Friday, December 24, 2004

Theories are increasingly theoretical

This text follows my discussions with Nima Arkani-Hamed and David Goss.

Some people don't like the fact that the arguments in string theory are increasingly theoretical in nature, and that our theories seem to give us less exactly calculable sharp predictions that are verified experimentally.

However: it's not just string theory: the whole particle physics has been becoming increasingly theoretical and string theory just continues in the same direction. What do I mean?

QED, Electroweak theory, QCD: increasing groups, decreasing accuracy

The peak of the old-fashioned quantitative predictivity of very particular facts in physics was QED which stands for Quantum Electrodynamics. You know that people could have calculated its predictions already 50 years ago, including the quantum loop corrections, even though they did not quite understand why their methods were working (The Renormalization Group), and the most precise predictions - like the anomalous magnetic moment of the electron - have been successfully tested with the accuracy of 13 decimal places!

Then the physicists found the electroweak force that naturally predicted the neutral currents, W bosons, the Z boson, and so forth. It is also a relatively very predictive theory (Glashow, Salam, Weinberg) although its predictions were never tested as exactly as for QED. Nevertheless, all the cross sections and decay rates are measured rather precisely, the electroweak scattering is "clean".

Another step: QCD

Then you go to QCD which is now an accepted and "Nobelized" part of our theoretical canon. QCD, in some sense, confirmed the things people "guessed" by other means, and one might criticize it using some very similar words as those often applied to string theory by its critics.

You know, QCD is claimed to be a theory of the strong force, but it talks about the gluons, quarks, and especially their three colors, three concepts that were never directly seen; and according to QCD no one will ever see either of them. Also, no one has been able to calculate the properties of the proton, neutron, and nuclei - which used to be thought to be the objects from the strong interaction - from this theory too well. The actual calculations often rely on some properties of the quark and gluon distribution functions, and the critics might say that these functions have never been really derived from QCD. Even if one accepts the existence of quarks, they were not really invented by QCD: Gell-Mann received a Nobel prize for quarks in 1969, five years before QCD was proposed. The new quark flavors, such as the c-quark found in the J/psi particle, were naturally predicted by the electroweak theory (the GIM mechanism from 1970), not by QCD. In this respect, QCD seems to have had no "striking" new predictions. So why do we say that QCD is a good theory of the strong interaction?

The low-energy properties of the hadrons have not been calculated accurately enough simply because QCD is a pretty difficult machine to calculate with at low energies - but this difficulty is a fact of Nature. In the same way, the vacuum structure in string theory is also rather complicated, which also seems to be a fact of Nature. At high energies, the quarks are almost free (due to asymptotic freedom, which is really what our friends got their 2004 Nobel prize for). If the quarks are free, perturbation theory is great and one can easily and precisely calculate the high-energy events. But for the effects important for the nuclear physics, the interaction is strong - more or less by definition. QCD is a strongly coupled theory at longer distances. The perturbation theory breaks down and the nonlinear equations of QCD are just very difficult - some progress can be obtained numerically using lattices and some other tools (the AdS/CFT correspondence has become the most powerful new tool).

In this sense, I believe that one could use nearly the same criticism not only against string theory, but also QCD itself. However I feel that it's not hard to realize that in the QCD case, it would be unreasonable. Not only because of the Nobel prize!

So what does QCD predict that makes us sure that it's right? It predicts the jets in the high energy collissions - "dressed" quarks and gluons. But people qualitatively knew about these things experimentally already before QCD, so it was not a real prediction. They also knew about the organization of strongly interacting particles into families (with different composition of quarks, depending on the particular member of the family - i.e. of the multiplet). So this was not a "real" prediction either. QCD was constructed to agree with the scaling laws - it was an input and one of Gross's motivations - but it did not predict much afterwards, as long as one talks about some completely new, visible effects.

The advantage of QCD is claimed to be beauty - it is a nice SU(3) Yang-Mills theory - and the pure QCD has no dimensionless parameters - the same virtue as string theory: the original dimensionless coupling is converted into a dimensionful scale by the dimensional transmutation. Yang-Mills theory seems to be the unique way how to obtain asymptotic freedom (vanishing of interactions at very high energies) from a quantum field theory.

David Gross likes to say that a theory without dimensionless parameters (QCD) can now explain all the "anthropic" mysteries from nuclear physics. Nima Arkani-Hamed correctly points out that it's not quite correct because the various "coincidences" relating the masses of the nucleons etc. depend on all these small parameters like the quark bare masses. Well, I am not terribly happy to admit that Nima's objection is fair because his objection is a small argument in favor of the anthropic thinking. Nevertheless I must admit that Nima is right because he is. ;-)

The success of QCD is that it is really the only theory that explains the data that had been known already before QCD was found - and it's able to put these data into a coherent framework. And it is a very beautiful theory - it has nice symmetries and no dimensionless parameters in its "pure" version. These things were enough for the authors of QCD to know that it was correct as early as in 1975.

We're saying the very same things about string theory. String theory is really the only theory that can agree with the existing facts about quantum field theory but also with physics of general relativity i.e. with gravity. Of course, there is a difference between QCD and string theory is that QCD has given us some new predictions that were unavailable for the previous rules to understand the strong interactions, and these predictions are tested at the 1% accuracy, while string theory is still waiting for the right experiments that will eliminate its critics. Let me be more specific: the 1% accuracy was only achieved in the 1990s, twenty years after the fathers of QCD knew that QCD was correct.

Nevertheless, you see that the character of our theories is evolving in a particular direction - even if we study the evolution within the Standard Model itself. String theory is just one more step in this progression; it certainly implies no "qualitative" change in our understanding what physics theories are good for. We're marching towards more strongly coupled - and more difficult to calculate - theories that may look "richer" but that are also increasingly more constrained, and we are using increasingly complex mathematics - and the observations about the uniqueness of the consistent solutions of our problems - as our arguments. It is happening simply because the naive, simple math that can be easily calculated and compared with the experiments was already calculated a long time ago.

As our theories become more mathematical and abstract - which is a necessary process, as I tried to explain - the number of the people who actually understand the logic behind these new steps decreases. Not too many "ordinary" people understand relativity; quantum mechanics is even more difficult for most physics fans. Quantum field theory requires a special training, among other things, and in the case of string theory it is simply true that a PhD degree from theoretical physics is not a sufficient condition to understand the inevitability of its claims. I agree with the critics of string theory that a theoretical physics PhD should be enough to understand string theory, but my ideas how to achieve this goal are very different from theirs. ;-)

As our theories are becoming more mathematical, we are simultaneously revising the concepts dramatically and we are finding new connections between the previous concepts, and their limitations that looked impossible previously. The latter was happening in every revolution of physics, including the quantum revolution.

So I don't really understand what is it exactly that makes so many people feel so uneasy about string theory and why. Of course, I understand why people may be frustrated that the progress is slow, but it's harder to see how can string theory be blamed for it. Where we're going - in the perspective of a decade or so - is arguably the right way, and all philosophical properties and trends of this progress agree with what has been proved fruitful in the past and recently.

Much of the recent progress, including the construction of QCD, was about pushing "reductionism" as far as we can. We could not be satisfied with a list of 200 strongly interacting "elementary" particles and their messy interactions; people eventually convinced themselves that the right elementary particles are quarks (and gluons), although the hadrons remain a good description at low energies. In a similar fashion, we cannot be satisfied with the list of the elementary particles of the Standard Model plus the graviton, whose interactions furthermore don't work at the loop level, and this is why we are happy to reduce these concepts further to the level of strings (and their non-perturbative friends) - because this reduction seems possible which is itself a shocking, nontrivial fact. Again, the previous language of low-energy effective theories remains good at long distances.

String theory marvellously has all the desired qualitative features and the quantitative power to explain everything we know about the real world, and we know that the unification of quantum field theories with gravity is a very difficult task and a generic proposed theory usually solves nothing at all, while string theory seems to solve a lot. This is why we "know" that string theory is probably correct, even though it may take decades or even centuries to convince the critics. But the situation is qualitatively analogous to QCD. The difference is that string theory is even more dependent on theoretical arguments than QCD, and it works with much higher energy scales. But there is no qualitative phase transition in the definition of physics!

We may be unhappy about the particular developments in the last 1 year or perhaps even 5 years or something like that. But every time I see what the alternatives could be, it reassures me that we are on the right track. The alternatives usually want to return science at least 40 years into the past, and perhaps to the 19th century.

It's hard to convince anyone about the analogy if he or she does not feel it this way, but let me try anyway. There are creationists who reject evolution. Let's call them the 1860 crackpots. There are people who reject special relativity, right? Let's call them the 1905 crackpots. Some of these insist on the luminiferous aether (even though some of them may call it spin foam). Then there are people that reject general relativity, the 1916 crackpots, and quantum mechanics, the 1926 crackpots. Then there are thinkers who reject the (divergent) loop diagrams and their regularization; let's call them the 1949 crackpots, and who reject quarks, who are called the 1973 speculative colleagues.

As I go towards the present, physics of these topics becomes increasingly difficult, requires higher education, expertise - and I think that something remotely similar exists in any other sufficiently complex field of science, including e.g. number theory, too. Proving the Fermat Last Theorem is a pretty fancy thing that requires some new technology, does not it?

The people who reject our understanding collected in the last 20 years that string theory is the only way to exceed the limitations (and repair the divergent behavior) of quantum field theory and classical GR - and who reject hundreds of the particular more detailed insights about string theory and quantum field theory that we've made and we will never unlearn - are, of course, not quite as clear crackpots as the previous categories because they only failed to follow (or decided to deny) the last 20 years and the questions studied by string theory are still "work in progress". But ignoring these insights still seems as a pretty bad starting point for making contributions to physics - or trying to direct physics - in 2004.

What I find more obvious is that the people who want to ignore string theory actually want to neglect some older, well-established insights as well - the renormalization group, semiclassical gravity (of Hawking), and others - perhaps even perturbation theory or the S-matrix as the important concepts in quantum relativistic physics.

One may ask why I feel so sure that string theory is most likely on the right track. It is a combination of both aspects: the impressive power of string theory demonstrated in many contexts, but also the naive picture of physics that the proponents of "alternatives" want to advocate. One must always choose some principles when he or she tries to go beyond the known realm. But the non-stringy people in physics just generally choose principles that look very simple-minded and obsolete. It's pretty hard to explain non-technically and exactly why I almost always feel so certain about it. I understand why the people feel that my certainty looks like "religion" - it would also look like religion to me if I did not know most of the things I know, or if they were not organized in my brain the same way.

Aether, hidden variables: repeating the errors forever

But it's like if you remember some error that you did 15 years ago, and you later understood perfectly why it was silly and how your viewpoint on the problem was uninformed and narrow-minded and 19th-century-like (or perhaps it was not you, just some other people around). Today, you may understand that all your confusion 15 years ago was unjustified, and that there exists a completely meaningful and rigorous answer to all your questions you had - and these answers are often different than you thought. Also, you may realize today that you used to neglect a huge amount of important knowledge - you were just too ignorant about too many things - which invalidates all your previous reasoning.

And suddenly, 15 years later, someone comes with the same or even more unlikely approach and claims that it is an important idea that is meant to revolutionize physics.

Like those loop quantum gravity people. Most of them probably don't know that Maxwell did not write just his equations; he constructed a few discrete models of aether. George FitzGerald even constructed working models of such an aether that produced the transverse electromagnetic waves! And this model really worked. Such problems involving gears and wheels were what the 19th century physics was about. All this aether, something discrete that fills the vacuum, was exactly the trash that Einstein had to throw away, and this non-trivial act was one of the main reasons why Einstein was such a revolutionary. Of course, Einstein could have done it because he was standing on the shoulders of giants, including Hendrik Lorentz.

And then 100 years later someone comes and proposes a new model of aether, a discrete substrate filling the vacuum. Now it should explain gravity instead of electromagnetism. A difference is that the "modern" models, unlike FitzGerald's model, quite obviously do not work and cannot give you the right physics. No 21st century FitzGerald will be able to construct a mechanical model of a spin foam that behaves like general relativity - because it does not behave this way. These models cannot agree with special relativity because of the very same reasons as the 19th century aether. Another difference is that it is not 1860, but 2004. The progress in science was not so terribly non-linear after all - and it is going in some direction. There are just too many people who want to revert science and return us to the trees. In many cases, one can easily decide that certain progress would be "negative".

In physics, we have learned something, and it is impossible to "unlearn" most of these insights. There is a lot of recent insights that will stay with us even if string theory will be proved irrelevant for the experiments. But let's not be too pessimistic. String theory agrees with all the basic (and often also with the non-basic) discoveries and contains all the methods of the previous successful theories - quantum field theory, general relativity, gauge theories, chiral fermions organized into families, Higgs mechanism, confinement, relations between them, Renormalization Group effects, non-perturbative physics, the S-matrix. It's the only known theory different from the old, incomplete framework of quantum field theory that can do everything good that the old theories were able to do as well.

The self-described "competitors" just don't care about the actual physics - I really mean primarily experimental physics. They don't really care whether their theory has something new to say about QCD, general relativity, black holes, particle spectrum, scattering amplitudes - the physical phenomena that really exist. They don't even care whether their theory is consistent with the older insights. They prefer to extend some obsolete and narrow-minded dogmas - such as "the world is discrete" or "the vacuum must be made of something" - dogmas that have really nothing to do with the discoveries physics made in the last 200 years. Dogmas that have been more or less falsified. And that makes a difference.

Some people want physics to become "postmodern" and allow hundreds of different trends that revive various old theories of aether, Lorentz-FitzGerald contractions, hidden variables, and many other wrong things from physics of the past that our heroes had to struggle with for so long before they saw the new light.

I would really prefer if theoretical physics were interrupted completely rather than becoming a "diverse" arena of all these pseudoscientists who are rejecting random principles we learned - as well as the majority of the actual data - and who keep on constructing toy models with very limited ability to agree with anything we actually observe: interrupted physics can continue in the future once people become more reasonable and creative. On the other hand, a return to the proto-science or even pseudo-science would effectively convert the culture of theoretical physicists into the culture of intellectual monkeys once again.

The string theorists know what they're doing and how their theory fits all successful - and experimentally verified - previous insights about Nature; others don't. Our civilization certainly does not have enough resources to pay for all conceivable proto-theories that are comparably attractive as loop quantum gravity - simply because the space of such not-terribly-serious ideas off the track is virtually infinite.

Concerning string theory: don't get me wrong: I am far from being certain that we will have great new successes in the next 2 years, for example. And it's not clear in advance what the LHC will see. I am not even sure whether the number of string theorists is already too high or still too small. But most of my statements are based on a comparison of string theory with the alternatives, and in this respect, my feeling is that there is no rational justification at this point why the alternatives should "grow".


  1. Wow Lubos, you write beautifully and exceptionally clearly when explaining the physics! The first 18 or so paragraphs are right on the mark.

    It's when you switch to your "sinners in the hands of an angry God" theologian mode that you lose me. It seems pretty reasonable to me to apply the crackpot label to those who disbelieve theories that have been verified in many experiments (like Darwin's natural selection, Einstein's relativity, and quantum mechanics). When you start applying that label to anyone who has a different approach than yours to unsolved physics problems, that's theology and not physics. The fact that you often resort to violent metaphors (at least I hope they are metaphors) when discussing rival ideas - the phrase "drown in their own blood" sticks in my mind - seems incompatible with the kind of objectivity science has always required.

    You say: So I don't really understand what is it exactly that many people feel so uneasy about and why.I hear from a few undergraduates at a top physics school that they have decided against string theory (or physics altogether) because it doesn't look like anything important seems likely to be discovered anytime soon. The total intellectual effort (in man years) that has been invested in string theory to date is probably greater than had been invested in all of theoretical physics before 1930. No definitive predictions that look testable in the near future are known. No string theory model is known that captures all the known features of the standard model. I am just an ignorant outsider, but it looks to me like another string theory revolution is needed.


  2. I very much enjoyed the beggining of your post. You have a coherent, rational defense of string theory. This is when you're at your best.

    Then you revert once again to calling people stupid. I think this is when you're at your worst.

    Here's a question: Do you really think the loop quantum gravity people are stupid? Do you really think, for example, that Lee Smolin is an idiot? Why is there so much rancor in theoretical physics? We have the best jobs in the world! We get to sit around and think about things! We even get paid for it.

    Why couldn't you, for example, sit down with Lee for a few beers. You could tell him why you don't like Loop gravity, and he could tell you why he does. Probably in the course of conversation you'd learn a thing or two, and so would he. You might also make another friend.

    The point is, our field should be one of an open exchange of ideas, not useless name calling. Perhaps I am just a grad student who is yet to be disillusioned...

  3. I am not a string theory skeptic, but this is ridicolous.
    First of all, QCD did make several predictions. The quark model started being taken really seriously after the discovery of the Omega (spin 3/2 sss state), and QCD itself was believed after the discovery of the J/psi, predicted by QCD as an explanation for the absence of FCNCs.

    The connection of QCD with experiment is an extremely active field. It is hampered by the fact that QCD is very difficoult, but progress is being made both on the theoretical, phenomenological and experimental front. The theory does not just explain known hadrons, but is now giving us tools we can use to experimentally model "unusual" states of matter (color superconductivity, quark-gluon plasma, etc.)

    Nothing comparable exists in string theory. Not even the equivalent of the quark model (a way to "classify" existing low-energy physics). It might in the future, but not yet.
    So being skeptical of string theory is not "crackpotism", it's a legitimate minority opinion.

  4. I think it is not concievable for some people to encompass the whole story into a solution and framework, that would be acceptable position to begin from?

    As a commoner, this was most troubling, to see vast branchs of explorations, with no consistent framework to begin from(what is the geometry and topological considerations that arises)? Tearing versus discrete?

    So I understand now, that the ideas of emergent properties , not only from people like laughlin, but from where string theorists would derive the "principals" that brings this cohesiveness together, was very important. I am still learning.

    Cosmology is asking the same question of string theory, and without this dimensional view, there is no way to map a beginning as far as I can tell(particle mapping of collsions))? But I am still very ignorant in these matters and hope to help identify the beginnings of this emergent property in what test runs are made by Nima and others in mapping theoretcial positions of experimentation coming on line :)

  5. Hey! Thanks for your feedback.

    For the critical person who talks about the quark models. Interesting feedback, except that its main message is wrong as you will see if you read this whole answer!

    What I would agree with is that the counterparts of these discoveries (like "sss") from QCD in string theory have not been seen yet. But it's not true that string theory does not have such things. It has a lot of them, although we don't even know for sure at which scale they will appear. No doubt, they will only appear at a higher scale.

    It's just a matter of fact that you need a higher scale. By definition. The low-energy physics has been understood, and new physics only occurs at higher energies. Every theory that is meant to attack similar questions as string theory will have to face the same "complication". It's just impossible to use this as a criticism against string theory. You can use it as a complaint against the very idea that theoretical physics should continue.

    "The connection of QCD with experiments is an extremely active field." Well, i think it is not a too interesting field, and I don't believe that something really impressive is gonna be found here. But it's also true that "The connection of string theory with experiments is an extremely active field," and in the latter case I think it is much more interesting.


    Concerning J/psi, I am happy that you wrote it because it shows how clearly wrong the critics are once they're forced to go to technicalities. The J/psi and the absence of FCNCs was not predicted by QCD but by the electroweak theory, years before QCD! Just to make it clear: J/psi was the first particle that contained the c-quark. The only reason why the c-quark has to exist is that the s-quark must have a partner in the doublet of the SU(2) symmetry of the WEAK interactions, and the electroweak theory does not work well without the c-quark! The existence of the c-quark was realized by the GIM mechanism in the electroweak theory, years before QCD was found, and QCD does not care a single bit whether there are u,d,s quarks or four quarks because it is isospin blind!

    The example you mention is actually another excellent manifestation of the fact that the similar predictions came from the electroweak theory, and almost never from QCD.

    I hope that it invalidates your posting sufficiently clearly.

  6. For those readers who don't understand why David et al. were irritated by the second half of the article:

    I reflected the feedback and tried to soften the second part of the article significantly. Enough so that they should be happy now, although obviously ANY ambitious work in progress will always be a slightly controversial topic.

  7. For QCD, there are phenomena observed, for which it appears QCD contains an adequate explanation - even if we cannot compute these to great precision.

    I see the situation as analogous to fluid dynamics - the basic phenomena in the laboratory, and the underlying laws are known, and increasing precision of computation is where the game is.

    Unfortunately, string theory is nothing like that. String theory does predict a variety of phenomena. But the phenomena predicted by string theory within the experimental regime that we have access to, are not necessary phenomena of string theory; if these phenomena do not show up, it does not invalidate string theory. And we have no experimental phenomena for which string theory is a quantitative explanation (qualitative, yes, and numerological, yes) (I don't consider three coupling constants running into one value to be an experimental phenomenon as of yet, because it is based on extrapolation over a huge scale of energies). So, string theory is not in the situation of QCD.


  8. You're not right when you say "string theory is nothing like that", Arun. It's just a misunderstanding of some basic points of the theories and their goals.

    Both QCD and string theory are nice structures that however require some input to make full real predictions. In QCD we need various continuous parameters - quark masses etc. In string theory, we need to know the discrete choice "which vacuum do we live in". Maybe we won't need it at the end and there will be a unique prediction for the vacuum selection, but we simply need the discrete piece of information today.

    In both cases, there exist predictions that are independent of this input.

    You're also very wrong if you think that there is any difference between QCD and string theory regarding the possibility that "the phenomena may be explainable by other theories". You can explain the phenomena of the strong interactions by a convoluted theory with fundamental hadron fields, and their interactions, as long as you don't care that this theory will break down at high energies, and infinitely many irrelevant terms will be undetermined. At high energies, you will patch it with a different description that fits the Bjorken scaling, and you may be satisfied.

    The same thing holds for string theory. It is an elegant structure that unifies many different low-energy fields into the rubric of possible modes of a more fundamental object. Nevertheless if we are comparing it with the real experiments, we always prefer to use the language of low-energy effective actions - simply because we know that string theory reduces to them, much like QCD reduces to baryons and mesons at low energies. The low energy actions themselves can almost always "mimick" the stringy physics, but this physics will simply look unnatural in this language.

    If you think that there is some serious qualitative difference between these theories in high energy physics, be sure that you are misunderstanding something absolutely elementary about the meaning of string theory.

  9. I have not been following QCD for many years, so this might be a naive question: Do you know if the quark potential can be derived from the theory, or if they still stick with the Cornell potential for Charmonium or any other Ansatz?

    Mike Ros

  10. Lubos,

    In a hypothetical future scenario where string theory research falls of out favor, what concepts from string theory do you think will survive afterwards?

    In a hypothetical "year 2100" scenario where hardly anything from 20th century physics survives intact as it's known today, the few things I can see surviving intact would perhaps be the path integral formalism and the notion of anomaly cancellations. With respect to string related concepts, perhaps holography could survive intact in a hypothetical "year 2100". The Beckenstein-Hawking entropy result appears to be a very deep result.

  11. Most of the important things will survive, even if string theory falls out of favor.

    Just like Newton's laws and Maxwell's theory survived as limiting descriptions even in the 20th century, the Standard Model and General Relativity are guaranteed to survive as the low-energy descriptions even in 2100. I have no idea why you think that the anomaly cancellation and the path integral are the only concepts that will survive.

    The whole mathematical portion of string theory, including the detailed properties of the Calabi-Yau manifolds and string physics on them, will always survive at least as very important part of mathematics, and these things will always be studied as mathematical or theoretical physics simply because they are.

    Holography will survive, too. Gauge theories will always be important for our description of reality, they will never go away either, and it will also be important to know how they behave in different limits, for example for a large number of colors. In this limit the AdS/CFT correspondence holds, and it describes physics of the gauge theory including all details as a gravitational theory with all the string phenomena that string theory predicted independently.

    There is just no way how this can go away, except for the possibility that the people will lose their brain and ability to understand mathematical facts.

    You know, just the very fact of the importance of gauge theories more or less implies that all concepts of string theory - excited strings, D-branes, NS5-branes, bubbling topology, quantum mechanics of black holes, black hole thermodynamics, critical transitions - all these things are guaranteed to stay with us in physics.

    Of course, stringy phenomenology is a completely different question. Once we have some data from higher energies, at least 99% of the particular models will be understood as wrong and irrelevant. But this is true for all of phenomenology beyond the Standard Model, not just for stringy phenomenology!

    To summarize: all important mathematical features of string theory are guaranteed to survive as a part of physics even if string theory falls out of favor. If you only quote the Bekenstein-Hawking entropy, it just means that you misunderstand everything in string theory. In string theory, one can reproduce the entropy formulae from totally different, dual starting points. They're, more or less by definition, equally fundamental as the simple "area" approach to the entropy. It can't be otherwise.

    What I wanted to convey by this text is that some people are just overly self-confident about their ability to judge the theories they don't understand. If someone has a physics PhD, it does not mean that his comments about string theory will be valuable. There are many people who understand Newton's laws, less people who understand the Hawking-Bekenstein formula, and even less people who follow string theory. But this certainly does not mean that these insights are decrasingly deep! It just means that they're increasingly difficult.

  12. Hi Mike Ros,

    it's not my field, but see some recent papers about the potential quark models and their relation to QCD:

    I don't know what's the right short summary.

    All the best

  13. Neils Bohr once said (to other theoretical physicists)that as physicists they had a great advantage over philosophers, in that they had all published something that was subsequently shown to be wrong! I'm guessing that he had in mind the humility this engendered.

    Maybe that accounts for the fact that he and Einstein were able to conduct their long argument over quantum mechanics with the utmost civility and good humor. Of course it might have helped that tenure was not at stake for either.

    It's been observed that academic debates tend to be most bitter where facts are least in supply. Thus philosophers and literary critics debate ferociously, and theologians, including the recent Stalinist-Leninist variety, are happy to slaughter millions over slight differences in interpretation.

    I hope physics isn't coming to that, but we are suffering from a dearth of new data.

    I do think that cooling your jets would make you a lot more persuasive, Lubos. We all learned debate by insult in primary school, but most of us find it less attractive as we get older.

  14. Lumo saith: If someone has a physics PhD, it does not mean that his comments about string theory will be valuable.In this respect string theory is very different from climate science, where all the good comments come from MD's, history PhD's and string theorists;-)

  15. Dear imperialist pig (this is not an insult, it is your name) ;-),

    you may think that you are joking :-), but actually what you say is absolutely true and rather important. Climate science is really very different from string theory. The best climate science is done by political scientists, economists, and assorted physicists, while one must really study some background for years to get into string theory.

    All the best

  16. Lubos,

    as others have pointed out already you make many good points
    and in general you are a smart advocate of superstring theory
    at the same time keeping its critics honest.

    However, I think you are too negative on QCD. As far as I
    understand hadron spectroscopy is how theory (lattice QCD
    and other calculations) and experiment are compared.
    Recently a lot of progress has been made and these are
    difficult (grand challenge) but straight-forward projects.
    As you have pointed out these projects are not even too
    interesting, simply because QCD spectra and experiments match within the error bars (otherwise this are would all of a sudden be very hot!). It is not about one particle (you mentioned J/psi) but the whole spectrum.

    Obviously there is no such thing for superstrings and even
    worse (in my opinion) there is no experimental evidence yet
    for the main ingredient: super-symmetry.

    And this is another difference: A lot was know about the
    underlying symmetries before the standard model was put together. In the case of supersymmetry the symmetry was
    conjectured first and now we are waiting for some evidence.

    By the way, I always assumed that "discrete models" of
    quantum gravity were supposed to be calculation tools,
    what lattice QCD is to QCD.
    I do not think that many people would take "discrete models"
    to be a true image of reality ...

    Best regards,
    Wolfgang Beirl

  17. lumo said: The best climate science is done by political scientists, economists...That statement is absurd for the same reasons as claiming that string theorists are too brainwashed to critically examine their own theories. You can't do any kind of science without understanding the details, as Feynman liked to point out. Your friend Lomborg goes wrong repeatedly for just that failure. True many physicists have contributed to climate studies, (meteorology and oceanography are branches of physics), but only those who have gone to the trouble of mastering the details.

    You don't need to know algebraic topology or differential geometry (or even QFT or GR)to understand the physics of the climate system but you do need to understand fluid dynamics, thermodynamics, and radiative transfer as well as a lot of specific knowledge and understanding of the measurements involved and how climate models work.

    Of course anybody who knows a little intuitive psychology can figure out how to tell you what you want to hear. So hey, don't listen to me. I sure Rush Limbaugh can teach you all the climate science you want to know.

  18. Dear capitalist p.i.g.,

    we differ because I prefer the results, not vague prejudices. If you look at the list of the 25 most influential scientists (TIME), you will find Witten for string theory, indicating that it is useful to be mathematically experienced, educated, and super-smart, but for the future planning of the planet & predictions of the future impacts of the climate, you will find a political scientist by profession, Bjorn Limborg.

    Some people may study and be brainwashed by other alarmists, but these people are simply not so scientifically productive.

    I certainly don't believe that the average alarmists understand thermodynamics, radiative transfer, and fluid dynamics well, and even if some of them did, it is not enough to predict the climate variations.

    Yes, I am not dreaming about hearing once again all the alarmist teachings that I've heard about 1000 times already. And yes, I also think that Rush Limbaugh has probably more reasonable opinions about the climate than the people who play with their computer games that they call "models".


  19. Dear Wolfgang,

    thanks for your comments. It's a bit incomprehensible to me how can you create a calculational technique for a theory that does not exist. Lattice QCD only works because it is a discrete version of QCD, a quantum theory of exists.

    If you make lattice general relativity, it will not work because "quantum general relativity" does not exist as a theory unless you add all the necessary new physics at the Planck scale.

    We don't have an experimental confirmation of SUSY and other things predicted by string theory, but we also don't have any experimental indications that string theory contradicts something.

    Happy Weinachten,

  20. Lubos,

    I just do not see how you can "add all the necessary
    new physics" without experimental evidence and hints
    what the new physics actually is.

    Are you saying that you can proof that superstring
    theory is the only consistent theory at high energies ?

    No physicist in the 18th or even 19th century was able
    to "guess" that electrons, photons etc. are "necessary"
    or even that relativity was "necessary" based on their
    knowledge of the "low-energy limit" (falling apples etc. 8-)

    As I wrote already, I wish you good luck (and I really mean
    it) with your project, but I believe supersymmetry only when
    I see it. You have indicated that the LHC should do the
    trick ...

    Merry Christmas und eine Froehliche Weihnacht,
    Wolfgang Beirl

  21. Lubos,

    I can see you won't be persuaded by me.

    Merry Christmas or whatever you celebrate.

    From Da Pig

  22. Lubos,

    What is the experimentally accessible phenomena which requires string theory?
    What was the experimentally accessible phenomena which required a theory of strong interactions?

    One doesn't have to know any string theory or QCD to know that the experimental situation available at the time of development of the two theories is very different. The particle data book was filled with stuff begging for an explanation.

    What is the most striking prediction of string theory in the experimentally accessible regime? What were the most striking predictions of QCD? Some knowledge is needed here, but the situation is clear enough.

    Regarding QCD requiring external input such as quark masses - QCD was not meant to be a theory of everything. A Theory of Everything is on shaky grounds when it cannot provide a principle to decide between vacua.


    I see one of two possible answers to the above :
    a. something concrete, or
    b. "the above shows ignorance of string theory".


    IMO, General Relativity is more akin to string theory than QCD, in terms of its development. There was no experimental results pointing to anything wrong with Newtonian gravity; the real motivation for seeking GR was that Newtonian gravity is incompatible with Special Relativity. With regard to GR, we were fortunate in that a few observations were immediately within reach.


    Anyway, perhaps we should declare a moratorium on arguments on the validity of string theory until we see what the LHC shows.

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  24. You are only partially right about J/psi being an electroweak, rather than a QCD, discovery. It is true that the calculation was electroweak. However, it's effect was to convince the particle physics community that quarks were not simply a classification scheme but actually real particles. Heavy quark spectroscopy was the cherry on the icing, since the fact that running coupling implied energy levels of the predicted heavy quark states were perturbative was predicted before the J/psi discovery.
    The fact that SU(2) is "simpler" than SU(3) is due to symmetry breaking. Otherwise, it would also be asymptotically free, non-pertrurbative and probably confining as well.

    Is there "anything similar" in string theory? Other than the observation of KK states, I don't see any, since other "string theory" results (extra dimensions, SUSY, SUGRA) predate string theory and are not necessarily associated with it.

    Can this failure of string theory be excused by the necessity of a higher energy scale? Actually, not really. The cosmological constant is a big problem because the energy scale argument does not hold. Even if at the string scale the cosmological constant is zero, the EFFECTIVE LOW ENERGY VACUUM should have a QFT-calculable cosmological constant, which as far as we know is WAY bigger than ANYTHING observed. String theory has, as yet, NO answers to this, other than the phenomenologically useless anthropic argument and proposals (Dvali-Gabadadze-Porrati) "inspired" by string theory but not really part of it.
    As far as a TOE goes, that's a pretty damn big failure.

    Quarks/QCD, as you correctly say, was in contrast able
    to describe all of the existing physics.

    I am amused by your contention that string phenomenology, with no hint of an experimental connection as yet, is "more interesting" than QCD research. At the moment, there is an invasion of string theory types into QCD, since trying to connect to QCD via the ADS/CFT correspondence (A LOOONG shot, through I'd be very happy if it succeeded) is the only hope they have to connect the theory to the real world.

  25. Dear Wolfgang,

    yes, I am saying that we know that physics of string/M-theory is the only way how to tame general relativity in four (or more) dimensions in the quantum regime. We don't have a rigorous proof, but we kind of know it.

    More importantly, even if you thought that you see "something" else in the experiment, it will never allow you to make anything meaningful out of an *inconsistent* theory of "something" - like a quantum theory of gravity that contains nothing stringy.

    Yes, I agree that no one in the 18th century was able to predict the existence of electrons etc. And I therefore also agree that string theory is exceptionally amazing in this respect.

    Arun, also for you. The experiment that makes string theory inevitable is Newton's falling apple (together with the Standard Model), and it is just a matter of mathematical thinking to understand why. Theoretical physicists always needed to think mathematically, but they just need to do it more and more seriously, as the article explains more than clearly, I am sure.

    Your wrong conclusions show that you're not thinking about physics and the logical connections about its concepts seriously enough.

    String theory is not primarily a theory of the experimentally accessible regime even though we may see one of the major scenarios at the LHC: string theory is the only candidate for a theory of everything and a theory of quantum gravity which is not directly experimentally accessible. Its new phenomena simply operate at a higher scale, and they have higher requirement for the experiment.

    QED had low requirements for the experimentalists because QED is only an approximate theory of the "conventional" phenomena that are essential for electromagnetism, chemistry, and life. As the scale of the theories grows, the requirements grow as well. But this is not a problem of the theories: it is an inherent property of the questions themselves.

    You ask "What were the striking predictions of QCD?" I am not sure whether you read the article, or whether you just repeat the sillyness about "unprecedented disconnectedness of string theory from physics" without reading anything new. The article was explaining that there were no "striking" predictions of QCD, and all candidates what they were supposed to be, listed below the article, were actually predictions of the electroweak theory.

    The situation of QCD was qualitatively analogous to string theory in the sense that QCD is the only working quantum field theory that could agree with the phenomena that were known before, much like string theory is the only theory that can work with all of them including quantum gravity. But the new basic objects phenomena predicted by QCD can never be seen in isolation - the gluons, the quarks, their colors. QCD implies that they cannot be seen.

    If you were using the same naive thinking that you use against string theory, but against QCD, you would conclude that it is a theory based on objects that were never seen, and its description of the observed objects and phenomena is just qualitative, handwaving, and depends on many assumptions about distributions of quarks on the nucleons and so forth.

    You don't realize that because you were probably never thinking about QCD seriously enough. You think that one can just jump directly to string theory and say something meaningful about it. But it's not possible. One must first understand quantum field theory and GR well, and only afterwards she can start to appreciate what string theory does.

    The fathers of QCD - and many others - knew that the theory was correct even without the confirmed "striking" predictions. One really had to wait until the 1990s to see some cross sections - irrelevant for many others - to be measured with the 1% accuracy, and this is what forced the critics of QCD to disappear. Sorry, Arun, but the less smart one is, the more striking confirmed predictions one needs to understand that a correct statement is correct.

    A good physicist should always realize that he can be being foolish. You seem to be proud about being foolish which is simply a wrong approach to any difficult question.

    Happy Christmas (and other holidays for others)

  26. I've erased a silly posting by Quantoken that claimed that he had found a serious error in a rather well-known paper about a completely different topic than what is discussed here - but of course he won't tell you what the error is supposed to be. Given the fact that he does not follow mathematics even 3 levels below that paper, I feel that such comments add too much noise to this focused discussion.

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  28. For the QCD person:

    I was saying that J/Psi and the GIM mechanism were not based on any argument involving QCD whatsoever and they could not help to prove QCD. What I say is true, and not just "partially" true. One reason is simple: the GIM mechanism was already published in 1970. The GIM mechanism has nothing to do with "colors" of quarks, the strong force between the quarks, or anything like that which we call QCD.

    The argument of GIM was an electroweak argument, and it does not really depend on the question whether the quarks are "elementary" or just "thought" constituents, more precisely it does not depend at all on the character of the force that keeps them together. It does not depend on QCD, assuming that we mean the same thing by QCD.

    I consider the previous two paragraphs as answering - invalidating - the first part of your posting. Your posting then continues along the usual lines: other features of string theory predate string theory and preposterous Al-Qaeda-like rants about "failures".

    First of all, it is always like that in physics that the concepts usually appear before the full theory is seen. Second of all, string theory is, on the contrary, a counterexample in many respects.

    The QCD example happens to be discussed above. The concept of quarks appeared years before QCD and Gell-Mann got a Nobel prize for it in 1969. They became just a small part of QCD five years later or so.

    The extra dimensions predate string theory, yes - the string theorists are continuing many threads in physics and the Kaluza-Klein thread is one of them. But supersymmetry (in the Western world) was discovered in the context of string theory, in Ramond's homework problem to incorporate fermions into (bosonic) string theory. Most of the fancier details about extra dimensions also appeared in string theory. The fact that one can isolate an idea from string theory to make a simplified presentation for a layman does not mean that the idea is not a discovery of string theory.

    I insist that the people who keep on copying the confused statements about the alleged "failure" of string theory from each other don't understand physics well enough to participate in this discussion meaningfully, and I wish them happy holidays DESPITE their ignorance.

  29. I've erased another anonymous off-topic political stupidity about someone not being objective because of links with evil corporations. These blogs should have some kind of artificial intelligence to identify morons and deny them posting. ;-)

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  31. QCD:

    Experimentally verified predictions:

    Scaling violation in deep inelastic scattering
    Corrections to Bjorken sum rule
    Correction to Gross-Llewyllen-Smith sum rule
    Hadronic width of tau lepton
    b-b-bar threshold production
    Prompt photon production in pp and p-bar collisions
    Lattice guage theory calculations of heavy quark spectra
    Heavy quarkonium decays
    Shape variables characterizing jets at different energies
    Total e+-e- annihilation cross-section
    Jet production in semi-leptonic and hadronic processes
    Energy dependence of photons in Z decay
    Electroweak radiative corrections

    Consistency with current algebra (effective low energy theory of pions)

    etc., etc.

    All this comes from a simple theory that can be written down (non-perturbatively) in one line, involves exceedingly beautiful mathematics (gauge theory + Dirac equation). Only free parameters are quark masses. High energy behavior well understood and completely consistent.

    String theory:

    Experimental predictions:

    Not a single one

    Theory can't be written down in a single line, because (non-perturbatively), no one knows what it is.

    Merry Christmas!


  32. Peter, many of these things are discussed in the article. Some other predictions are real, of course, but they were only confirmed recently, much like string theory's predictions that will be confirmed in the future. Superstring theory is simply a newer theory than QCD, and everything is logically moved to the "future".

    Your statements about "simplicity" of QCD vs. string theory only reflect the fact that you don't understand string theory. String theory can often be simpler than QCD, and some things in QCD are harder. Of course, for you who knows nothing about string theory, it's not simple - but this observation has no scientific consequences.

    There are many people who hate relativity or quantum mechanics because it is not simple enough for them - for these simpletons. I called them crackpots, and I also explained why the case of string theory is analogous, just on a higher level. You're a crackpot with respect to the last 20 years of physics, Peter.

    Try to read Quantoken's postings on your blog carefully - and you will see how qualitatively similar to this crackpot you actually are! It's just a slightly different level of crackpotness.

    Happy Christmas anyway,

  33. Incidentally, the exact, non-perturbative definition of M-theory can also be written on one line, see e.g.

    Such amazing discoveries would have never been made if the people responsible for organizing physics were listening to crackpots like you.

  34. Lubos,

    I'll reiterate my previous sentiments. Using "crackpot" to describe somebody generally connotates that not only are they stupid, but they are crazy.

    Do you really think it is crazy to have objections to string theory?

    Do you really think it elevates the intellectual debate to call other, clearly intelligent people, crackpots?

    You do a very good job of defending string theory when you are not name calling. I think we would all prefer if you stuck to that.

  35. Dear David Guarrera,

    thanks for your feedback. Concerning craziness, it depends how you define "crazy". :-) I am using the word "crackpot" in a rather standard fashion, see for example

    to check that my usage of the word does not differ from the usual definition in a significant way - perhaps not at all. And you know I've been trying to be careful to reduce the usage of the word for those whose critical barrier in physics, which is very hard to overcome, is around 1975 ;-). OK, I will try to be even more friendly to them!

    Happy Christmas,

  36. Here's a crackpot, Lubos:

    "The Einstein's theory , in fact ,regards matter as an event
    which include time effect and space effect , the effect is one
    which people reaserch most in all kinds of effects of matter .
    Besides people reaserch the relative effect with time-space concept
    , they are heat effect , smell effect , sence oof taste effect
    , colour effect , vanish annnd production of matter effect .but
    these effects ,in essence ,connect ,such as ,when photon produces
    the time-space effect ,it also has heat effect..."

    cut and pasted from an email that was appearently just sent to a whole bunch of physics MIT people. My first crackpot email, how exciting!

  37. Dear David,

    I am so jealous about your romantic feelings ;-) of the first crackpot e-mail - the nice belief that there may be something deep in it and that you will gain a new perspective how to look on the world!

    And I am sorry in advance for your change of the opinion about these e-mails once you get 2004 of them like me. ;-)

    All the best

  38. I don't agree that physics is a progression from theories which are easy to compute and give accurate predictions to theories which are hard to compute and give less accurate predictions. You give the sequence Newtonian mechanics, relativity, quantum mechanics, quantum field theory, QED, electroweak theory, QCD, string theory, but you left out other subfields. Turbulence is still very messy, even though the Navier-Stokes equation was discovered during the 19th century. Computing the spectrum of multielectron atoms in atomic physics isn't easy, much less predicting chemistry from first principles. We just happen to be lucky we can predict the results of QED to such accuracy.

  39. I think that your example provides very good support for my thesis, too. The Navier-Stokes equations are, in some sense, very analogous to QCD. They are not *just* equations to describe turbulence. They describe fluid dynamics, including the simple, slightly perturbed linear phenomena. In this regime of small "Reynolds number", so to say, they are much more simple and "perturbative" than even high-energy QCD, and therefore there could be found in the 19th century by analyses of nearly frictionless liquids.

    Their application to the highly turbulent regime is a strongly coupled problem analogous to strongly coupled QCD, and of course it takes much longer to calculate this stuff.

    The accuracy success of QED is not just "luck". It has a rational explanation. QED is very close to fundamental physics and so its processes are "clean" and inherently accurate (unlike processes with some dirty composite matter); on the other hand, the coupling of QCD is very small which makes perturbation theory excellent. If the coupling is more like alpha=1/10, then one cannot really neglect the possible Landau poles etc. and one cannot get as exact results as from QED.

  40. I wanted to say "coupling of QED is small", of course. ;-) The third necessary feature that makes tests of QED so accurate is that the QED phenomena are easy to generate - the whole life and technology is based on them. What I want to say is that once one knows theories AB,CD,EF, how they work theoretically, he can also predict which of them will be accurately tested (knowing some basics of available technology, which, in some sense, is also included in the equations).

  41. Why we're lucky QED can be computed to such high accuracy

    1. The coupling constant e is small, about .1

    2. The nature of QED Feynman diagrams is such that internal vertices come in pairs and so, the perturbation series is in powers of e squared.

    3. The mass of the next lightest charged particle ("bare" quarks are confined), the muon is hundreds of times the mass of the electron and there are only a small number of charged light pions. The other charged particles have masses thousands of times that of the electron.

    4. For the neutral pion, the photon self-energy contribution from the diagram where a photon splits into a quark-antiquark pair which then annihilates to give back a photon has to be added similar diagrams where an arbitrary number of gluons are interchanged because QCD is strong. Nonperturbatively (with respect to the QCD coupling only!), this means the contribution from neutral pions is small.

    5. The weak interaction is very weak at the scale at which QED experiments are being performed, due to the decoupling of the W and Z bosons.

    6. Electrons aren't "colored" (carry a chromodynamic charge)

    7. Gravity is completely negligible.

    8. QED is renormalizable

    We would have to consider ourselves very lucky if there is another theory with this much "luck".

  42. Your comment about QED being clean because it's closer to fundamental physics contradicts the thesis of your main posting. :)

  43. For the contributor with the numbered list: the list is a kind of astrology. Let me try to explain you these "observations" of yours.

    1. I've mentioned the small coupling constant in the article

    2. It's not just QED where you get an expansion in powers of e^2 which is proportional to alpha. In *any* gauge theory, the diagrams "conspire" in such a way that the real physical observables are always expansions in terms of g^2. It's because you can write the Lagrangian in a way that only depends on g^2, as 1/g^2 Tr (Fmn.Fmn)...

    3. Muons are 206.8 times heavier than the electron. If there were several light charged leptons with masses comparable to the electron, be sure that it would not destroy the accuracy of QED calculations. The existence of light pions comparable to the electron would make the calculations tougher indeed, but the reason is not in QED itself: the reason is that you would have to include QCD to your QED calculations.

    4. Contributions from intermediate pions is small because they are approximate Goldstone bosons, but it has nothing to do with the overall simplicity of QED and hardness of QCD calculations.

    5. This point only says that QED is a good effective theory, and separated from the weak interactions. Of course if it were not a good effective theory, we would have to find another one. But be sure that today, even if the W and Z bosons were much much lighter, we would be able to make the same exact calculations of electrons' properties simply because we DO understand the weak interactions, too.

    6. The sentence "electrons are not colored" just means that "QCD does not operate at the electron scale" - it's a part of the definition of QED and QCD. Once again, if QCD electrically charged objects were as light as the electron, one would have to consider QCD and QED simultaneously, but then you could not say that "QED" calculations are difficult simply because the right calculations would NOT be just QED, but mostly QCD. It would still be QCD that is difficult.

    7. Gravity is negligible even in QCD, but this is not enough to make the calculations easy.

    8. QED, as well as the rest of the Standard Model, is renormalizable. QED is however perturbatively inconsistent unlike QCD. QCD is *more* consistent than QED.

    Your last "luck" comment is also confused. Of course that only the theory at very low energies - comparable to the lightest particle charged under something - is as simple as QED, and there can only be roughly one simple theory like that. But it does not mean that we can't find other theories. Well, we know that we have other theories, too.