Why is String Theory Considered to be a Scientific Theory?

[inflector]

FIRST: Please, I don't what this to be a String Theory bashing thread. I've seen and even participated in plenty of those. I think that String Theory is interesting and that if and when we find a Theory of Everything that it is likely that String Theory will at a minimum have pointed us at least partly in the right direction. But from what I've read, I can't understand why it is considered to be a theory.

When I first started researching physics a few years back, I noticed that there were lots of discussions about the meaning of "theory" in science being different than in normal English usage. That in science a theory is something more. This came up especially in discussions of Creationism and the meaning of the words: "theory of evolution."

In this vein, for example, we find:

In the sciences, a scientific theory comprises a collection of concepts, including abstractions of observable phenomena expressed as quantifiable properties, together with rules (called scientific laws) that express relationships between observations of such concepts. A scientific theory is constructed to conform to available empirical data about such observations, and is put forth as a principle or body of principles for explaining a class of phenomena.

for the first paragraph at http://en.wikipedia.org/wiki/Scientific_theory

Okay, on that basis, I can understand why String Theory might be considered a theory. It is a collection of concepts that purports to express relationships between observations.

But later in the same article under the heading: "Essential criteria," one finds:

The defining characteristic of a scientific theory is that it makes falsifiable or testable predictions. The relevance and specificity of those predictions determine how potentially useful the theory is. A would-be theory that makes no predictions that can be observed is not a useful theory. Predictions not sufficiently specific to be tested are similarly not useful. In both cases, the term "theory" is hardly applicable.

(emphasis mine)

So how does String Theory meet this criterion? It is my understanding that String Theory cannot currently be falsified. Am I wrong in this understanding? If not, why is the last sentence not applicable in the String Theory case?

Finally, in looking at crank and crackpot ideas which seem to pervade google results and the relevant discussions about these ideas here and on BAUT, for example, one sees that scientists have thought long and hard about what makes something scientific versus a crackpot idea. In fact, John Baez has even had a list for many years called http://math.ucr.edu/home/baez/crackpot.html" which I see referred to from time to time when appropriate. This index gives points for qualities that determine if a proponent is a crackpot. The very last item, the one with the most points, therefore the dead giveaway that an idea is a crackpot idea is:

50 points for claiming you have a revolutionary theory but giving no concrete testable predictions.

So I want to know why String Theory gets a pass on this? Why is it considered to be a scientific theory by most physicists despite their obvious contempt for ideas that are not testable under other circumstances?

I sincerely want to know why String Theory is different from other theories so it can be considered a scientific theory despite not meeting the definition's primary essential criterion (at least according to Wikipedia). I must be missing something here and I would like to know what it is.

 

[negru]

Do we really have to go through this every other week?

Ok, why is quantum field theory a theory? Does it make any predictions on its own? No, you have to feed it gauge groups first.The same for string theory, only you feed it vacua.

Or from the "tool" perspective, what do complex numbers predict? Nothing, you just use them. Same for string theory.

 

[inflector]

Do we really have to go through this every other week?

Ok, why is quantum field theory a theory? Does it make any predictions on its own? No, you have to feed it gauge groups first. The same for string theory, only you feed it vacua..

So what is the difference precisely? What is it about Quantum Field Theory that makes it okay for us to call it a theory? Does the combination of string theory and vacua make predictions? If so what are they?

Are you saying that Wikipedia's definition is wrong (wouldn't be the first time), is that what it is? If so what criteria do you prefer? If this comes off as flip that is unintentional, I really am trying to get to the bottom of this.

Or from the "tool" perspective, what do complex numbers predict? Nothing, you just use them. Same for string theory

Complex numbers aren't a scientific theory they are a mathematical concept.

Perhaps this is the problem. I am confusing mathematics with science. Is it that String Theory is a theory in the sense that number theory is theory? It's not a scientific theory in the sense I quoted above but theory in the sense of a set of mathematical tools?

Is the problem that mathematical terms and traditional scientific terms are blended at times?

My real problem is that I was fine with the word "theory" to begin with. I thought it made perfect sense to call it a theory using the English language non-scientific sense of the word, i.e. an idea that one thinks might be true. But I have seen many others, especially in discussions about crank or crackpot ideas that these are not theories because they make no predictions (among other reasons) and that in science the word "theory" has much more specific meaning, and that in science the word "theory" is reserved for ideas that have some empirical proof and that can be tested over time to greater degrees of accuracy.

But it seems that we have more than one concept going on here even in the "scientific" sense of the word. Is this just a matter of some scientists being opportunistically precise at times but not so at others?

Let's broaden it out since you include quantum field theory. In what sense are some of these ideas and frameworks for explaining observed behavior—like QFT—"theories" in the scientific sense of the word? What precisely distinguishes them from crackpot ideas? Is it that they are modeled using mathematics and that they have been peer reviewed, that they are scientifically plausible? Some combination of the above?

I'm perfectly happy calling String Theory just that, it is what I learned it to be, I just want to know why there seems to be an inconsistency in the usage of the term while at the same time many seem to want a very precise definition.

 

[Haelfix]

There is nothing unique about string theory here, indeed the vast majority of modern physics research fails to be falsifiable in practise. For instance, the theory of Hawking radiation and black hole evaporation is widely believed by most black hole researchers out there, however it will never be tested directly. Of course that its important to eventually find a testable portion, but unfortunately that might be simply a limitation of human scales rather than a failing of the actual theory itself.

Moreover, the mathematical part of string theory exists and will always be a part of physics, whether now or in 10,000 years. It is an important and nontrivial part of quantization (see eg Witten's latest paper), as well as being a part of the nonperturbative behaviour of certain gauge theories (which we know is important in nature).

Complaining about its testability in that context is akin to complaining about the existence of Fadeev-Popov ghosts in Yang Mills theory.

 

[inflector]

There is nothing unique about string theory here, indeed the vast majority of modern physics research fails to be falsifiable in practise. For instance, the theory of Hawking radiation and black hole evaporation is widely believed by most black hole researchers out there, however it will never be tested directly. Of course that its important to eventually find a testable portion, but unfortunately that might be simply a limitation of human scales rather than a failing of the actual theory itself.

Are you too saying the the wikipedia definition is good in theory but not in practice?

Shouldn't we somehow differentiate between what we only think might be true, even if for very good reasons, and what has direct empirical evidence? I'm new here but I thought that was the very distinguishing characteristic between science and belief or religion, i.e. that in science there is a tentative nature to current beliefs, that they can be changed by evidence. That we are willing to discard even cherished ideas if shown evidence. Perhaps I've been hanging around skeptics too long but that seems to me to be the relevant factor distinguishing science from religion or pseudoscience like astrology.

In the case of Hawking radiation, it seems that there are at least some predictions that could be falsifiable in principle if we could send probes to the center of the galaxy, for example. I realize this isn't likely to happen without some major new physics, but at least there is a framework for what is supposed to be happening that we can check under some regimes. Further, if radio telescopes improved by many orders of magnitude, we could probably detect the radiation directly. So there are criteria by which one could prove it wrong.

Moreover, the mathematical part of string theory exists and will always be a part of physics, whether now or in 10,000 years. It is an important and nontrivial part of quantization (see eg Witten's latest paper), as well as being a part of the nonperturbative behaviour of certain gauge theories (which we know is important in nature).

I never disputed this. This thread is not about whether or not String Theory is useful or promising.

Complaining about its testability in that context is akin to complaining about the existence of Fadeev-Popov ghosts in Yang Mills theory.

I'm not complaining at all. I don't care. I just am trying to understand the inconsistency.

As a relative newcomer to physics, this seems like a slippery slope. Who gets to decide what is a "scientific theory" and what isn't? If it were me defining the terms, I would not be so precise, but I didn't define them, some scientists and philosophers before me did. I am just trying to understand why there is an inconsistency in the usage.

Two replies so far and no answer to this question. This makes me believe that the precision in the term is fine when one is trying to bludgeon a crackpot or creationist but that in practice things are much fuzzier.

I am still hoping for a more rational distinction and further, that if one exists that someone will make the effort to correct Wikipedia so that people who are trying to learn will understand that the term "scientific theory" is fuzzier than one would be led to believe by reading the current article there.

 

[humanino]

Who gets to decide what is a "scientific theory" and what isn't?

The scientific community at large, obviously.

That includes outdated "scientific" theories from antiquity.

The definition of what science is has not been carved in stone by a prophet in the past. As Weinberg puts it indeed, in science we do not have prophets, but we do have heroes. You are most welcome to contribute and become one, by having influence on the status of string theory.

 

[marcus]

Inflector, thinking back to when I was in school (in math) there were fields of applied math that we called
probability theory
game theory
decision theory
recursive function theory
dynamical systems theory

One of these math chunks was not a scientific theory in the way described in your post. A branch of applied math might be USED to make predictions, if you FED it some stuff, as one of the posters here said. But by itself it wasn't empirical science---it wasn't a theory of nature but rather a corpus of mathmatical appliances---theorems, definitions, equivalences ("dualities"), computational techniques, interesting unsolved questions...

If as someone said QFT does not by itself make testable predictions, then perhaps by itself it is just a branch of applied math Until you "feed" it some definite gauge groups---and maybe some numerical parameters. At least it comes with selection principles (directions on the package) that tell you how to determine those plug-in parameters. Some "theories" don't.

You seem to be wanting to nail down the fuzzy doubletalk. Why is "string theory" called by that name if it is not a physics theory, not a scientific theory, not an empirical theory of nature?

Partly it is just the customary way people talk, which is hard to change or make entirely consistent. It seems as if "string theory" is not a theory of nature. As David Gross has said "we don't know what string theory is!" So it is not a coherent well-defined scientific theory. But it does resemble a large active branch of applied math---and it seems natural to a lot of people to call it a "theory" in that sense. It sounds right to them.

That's my take on it. There's a lot of disingenuous doubletalk that you can't nail down.

 

[Naty1]

Lots of these should probably be called "hypothesis" ...or something similar.

Poor 'ol Bill Unruh never got his "effect" such a noble epithet...

And likewise Jacob beckenstein...saddled with the monicker "bound"...

no need to get panties in a bunch over these imprecise terms...lots better things to argue and haggle over...

 

[atyy]

There are things that if are seen would favour string theory, it's just that the experiments may be beyond present technological capabilities. Quantum electrodynamics would not have been testable 130 years ago, that doesn't mean that if someone has come up with the theory then, that it wasn't a scientific theory until the 20th century.

There are also things that are seen that would falsify string theory.

These are discussed in section 4 of http://arxiv.org/abs/hep-th/0701050 .

Also, it is known that our best theories which do work break down at a high enough energy, and some theory must replace them. So by mathematical consistency alone, we have to search for a new theory. At present, string theory is without doubt an approach that has taught us a lot about whatever the true theory of quantum gravity is, even if it ultimately turns out that string theory does not model nature. Let me note that there are people who work on stuff related to loop quantum gravity who agree.

http://arxiv.org/abs/0705.0705: "I think in particular that string theory has been very successful as a brain storming tool. It has lead already to many spectacular insights into pure mathematics and geometry. But my personal bet would be that if somewhere in the mountains near the Planck scale string theory might be useful, or even correct, we should also search for other complementary and more reliable principles to guide us in the maze of waterways at the entrance of terra incognita."

http://arxiv.org/abs/gr-qc/9508064 (I don't know if this guy still agrees with what he said back then, but this is what he wrote in 1995): "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."

 

[dx]

String/M theory has already had tremendous influence on our understanding of the established part of physics, including particle physics and cosmology. Whether you want to call it a 'theory' or not, it certainly is scientific.

 

[bcrowell]

There are also things that are seen that would falsify string theory.

These are discussed in section 4 of http://arxiv.org/abs/hep-th/0701050 .

My reading of that section of the paper is that it's a list of "wouldn't it be great if..." ideas for what we could conceivably see that might support string theory -- with an admission that none of them are very likely.

Also, it is known that our best theories which do work break down at a high enough energy, and some theory must replace them. So by mathematical consistency alone, we have to search for a new theory.

But if the only guide is logical self-consistency, then it's not science, it's philosophy. The problem is that "at a high enough energy" could end up meaning at the Planck scale. There is no foreseeable technology that can probe the Planck scale. Any beings who could manipulate matter at Planck energies would be gods, not humans.

At present, string theory is without doubt an approach that has taught us a lot about whatever the true theory of quantum gravity is, even if it ultimately turns out that string theory does not model nature.

I'm not convinced that this is true. It may have taught us absolutely nothing about the true theory of quantum gravity. It may have hindered us from finding a true theory of quantum gravity, in the same way that Aristotelian physics hindered Galilean physics from being created.

My guess is that 500 years after I'm dead, if there is anything like H. sapiens still around, they will not have figured out quantum gravity.

 

[atyy]

But if the only guide is logical self-consistency, then it's not science, it's philosophy. The problem is that "at a high enough energy" could end up meaning at the Planck scale. There is no foreseeable technology that can probe the Planck scale. Any beings who could manipulate matter at Planck energies would be gods, not humans.

My view is that we should consider all hypotheses consistent with the data. We should classify the hypotheses according to how future data would rule out classes of them.

Or do you think we are done if the LHC discovers the Higgs and nothing else?

I hope you agree that no quantum gravity research should be funded.

 

[atyy]

I'm not convinced that this is true. It may have taught us absolutely nothing about the true theory of quantum gravity. It may have hindered us from finding a true theory of quantum gravity, in the same way that Aristotelian physics hindered Galilean physics from being created.

Do you think these statements are false?

http://arxiv.org/abs/0809.4266
"These dualities become especially powerful when combined with string theory [1]. It is an occasional misconception, however, that the existence of holographic dualities is contingent on the validity of string theory. This is not the case."

Or that this approach is misguided?

http://arxiv.org/abs/1006.1902
"Ever since the seminal work of Bekenstein and Hawking, it has been clear that there is a deep and fundamental relation between gravity, thermodynamics and quantum mechanics, while its detailed form and origin was and is largely mysterious. ... It seems likely that the basic triangular relationships transcend string theory and AdS/CFT, although lessons from string theory are likely useful guides for unraveling the more general picture. It is our hope that the attempt here to generalize fluid/gravity duality away from the stringy context to its most essential ingredients may be useful in understanding this triangle."

 

[MTd2]

I would argue that String theory is not like a framework like QM and QFT but has its fundamental degrees of freedom and expectations values well determined, even if it is on principle.

But I have to agree with bcrowell. Completely. And I would add the same could be said of any attempt to try to find QG or a unifying theory of everything. I had already though of something similar, including the Aristotelian physics, but I could put it into words. I guess the only way out is to diversify business. Try out as many possibilities as possible. In the long wrong we could benefit from a better mathematical comprehension, way beyond what String Theory provided.

 

[atyy]

I guess the only way out is to diversify business. Try out as many possibilities as possible. In the long wrong we could benefit from a better mathematical comprehension, way beyond what String Theory provided.

I think bcrowell implied the opposite. Well, not quite, I would understand his comments to mean that no possibilities should be researched.

 

[MTd2]

Being hopeless doesn't mean something should not be researched. At least this is hot it is for me.

 

[atyy]

Being hopeless doesn't mean something should not be researched. At least this is hot it is for me.

Is your view similar to this:

My view is that we should consider all hypotheses consistent with the data. We should classify the hypotheses according to how future data would rule out classes of them.

 

[MTd2]

Yes, with both of them. I really see no contradiction. I just think that one can always postpone ruling out a theory almost forever.

 

[mitchell porter]

So I want to know why String Theory gets a pass on this? Why is it considered to be a scientific theory by most physicists despite their obvious contempt for ideas that are not testable under other circumstances?

I sincerely want to know why String Theory is different from other theories so it can be considered a scientific theory despite not meeting the definition's primary essential criterion (at least according to Wikipedia). I must be missing something here and I would like to know what it is.

The issue is discussed elsewhere in Wikipedia:

http://en.wikipedia.org/wiki/String_theory#Is_string_theory_predictive.3F

We can approach it at several levels. First, as negru says, you can think about it like quantum field theory, with the choice of background geometry in string theory being analogous to the choice of symmetry groups, representations, and couplings in a QFT.

Does QFT in the abstract, without any of those specifications, make predictions? It implies quantum mechanics and relativity (though usually we'd go the other way and say that QM and relativity together imply QFT). In a weaker way it implies fermions and bosons, antimatter, and various other generic phenomena. But to go any further you have to specify a particular QFT, like QED or the whole standard model. Then and only then can you get significant quantitative predictions.

String theory in the abstract also has qualitative implications. It "implies" quantum field theory, so all the generic features of QFT carry across. But more specifically it also implies the existence of gravity and gauge fields, which correspond to the spin-2 and spin-1 states of the string. To pursue the analogy further: you won't get predictions any more specific than that, until you specify the background geometry through which the strings move. When you do that, you finally have a fully specified string model, which makes definite quantitative predictions.

So one perspective is that string theory, like quantum field theory or even classical field theory, is a template for theory construction. These "templates" are theories at the level of ontology, because they make qualitative hypotheses about what exists (classical fields, quantum fields, strings). Most falsification in physics occurs at the level of equations, where you fill out the template in a particular way, and calculate the consequences. Your QFT doesn't work, you make another. Your string model doesn't work, you make another.

But the ontological level is not entirely removed from the possibility of falsification. QM was invented because of the problems faced by classical field theory in explaining the atom. Strings became popular because of the problems faced by quantum field theory in explaining gravity. And we should remember that no string model has yet been exhibited which exactly reproduces the standard model! All we have are geometric backgrounds in which the strings reproduce some of its qualitative features. It looks unlikely, but it is still conceivable that it is actually impossible to get all the particle masses, etc, out of a string-theory background. If this is true, then it can be demonstrated mathematically, by enumeration of cases (can't do it this way, can't do it that way, there are no other options). So this is already a form of falsifiability of string theory - string theory as a general framework, not just the specific models - though it would take (is taking!) decades to run through the possibilities, and most people expect that the exact standard model will turn up somewhere, and possibly in many different geometries. In that situation, the different stringy realizations of the standard model would have to be distinguished by their high-energy behavior.

There is another aspect to the situation in string theory, which makes it different from QFT, and this is the perspective that while there are infinitely many QFTs, there is only one string theory. The difference between strings on one background and strings on another background is like the difference between a universe that expands forever and a universe that collapses. They are different hypotheses about nature, but they are both expressed in terms of general relativity.

But in general, the analogy with quantum field theory is a good one. QFT is a framework which synthesizes quantum mechanics and special relativity. A particular QFT can be falsified by its particular predictions, but falsifying the QFT framework is a much deeper challenge, and it really requires the discovery of a whole new framework which reduces to QFT under ordinary conditions. String theory, analogously, is a framework which synthesizes gauge fields and gravitation. A particular string model can be falsified by its particular predictions, but falsifying the string-theoretic framework is a lot harder, precisely because it does generically reduce to what we already see at low energies.

 

[Fra]

Just to add my opinion, without bashing or not bashing ST.

So I want to know why String Theory gets a pass on this?

I think your question partly has a very simply answer, but there is deeper aspect on description of the scientific process, or ANY learning process, or inference process is more complex that may be discussed.

(1) As already mentioned the simple answer is that ST is first of all a research discipline, and as to what it survived so long, is a due to rest of the community and funding mechanisms. Why they are still funded has to do also with the entire sociology and commercial forces in science. Wether we like it or not, science aren't immune to realyworld constraints such as economy, politics etc.

(2) Leaving that apart, the question of wether this simple falsification idea, of Popper at all is an aqequate description of a genereal inference process, I think the answer is no. I'm not a philosopher and I haven't read all views on this but I did read Poppers main book on this and IMO his view is too simple as he rejects the induction process to psychology outside of science. Poppers problem is that he wanted to describe the non-deductive process of science as close to a deductive process as possible. The big hole in Poppers abstraction is that lack of analysis of what happens when a theory is "falsified" and how a new "hypothesis" is generated. Because this is the core of learning. The falsification part is almost trivial in comparasion. Popper dismissed this question to beeing outside science.

/Fredrik

 

[MTd2]

String theory, analogously, is a framework which synthesizes gauge fields and gravitation.

Or what some think it is gravitation, or quantum gravity. Right now, we don't know which is the right one, experimentally, with or without gauge unification. And, on practice it doesn't do any better than QFT or QFT + SUSY.

So, as to bcrowell said, it could be a way to misguide from the right path, just like Archimedean Physics compared to Galilean Physics.

 

[atyy]

(2) Leaving that apart, the question of wether this simple falsification idea, of Popper at all is an aqequate description of a genereal inference process, I think the answer is no. I'm not a philosopher and I haven't read all views on this but I did read Poppers main book on this and IMO his view is too simple as he rejects the induction process to psychology outside of science. Poppers problem is that he wanted to describe the non-deductive process of science as close to a deductive process as possible. The big hole in Poppers abstraction is that lack of analysis of what happens when a theory is "falsified" and how a new "hypothesis" is generated. Because this is the core of learning. The falsification part is almost trivial in comparasion. Popper dismissed this question to beeing outside science.

I've read that at least Popper's view was scientific, because it has been falsified by Dirac's lone monopole;)

 

[Fra]

I've read that at least Popper's view was scientific, because it has been falsified by Dirac's lone monopole;)

Yes :) The real interesting part isn't just the pure philosophy of science but also to what extent this could give suggestions of how to design physical models, in particular when one is considering like theories of theories, or "frameworks", that may even exist in competition, then the issue isn't just falsification of specific predictions, it's more about survival and beeing able to adjust in response to feedback when and if it comes.

It's more like in biology. Rats are abundant not because they are always right or do the right thing, they are around because they are masters at taking the optimal corrective action when they are wrong. To falsify a speice I guess you need to extinguish it completely. The only fatal behaviour for such system isn't to be wrong, it's to fail to learn when feedback tells you do.

To kill the entire hypothesis generator machinery was killed because one theory made one false prediction would be extremely non-optimal. Ideally the falsification, should suggest and optimal revision of the theory, so it can evolve and improve. So again, such "theory generators" would be "killed" only if they failed to learn so as to eventually get outcompeted.

The level of specific skills, and the levels of "flexibility" needed optimally is again depending on the context.

But the nice thing is that all these things known to evolutionary biologists can actually have a meaning in terms of abstract inference, and by extension to inference and measurement theory to fundamental physics, and in particular it's structure and nature. This is exactly where my own focus is atm.

/Fredrik

 

[bcrowell]

Or do you think we are done if the LHC discovers the Higgs and nothing else?

Fields of research are like plants and animals. They grow, mature, and eventually die. At the turn of the 20th century, grad students were writing PhD theses on the motion of tops on inclined planes, and a Nobel prizes in physics was awarded for "invention of automatic valves designed to be used in combination with gas accumulators in lighthouses and buoys." A more recent example is that the field that I did my PhD and postdoc in (low-energy nuclear structure) has become moribund (in my opinion), because the experimental techniques have been exploited to the point of diminishing returns. I think accelerator-based particle physics is clearly a very extreme example of an experimental technique that has been exploited to the point of diminishing returns. We should not expect that any field of physics will remain healthy indefinitely: not lighthouse design, not low-energy nuclear structure, and not accelerator-based particle physics.

My guess is that there is a desert between the electroweak scale and the Planck scale, with absolutely nothing interesting in it. But maybe I'm wrong, so maybe it's worth funding at least some research into other approaches, e.g., ultra-high-energy cosmic rays.

I think bcrowell implied the opposite. Well, not quite, I would understand his comments to mean that no possibilities should be researched.

Well, if I was working at a US funding agency like DOE, I would recommend zero funding dedicated explicitly to quantum gravity research. If my recommendations were followed, it certainly wouldn't mean that all research on qg would cease. Once someone has tenure, you can't stop him from working on something that he finds compelling and that he can work on with nothing more than pencil and paper.

 

[atyy]

Fields of research are like plants and animals. They grow, mature, and eventually die. At the turn of the 20th century, grad students were writing PhD theses on the motion of tops on inclined planes, and a Nobel prizes in physics was awarded for "invention of automatic valves designed to be used in combination with gas accumulators in lighthouses and buoys." A more recent example is that the field that I did my PhD and postdoc in (low-energy nuclear structure) has become moribund (in my opinion), because the experimental techniques have been exploited to the point of diminishing returns. I think accelerator-based particle physics is clearly a very extreme example of an experimental technique that has been exploited to the point of diminishing returns. We should not expect that any field of physics will remain healthy indefinitely: not lighthouse design, not low-energy nuclear structure, and not accelerator-based particle physics.

My guess is that there is a desert between the electroweak scale and the Planck scale, with absolutely nothing interesting in it. But maybe I'm wrong, so maybe it's worth funding at least some research into other approaches, e.g., ultra-high-energy cosmic rays.


Well, if I was working at a US funding agency like DOE, I would recommend zero funding dedicated explicitly to quantum gravity research. If my recommendations were followed, it certainly wouldn't mean that all research on qg would cease. Once someone has tenure, you can't stop him from working on something that he finds compelling and that he can work on with nothing more than pencil and paper.

That sounds terribly pragmatic and common sensical. I certainly wouldn't argue too much with that. But whether string theory is a scientific theory, and worth investigating in principle (if we had unlimited resources) is a different sort of question, isn't it?

 

[Haelfix]

"was working at a US funding agency like DOE, I would recommend zero funding dedicated explicitly to quantum gravity research"

Certainly you can make a case that too much money has been spent on quantum gravity research in the past twenty years. I disagree, but nonetheless a case can be made.

Otoh, i'd point out that historically some of the most important results in all of physics including some Nobel prizes have come out of this line of research.

As an example, 'T Hooft and Veltman proved the renormalizability of Gauge theory as a 'warmup' before the quantization of gravity.

"But if the only guide is logical self-consistency, then it's not science, it's philosophy"

I disagree. There are many results in science that are not directly provable, but rather implications of theories that are proven. I don't need to setup an experiment on the moon to know that an apple would still fall there.

Of course you may point out that string theory is rather a larger stretch than claiming the universality of Newton's laws, but I'd retort by saying its a question of degree. String theory respects GR, it respects quantum mechanics and is in some sense the only *known* mathematical combination of the two that also has the capacity to explain all the details about the universe*.

(*Asymptotic safety is the other logical alternative, although it certainly has some theoretical issues. But regardless, it would be a rather unfortunate result for physics if it turned out to be true as there would be no hope of ever pinning down the dynamics of quantum gravity)

 

[inflector]

Asymptotic safety is the other logical alternative, although it certainly has some theoretical issues. But regardless, it would be a rather unfortunate result for physics if it turned out to be true as there would be no hope of ever pinning down the dynamics of quantum gravity

Why does Asymptotic Safety imply that one can't pin down the dynamics of quantum gravity?

 

[MTd2]

Of course you may point out that string theory is rather a larger stretch than claiming the universality of Newton's laws, but I'd retort by saying its a question of degree.

A big desert, maybe.

 

[MTd2]

Why does Asymptotic Safety imply that one can't pin down the dynamics of quantum gravity?

Not true. AS is a mathematical property yet to be proven of a classical theory. Quantum gravity would deviate the classical solution at some level, but proving that would be as hard as proving string theory.

 

[atyy]

If no money were spent on quantum gravity, I think string theory would still survive in condensed matter:)

 

[MTd2]

This is something I'd like ZapperZ to answer.

 

[Kevin_Axion]

If no money were spent on quantum gravity, I think string theory would still survive in condensed matter:)

Because of AdS/Condensed Matter duality?

 

[atyy]

Because of AdS/Condensed Matter duality?

Yes. There are also some incredibly cute things from like http://arxiv.org/abs/0902.3996 and the Cardy formula comes a statistical mechanician.

 

[Haelfix]

Not true. AS is a mathematical property yet to be proven of a classical theory. Quantum gravity would deviate the classical solution at some level, but proving that would be as hard as proving string theory.

Umm no. AS is a perfectly reasonable conjecture about the *quantum theory*. It will be as you say difficult to prove, but if they manage to keep finding computer evidence for it, as well as independant analytic analysis of various toy field theories, at some point evidence could hit a tipping point. We're still far from that though.

The ultimate problem is the dynamics will never be known unless you do Planck scale experiments. In other words you have to fix the physics of the constraint surface. Hence why I say that it would be unfortunate, and would in practise be the end of quantum gravity research (like Ben implies), b/c unlike other theories out there, there is no independant way to derive or guess what those parameters are.

Historically that was one of the big reasons people went for String theory in the first place. All of its free parameters were fixed, and in principle you could hope for a unique prediction once the vacuum selection principle was figured out

 

[MTd2]

AS is a property like perturbative renormalization is. A given theory may or may not have it. The difference between those it is that for the latter one can see it working as when coupling constants are small, while AS one needs to go to the highest energies, in the case of GR.

It seems that SM is also AS, but I am not sure under what conditions. It seems that Higgs field do have this property. Marcus can tell more than I.