Is String Theory the Monopolistic Theory in Modern Physics?

[atyy]

String theory has successful physics!

The first microscopic calculation of the black hole entropy, and the first UV complete theory of quantum gravity, which is a specific instantiation of the holographic principle.

Neither of these have been accomplished by any other approach to quantum gravity.

String theory may not model our universe, but it may be like two other theories which did not model high energy physics - Nordstrom gravity and Wilson's renormalization group in statistical physics - that pointed the way.

Nordstrom gravity was the first relativistic theory of gravity. It is inconsistent with observations, but it showed that it was possible. Einstein and Fokker reformulated it as a theory of curved spacetime, a prelude to general relativity, which is consistent with observation.

Wilson's renormalization group was first applied to condensed matter, and not to high energy physics. Although the details are different, the concepts carry over to high energy physics, and explain why we no longer think of renormalization as the mysterious process of cancelling infinities.

Thus simple but wrong models can provide powerful conceptual insights that point forward. We do not know if string theory models the real world or not. But its indelible conceptual contribution to physics is no longer debatable.

 

[nicoo]

The first microscopic calculation of the black hole entropy

Is this for non-extremal black holes ?

Neither of these have been accomplished by any other approach to quantum gravity

AFAIK LQG recovers (non-extremal) BH entropy for non-extremal BH...

 

[atyy]

Is this for non-extremal black holes ?

No, string theory has only calculated (in a microscopic way) the black hole entropy for extremal black holes.

AFAIK LQG recovers (non-extremal) BH entropy for non-extremal BH...

I'm not sure the LQG calculation is right, but if it is, it is not a microscopic counting, and is essentially semiclassical, and so does not progress beyond Hawking's calculation.

 

[Ilja]

String theory has successful physics!

The first microscopic calculation of the black hole entropy, and the first UV complete theory of quantum gravity, which is a specific instantiation of the holographic principle.

Neither of these have been accomplished by any other approach to quantum gravity.

A strange understanding of successful physics. Computing something we have no chance to measure anyway is a nice exercise in mathematics, not in physics.

And, being UV complete may be a nice theoretical property, but I'm not impressed too. Or do you want to propose a way to distinguish UV-complete theories from UV-incomplete theories by observation?

Thus simple but wrong models can provide powerful conceptual insights that point forward. We do not know if string theory models the real world or not. But its indelible conceptual contribution to physics is no longer debatable.

I agree that it can. That means, there is a possbility of some contribution to physics. But something indelible? Indelible are yet only the contributions to mathematics.

 

[atyy]

A strange understanding of successful physics. Computing something we have no chance to measure anyway is a nice exercise in mathematics, not in physics.

Sure, one can take that point of view. For consistency, if LQG or Schmelzer do a successful microscopic calculation of the black hole entropy, then you will also say that that is not physics.

And, being UV complete may be a nice theoretical property, but I'm not impressed too. Or do you want to propose a way to distinguish UV-complete theories from UV-incomplete theories by observation?

UV complete and incomplete theories are in principle experimentally distinguishable. As for whether something will ever be technologically possible, let me point out that it is still unverified whether chaos is ever responsible for apparent irreversible behaviour in classical systems - yet no one criticizes that as not physics. I'll respect your point of view if you do :)

 

[mitchell porter]

On whether string theory is how reality actually works...

In our current state of half-knowledge half-ignorance about reality and about string theory, it is at least possible that this is so. String theory on a specific background can contain all the necessary ingredients to explain what we see. The immediate problems are

(1) we have no specific string model which predicts the exact values of the standard model parameters, and in fact it's not presently possible to calculate those properties of a "string standard model" to that degree of precision

(2) string models also contain other stuff that isn't seen so far.

Even when they don't contain "exotics", completely new particles with no relation to the known ones, they do contain superpartners of the known ones, and these have not been seen directly. (In various places there continue to be discrepancies between SM theoretical prediction, and experiment, so it's conceivable that we are already seeing the indirect effects of beyond-SM particles, but these discrepancies may also just go away as calculation and measurement improve.) The scalar "moduli" fields arising from the size and shape parameters of extra dimensions are another potential source of trouble, which also could be behind osberved new physics (e.g. aspects of the dark sector).

We can acknowledge these problems while also acknowledging that they are nothing like a falsification.

The following fact is also often presented as a problem for string theory:

(3) there are zillions of possible string models (possible shapes for the extra dimensions, etc).

The argument is that string theory can't be falsified, because there is always another model, so it isn't science.

I feel that the clearest way to interpret the significance of fact 3, is to think of two levels of ambition, in the application of string theory to fundamental physics. First is the quest to find a string model, among the zillions, that just matches reality - which gives the right ratio of electron mass to muon mass, and so on. Then comes the quest to find a reason why the world is like that, rather than some other way.

We are still very far from meeting even the first level of ambition. See my characterization of string phenomenology in #28, and also the second half of "fact 1" in this comment. There is definitely progress in string phenomenology, today's best string models look much more like the real world than those of the mid-1980s, and the ability to calculate has made great progress, but we are still very far from calculating quantities like that electron/muon ratio to several places.

Regarding the second level of ambition, the anthropic principle gets a lot of attention for the past ten years. This is the part that critics of string theory find really threatening: what if, instead of theories that make sharp falsifiable predictions, we settle into a new dogma in which there are handwaving arguments that various qualitative features of the standard model are generic in the landscape of string vacua? But before we debate the merits of anthropic thinking, I would like to say that it is very far from being shown that this is how string theory must work.

The issue of "vacuum selection" is basically an issue of cosmology - what is the large-scale structure of the world in string theory. And we just don't know whether the branching world of eternal inflation is how it works (and even if that is how it works, we don't know that it then goes on to ergodically explore the landscape). There are issues about de Sitter space, there is the question of where inflation's initial conditions come from. There may be a wavefunction of the universe which shows a sharp preference for particular vacua. None of this is worked out properly, it is all discussed very heuristically, and we probably won't really know which heuristics were the right ones - and thus, which picture is the right one - until there is further progress with the mathematical and conceptual fundamentals of string theory.

So at the second level of ambition, we may eventually get sharp predictions produced by cosmological initial conditions. But OK, suppose eternal inflation is how it works, and instead we get a prediction that the zillions of different vacua are being realized in different places, and all the properties of the standard model have a high degree of contingency.

A historical precedent would be the attempts to understand the detailed arrangement of the solar system as expressing some divine intention or natural principle. Today we can see that stars and planets come in numerous arrangements; they all obey the same celestial mechanics, but the particular arrangement we have in this solar system is not the only way it is. There are principles visible in it, e.g. Bode's law, and perhaps it represents a common "type", but still, ultimately there's no deep reason why e.g. the outer planets have the tilts and eccentricities that they do.

The same may apply to the standard model. There may be patterns there arising from deep relationships, and there may be other patterns which are just contingent randomness, which are only constrained by the anthropic necessity that physics be consistent with the existence of observers and/or atoms and/or a long-lived universe. The random patterns would be the junk DNA of particle physics.

Again returning to our current state of half-ignorance half-knowledge: in that state, this must be regarded as possible. We don't know it's true, we don't know it's false, we don't even know whether it is what string theory actually predicts. Anthropic string theory can't be excluded apriori - though any predictions it generates are liable to be suspect - and it leaves the first level of ambition untouched and still valid; one can still hope to identify a specific string vacuum which predicts measurable quantities to arbitrary precision.

Returning to that first level of ambition, where we just try to match experiment and make predictions, and don't especially care if our model is one among zillions of models - we try to falsify the model, and not all of them at once - I started by saying that it's possible that the existing program of string phenomenology may yet pay off. Maybe we'll see superpartners, and so on.

But, it is also possible that some of the guiding phenomenological assumptions are wrong. This is related to my comments #25 and #28, where I tried to explain that string phenomenology derives many of its ideas from a consensus about what to look for and what needs explaining, that originates outside string theory, in the culture of particle physics. There has been a prevailing assumption that the bare standard model is an unnaturally tuned theory, that there needs to be something else in order to make it "natural", and that superpartners with masses within an order of magnitude of the Higgs fit the bill. All this is in question now. The anthropic alternative is currently dominating discussion - the idea that the Higgs is tuned to a thousandth or a millionth part, in order to make atoms and a long-lived universe possible - and so anthropic string phenomenology is also getting a boost. But there are actually many other ideas on the margins, and it may be expected that they will also get their turn to be investigated, and that many surprises will show up.

Just at the level of field theory, there are many other paths to explore, and it's also true of string theory. People may begin to look at new parts of the landscape, and they may even look at neglected versions of string theory itself. There are many unexplored options, at that first level of ambition for string theory, and it may be that the experience of repeatedly being disappointed by the failure of superparticles, large extra dimensions... to show up, will have been necessary to drive people off the beaten track and look at the other possibilities.

 

[atyy]

@carlgrace, are you a condensed matter person?

 

[Ilja]

For consistency, if LQG or Schmelzer do a successful microscopic calculation of the black hole entropy, then you will also say that that is not physics.

Full agreement. The only thing done by Schmelzer in arXiv:0908.0591 is to derive from his model the number of fermions of the standard model, and the gauge group and action as one of the maximal possible compatible with a few principles. There is no computation of black hole entropy.

UV complete and incomplete theories are in principle experimentally distinguishable.

Interested in further information. As I understand, the UV limit is a limit of the critical distance going to zero. So this limit can be taken theoretically, but, if in reality we have a finite critical distance, as the atomic distance in condensed matter theory, then there is no such limit in physical reality.

If a proposed large distance theory has such a limit or not is, of course, a physically interesting question, because it usually has some influence on physical observables larger than the critical distance. But there is no reason to think that the very existence of such a limit makes a theory superior to others. Or even indelible.

let me point out that it is still unverified whether chaos is ever responsible for apparent irreversible behaviour in classical systems - yet no one criticizes that as not physics. I'll respect your point of view if you do :)

"No one" is wrong. Theorems about ergodicity have been criticized for having nothing to do with physics, for the reason that the time necessary for ergodicity to become relevant would be astronomically large for realistic systems. Sorry but I don't remember where I have read this. Related with Bayesian interpretation of probability AFAIR.

 

[tom.stoer]

This is one of the threads losing focus. What shall we discuss:
1) missing experimental support / verification / falsification? this applies to all theories containing quantum gravity
2) landscape? this applies to all known theories (gravity, gauge, SUSY); the only difference is that all these theories have their landscape problem in their fundamental defining equations (higher order curvature terms, different field content, symmetry groups, generations) whereas the string theory landscape exists on the solution side (which is at least some progress)
3) politics / funding / hiring / ...? then it's not the right forum to discuss

Besides that, there ARE problems in string theory:
- no fundamental defining equation known
- no background independent formulation (Ansätze from AdS)
- no probability measure on the landscape
- or if you don't like the landscape: no principle to narrow it down or to achieve uniqueness

Even string theorists (e.g. Gross) could agree on that

 

[tom.stoer]

... it is not a microscopic counting, and is essentially semiclassical, and so does not progress beyond Hawking's calculation.

There are calculations with next-to-leading-order / logarithmic corrections; and I don't see why you think that LQG calculations are not based on microscopic counting

 

[ShayanJ]

This is one of the threads losing focus. What shall we discuss:
1) missing experimental support / verification / falsification? this applies to all theories containing quantum gravity
2) landscape? this applies to all known theories (gravity, gauge, SUSY); the only difference is that all these theories have their landscape problem in their fundamental defining equations (higher order curvature terms, different field content, symmetry groups, generations) whereas the string theory landscape exists on the solution side (which is at least some progress)
3) politics / funding / hiring / ...? then it's not the right forum to discuss

Besides that, there ARE problems in string theory:
- no fundamental defining equation known
- no background independent formulation (Ansätze from AdS)
- no probability measure on the landscape
- or if you don't like the landscape: no principle to narrow it down or to achieve uniqueness

Even some string theorists (e.g. Gross) would agree on that

You're right tom...but the reason that string theory is being criticized this much is that its fame doesn't seem to be in accordance with what it has to offer!
String theory is just one of the candidates of quantum gravity theory...and as developed and unconfirmed as others!But other candidates just seem to be in its shadow...at least in public...
I know...you may tell string theory can be a TOE but...well...the very existence of a TOE is in doubt!

 

[tom.stoer]

You're right tom...but the reason that string theory is being criticized this much is that its fame doesn't seem to be in accordance with what it has to offer!
String theory is just one of the candidates of quantum gravity theory...and as developed and unconfirmed as others!But other candidates just seem to be in its shadow...at least in public...
I know...you may tell string theory can be a TOE but...well...the very existence of a TOE is in doubt!

I know that and I can agree to that. I started a thread on this topic some years ago. But nevertheless WE should try to distinguish physics and other issues like funding.

Let's have a look at Smolin and Woit: they are right with some physical concerns. But instead of discussing them in detail they move forward to funding / hiring / ... and partially their own research interests. As soon as they do that they lose credibility b/c people do no longer believe in their arguments against strings based on physical reasoning but they think that this was only an intro to provide a basis for other discussions.

That's why I wrote

... What shall we discuss: ...
3) politics / funding / hiring / ...? then it's not the right forum to discuss

 

[Ben Niehoff]

- no background independent formulation (Ansätze from AdS)

This one I have to disagree with, and I don't know why it gets repeated so much. Perturbative string theory is a theory entirely on the worldsheet. One writes down a certain 2-dimensional SUSY CFT with 8 scalars (10 scalars plus a gauge symmetry that reduces the physical content to 8) and 8 fermions. Vanishing of the beta functions tells you that, at low energies, these 10 scalars can be interpreted as coordinates on some spacetime which must solve Einstein's equations. The 8 physical scalars correspond to degrees of freedom of a massless, stringlike object propagating in this spacetime.

But this is a low-energy approximation that pops out entirely from the worldsheet theory. The worldsheet theory is 2-dimensional coordinate-invariant, so I hope you would agree from classical differential geometry (and GR) that one can discuss the 2-d theory without making any reference to a putative 10-dimensional ambient space.

But what's more, is that these 10 scalars only have an interpretation as geometry in the low-energy approximation. At higher energy, the very notion of a background geometry breaks down!

So string theory is quite background-independent, to the extreme that technically speaking, there is no background geometry at all.

 

[Ben Niehoff]

Wow, you must not get out much. All I've seen is reasoned, calm critique with just a touch of dramatic exaggeration. Maybe you should actually read it instead of jumping to conclusions.

My impression was based on the first 5 posts. I see the thread become more sensible afterward. Still, I take issue with people forming such strong negative opinions about something they do not understand at all, based on a few popsci books and videos.

Would you consider Brian Greene to be an unreasonable person? Is he making fantastic claims to attract viewers and advertising dollars?

I am not going to engage in appeals to authority, this or that person said this or that thing, etc. String theory is not a monolithic enterprise. And neither do I expect a popular science talk to represent the full nuances of anyone's scientific opinions.

If you want to know what string theory is and what it is used for, you have to skip Ted talks and read the arXiv.

 

[tom.stoer]

The worldsheet theory is 2-dimensional coordinate-invariant, ... At higher energy, the very notion of a background geometry breaks down! So ... technically speaking, there is no background geometry at all.

So what is the fundamental formula (action integral, ...) fully defining string theory in a background-independent way?

 

[Ben Niehoff]

So what is the background-independent formula fully defining string theory?

If you're now demanding the One Equation then I don't know what to tell you. That is a separate issue entirely, and I think string field theory is still not too well understood, except in a few generalities. Although I do think Witten wrote down an action for it involving Moyal star products.

The worldsheet action (as a SUSY, nonlinear sigma model) is a standard textbook thing, and you can look it up in, say, Becker, Becker, Schwarz.

As for M-theory, as far as I remember, people are still trying to work out what the actions of M2- and M5-branes might be.

I'm not sure the lack of a One Equation is even a problem. I don't see anyone being particularly bothered that QED has no full, non-perturbative definition.

 

[Ilja]

But nevertheless WE should try to distinguish physics and other issues like funding.

The point why in string theory discussions issues like funding become important is very simple: It is part of the great sociological success of string theory.

There are good general arguments that the modern funding of science - young scientists get only short time jobs and therefore have to care all the time about their ability to get a new job - has negative consequences, because it supports established mainstream directions and makes it almost impossible for young scientists to develop for many years some new, own idea. This is probably the only domain where the communist system was better: the Soviet scientist has had a low-paid but safe job, so it was possible for him to develop new ideas over a long time without publish or perish pressure.

In experimental science this may be not that dangerous, where we have observation which forces a leading mainstream direction to correct its errors. In fundamental science, the result may be fatal, namely an accidental choice of a direction which gets all the job offers, so that even people not really believing into this direction have to work in this direction if they want to work in fundamental physics at all.

We all want independence of science. But the standard way to make people independent is to give them security, especially job security. If we want judges to be independent, we give them job security, so that they cannot be fired if they make a politically unpopular decision. Instead, the job of a scientist is less secure than that of an average worker, so independence of science is, essentially, dead, reduced to independence of a few mainstream directions of science.

So the problem is a very important one, and it is especially important in fundamental science, because fundamental science is much more speculative, and therefore to concentrate on a single mainstream direction is much more dangerous there.

And the classical example of such a speculative direction, which has become the only game in town, is string theory. So, for discussions about string theory and its alternatives this point is essential and important.

 

[Ben Niehoff]

And the classical example of such a speculative direction, which has become the only game in town, is string theory. So, for discussions about string theory and its alternatives this point is essential and important.

I really don't think this is true. There are, for example, a lot of people doing LQG, higher spin field theory, etc. In my department, we have people working on "2 time physics", which is highly speculative and probably only a handful of people do it.

And outside of quantum gravity, you also have people working on all kinds of beyond-SM stuff.

As someone about to apply for my first postdoc, and thus intimately aware of the job-insecurity game, I have to disagree with the notion that this shuts out new ideas. A new idea is not generally developed in the fashion of Einstein (working on something alone for years and then publishing it). A new idea is developed a little bit at a time, shared with the community, improved upon, etc. And if any new idea looks promising (or even just interesting), I guarantee a lot of people will jump into it and publish a flurry of papers, potentially making the originator(s) of the idea famous and important. This is what has happened in the last year and a half over black hole firewalls, for example.

I do think that it is more difficult for a new person in the field to come up with genuinely interesting new ideas, because to do so requires learning a lot of what has already been done. It takes time to build enough familiarity and mathematical tools.

 

[tom.stoer]

If you're now demanding the One Equation then I don't know what to tell you. That is a separate issue entirely, and I think string field theory is still not too well understood, except in a few generalities.

OK, I agree.

Although I do think Witten wrote down an action for it involving Moyal star products.

Any reference?

The worldsheet action (as a SUSY, nonlinear sigma model) is a standard textbook thing, and you can look it up in, say, Becker, Becker, Schwarz.

I'll have a look - but what I have seen so far is always in some way background-dependent. It's like non-rel. QM: yes, the theory can be defined for arbitrary el.-mag. fields, but the field is chosen by hand. So background independence is not te same as "for arbitary backgrounds".

I'm not sure the lack of a One Equation is even a problem. I don't see anyone being particularly bothered that QED has no full, non-perturbative definition.

QED, QCD, ... are not theories of quantum gravity, so the spacetime background is natural. But at least classically the theories ARE background independent - as long as you do not start perturbative calculations like scattering, lamb-shift, scaling-violation in DIS etc. So the definition is background-independent, only for specific solutions you chose specific backgrounds.

And by the way: GR is background-independent, so a theory of QG should be as well ;-)

 

[Ben Niehoff]

I'll have a look - but what I have seen so far is always in some way background-dependent. It's like non-rel. QM: yes, the theory can be defined for arbitrary el.-mag. fields, but the field is chosen by hand. So background independence is not te same as "for arbitary backgrounds".

I'm not sure what you're getting at here, because I just explained how the worldsheet theory is, in fact, background-independent. (And yes, for real; not just "for arbitrary backgrounds").

I think some of the confusion comes from the fact that introductory textbooks will usually draw pictures of strings propagating in spacetime, and will refer to the 10 scalars as "spacetime coordinates", talk about things like "induced metrics", etc. But these are all words that apply only in the low-energy limit.

In reality the worldsheet theory contains a bunch of scalar fields that interact in some conformal field theory. The theory is fully quantum, and only in the classical limit can these fields even be viewed as smooth functions, let alone coordinates on a manifold. In the full theory, they are a quantum mess.

And by the way: GR is background-independent, so a theory of QG should be as well ;-)

Agreed, certainly, and on this issue I think string theory one-ups GR. GR, after all, assumes that the background is a smooth, 4-dimensional manifold. String theory doesn't even assume the existence of these basic structures (smoothness, topological, etc.)

 

[Ben Niehoff]

Any reference?

The Wiki article on string field theory has Witten's action as well as a bunch of other stuff. I assume it has some real references at the bottom. It's not what I work on, so I can't help you much beyond pointing you there.

 

[tom.stoer]

I'll think we need to start a new thread

 

[atyy]

Interested in further information. As I understand, the UV limit is a limit of the critical distance going to zero. So this limit can be taken theoretically, but, if in reality we have a finite critical distance, as the atomic distance in condensed matter theory, then there is no such limit in physical reality.

If a proposed large distance theory has such a limit or not is, of course, a physically interesting question, because it usually has some influence on physical observables larger than the critical distance. But there is no reason to think that the very existence of such a limit makes a theory superior to others. Or even indelible.

UV complete and incomplete theories are experimentally different in the sense that UV incomplete theories are "not even wrong" above a certain energy. They don't make predictions. OTOH, UV complete theories are falsifiable above a certain energy. I agree that the energy above which this happens for gravity is so high, having a UV complete theory of gravity may never be relevant. Anyway, this criticism doesn't apply to string theory alone, but to all approaches to quantum gravity. I do consider it reasonable to argue that all quantum gravity research is not worth public spending on - it's something like public funding for the arts. Anyway, Bach had trouble with getting funding in his day too ...

Anyway, funding aside, my point of view is that conceptual understanding is important too. For example, nowadays we can get almost everything using classical GR and quantum field theory. The other way of calculating things is quantum GR as an affective field theory, and the one can calculate the quantum corrections to classical GR. The quantum corrections are probably too small to be observed even within the next 100 years, so one could say we don't need quantum GR, and it is "not science", since it is just as good as classical GR for everything we see. But quantum GR is the more satisfactory framework than classical GR, so I prefer having it as the basis of classical GR. Similar to Wilson's renormalization group explanation which is "not science" in the sense that the calculations remain exactly the same as "removing mysterious infinities", but everything makes sense after Wilson, so I prefer it.

"No one" is wrong. Theorems about ergodicity have been criticized for having nothing to do with physics, for the reason that the time necessary for ergodicity to become relevant would be astronomically large for realistic systems. Sorry but I don't remember where I have read this. Related with Bayesian interpretation of probability AFAIR.

The ergodicity problem is related, but I was thinking more specifically of microcopic chaos and apparent irreversibility. I was thinking of this experiment http://www.nature.com/nature/journal/v394/n6696/abs/394865a0.html , which was subsequently shown to fall short of its goal due to some loopholes.

There are calculations with next-to-leading-order / logarithmic corrections; and I don't see why you think that LQG calculations are not based on microscopic counting

The attempts to calculate the black hole entropy by state counting in LQG do get log corrections, but not the factor of 4 in the Bekenstein-Hawking entropy.

The calculation by Bianchi that does get the factor of 4 has no state counting.

 

[julian]

How can you state the assumptions of string theory when nobody knows how to define a theory. As far as I know M-theory is still a `mystery'. By world-sheet theory do you mean perturbative string theory cus this is not background independent because basic structures - Hilbert space, algebra of observable operators, etc depend on the background spacetime chosen. To be background-independent the Hilbert space etc must simultaneously accommodate all backgrounds. Guessing that's the idea of string field theory. There is a difference between being able to define the theory on arbitrary backgrounds and background independence. Perturbative string theory can't even be defined time-dependent spacetime without getting Tachyons.

 

[carlgrace]

@carlgrace, are you a condensed matter person?

I am. Did I say something to give myself away? ;)

 

[julian]

Just to quote Smolin's - `appealing to authority' -sorry - "The case for background independence" http://fr.arxiv.org/pdf/hep-th/0507235

"Some string theorists have also claimed that string theory does not need a background independent formulation, because the fact that string perturbation theory is, in principle, defined on many different backgrounds is sufficient. This assertion rests on exaggeration and misunderstanding. First, string perturbation theory is so far only defined on stationary backgrounds that have timelike killing fields. But this is a measure zero of solutions to the Einstein equations. It is, however, difficult to believe that a consistent string perturbation theory can be defined on generic solutions to the Einstein equations because, in the absence of timelike killing fields, one cannot have spacetime supersymmetry, without which the spectrum will generally contain a tachyon21."

He then goes on to say:

"More generally, this assertion misses completely the key point that general relativity is itself a background independent theory. Although we sometimes use the Einstein’s equations as if they were a machine for generating solutions, within which we then study the motion of particles of fields, this way of seeing the theory is inadequate as soon as we want to ask questions about the gravitational degrees of freedom, themselves. Once we ask about the actual local dynamics of the gravitational field, we have to adopt the viewpoint which understands general relativity to be a background independent theory within which the geometry is completely dynamical, on an equal footing with the other degrees of freedom. The correct arena for this physics is not a particular spacetime, or even the linearized perturbations of a particular spacetime. It is the infinite dimensional phase space of gravitational degrees of freedom. From this viewpoint, individual spacetimes are just trajectories in the infinite dimensional phase or configuration space; they can play no more of a role in a quantization of spacetime than a particular classical orbit can play in the quantization of an electron."

 

[Ilja]

I really don't think this is true. There are, for example, a lot of people doing LQG, higher spin field theory, etc. In my department, we have people working on "2 time physics", which is highly speculative and probably only a handful of people do it. And outside of quantum gravity, you also have people working on all kinds of beyond-SM stuff.

Maybe the situation has already changed a little bit, given that the failure of string theory has been already recognized more widely. That would be nice.

As someone about to apply for my first postdoc, and thus intimately aware of the job-insecurity game, I have to disagree with the notion that this shuts out new ideas. A new idea is not generally developed in the fashion of Einstein (working on something alone for years and then publishing it). A new idea is developed a little bit at a time, shared with the community, improved upon, etc. And if any new idea looks promising (or even just interesting), I guarantee a lot of people will jump into it and publish a flurry of papers, potentially making the originator(s) of the idea famous and important. This is what has happened in the last year and a half over black hole firewalls, for example.

This is possible if the new idea is sufficiently close to the mainstream. My approach was too far away from it - I have used a preferred frame, and that's almost forbidden. But, even worse, once there was a preferred frame, I have continued to move into this direction and tried for a condensed matter interpretation of the GR and SM fields. That was already far too much, an ether theory. So my experience was that I was completely alone. And even after I have succeeded to publish the results in Foundations of Physics, there was almost no reaction.

Quite surprising, if you look at the results. The particular quite simple model gives all the SM fermions, exactly three generations, and the SM gauge group with its action on the fermions. Together with a lot of massive scalar fields, so I'm yet not afraid of the Higgs observation. Imagine something similar reached by string theory, there would be thousands of papers about it. I have also needed a lot of time to complete the model, but I was unable to publish any intermediate results. That I was able to publish the final paper arXiv:0908.0591 was also only a happy accident: I have been able to solve a final large problem - to include weak interactions appropriately - weeks after receiving a review which required major revisions - because the model was yet unable to include weak interactions. Without this accidental coincidence, I guess the final paper would have been rejected because the Higgs sector is not handled appropriately.

Up to now, I haven't seen any reaction in form of people jumping in or papers about it. An ether theory is, obviously, even if published in Foundations of Physics and considering relativistic gravity and the standard model, too far away from the mainstream, too much anathema.

 

[atyy]

Just to quote Smolin's - `appealing to authority' -sorry - "The case for background independence" http://fr.arxiv.org/pdf/hep-th/0507235

"Some string theorists have also claimed that string theory does not need a background independent formulation, because the fact that string perturbation theory is, in principle, defined on many different backgrounds is sufficient. This assertion rests on exaggeration and misunderstanding. First, string perturbation theory is so far only defined on stationary backgrounds that have timelike killing fields. But this is a measure zero of solutions to the Einstein equations. It is, however, difficult to believe that a consistent string perturbation theory can be defined on generic solutions to the Einstein equations because, in the absence of timelike killing fields, one cannot have spacetime supersymmetry, without which the spectrum will generally contain a tachyon21."

He then goes on to say:

"More generally, this assertion misses completely the key point that general relativity is itself a background independent theory. Although we sometimes use the Einstein’s equations as if they were a machine for generating solutions, within which we then study the motion of particles of fields, this way of seeing the theory is inadequate as soon as we want to ask questions about the gravitational degrees of freedom, themselves. Once we ask about the actual local dynamics of the gravitational field, we have to adopt the viewpoint which understands general relativity to be a background independent theory within which the geometry is completely dynamical, on an equal footing with the other degrees of freedom. The correct arena for this physics is not a particular spacetime, or even the linearized perturbations of a particular spacetime. It is the infinite dimensional phase space of gravitational degrees of freedom. From this viewpoint, individual spacetimes are just trajectories in the infinite dimensional phase or configuration space; they can play no more of a role in a quantization of spacetime than a particular classical orbit can play in the quantization of an electron."

Smolin did not argue that string theory was worthless. He argued that string theory had been oversold by some string theorists, and that approaches other than string theory are also valuable.

In fact, Smolin had worked on possible relationships between LQG and string theory, which indicates he considered it worthwhile. I suggest reading these 3 papers that indicate the relationship between LQG and string theory is still worth studying.

http://arxiv.org/abs/0905.3627
http://arxiv.org/abs/0907.2994
http://arxiv.org/abs/1212.5183

The link between LQG and strings in the second is indirect, but tensor networks were recently used by Hartmann and Maldacena, both of whom are stringists. The last is a collaboration between an LQGer and a stringist.

 

[atyy]

I am. Did I say something to give myself away? ;)

Well, I saw you had lots of posts in the squalid state section:)

My discipline (living organisms) is even more squalid than yours, but may I suggest why condensed matterists should consider string theory their Trojan horse?

Two condensed matter viewpoints I've heard are:
1) No theory of everything!
2) Everything is emergent!

With the string landscape, string theory has taught HEP people that there will be multiple vacua - just like condensed matter is in large part a study of the multiple low energy "vacua" that correspond to different materials, ie. string theory is the apotheosis of squalidity. (John McGreevy makes a similar analogy: Monovacuist (n): One who believes that a theory of quantum gravity should have a unique groundstate (in spite of the fact that we know many examples of much simpler systems which have many groundstates, and in spite of all the evidence to the contrary)).

Then with AdS/CFT presently providing the only non-perturbative definition of string theory, that means that string theory is defined by QFT, which perhaps can be emergent from a condensed matter type of lattice model. (A string theorist who's said something like this is Raman Sundrum: Let us ask whether the pre-requisite CFT can itself emerge from something even more basic and less symmetric. It is common for equilibrium condensed matter systems, which may be discrete lattice theories at short distances, to approach conformal field theories in the IR, at second order phase transitions. Because time is out of the picture at equilibrium, these are Euclidean CFTs. However, in real-time systems even the approach to emergent special relativity, let alone Lorentzian conformal invariance, is subtler.)

So secretly, condensed matter rules the "TOE". OK, I'm half kidding, but just half;)

 

[Ben Niehoff]

By world-sheet theory do you mean perturbative string theory cus this is not background independent because basic structures - Hilbert space, algebra of observable operators, etc depend on the background spacetime chosen.

By world-sheet theory, I mean the CFT defined on the worldsheet. I.e., it is a 2d CFT with various fields living on some Riemann surface with punctures. Specifically it is a kind of nonlinear sigma model.

There is no background spacetime.

You need to remove cartoony pictures in which a "string" propagates on some "background" from your head. That is not how string theory is understood today.

To be background-independent the Hilbert space etc must simultaneously accommodate all backgrounds.

GR doesn't even do that. I guess GR is not background independent.

Of course, really we're talking nonsense here, because GR is a classical theory and there is no notion of Hilbert spaces in the first place. If "background independence" is a quality we say GR should have, then certainly it is not a notion that is defined by referring to Hilbert spaces.

But I have yet to see a clear, precise definition of what "background independent" is actually supposed to mean except "a property we want LQG to have but string theory not". I have occasionally seen it paraphrased as "no prior geometry", but as you see here, when I explain how string theory has no prior geometry, the definition of "background independent" weasels its way into something else.

Perturbative string theory can't even be defined time-dependent spacetime without getting Tachyons.

Patently false. Worldsheet SUSY is enough to kill the tachyons. Low-energy solutions to supergravity need not be supersymmetric.

Think about that for a moment. If a theory has a certain symmetry, must all solutions of that theory have the same symmetry? For example, Newton's law of gravity is rotationally invariant. Is the motion of the solar system rotationally invariant?

Just to quote Smolin's - `appealing to authority' -sorry - "The case for background independence" http://fr.arxiv.org/pdf/hep-th/0507235

I don't know why Smolin appears to believe that string perturbation theory requires us to choose, by hand, a spacetime in which the string propagates. Maybe it's because he hasn't done string theory in a long time, and his understanding of it is not up to date. (Nor would I expect his understanding to be up to date; despite his being a very smart guy, he does, after all, work in an entirely different field now.)

 

[carlgrace]

Well, I saw you had lots of posts in the squalid state section:)

My discipline (living organisms) is even more squalid than yours, but may I suggest why condensed matterists should consider string theory their Trojan horse?

Two condensed matter viewpoints I've heard are:
1) No theory of everything!
2) Everything is emergent!

With the string landscape, string theory has taught HEP people that there will be multiple vacua - just like condensed matter is in large part a study of the multiple low energy "vacua" that correspond to different materials, ie. string theory is the apotheosis of squalidity. (John McGreevy makes a similar analogy: Monovacuist (n): One who believes that a theory of quantum gravity should have a unique groundstate (in spite of the fact that we know many examples of much simpler systems which have many groundstates, and in spite of all the evidence to the contrary)).

Then with AdS/CFT presently providing the only non-perturbative definition of string theory, that means that string theory is defined by QFT, which perhaps can be emergent from a condensed matter type of lattice model. (A string theorist who's said something like this is Raman Sundrum: Let us ask whether the pre-requisite CFT can itself emerge from something even more basic and less symmetric. It is common for equilibrium condensed matter systems, which may be discrete lattice theories at short distances, to approach conformal field theories in the IR, at second order phase transitions. Because time is out of the picture at equilibrium, these are Euclidean CFTs. However, in real-time systems even the approach to emergent special relativity, let alone Lorentzian conformal invariance, is subtler.)

So secretly, condensed matter rules the "TOE". OK, I'm half kidding, but just half;)

Wow that's a fascinating article! Thanks for sharing it. You know, the level of discourse between my colleagues and me regarding string theory is pretty much at the level of Brian Greene and Lee Smolin books, but I would agree with you about the idea we are quite suspicious of even the idea of a theory of everything (even if, as you imply condensed matter *is* the theory of everything haha). Also, the concept of everything is emergent sure resonates with us.

Thanks for the link!

 

[tom.stoer]

I'll think we need to start a new thread - https://www.physicsforums.com/showthread.php?t=700495

 

[Demystifier]

To all who say they wish to become a String Theorist (but may not have even graduated from college yet), I ask a serious question:

Why would you want to do such a thing?

Because string theory is the theory of everything, and who doesn't want to know everything?

 

[schema]

I am deeply rooted in relativistic physics. I reject string theory on all levels due to my bias. This is basically what I conclude from hours and hours of Michio Kaku pontification: " Our entire, long standing, anomaly riddled theory is hinged apon the existence of gravitons, for which there is -and probably never will be- proof of. But we are right, trust us."
Everyone is out searching for a unified theory, and I hate to say it, but string theory is the best game in town right now. I heard conjecture somewhere about string theory and how the theorists are abandoning the idea of the strings in favor of variations of M-theory. I am not sure if that is true or not, but I concure none-the-less. I never understood the significance of 7 smaller dimensions. I can't fathom the dimenions suggested in M-theory. I guess they have to make the math make sense somehow.

 

[friend]

Everyone is out searching for a unified theory, and I hate to say it, but string theory is the best game in town right now. ...

"unified theory..." I strongly suspect that we are not going to know how to unified all the quantum fields until we know where quantum theory comes from to begin with. And String theory doesn't even start to address this.