Why does Nima A-H say unlikely to tell us anything about particle physics?

[marcus]

Why does Nima A-H say “unlikely to tell us anything about particle physics”?

Nima Arkani-Hamed is a brilliant young phenomenologist (b.1972) who left a faculty position at Harvard this year to join the Princeton Institute for Advanced Studies. My impression is that the work he is best known for (which postulates large extra dimensions) has been string-inspired, so I think of him as one of a handful of stars in the greater string community.

I'd like it if someone could explain the physical reasons underlying his recent statement that string theorizing was “unlikely to tell us anything about particle physics”.

There must be some solid physics grounds underlying that remark, made in the closing talk of a string mini-symposium at Princeton yesterday. It's not the kind of thing he would have been saying 2 or 3 years ago. So what has changed?

Here is the relevant piece of Peter Woit's report on the symposium:
==quote==
Finally, Arkani-Hamed ended with the statement that string theory is useful as a way to study questions about quantum gravity, but “unlikely to tell us anything about particle physics”. This is an opinion that has become quite widespread among theorists, but news of this has not gotten out to the popular media, where the idea that string theory has something to do with the LHC keeps coming up...
==endquote==
http://www.math.columbia.edu/~woit/wordpress/?p=1285

Other people on yesterday's panel at Princeton were Michael Douglas, Tom Banks, and Nathan Seiberg.
Contextual information:
http://pcts.princeton.edu/pcts/bigbang/bigbang.html

 

[BenTheMan]

What does any theory of quantum gravity tell us about particle physics?

It's just a question of dimensional analysis, I think---even if we have a grand unified theory, the scale that things unify at is well below the Planck scale, which means that effects on GUT physics is in the less than a % range, just by looking at effective operators.

Conversely, I could ask: what does quantum field theory tell us about the standard model? Nothing---it doesn't predict masses, it doesn't predict coupling constants, it doesn't predict quarks or leptons or chiral symmetry breaking or CP violation or neutrino masses or (...) Quantum field theory is a set of rules: it says, if you want to make a theory of particle physics, this is what you need. It is the opinion of a growing number of people in the field that string theory is a set of rules for building consistent models of quantum gravity. Just like we never expected QFT to predict the standard model, neither should we expect string theory to do the same thing. QFT tells you about the structure of the SM, just as string theory will tell you about the structure of your quantum theory of gravity.

The reason that we don't know what to expect out of the LHC is not that we don't understand QFT. We understand QFT SO well that we can build thousands of theories for what we MIGHT expect. Is the fact that we don't know what we're going to see at the LHC a failing of QFT?

 

[BenTheMan]

And, without putting words into his mouth, Nima would call himself a physicist. He is interested in physics. I don't think he'd consider himself a string theorist.

And, rereading your first post more thoroughly, I see that Peter Woit has already explained the view point.

 

[Haelfix]

He probably just means that seeing Stringy quantum gravity signatures/echoes (other than weakscale SUSY) at the LHC is a longshot. Its always been a longshot and that's really nothing new. Its akin to figuring out galaxy formation using nothing but QCD physics and dynamics absent any knowledge about the gravitational force or electromagnetism. --eg good luck with that.

Even the stringy inspired models like RS, LED, quiver gauge theory, etc (as they stand today) would be elucidated more through effective field theory and precision measurement than any a priori fundamental string work (and no one knows how to rigorously embed them as they stand).

What he clearly does not mean is that string theory as a formalism is useless for particle physics. Thats already been falsified via the QCD/string relations, progress on nonperturbative YangMills theory, dualities and so forth.

 

[ensabah6]

What he clearly does not mean is that string theory as a formalism is useless for particle physics. Thats already been falsified via the QCD/string relations, progress on nonperturbative YangMills theory, dualities and so forth.

If the universe is non-SUSY and 4D, and elementary particles are not strings, would string theory as a formalism still be useful for particle physics via the QCD/string relations, progress on nonperturbative YangMills theory? In other words, I understand there exist "QCD/string relations" that has usefulness in HEP, but this could be true and string theory is the wrong physical theory, and "useless" in a more fundamental sense of providing the physically correct picture and laws.

 

[Haelfix]

Ensebah yes, that's clearly a possibility. String theory might not be the correct description of quantum gravity, but nevertheless be correct for describing various particle physics in a dual picture.

Another interesting and controversial possibility is that *every* weakly coupled gauge theory has some sort of stringy dual, at which point the whole thing ceases to really be an argument at all.

 

[Fra]

I think these questions puts the focus on something good, i have a question and maybe someone can answer this.

Let's suppose the string reasoning aspires on something profound, more than just a single theory, what can it be? This is the question I asked myself when I first heard about string theory.

It proposes a basic microstructure, and it's "simplicity" suggest a basic "natural action", who's possibilities should generate all possible mictrostructure and actions seen in nature. Ie. give a simlpe structure, there is certainly a bound to what can happened to it (kinematics). The dualities between strings, branes etc, are sort of a kind of way to show they the simple idea might generate more complex picture from the same fundamental picture. ie. unification. This in itself is not a bad idea at all, it's is not too far from my personal conceptual vision, but what about the details?

A problem IMO is that I don't think string theory seems to me at least, to satisfy this vision. This critics IMO sits at theoretical and conceptual level, not just a the level of lack of testable predictions, although I think the two critics are connected.

IF you start ask things like, what is the simplest possible distinguishable theory, what is the simplest possible distinguishable action and what is the context of that? Then if string theory is the answer to the mentioned profound vision, should we not at this level expect to find that the string corresponds to something elementary?

For example, if we really want to find an elementary object, and find out a corresponding natural action, isn't it somehow odd to suggest a continuous string (continuum) as elemetary?

This is the first thing that bugs me about string theory. Could it simply be, that this continuum pulled out of nowhere, is partly responsible for the landscape problem?

-> Has any string theorist tried to seriously merge this by trying to finding some "M-theory" in terms of string bits, that smolin I think has mentioned?

And the points would be to further decompose strings into something even more fundamental, which might naturally suggest that a string is just the limit of a collection of points, almost like a normalized discrete distribution. Maybe strings are the simplest possible CONTINUUM structure, that could make sense even to me. But the question is if there is physics going on, where the continuum notion itself isn't valid? I think this would be an interesting thing to investiage. This way it seems also easy to see how the bits can easily rearrange from a dense string to a thin branes etc, and at the same time the energy of their dynamics will be different.

One practical part is that one does not have to choose from a continuum of background measuers, at least one can resort to combinatorics. And the overall number N, put's a hard limit of the complexion numbers at any stange. This might still recover a continuum picture as N -> inf for those that insist on it. And long with the limit, we should get the correct limit of the continuum measures as well. I think this might also have a few things to say about thte fundations of QM, since if we do it this way, the concept of probability itself, must also be discretized. And the continuum is merely a limiting case.

At least to speak for myself, if I am to accept string reasoning as something bigger than just a theory, ie. as a model for how ANY theory of physics must be constructed - which in itself IMO is a grand visions I would support - there are many unasked questions in string theory, that I see as main motivators.

/Fredrik

 

[BenTheMan]

Another interesting and controversial possibility is that *every* weakly coupled gauge theory has some sort of stringy dual, at which point the whole thing ceases to really be an argument at all.

Do you mean---every weakly coupled gauge theory has a stringy completion? I'm a bit confused about "string dual" as used in this context.

 

[Haelfix]

Its just a conjecture I've heard people mumble about in the past during lectures. Eg every weakly coupled gauge theory might be dual to a string theory (not necessarily critical and sometimes with exotic backgrounds).

I don't know the status of that idea actually.

 

[p-brane]

To be clear, they were talking about the landscape, not string theory in total. As a reminder, the landscape was introduced to deal with the discovery that the universe is accelerating. The idea is to assume that this is some kind of vacuum energy and then attribute it to the vacuum energy of moduli in backgrounds where these are positive. But it may be that this attribution is incorrect and the origin of the observed acceleration is something we have not yet discovered. It may even be that the landscape is correct but for some reason the apparent positive cosmological constant isn`t actually do to the moduli even if they are stabilized at positive values. The gravitational physics of the vacuum must be understood before we can really assess the meaningfulness of the landscape. I think the ideas of Tom Bank`s are the most interesting on the subject of the landscape.

 

[BenTheMan]

I guess I'm confused about what "stringy dual" means. Presumably, you do something like g -> 1/g, and then use AdS/CFT or something?

 

[p-brane]

I guess I'm confused about what "stringy dual" means. Presumably, you do something like g -> 1/g, and then use AdS/CFT or something?

AdS/CFT is not a weak/strong duality. It is a duality between large N super Yang-Mills, which is a weakly couple gauge theory, and stringy supergravity. I think Haelfix was just saying that all weakly coupled gauge theories may have some sort of stringy dual, though they won`t in general be captured by AdS/CFT or in terms of stringy supergravity.

 

[Coin]

To be clear, they were talking about the landscape, not string theory in total.

Well... this was something I was wondering about. In Woit's description of what happened, A-H started out talking about the landscape and the anthropic principle (the subject of the panel), and voiced (as more than one person there did) the opinion that anthropics is relevant to the cosmological constant but doesn't make sense to apply to low-scale physics problems like EWSB. But then after making these remarks, as per Woit's report, A-H closed with a comment that broadened the scope to string theory in general and suggested string theory should be viewed as a tool for studying QG rather than particle unification. It is this (somewhat more controversial) comment that grabbed Marcus' attention.

Of course we have only Woit's paraphrasing to go on, so it is possible he just misunderstood A-H's intent? I haven't found any reports on this roundtable except the one at Not Even Wrong.

 

[ensabah6]

Ensebah yes, that's clearly a possibility. String theory might not be the correct description of quantum gravity, .

I was thinking of HEP -- gravity may be described by GR geometrically, but gravity itself may not be geometry but something fundamentally different.

 

[arivero]

I think that the point is that string theory formulated as a theory of everything is simply a theory able to fuse gravity and GUT QFT, but unable to predict the parameters of GUT QFT. Furthermore, GUT QFT itself is not guaranteed to be able to predict the mass parameters of Nature QFT; it is a theory of the coupling constants, and it relates to the mass only via the Higgs mechanism, presently still unconfirmed.

Or course, the door is open for string theory, as a theory of unique structures, to be used in other way, closer to High Energy Physics. But there is not any orthodox way to do it. The QCD string is not a superstring theory (albeit some crackpot -me- likes to assert the contrary). As for Gauge/AdS or QCD/CFT or whatever dualities, they do not focus in the foundational part of particle theory. Other dualities (tell g->1/g) could be used to select fairly unique Quantum Field Theories, but nobody has given a connection to the Standard Model. And besides, it is not really string theory, it is just another game that people who does strings happens to like too.

 

[marcus]

There's been a big change of mind, so I'm asking what is the underlying physics could have motivated that. Let me repeat the question.

It's not the kind of thing he would have been saying 2 or 3 years ago. So what has changed?[I mean objectively, to motivate the change of mind.]

Here is the relevant piece of Peter Woit's report on the symposium:
==quote==
Finally, Arkani-Hamed ended with the statement that string theory is useful as a way to study questions about quantum gravity, but “unlikely to tell us anything about particle physics”. This is an opinion that has become quite widespread among theorists, but news of this has not gotten out to the popular media, where the idea that string theory has something to do with the LHC keeps coming up...
==endquote==

According to the quote, he is not merely talking about string landscapery here, because the main stringy tool to study quantum gravity is AdS/CFT and stuff like that, not the landscape. As Coin pointed out, he is making a more general statement.

... In Woit's description of what happened, A-H started out talking about the landscape and the anthropic principle (the subject of the panel),... But then after making these remarks, as per Woit's report, A-H closed with a comment that broadened the scope to string theory in general and suggested string theory should be viewed as a tool for studying QG rather than particle unification..

...What he clearly does not mean is that string theory as a formalism is useless for particle physics. Thats already been falsified via the QCD/string relations, progress on nonperturbative YangMills theory, dualities and so forth.

That's right. He was talking about superstring/M itself, not math formalism spin-offs. Arivero commented on this. The QCD string is using some mathematical techniques, some spin-off formalism---but is not properly superstring/M. The math formalism used as a technique of calculation (to solve other problems without addressing fundamental issues) could be called "stringalism". Here's a relevant part of Arivero's comment.

...Or course, the door is open for string theory, as a theory of unique structures, to be used in other way, closer to High Energy Physics. But there is not any orthodox way to do it. The QCD string is not a superstring theory...As for Gauge/AdS or QCD/CFT or whatever dualities, they do not focus in the foundational part of particle theory...

We should try not to get distracted by side issues and miss the main point. Here's a comment from Woit's blog that I think captures the essence:

==quote from Sumar Ongi==
I may be wrong, but I think not too long ago Arkani-Hamed was actively working with landscape-inspired models. (I even vaguely remember his calling this kind of work “staring the monster in the face”, or something like that, in an interview.) If so, his change of opinion about those models (and about the relationship between ST and Particle Physics) is quite remarkable. More generally, it seems to me that all the opinions quoted above from well known theorists about these matters reflect a tidal change of mind in the string community.
==endquote==
http://www.math.columbia.edu/~woit/wordpress/?p=1285#comment-45748