What are unified theories trying to match at lower energies?

[crackjack]

Also the ambition of ST is higher, therefore the two things you compare aren't even fair competitors?

All the more reason to not strike off ST without striking off SUSY/GUT first.

Yes SUSY/GUT is not something we know, which means ST must connect directly to the more firm ground, which to me means agreement with the subset of predictions of the SM that is currently tested against experiment, nothing else.

Like I said ST can relay the baton only to SUSY/GUT which should then relay it further to SM.

 

[crackjack]

@ tom.stoer

What you say is all true, but such problems abound all theories under development - especially the very ambitious ones. In fact, if such issues (what I would call open problems) were not present, the theory would have been considered finished. They are all valid reasons for rejecting the theory (even partially) only when there are no-go theorems (with reasonable assumptions) which prevent these objectives from being achieved.

On the other hand, for each of your point, I can give you similar issues with SUSY/GUT. And yet, most of those who oppose ST do not oppose SUSY/GUT as much.

I am not defending ST, but just trying to understand if there are valid reasons (and what they might be) for writing off ST (without striking off SUSY/GUT).

 

[arivero]

SUSY/GUT is a minimal, from experiment upwards, program, and while you can argue that it does not predicts, its group theory helps to explain the structure of charges in the standard model. SUSY/GUT is not a program to pick a random lie group, in this sense its "landscape" is as big as the one of string theory, or biggest (at least string theory is limited to D<26 and G < E8xE8) and of course such approach would be absurd.

The weak points of S/GUT is, to me, the "U". It could be that the group theory chain is not really related to a traversal of a high energy desert of, how many, 20 orders of magnitude?

On the other hand, it is true that the big picture is that ST relays the baton down to SGUT and then down to SM. But perhaps the alternative way is that ST relays to KaluzaKlein and then ir to SM, without the "U" of GuT.

 

[Physics Monkey]

In my opinion, it would be totally unprecedented if the physics we observe at ordinary energy scales (even up to a TeV) were to somehow uniquely determine the structure of the world at arbitrarily high energies. I think nothing like this has ever happened before, and we don't really have a good reason to believe it will happen now. Nevertheless, this uniqueness myth is a part of string culture and has in the past motivated much string research (and perhaps still does). Now I don't want to take this cartoon characterization too seriously, but the influence is there. And we need look no further than the first chapter of Polchinski to find this uniqueness myth in all its glory (not that Polchinski really believes it). The amusing thing is that string theory destroyed its own myth with the stringy landscape.

Of course, we could in principle see Regge trajectories of strings at the LHC or at whatever comes after the LHC, but this seems unlikely. We may see SUSY or extra dimensions or who knows what, but except for actually seeing the strings, I predict that we won't need string theory to understand what's going on for a long time (except perhaps indirectly via ads/cft for example). And even if we assume strings, the presence of the landscape means we probably still won't be able to say much about what comes after the next generation of experiments.

It seems to me that string theory is in the process of changing form. String theory now appears less like a specific theory of the world, like the standard model, and more like a framework for understanding a large class of worlds, like the framework of quantum field theory. I think our reasons for studying string theory are changing as well, and similarly, our old expectation that string theory is simply right or wrong is also outdated.

 

[arivero]

Just a point, even without disputing the validity of your argument.

I predict that we won't need string theory to understand what's going on for a long time

I think that the orthodoxy, or at least Witten et alikes, do not expect string theory to be the main piece.

Lets take Weinberg's volume three. He explains that Supergravity forces the existence of a tensor A_{MNR} in 11 dimensions, and that this tensor has two roles:

- it stabilizes the compactification to 4+7 dimensions.
- its sources, via the "generalisation" of maxwell equations, are not charged point particles, not charged strings, but charged membranes.

Then Witten approach comes to tell us that when descending from 11, where sugra lives, to 10, where we can formulate chirality, this membrane, ie this source of the 11D tensor implied by supergravity, wraps to build strings, and that this is the object which is being classifiyed in 10D.

And then the usual lore.

But strings as fundamental objects are not there. What is fundamental is n-dimensional supersymmetry. In turn, you can look for mathematical uniqueness of this structure, n-supersymmetry, via Evans interpretation (as division algebras) or Duff "Brane scan"

 

[Fra]

All the more reason to not strike off ST without striking off SUSY/GUT first.

It sounds to me like you see only two ways here, this is what confuses me. I certainly see more than these two ways. Relative to my preferred way, ST is stil not "sufficiently" ambitious as it still relies on the old QFT framework.

I think there are two natural two fronts of research: the front starting from a minimally speculative experimental basis; and a front more radical that aims to address the deeper conceptual issues.

Like I said ST can relay the baton only to SUSY/GUT which should then relay it further to SM.

I guess I didn't get your arguments for this. If ST can't connect to actual experiments, what does that have to do with SUSY based GUTs? (which we agreed are also so far just a set of theories; and there is no unique connection point)

Also, my personal opinon is that a new program does not need to match the exact mathematical structure of the SM, since this is not proved, it's merely consistent with observations - so far, but as we know there are domains where we lack experimental data. One only needs to mathc the experimentally verified parts of the SM. It's quite possible that the symmetries in the SM, as well as the spacetime structure, can be explained as emergent in particular limits (this is my personal guess).

/Fredrik

 

[tom.stoer]

Let's look at other theories with similar problems as in string theory. In Loop Quantum Gravity there is a unique description of "spacetime" - unfortunately it's a spin network to which we can neither associate dimension, nor space and time as we know it. Therefore we expect that all these low-energy entities will emerge in some appropriate limit. Then we have similar problems as with string: We must show that these entities do really emerge - at least their experimentally known signatures. The key difference is that Loop Quantum Gravity does not rely on the emergence of "intermediate theories" like SUGRA, SUSY, etc. but that it tries to derive the low-energy physics directly from its fundamental entities. So if this works then it's a success for LQG, if it does not work, it's a failure of LQG.

In string theory you always have these "intermediate theories". The question is: what is string theory good for if it only generates these theories? What is the benefit?

 

[Fra]

No matter how this is turned around, there are I think no excuses for producing not something constructive that connects to measureable things. But I some problems might be impossible to answer in one ago, and then I see a sense in that the answer is another, but refined question, and in such a question the measure of success should be one of relative progress, or evolution.

Maybe we still don't understand the nature of the relation between a high energy view and a low energy view? I am at least very much in doubt about that.

I think of the so called high energy TOE picture, as a low complexity limit if you talk about the inside observer (ie matter). But this picture itself, has IMO no objective status, since this low complexity limit refers to another observer of higher complexity.

I think that part of the puzzle might be that there is no objective absolute meaning of energy scales, no more than there is an objective meaning of "complexity scales". Instead all we can do is try to understand the relation between the lowest and higher distinguishable scale in each case, and understand how this picture again scale as this "case" changes.

After all, the context of all our theories, are Earth based. This is the constraining factor for both particle experiments as well as cosmological observations.

I think we might need to start asking some of these "open questions" in a different way. We probably don't even quite understand what we ask, even though the questions seems simple to phrase, at least that's the case if I speak for myself :)

An implication of this thinking is that the idea of an objective universal super-symmetry that exists in a realist sense may simple be the wrong picture. The nature and meaning of the symmetries of the laws of physics may be more subtle that the conventional picture. I am increasingly more convinced that this is the case, and this is why several of the existing programs to me, are not just infruitful, they might even be considered to be asking the questions in a wrong (the old) way.

I think the revolution we need, is not an invention of another mysterious supergroup of some kind, it's rather a deeper understanding of the nature of symmetry.

/Fredrik