Why I am REALLY disappointed about string theory

[mitchell porter]

There's a great little introduction to M-branes on the arxiv today http://arxiv.org/abs/1012.0459" .

There are two main perspectives we can take on extended objects such as the membrane and fivebrane. We can look at them as solutions of 11-dimensional supergravity (these solutions will also have near horizon limits) and look at the field theories on their worldvolumes. This is at the heart of the AdS/CFT correspondence. The degrees of freedom on the worldvolume are goldstone modes from broken symmetries, including supersymmetries. Requiring that the Bosonic and Fermionic degrees of freedom match to give a supersymmetric worldvolume theory puts very strong constrains on the allowed extended objects, importantly the maximal dimension this can occur in is 11. Here the 8 scalars from broken translations in the directions transverse to the brane match with 8 Fermions from the broken supersymmetry. A fivebrane thus has only 5 scalars but will still have 8 Fermions if it preserves half the supersymmetry. The three additional Bosonic degrees of freedom come from broken gauge symmetries of the three-form C. This leads to a 2-form with anti-self-dual field strength on the fivebrane worldvolume. This makes the fivebrane worldvolume theory difficult to formulate. (Chapter 3)

 

[mitchell porter]

Another addition to this old discussion about the fundamental formulation of string theory:

I have been studying the AdS/CFT correspondence, and I am only now realizing how central it is to answering that question! d=4 N=4 super-Yang-Mills theory is believed to be exactly equivalent to Type IIB string theory on AdS₅ x S⁵. The fifth, "AdS" dimension; the five further compact dimensions; string states and brane states in this ten-dimensional space - they're all entirely constructible from operators in the four-dimensional theory, which lives on the boundary of the AdS space. So if you want to understand Type IIB string theory - at least on an AdS background, which I admit is an unusual space - study this four-dimensional super-gauge theory!

As for M theory, the three-dimensional http://arxiv.org/abs/0806.1218" is a recent review of one aspect of ABJM (when it talks about Type IIA on AdS₄ x CP³ as the dual theory, that's the same as the M-theory description, but with the eleventh dimension removed from the S⁷ so it becomes CP³), and it makes it clear how much struggle and hard work is involved in extracting each increment of additional insight from these theories, even though we can write down the whole equation.

 

[Calrid]

I personally have nothing against string theory even though of course technically it is not a scientific theory or even a very good hypothesis yet as it makes no measurable predictions.

The only thing that disappoints me is the fact that it seems to be very esoteric so only String Theorists can understand it, and hence it seems to be defended with a sort of religious zealotry, it is not as open as one would like, perhaps that is something that will change with evidence when theory meets experiment. Would be fine if it was dark/energy matter theory, that has some sort of evidence although nothing concrete. But I fear strings is putting the cart before the horse, and M-theory the car before the horse, and I don't think that is good science if it remains untestable, for well the foreseeable future at least.

As long as Scientists understand where philosophy ends and science begins though then I am fine with the whole thing. It should really be in maths departments if it is not applied (and again it can be if not to a ToE or gravitation as yet). I think mostly it is good science - if it tackles applications to science such as quantum chromodynamics issues at least - in a model and or environmental predictions or materials science.

It's a promising idea but as yet that is all it is, the disappointment probably comes from expecting too much from too little if you see what I mean.

Anyway this is mostly a popcorn post. I am by no means an expert on this subject. I just find the "theory" interesting.

 

[tom.stoer]

@mitchell porter: thanks for these interesting ideas. It could very well be that this is a direction where progress can and should be made. What you are saying - better: what I understand from it - would mean that:
- there may be a full, non-perturbative formulation on a certain class of backgrounds
- there may be a duality between strings and SUSY-gauge-theories
- there is still no selection principle for this special background (or class of backgrounds)

If this is correct, then the following questions apply:
- why a certain class of backgrounds / topological sector? unfair questions: nobody asks this for QED or GR
- why strings and not only SUSY-gauge-theory?

Perhaps string theory is only a different perspective to look at the same thing

 

[Physics Monkey]

The only thing that disappoints me is the fact that it seems to be very esoteric so only String Theorists can understand it, and hence it seems to be defended with a sort of religious zealotry, it is not as open as one would like, perhaps that is something that will change with evidence when theory meets experiment.

Have you ever tried looking at Barton Zwiebach's book "A First Course in String Theory"? Granted, this book is a lot about the classical mechanics of relativistic strings and membranes, but it really does give some nice intuition for those subjects. One can learn a surprising amount about certain solutions of string theory just using classical mechanics (SUSY helps make this true).

And speaking personally, I've found that string theorists are actually among the friendliest and most open groups in physics. The people I know are generally a pretty laid back group, far from zealots, and I suspect its partially because a lack of direct experimental contact forced them to adopt a more conciliatory stance. Of course, my experience could be limited.

 

[Calrid]

Have you ever tried looking at Barton Zwiebach's book "A First Course in String Theory"? Granted, this book is a lot about the classical mechanics of relativistic strings and membranes, but it really does give some nice intuition for those subjects. One can learn a surprising amount about certain solutions of string theory just using classical mechanics (SUSY helps make this true).

And speaking personally, I've found that string theorists are actually among the friendliest and most open groups in physics. The people I know are generally a pretty laid back group, far from zealots, and I suspect its partially because a lack of direct experimental contact forced them to adopt a more conciliatory stance. Of course, my experience could be limited.

I didn't mean to suggest they were ornery cusses or mavericks. Just that you have to be very well educated to really contest anything here. Smolin and Woight do a good job, and I sometimes wonder if they are not doing better science than those who are on board. At least atm. I just meant that as far as peer review goes this is often only open to those who are already invested in the science, unlike say materials science or QED issues where anyone with a good grounding can play.

Even Kaku has said he is disappointed at where string theory is now given the promise it held. As long as this is taken as constructive criticism there's no need to really defend it with so much vigour. Just go back and do what all good Scientists do, prove it. Easier said than done but tenacity may pay off. Let the cards fall where they may. Rome wasn't built in a day. I just warn strongly against house of cards theories, built on foundations that are not strong.

And yeah I really wish I had the time to study this even at a basic level. Although I do understand the basics already to some extent. But us undergrads have to focus on the now sometimes, there is a lot to learn already.

 

[atyy]

Smolin and Woight do a good job, and I sometimes wonder if they are not doing better science than those who are on board.

Smolin: it seems that any acceptable quantum theory of gravity, whatever its ultimate formulation, is likely to reduce to a perturbative string theory in the appropriate limit. http://arxiv.org/abs/gr-qc/9508064

Basically, strings are amazing because:

1) They contain gravity
2) It uncovered that gravity in some universes can be described by a good-old quantum field theory!
3) It uncovered a link between quantum field theory and Penrose's twistors!

We do not know yet if strings can describe our universe, but the hope is that by studying the clues above, we will be led to a theory that does.

 

[marcus]

For a frank but sympathetic view of the String program in broad context of LHC particle physics and cosmology here is Steven Weinberg's talk to the national Science Writer's conference at Austin TX in October 2009:
http://blogs.physicstoday.org/newspicks/2009/10/higgs-discovery-likely-at-lhc.html

Drag the time button to around minute 48. The comment on String (set in broader context of physics today and also in historical context) begins at minute 50, after a question by science writer Tom Siegfried.

Here's what looks like a more watchable copy on YouTube:


Great guy! The most impressive elder statesman physicist I know of---excellent speaker.

I think the whole 55 minutes is well worth watching. He does a great job of surveying both ground-based particle physics and cosmology, plus gives evidence for dark matter and says what he thinks will turn out to be the dark matter particle and what characteristics we can infer or guess about it. Then shortly after minute 48 he gets into discussing String (which he worked on in the 1980s and on which he has a combined insider/outsider perspective).

Sometimes it's slow about letting you drag the time button. You drag it and then may have to wait a few seconds. A red stripe will appear to the right of the button. You may then have to nudge the button slightly to the right to get it in the red stripe in order to get it to start playing.

 

[Physics Monkey]

Anybody there to summarize from a string theory perspective what the essential conclusions are?

Although I didn't participate in the bulk of this discussion, I can attempt to summarize very very briefly the successes, failures, and prospects of string theory as I see them. When I refer to strings I generically mean the whole apparatus of stringy associated topics.

1. Quantum Gravity (Theory)
-Strings provide a perturbative theory of quantum gravity around many backgrounds. Thus there is some non-trivial answer possible when asking to unify quantum mechanics and gravity.
-Strings provide answers to non-perturbative questions such as black hole microstate counting.
-Strings allow topology change, showed gravity could emerge from simple models (e.g. matrix models), and many other non-trivial insights into the nature of "quantum geometry".
-Universality, by which I mean the emprical fact that consistent low energy theories containing gravity are often realizable in string theory e.g. the recent landscape work in 6d theories.
-Trouble (as well as inspiration) from positive cosmological constant, although this is bound up with the landscape issues mentioned below.

2. Gauge Theory
-Strings led or inspired a number of non-perturbative advances in gauge theory, especially susy gauge theory. To the extent that susy gauge theories contain phenomena that appear generically in other theories, the powerful string-inspired insights into gauge dynamics have been very useful theoretically.
-Gauge/gravity duality is one obvious example.
-New perturbative methods in gauge theories is a more recent example.
-Dualities in gauge theories.

3. Landscape Issues
-Strings really raised awareness of the possibility of many solutions with no obvious selection criteria. Ironically, this is exactly the opposite of what is promised from strings in many pop sci books.
-The landscape is real in at least one context, namely holographic duality, where parts of the string landscape are dual to parts of the landscape of non-gravitational many body systems. The latter landscape is very well established.

4. Experiment
-Strings have not made direct contact with experiment.
-There have been many inspirations arising in part from string theory e.g. string cosmology, ideas in heavy ion experiments, etc.
-I personally think its unlikely we will directly test the predictions of string theory as a simple theory of quantum gravity any time soon. Data from low energy physics just isn't constraining enough, it never is. There are outside chances from cosmology, etc. but I just don't see it happening. I think the landscape is real and hence we will likely not be able to isolate our vacuum from the many solutions of string theory.
-I think it is much more likely that strings will make indirect experimental contact through holographic duality. Given 30 more years development, I could easily imagine doing indirect (dual) table top experiments on quantum gravity. Or vice versa, that we can use strings to say something about a real physical system without gravity.

5. Prospects
-Strings have been intertwined with important advances in quantum field theory and gravity for decades. This won't go away anytime soon, although the rhythm of string research waxes and wanes over time.
-It's not just a matter of strings being "falsifiable". Even if our world were conclusively not stringy (something I can almost guarantee will never be shown), string theory would still be useful and interesting if only for holographic duality. Indeed, it might even still be experimentally relevant, although for a purpose not originally envisioned.
-String theory embodies a collection of mathematical and physics insights about gravity, quantum field theory, and much more, and so it will never just go away. I think of it now as more of a framework for different theories rather than a single theory. It's like a beautiful super-structure into which much else fits in often surprising ways.

My wife recently read Brian Greene's first book, but she didn't like it and I have to agree with her criticism. The book makes many promises in the name of strings, predicting particle masses, being the single grand theory of gravity, etc., and none of these promises are fulfilled in the book. Indeed, string theory has still not fulfilled these promises. In fact, I would be comfortable betting that string theory will never predict the electron's mass. On the other hand, I think string theory has given us much we didn't expect, something far more interesting. And since our job in physics is not just to crank out numbers for the latest random experiment being done, but to understand the deep structure of nature, I think string theory will continue to play a role in our search for that structure.

 

[suprised]

I thought this was the thread were you'd agreed to hold yourself back, marcus, in order that others can have a scientific discussion?

 

[fzero]

For a frank but sympathetic view of the String program in broad context of LHC particle physics and cosmology here is Steven Weinberg's talk to the national Science Writer's conference at Austin TX in October 2009:
http://blogs.physicstoday.org/newspicks/2009/10/higgs-discovery-likely-at-lhc.html .

Since marcus feels that Weinberg's statements are important, I thought I'd take a couple of minutes to transcribe them for the record. I won't offer an opinion on the substance, but might join the discussion at a later point if there's some concrete issue that I can help shed light on.

It's developed mathematically, but not to the point where there is anyone theory, or to the point where, if we had one theory, we would know how to do calculations to predict things like the mass of the electron or the masses of the quarks. So, I would say, although there has been theoretical progress it's been….I find it disappointing.

One of the hopes would be that the LHC would provide a clue to something we're missing in superstring theory and I think there supersymmetry is the most likely place to look. One of the troubles with superstring theory is that, although in a sense the theorists think there's only one theory, there are an infinite number of approximate solutions of it and we don't know which one corresponds to our world. But at least in a large variety of the solutions of superstring theory there is supersymmetry visible at low energies. And if we see supersymmetry at low energies, the superstring theorists may be able to derive from it some type of clue as to how to try to solve these theories.

But I haven't talked about it in this lecture because I don't see how that would work. But I couldn't say that that was likely with any sincerity and certainly the LHC and any other accelerator that we can imagine being built would not get up to energies which are high enough so that we can directly see the structures that are described by superstring theory: the strings or the D-branes or whatever it is. Those will not be accessible at the LHC, so any clue that we get will be very indirect.

I have myself, well I was working on superstring theory in the 80s and gave it up because I moved into cosmology, which, in the last couple of decades, had the excitement that elementary particle physics had in the 60s and 70s: A wonderful coming together of theory and observation. Cosmology now reminds me of the excitement that I felt when I was younger and doing particle physics.

It's a pity that superstring hasn't developed better. I still think it's the best hope we have. I don't know of anything else. My own work very recently has been trying to develop an alternative to superstring theory as a way of making sense of quantum gravity at high energies. But even though I'm working on this, I still find superstring theory more attractive, but not attractive enough.

 

[Haelfix]

I just learned from one of my students, that Susskind has made available a semester long string theory course on Itunes and that it is accessible and made for laymen.

I watched a few from a previous semester dealing with Stat mech, and they were excellent (Lenny is a great teacher), so highly recommended.

One of my eternal frustrations with elements on this board is that people want to delve into and discuss complicated research programs in quantum gravity, without having a firm (or even partial) grasp of the very basics like general relativity, quantum field theory, much less technically difficult but required knowledge like QFT on curved spacetime and eg the Wheeler-De Witt equation in cosmology.

So take the hint, go to itunes and keyword susskind and/or Stanford University.. Even better, start from the beginning and follow the progression (GR->quantum mechanics --> field theory --> string theory), he keeps the mathematics to a minimum.

 

[marcus]

Fzero, thanks so much for transcribing that!
I think it is an especially interesting statement in part because of Weinberg's stature as a physicist but maybe even more for what I feel is the balance, objectivity, frankness, sympathy. He has always been a friend to String researchers and clearly wants to stay friends with them. He avoids any feeling of hostility, he remains kind. This and other Weinberg videos have made me admire him as a person.

I think it's great to have the sample you transcribed, but I would also urge anyone here who has a free hour to watch the whole talk from beginning to end. He looks at physics steadily, from a lifetime of experience, and sees it whole, and speaks with a certain frank honesty.

 

[fzero]

Fzero, thanks so much for transcribing that!

I actually found an alternate transcription a few minutes ago at:

http://blogs.physicstoday.org/newspicks/2009/10/higgs-discovery-likely-at-lhc.html

So I probably could have saved some time if I'd searched for that sooner.

I think it is an especially interesting statement in part because of Weinberg's stature as a physicist but maybe even more for what I feel is the balance, objectivity, frankness, sympathy. He has always been a friend to String researchers and clearly wants to stay friends with them. He avoids any feeling of hostility, he remains kind. This and other Weinberg videos have made me admire him as a person.

I don't believe that there is any hostility to begin with. Weinberg certainly has all of the qualities you describe, but he's not someone that is going to pull punches when confronted with bad science. He's well beyond the point in his career where he has to worry about friends or funding. He continues as PI on the NSF grants that fund string theory research at Texas and therefore bases his integrity on a belief that there's a value in pursuing that research.

 

[marcus]

bases his integrity on a belief that there's a value in pursuing that research.

Quite so.

 

[marcus]

I actually found an alternate transcription a few minutes ago at:

http://blogs.physicstoday.org/newspicks/2009/10/higgs-discovery-likely-at-lhc.html

So I probably could have saved some time if I'd searched for that sooner. ...

Ooops! I gave that link back in post #458 and didn't notice that physicstoday.org already had a transcript of that very passage! Had I noticed I could have saved you the trouble. In any case it's nice to have. Here is what physicstoday.org gives. It may have some differences from what you transcribed (but I haven't noticed any):

==quote Weinberg at 2009 Science Writers conf.==
"It’s developed mathematically, but not to the point where there is anyone theory, or to the point that even if we had one theory we would know how to do calculations to predict things like the mass of the electron, or the masses of the quarks. So, I would say, although there has been theoretical progress... I find it disappointing. One of the hopes would be that the LHC would provide a clue to something we’re missing in superstring theory and I think that supersymmetry is the most likely place to look."

"One of the troubles with superstring theory is that although in a sense the theorists think there is only one theory, there are an infinite number of approximate solutions of it and we don’t know which one corresponds to our world. But at least in a large variety of the solutions of superstring theory there is supersymmetry visible at low energies, and if we see supersymmetry at low energies, superstring theorists may be able to derive from it some kind of clue as to how to solve these theories. But I haven’t talked about it in this lecture because I don’t see how that would work... I mean I couldn’t say that it was likely with any degree of sincerity, and certainly the LHC and any other accelerator that we can imagine being built will not get up to energies which are high enough so that we can directly see the structures that are described by superstring theory, the strings or the D-branes or whatever it is. Those will not be accessible at the LHC, so any clue we get will be very indirect."

"I myself, well I was working on superstring theory in the 80s and gave it up because... I moved into cosmology, which in the last couple of decades has had the excitement that elementary particle physics had in the 60s and 70s, a wonderful coming together of theory and observation. Cosmology now reminds me of the excitement that I felt when I was younger and doing particle physics... and it’s a pity that superstring hasn’t developed better. I still think it’s the best hope we have, I don’t know of anything else. My own work very recently has been trying to develop an alternative to superstring theory as a way of making sense out of quantum gravity at very high energies. But even though I’m working on this I still find superstring theory more attractive, but not attractive enough…"
==endquote==

 

[Calrid]

Smolin: it seems that any acceptable quantum theory of gravity, whatever its ultimate formulation, is likely to reduce to a perturbative string theory in the appropriate limit. http://arxiv.org/abs/gr-qc/9508064

Basically, strings are amazing because:

1) They contain gravity
2) It uncovered that gravity in some universes can be described by a good-old quantum field theory!
3) It uncovered a link between quantum field theory and Penrose's twistors!

We do not know yet if strings can describe our universe, but the hope is that by studying the clues above, we will be led to a theory that does.

Smolin is still a critic of String theory, but then he favours LQG so to him it would be a competing theory. I don't think any comment cherry picked from the web is going to change that. Relational quantum mechanics is really Roveli's brain child, although Smolin supports it.

There are some strong arguments against strings, that is undeniable. I've seen a paper that raises several salient criticisms, although don't ask me to fish it out, it was 5 years ago I saw it.

That said criticism is perhaps one of the most valuable checks and measures science has, I just wonder if String Theory really gets enough being as its so esoteric and mathematically dense that few Scientists really understand it outside of those who are already committed to it.

 

[marcus]

...I don't think any comment cherry picked from the web is going to change that...

That particular cherry was dated 1995, which is a rather old cherry
Also "likely to reduce to a Theory X in the appropriate limit" is hardly a ringing affirmation.

Your nickname suggests connection with a famous city. If, in fact, you lived there then you would have just experienced a remarkable time in that city's history.

 

[Calrid]

That particular cherry was dated 1995, which is a rather old cherry

Yeah come on though, you know the strong criticisms Smolin has levelled against strings, he's mooting a competing theory, he's hardly in their corner.

By now, just about everyone has heard of string theory. Even those who don't really understand it--which is to say, just about everyone--know that it's the hottest thing in theoretical physics. Any university that doesn't have at least one string theorist on the payroll is considered a scientific backwater. The public, meanwhile, has been regaled for years with magazine articles breathlessly touting it as "the theory of everything." Brian Greene's 1999 book on the topic, The Elegant Universe, has sold more than a million copies, and his Nova series of the same name has captivated millions of TV viewers.

But despite its extraordinary popularity among some of the smartest people on the planet, string theory hasn't been embraced by everyone--and now, nearly 30 years after it made its initial splash, some of the doubters are becoming more vocal. Skeptical bloggers have become increasingly critical of the theory, and next month two books will be hitting the shelves to make the point in greater detail. Not Even Wrong, by Columbia University mathematician Peter Woit, and The Trouble with Physics, by Lee Smolin at the Perimeter Institute for Theoretical Physics in Waterloo, Ont., both argue that string theory (or superstring theory, as it is also known) is largely a fad propped up by practitioners who tend to be arrogantly dismissive of anyone who dare suggest that the emperor has no clothes.

There were good reasons for the theory's appeal when it first emerged in the late 1970s and early '80s. At the time, physicists found themselves facing a crisis: the two most important ideas of 20th century physics, relativity and quantum theory, were known to be fundamentally incompatible. Quantum theory describes the universe as intrinsically discontinuous: energy, for example, can come in bits just so small, but no smaller. Relativity treats time and space and gravity as a smooth, unbroken continuum. Each theory has its purposes, and they usually don't overlap. But when dealing with very large masses or time periods that are infinitesimally small, like the core of a black hole or the first moments after the Big Bang, neither quite works.

Read more: http://www.time.com/time/magazine/article/0,9171,1226142,00.html#ixzz1HD9oydVZ

He wrote a book which was basically an attempt to debunk the fad science that was strings in 2006. I think cherry picking comments from when he was still working in strings before he became apostate really tells us nothing. He also runs a blog that basically debunks claims of string theorists, such as that evidence would turn up at CERN and various other claims about the applicability of the theory.

You're reaching here.

The Trouble with Physics, on the other hand, was strongly critical of string theory and of its prominence in contemporary theoretical physics. Smolin suggests that string theory suffers from serious deficiencies and has an unhealthy near-monopoly in the particle theory community. He called for a diversity of approaches to quantum gravity, and argued that more attention should be paid to loop quantum gravity, an approach Smolin has devised. Finally, The Trouble with Physics is also broadly concerned with the role of controversy and the value of diverse approaches, in the ethics and process of science.

Smolin's thesis found support in one corner. In the same year as that in which The Trouble with Physics was published, Peter Woit also published a book for nonspecialists, whose conclusion was similar to Smolin's, namely that string theory was a fundamentally flawed research program.[11]

http://en.wikipedia.org/wiki/Lee_Smolin

Also "likely to reduce to a Theory X in the appropriate limit" is hardly a ringing affirmation.

Of course but then some people have said that it is a theory of anything, Woight I believe.

Everything reduces to maths if you go far enough into any model, and you have a model that can model not everything but anything. This is hardly much of an endorsement for strings either. Something Smolin makes all too clear in his book.

In context though Smolin thought that LQG would be indistinguishable from strings, without actually needing any of the tiny vibrating particles strings implies.

Again although I have nothing against String Theorists, savvy critics like this are vital if the field is going to be properly monitored. I still wonder if it is really though.

 

[atyy]

There are some strong arguments against strings, that is undeniable. I've seen a paper that raises several salient criticisms, although don't ask me to fish it out, it was 5 years ago I saw it.

That said criticism is perhaps one of the most valuable checks and measures science has, I just wonder if String Theory really gets enough being as its so esoteric and mathematically dense that few Scientists really understand it outside of those who are already committed to it.

I'm a biologist. I spent years liking the underdog LQG from reading Scientific American articles in the gym without having any idea what the formalisms of LQG or strings were. But I finally decided to learn a little string and LQG a few years back after hearing all my friends talk about Smolin's book for weeks without my understanding a single word. Guess what? While being a string practioner requires real skill, the achievements of string theory were clear enough to me - a lay person - that strings are way ahead of LQG.

BTW, one of the achievements of strings is to demonstrate that theories without gravity can actually contain gravity. So LQG may yet have gravity ...

 

[Calrid]

I'm a biologist. I spent years liking the underdog LQG from reading Scientific American articles in the gym without having any idea what the formalisms of LQG or strings were. But I finally decided to learn a little string and LQG a few years back after hearing all my friends talk about Smolin's book for weeks without my understanding a single word. Guess what? While being a string practioner requires real skill, the achievements of string theory were clear enough to me - a lay person - that strings are way ahead of LQG.

BTW, one of the achievements of strings is to demonstrate that theories without gravity can actually contain gravity. So LQG may yet have gravity ...

You seem to be under the mistaken impression that I am trying to say that one theory is better than another I am merely establishing Smolin as a critic.

Besides Smolin and his team are a small research group, and LQG doesn't have the same fan following so big whoop basically.

My contention was only that strings needs more openness and criticism from others. I wasn't trying to say any particular theory is the best. Mind you strings isn't and never has been the only game in town, this is of course a myth. I'm not even anti strings but like most people there seems to be this almost Zealot like defence of what after all is merely a hypothesis. Again I think this is unhealthy.

 

[atyy]

You seem to be under the mistaken impression that I am trying to say that one theory is better than another I am merely establishing Smolin as a critic.

Besides Smolin and his team are a small research group, and LQG doesn't have the same fan following so big whoop basically.

My contention was only that strings needs more openness and criticism from others. I wasn't trying to say any particular theory is the best. Mind you strings isn't and never has been the only game in town, this is of course a myth. I'm not even anti strings but like most people there seems to be this almost Zealot like defence of what after all is merely a hypothesis. Again I think this is unhealthy.

But can you support the insinuation that there isn't enough openness and criticism? That it is understood by only a few insiders? That it is being supported by zealots?

No one is saying that there shouldn't be openness and criticism. No one is saying that it there should be zealot like support for it.

I do believe Smolin's "The rise of string theory and the fall of a science" was gross exaggeration. (OK, I'll also say that Ed Witten's public advertisements in the late 80s were also overdone.)

 

[atyy]

In context though Smolin thought that LQG would be indistinguishable from strings, without actually needing any of the tiny vibrating particles strings implies.

BTW, although coming from a different place from Smolin, there are some researchers who are trying to find connections between LQG and string theory (when you read these, bear in mind that string theory has been linked to certain sorts of quantum field theories by AdS/CFT).

http://arxiv.org/abs/0907.2994
http://arxiv.org/abs/0905.1317
http://arxiv.org/abs/1102.5524

Or more schematically on the last page of http://www.emergentgravity.org/drupal/sites/default/files/EGIV_presentations/Vidal.pdf , as well as the last page of http://dao.mit.edu/~wen/talks/10Sum.pdf .

BTW, this is an even smaller group of researchers than LQG!

On the "mainstream" LQG side, these researchers make it clear they are watching out for links too http://arxiv.org/abs/0905.3627

 

[Calrid]

But can you support the insinuation that there isn't enough openness and criticism? That it is understood by only a few insiders? That it is being supported by zealots?

No one is saying that there shouldn't be openness and criticism. No one is saying that it there should be zealot like support for it.

There is though, for even suggesting string theory should be more open to criticism you have jumped on me as if somehow suggesting something like that is inapt? More criticism more analysis, more peer review. This is not science you know!

I do believe Smolin's "The rise of string theory and the fall of a science" was gross exaggeration. (OK, I'll also say that Ed Witten's public advertisements in the late 80s were also overdone.)

A lot of people don't. I for one am glad media like this exists it does expose some of the overblown claims and exaggerations from this hypothesis.

This is healthy. I have posted on various forums and the level of sheer frenzy criticising this hypothesis sets of is sometimes beyond belief. To the point where now I don't even bother even mildly criticising it on some forums, to avoid the inevitable wall of fanatics and flame wars that tend to result. I will say this though it took me two years of endlessly quoting philosophy of science to even get people to admit that a theory without evidence is not a scientific theory. How stupid is that? I think the problem is that most people actually believe the hype that this is indeed a theory like the big bang theory when clearly it is not. This is also an unhealthy way to go about researching what is still no more theoretical than the Higgs Boson.

My criticism is not of the research itself or the mathematics or the bold claims, although some are completely fanciful and based on suppositions upon ideas. It is of the fanatical following and the lack of openness or criticism from those neutral or even anti that comes along with this "theory".

 

[atyy]

My criticism is not of the research itself or the mathematics or the bold claims, although some are completely fanciful and based on suppositions upon ideas. It is of the fanatical following and the lack of openness or criticism from those neutral or even anti that comes along with this "theory".

OK, good. We can agree to disagree on the sociology, why don't we discuss the science? Eg. what do you think of the interesting points mitchell porter, tom.stoer and Physics Monkey made in posts #452, #454 and #459?

 

[tom.stoer]

Perhaps it's time to step into again:-)

Let's look at Greene's and Smolin's books: reading Greene one could get the impression that string theory is the solution of everything; why was it then necessary to write a second book? Reading Smolin one could get the impression that nearly everything is wrong and that nearly nothing will work (ever); so why are there so many interesting and certainly "not wrong" results from string theory? Smolin's book started an interesting debate, but of course it's not free from a "hidden agenda". I avoided the names Smolin and Woit - for some very good reasons: I don't want to have fights on behalf of somebody, instead I wanted to have an unbiased exchange our own ideas (not the ideas of Smolin, Woit, Witten, Susskind, Distler, ...) So my conclusion was that discussing (or even mentioning) these books (or names) is just a waste of time!

I just checked my very first post in this thread and I think it is still valid: I wanted to express why I would really like to be a fan of string theory, why it is so hard to become a fan (or to stay a fan looking at facts only), and I wanted to invite the string community here in this forum to provide support for their theory.

This worked rather well. There were very many intersting thoughts from forum members, last but not least Physics Monkey trying to summarize main achievements. Is it unfair to ask if you could write a similar review explaining the major problems? It should be done - and it should come from an insider. fzero, thanks for mentioning Weinberg! This is something that goes into the direction of David gross I mentioned a couple of times.

 

[Calrid]

OK, good. We can agree to disagree on the sociology, why don't we discuss the science? Eg. what do you think of the interesting points mitchell porter, tom.stoer and Physics Monkey made in posts #452, #454 and #459?

I don't think I could really provide any insight not being an expert in this area. So anything I post would probably be no more than hmm sounds interesting where can I read about this.

 

[Calrid]

Perhaps it's time to step into again:-)

Let's look at Greene's and Smolin's books: reading Greene one could get the impression that string theory is the solution of everything; why was it then necessary to write a second book? Reading Smolin one could get the impression that nearly everything is wrong and that nearly nothing will work (ever); so why are there so many interesting and certainly "not wrong" results from string theory? Smolin's book started an interesting debate, but of course it's not free from a "hidden agenda". I avoided the names Smolin and Woit - for some very good reasons: I don't want to have fights on behalf of somebody, instead I wanted to have an unbiased exchange our own ideas (not the ideas of Smolin, Woit, Witten, Susskind, Distler, ...) So my conclusion was that discussing (or even mentioning) these books (or names) is just a waste of time!

I just checked my very first post in this thread and I think it is still valid: I wanted to express why I would really like to be a fan of string theory, why it is so hard to become a fan (or to stay a fan looking at facts only), and I wanted to invite the string community here in this forum to provide support for their theory.

This worked rather well. There were very many intersting thoughts from forum members, last but not least Physics Monkey trying to summarize main achievements. Is it unfair to ask if you could write a similar review explaining the major problems? It should be done - and it should come from an insider. fzero, thanks for mentioning Weinberg! This is something that goes into the direction of David gross I mentioned a couple of times.

I think that's a bit of a straw man. No one thinks the whole thing is wrong, not even Smolin. His book like woights is more pointing out the overblown claims of evidence turning up at CERN and takes the same tack as Woight does in: Not even Wrong. It's a polemic style certainly but again away from the same over blown claims String Theorists have on the opposite side. It does have some very good points to make. You just have to cut through the propagandist oxidised guff to find the nuggets.

I agree though as books go its certainly in the same style as the pro string campaigners. Woights denser scientific contentions and lack of appeal to the laymen, makes it a more substantive critique IMO.

 

[mitchell porter]

In fact, I would be comfortable betting that string theory will never predict the electron's mass.

How much do you want to bet? i.e. please explain this opinion.

 

[atyy]

How much do you want to bet? i.e. please explain this opinion.

How's he going to collect if he's right?

 

[marcus]

.
My wife recently read Brian Greene's first book, but she didn't like it and I have to agree with her criticism. The book makes many promises in the name of strings, predicting particle masses, being the single grand theory of gravity, etc., and none of these promises are fulfilled in the book. Indeed, string theory has still not fulfilled these promises. In fact, I would be comfortable betting that string theory will never predict the electron's mass. On the other hand, I think string theory has given us much we didn't expect, something far more interesting. And since our job in physics is not just to crank out numbers for the latest random experiment being done, but to understand the deep structure of nature, I think string theory will continue to play a role in our search for that structure.

How's he going to collect if he's right?

It's usual in these situations to remove infinities by introducing a cutoff.

From what Physics Monkey says, he would also be comfortable betting that the electron mass not be derived from Stringery by 2017, say. So then if anyone takes the bet it would be possible to collect on it.

 

[tom.stoer]

Yes.The mountain has given birth to the mouse.

any substantial contribution?

 

[Fra]

How's he going to collect if he's right?

How are we even going to establish if he is right, when the predictive systems (the theory) is merely solutions in another theory.

Would "finding ONE theory that predicts electron mass" count?

Or do we also need to explain this particular choice of solution to have made a complete prediction (or postdiction)?

/Fredrik

 

[Physics Monkey]

How much do you want to bet? i.e. please explain this opinion.

Haha, I don't know, I might be willing to bet quite a bit. I seriously doubt any prediction will be forthcoming. Basically, I think its crazy to think that structure of the world at a few GeV tells us much of anything about the structure of the world at 10^{18} GeV (and vice versa). I think such a situation is unprecedented in the history of science

This is because I suspect the landscape is a real thing. Does anyone really think that string theory, with all its incredible richness, can't accommodate a bit heavier of an electron, or an extra generation of very heavy particles, or any number of other minor (or even major) tweaks?

For example, suppose you have a vacuum that closely resembles our world.
1) I would imagine that there are hundreds more with slightly different low energy parameters.
2) And similarly, I would imagine that there are hundreds with the same low energy physics but with varying physics at higher energies.

1) is very familiar from condensed matter physics where we often have continuously variable parameters.
2) is an example of low energy universality i.e. the relative independence of low energy physics from the high energy details.

Holographic duality tells us that this intuition also applies to some part of the string landscape.

Another example from condensed matter physics. Suppose you were a tiny organism living inside a material in one of the labs down the hall. You discover through a series of ingenious high energy experiments your proposed theory of everything: electrons hopping on a lattice interacting electrostatically. You look for symmetry principles telling you that the lattice you found is the perfect lattice, that it had to be that way (haha, that actually sounds a lot like string theory). You write a popsci book declaring that you will soon compute all the interesting constants of nature: the speed of phonons, the mass of low energy quasiparticles, etc. But then you discover that far from giving a unique answer, the high energy theory predicts all kinds of worlds you've never seen: different lattices, different phonon speeds, different critical temperatures for the various phase transitions you've observed, different phases you've never seen. And you finally realize that knowing the high energy theory doesn't ultimately tell you that much about your low energy world. But that's ok because the high energy theory is still interesting and useful and it opens your eyes to wonderful new possibilities. That is what I understand string therory to be.

 

[Physics Monkey]

Haha, as a place to start, I would be willing to bet US $100 that no string construction will predict the electron's mass in the next 20 years. By predict I mean roughly the following: we may find a vacuum that has the electron's mass right (that wouldn't surprise me), but string theory should have given some reason why this vacuum should be preferred to any other. Or perhaps shown how the vacuum/landscape language I'm using is misleading.

I'm a reasonable monkey physicist , I would be happy to pay up if anything remotely predictive came out of string theory (regarding high energy particle physics).

 

[negru]

Haha, as a place to start, I would be willing to bet US $100 that no string construction will predict the electron's mass in the next 20 years. By predict I mean roughly the following: we may find a vacuum that has the electron's mass right (that wouldn't surprise me), but string theory should have given some reason why this vacuum should be preferred to any other. Or perhaps shown how the vacuum/landscape language I'm using is misleading.

I'm a reasonable monkey physicist , I would be happy to pay up if anything remotely predictive came out of string theory (regarding high energy particle physics).

This sounds overambitious for now. However, having a landscape doesn't mean one can't make predictions. People who say that really haven't thought about the topic too much or are just playing dumb. It doesn't matter if there are 10^500 solutions, or a continuum of them.

As long as the number of parameters needed to describe the vacuum is less than the current number of parameters in the Standard Model, it's more than enough. There's nothing bad about having a landscape. In my opinion, like in yours, having a landscape of solutions is the most sensible thing a theory of everything can have. We shouldn't we be able to have a theory in which the electron mass is not X, but X+0.0000000001 ? Like there is no sensible reason why the Earth's mass is X, but there is nevertheless a mechanism which can explain how it got that mass.

In fact. the improvement from 23 something parameters to 10^500 different solutions is quite big. The latter has measure zero in the former. It's infinitely better. Calling it a plague is really disingenuous. Assuming that some solutions come close to reality of course.

 

[A. Neumaier]

I'm a biologist. I spent years liking the underdog LQG from reading Scientific American articles in the gym without having any idea what the formalisms of LQG or strings were. But I finally decided to learn a little string and LQG a few years back after hearing all my friends talk about Smolin's book for weeks without my understanding a single word. Guess what? While being a string practioner requires real skill, the achievements of string theory were clear enough to me - a lay person - that strings are way ahead of LQG.

Please let us know according to which standards you measure being ''ahead''?

And which achievements show clearly enough that strings are way ahead of LQG?

 

[negru]

Please let us know according to which standards you measure being ''ahead''?

And which achievements show clearly enough that strings are way ahead of LQG?

Just to point out the first that comes to mind, string theory contains gravity while it remains to be shown that so does LQG.

 

[Fra]

It doesn't matter if there are 10^500 solutions, or a continuum of them.

Because 10^500 are still too many to be searchable in any rational manner?

Like there is no sensible reason why the Earth's mass is X, but there is nevertheless a mechanism which can explain how it got that mass.

What is the mechanism whereby the string action relative to a background or vacuum, couple to the action and evolution of of the background?

The problem isn't that there is some kind of landscape per see, the problem is that there is no measure on this landscape that allows evolution and learning.

What I would like to see is a fit learning algorithm that can make progress and converge in the landscape.

Say a rational search algorithm, and then show that this converges faster than the landscape grows further.

If ST can provide a mechanism for this, then there is notthing wrong per see with a landscape. To compare with evolution, the landscape itself should be observer dependent and be large enough to ensure variation, but not large enough to cause for the observer to get lost in his random walk.

I do not object to non-unique inferences at all, on the contrary do I embrace it. It's a part of the inferencial perspective I am into, but the algorithmic search is important. So important that I don't think it's reasonable to take the such a program conceptually serious until this gap is filled in. This isn't about beeing ambitous, it's just that before one engages in such sophisticated version of theories of theories, modifying the falsification criterias etc one must get it right, or it's better to stick with the old unique theory framework.

/Fredrik

 

[Fra]

Because 10^500 are still too many to be searchable in any rational manner?

Since I don't have the technical insight in exactly how these theories are counte in ST, I wonder this.

What structure does there exists in this set?

The structure, such are order relations etc, can be exploited to improve search and learning. In this sense, it's even POSSIBLE that it's faster to handle a continuum as one degree of freecom, rather than as one big SET with nor order relations defined. So cardinality alone doesn't settle wether a continuum is worse than a finite set when it comes to search times.

/Fredrik

 

[tom.stoer]

Just to point out the first that comes to mind, string theory contains gravity while it remains to be shown that so does LQG.

What about LQC which has FRW cosmology as a limiting case? What about deriving graviton propagators from spin foam? Of course not everything is completed in LQG, but afaik there is no fully developed string theory on dynamical backgrounds, either; so both programs are work in progress.

But this is not relevant here. I don't think we should compare LQG and string theory in this thread (we can do that, but not here, as we would immediately start to argue about LQG; this is a different subject). I started and continued this thread w/o mentioning LQG (hopefully :-), simply to avoid this never-ending discussion from the very beginning.

 

[atyy]

Please let us know according to which standards you measure being ''ahead''?

And which achievements show clearly enough that strings are way ahead of LQG?

Just a quick answer and I'll refrain after this, since following tom.stoer's note we shouldn't get into this discussion here.

From my lay point of view, strings are ahead becasue
1) Strings have gravity.
2) The AdS/CFT correspondence is a non-perturbative formulation of quantum gravity in some universes, and a concrete example of the emergence of space

LQG has not been shown to contain gravity. The theory is not well-defined ( http://arxiv.org/abs/1010.1939 , Eq 26). The "classical limit" addresses only kinematics, not dynamics ( http://arxiv.org/abs/1101.5061 ). My interest in LQG is that since string theory has shown that theories without gravity can contain gravity, maybe the LQG formalism does contain gravity, if it is interpreted differently.

 

[atyy]

Haha, as a place to start, I would be willing to bet US $100 that no string construction will predict the electron's mass in the next 20 years. By predict I mean roughly the following: we may find a vacuum that has the electron's mass right (that wouldn't surprise me), but string theory should have given some reason why this vacuum should be preferred to any other. Or perhaps shown how the vacuum/landscape language I'm using is misleading.

I'm a reasonable monkey physicist , I would be happy to pay up if anything remotely predictive came out of string theory (regarding high energy particle physics).

A tangent: I think condensed matter folks have long wanted a landscape (eg. Wen's manifesto at the start of his QFT text, if I'm reading it right). So from that point of view, the landscape is an achievement of strings?

Edit: I wrote the above before seeing these:

For example, suppose you have a vacuum that closely resembles our world.
1) I would imagine that there are hundreds more with slightly different low energy parameters.
2) And similarly, I would imagine that there are hundreds with the same low energy physics but with varying physics at higher energies.

1) is very familiar from condensed matter physics where we often have continuously variable parameters.
2) is an example of low energy universality i.e. the relative independence of low energy physics from the high energy details.

Holographic duality tells us that this intuition also applies to some part of the string landscape.

Another example from condensed matter physics. Suppose you were a tiny organism living inside a material in one of the labs down the hall. You discover through a series of ingenious high energy experiments your proposed theory of everything: electrons hopping on a lattice interacting electrostatically. You look for symmetry principles telling you that the lattice you found is the perfect lattice, that it had to be that way (haha, that actually sounds a lot like string theory). You write a popsci book declaring that you will soon compute all the interesting constants of nature: the speed of phonons, the mass of low energy quasiparticles, etc. But then you discover that far from giving a unique answer, the high energy theory predicts all kinds of worlds you've never seen: different lattices, different phonon speeds, different critical temperatures for the various phase transitions you've observed, different phases you've never seen. And you finally realize that knowing the high energy theory doesn't ultimately tell you that much about your low energy world. But that's ok because the high energy theory is still interesting and useful and it opens your eyes to wonderful new possibilities. That is what I understand string therory to be.

As long as the number of parameters needed to describe the vacuum is less than the current number of parameters in the Standard Model, it's more than enough. There's nothing bad about having a landscape. In my opinion, like in yours, having a landscape of solutions is the most sensible thing a theory of everything can have. We shouldn't we be able to have a theory in which the electron mass is not X, but X+0.0000000001 ? Like there is no sensible reason why the Earth's mass is X, but there is nevertheless a mechanism which can explain how it got that mass.

 

[qsa]

The reason why you can have a landscape problem in string(or any other similar in nature) is that the model is not designed from a fundamental entity. it is designed with a make blieve properties,just like point particles carrying charge, spinning(we don't know how) , has a mass! , it's a field and a particle and the list goes on and on. We do that by playing the diff equation game, the price you pay is that now you can twist things all the hell and create parameters and play with them as you please. These models are inherently free for all.

As a matter of fact if the universe is designed from a funamental entity, it will become clear that no much freedom exists in designing the universe. So, this entity is unique and whatever relations among these entities will be bounded, and that is how you get a finite orderly universe that is comprehensible. Sorry, Einstein.


.

 

[negru]

I agree that if we can't search the landscape it won't be very useful. But yeah, otherwise to me it's a good thing to have one. Having a unique solution might have generated nobel prizes more quickly, but I wouldn't have found that option more satisfying than our current predicament. There's no obvious reason why the parameters should have the unique values they have. We'd just like them to have specific values to test out theories - but what we want or need says nothing about how things actually work.

 

[tom.stoer]

I agree that transforming a landscape of theories into a landscape of solutions is a big success of string theory. But there are several problems:

1) up to know you do not have a unique theory from which you can derive one landscape and its vacua; instead you have different approximate theories (related via exact or approximate) dualities which by themeselves define a (small) landscape of theories
2) up to now you are not able to calculate (not even approximately) any of the physical parameters we are interested in; you cannot derive the low-energy mass spectrum and the coupling constants in the (nearly realistic) vacua
3) once a set of vacua is known, there should also be some selection principles forcing the theory to adopt some vacuum states

In condensed matter physics there is one single theory (1) from which - via certain approximations - one can derive effective theories describing condensed matter states. One is able to calculate (approximately) certain physical parameters like density, heat capacity, ...(2). And one is able to specify certain conditions (temperature, pressure, ...) that force the system under consideration to be in a specific state (liquid, magnetized, ...) or show certain excitations or effective degrees of freedom (phonons, polarons, ...). In string theory the unique theory is not known, the calculations are not possible, the effective degrees of freedom are partially known but the selction of specific vacua, the transition between vacua etc. are not known.

 

[negru]

Yes I completely agree, and I also agree with the ordering of your points. We should first understand what the unique framework would be, if there is one, or what else dualities can teach us, before we even attempt to ask whether a selection principle exists.

Which is why I'd personally consider any work on selection principles or computing parameters to be way premature, and likely a waste of time. And this is why we still need to focus on more subtle issues, as well as ads/cft. Again it is my opinion that we won't find out too much from strings before fully understanding ads/cft - which is why many string theorists are working SYM for the moment. This however is something that most critics will complain about.

 

[tom.stoer]

Yes I completely agree, and I also agree with the ordering of your points. We should first understand what the unique framework would be, if there is one, or what else dualities can teach us, before we even attempt to ask whether a selection principle exists.

Thanks. This is one of the posts which distribute the disappointment I felt when starting the whole discussion.

 

[Fra]

We'd just like them to have specific values to test out theories - but what we want or need says nothing about how things actually work.

What I tried to convey is that AFAIC if you attempt something as bold as a theory of theories, then you are doing inference. It's more than just physics, and with this I don't mean it's mathematics, I mean it's a general theory of rational learning to describe any scientific process. And in this perspective the theory is an interaction tool for learning and interactions.

Here, how things "actually work" refers to how inference works, not how an electron behaves.

(although with a separate conjecture, that I personally make, you can make this even more radical by thinking that any physical interaction IS an inference. In this perspective, it is not the case that our inference says nothing about how things "actually work". The very radical point here is that it does)

I've expressed this view before, but my point is that if you take ST to really by the grand thing that at least some people think of; like the theory of theories... in a deeper sense. Then one must also take the inferencial perspective serious, or it simply doesn't stick together.

For me, this "vision" of ST as theory of theories, is not bad at all, it's just that it was NOT how string theory was started. I think it's at best something that may be some conclusion from failing to find a unique theory. But still, most arguments I have read from string people, does not seem to acknowledge these points.

In short, there is a lack of understanding what the landscape really means in terms of inference. That's my firm opinon. Paradoxally, string theory may hint this, and some other camps critique it, but it seems that string theory isn't the right "theory of theory", becuase some basic traits are missing.

Fill in those gaps, and I would be prepared to take it more seriously as an inferencial theory.

/Fredrik

 

[Fra]

1) up to know you do not have a unique theory from which you can derive one landscape and its vacua; instead you have different approximate theories (related via exact or approximate) dualities which by themeselves define a (small) landscape of theories

I have to admit that from my perspective, I do not think the lack of a unique meta theory of theories is a problem because I don't see how such a meta theory could be inferred as independent of the inference system used to infer it. It just doesn't make sense.

The only way it could be realized is as a conjecture, or element of structural realist reality. But those kind of things is exactly what I think are non-inferencial.

I think the landscape itself must evolve and can only be described from the point of view of an observer; and this evolution can not be predicted by the same observer. But it can be predicted by other observers.

So the landscape of theories defining the first layer of landscapes is then just corresponding to the observers. IMHO the landscape problem should thus related to the population of inference system in nature and is thus perfectly analogous to evolution. But this is not the same as the antrophic principles.

A landscape ~ diversity of inferencial systems

Not all points in the landscape are viable for the same reason we do not find one-legged lions in nature - even though such a lion would be perfectly "consistent". This is why consistency is an insufficient selector here, we need to account also for the fitness.

Unfortunately, i don't see that string theory as it's formulated, starting with the continuum etc, is able to provide the analysis needed to push this to the next level. Too many things needs to be reworked that it probably wouldnt' be string teory anymore except as a limiting inferencial model where the continuum is realized. But I am convinced that the resolution of the problems here lie at the level prior to the continuum.

/Fredrik