A workshop in Paris/Saclay is taking place this week entitled Wonders of Gauge Theory and Supergravity and the talks are now online. They show that some exciting new things have been happening in the study of gauge theory and supergravity amplitudes, and I’ll make the prediction that this field will attract a huge amount of attention in the coming years (at least until the LHC experiments announce results incompatible with the Standard Model…).

Perhaps the most remarkable part of this whole story is the mounting evidence that N=8 supergravity amplitudes are finite in perturbation theory. Remember the standard story about how quantum theory and general relativity are incompatible that has dominated discussion of fundamental physics for years now? Well, it turns out that this quite possibly is just simply wrong. See Zvi Bern’s talk on UV properties of N=8 supergravity at 3 loops and beyond for the latest about this. Bern shows that divergences everyone had been expecting to occur at 3 loops aren’t there, and gives evidence that they might also be absent at higher loops. He even sees this as a phenomenon not special to N=8 supergravity, but also occurring in theories with less supersymmetry, e.g. the N=5 and N=6 theories. Among the other talks, Nima Arkani-Hamed’s is also about this, advertising the idea that N=8 supergravity is the Simplest QFT.

Much of this story is about the N=4 SYM amplitudes and new insights into them and their relations to supergravity amplitudes, with some of this research growing out of and motivated by the AdS/CFT conjecture of the existence of a string dual to N=4 SYM. Quite a few of the talks are interesting and worth trying to follow, with a much higher proportion of new ideas than is usual at particle theory workshops in recent years.

To go out on a limb and make an absurdly bold guess about where this is all going, I’ll predict that sooner or later some variant (“twisted”?) version of N=8 supergravity will be found, which will provide a finite theory of quantum gravity, unified together with the standard model gauge theory. Stephen Hawking’s 1980 inaugural lecture will be seen to be not so far off the truth. The problems with trying to fit the standard model into N=8 supergravity are well known, and in any case conventional supersymmetric extensions of the standard model have not been very successful (and I’m guessing that the LHC will kill them off for good). So, some so-far-unknown variant will be needed. String theory will turn out to play a useful role in providing a dual picture of the theory, useful at strong coupling, but for most of what we still don’t understand about the SM, it is getting the weak coupling story right that matters, and for this quantum fields are the right objects. The dominance of the subject for more than 20 years by complicated and unsuccessful schemes to somehow extract the SM out of the extra 6 or 7 dimensions of critical string/M-theory will come to be seen as a hard-to-understand embarassment, and the multiverse will revert to the philosophers.

Many of the titles of the talks at Strings 2008 have recently been announced. The plenary talks will include several talks mostly not about string theory, including 3 about the LHC and one by Lance Dixon on the amplitudes story. It seems that the string theory anthropic landscape is a topic the conference organizers don’t want anything to do with, since the only person from the Stanford contingent speaking will be Kallosh on prospects for getting something observable out of string cosmology models of inflation. As for what is popular, it clearly helps a lot to be from one of my alma maters, with Princeton (7 speakers), and Harvard (3 speakers) the best-represented institutions.

**Update**: For an extensive rant about this, see here.

**Update:** Last week was Paris, this week it’s Zurich. Amplitudes are all the rage this summer.

Something I hear about occasionally is some group of researchers somewhere who believe gravity has a “UV fixed point”, which would apparently make gravity renormalizable. Is this “UV finiteness” thing Zvi Bern is describing the same thing or is that a different research program?

Coin,

The “UV fixed point” proposal is something different, using the renormalization group to tell you how to handle divergences. In what Bern is describing, the divergences are just not there, presumably for some still poorly understood symmetry reason.

Pingback: Of being bold « A Quantum Diaries Survivor

Let me see if I understood the main objective of this line of research. They are trying to prove that N=8 supergravity is renormalizable, and use this to prove that other supergravity theories with lower and lower N can also be renormalizable, until they get N=0, which is just the usual gravity. Is this correct? Why not?

OMG, this is so funny. You and Smolin criticize string theory for not being fully defined and not having precise rules for computing multiloop diagrams and you think N=8 sugra is in better shape? You think the theory will make sense non-perturbatively without string theory? You think you’re going to get the SM?

You’re deluded.

Daniel,

The main point of this research program is that it is showing that something is going on in these higher-loop calculations making the divergence structure much better than expected, something which is not fully understood. There is evidence that at least some quantum gravity theories with enough supersymmetry are finite or renormalizable. I haven’t seen any claims that this will be true for N=0. For trying to unify gravity and the SM, you actually don’t want the N=0 theory by itself to make sense, but rather have consistency determine the extra fermionic and other fields that we observe.

“majorana”,

“OMG, this is so funny.”

You know, you give all appearances of being a 12-year old whose knowledge of this subject is based on reading Lubos’s blog. If you actually know anything about this, and want to have a serious discussion about science, please stop with the juvenile behavior.

Hi Peter,

“I haven’t seen any claims that this will be true for N=0.” Well, I point this out: p. 35 (No known susy armunte explains these cancellations). p.38, p.40

I thought that it was a claim…

Daniel,

What I meant is that I didn’t see any claim that for N=0 you would get cancellations at all orders. It is certainly true that even for N=0 Bern et. al. are finding unexpected cancellations.

Arkani-Hamed aparently previously gave the same (very nice) talk at Perimeter Institute, which is online on video here:

http://streamer.perimeterinstitute.ca/mediasite/viewer/NoPopupRedirector.aspx?peid=ff9a6fa8-bfde-44b3-a91b-bba7c382720f&shouldResize=False

Hi Peter,

He does not claim, but at least this is the impression I got when I read everything, with p. 42 on my mind… Nevermind then…

Interesting stuff. But it is my understandind that phenomenologically, theories with N>1 are not viable because they necessarily treat left-handed and right-handed spinors the same way so they cannot accomodate the SM.

So what is the situation:

a) For now this is seen as a purely formal exercise to try to understand these mysterious cancellations (that apparently go beyond the usual SUSY cancellations) with no goal to connect to phenomenology

b) This is only the first step of a program in which the second step would be to somehow reduce the N=8 theory to N=1. (by breaking 7 of the SUSY explicitly? By another approach?)

c) Something else that I am completely missing

Thank you for the very informative blog, btw.

Patrick,

For now the situation is a), Bern says this pretty explicitly. As you note, there are well-known problems with getting the SM out of N=8 supergravity. A lot of people worked on these in the early 80s, but most gave up once the conventional wisdom became that the theory was non-renormalizable, and string theory got going. Maybe some people will start looking at these problems again. In any case, it would be very intersesting to know what is causing this, presumably some sort of non-obvious symmetry in these theories.

Hi Peter,

Two questions:

(1) I’m new to hep. Can you suggest a suitable text to study gauge theory (perhaps the one you studied from)?

and

(2) When is the LHC expected to officially start (at least tentatively)? And how long is the data-gathering process expected to last? If no Higg’s particle/s is/are found, will it mean another odd no. of decades before another set of experiments are to replace the current set?

Would love to read your feedback.

Regards,

Jeffrey

Jeffrey,

Among the newer QFT books that discuss gauge theory, the one I like best is probably the one by Nair.

Latest about the LHC is that it will be ready to try injecting beams in August, first physics collisions maybe by October, with a month or two of data-taking at low luminosity this year. This won’t be enough to find the Higgs. That will take another year or two of data-taking.

Getting to higher energies than the LHC is going to take a long time. The best prospect is probably to increase the LHC energy by using new magnets, but that is probably at least 20 years off if not more.

“Perhaps the most remarkable part of this whole story is the mounting evidence that N=8 supergravity amplitudes are finite in perturbation theory. Remember the standard story about how quantum theory and general relativity are incompatible that has dominated discussion of fundamental physics for years now? Well, it turns out that this quite possibly is just simply wrong.”

Is perturbative finiteness anything more than a red herring? Of course, it might have useful spinoffs for computing QCD amplitudes etc., but as a fundamental theory, I fail to see the point.

In partidular, it is known that in string theory, just N=8 sugra cannot be consistently decoupled at low energies (Green, Oooguri, Schwarz). So the full UV of pure N=8 sugra is not understood at all, to say the least.

Besides, the perturbative finiteness of N=8 is not that surprising because type I open string amplitudes “square” to type II closed string amplitudes (Kawai, Lllewelyn, Tye), and in d=4 open strings result in the finite N=4 gauge theory, while the closed strings give rise to the supergravity.

What I am surprised by is the claim that some theories other than N=8 might also be finite (even if only perturbatively). Does it have any understanding in terms of strings? Is there something basic that I am missing ?

If the current program of N=8 supergravity amplitudes being finite works out, it will nevertheless be a four-color-map theorem type of proof. What is encouraging is that there may be a new physical principle to be discovered.

Peter, you are going soft on us. This is just another wacky theory with no connection to reality. It does not because valid or useful just because some of the infinities cancel.

chethan krishnan wrote:

What I am surprised by is the claim that some theories other than N=8 might also be finite (even if only perturbatively).Where did you encounter this claim? The claim that is circulating seems to be (a) N=8 may be finite; (b) N<8 is less divergent than expected (but not finite).

Chethan,

If “the perturbative finiteness of N=8 is not that surprising” because of a simple string theory argument, how come just about all elementary string theory discussions motivate the subject by saying that you need string theory to get perturbative finiteness?

I just don’t see any evidence that you need string theory to define N=8 supergravity, or the relevance of the string theory decoupling argument.

N=4 SYM can be defined as a 4d QFT completely independently of string theory, and I don’t see why the same shouldn’t be true of N=8 supergravity. More interesting though than N=8 supergravity is the larger phenomenon that seems to be going on here of gravity QFTs having some sort of still not understood extra structure or symmetry that may make them finite or renormalizable.

I think that the problem to get the SM from N=8 sugra is that the R-symmetry sector can not accomodate the SM gauge groups, there are some old papers of kaku-townsend-van nieven…. about it.

anon. says:

“The claim that is circulating seems to be (a) N=8 may be finite;

(b) N<8 is less divergent than expected (but not finite).”

So you are saying that the finiteness hope is there only for N=8 sugra, but not so for other theories. But then I do not understand Peter’s claim:

“More interesting though than N=8 supergravity is the larger phenomenon that seems to be going on here of gravity QFTs having some sort of still not understood extra structure or symmetry that may make them finite or renormalizable.”

Peter also says:

“N=4 SYM can be defined as a 4d QFT completely independently of string theory, and I don’t see why the same shouldn’t be true of N=8 supergravity.”

Does this mean that you consider a Feynman diagram expansion a full description of the theory? The only possible non-perturbative completion of N=8 sugra that I know of is in string theory, and there it does not decouple. That does not bother you?

“If the perturbative finiteness of N=8 is not that surprising because of a simple string theory argument, how come just about all elementary string theory discussions motivate the subject by saying that you need string theory to get perturbative finiteness?”

There are many field theories which are finite. Nobody invokes string perturbation theory to argue their finiteness. The string theory lesson here is NOT about the finiteness of N=8 itself, but that the gauge theory divergence structure here might be related to the gravity case, and so the gravity case might not be as bad as one would have imagined naively.

“N=4 SYM can be defined as a 4d QFT completely independently of string theory, and I don’t see why the same shouldn’t be true of N=8 supergravity.”

Because of gravity! Gauge theories usually have good non-perturbative definitions. Not so for gravity.

Just some comments:

– The expectation that N=8 diverges at three loops is based on 80-ies technology in proving nonrenormalisation theorems. People like Stelle and collaborators have updated their techniques recently and also find that the 3-loop divergence is not there. I’m not quite sure what the latest prediction is. They are still hoping to find a symmetry proof of finiteness.

– A topologically twisted (?) version of N=8 is not interesting phenomenologically, I think.

– At the exact same conference Berkovits announced that he and Maldacena have some understanding of the appearance of ‘dual conformal invariance’ in N=4 multiloop computations. This understanding is based on string theory through the AdS/CFT correspondence.

Peter,

Those links are dead. Am I the only one who can’t access them?

Chethan,

I just don’t believe claims you’re making about “non-perturbative completion”.

The main claim always made for string theory is that it is a “UV completion” of supergravity. This is based on the conventional wisdom that supergravity is ill-defined at high energies due to divergences in higher loop terms of the perturbation expansion, something which is not supposed to happen in superstring perturbation theory. If Bern et. al. are right, N=8 supergravity doesn’t need a “UV completion”, it is UV complete, in the conventional sense of that term, which is about perturbative calculations.

Sure, for both QFT and string theory, a truly well-defined theory requires a formulation more general than perturbation theory. It’s one of the fundamental problems of string theory that this is missing for string theory, so I don’t see how one can claim that the significance of string theory is that it provides a “non-perturbative completion”.

In AdS/CFT, a perturbative string is supposed to allow a useful approximation to the strong coupling limit of the gauge theory, but there the situation is exactly the opposite of what you want. The gauge theory by itself appears to be defined non-perturbatively, at all couplings (e.g., use a lattice), whereas the string theory is only defined at weak coupling. Here the QFT provides a “non-perturbative completion” of the string theory, not the other way around.

EDT,

That server seems to be down now, presumably it will come back up at some point. If it doesn’t, and the web-site for that conference has moved, please let me know and I’ll update the links.

FWIW, it has been suggested recently (link below) that the “still not understood structure” responsible for finiteness involves string theory dualities and symmetries, which points to a much more prominent role for string theory than the role allowed in the post here.

http://arxiv.org/abs/0806.1726

Though it MIGHT be a logical possibility to think about N=8 supergravity as a standalone theory, it really makes a lot of things (BPS solitons for example) significantly more artificial. From such a point of view, it is merely an extraordinary coincidence that the theory seems to have a natural 11 dimensional origin.

Maximal supersymmetry is most natural in 11 dimensions. If we are working with maximal local supersymmetry, it takes a lot of effort to NOT believe in string theory, considering all we have learnt in the last decade or so. Most of what we have learnt is based purely on supersymmetry.

Hi Peter:

Read your book but I am confused on a couple of things that are unclear to me. Maybe you can help. I understand that ST is referred to as not A QFT. But I read one QFT book (the first section) that suggested ST was a QFT but just used a different way of treating time as opposed to standard QFT. If ST is not a QFT are there still virtual particles in ST? Please help. Also I have read that GR is/is not a gauge theory. Obviously only one viewpoint is correct. Thanks.

Hi Peter,

I found an argument somewhere else…

“You might propose to break the N=8 supersymmetry spontaneously. However, N=8 supersymmetry cannot be spontaneously broken to N=1 or N=0 supersymmetric – the latter being realistic choices. In fact, even N=2 supersymmetry is too constraining and cannot be spontaneously broken to N=1 or N=0, at least not by field theoretical methods in four dimensions. If you can’t break the N=8 supersymmetry spontaneously, you may want to break it explicitly. However, in that case, you lose the cancellations completely [unless you break it explicitly by using M-Theory/String Theory.”

I would add, unless cancellations at lower N are also a myth. Is that correct? Or is there more to it?

Daniel,

As I keep repeating, completely independently of its divergence problems, no one knows a viable way to get the SM out of N=8 supergravity. You can try and do this by adding explicit breaking terms, but then it’s no longer N=8 supergravity. What’s interesting here is NOT just the specific result about N=8 supergravity, because we know that theory by itself is not enough to give us what we need. What is interesting is that the assumption the whole field has been built on for 25 years, that QFT is inherently incompatible with quantum gravity because of high energy divergence problems, appears to be wrong. The interesting question is what is causing this, and how to identify the class of QFTs that don’t have divergence problems and can give quantum gravity. Maybe one of these will be viable.

One interesting sociological question is how long it will take for string theorists to stop repeating the claims about incompatibility of QFT and gravity that they have convinced almost everyone about, now that these claims appear to be wrong.

p falor,

String theory is a quantum theory, but not a theory of quantum fields on space-time, and thus not a QFT. You can try and formulate it as a theory of quantum fields on the infinite dimensional space of all paths in space-time (“string field theory”), but that’s not what you normally mean by QFT.

Perturbative string theory is given by an infinite sum of terms, each term of which can be thought of as a QFT on a 2d surface of a given topology. This QFT is a conformal QFT, and you have to couple it to a 2d quantum gravity on the surface (i.e. integrate over all possible metrics on the surface). But string theory is supposed to be something non-perturbative, and so go beyond this infinite sum of terms, each of which can be thought of as a (different) QFT.

When we say “gauge theory”, we normally mean a theory with internal gauge symmetry (mathematically, the symmetry of all “vertical” automorphisms of the bundle structure), and the Yang-Mills action (curvature-squared). Gravity isn’t a gauge theory of this kind. But there are two ways you can try and think of it as something similar. One is to use the bundle of orthonormal frames, which has a standard gauge symmetry. But this bundle has extra structure (the vierbeins), not available in usual gauge theory, and this gets used to construct the Einstein-Hilbert action. So, this goes beyond the usual gauge theory: there are extra fields and different terms in the action. A second way to go is to try and think of the infinite dimensional symmetry of space-time coordinate reparametrizations as something like a gauge symmetry. One can do this, but it is a different kind of symmetry than the standard gauge symmetry.

Sorry for the somewhat technical answer, but it’s a rather technical question…

Peter,

I see you’ve posted a link to Motl’s insistent plea that N=8 Supergravity is intimately tied to his beloved String Theory. Do you have a technical response?

manyoso,

I’ve already responded to the claims that “you need string theory to define N=8 supergravity non-perturbatively” above. See my response to Chethan.

As for the rest of what Lubos writes, he goes on and on about all sorts of things that just aren’t relevant. Sure, there are all sorts of interesting relations between N=8 supergravity and string theory, but they don’t add up to “string theory is needed to define non-perturbative N=8 supergravity”. String theory itself has a much more serious unsolved problem of how to define it non-perturbatively than supergravity does. It’s true that perturbative supergravity for instance can’t completely describe black holes, but this is equally true of perturbative string theory.

In any case, for the N’th time, what’s important here is not the special case of N=8 supergravity, which in any case isn’t a viable unified theory. What is important is that it appears that there are QFTs for quantum gravity which don’t have the expected divergence problems. This is big, exciting news and opens up a lot of non-string theory possibilities to investigate for anyone interested in quantum gravity. Lubos and others are just trying to obfuscate this simple fact.

I guess the techinical response is shown in the conference articles… I would just like to find what is the reason many of these guys suspects that the renormalizations effects are not due to supersymmetry. I mean, I would like to find an article, in which a calculation makes them to think like this.

Peter,

“What is interesting is that the assumption the whole field has been built on for 25 years, that QFT is inherently incompatible with quantum gravity because of high energy divergence problems, appears to be wrong.”

As of today, quantum gravity remains a theory plagued with many unsolved computational challenges, with no firm foundation and no experimental basis. Apart from renormalizability, there are many unsettled questions such as: Does quantum gravity require smooth manifolds and can it be consistently built in a perturbative framework ?. On the contrary, there is overwhelming evidence that perturbative quantum field theory is a correct framework, at least at the Standard Model level. If such a huge gap exists, on what basis can one say that the two appear to be compatible?

Regards,

Observer

Hi Peter:

Thanks for the reply. I believe it does help improve my understanding. Still there is one other issue that I am really have a hard time understanding that you alluded to in your book. The CC problem. Now I understand a QFT calculation gives the CC 60-120 orders of magnitude too large. This calculation is based on supposely sound physics principles. The same ones that are used to calculate the Hawking radiation from a black hole that is apparently accepted by most (all) HEP theortical physicists.

How can the current QFT frame be valid, i.e., assumptions etc, if this supposely basic calculation is so wrong? Maybe the vaccuum energy doesn’t couple to gravity but this would counter GR correct?

But more puzzling to me (you may not be able to answer this) is that in ST assuming a landscape, a vacuum state is being searched for that has a small CC. This suggests to mean that ST does not view the vacuum energy the same way as a QFT does. How can one a assign a value to a basic calculation? Shouldn’t the theory determine the value?

All of this maybe very obvious to those in the know but to someone like me I am hopelessly confused. Can you help? Thanks.

Observer,

I’m not claiming that QFT and gravity are definitely compatible, just noting that the conventional reason given for claiming that they are definitely incompatible now appears to be wrong.

Maybe these unexpected cancellations in supergravity will ultimately throw new light on the failure of past efforts to calculate a sensible vacuum energy density in QFT.

Just a thought…

Now I understand a QFT calculation gives the CC 60-120 orders of magnitude too large.This isn’t true. The cosmological constant is a superrenormalizable parameter that can be set to whatever you want. The problem is that making it small is a huge fine tuning.

Like any useful toy model of gravity, N=8 sugra has black holes. Because of the gauge fields that are automatically there in the theory, these can be charged black holes. The only way a purely perturbative notion of consistency can exist for N=8 sugra, is if one declares by fiat that these perfectly reasonable objects in the classical theory are not states in the quantum theory. This is very hard to justify, especially in a supersymmetric theory because in supersymmetric theories, these ideas have produced perfect agreements with classical black hole entropies etc.

On the other hand, if one includes these as states in the theory, and imposes the uniqueness of the wave function, the theory (or rather its BPS sector, which has sufficient structure to make the comparison non-trivial) is exactly the same as what one expects from string/M theory. People who believe in string theory believe that this coincidence (and many others) could not have happened if there was no truth behind it. It must be admitted that this kind of evidence is still circumstantial, but I personally find it highly non-trivial and therefore compelling.

I think Peter is right that the oft-cited problem of gravity is its perturbative non-renormalizability. Often the sales-picth for string theory has been that strings are finite. But I think people focussed on this issue because before N=8 came along, this was argument enough! But now that there exists a theory that is perturbatively (perhaps) finite, we really have to emphasize that it is the full quantum consistency that we are after. Perturbative finiteness is necessary, but certainly not sufficient.

The good news is that even without having a FULL non-perturbative definition of gravity/string theory, we can still make non-trivial statements about non-perturbative aspects from quantum consistency alone. This is the sense in which string theorists say that N=8 is not “non-perturbatively” complete. This statement certainly does not imply that we have the full non-perturbative definition of the theory. The claim is that in the only quantum mechanical framework where we know how to make sense of the classical BPS objects of the theory, it does NOT make sense to just look at the supergravity multiplet – because there are extra massless fields around.

String theory might or might not be directly useful for building a model for our universe. I think arguments against string theory have maximum mileage when they focus on the practical usefulness of string theory in understanding our vacuum. But as a theoretical superstructure, I think it is hard to argue against it.

Aaron Bergman:

According to Sean Carroll

“Unlike supergravity, string theory appears to be a consistent and well-defined theory of

quantum gravity, and therefore calculating the value of the cosmological constant should,

at least in principle, be possible.”

He isn’t suggesting that the CC is a constant to be defined but something that can be calculated from a theory. Other references clearly imply(at least to me) that the vacuum energy should be calculable from QFT.

Obviously I am missing something but would appreciate a simpler explanation of what that is.

Thanks.

p falor,

The idea is that vacuum energy gets contributions from all the fields in the theory, like electrons, photons, etc. Lets say that before you added these contributions, the “bare” CC was some number A (we do not know its value beforehand). After you add the contributions from everything, lets say that the result is A+B. It turns out that in quantum field theory, B is usually a huge (computable) quantity. But the final result for CC, namely A+B, is experimentally known to be a miniscule number. This means that we are forced to choose (“fine tune”) A to be a huge, but negative number almost (but not exactly) equal to B so that the sum of A and B is the small experimentally measured value of CC.

We say that the “natural” scale of the cosmological constant is that of B, and that the observed CC is absurdly small.

What Aaron is saying is that because CC is a certain kind of parameter, you are in fact allowed to tune it. You are not in a situation where there is actually a contradiction, because you are not tuning something that is NOT allowed to be tuned. In that sense, the CC problem is not really a problem, because it just refers to the unease that physicists feel, when they do this fine-tuning.

What Sean was talking about was the hope in string theory that perhaps CC would be automatically fixed by the theory itself. But nowadays, it seems much more likely that CC in string theory is essentially like a parameter in field theory. So it might not be dynamically determined. This just means that string theory is much more like like any other theory than many string theorists hoped: it needs experimental input to fix its parameters.

somebody,

I’m not disagreeing that how to come up with a consistent non-perturbative N=8 supergravity is an issue, just with the idea that we know that the answer to this is non-perturbative string theory, since we don’t even know what non-perturbative string theory is (the BPS sector story provides an interesting possible constraint, but you still don’t know what the theory is that you are claiming solves the problem, or even whether there is more than one way of doing this).

p falor,

Sean Carroll’s statement makes two assumptions. The first is that gravity QFTs inherently are non-renormalizable (which now appears to be wrong), the second is the existence of a single consistent non-perturbative version of string theory (which doesn’t yet exist). If such a single consistent string theory existed, presumably you could calculate the CC in it. Nowadays, with the “landscape”, most string theorists seem to have adopted the ideology that the CC actually can’t be calculated, since there are essentially an infinite number of very different grounds states, all with different CC.

As for the argument that all we have to do to get predictions about string theory is “fix a parameter”, sorry this is nonsense. String theory does not provide a prediction about what will happen at the LHC in terms of some number of parameters that need to be fixed by experiment. It predicts nothing about what will happen at the LHC, no matter how many parameters you give it.

Off-topic, but perhaps worth mentioning: Lincoln Wolfenstein has reviewed in PhysicsWorld a new book for a general audience by Helen Quinn and Yossi Nir, which focuses on matter-antimatter asymmetry but also appears to offer a nice overview of the Standard Model.

Somebody, Peter,

I think I see Somebody’s point: QFT calculates a very large number, but the observed CC is very small (relatively) so to make the calculation come out we just subtract another very large number. And presto it works. My point was more along the line since QFT supposely can calculate the vacuum energy generated by the virtual particles but gets the wrong answer is there some reason to doubt the virtual particle framework of QFT?

Others have suggested that there is only suggestive evidence for virtual particles as predicted by QFT. Since the 1920’s particle physicists have been aware of this CC potential problem but ignored it until yhe last 25 years or so. It sounds like the problem is just going to be defined away. True the constant that must be added to the calculated vacuum energy has to be very fined tuned but what is the current alternative?

> My point was more along the line since QFT supposely can

> calculate the vacuum energy generated by the virtual particles

> but gets the wrong answer is there some reason to doubt the virtual particle

> framework of QFT?

A few pointers:

1. As I said, QFT does NOT make a wrong prediction for CC. The freedom of adding the bare value is not an option, it is a must. In the case of other renormalized couplings also (like fermion mases, charges etc.) we do this. But in those cases, the bare piece and the computed piece and the actual value are all roughly the same order of magnitude.

2. QFT is simply the most spectacularly successful theory there is in everything else.

3. Any local, quantum mechanical theory that respects special relativity at low energies will look like a QFT. This is almost a theorem, but maybe there is some way to tweak it, in which case you have to deal with #2.

p falor,

Virtual particles appear in any QFT calculation above tree level, and many of these (especially in QED) are tested to very high precision. You can’t get rid of either virtual particles or QFT.

The fact that we can’t calculate the CC is not surprising, since we don’t have a viable unified theory of gravity and the SM. What is (a little) surprising is that naive order of magnitude estimates are completely wrong. This is a hint, but not much more, and we don’t know what it means. About all one can say is that it means that there’s more to the solution of this problem than a naive gluing together of the SM and GR would indicate.

Peter, you say:

“The fact that we can’t calculate the CC is not surprising, since we don’t have a viable unified theory of gravity and the SM.”

Is there any legitimate basis for the assumption that the vacuum of GR and the vacuum of QFT are identical entities? GR refers to macroscopic scales and ceases to be relevant below the scale of nuclear processes. Why then do you expect that a consistent quantum theory of gravity is the root cause of the CC problem?

Best regards,

Observer

While we’re at it, let me (mis?)interpret what Peter said another way. I took it that if some physical model tells us that two quantities are fine tuned so as to cancel to 120 (or 60?) decimal places, and that such fine tuning is an extremely improbable circumstance, then instead it is the physical model itself that should be regarded as being extremely improbable to be the right one. In other words, maybe when a unification of GR and QFT is found, it will be understood that the concept of fine tuning was simply mistaken.

Observer, commonsense,

You really need some way of putting QFT together with GR to make the question of calculating the CC a well-posed one.

The “10^60” number one sees refers to SSYM, where supersymmetry breaking will introduce a CC of the wrong scale. I think supersymmetry breaking is already a deadly problem for conventional SSYM models, this is just one more reason not to believe them.

The “10^120” number is based on assumptions about quantum gravity and the Planck scale, there the lack of a convincing theory is the problem.