Are superstrings the only option? What about GR approaches?

[Robert100]

Are superstrings (and related M-theory) the only option for a theory of everything? Wikipedia states:

"The only mainstream candidate for a theory of everything at the moment is superstring theory / M-theory; current research on loop quantum gravity may eventually play a fundamental role in a TOE, but that is not its primary aim. These theories attempt to deal with the renormalization problem by setting up some lower bound on the length scales possible. Also, early 21st century theories of everything tend to suppose that the universe actually has more dimensions than the easily observed three of space and one of time. The motivation behind this approach began with the Kaluza-Klein theory in which it was noted that adding one dimension to general relativity would produce the electromagnetic Maxwell's equations. This has led to efforts to work with theories with large number of dimensions in the hopes that this would produce equations which are similar to known laws of physics. The notion of extra dimensions also helps to resolve the hierarchy problem which is the question of why gravity is so much weaker than any other force. The common answer involves gravity leaking into the extra dimensions in ways that the other forces do not."

As far as I have been able to determine, no other ToE's are being worked on. I know that Peter Woit is (in)famous for his blog "Not Even Wrong", but is Woit offering any options? Are Woit and his supporters offering other paths to a possible ToE that is anywhere near as far along as superstrings? Are any of these other paths actually being worked on by any practicing physicists? Are there any recent (last 5 years) peer-reviewed articles on such alternate paths?


The Wikipedia article hints at other attempts to work out a possible ToE. Are any of the following accepted as being a possible ToE by mainstream physicists?

"There have been several attempts to advance the general theory of relativity as a theory of everything. As mentioned above, Einstein was responsible for one of these: in collaboration with Rosen he attempted to model particles as tiny wormholes, hence the term Einstein-Rosen Bridge. Wormholes have also been proposed at various times (for instance, by Shimony and by Durand [1]) to explain Bell violations not as superluminal influences but influences that take a shortcut through a wormhole. Such theories face a number of hurdles: the creation of wormholes changes the topology of spacetime by creating a new "handle" which implies violations of causality (see Hadley [2]), and the general theory of relativity predicts its own breakdown at a Gravitational singularity by theorems of Stephen Hawking and Roger Penrose. A recent effort to surmount this hurdle notes that the equivalence principle can be applied along curves rather than at a single point (Iliev [3]), which would imply that time dilation of (1 − v2) − 1 / 2 is indistinguishable locally (along the curve) from a relative velocity v and the unbounded time dilation observed as an event horizon emerges at the center of a collapsing star implies that the center is in reality as well as appearance receding at a velocity approaching the speed of light, producing a bubble-like local inflation of the star's interior (Monroe [4]). This approach skirts the trapped surface assumption of the singularity theorems of Hawking and Penrose. It posits that quantum behavior is an emergent phenomenon in general relativity caused by a stochastic gravitational background radiation (Calogero [5]), in which colliding gravitational waves create virtual particle pairs in the form of wormholes (Griffiths [6]), and the interference pattern in the two-slit experiment is caused by interference between gravitational wavefronts."


I would be really surprised if superstrings was the only ToE being worked on, yet I also get the idea that Woit doesn't have any actual alternatives, at least not that are developed and studied by peer reviw.

In a posting to Not Even Wrong Woit remarks:

"The SU(2) gauge symmetry is supposed to be a purely internal symmetry, having nothing to do with space-time symmetries, but left and right-handed spinors are distinguished purely by their behavior under a space-time symmetry, Lorentz symmetry. So SU(2) gauge symmetry is not only spontaneously broken, but also somehow knows about the subtle spin geometry of space-time."

According to Wikipedia, Woit believes that a proper investigation of what can be done using the geometry of spinors in just four dimensions (along with many other possibly fruitful ideas) has been prevented by an obsession with extra-dimensional speculations.

This seems to imply that no professional physicists are actually pursuing this, and it is really just a hunch. If so, then Woit seems to attacking superstrings without offering a better alternative! What's going on here?

Robert

 

[josh1]

What's going on here?

What do you think is going on Robert? Just visit the sites of physics departments of the top universities to see what's going on. You'll find it's string theory and not other approaches. Unfortunately, there are a lot of people here, especially marcus and selfadjoint, who basically mislead people who don't know enough physics to know the difference. The reason these guys, especially marcus, don't like string theory is that it's simply too hard for them to follow. The following post by marcus is typical of the tactics used. Don't trust anyone at this site to be honest about why strings actually dominate. In fact, don't even trust me. You need to go directly to the scientists that actually do this sort of research. Peter woit is not one of them, and marcus certainly isn't and has never posted a single thing indicating he actually understands anything about string theory on a technical level. Ask him some real physics questions and he'll make excuses rather than just explaining the physics. On the other hand, I'll answer any physics questions you have, on string and any other approach to quantum gravity. But again, don't trust me or anyone else here. Email some of the researchers out there about what they think of peter woit or this forum. I'm pretty sure the term crackpot will come up alot. I think it's worse than that. For example, Peter woit is a liar. He repeatedly pronounces that string theory is a failure, but he knows full well that we don't yet understand string theory. All other approaches have been understood for a decade and it is clear to all but a very few that these other approaches we're pretty much dead on arrival.

 

[marcus]

Are superstrings (and related M-theory) the only option for a theory of everything? Wikipedia states:
...

Wikipedia is not such a good place to start. Wiki's articles relating to quantum gravity have parts which have been repeatedly tug-of-war edited and re-edited so as to in some cases down-play the interest and importance of non-string approaches. IMO Wikipedia is not as authoritative or reliably unbiased in that department as it is in many other areas---some passages may give quite a wrong impression. In the case of several Wikipedia articles, I've found it interesting to click on "history" which gives the history of the article, how it was written, who changed what, who then changed it back---and so on. A kind of "scrimmage".

Peter Woit has his own research but his blog does not cover a wide spectrum of research news in non-string QG to any great extent. The blog is an interesting and fun place but if you want a broad introduction to non-string QG research efforts you should begin somewhere else.

the last big conference in non-string QG was October 2005 and the talks are on-line, at least were the last time I looked. go to the website of the Loops '05 conference and check it out. If the site is up, check out "program" and click on individual talks

recently quite a bit of emphasis has been given to including matter and deriving matter from the quantum space(time) models

a book to look out for, scheduled for 2006 publication by Cambridge University Press, is a collection of essays describing not just one approach but several different lines of non-string QG research. It also contains chapters by string theorists. The book is edited by Daniele Oriti. Around 10-or-so of the chapters are available online at the preprint archive. the book title is something like
Approaches to Quantum Gravity, towards a new understanding of space time and matter
(see for example the preprint chapter mentioned here:
https://www.physicsforums.com/showpost.php?p=980353&postcount=481)

several of the approaches have matter fields and spacetime geometry arise from the same "stuff"---non-string QG approaches are certainly developing towards a UNIFIED picture of quantum spacetime geometry and matter interactions----a quantum spacetime-matter dynamics. but they certainly are not there yet. the number of people researching in non-string QG is increasing (still quite small compared with the string research establishment however!)

for an introductory overview of nonstring QG there is a series of 25 video lectures by Lee Smolin and two other people. it is free for download at perimeterinstitute.com

neither Wikipedia (as currently edited) nor Peter Woit are really in the business of giving a broad accurate picture of non-string QG research or of the most active alternatives to string theory. Both of them are excellent and I have the highest regard for them as web resources. But Peter Woit has his plate full just keeping track of all the furor surrounding the String Landscape and the slide towards the Anthropic Principle, plus all the other stuff he watches and discusses with commentors. He would have neither the time nor the space in his blog to report the QG scene. It is a great blog but it is not its job to tell you about spin-networks, GFT (group field theory), spinfoam models, doubly special relativity, loop quantum cosmology, black hole singularity removal, quantum bounce replacing big bang, prospects for testing non-string QG models by gammaray burst observations and a lot more. If you want to know about alternatives to string you need a whole other news service and there almost isn't one

 

[marcus]

I was too long-winded before. I'll try to answer in brief

Are superstrings the only option?

No, interestingly enough. Non-string QG is getting into the unification business.

Are superstrings (and related M-theory) the only option for a theory of everything?

On the contrary, alternatives are being worked on. But it is not clear to me that ANYBODY'S theory, as it stands, is going to work out as an option. Everything in the field is going to take more work. AFAICS the key step is TESTING. As science teacher, you know that a theory of spacetime and matter has to be brought to the point that it makes unambiguous predictions that can be tested. To be science, it has to bet its life that a practical future experiment will turn out a certain way, and if it turns out different, it dies. If a theory is so mushy that it can happily accommodate any possible future outcome of any future experiment then it is not predictive in the sense of making hard predictions---not falsifiable. Since science theories cannot be ultimately verified, testability amounts to being falsifiable.

So right now theorists are scrambling to get falsifiability for their theories. A recent string paper about this was by Distler. A recent non-string paper was by Smolin. (if you want archive numbers, just say).

And of course the non-string approaches are being pushed to get UNIFICATION too. At this stage the aim seems to be to get MATTER to arise from their models of spacetime. Laurent Freidel has in certain cases gotten the Feynman diagrams usual in the quantum field theory of matter to emerge in a spinfoam spacetime model. John Baez has two recent papers of a similar nature. No Feynman diagrams there yet, but matter cropping up in 4D spacetime models---i.e. no extra dimensions. May be some crossover with string theory, but without assuming extra spatial dimensions. Can't tell if any of the new non-string spacetime-and-matter approaches will be able to make it to the finish line. Too early.

As far as I have been able to determine, no other ToE's are being worked on.

No way! The non-string QG guys are really scrambling now. It's a fun field to watch. Do you want URLs to recent work to get a sample of it?

 

[Chronos]

What do you think is going on Robert? Just visit the sites of physics departments of the top universities to see what's going on. You'll find it's string theory and not other approaches. . .

String is still popular, but fading. The landscape/anthropic mess has disillusioned many who prefer to avoid blurring the line between science and philosophy.

. . . Unfortunately, there are a lot of people here, especially marcus and selfadjoint, who basically mislead people who don't know enough physics to know the difference. The reason these guys, especially marcus, don't like string theory is that it's simply too hard for them to follow. . . [marcus] has never posted a single thing indicating he actually understands anything about string theory on a technical level. Ask him some real physics questions and he'll make excuses rather than just explaining the physics . . . Peter woit is a liar.

Is the concept of refraining from 'ad hominem attacks' too hard to follow?

 

[f-h]

First of all, LQG is first and foremost a theory of Quantum Gravity not of Everything.

Matter may or may not arise in natural ways, but that's not the primary aim, and while it's an active field of inquiry it's far from established.
The following sentence:
"These theories attempt to deal with the renormalization problem by setting up some lower bound on the length scales possible."
Is of course wrong, the minimal length scale is not set up, but a consequence of the theory, implying why perturbative attempts to quantize Gravity failed.

Josh1, it's not difficult to see that above and beyond whatever scientific merits it might or might not have, there is a significant sociological inertia to the idea. This is normal but it's the first time in science that such an idea has become so dominant on the theoretical side without experimental evidence. This warrants suspicion from the outside, suspicion that can not be waved away by claiming that the critics do not understand the technical details. Once you understand the technical details of String Theory, you are looking for a PostDoc... in String Theory.

In fact the extreme technical sophistication makes me warry about ST. Compare to LQG, you go through a technically highly sophisticated quantisation procedure, but at the end (or at the first intermediate stop) you arrive at a physical picture that is simple and straightforward. To me the fact that technical complexity of the theory is not just increasing in time but comes back down again, too is an indication that something right is going on.

 

[josh1]

String is still popular, but fading. The landscape/anthropic mess has disillusioned many who prefer to avoid blurring the line between science and philosophy.

No, strings are not fading. This is just the kind of B.S. peddled here as fact.

Is the concept of refraining from 'ad hominem attacks' too hard to follow?

Not at all. The problem here is that there are all kinds of ways to behave rather badly towards other members without setting off any alarms, and some members here - two in particular - are masters of this.

 

[Careful]

** What do you think is going on Robert? Just visit the sites of physics departments of the top universities to see what's going on. **

How many people were working on :
(a) discrete processes to ``explain'' the photo electric effect when Einstein came up with this ?
(b) relational approaches to physics when GR was born in 1915 ?
(c) the renormalizability of Yang Mills when 't Hooft proved this in his doctoral thesis?
(d) mathematics as the basic language for natural philosophy before Newton wrote his principia ?
etc...
Having settled that, let me enjoy

**
On the other hand, I'll answer any physics questions you have, on string and any other approach to quantum gravity. **

since I have plenty of questions about string theory (here are a few of them):

(a) where/when is it shown that string field theory is renormalizable ?
(b) does string theory reproduce the standard model of particle physics and why (not)?
(c) how dependent is string theory upon supersymmetry ? (one of these social concepts which can survive 30 years without any experimental support whatsoever)
(d) who has *any* clue whatsoever how to do string field theory in the strong field regime *nonperturbatively*?

There are of course the more burlesque questions such as why should matter be the vibration of a string now ? It would be much better if on this forum less people would say how great/unique their favorite approach is and explain more in a calm/straightforward way what the ups/downs are.

Cheers,

Careful

 

[Mike2]

since I have plenty of questions about string theory (here are a few of them):

(a) where/when is it shown that string field theory is renormalizable ?
(b) does string theory reproduce the standard model of particle physics and why (not)?
(c) how dependent is string theory upon supersymmetry ? (one of these social concepts which can survive 30 years without any experimental support whatsoever)
(d) who has *any* clue whatsoever how to do string field theory in the strong field regime *nonperturbatively*?

I think more fundamentally String/M-theory needs to answer how strings or branes exist in the first place. How can it be physical to have fields existing only on submanifolds since this implies a discontinuity in the field as one approaches the submanifold (branes) from a space where the field is zero to the brane where the field goes instantly to some value. Can such discontinuities be physical?

And where does the flat spacetime come from which acts as the background to calculate vibration modes, etc?

 

[Careful]

**I think more fundamentally String/M-theory needs to answer how strings or branes exist in the first place. **

I don't see any problem in taking the most simple proposal for matter and study it's consequences. One needs to assume something in order to make progress.

**
How can it be physical to have fields existing only on submanifolds since this implies a discontinuity in the field as one approaches the submanifold (branes) from a space where the field is zero to the brane where the field goes instantly to some value. Can such discontinuities be physical? **

One can joke that the big invention of string theory is the repacement of a \delta^3 by a \delta^2 and one could imagine that at the most fundamental level, things aren't a three dimensional continuum (string theory has to get rid of embedding space of course - see Dijkgraaff et al.). Again, going over to three dimensional continuum models (such as fast spinning fluids and so on) is much more difficult and you are immediatly left with the burden to explain why matter appears in quantized form to us (not that ordinary QFT explains that, it takes it as an assumption).

But *in the first place*, any reasonable candidate theory has to explain why accepted wisdom is a good approximation to experimental outcome. IMO, it is fine - and even necessary - to overlook certain theoretical desirata as long as this does not conflict common sense.

Careful

 

[George Jones]

No, strings are not fading. This is just the kind of B.S. peddled here as fact.

Maybe string theory is not fading now, but a slow fade is inevitable if sharp evidence for string theory is not found. The fade will be slow, because, as you say, string theorits are firmly entrenched at top universities, and thus are firmly entrenched on hiring committees at top universities.

As f-h says,

Josh1, it's not difficult to see that above and beyond whatever scientific merits it might or might not have, there is a significant sociological inertia to the idea. This is normal but it's the first time in science that such an idea has become so dominant on the theoretical side without experimental evidence.

An interesting exchange on sci.physics.strings occurred about a year ago between "We Pretty" and Lubos Motl. We Pretty posted first. Here is http://groups.google.ca/group/sci.physics.strings/msg/5ee7aff5697efe2f?hl=en&":

> What is the answer to the question:
> Is there an experiment now or in near future such that
> the results according to string theory would differ from
> the results according to current accepted theories such
> that if the predictions of string theory were not observed
> it would imply that string theory is false ?

I think that this is a pretty sharply formulated question, and my opinion
is that the answer to this strong question is unfortunately No, there is
no such known experiment, and string theory is not falsifiable in this
strong sense. If someone thinks that I'm wrong, it will be great to learn
why.

> I know that when the question was asked with relativity the
> answer was a definite yes. The answer was not something like
> "yes, _if_ the speed of light is low enough or if the hyper
> space field is strong enough".

Fair enough. We no longer live in the world where things are as easy as
special relativity. Special relativity is based on 1-10 papers that are
probably still more important than the 15,000 papers of string theory. But
that's how the life goes. The society is also much richer than it was 100
years ago, so it's not shocking that it typically can afford to fund
research where the expected gain per dollar is smaller than from the
salary for the Swiss patent clerk.

> Perhaps a question "What is a non-trivial prediction of a theory"
> could be in FAQ and the answer something that would result yes in
> the previous question.

There are many striking predictions that assume certain assumptions -
assumptions whose validity we're not able to verify with the current
understanding of the theory - but nevertheless these are predictions such
that if they're confirmed, they will go very far to support the theory.

> I'm almost sure that when physicists cannot agree on whether or
> not string theory can make predictions it is not because they do
> not understand the theory but because they do not agree on what
> is a prediction.

I wish the world were as simple as you think. There are many contexts in
which our understanding is so shallow that even string theorists do not
agree whether something is a prediction of string theory or not - they
disagree whether it follows from string theory, regardless of the
terminological detail whether one calls it a "prediction".

> I do realize the difference between the answers to the question
> about the predictions of string theory. I think that string
> theory cannot make non-trivial predictions defined as I did

As I said, I think you're right.

> but I still would be happy to finance the research of string theory with
> my tax dollars if I'd pay my taxes in dollars. I have got an impression
> that string theory is cool and therefore should be researched.

I really think that we should be more honest in saying what can be
expected from some direction of research, and what cannot. String theory
is indeed cool. It's a source of great and often unexpected ideas that
seem to connect virtually all good ideas in high-energy theoretical
physics and many branches of mathematics.

All the best
Lubos

Even if string theory is correct, without experiemntal evidence, it will begin a slow fade. The question is when. For example, suppose the first hard experimental evidence is seen 50 years from now. Physics is a sociological as well as scientific endeavour, and even in this hypothetical "string theory is true" scenario, string theory will fade.

Of course, the LHC could soon find hard and accepted evidence for supersymmetry, in which case string theory will not fade.

I am neither pro string nor anti string, and I had hopes to work my way thoroughly through Zwiebach and Polchinski, but it now appears that this is not going to happen my lifetime. I do not, however, feel that such a programme is a waste of time.

Without further evidence, my feelings a decade from now might be different, though.

Regards,
George

 

[Mike2]

One can joke that the big invention of string theory is the repacement of a $$\delta^3$$ by a $$\delta^2$$ and one could imagine that at the most fundamental level, things aren't a three dimensional continuum (string theory has to get rid of embedding space of course - see Dijkgraaff et al.)

It sounds like the problem still remains with $$\delta^2$$

 

[selfAdjoint]

All of this discussion, pro and con string theory is just the sort of "here is my belief and it's better than your belief" stuff we are used to seeing from newbys on the philosophy subforums.


Let researchers continue to research and let onlookers refrain from making large comments, until we see what LHC has to show us (there might even be some shocks yet from Fermilab!).

And we on PF should continue to be VERY interested in any theoretical demarches that are announced, without nailing our hearts to any of them.

 

[Careful]

It sounds like the problem still remains with $$\delta^2$$

What specific problem are you alluding to here? Could you also provide some explanations?

 

[marcus]

... see what LHC has to show us (there might even be some shocks yet from Fermilab!).
...

not to forget possible shocks from astronomical observations (such as nearterm by the gammaray burst instrument GLAST). Not seeing a slight play in the speed of light at very high energies over astronomical distances would cause real trouble for non-string QG

general agreement with your views on large pronouncements (but urge tolerance for outbursts evoked from any and all sportsfans in the heat of the moment, it's exciting stuff)

 

[Mike2]

What specific problem are you alluding to here? Could you also provide some explanations?

I take it you mean that $$\delta^3$$ refers to the infinity seen while approaching zero distance to point particles from anywhere in 3D. And $$\delta^2$$ refers to the instantaneous change in fields while approaching a surface (of a worldsheet) from anywhere in 3D. If the first poses a problem in physics, I don't see how the problem is cured by the second.

 

[Careful]

I take it you mean that $$\delta^3$$ refers to the infinity seen while approaching zero distance to point particles from anywhere in 3D. And $$\delta^2$$ refers to the instantaneous change in fields while approaching a surface (of a worldsheet) from anywhere in 3D. If the first poses a problem in physics, I don't see how the problem is cured by the second.

Ask that to a string theorist (!), it is not so obvious at all (think about the matrix models where you have this nonexisting double scaling limit).

Cheers,

Careful

 

[f-h]

It sounds like the problem still remains with $$\delta^2$$

Maybe, but the problem is precisely one infinity smaller then with $$\delta^3$$ ;)

 

[josh1]

** What do you think is going on Robert? Just visit the sites of physics departments of the top universities to see what's going on. **

How many people were working on :
(a) discrete processes to ``explain'' the photo electric effect when Einstein came up with this ?
(b) relational approaches to physics when GR was born in 1915 ?
(c) the renormalizability of Yang Mills when 't Hooft proved this in his doctoral thesis?
(d) mathematics as the basic language for natural philosophy before Newton wrote his principia ?
etc...

Uhm, although I'm unsure about their appropriateness or accuracy, I'm guessing that with these historical remarks, you're making the point that judging the validity of research on the basis of whether or not it's mainstream is not a good idea. Yes, that would be stupid of me if I we're judging this way, but really, I was just trying to warn Robert that some of the members here try to convince people that string theory is no longer mainstream and other approaches are somehow knipping at it's heels. This isn't the case. So if someone does want to know truly what is going on, go visit the sites of some of the major research centers. That's all I was saying. However, LQG is certainly not mainstream compared to strings, and in this case at least, the reason is that people know that LQG is almost certainly unphysical.

Having settled that, let me enjoy

I guess we’ll see.

where/when is it shown that string field theory is renormalizable?

I think it's time for you to leave the 70's and 80's behind and embrace the brave new world of effective field theories where theories that we're viewed as renormalizable in the original sense are now correctly viewed as including only the lowest order terms of a lagrangian that will in general contain infinitely many higher order terms, these just being suppressed at higher energies. If for each term there is a counterterm, then effective field theories are as renormalizable as one's that are renormalizable in the original sense.

We can understand why LQG is wrong from the effective field theory point of view in that the einstein-hilbert action is just the lowest order terms in an effective field theory. Yet the assumption of LQG is that these are the only terms, and this is impossible to reconcile (at least in any natural way) with the other interactions. It just makes no sense to expect the einstein-hilbert action will remain a valid basis for quantization of gravity all the way up to Planck energies.

who has *any* clue whatsoever how to do string field theory in the strong field regime *nonperturbatively*?

There is sometimes a bit of confusion about what the significance of the term “nonperturbative” is when used in the same sentence as the term “string field theory”. String theory is formulated as a first quantized theory, these being inherently perturbative in form. On the other hand, quantum field theories are second quantized and are framed in a nonperturbative way - in the form of a lagrangian of field operators. So with string field theory, we’re looking for an inherently nonperturbative form of string theory, but we’ve not yet succeeded.

does string theory reproduce the standard model of particle physics and why (not)?

As is well known, we've been unable to produce the standard model using string theory. But this is very far from knowing that it's impossible to do so. For example, there are various compactifications of the weakly coupled E₈xE₈ heterotic theory that produce a picture too much like the one obtained from the standard model to be dismissed as insignificant in a theory as vast and poorly understood as strings. However, generating any kind of predictions from this particular model requires we either discover how to stably break supersymmetry at weak coupling, or pursue the strongly coupled theory. But at present there seems to be so many possible ways of handling the first problem that none of them seem natural. The latter on the other hand currently seems intractable.

how dependent is string theory upon supersymmetry ? (one of these social concepts which can survive 30 years without any experimental support whatsoever)

In fact - and this is commonly not appreciated - string theory and supersymmetry are completely independent ideas. It's just that the nonsupersymmetric part of moduli space is far less tractable than the supersymmetric one.

why should matter be the vibration of a string now ?

 

[Careful]

** I was just trying to warn Robert that some of the members here try to convince people that string theory is no longer mainstream and other approaches are somehow knipping at it's heels. This isn't the case. So if someone does want to know truly what is going on, go visit the sites of some of the major research centers. That's all I was saying. However, LQG is certainly not mainstream compared to strings, and in this case at least, the reason is that people know that LQG is almost certainly unphysical. **

Right, but if that is all what you want to say, why make such a fuzz ? Bottom line is that (wrong) public opinion is not going to change anything to how research is done. Personally, I don't see any value in a discussion over which dog got the most juicy bone, or whether some nasty bugs are slowly consuming the latter.


**I think it's time for you to leave the 70's and 80's behind and embrace the brave new world of effective field theories where theories that we're viewed as renormalizable in the original sense are now correctly viewed as including only the lowest order terms of a lagrangian that will in general contain infinitely many higher order terms, these just being suppressed at higher energies. **

Bad start , first don't tell me how modern I should be (we are both intelligent people and make our own conscious well motivated choices), second my definition of renormalizability is the modern one (which you can find on Baez' webpage).

**
If for each term there is a counterterm, then effective field theories are as renormalizable as one's that are renormalizable in the original sense. **

So, here you tried to avoid my question about renormalizability of string field theory.

**
We can understand why LQG is wrong from the effective field theory point of view in that the einstein-hilbert action is just the lowest order terms in an effective field theory. Yet the assumption of LQG is that these are the only terms, and this is impossible to reconcile (at least in any natural way) with the other interactions. It just makes no sense to expect the einstein-hilbert action will remain a valid basis for quantization of gravity all the way up to Planck energies. **

That is not known, it *might* be that the renormalization flow around a nontrivial background (say de Sitter), has a non gaussian UV fix point (I never checked myself the work of Reuter though). Another possibility is that quantum mechanics is wrong at these gigantic energies. EDIT : another possibility is to quantize using a different state (eg. the Ashtekhar Lewandowski state).

**
There is sometimes a bit of confusion about what the significance of the term “nonperturbative” is when used in the same sentence as the term “string field theory”. **

Nonperturbative simply means that if you take a (quantum) ``field´´ theory (a theory with an infinite number of particles), split the lagrangian in a free and interacting part and treat the interactions as perturbations in your expression of interest (a state sum in statistical physics, vacuum expectation values in QFT), then either (a) the perturbation series converges (b) the series is the formal expression of a well defined, known, (non analytic) function.

**
String theory is formulated as a first quantized theory, these being inherently perturbative in form. So with string field theory, we’re looking for an inherently nonperturbative form of string theory, but we’ve not yet succeeded. **

Well, I am not a string theorist, but people have definately studied the subject vis a vis its correspondance to 1+1 dimensional (topological) quantum gravity with topology change (without succes).

**
In fact - and this is commonly not appreciated - string theory and supersymmetry are completely independent ideas. It's just that the nonsupersymmetric part of moduli space is far less tractable than the supersymmetric one. **

Right, but you have to admit that analytic calculations get pretty fast hopeless once you abandon supersymmetry.

**

**

hehe, did it become difficult to imagine otherwise ?

 

[john baez]

there's more to life than the einstein-hilbert action

We can understand why LQG is wrong from the effective field theory point of view in that the einstein-hilbert action is just the lowest order terms in an effective field theory. Yet the assumption of LQG is that these are the only terms, and this is impossible to reconcile (at least in any natural way) with the other interactions.

This is not an assumption of loop quantum gravity. It may have been in the early days, when some people were a bit naive, but it sure isn't now. You can take my word for this - I've discussed this issue with Ashtekar, Rovelli, and Smolin, and none of them believes this assumption you are crediting them with. Or, you can read papers on the subject. For example, already in the simple case of 3d quantum gravity, loop quantum gravity gives corrections to the Einstein-Hilbert Lagrangian:

Etera R. Livine, Simone Speziale, Joshua L. Willis
http://arxiv.org/abs/gr-qc/0605123"

Some clarification is in order - I don't like the title of this paper, since 3d gravity doesn't have gravitons. What they're really talking about is the calculation of 2-point functions in 3d gravity, which they are doing in parallel with Rovelli's work on 2-point functions in 4d gravity. (Speziale is a student of Rovelli; he's working here at the Perimeter Institute with Livine.) And, they can compute the correction terms to these 2-point functions:

We compute explicitly the next to leading and next to next to leading orders, corresponding to one-loop and two-loop corrections. We show that while the first arises entirely from the expansion of the Regge action around the flat background, the latter receives contributions from the microscopic, non Regge-like, quantum geometry. Surprisingly, this new contribution reduces the magnitude of the next to next to leading order. It thus appears that the spinfoam formalism is likely to substantially modify the conventional perturbative expansion at higher orders.​

The Regge action is the discretized version of the Einstein-Hilbert action, but here they are seeing corrections - check out the nice graphs!

Rovelli is trying to do the same sort of calculation in 4d gravity, where it's much harder:

http://arxiv.org/abs/gr-qc/0508124"

If you read this paper you'll see I've been hassling Rovelli about the details of this computation.

 

[Careful]

**This is not an assumption of loop quantum gravity. It may have been in the early days, when some people were a bit naive, but it sure isn't now. Some clarification is in order - I don't like the title of this paper, since 3d gravity doesn't have gravitons. What they're really talking about is the calculation of 2-point functions in 3d gravity, which they are doing in parallel with Rovelli's work on 2-point functions in 4d gravity ... Surprisingly, this new contribution reduces the magnitude of the next to next to leading order. It thus appears that the spinfoam formalism is likely to substantially modify the conventional perturbative expansion at higher orders. The Regge action is the discretized version of the Einstein-Hilbert action, but here they are seeing corrections - check out the nice graphs! **

Ok, but in a clear sense, this is still quantization using Einstein Hilbert alone (you simply take a different vacuum state). This toy model, is it causal dynamical triangulations ? Does one take a continuum limit ? Is a cosmological constant necessary to ensure nonperturbative renormalizability or is that not studied ? So, I guess what you say is that they have a toy theory using regge calculus, work with fixed embeddings in 2+1 Minkowski and expand the value of the path integral weights in the gravitational constant G; moreover the result corresponds (in perturbation theory) to a second quantized field theory (with respect to the Fock state) with additional terms in the action. If so, then this is not surprising at all (since you have different states anyway).

As you know mr. Ricci is very well behaved in conventional expansion, it is however mrs. Weyl (in 4-d of course) who is troublesome (and poorly understood as all woman are). In the same paper you are referring to, did the authors compute expectation values of mr. Ricci nonperturbatively (and if yes, were they finite?)?

Cheers,

Careful

 

[marcus]

Etera R. Livine, Simone Speziale, Joshua L. Willis
http://arxiv.org/abs/gr-qc/0605123"

Some clarification is in order - I don't like the title of this paper, since 3d gravity doesn't have gravitons. What they're really talking about is the calculation of 2-point functions in 3d gravity, which they are doing in parallel with Rovelli's work on 2-point functions in 4d gravity. (Speziale is a student of Rovelli; he's working here at the Perimeter Institute with Livine.) And, they can compute the correction terms to these 2-point functions...

must say I like the Livine Speziale Willis paper, and was reading it yesterday with some interest. I was pleased to see it mentioned by you today---along with some helpful insight.

compliments to them on this paper.

======

Etera Livine is 25. What will quantum gravity look like when he's 50?

 

[Careful]

must say I like the Livine Speziale Willis paper, and was reading it yesterday with some interest. I was pleased to see it mentioned by you today---along with some helpful insight.

compliments to them on this paper.

======

Etera Livine is 25. What will quantum gravity look like when he's 50?

Since you have read the paper, you could also anwer my questions ?

 

[marcus]

Since you have read the paper, you could also answer my questions ?

I doubt it I can try, but the most satisfactory result would be from your giving full attention to the paper itself.

You ask:
This toy model, is it causal dynamical triangulations?

I don't think so. AFAICS it does not resemble what Loll does in CDT at all. Maybe you have looked deeper and seen some connecton (and there surely must be some connection). But in a straightforward sense it is not CDT.

I think it is "toy" only because it is 3D rather than 4D. Otherwise it is not at all a toy model. I think that word in the title is actually a little confusing and that is more that they are studying the 3D case as a "warmup", because similar techniques might in some cases carry over.

I hope to have time to read some more in the paper today, maybe I can respond to more of what you said.

============
BTW I have never heard from Loop gravitists that they expect just the simple E-H action with no quantum corrections. This issue may be actually a non-issue, like someone started a rumor that they beat their wives.
So from time to time in conversation someone who doesn't know the QG very well is asking "And do you in fact not beat your wife? When did you stop? Show me how you don't beat your wife?"

So reference to this recent paper may not be needed to prove a point, it is just in this context an example of non-string QG people going about their normal business----and, for example, calculating quantum corrections.

As I read your post, it suggested to me that you are mainly concerned with this E-H issue (or non-issue). If you are mainly interested in that, then perhaps there is no need to look at this particular paper in detail! AFAIK there never has been reason to expect only pure E-H with no quantum corrections, or at least the Loop people have not seen any reason to expect such a limitation.

I should try to find some kind of recent standard introduction to LQG. Something like the 25 Smolin lectures, but in the form of book or monograph rather than in video.

Watching QG developed in these lectures gives some idea of what latitude they have with the action, but that doesn't do you any good, I think, because I can't give you a spot page reference to a precise place where Smolin says "and also we do not beat our wives."

 

[f-h]

We can understand why LQG is wrong from the effective field theory point of view in that the einstein-hilbert action is just the lowest order terms in an effective field theory. Yet the assumption of LQG is that these are the only terms, and this is impossible to reconcile (at least in any natural way) with the other interactions. It just makes no sense to expect the einstein-hilbert action will remain a valid basis for quantization of gravity all the way up to Planck energies.

As apparently this is not getting through, I will repeat what has been said hundreds of times elsewhere: The Einstein Hilbert action is not crucial to LQG. LQG is an extension of the concept of a QFT which includes ordinary QFT in the appropriate limit (at least in 3D). Central is the implementation of (so far unfortunately only part of) the diffeomorphism group as a full quantum symmetry.
For this kind of Qunatum Field Theories LQG is unique by the LOST theorem.
Dynamics are so far a difficult and unkonw field, it is however expected that when one does retrievce an effective field theory expansion of LQG one will find contributions from all terms compatible with the symmetries in the effective action.

One of the big reasons Supersymmetry is considered physically viable is that it is one of the few nontrivial modifications/extensions of Poincare symmetry possible. Another nontrivial modification possible is q-deformation. In LQG generalized QFT q-deformed Poincare groups do appear as symmetry of the effective field theory.
Moreover this is a symmetry with a natural scale, that is, it doesn't need to be broken like supersymmetry.

I certainly resent the comment that it's understood that LQG is unphysical *as opposed to strings*.
The simple matter of fact is that both seem extremely unphysical to me, but LQG with the better potential to get somewhere without experiment because it's proponents are more conceptionaly minded.

Edit:
My bad for answering before reading the replies already saying the same thing, sorry ;)

 

[marcus]

My bad for answering before reading the replies already saying the same thing, sorry ;)

not sorry. I like your response better than mine anyway (and I was just picking up on this new paper by Livine et al, which isn't essential to the discussion, and what Careful said)

 

[wolram]

What is so laking in all this bull baiting is tests, i think it is all a load of cods wollop, string, lqg, is just so much hot air, and is going no where until some
one puts up or shuts up, a test in this centuary please.

 

[Careful]

not sorry. I like your response better than mine anyway (and I was just picking up on this new paper by Livine et al, which isn't essential to the discussion, and what Careful said)

I just gave a quick glance at the paper, the authors use some version of the Pozano Regge model which is definately different from causal dynamical triangulations (they use a fixed reference edge length). How they choose their reference background state is not so clear however (at first sight, one does not make a clear link to the usual Gaussian state). Anyway, perhaps f-h can comment more on this (he must have enjoyed a full discussion about this previously) - sorry I have no time to decently read all papers so sometimes one just has to learn by asking questions.

Cheers,

Careful

 

[marcus]

What is so laking in all this bull baiting is tests, i think it is all a load of cods wollop, string, lqg, is just so much hot air, and is going no where until some
one puts up or shuts up, a test in this centuary please.

thank you, wolram.
It is always good to be reminded that what we are talking about is empirical science---where theory is useless unless it can manage to make falsifiable predictions

but the O.P. here (Robert100) was not asking about that as much as about "is superstring/M the only body of theory being developed that takes on the unification issues?"

I was interested by the O.P. because, if I remember right, Robert100 says he is a high school science teacher. We haven't seen signs of him lately making me wonder if he and his interest is for real.

The goal of unification can be pictured in various ways----different research lines will want to picture it differently----and that is interesting in itself.

Carlo Rovelli in the preface to his book "Quantum Gravity" states the goal this way:
to finally get to a GENERAL RELATIVISTIC QUANTUM PHYSICS
(that is a comprehensive quantum physics including what is now the standard particle model but which obeys 1915 GR instead of merely 1905 SR)

I think the answer to the O.P. question is clearly no. In other words, SST is NOT the only reasearch line being pursued that could lead to a unified picture of QM and GR.

Indeed it's hard for me to see how SST offers to do that because of the difficulty it has realizing essential features of General Relativity---for a recent (May 2006) discussion see Washington Taylor String Field Theory http://arxiv.org/hep-th/0605202 , prepared for Oriti's book. But opinions vary and, as in any field, depend somewhat on hopes for future results.

I appreciate your impatience for experimental tests and expect that they will be coming along presently----as early as 2007 or 2008 for GLAST.
You may or may not be pleased with the results. GLAST may actually shoot down some versions of non-string QG that predict a slight increase in the speed of light at very high photon energy. Then, with our hopes blasted (or "GLASTed") some of us will feel just as discontented as before. but it's how progress is made.

 

[selfAdjoint]

I was interested by the O.P. because, if I remember right, Robert100 says he is a high school science teacher. We haven't seen signs of him lately making me wonder if he and his interest is for real.

I'd be lying low too if I posted a simple question and got back a load of quasi-political posturing. You and John Baez gave straight answers within your rescript, but it seems to me that everyone else, and I do not exclude myself, was in a pi**ing contest.

The answer John100, is nobody knows, and it may be way too soon to even ask the question. Blame the physicists of the late 20th century for giving people the false impression that a TOE was right around the corner. And blame us for running a stupid election campaign instead of saying so.

 

[john baez]

Ok, but in a clear sense, this is still quantization using Einstein Hilbert alone (you simply take a different vacuum state).

Hmm.

This toy model, is it causal dynamical triangulations?

No, I said it was a spin foam model. You can see causal dynamical triangulations as a funny kind of spin foam model if you like, but this is not that. It's the Ponzano-Regge model of 3d gravity, the grand-daddy of all spin foam models.

Does one take a continuum limit?

No need - 3d gravity is topological, so a fixed triangulation gives exact answers, and refining the triangulation doesn't change them.

Is a cosmological constant necessary to ensure nonperturbative renormalizability or is that not studied ?

A cosmological constant is not needed, though you can put it in if you like. They consider the case where the cosmological constant is zero, because it's simpler.

So, I guess what you say is that they have a toy theory using regge calculus, ...

They use the Ponzano-Regge model, where amplitude for a 3-simplex is given by the 6j symbols. These are asymptotically given by the cosine of the Regge action, so there's a relation to the Regge calculus, but it's not the same thing.

work with fixed embeddings in 2+1 Minkowski ...

No embedding in Minkowski spacetime.

Sorry to be negative-sounding. Try looking at the paper - I gave a link so people could easily check it out. There's no need to guess what's going on.

 

[CarlB]

As far as I have been able to determine, no other ToE's are being worked on.

What will finally bury string theory is not another complicated theory that no one can compute anything with. String theory can fail to compute results right up there with the most airy theory anyone can imagine.

What will destroy the various complicated ways of explaining elementary particles is their failure to produce the elementary relationships between the very carefully measured arbitrary parameters of the standard model.

In addition to the various coincidences in the mixing angles, my favorite is the Koide relationship among the masses of the charged leptons, which, back in 1982, predicted the tau mass to within a fraction of a standard deviation for the 2006 data:

http://www.arxiv.org/abs/hep-ph/0505220

Now the thing that should be noted in the above is that the Koide relationship is for the measured masses. It is not that the bare masses are simple, but instead the measured masses. This suggests that our understanding of mass is simply wrong. And consequently, Koide's paper has received relatively little attention, with the assumption that it was just a lucky coincidence.

But if you are hoping for a simple structure behind the elementary particles, this is the place to start looking. And more recent work has extended the above coincidence to yet another 6-digit coincidence in the lepton masses, and relations that apply to the neutrinos:
http://www.arxiv.org/abs/hep-ph/0605074
http://www.brannenworks.com/MASSES2.pdf

The analogy with classical mechanics is the remarkable coincidences among the wavelengths of the light emitted by the hydrogen atom. Sure a lot of people were working on explaining it with classical mechanics, but that wasn't the solution.

Carl

 

[CarlB]

I would be really surprised if superstrings was the only ToE being worked on, yet I also get the idea that Woit doesn't have any actual alternatives, at least not that are developed and studied by peer reviw.

There are plenty of people applying Clifford algebra to the elementary particles. And this paper applies Clifford algebra to gravitation:
http://www.arxiv.org/abs/gr-qc/0405033

Now if you read the above, you will find that they have repackaged GR into a form where it is both easier to use and has a simpler ontology. This is we should expect in a new foundation for physics, not by piling renormalizations on renormalizations like bad lies told by a drunken spouse.

The big problem with the foundations of physics is that they are all very stiffly coupled one to another. Among the believers, this is evidence for robustness, but any engineer can tell you that it is a recipe for fragility. If any part of the foundations are wrong, then many parts of the foundations are wrong, and the effort to rewrite physics to a new foundation will be extensive and difficult.

On the other hand, there is plenty of evidence that there are alternatives to the foundations of physics. For example, see the "Euclidean relativity" links here:
http://www.euclideanrelativity.com/links.htm

Carl

By the way, the ability of "peer reviw" to pass judgement upon revolutionary physics theories is, well, not exactly a proven ability.

 

[Sauron]

I just wante answer the first post of josh classifilying strings as "hard" and LQG as easy.

Why do you think so? In the early times string physics could make a diference betwen particle physicians who didn´t know modern math and tehmselves.

But LQG people have the same degree of math sosfistication than string people. In fact i would say they are better mathematicians, or at least much more rigurous ones.

Ah, I found timeto read the Lubös objections to the graviton article of Rovelli et all. Interesting, but i find a mayour drawback in it´s critics.

It comes from his cliams about the meaning of difeomorphism invariance. He,i think, is using the old fashioned one that appears in the seventies articles oin perturbative quantum gravity, resembling gauge invariance. And in LQG is used a diferent meaning of difeomorfismin invariance. I believe that it would be interesting to state with precision how these diferent aproachs to the same idea are compatible or not.

Other thing, people seems to be forgeting that unification as currently stated depnds in the fact that coupling onstants value agree at a certain energy level. And that is ultimately an experimentalquestion which can be truth or not, and we still don´t know for sure.