String theory and standard model in CERN

Question

I don't know how to say it, but in the TV dominatrices and the popular science books we see the string theory as "the best theory to explain everything", and as "the only game in town"... etc. And yet, in CERN -The largest physics laboratory-, they are just using the Standard Model (SM) as the only way to find the theory of everything (ToE), and sometimes they call it: "theory of almost everything".

Also, they didn't even mention anything about string theory. (as if it doesn't exist!)

So my question is:

Why? Isn't it easier for them to take the string theory as a Theory of Everything?

Also, the Standard Model is not able to explain the gravitational force from the SM equation! So, the SM is essentially incomplete! Why do they keep working on the SM? Am I missing something here?

-P.S.: I'm an engineer, not a physicist! So, I don't know much about the details.

Answer 1

As with many discussions about string theory, it is sometimes good to recall some reality:

It was over 50 years ago that the Higgs mechanism was proposed. Compared to fully-fledged theories such as string theory, the Higgs mechanism is a tiny add-on to the observed standard model (as it was then). It took 50 years for experiment to get to the point of seeing it, and in fact so far just a first glimpse of it. For 50 years, the Higgs mechanism was speculation not confirmed by experiment. It had all theoretical backing behind it, theory all pointed to it being true, but it couldn't be checked experimentally for 50 long years. For 50 years, you were free to make TV documentaries about particle physics without mentioning the Higgs mechanism, if you thought it was too outlandish a proposal to have a chance of being confirmed. Then finally experiment reached its energy scale and there it was. 50 years later.

As you all know, there are scenarios thinkable where more beyond-the-standard-model-physics is right around the corner, but nothing to rule out that it takes another 50 years to see the next piece of "new physics". That's just a fact of our short life.

But that's not necessarily as bad as it may sound. While "new physics" may remain specuative for a long time to come, here is a well-kept secret to take note of: even old physics isn't fully understood yet. And string theory can help here, and theorists know (though TV stations may not yet have gotten the message).

For instance, computation of scattering amplitudes even in the known and confirmed standard model is still a challenge, if only you are ambitious enough. String theory has helped with understanding some subtle points in plain Yang-Mills perturbation theory. See the links at string theory applied elsewhere -- QCD scattering amplitudes. In particular check out the remarkable story linked to there, told by Matthew Strassler in his post From string theory to the large hadron collider, which is about how string theory insights into QCD scattering amplitudes helped raise the precision of loop computations to the level that it was possible in the first place to separate signal from background in the LHC. He cites people who were involved as saying that without these string theory insights the Higgs might have been produced, but not identified at the LHC. Have a look, it's an interesting story.

Another thing may be worthwhile to remember from time to time: while we are all fond of proclaiming that we understand fundamental particle physics via quantum Yang-Mills theory, fact is that quantum Yang-Mills theory is still an open theoretical problem. We know that we don't understand some very fundamental facts about qauntum Yang-Mills. It's a "Millennium problem" Yang-Mills existence and the mass gap.

Now, one thing that string theory has become after its "second revolution" is something like a map of the space of Yang-Mills like-field theories and various "dual" theories. Via D-brane physics, KK-reduction, AdS/CFT, etc. Yang-Mills like theories appear in various guises in various corners of string theory, and their embedding into string theory geometrically explains subtle equivalences between these, such as electric/magnetic duality, etc. If you haven't seen it before, check out at http://ncatlab.org/nlab/show/gauge+theory+from+AdS-CFT+--+table at least part of this string-theoretic "map" of the space of quantum field theories related to Yang-Mills theory. While this hasn't solved the mass gap problem yet, clearly, one may start to feel that the deeper nature of Yang-Mills theory is slowly but surely being probed here.

The punchline here is the following: besides being a framework for models of quantum gravity and gauge unification, string theory is a piece of theoretical physics that sheds light on the nature of quantum field theory as such. While experimentalists and public media are busy with indulging in the Higgs physics now that they waited for half a century, maybe theoreticians can use the time before the next accelerator to step back and think a bit more about the still open more fundamental issues of quantum physics. That's where string theory has already helped, and I think will help in the future. Of course you won't see this on public TV.

(Generally, it is surprising these days how not only the public media but also the broad community's attention is consistently attracted to the shallow and ignoring the deep advances that do happen in fundamental physics. For instance there is loads of excitement about, say, the firewall essay contest, but the really interesting advances, such as for instance in genuine mathematical characterization of string theory vacua here remains a topic among a tiny group of specialists. At the same time everybody has an opinion about the "landscape", and everbody else has the opposite opinion. What is needed instead is more decent theoretical work on the foundations of quantum field theory and, inevitably then, string theory.)

Answer 2

The Standard Model is a quantum field theory that explains almost everything we know about except gravity, and you can do very precise computations using it (even more precise, now that we know the mass of the Higgs). However, there are good reasons to believe that gravity cannot be incorporated into a quantum field theory.

String theory attempts to incorporate gravity and the standard model in the Theory of Everything, and is supposed to predict everything if you can find the right way of folding the extra dimensions up. However, there are so many possible ways to fold these up that it currently cannot predict anything that can actually be measured with current or near-future experiments, and some people believe it may never be able to. Not everybody believes that String Theory is the correct Theory of Everything, but nobody has come up with another generally accepted way to combine gravity and quantum field theories.

Answer 3

I think I will add my two bits as an experimentalist.

The Standard Model is not the result of brilliant physicists thinking deeply and coming up with a theoretical self consistent formulation that miraculously fitted the data. It is the result of strong interplay between experiments and tentative models that coalesced to what is now the standard model. In its creation it has incorporated a large number of constants from the experimental data, because the theory comes with a number of unknown constants. In a very real sense the standard model is a shorthand encapsulation of almost all the known data of particle physics, some incorporated, some predicted, and at the same time it is able to predict by calculations phenomena not measured yet. This is, for the energies available to us up to now with the LHC, quite sufficient.

There are indications that the SM cannot describe everything in elementary particles even at these low energies as with some CP violation discrepancies.

Theory follows its own path: theorists are not satisfied with the phenomenological success of the SM, as for high enough energies calculations break down due to infinities not wrapped up. Even before strings and string theory supersymmetry was seen as an attractive solution to resolve infinities at high energies inherent in the SM. At the same time because of cosmological data theorists are pursuing the quantization and consistent unification of all four forces at many fronts. String theory has the necessary group structures to accommodate the SM and Supersymmetry ( a mirror of the SM yet to be seen experimentally) and is able to incorporate gravity in the whole scheme. At the moment it is the only scheme able to do all this. Hard working phenomenologists are trying to predict effects at LHC energies, but with no success at the moment, since the SM is enough to explain whatever has been measured ( or published, there may be exciting stuff down the pipe)up to now.

As an engineer you will understand that the experimentalists follow the SM blue prints looking for discrepancies to announce new physics. At the moment there is none, so most of them are not talking about strings, although there are groups testing phenomenological predictions from large extra dimensions in string theory.

Answer 4

The targetted collision energy of 14 TeV for the LHC is most appropriate to investigate physics around the electroweak scale about 246 GeV and somewhat above. For the up to now by the LHC probed energy of 8 TeV and considering the analysis of the data done so far, the standard model seems to work rather well at these scales so it seems natural to parameterize the observations by the standard model until new physics shows up.

String theory however is first of all a quantum gravity theory and large observable effects of it would most naturally appear at the quantum gravity scale which is much higher (about the Planck scale of about $10^{19}$ GeV) than the by the LHC directly reachable energy scale.

Nevertheless, it could well be that string theory and other higher energy BSM physics has low energy effects that can in principle be seen at the LHC. High energy phenomenology is the part of particle physics which tries to predict such effects. For example string phenomenology is a growing industry and this paper gives a somewhat broader overview about potentially observable low energy phenomenological effects of Planck scale physics.

Technically speaking the standard model is a good effective theory to describe physics around the electroweak scale, and it is expected to be linked by renormalization to a more general theory (for example string theory) needed to describe physics at the Planck scale. Any unified theory that is any good has to be able in principle to reproduce the standard model as an effective theory.

To come to the point, since the GUT or quantum gravity scale (where the strength of gravity is expected to become comparable to the other three forces) are widely separated from the energy scale probed by the LHC up to now and no strong hints of new physics has turned up so far, I agree with Matt Strassler that it is better to focus on standard model physics in texts about CERN and the LHC targetted at an audience of non experts. But this does not mean that doing BSM physics to try to understand how our universe fundamentally works and looking for hints of it is not important and as Urs Schreiber nicely explains, string theory is even useful for doing standard model calculations that would not have been possible without these insights. And as Lubos says in a comment (if I understand him right), as string theory in principle contains the standard model as an effective theory at the electroweak scale, trying to apply pure string theory calculations to the observations at the LHC is just a more complicated reparameterization.

Of course books like the Elegant Universe linked to in the question (which is considered a good one by experts) have their legitimacy too, to give people who are curious about BSM physics, quantum gravity, black holes, early universe cosmology etc a good introduction to a possible way to describe these things theoretically.

Answer 5

String theory cannot perform nearly any practical computations by itself. For example, you cannot predict what is the probability of producing some muons at certain angles from colliding electrons using String Theory - noone knows how to do it.

However, some people argue that ST reduces to standard model in most cases. It hasn't been explicitly computed, though.

And so, ordinary phisicists say they use standard model and they use standard model while string theorists use standard model and say they are using String Theory.