LHC, supersymmetry and strings

[Demystifier]

One of the things that LHC will experimentally search for are supersymmetric partners of the Standard-Model particles. Maybe it will found them, maybe it will not. But what if it will? The existence of supersymmetry does not imply the existence of superstrings. However, string theory is consistent only if it is also supersymmetric. In other words, string theory implies supersymmetry, that is, the existence of supersymmetric partners.
Thus, if LHC finds supersymmetry, one of the predictions of string theory will be experimentally confirmed. If that happens, will you give more credit to string theory? Will you start to think that string theory is more likely to be correct than you thought so far?

 

[ccdantas]

Yes, I'd find it (a little) more appealing. But I do not agree with you that ST predicts supersymmetry. As far as I understand it, supersymmetry is a precondition for constructing ST. I'd really find it much more appealing if ST could offer clear and testable predictions, that is, if it could be falsifiable.

 

[josh1]

I do not agree with you that ST predicts supersymmetry. As far as I understand it, supersymmetry is a precondition for constructing ST.

Can you clarify the difference between prediction and precondition?

Yes, I'd find it (a little) more appealing. I'd really find it much more appealing if ST could offer clear and testable predictions, that is, if it could be falsifiable.

So if astronomers discovered a gigantic cosmic string (this is edward witten's favourite way of varifying string theory) you would be only a bit less dismissive of string theory unless someone could provide a way to falsify string theory?

 

[arivero]

So if astronomers discovered a gigantic cosmic string (this is edward witten's favourite way of vE?rifying string theory)

So now Witten is astronomer? I'd thought his favorite way should be to find some proof of 11-dim gravity in a way such that the extra seven dimensions are acted by SU(3)xSU(2)xU(1).

 

[Thomas Larsson]

IF the LHC finds superpartners, it would be a great triumph for string theory. However, that is a very big IF. So far, all experiments looking for SUSY effects have seen exactly nothing. In fact, only in the past two days there have been four eprints reporting on null searches for SUSY, namely

http://www.arxiv.org/abs/0710.2372
http://www.arxiv.org/abs/0710.2378
http://www.arxiv.org/abs/0710.2502
http://www.arxiv.org/abs/0710.2897

 

[ccdantas]

Can you clarify the difference between prediction and precondition?

precondition: a condition that must be fulfilled before other things can happen or be done
prediction: an indication of the outcome of an experiment in a laboratory setting (or based on the observation of a phenomenon in nature)

So if astronomers discovered a gigantic cosmic string (this is edward witten's favourite way of varifying string theory) you would be only a bit less dismissive of string theory unless someone could provide a way to falsify string theory?

I'm not waiting for the day that ST will be falsified. I'm only expecting that ST can be formulated as a falsifiable theory, like any theory is supposed to be. There is a big difference between the two. Think about it.

 

[Fra]

I think I objected to this in another thread. If you take the view that ST really isn't a theory (that makes straight predictions), but rather a framework with contains many possible (but somewhat specific) theories or types of theories, then the falsification should be applied to the framework, because there is no theory. It's a framework of theories.

So what does falsification of a framework possibly mean?

Perhaps that no successful theory exist in the framework? seems reasonable?

But then the "string strategy" really is seriously incomplete unless this framework is equipped with a method or principle that allows theories to evolve or be selected in the framework.

Strongly constrained but pre-tuned (narrowed down) frameworks can do with a random selection and still be very fit, but as the framework inflates in complexity the entire strategy stalls unless there is a guiding principle. That's my opinion at least.

Given we have such strategy, and the framework as such is good, it seems the falsification should be replaced by a quantiative measure of support, where a good strategy is a "quick learning" strategy or a "quick selection".

I figure that in this way, the efficient strategy (good framework and good adaption) is the winning one that will dominate, all others will be less frequent.

I think this applies not only to human endavours but also to how evolution in nature works. The poppian thinking of falsification feels sort of out of date.

/Fredrik

 

[Demystifier]

I'm not waiting for the day that ST will be falsified. I'm only expecting that ST can be formulated as a falsifiable theory, like any theory is supposed to be. There is a big difference between the two. Think about it.

But string theory is already formulated as a falsifiable theory. For example, it clearly predicts that there are particles with masses of the order of the Planck mass. The fact that experimentalists are not able to verify this because their accelerators are not strong enough does not make string theory unfalsifiable. If such energies were experimentally available but such particles were still unseen, that would be a clear experimental proof that string theory was wrong.

The problem, of course, is to find a LOW energy prediction of string theory. But LQG, for example, suffers from the same problem. I do not understand why it is considered a drawback of string theory but not of LQG. Double standard?

 

[josh1]

precondition: a condition that must be fulfilled before other things can happen or be done prediction: an indication of the outcome of an experiment in a laboratory setting (or based on the observation of a phenomenon in nature)

If supersymmetry is as you say a “precondition for constructing string theory”, string theory requires supersymmetry to actually exist in nature to make sense. If supersymmetry doesn`t exist in nature, string theory can never make any sense as a physical theory since it is as you say a “precondition for constructing string theory”. Therefore should one not say that string theory does indeed predict supersymmetry, since supersymmetry is as you say a “precondition for constructing string theory”?

There is a big difference between the two. Think about it.

IMHO, there is no substantial difference between the two in how you’re using them here.

I'm only expecting that ST can be formulated as a falsifiable theory, like any theory is supposed to be.

Suppose a theory was falsifiable in principle, but not in practice. Would this satisfy you, because it’s possible that this may turn out to be the case. It`s even logically possible that there are correct theories of our universe that are not falsifiable even in principle. It may turn out that the only basis on which our belief in a theory can ever rest is it’s explanatory power. I hope this does not turn out to be the case, but how can we be sure it won`t?

Is this the main source of your unease with string theory as far as you understand it?

 

[ccdantas]

The problem, of course, is to find a LOW energy prediction of string theory. But LQG, for example, suffers from the same problem. I do not understand why it is considered a drawback of string theory but not of LQG. Double standard?

I think it is a problem for both: ST and LQG. And I never said otherwise.

Although I find LQG approaches more interesting (because my background mostly comes from GR, not particle physics), it is not the case that I find LQG less problematic than ST.

Actually, if I would go to work on quantum gravity, I'd study both approaches (their main fundamentals), find useful tools and ideas from them, but I'd also think about other ideas. In special, I'd go far back into their ontological status. I'm starting to feel like I am a 'structural realist' when it comes to how our scientific knowledge evolves over difficult matters.

 

[ccdantas]

Suppose a theory was falsifiable in principle, but not in practice.

Well, I suppose there are many currently accepted definitions of what science is.

I am a professional scientist (an astrophysicist turned into software engineer turned into condensed matter physicist) and use the traditional scientific method in my daily work.

But I respect other interpretations. So it's fine for me if there are people working under the frame of 'a theory falsifiable in principle'. I myself crossed a bit that territory some times.

 

[ccdantas]

One thing that is important is to have a clear idea of what you are doing. Is it science or not? What is science?

http://egregium.wordpress.com/2007/06/11/what-is-science-for-me/

http://egregium.wordpress.com/2007/06/08/what-is-science-for-you-up-to-50-words/

 

[josh1]

I am...an astrophysicist turned into software engineer turned into condensed matter physicist

Were these first or second order phase transitions?

 

[marcus]

...
The problem, of course, is to find a LOW energy prediction of string theory. But LQG, for example, suffers from the same problem. I do not understand why it is considered a drawback of string theory but not of LQG. Double standard?

Demy, isn't it true that string is a framework within which various theories with various theories might be constructed? Perhaps it would help to restate the problem. It might be too much to ask that there be a collective prediction that would refute the whole philosophy. But I leave that up to you since you know more about string thinking.

In any case I would say that LQG stands for a bunch of (different but related) approaches.
Different theories within the LQG cluster might make predictions and be falsifiable. The problem (as I would restate it in the LQG case) is to take one particular approach and see if you can refute it, experimentally or otherwise.

For example the approach that Lee Smolin has been working on since 2005 (and a halfdozen other people) depends critically on 4D. If evidence of extra dimension were seen this would shoot it down.

It also has only the ordinary particles---no room for partners. So if evidence of SUSY were found, that would shoot it down.

 

[Demystifier]

Demy, isn't it true that string is a framework within which various theories with various theories might be constructed?

In any case I would say that LQG stands for a bunch of (different but related) approaches.

I would say there is no much qualitative difference between strings and loops in that regard. The difference is only quantitative, in the sense that the string approach contains a much larger number of (different but related) possibilities. This is the price payed for the fact that the string program is much more ambitious than the loop program.

If the ONLY goal of the string approach was to have a consistent quantum gravity, then I would certainly prefer loops over strings. But the string approach is much more than a quantum theory of gravity.

 

[Demystifier]

Anyway, marcus, would YOU find strings more appealing if supersymmetry would be found by LHC?

 

[marcus]

...This is the price payed for the fact that the string program is much more ambitious than the loop program.

If the ONLY goal of the string approach was to have a consistent quantum gravity, then I would certainly prefer loops over strings. But the string approach is much more than a quantum theory of gravity.

I didn't realize that you think the "loop program" is only to make a consistent quantum gravity. If you think that, then naturally you would suppose that the string program is much more ambitious.

 

[josh1]

...isn't it true that string is a framework within which various theories with various theories might be constructed?

As has been repeated many times in this forum, string theory appears to be based on a unique but as of yet undiscovered set of principles with different solutions describing different phenomenologies.

…LQG stands for a bunch of (different but related) approaches.

I too use the term LQG this way. But I’m unaware of any serious LQG analog of the developments in string theory that suggest that these approaches are parts of the same theory. This shouldn’t be surprising though since IMHO research in LQG has always been characterized much more by free-wheeling invention than natural discovery.

 

[Demystifier]

As has been repeated many times in this forum, string theory appears to be based on a unique but as of yet undiscovered set of principles with different solutions describing different phenomenologies.

Are you talking about dualities and the hypothetic M-theory? I think it is more honest to say that "it is believed" or that "there is some evidence", rather than "appears to be".

 

[josh1]

Are you talking about dualities and the hypothetic M-theory?

Why? Have you ever heard of anyone arguing for the uniqueness of string theory on some other basis?

I think it is more honest to say that "it is believed" or that "there is some evidence", rather than "appears to be".

I disagree.

 

[Demystifier]

I didn't realize that you think the "loop program" is only to make a consistent quantum gravity.

What are the other goals of the loop program?

 

[Demystifier]

Have you ever heard of anyone arguing for the uniqueness of string theory on some other basis?

No. But have you ever heard of anyone saying that there is a PROOF that the single unifying M-theory exists?

 

[marcus]

As has been repeated many times in this forum, string theory appears to be based on a unique but as of yet undiscovered set of principles with different solutions describing different phenomenologies.

That is a nice idea. When the "unique but as of yet undiscovered" set of principles is discovered then there will actually be a string theory (not just a philosophy or ideology or intellectual framework). And THEN it may actually be possible to test the whole shebang and falsify it.

I like that prospect and hope you are right. Regretfully, so far it only "appears" to be based on some unknown but imagined set of principles, as you say.

I too use the term LQG this way. But I’m unaware of any serious LQG analog of the developments in string theory that suggest that these approaches are parts of the same theory...

There is Martin Reuter's QEG (asymptotic safe quantum gravity). There is Renate Loll CDT. There is Lee Smolin dynamical knotted graph combination of particles and geometry. There is Laurent Freidel combination of spinfoam and Feynman diagram (also putting matter and geometry in one format.) There is the approach pursued by Derek Wise and others (matter and geometry). Then there is the current attempt to merge Alain Connes NCG Standard Model with something like the conventional LQG setup.

Many of the approaches being actively pursued combine matter (e.g. the Standard Model basket of particles) with geometry. Some, like Reuter QEG do not. There is no easy way to see them all as related. Sometimes people point out analogies and generalize. Daniele Oriti has an approach which subsumes several different approaches.

But I would tend to agree with Josh that there is no one "LQG program" that one can describe. Also it seems to me that the different lines of research presented say at Loops '07 are exceedingly ambitious. Look at Garrett Lisi's work for example. Or Lee Smolin's.

So I would tend to disagree with Demy here---because he talks about a single "LQG program" and seems to think it does not involve the Standard Model. All these different people do not have a single program, I would say, but at least the majority clearly have a single GOAL.
That involves defining a quantum state of geometry-and-matter.

I cannot speak for the individual programs of course but personally I do think several of these current efforts would be destroyed if evidence were found of extra spatial dimensions. It would not be simply a matter of changing a few numbers and presto our theory applies to higher dimension.

 

[marcus]

What are the other goals of the loop program?

There is no one single "loop program", Demy, as far as I know. There are the various research lines presented at the Loops '07 conference. The common goal---please read my preceding post #23---of several of the most active lines of research seems to be a quantum theory of geometry and matter. Defining a dynamical state of geometry+matter.

 

[josh1]

That is a nice idea. When the "unique but as of yet undiscovered" set of principles is discovered then there will actually be a string theory (not just a philosophy or ideology or intellectual framework).

Er…this isn’t exactly what I had in mind (I'm being nice here. In fact I'm being very very nice here).

Regretably…it only "appears" to be based on some unknown but imagined set of principles, as you say.

It’s not merely “appearance”! This was and continues to be the conclusion drawn from an extremely powerful series of discoveries about nonperturbative string theory initiated by witten in the early nineties and developed in great detail over the next decade. Nobody has ever found any inconsistency in any of these developments and the view that the original conclusions drawn from this work about the uniqueness of string theory are wrong in no way represent the mainstream attitude of string theorists towards their field. Just look at any of the recent textbooks on the subject since these reflect the standard views towards string theory and explain the results that are currently viewed as the standard ones in the field.

What is “regrettable” is that there’s really no substitute for a detailed understanding of these discoveries and their elaboration over the last decade to convey the reason why the attitude of string theorists continues to be so positive despite the fields current challenges. Moreover, in addressing these challenges there have already been great advances that needed to be made in any event. One very important one is the discovery of the idea of flux compactification which shows how to stabilize all of the moduli that characterize a given solution’s associated phenomenological content and do so in a way that stably breaks supersymmetry.

… I do think several of these current efforts would be destroyed if evidence were found of extra spatial dimensions.

The same with the discovery of supersymmetry, as I believe you recently mentioned.

 

[marcus]

... the string program is much more ambitious than the loop program.

If the ONLY goal of the string approach was to have a consistent quantum gravity, then I would certainly prefer loops over strings...

Smolin's group represents a considerable part of loop community research. We have a thread here at PF about the two recent papers. "Matter as twists in geometry"
https://www.physicsforums.com/showthread.php?t=190053

the goal is to determine the fundamental degrees of freedom describing space and particles so that one gets a dynamical quantum state of geometry and matter

Arguably that goal is more ambitious, rather than less----because the competition only models particles, it doesn't have a quantum state of spatial geometry.

Demy, you brought up the comparison of different research programs' ambitiousness and I'm glad you did because I think it is an important point for us to pursue. You indicate that ambitiousness is one thing that determines how you rate the various research lines.

So I should probably make explicit what Smolin's group is working on. it is a natural extension of LQG spin networks to include matter. A PF member named lqg is one of the people involved and he posted here recently. The spin network in question is a four-valent ball-and-tube network which behaves as if it is embedded in a 3D manifold. The dimensionality is critical, because the network must be able to have just those knots which are possible in 3D. (These would come undone if it were embedded in space of 4D.) Essentially it is a topological theory of matter. Certain topological numbers are conserved and these may correspond to particle quantum numbers.

Naturally I am not claiming that this approach is GOOD or likely to SUCCEED or that it is the FIRST topological theory of matter. Those are separate issues. All I am doing now is to point out that the spin network is a time-honored way to describe the quantum state of spatial geometry, in the loop community, and what is being investigated is how to make a spin network also serve as a theory of matter.

The four-valent ball and tube networks evolve by Pachner moves. This is their time-evolution. As far as I know the researchers have not yet assigned amplitudes to different Pachner moves, so they are not at the point of being able to write a path-integral. But one can see the goal and the direction very clearly: they want a quantum dynamics of geometry+matter (represented by a single object).

This research line is not the only currently active one that aims at this goal. So as to keep this post short I will save parallel approaches for another time.

 

[Demystifier]

Marcus, thank you for pointing out to me that the loop program is wider than I thought.
Is there a kind of a review (not too technical if possible) of various such beyond-gravity achievements of the loop approach?

 

[ccdantas]

I'm sure Marcus will answer that soon. I don't know of any review summarizing the various approaches in the loop program, but I know of an upcoming book with various contributions to the program (including some outside the program) that Oriti is editing ("Approaches to Quantum Gravity: Towards a New Understanding of Space, Time and Matter"). Some chapters are available through the arxiv.

http://arxiv.org/find/all/1/co:+oriti/0/1/0/all/0/1

 

[ccdantas]

Well, perhaps this paper (recent advances and FAQ) by Ashtekar may also interest you:http://arxiv.org/abs/0705.2222

 

[ccdantas]

Oh, now I see that you are interested in the "beyond-gravity achievements" of the program. I don't know whether you can specifically find those in the papers that I have indicated. I'll wait for Marcus' response...