consistent with both GR and the standard model. While it is true that the low energy physics can not uniquely specify the UV completion, it is true that the low energy physics implies certain constraints on the microscopic physics.

Haelfix was making very general statements about the constraints that any theory of quantum gravity must satisfy if it is to be consistent with what we currently understand about gravity. If things are drastically different, as they may be (but probably aren't), then we have a lot of re-thinking to do.

But personally I'm of the opinion that most people are more or less on the right track.

 

[BenTheMan]

And no, I'm not single.

But if you play your cards right, I'll send you a pair of my used underwear.

 

[negru]

By the way. Whatever other theory you find would still have to explain all the features string theory uncovered so far (like the relation between gluons and gravitons) as well as why adding some extra symmetry (susy) gives stuff like ads/cft. Since we know all these things have to be true and explained (whether real or not - they're consistency checks which have to hold), it seems to me very likely that any other theory will just be a particular solution (or face) of string theory.

 

[MTd2]

What if there are no gravitons?

 

[negru]

I don't think you can avoid gravitons in any quantum theory of gravity. Plus you can always just stick to the effective quantum gravity, which is fine by any standard and has gravitons.

I mean, you have gravitational waves (think a nobel or some went to that). What's the quanta of the gravitational wave then?

 

[MTd2]

Curiously, denying gravitons, at least at fundamental level, ended being what Erik Verlinde means by emerging gravity. He is now saying that the only fundamental string is the open string.

 

[BenTheMan]

What if there are no gravitons?

Then you have to understand Quantum Mechanics as emergent, which is (as far as I can tell) what the loop quantum gravity people are interested in.

 

[atyy]

Curiously, denying gravitons, at least at fundamental level, ended being what Erik Verlinde means by emerging gravity. He is now saying that the only fundamental string is the open string.

So back to where he started?

 

[atyy]

Then you have to understand Quantum Mechanics as emergent, which is (as far as I can tell) what the loop quantum gravity people are interested in.

Nah, Carlo Rovelli has been trying to get gravitons. How can gravitons not exist - I mean quantum GR is a good effective theory at low energies. The only questions is whether the gravityon is fundamental or emergent, isn't it?

 

[MTd2]

Then you have to understand Quantum Mechanics as emergent, which is (as far as I can tell) what the loop quantum gravity people are interested in.

Quantum Mechanics as emergent is something from t'Hooft's dissipative mechanics. Don't you mean GR?

 

[negru]

If it's emergent or not (and I wouldn't trust Verlinde at all on this one) the graviton still exists. For effective gravity it wouldn't really matter. If you don't like the graviton, you'd have to change the basic QM principles.

 

[MTd2]

Why changing QM? The 2-form of Einstein tensor would be reinterpreted as a gauss law entity, it is not a field therm. You might think of gravitons as something like phonons in a lattice, because you mean effective. But I am not sure if space is cold enough so that there are coherency effects on whatever exists as a fundamental lattice.

 

[tom.stoer]

As long as they are not observed experimentally gravitons are purely mathematical entities. There's no physical reason except for analogy (which is not a bad reason!) to expect them to exist.

In LQG there are NO gravitons at the fundamental level.