Why is String Theory Considered to be a Scientific Theory?

[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 independent 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 independent 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.

 

[atyy]

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

 

[Haelfix]

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.

Asymptotic safety is the conjectured existence of a nontrivial (non Gaussian) fixed point controlling the quantum UV behavior of Einstein Hilbert gravity. The theory is not perturbatively renormalizable in the power counting sense. However under exact renormalization group flow presumably its divergence structure simplifies in the vicinity (IR repulsive orbit) around the fixed point such that the parameter space of the infinite amount of counterterms develops relations between them and hence the theory space becomes finite.

A nonrenormalizable theory would require an infinite amount of measurements at each scale in order to derive a prediction, whereas here because of the aforementioned relations between the coupling constants the theory simplifies and all you need are a finite amount of measurements. In short you get a predictive theory.

Of course, you still have to *do* those measurements, in order to pin down the nature of the fixed point set. Otherwise you are forever doomed to wander around any (infinite) number of UV repulsive directions.

As for whether the Standard model is asymptotically safe. The answer is no, the standard model is a renormalizable theory. There are however proposals for a Higgsless completion for the electroweak nonlinear sigma model that exploit the use of a fixed point.

 

[MTd2]

There are 2 factors that one needs to keep track, the cosmological and Newton "constants". Those form a surface to which all those infinite coupling "constants" converge too. So, it is really not impossible to find the asymptotic safe point :).

If you want to call a mathematical property a theory is up to you.

As for the SM, isn't there a divergence due the value of Higgs?

 

[Haelfix]

There are 2 factors that one needs to keep track, the cosmological and Newton "constants". Those form a surface to which all those infinite coupling "constants" converge too. So, it is really not impossible to find the asymptotic safe point :).
If you want to call a mathematical property a theory is up to you.
As for the SM, isn't there a divergence due the value of Higgs?

No that's not necessarily correct. The critical surface dimensionality in general depends on how many attractive directions there are. That is something that only experiment can conclusively tell you. In the approximation where you truncate all the higher derivative terms (generated by radiative corrections) and only keep the EH part, then presumably the critical surface is obviously at most 2 dimensional (depending on if the CC and Newtons constant are attractive, which they showed to be true in the original papers). In general for arbitrary many matter couplings and for arbitrarily many derivative terms, the surface dimensionality will vary in principle or is unknown. However the good news is that you only have ot make a finite number of experiments.

"As for the SM, isn't there a divergence due the value of Higgs"

Ummm...

 

[relativityfan]

hi, I wonder how popular string theory is?. it is a beautiful theory but it receives often criticism, is there any poll for the popularity regarding whether string theory is the right direction for quantum gravity?

 

[marcus]

I don't know of any popularity poll. Who would be the best group of respondents to such a poll? Maybe past winners of the Nobel prize in physics?

Here is an interview on Bloggingheads with Frank Wilczek (a prominent theorist who shared the Nobel for quantum chromodynamics)
http://bloggingheads.tv/diavlogs/30297
It has a six minute segment you can jump to that gives his view of string---"it's not really a theory" (in the usual sense of a bunch of equations and a mapping between the equations and reality).

This goes quite a ways towards answering the topical question--Why is String Theory Considered to be a Scientific Theory?--that started the thread.

If you listen to Wilczek, a central mainstream figure in theoretical particle physics, then your basic conclusion is it is NOT considered to be a scientific theory, by people who know what they are talking about.

That's all right---it's work in progress, an actual scientific theory in the conventional sense might eventually emerge. And there are branches of mathematics normally called "theory" that have no direct connection with the natural world: that are not scientific theories in the sense Inflector set out at the start of the thread.

My take on this is that it would help if people would try to speak precisely and acknowledge that it is NOT a scientific theory---that it is rather a body of mathematical theory with application in various areas but which has not, as yet, produced a theory of nature by accepted standards of Physics. I take Wilczek's word on this.

 

[negru]

Funny how he doesn't like these extra dimensions but he's teaching a susy course at mit

 

[MTd2]

Going from SUSY to extra dimensions is a big stretch for a lot of people.

 

[negru]

Susy is extra dimensions, they're just fermionic

 

[MTd2]

Susy is extra dimensions, they're just fermionic

I don't understand. Extra dimensions means those from superstring theory.

 

[negru]

Extra dimensions means any dimensions beyond the usual. If you make them fermionic, you get susy.

That's why susy is an extension of poincare (spacetime) symmetry, not an internal symmetry.

 

[MTd2]

When someone says dimensions, I think of the configuration space. Wilczec probably also means this.

 

[tom.stoer]

One can write SUSY using superspace / fermionic extra dimensions. That's rather formal.

But if you look at the bosonic string in 26 dimensions, you will see that there is NO differenc between the first 4 and the the remaining 22 dimensions. That's what at least at this level there's a big differenc between the fermionic dimensions from SUSY and the bosonic dimensions in ST.

I think talking about these dimensions is misleading. What about the following: In QCD one has eight gluons, i.e. an additional index a=1..8 for the gauge field; do you think that the gauge field lives in 4*8 = 32 dimensions?

 

[negru]

But you're mixing internal symmetries with spacetime symmetries, so it's not the same thing.

What I was trying to point out is that (at least to me) susy is more far fetched than usual extra dimensions.

What nature chooses that's her business. I'm just talking about the complexity of extra dimensions. In this sense, susy is more complicated than extra bosonic dimensions. So unless you have any reason to suspect that nature doesn't have bosonic dimensions, you don't have a case. Just like extra dims are small and you need high energy to see them, susy is broken and requires high energy to see. Same thing from a practical point of view, but one idea is crazier than the other.

 

[tom.stoer]

But with SUSY you are not _forced_ to use a notation (superspace) involving dimensions. That's all I am saying.

 

[MTd2]

Dimensions with anyonic dimensions...

 

[negru]

But with SUSY you are not _forced_ to use a notation (superspace) involving dimensions. That's all I am saying.

But it's equivalent, so it's the same thing.

 

[marcus]

Funny how he doesn't like these extra dimensions but he's teaching a susy course at mit

Going from SUSY to extra dimensions is a big stretch for a lot of people.

Where, in the interview, does Wilczek say that he "doesn't like these extra dimensions"?

It seems to me that Wilczek is trying to speak accurately in this interview and to avoid expressing dislike, or any kind of biased emotional attitude.
The point is not to express a leaning "for" or "against". You trivialize things when you interpret everything in terms of attitudinal push and pull. There are facts and distinctions that need to be clarified.

We have to learn how to live with a certain confusion of language, which Inflector has pointed out: it is confusing to call it "string theory" because it is not a physics theory in the usual sense.

It does not demean or diminish string research activity or string math to say this. It does not need to evoke immediate defensiveness, excuses, obfuscation, distraction, testimonials to how hopeful and promising, paeans to greatness, attacks on rivals, etc. All that stuff is irrelevant to the main issue.

 

[inflector]

Thanks for the link to the bloggingheads.tv episode. I had seen that one but forgot that they addressed string theory specifically. The point I remembered was about 39 minutes towards the end of the section: "Light, matter—what’s the difference?" where they had this conversation after Wilczek started explaining about the Higgs field and Wright noted that is sounded like he was talking about aether:

WRIGHT: Weren't we taught in junior high school that the idea of the aether was discredited?

WILCZEK: The aether was never discredited. The aether is more dominant now in physics than ever. There were some very specific detailed models of what this medium—that we call empty space and see as emptiness in everyday life—is that had flaws and they were discredited.

But our modern physics is absolutely built from A to Z on the idea that space is a rich medium full of structure.

WRIGHT: So there is no empty space?

WILCZEK: There is no empty space in the sense of void. Space is full of fields that can vibrate that have spontaneous activity and really that's the primary ingredient of reality. Particles are just excitations of this medium, kind of bubbles or waves inside this medium which is always there and everywhere. It's really in modern physics the primary ingredient of reality. Particles or what could be considered ordinary matter are just secondary manifestations.

I found that to be an absolutely fascinating perspective and one that I hadn't seen put in those terms before.

My take on this is that it would help if people would try to speak precisely and acknowledge that it is NOT a scientific theory---that it is rather a body of mathematical theory with application in various areas but which has not, as yet, produced a theory of nature by accepted standards of Physics. I take Wilczek's word on this.

That would be my take as well. Doing otherwise actually hurts the cause of science in ways that those immersed in it cannot fully comprehend.

If you look at Robert Wright's puzzled look when he talks about extra dimensions and such, that's a good indication of the way that String Theory comes across to a non-believer. And Wright is a very smart guy, he's no dummy when it comes to science. But his eyes glaze over when he thinks about the extra dimensions of String Theory.

To a non-scientist, String Theory comes across as extremely complicated and very disconnected from everyday reality. Pictures of the Calabi-Yau shape (like http://members.wolfram.com/jeffb/visualization/calabi-grid.gif) might be pretty but they don't help make String Theory seem accessible or relevant to normal life. When the public at large doesn't understand science or even feel like that they could possibly ever do so, this can't help the overall budgets for science during times of economic pressure. Witness the pending budget cuts in the UK.

So it seems like a bad idea to spend a lot of time and effort publicizing a theory that isn't even actually a scientific theory yet. All the popular science books on String Theory haven't helped the cause of physics in the larger world, IMHO. The hype doesn't help over the long term.

The popular science web sites are even worse in this regard, every single paper related to quantum mechanics is one that "might possibly help scientists build quantum computers to solve every computer problem in microseconds" and every materials science paper is one that "might lead to radically new materials that will let us build bridges to the moon and elevators to space." After a while it all sounds like BS and that scientists are mostly full of BS. Even when it is not the scientists doing the hyping, in general.

Far better, to acknowledge the reality of the current state of affairs. It may not sell as many books and it may not get as many readers for your blog entry, but telling the truth won't hurt the credibility of science itself.

That way when the world needs to trust science, like say when we are warming up the planet and science points to human development as the cause of the problem, and science also predicts a future with enormous problems that we can avoid if we act in time, that way during these times of crisis, the public at large might actually believe what the scientific consensus says and act.

 

[inflector]

It does not demean or diminish string research activity or string math to say this. It does not need to evoke immediate defensiveness, excuses, obfuscation, distraction, testimonials to how hopeful and promising, paeans to greatness, attacks on rivals, etc. All that stuff is irrelevant to the main issue.

Yes, that's it exactly.

 

[atyy]

Compactification of the twisted heterotic string
Nair VP, Shapere A, Strominger A, Wilczek F
Nuclear Physics B, 1987

Resolution of cosmological singularities in string theory
Larsen F, Wilczek F
Physical Review D, 1997

An action for black hole membranes
Parikh MK, Wilczek F
Physical Review D, 1998
"Here we use the tree-level effective action obtained from string theory after compactification to four macroscopic dimensions."

Saltatory relaxation of the cosmological constant
Feng JL, March-Russell J, Sethi S, Wilczek F
Nuclear Physics B, 2001
"...Microscopic considerations from string/M-theory suggest two major innovations in the framework. ... By requiring stability on the scale of the lifetime of the universe, rather than absolute stability, we derive a non-trivial relation between the supersymmetry breaking scale and the value of the cosmological term. It is plausibly, though not certainly, satisfied in Nature."

 

[atyy]

That way when the world needs to trust science, like say when we are warming up the planet and science points to human development as the cause of the problem, and science also predicts a future with enormous problems that we can avoid if we act in time, that way during these times of crisis, the public at large might actually believe what the scientific consensus says and act.

Yes, of course. Nonetheless, one should separate questions of science and sociology.

http://insti.physics.sunysb.edu/~siegel/vs.html "Don't blame the product for the advertisement. String theory has been grossly over-sold. It isn't even a "theory" yet, just a "model". It hasn't solved anything, much less everything. But just because it isn't everything doesn't mean it isn't anything. It has many interesting features, some of which have been reproduced in particle theory and proven useful, some of which haven't but would be desirable in a more realistic theory."

BTW, Warren Siegel has the other famous index complentary to Baez's http://insti.physics.sunysb.edu/~siegel/quack.html

Also, the guy who wrote the crackpot index you mention also writes in "Calabi-Yau Manifolds and the Standard Model" http://arxiv.org/abs/hep-th/0511086 "This is rather striking, because string theory focuses attention on spacetimes of this form, at least if we work in the Euclidean signature."

And notice Siegel does not say "string theory is not a theory", he says "string theory is not a "theory"".

He specifically says "At this point in time, there are no more-promising solutions to many of the problems of high-energy physics. Of course, alternatives exist, and you are welcome to try some (I do), but most of the complaints leveled against string theory can be applied to them even more strongly. (In particular, watch out for theories that claim to solve some problem simply because it is too difficult to even see if they have that problem.)"

 

[negru]

That way when the world needs to trust science, like say when we are warming up the planet and science points to human development as the cause of the problem, and science also predicts a future with enormous problems that we can avoid if we act in time, that way during these times of crisis, the public at large might actually believe what the scientific consensus says and act.

Not that it's related, but I personally don't want people to blindly follow science, nor scientists to tell people what they should do. Because this will necessarily lead to the corruption of science. Science hasn't worked so far because the scientists are perfectly moral and truth seeking creatures. It worked exactly because scientists only put forward their ideas, and whoever was interested bought them. If the science they bought was correct, they made use of it, and that ensured the success of correct science.

So if we want people to trust science, we'd best keep to presenting objective truth (as best as we can) with absolutely no "advice" attached.

 

[arivero]

One can write SUSY using superspace / fermionic extra dimensions. That's rather formal.

But if you look at the bosonic string in 26 dimensions, you will see that there is NO differenc between the first 4 and the the remaining 22 dimensions. That's what at least at this level there's a big differenc between the fermionic dimensions from SUSY and the bosonic dimensions in ST.

I think talking about these dimensions is misleading. What about the following: In QCD one has eight gluons, i.e. an additional index a=1..8 for the gauge field; do you think that the gauge field lives in 4*8 = 32 dimensions?

First, you are not doing the account properly. Each gluon is a generator of symmetries, associated to a real parametero, so there are 8 dimensions. You could always to do a group G living in GxG, so the answer naive is 16 dimensions for QCD.

But you can ask what is the minimum dimension where it can live as spacetime symmetry. The answer is that you must quotient by a maximal nontrivial subgroup, for QCD it is SU(3)/ SU(2)xU(1) (just a coincidence, as far as we know). So from the 8 gauges you must not multiply times 2, but substract 3 and 1, and then you get that SU(3) can live a an spatial symmetry of a 4 dimensional manifold. An explicit example is CP2.

 

[Haelfix]

The fog is getting thick once again in this thread.

Whether string theory is a 'theory' or not is nomenclature (you can call it a research direction or a program). It does however make falsifiable predictions (just at a scale that we will never measure), it is both consistent and unique as a mathematical structure, there are observations that would instantly favor it over others (for instance KK resonances at the LHC or cosmic strings ) and there are theories that presumably do live in the swampland (eg D=4 N=2 Supergravity)

The point being if any theory in quantum gravity deserves to be called a 'theory', then this is it.