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Why is String Theory Considered to be a Scientific Theory?

by inflector
Tags: considered, scientific, string, theory
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mitchell porter
#19
Oct9-10, 12:19 AM
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Quote Quote by inflector View Post
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_..._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
#20
Oct9-10, 07:00 AM
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Just to add my opinion, without bashing or not bashing ST.

Quote Quote by inflector View Post
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
#21
Oct9-10, 10:38 AM
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Quote Quote by mitchell porter View Post
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
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Oct9-10, 01:15 PM
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Quote Quote by Fra View Post
(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
#23
Oct9-10, 03:04 PM
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Quote Quote by atyy View Post
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 extremly 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
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Oct9-10, 09:37 PM
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Quote Quote by atyy View Post
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.

Quote Quote by atyy View Post
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
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Oct9-10, 09:51 PM
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Quote Quote by bcrowell View Post
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
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Oct9-10, 10:14 PM
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"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
#27
Oct9-10, 10:21 PM
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Quote Quote by Haelfix View Post
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
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Oct9-10, 10:32 PM
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Quote Quote by Haelfix View Post
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
#29
Oct9-10, 10:43 PM
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Quote Quote by inflector View Post
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
#30
Oct10-10, 12:36 AM
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If no money were spent on quantum gravity, I think string theory would still survive in condensed matter:)
MTd2
#31
Oct10-10, 12:38 AM
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This is something I'd like ZapperZ to answer.
Kevin_Axion
#32
Oct10-10, 01:14 AM
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Quote Quote by atyy View Post
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
#33
Oct10-10, 01:24 AM
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Quote Quote by Kevin_Axion View Post
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
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Oct10-10, 12:13 PM
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Quote Quote by MTd2 View Post
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
#35
Oct10-10, 01:01 PM
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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.
atyy
#36
Oct10-10, 01:12 PM
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Quote Quote by MTd2 View Post
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.
Maybe.
http://arxiv.org/abs/0901.2459
http://arxiv.org/abs/0912.0208


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