One week ago, I argued that it is totally inappropriate to use the adjective "speculative" for theoretical frameworks such as supersymmetry.
It's a topic that's being discussed at many places which is why now, one week later, I will reopen these issues. Two days ago, Alok Jha of the Guardian wrote his text
leading HEP crackpots' website where Giotis, Urs Schreiber, and others kindly debate some nasty and stupid physics haters. Incidentally, the subtitle in Jha's article calls SUSY "the elusive followup theory to the Higgs mechanism". First, it isn't the only or most accurate way to describe SUSY which is mostly independent of the Higgs issues. Second, it isn't "the" only followup theory to the Higgs mechanism. Third, if it were "the [right] followup", it shouldn't be shocking if we need more than 1 year after the discovery of the Higgs boson to discover SUSY. One year is a short time in the history of physics.
But the basic point that Giotis began to emphasize to these demagogues is that the existence of SUSY in Nature and the discovery of SUSY at the LHC are two completely different questions. The LHC is an accelerator that allows us to reach energies that are one order of magnitude greater than the energies accessible at the previous top collider, the Tevatron. But even at the logarithmic scale, you would need to make about 15 analogous steps to get close to the fundamental scale (or the string scale).
The LHC may look expensive to some people but it's just far from a tool allowing us to directly test the most fundamental questions about Nature. Whether this increase of the log(energy) by 1/15 of what we would like is enough to find groundbreaking discoveries isn't and couldn't be clear. The LHC has found the Higgs boson and it is not "impossible" that this is it.
Columbus may have seen no traces of a new (or old and exotic) continent 2,000 kilometers West from Spain but that didn't imply that there were no new continents in that direction.
In Spring 2007, I listed some probabilities of different ambitious propositions in high-energy physics. It's still about right but I would say that my numbers were slightly optimized to be acceptable for the bulk of high-energy physics theorists. At any rate, you see that the probability that string theory is right is closer to 100% than to 50% (I would insert a higher number today); the probability of SUSY at the GUT or lower scale was quoted as 70% (and yes, I would write the same number today, with most of the remaining 30% corresponding to SUSY that only gets restored between the GUT scale and the Planck scale); and the probability of SUSY at the LHC was 50%. Note that it has never been "guaranteed" that the LHC would see SUSY and almost none of the SUSY champions has ever claimed such a thing. I surely haven't. I have always viewed SUSY at the LHC as a "damn plausible" possibility which is worth thinking about because it's so exciting.
Science produces insights, people write new papers, they have time to think about the old ones, so the probabilities are evolving. But there has been no complete revolution since 2007 so the current numbers are qualitatively the same. Of course that the probability that the LHC will find SUSY has moderately decreased because the LHC has already depleted a fraction (but far from 100%) of its ability to find new physics. The claims that theorists are completely negating their logic and circumvent the rules are just lies.
Now, of course that it would be great fun if the LHC started to produce tons of new physics. But something's being "fun" is a totally different thing from its being true – and from its being a prediction of a theoretical framework. Individual models may predict SUSY at the LHC – and parameter spaces of these theories may at least claim that "SUSY at the LHC is very natural". But it is not really clear which of these parameter spaces of classes of theories is right so science in general isn't able to predict when the supersymmetry or any other example of new physics will be discovered with any certainty at this point – and whether it will be at the LHC.
Ben Allanach, a top SUSY phenomenologist, said the following to the Guardian:
If the worst happens, and supersymmetry does not show itself at the LHC, Allanach says it will be a wrench to have to go and work on something else. “I’ll feel a sense of loss over the excitement of the discovery. I still feel that excitement and I can imagine it, six months into the running at 14 TeV and then some bumps appearing in the data and getting very excited and getting stuck in. It’s the loss of that that would affect me, emotionally.”It's obvious and right that folks like Allanach are excited according to the experimental data that are arriving. Again, excitement is something else than the truth, too. Truth may seem unexciting for quite some time but it's still the truth and to find a new great truth, one (and the mankind) often has to be very patient, clever, and hard-working. By the way, by "something else to work on", Ben Allanach surely doesn't mean "other kinds of new low-energy physics" because this will have been largely falsified, too.
At any rate, the supersymmetric model building isn't the only part of research of SUSY or the only body of knowledge we have about SUSY. SUSY is a much grander principle whose importance goes well beyond the LHC and its limits. It seems necessary to produce fermions, ban tachyons, and improve some hierarchy-like problems according to the only known (and, most likely, the only mathematically possible) unifying theory of quantum field theory and general relativity – an overarching theory we still call string theory. From this broader, top-down viewpoint that I prefer, phenomenologists are just "engineers" who are working on specific ways to discover SUSY (and questions what exactly will be seen) in the near (or not so near) future.
But the SUSY research doesn't really boil down to these phenomenologists' work and it didn't start with that.
In the early 1970s, SUSY began in the USSR when some mathematical physicists asked whether spacetime symmetries greater than the Poincaré symmetry may be compatible with everything we know; and it started in the West when Pierre Ramond decided to incorporate fermions to the (previously) bosonic string theory. The Soviet guys found exactly one possible extension of the known spacetime symmetries, a graded Lie algebra known as supersymmetry. And Ramond found out that the world sheet SUSY was needed to add the fermions to string theory. Later, Ramond's (and Neveu-Schwarz's) string theory was shown to predict spacetime supersymmetry, too (with GSO projections). Years before that GSO's development, people walking in the footprints of Wess and Zumino were already building 4D supersymmetric quantum field theories. In the early 1980s, other folks like Savas Dimopoulos and Howard Georgi – soon accompanied by folks like Gordon Kane and Howard Haber – were reconciling supersymmetry with the Standard Model and looking what new things result from this union.
Of course that the Standard-Model-like supersymmetric models are the most relevant ones for the interpretation of likely and plausible experimental signals in the near future. But they're far from being the most fundamental supersymmetric models. Even in our Nature, SUSY is almost certainly realized by a more comprehensive theory than the MSSM – at least if you want to include higher-energy phenomena.
In the more formal QFT and string theory research, SUSY has been essential for most of the important other developments in the last 30 years, too. It was needed to make grand unification really convincing (due to the coupling unification in MSSM). It was needed to calculate some quantities that established dualities (S-duality, U-duality, Seiberg-Witten analyses of gauge theories), it was essential for the twistor prescriptions for gauge theories, to calculate the correct black hole thermodynamics from a microscopic theory, to establish and study almost all specific theory pairs in AdS/CFT. Matrix theory requires SUSY as well and there are many more examples. SUSY cures or improves certain "systemic" inconsistencies and hierarchies and the modern research has simply shown that "Yes SUSY" is a better default assumption than "No SUSY". The progress leading to this paradigm shift was theoretical in character but it was totally scientific, anyway.
The continuing validity of the Standard Model is just another example of the wisdom that successful theorists often underestimate the range of validity of their own theories. This 1994 paper by David Gross summarizing the 1938 Warsaw conference showed lots of confusion that was around at that time. With the shining exception of the early de facto string theorist Oskar Klein, all top physicists were eager to falsify quantum mechanics and/or quantum electrodynamics at nearby scales, typically the Compton wavelength of the electron (distance in between the size of the atom and the nucleus). Of course that we know that the validity of QED goes much further than that. Those big shots should have taken their discoveries more seriously than they did. Sometimes old physicists who have already achieved enough want the younger theorists to share their relatively simple path to fame and wealth but Nature isn't obliged to obey. After some discoveries, it may often take a longer time for new discoveries to emerge. It's not necessarily the current physicists' fault; it's often just a fact about Nature's inner workings.
The case of the Standard Model may very well be analogous. People are obsessed by the wishful thinking that they will see lots of discoveries in the rest of their lives which is an assumption requiring that the discrepancies between the experiments and the established theories will be found rather soon. But it doesn't have to be the case. However, if the discrepancies aren't found quickly, it doesn't mean that there is no new physics or the search for new physics is futile. It just means that Nature eventually forces us to be more patient as She unmasks that some guesses about the speed of new discoveries were too optimistic. The "right amount of patience" is something that only Nature is allowed to determine and we ultimately learn what the amount was. It's totally dumb to impose any Stalinist or Smolinist five-year plans or five-year deadlines if we really don't know where (how far) Nature has hid the new treasures.
Phenomenologists who made bets on new physics around the corner shouldn't be surprised. If 1,000 phenomenologists propose 1,000 different, inequivalent models for new physics beneath a threshold, e.g. 1 TeV, then it's guaranteed that at least 99.9% of them would ultimately be shown wrong. We didn't need the LHC to arrive to this conclusion. High-energy physics was producing lots of hypothesized and competing models of new physics because the development of these ideas was still cheaper than the construction of the experimental gadgets (the price of the LHC equals 100,000 annual salaries of a phenomenologist – e.g. 1,000 phenomenologists funded for 100 years which is a lot of papers, of order 100,000 as well). This price comparison (the fact that relevant HEP experiments have become relatively expensive) is the reason why it's inevitable that model builders have probed many possibilities in advance, most of which are guaranteed to be excluded by the LHC, so the composition of the phenomenological literature was guaranteed to paint a "too optimistic" projection of the coming experimental discoveries of new physics. These comments seem sort of obvious to me, I have always kept them in mind, but it seems that many other people were not.
In each article of this sort, I have to emphasize that SUSY is just treated as a scapegoat. The LHC data are compatible with the Standard Model so far which reduces the probability that any theory of new physics that is too radical is right; the LHC razor cuts through the space of possibilities within each paradigm of new physics. SUSY is the most well-motivated principle of new physics but among the experimentally relevant proposals of physics Beyond the Standard Model, SUSY is not being hurt more than others. In fact, just the opposite is true: the relative weight of SUSY in the signals of new physics that have a chance to be found first is increasing as other theories are being excluded more "sharply" than SUSY. It's something we have known before the LHC began its operations, too.
John Ellis is quoted by the Guardian:
John Ellis, a particle theorist at Cern and King’s College London, has been working on supersymmetry for more than 30 years, and is optimistic that the collider will find the evidence he has been waiting for. But when would he give up? “After you’ve run the LHC for another 10 years or more and explored lots of parameter space and you still haven’t found supersymmetry at that stage, I’ll probably be retired. It’s often said that it’s not theories that die, it’s theorists that die.”The claim that theorists die but theories do not is catchy and cute but I don't really think it's an accurate description of the real world. Physicists are sometimes right, sometimes wrong, sometimes they die, sometimes they are born, sometimes they share their opinions with similar physicists, sometimes they don't, sometimes they're more right than younger physicists, sometimes they're less right than younger physicists, and so on. There isn't any general rule that would say that older physicists have to be wrong or something like that.
Moreover, what the death of old theorists is good for in most cases isn't really the death of wrong theories. Instead, the death of old theorists is often needed for the new and better theories to be allowed to live. Physicists like Albert Einstein would decelerate the progress in quantum mechanics if they were around for too long and in too high numbers; physicists like Paul Dirac would discourage younger physicists from computing renormalized corrections in quantum field theories because they didn't "believe" renormalization; physicists I could name but I won't name would slow down or cripple the research of string theory if these old chaps weren't dying sufficiently quickly, and so on.
I have already mentioned that in some cases, the old theorists are too conservative in the sense that they're no longer able to absorb the new theories and their logic, so they just use their authority to "refuse" them and be loud about it. On the other hand, I have also mentioned that old theorists often tend to expect more speedy revolutions than what Nature actually recommends. So old theorists may err in both directions. It's silly to assume that they always err in the same way. If this were the case, they would have to be completely stupid not to learn anything from similar if not "always the same" mistakes by their predecessors.
What I need to point out about Ellis' indication that he won't say that SUSY is wrong in his lifetime is that as far as I see the reality, he simply has very good scientific reasons to keep his opinions. It's something I have explicitly said many times myself. Long before the LHC began its operation, I was saying that the LHC may find no new physics but I would still be confident that string theory and SUSY are right in Nature.
It's pathetic for the aggressive cranks to interpret Ellis' words as a sign of dishonesty. The data coming from the LHC show that the proponents of any new physics have to be more patient than the (strongest) optimists were expecting. The data don't selectively imply that SUSY is wrong simply because SUSY models compatible with all the data exist. They just imply that the Standard Model is useful and accurate enough in a larger class of contexts but we know it's not quite accurate so it's a matter of when and what, and not if, the new physics starts to emerge. You can't rule out a well-defined theory such as a class of SUSY models differently than by falsifying it, by showing it's wrong, by finding a contradiction. Vague demagogic slogans about their "not even wrong" aren't enough for well-defined theories. Wolfgang Pauli has used the "not even wrong" slogan for largely ill-defined (and also fundamentally misguided because postulates-of-QM-denying) musings by David Bohm, not for a Pauli-style well-defined physics ideas such as SUSY. It's really an example of chutzpah to use Pauli's dismissive anti-Bohm slogan against a theory of the very kind that he would love (or discover!) if he hadn't died in 1958 (symmetries depending on the spin in new ways – which other physicist in the 1920s would have loved that?).
The Baltic American counterpart of Alexander Unzicker has not only offered this nasty and unfair criticism against Ellis. He also wrote:
The LHC will be in operation until 2030 or so, and you can always start arguing that 100 TeV will be needed to see SUSY (see here), ensuring that giving up won’t ever be necessary except for those now still wet behind the ears.You see that it's a disgusting and dishonest demagogy, the kind of demagogy that the scum that keeps on reading that blog likes to hear. The single word that is most dishonest about the quote above is "start". People won't start to claim that 100 TeV is needed to (almost reliably) see SUSY. They have always claimed so. Some relatively recent phenomenological advances made the sub-100-TeV scale even more attractive. There are numerous reasons to think that something should be happening over there.
After all, the Superconducting Supercollider that was canceled almost 20 years ago was supposed to have the center of mass energy equal to 40 TeV which is pretty much of the same order as 100 TeV. So it's completely unfair to suggest that it would be a sign of the theorists' arrogance and their changing of the rules during the game if they wanted a 100 TeV collider in 2015. They – we – have always wanted one. It was the default energy scale for the accelerators that the theorists were actually planning. Political reasons forced the theorists to be satisfied with a 13 TeV collider more than 20 years later and many theorists would say that it could be enough to see new physics, too. But no doubt about that, at 13 TeV (or even 7-8 TeV), the risk that no new physics is found is of course higher than it is at 40 TeV.
I want to say that the suggestions by Shmoits and many similar people that the theorists are moving the lamp posts towards high energies is partly a lie, partly a tautology. It's partly a tautology because high-energy physics is about the research of high-energy phenomena and the more it knows, the more it moves towards higher energies. So some trend towards higher energies is inevitable as we're learning more. On the other hand, the "next big step" that the theorists were contemplating (and they were already building the collider) was always close to the vicinity of a TeV or dozens of TeV and if something has changed about this energy scale, it went down since the early 1990s or so, not up. Of course that there are also particular models that predicted new physics at low enough scales that would have shown up at the LHC by now but it didn't. They're excluded or their parameters have to be shifted towards less natural, heavier values. This is an inseparable part of learning: it's inevitable that there have to be periods in which the old theories just work so some specific enough scenarios with new physics have to be ruled out. But there are many classes of theories for which this hasn't really taken place.
People have been telling me about some advantages of precision machines, not too high-energy colliders and factories, and I have never bought into them. The energy frontier is still the most important place for progress. To increase the colliders' energy is the most natural way to increase the probability of new discoveries. This will probably be the case after the LHC, too. I am totally confident that the bulk of good phenomenologists agrees with me (I think that e.g. Nima Arkani-Hamed surely does).
Once the SSC was canceled, phenomenologists were sort of externally forced not to talk about "truly" high-energy colliders and these high energies themselves. But I don't really care. The cancellation of the SSC is an item in the history books and it doesn't mean that the mankind won't ever be allowed to build a super-20-TeV collider again. I don't find the old story too important today. The politicians don't have the power or the moral right to eternally constrain the scientists. They only decide about things at the timescale of their term. At longer timescales, things may be different. Of course that I want a 100 TeV or so collider. I have always wanted one but in the near future, due to some progress in the technologies, a real-world-priced collider that is this strong will become feasible again.
The ideas that one should "give up" this and similar research are really recommendations to abolish high-energy physics as a scientific discipline. There isn't any scientific evidence that this is sensible. Instead, the scientific evidence is overwhelming that the people proposing such things are dishonest stinky scumbags.
What I find unbelievably irritating are P.R.-like proclamations by individuals such as the crackpot-in-chief:
Urs, The problem is more your use of language, since if you made it clear to people that when you said “supersymmetry”, you meant something that can’t ever be tested, they wouldn’t take you very seriously. I think you’re right that some string theory/SUSY enthusiasts post-LHC will argue that nothing has shown them to be wrong, and nothing in their lifetime ever possibly can, but to the extent that they make this situation clear they will have a serious credibility problem.What this jerk doesn't understand – or understands but doesn't want to admit – is that scientific theories aren't judged by their being taken seriously by brainwashed violent imbeciles among the laymen, the kind of low-brow primates who keep on devouring "Not Even Wrong" and similar feces and happily smack their lips at the same moment. Science is accepting or eliminating hypotheses according to the best scientific evaluation of the available evidence.
The available evidence makes it more reasonable to be convinced that SUSY is a part of Nature and unless a clear discovery occurs in the near future, a few years of extra experiments only have a modest ability to change the scientific opinions about this question because the experiments we can pay for today are simply not and cannot be machines that directly and "reliably" touch God's face. SUSY may turn out to be experimentally inaccessible by the LHC but the same would clearly be true about the non-existence of SUSY. So the evil people's belief that SUSY isn't out there is equally "unscientific" according to their own criteria – of course that these malicious kibitzers never apply their own rules to themselves.
Much of the science as we know it is about effects that we're certain or reasonably confident about but we can't directly experimentally test them. The closer we move towards the cutting edge, the higher percentage these "untestable" insights and hypotheses represent. Their being "untestable" in the most naive sense doesn't mean that they're not valuable. Many of them, including SUSY, are precious. One can be a completely uneducated, uncultural, brainwashed person who isn't able to understand these facts but these idiosyncrasies can't invalidate the facts. Columbia University's copy of Alexander Unzicker has been saying for years that in this sense, "[practically, now] untestable" ideas can't be an essential (or even legitimate) part of science. But what he has been saying has always been a lie, it is still a lie, and the people who haven't been able to figure out that this main thesis sold by the populist is rubbish have always been severely limited morons, they are still severely limited morons, and they will remain severely limited morons. Science has never worked according to their primitive recipes and to make things worse for them, science's distance from these recipes is increasing with time because our deepening understanding of the space of ideas allows scientists to separate credible theories and hypotheses from the raw experimental data by ever longer chains of derivations and implications.
Similarly, it is not the scientists' job to avoid a "credibility problem" with stupid and dishonest people who have no idea, especially with people who haven't even been capable of figuring out that Shmoits' rants are pure trash. Science is something completely different than P.R. This mediocre scum has gotten used to the situation in which their dirty buttocks are being licked by the populist politicians and media every day and everything must be according to the random desires of these degenerated spoiled brats. But as long as science remains science, it will carefully avoid this kind of external control. You don't find SUSY theorists "credible enough", stupid and nasty "Not Even Wrong" reader? Why don't you eat your own excrements to get some relief? The people who are telling you that science proceeds according to its looking "credible" to your random mood swings and vitriolic fads are not scientists even though sometimes they love to fraudulently create the impression. They are hardcore demagogues of the Shmoit kind – garbage like you. Thank you.
As a bonus, to expose the characteristic "quality" of the science by the anti-SUSY kibitzers, let me quote a comical crackpot called "N. Takanishi" at "Not Even Wrong":
Giotis, Three decades ago, I proved that SUSY cannot be the fundamental symmetry of physics. The reasoning is as follows. Evidently, SUSY, if it exists, must be spontaneously broken, but there is no Nambu-Goldstone fermion. Hence, one must assume that super-Higgs mechanism works. That is, supergravity must be encountered. However, quantum supergravity cannot be consistent with SUSY. This is because the global super-charge generators have no space-time index, while the translation generator P_mu does have a space-time index in contradiction with the SUSY anticommutation relation.And it goes on and on and on. You see that he (or she) says that the supercharges have no spacetime indices. If he had asked his high school teacher, he would have known that supercharges have a spacetime index, a spinorial one. That's indeed needed for the SUSY algebra to be OK under the Lorentz symmetry and to have a spacetime vector (the momentum) on the right hand side. Completely rudimentary misunderstandings and ignorance – and I would say stupidity at a totally hopeless level – is what underlies the sand castle built by the anti-SUSY Mujahideens.