Of Particular Significance

5. Supersymmetry?

Matt Strassler [January 28, 2013]

Supersymmetry is certainly a great idea — it is the one symmetry of space-time that is consistent with the other symmetries of space-time but hasn’t been discovered  in nature. But is supersymmetry part of the real world? If it is, it must be hidden (often said to be“broken” [and “spontaneously” so] — see Figure 2 of this article). Why is that?

All particles are either fermions or bosons. A consequence of supersymmetry is that there are superpartner particles for all known particles; the superpartner of a fermion is a boson and vice versa. Were supersymmetry unbroken, the up quark and its superpartner, the up “squark,” would have to have the same mass, so we would have found the up squark long ago. So supersymmetry, if it is true, is hidden; but the superpartners are still out there, just with larger masses. We have to try to produce them at the LHC, and then look for indirect signs of them following their decay, much as we look for the Higgs particle by looking for its decay products.

One reason supersymmetry is popular is that it apparently makes Einstein’s old puzzle, of how to combine gravity with the other forces into a consistent whole, somewhat easier to solve. But if that were all it did, superpartner particles could have masses at the scale where quantum gravity becomes relevant — as much as 1015 (a thousand million million) times too heavy for them to be discovered at the LHC. But if the superpartners have masses around 1 TeV/c2 or below, making at least some of them accessible at the LHC (and let’s call this LHC-accessible supersymmetry), then supersymmetry might resolve one of the other huge problems in particle physics. This is the hierarchy (or “naturalness”) problem.

In the Standard Model, the fact that the Higgs field’s value (246 GeV), and consequently the masses of the W and Z particles (80.4 and 91.2 GeV/c2), are so much smaller than the mass scale associated with quantum gravity — by a thousand million million (1015) — appears to be an extreme accident. If you write the Standard Model (the equations that describe the known elementary particles and forces), and you change any of its details by a tiny, tiny amount, you will find that the Higgs field’s value (and the W and Z particle masses) will change drastically, either becoming (a) zero or (b) much, much larger — typically 1015 times larger. Getting the Higgs field to be 246 GeV seems extremely unnatural — it seems a little like balancing a pencil upright on a ball which is balanced on the nose of a seal which is balanced on the top of an iceberg. How does nature do this???

Well, if all the superpartner particles have mass near to 1 TeV/c² = 1000 GeV/c² or below, then it turns out that the math of supersymmetry works to get this balance automatically!  Basically the shift in the Higgs field’s value caused by any particular boson is nearly canceled by its superpartner fermion, and vice versa, as long as particle and partner have masses that aren’t too different.

So “LHC-accessible supersymmetry” might solve the hierarchy problem [i.e. the naturalness problem] and if it does, it predicts many or all of the superpartner particles should have masses near 1 TeV/c², putting them within reach of the LHC experiments. The challenge is that there are innumerable ways to hide [i.e. break] supersymmetry, each one of which gives a variant of supersymmetry with different details for the superpartner particles’ masses and decay patterns… and this makes it hard (painstaking, but not impossible) to completely rule out the idea of LHC-accessible hierarchy-problem-resolving supersymmetry.  One has to systematically rule out all of the possible variants, and that’s hard work.

One remark before we continue: it is important to distinguish supersymmetry in its most general form from “minimal supersymmetry”, in which the only particles accessible at the LHC are those we already know (except with five types of Higgs particles), plus their superpartners.

So far, the LHC experimenters have three strong pieces of information about supersymmetry, all negative, but none of them definitive. They are the following (pay special attention to the third):

The reason why supersymmetry forces the Higgs particle’s to be lightweight comes from the fact that (a) the mass is partially set by the strength of the Higgs interaction with itself, and (b) that strength is related, in minimal supersymmetry, to the strengths of the weak nuclear and electromagnetic forces, which we already have measured. (No such relation between these quantities holds true in the Standard Model itself.) If it weren’t for the fact that the top quark has a large mass, the lightest Higgs particle would actually have to be lighter than the Z particle, 91.2 GeV/c2. Effects from top quark “virtual particles” can pull the lightest Higgs particle’s mass up to 120, but not all the way to 125, unless some nasty adjustments are made to the theory, or one considers non-minimal supersymmetry.

John Ellis in his Higgs Symposium talk specifically discussed the impact of these measurements on a subclass of variants of Minimal Supersymmetry (the “Constrained Minimal Supersymmetric Standard Model”). I’ll let the experts read the talk and spare non-experts the details. Overall, my read is that only a small fraction of the variants of this subclass of models is still consistent with data.

But non-minimal variants of supersymmetry may have extra particles and forces that can shift the Higgs particle’s mass up to 125 GeV/c2. As Nima Arkani-Hamed described in his Higgs Symposium talk, the simplest non-minimal models that could do the job would have

  1. Extra spin-0 Higgs-like fields (and their superpartners), and/or
  2. Extra spin-1 Z-like fields (and their superpartners).

These extra “singlets”, as they are called, are not affected by the electromagnetic or by the strong or weak nuclear forces, but can interact with the Higgs fields and shift the mass of the lightest Higgs particle in the theory up to the 125 GeV/c2 observed in data. These singlets have experimentally observable consequences that can be sought at the LHC.

While the experimenters are looking for signs of singlets, standard searches for the superpartners of the known particles continue.  There is special focus right now on those superpartners which must be LHC-accessible if supersymmetry solves the hierarchy problem — the ones that interact most strongly with the Higgs fields.  These are the superpartners of the top quark (and bottom quark), the Higgs particles, the W and Z particles, and even the gluon (which interacts with the Higgs fields indirectly but sufficiently strongly to be important.)

Fig. 1: In Split Supersymmetry, the fermion superpartners (of Standard Model bosons) are LHC-accessible, while the scalars (spin-0 bosons, partners of the Standard Model's matter fermions) are 100-1000 times heavier.  This fails to solve the hierarchy problem, but does alleviate some other problems while retaining some good features of supersymmetry.
Fig. 1: From Arkani-Hamed’s talk: In Split Supersymmetry, the fermion superpartners (of Standard Model bosons) are LHC-accessible, while the scalars (spin-0 bosons, superpartners of the Standard Model’s matter fermions) are 100-1000 times heavier. This fails to solve the hierarchy problem, but does alleviate some other problems while retaining some good features of supersymmetry.

A rather different possibility, which has been around for a long time and which Arkani-Hamed likes (and I used to not be very fond of, but the Higgs mass measurement forces me to pay it more attention), is that supersymmetry does not entirely solve the hierarchy problem, but solves it part-way, with the remainder explained by a lucky accident or though a selection bias (such as the “anthropic” or “structure” principle, whereby the reason our part of the universe looks unusual is that (a) the universe is much more immense and diverse than we realize, (b) most regions are uninhabitable, and (c) only in rare regions with very unusual properties can there be anything like stars, planets, and evolution.)  This is the notion of “split supersymmetry”, whereby the fermion superpartners of the photon and the W, Z and Higgs particles remain relatively light and LHC-accessible, while the boson superpartners of the matter fermions are heavier by about a factor of 100 or more. (This kind of splitting arises very easily in theories of supersymmetry breaking, and in fact one typically has to work to avoid it.)  A complete solution to the hierarchy problem is abandoned, but it turns out this idea has some nice features too, which I’ll skip (but see Figure 1).

For our purposes, the key point is that this type of splitting of the superpartner particle masses can easily lift the Higgs particle’s mass to 125 GeV/c2. And as Arkani-Hamed pointed out, the absence of LHC-accessible boson superpartners will cause the fermion superpartners to have rather long lifetimes, long enough that they may travel a macroscopic distance (millimeters to meters) before they decay.  That means they would be discovered in searches for relatively long-lived particles that decay while traversing a detector like ATLAS, CMS or LHCb.  Not enough searches of this type have been done yet on the 2011-2012 data, so even without new data, there are opportunities for discoveries over the coming two years, as the already collected data is further analyzed.

Arkani-Hamed’s current view is that if supersymmetry exists and is LHC-accessible, then the key question is whether supersymmetry is natural though non-minimal, or minimal though not natural (Figure 2).  This will most likely be resolved (if supersymmetry exists and is LHC-accessible at all) during the 2015-2020 period, though progress might certainly occur sooner.

Figure 2: Arkani-Hamed's slide indicating the two possible implications of a Higgs with a mass of 125 GeV/c^2; either a complicated natural model emerges soon, or an unnatural but simple model may be the way to go.  (Of course supersymmetry might simply not be LHC-accessible, but that was not the point of this slide.)
Fig. 2: Arkani-Hamed’s slide indicating the two possible implications of a Higgs with a mass of 125 GeV/c^2; either a complicated natural model emerges soon, or an unnatural but simple model may be the way to go. (Of course supersymmetry might simply not be LHC-accessible, but that was not the point of this slide.)

82 Responses

  1. Professor, you wrote:

    “One reason supersymmetry is popular is that it apparently makes Einstein’s old puzzle, of how to combine gravity with the other forces into a consistent whole, somewhat easier to solve.”

    Am I interpreting this correctly by saying, if a particle exists to “unify” all the fundamental forces, including gravity, then that particle must be a superpartner of itself, perfectly identical?

  2. Can I simply just say what a comfort to uncover somebody who truly understands what they’re discussing on the net.

    You certainly realize how to bring an issue to light and make
    it important. More and more people should look at this and understand
    this side of your story. I was surprised that you are not more popular given that you certainly have
    the gift.

    1. You probably mean my techni-cousin … 😀 ?

      I’ve heared there are not many physicists that think the new particle is a techni-dilaton rather than a higgs. Nima Arkani-Hamed even said in a talk (as a joke) he would kill himself if it happend to be one 😉

  3. If there is a layman out there who misunderstands ‘naturalness’ but understands the Stone Weierstrass theorem then I should much like to meet them, or their math teacher who obviously went beyond the call of duty to educate their students.

  4. I have found that laymen frequently misunderstand the word naturalness and minimalism in physics. I like to explain it with an analogy (it’s a wrong analogy but it captures the essence). Think of an unknown function defined on some interval. Now it is a theorem in mathematics that every function that obeys certain nice properties can be arbitrarily approximated by a polynomial (Stone Weierstrass theorem). If you Didn’t know the function or it’s polynomial solution, but only knew that it was a physical solution to some real phenomena in nature you would first guess that it was minimal. Namely that it only involved a few terms, like x or x^2 and not thousands of complicated terms.. You would also guess that the coefficients in front of the polynomials were set to some physical scale, and of order one. This is what is called natural. So it would be unnatural if the solution involved polynomials with coefficients like .000000006. Eg it would be weird if in a problem involving the collision of planets, that you found a term involving atomic physics that wasn’t negligible. It’s not that it’s not possible, but it would look weird.

    Anyway there is more to it than that, but at least it gets people off the idea that natural means some sort of philosophical bias.

  5. Contrary to what you said , it is not only the weak/gravity force ratio , but also the cosmological constant and the universe phase space (according to penrose ) are extremely un-natural…the former by 120 orders of magnitude , the later by 10^123 orders of magnitude……

    1. I’ve said that elsewhere on this site; and in this article, I never said “it is only the weak/gravity force ratio”. This article is not intended to cover the problem of naturalness in great detail and discuss all the different issues involved. Please, let’s stick to the topic… which is supersymmetry at the LHC.

  6. Why naturalness / fine tuning / hierarchy are called problems ?
    Why we do not start from the a/m as facts …as the way nature IS ?
    Then why we do not try to include these facts in a sound , back to earth world view ?
    I mean why do we have to invent hundreds of speculations to force facts to yield to a pre-conceived materialistic world view ?
    Look at time , effort and money spent in the multy / meta , / extra verses just to deny creation of our extremely fine-tuned universe …..
    Look at the grand deception that man descended from the apes while identical DNA for man and apes is a supreme proof that humanness and apeness are not in DNA…..
    Among tens of hundreds of papers written by respected scientists about consciousness you will not find a single word starting from DNA as input going thru epigenomics , interactomes , gene regulatory networks , cells behavior ….etc to get consciousness as output…..
    Similar situation in cosmology / physics , starting from fields , particles , forces….etc as input , you will never find any mathematical structure results as output in any constant or law of nature….prove me wrong by showing all of us where we can find equations dealing with forces , fields , particles and results in the laws of Q.M or the constants of nature. ……you can put it by hand , but never in principle it can be generated naturally .
    My proof :
    M-theory……..TOE …..does it succeeded in doing that ? no my friend , no and never ….
    Arrogance ? no my friend but reflection on what is science…..the great flower of the human unique un-natural mind.
    Thanks with respect.

    1. Hi aa.sh,

      I believe you are generally a nice and very curious guy, which is a good thing.
      But here now you are ranting way too much about fundamental science and the people doing it … :-/

      Please dont do that on this nice site.

  7. Sorry Matt. but many sources contradict what you said that naturalness has a physical bases , sources deny this and clarify naturalness as ( bias to WHAT LOOKS RIGHT ) .
    By sources i mean web search under ( naturalness problem ).

    1. The web is a fine tool, but at least 90% of it is nonsense and probably more. This is no more true than in the area of science where even the most authoritative sites will use simplification and misconceptions. Information is only as good as its source and you will find many that are dubious indeed online.

  8. ( Who are you to tell scientist……….) , well , i am nothing , i am mere ordinary person , but it is not me who in 90% of cases deceive people by declaring as truth what is mere speculation , i am not talking about you Matt. , you know what i said about you in this particular article , i am talking about “scientists ” who deceive the innocent , who inforce their materialistic , naturalistic world view on the believers , who use science for atheism propaganda ….
    Science MUST stay neutral , science MUST declare as a fact what is proved as a fact beyond any shred of doubt .
    Cosmology and biology are now tools for atheism or agnosticism at least ….scientists MUST always clarify scope of science , declaring : we only are talking about how things interact , it is not our scope or capacity to talk about origins or why things happen , or what things are …..only how things interact.
    Science CANNOT speak about the meaning of the cosmos , only about the actions in the cosmos….
    Here we face scientists as preachers for scientism……for the cult of science , here is what i meant by saying ; be humble……and i repeat ; be humble this is a great virtue science deserve.

    1. Well, I am a “scientist”, and when you talk about “scientists”, you are talking about me. You are generalizing and stereotyping, and generally being a bigot. I don’t like bigotry anywhere, and I am not going to tolerate it on my website. And I don’t like you telling other people what they must or must not do. Please desist.

  9. How something began may or may not have some relevance as to how it works. Such is the case of, say vitamins, many believe those obtained ‘naturally’ from a source are better than those synthesized, but there has been no demonstrable difference in how they work. We also understand a great deal about life and the solar system while not being sure (But having some good ideas) about how they came about.

    Likewise a lot of theories in physics don’t care about where the system they’re used in comes from, fluid dynamics works wherever the liquid comes from. String theory (or something else) may give us an origin for the universe, or it may stop at some point and say ‘Beyond this point the theory breaks down’ We will see.

    1. One of my hobbies is astrobiology. Last year tremendous progress was made in my opinion, since people could elucidate the thermodynamics of replicators. RNA is currently the only molecule that fits the bill (England, in review but on arxiv) and its thermodynamics is analogous to crystallization (“Thermodynamic Basis for the Emergence of Genomes during Prebiotic Evolution”, Woo et al, PLOS Comp Biol).

      Bonus: Spontaneously assembled Shoztak protocells (lipid membranes, has spontaneous growth and division) with replicators are already a biological population from 2 components, they compete for lipids to the point they cannibalize, and fastest growing replicator eats lipids fastest. I.e. given an alkaline hydrothermal vent for continuous phosphate activation and strand separation, the process from chemical to biological evolution won’t stall like earlier found possible pathways could.

      Woo et al even get the timing correct. Today’s vent lifetimes max out at ~ 100 000 year, crystallization to a replicator pool takes ~ 30 000 year, and RNA in Shoztak protocells survives 4 years before hydrolysis.

      I.e. early cells can passively drift to infect neighboring alkaline vents until they evolve genes and make their own ATP from glycolysis, freeing them to live in the rest of the ocean.

      If something like that isn’t responsible for life, I will eat my (non-existent, being an a-hateist) hat.

      1. Possibly, I prefer the cold start to life myself. At this point in time I don’t really see any particularly outstanding theory, though there are some tantalizing hints. (In the same way I don’t accept the usual ‘Look at how many stars there are!’ argument for extraterrestrial life, which always seems to me to just be throwing big numbers at the problem.) But at the rate things are going, it’s quite likely that one of us will have to eat their nonexistent hat before the decade is out.

    2. “How something began may or may not have some relevance as to how it works.” – Doesn’t the term “how” imply at least some sort of “dynamics” and could easily be translated in “How did it work when it started working”? Which, of course, opens the possibility that it worked differently then as it does “now”. But the term “beginning” when used in physics or applied to the physically observed universe can NEVER mean an absolute beginning which, i believe, leaves us with two options: “beginning” applied to the physically observed universe always means some sort of “optical” boundary, not a real one. This optical boundary could potentially be “pushed further”, there can never be something physically untestable, unobservable, etc., in physics terms – in principle.Even if it were – it wouldn’t have had necessary ontological implications…
      I am very sorry if i trolled with a philosophical, kind of an off topic comment. Great site!

      1. This, of course, leaves no place for “scientific evidence” for the existence of god, but does no harm to the notion of creation and creator at all…

  10. In Fig. 1 – From Arkani-Hamed’s talk: In Split Supersymmetry – it is shown that dark matter is checked off, and so is Unification. Is this because dark matter may be associated with ‘singlets’? And can you elucidate what he meant by checking off Unification? Thanks. Sue

    1. I am afraid you have this backwards; Split Supersymmetry is the case without extra singlets.

      Garden variety LHC-accessible minimal supersymmetry has as two of its nice features

      1) possible unification of the non-gravitational forces: calculations in this theory show that the strengths of the electromagnetic, weak and strong nuclear forces are all the same [caveat: with a twist] at very short distances, comparable to the distances where quantum gravity becomes important.

      2) a candidate for a dark matter particle: a neutral massive stable particle (a mixture of the superpartners of the photon, Z and Higgs particles), affected by the weak nuclear force but not the electromagnetic or strong nuclear forces, whose abundance after the Big Bang turns out (after calculation) to be roughly the amount of dark matter observed in the universe.

      Those particular features (which are absent in the Standard Model) are preserved in Split Supersymmetry, even though supersymmetry is then no longer the complete solution to the hierarchy problem.

      And a benefit is that certain problems that garden-variety minimal supersymmetry tends to create, mostly because of the squarks and sleptons, are alleviated in Split Supersymmetry because those superpartners are heavier (and their effects therefore smaller) than in garden-variety LHC-accessible supersymmetry.

  11. As for ( extremely powerful DNA evidence ) , allow me to refer you to the most recent finds concerning DNA and the fact that it has NOTHING to do with morphogenesis or behavior or any mental capacity……..it is ONLY a partial protein synthesis prescription, so even identical human/ape DNA is a very strong proof that DNA has nothing to do with humanness or apeness , . here my friend you cannot argue with me , here you need intense dose of knowledge.
    As for your ( IT MAY turn that it is not necessary to be extra-physical….) this is just wishful thinking , as we are sure NOW that it MUST be extra-physical .
    As for Einstein and company , you know , they failed and i predict that it will persist , NO PHYSICAL CAN GENERATE PHYSICAL FROM NOTHING , physical comes always FROM physical……..give me just one example of a physical mechanism generating physical from true actual real pure NO-THING.

    1. Word salad. Humans _are_ apes, just not the same species as chimps, bonobos, et cetera.

  12. I think it is wrong to call ( directed specification ) extreme accident or biased selection……
    I think it is time to face reality with honest attitude by giving reality its proper description.

  13. On what foundation except prejudice it is decreed that physical constants MUST have the relative values of 1 ( naturalness ) ?
    Then on what bases naturalness of physical theories decreed to be a super-fact ?
    Then why it is resisted to accept what nature DECLARES ?
    IF weak force is 10^32 times stronger than gravity , who are we to say that this is un-natural ?
    ALL what we find in nature is natural by definition ……it is our prejudice which lead us stray .
    Those who explain fine-tuning by our existence are the ultimate aspect of ultimate prejudice , why ? because for us to fit the universe , the later must have been DESIGNED to fit us , saying that our existence IS the explanation of fine-tuning is declaring that science gone bankrupt and that prejudice is its supreme resort. period.

    1. “On what foundation except prejudice it is decreed that physical constants MUST have the relative values of 1 ( naturalness ) ?”

      It is not decreed, and no physicist will ever tell you that it is decreed. That’s your word.

      As for the notion that physical constants should be `natural’ — that is based partly on the observation that it is true for hadrons, and it is true in condensed matter physics, which have very similar mathematics. So it is not purely prejudice; we do see examples in nature that buttress the notion of naturalness. However, everyone in the field will tell you the arguments are not convincing. In short: you are inventing the word “decree” here to make a political point, which annoys me and probably most of my colleagues reading your comment.

      “decreed to be a super-fact” — your words, not mine.

      “IF weak force is 10^32 times stronger than gravity , who are we to say that this is un-natural ?”

      Because of the mathematics of quantum field theory, which is very well understood; and because there are no other examples known in nature, which indicates that our understanding is very good. However, the term “un-natural” is a physics term, not an English term — it does not mean “impossible in nature” or “contrary to nature”. It means “not expected in a generic quantum field theory (with or without gravity).”

      “ALL what we find in nature is natural by definition ……” — Now you are making word stew.

      “because for us to fit the universe , the later must have been DESIGNED to fit us , saying that our existence IS the explanation of fine-tuning is declaring that science gone bankrupt and that prejudice is its supreme resort. ”

      You are so over-reacting that I am beginning to think you ought to go take a nice vacation in the country. Here we are discussing various reasonable possibilities, and you’re worried about science going bankrupt. Well, I’m not.

  14. Dear friends :
    All evidence converge on a single fact : there are something un-natural , something beyond physics , something extra-physical operating in our universe….
    The problem is ; most scientists decided /decreed / declared that every thing MUST be natural , every thing must have as end explanation a physical one ….
    now they are forced to see that physics itself is un-natural…..
    No physical mechanism can generate a physical universe , this is a logical and mathematical fact…..
    scientists ; please try to transcend prejudice and try to see what is happening ….. the universe CANNOT be explained from inside the universe ….
    Fine tuning is a solid , firm fact ……you cannot explain it as an extreme accident…..this is THE ultimate prejudice based on fact free fantasies …….
    scientists : please ; some , tiny feeling of being humble facing the wonders of creation..

    1. Please, I really cannot tolerate this level of arrogance. Who are you to tell scientists to feel humble facing the wonders of creation? What do you think we do all day?

      As for the statement “now they are forced to see that physics itself is un-natural” — no, this has not yet been demonstrated, nor, were it demonstrated, would it lead to your conclusion that “No physical mechanism can generate a physical universe” because you are confusing the English word “natural” with its precise meaning in physics. The word “unnatural” does not in any way imply “unphysical”.

      And as for “All evidence converge on a single fact : there are something un-natural , something beyond physics , something extra-physical operating in our universe” — not all evidence is converging, and what it converges on is not a fact but a lack of a fact — and your statement of that “fact” again confuses “unnatural” in its precise physics meaning with “unphysical” — an unjustifiable logical step that you should not make.

      You are letting your own prejudices push you to draw conclusions that the data and logic do not warrant.

  15. As one is looking into the future (next 2 years), when LHC resumes operation at higher energy regime of ~ 7 TeV per beam, I see folks running LHC want to achieve and have a target of an integrated luminosity of 250 fb–1 /year. I wonder what the difference could have been had the collisions consisted of and in the future were to consist of p (p-), rather then proton proton. Would we have found some new phenomena sooner (besides SM H)?

    1. proton-antiproton collisions are indeed more sensitive to things that are produced in quark-antiquark collisions, such as heavy Z-like or W-like particles, and (in some circumstances) gluinos [and other heavy colored particles that can only be made in color-neutral or color-octet final states, if that language makes sense to you].

      Since the Higgs is predominantly produced in gluon-gluon collisions, and protons and anti-protons have the same number and distribution of gluons, there wouldn’t have been any difference in the Higgs discovery — though there would now be some small differences in precision Higgs studies.

      But antiprotons are hard to make in great abundance, and indeed this was the limiting factor at the Tevatron (the LHC’s predecessor).

      So I think this question is too ill-defined. If we could both have 6.5 – 7 TeV per beam AND have enough anti-protons to have 10^16 collisions per year, then we’d want that, yes. But the technology isn’t able to do it. So what are you really asking? Are you asking if it would be worth having antiprotons and fewer collisions per year? probably not.

  16. Quite possibly, though I must say that as a christian people have often assumed that I believed hat the universe was ‘fine tuned for life’ and that I have heard nothing but scorn for this view. There may well be a sizable segment of the physics community that would accept something like that, but it has been my experience so far that in absence of strong evidence tot he contrary a theory with any sort of ‘It just happened that way’ is considered incomplete and in need of a deeper explanation.

    1. “in absence of strong evidence to the contrary a theory with any sort of ‘It just happened that way’ is considered incomplete and in need of a deeper explanation.”

      That is because the basic assumption of science is that things do have an underlying logic and work according to rules. If you’re doing science, you will continue and you must continue to run with that assumption. If the assumption is wrong, science is the wrong tool and you will get the wrong answer. We do not know science is always the right tool to get to truth, and we will never know it.

      1. On the other hand there is in history no other competing tool that has worked, so it is effectively “right”, but fallible of course.

        We know the universe doesn’t need finetuning for life in all respects (weakless universe and other allowed parameter variation), so the religious “finetuning argument” as I call it isn’t correct. Most horrendous break with facts however is that the universe is exactly opposite to a universal biosphere (we are pond scum on a dust speck).

        The most likely environmental principle (selection bias) should be maximizing a likelihood over a distribution of allowed parameters (hopefully playing out by some dynamics like inflation), say, the weak anthropic principle. That could be said to “happen that way” but perhaps not be said to be a singular “accident”.

        Even so, split symmetry wouldn’t be finetuned (so not accidental) in all respects.

        To return to the topic at hand, I am curious if split supersymmetry would inherit the quasistable vacuum of a 125 GeV standard Higgs field? (Re “Selection bias or anthropic principle reasoning only needs to be applied specifically to the Higgs particle”.)

  17. Huh, this is a very interesting text 🙂

    For examply I did not know before reading this, that just the mass of the discovered higgs rules out most of the minimal supersymmetric models…

    These new singlets seem a bit “pulled out of thin air” to me, or are there some natural broder frameworks available, that naturally contain such new spin 0 or Z like particles?

    About the same issue I am curious concerning split supersymmetry. Is some kind of anthropic reasoning indispensable for it, or are there ideas about some broader frameworks out there which produce “naturally” such a spectrum of (s)particles ?

    I am very curios to know, and at Physics SE there is no point anymore in asking such things, so I’d like to see an answer here 🙂


    1. String theory, in any application to the real world, always generates singlets. Most string theory-based models generate a whole host of them. They might all be heavy or might interact with us very, very weakly, but they are almost certainly out there if string theory is right. And dark matter may well be made from singlets. [Indeed these were among the motivations behind my Hidden Valley papers.]

      For split supersymmetry — the spectrum of superpartners that arises there easily shows up in models (see Arkani-Hamed’s talk and his papers for references.) Indeed the usual spectrum people talk about, where the superpartners all have roughly the same mass, only arises in a relatively small fraction of models of supersymmetry breaking (and most such models have a problem with unobserved flavor-changing decays of quarks to other quarks with the same charge, and with large violations of the symmetry called CP.) Selection bias or anthropic principle reasoning only needs to be applied specifically to the Higgs particle, to keep it from being as heavy as the heavier superpartners, and bring it down to the mass scale of the lighter ones.

      1. Could we have a link to the Hidden Valley papers mentioned above. It seems interesting.

  18. It is a logical impossibility for any physical mechanism to generate Q.M. , and Q. PRINCIPLES since it must transcend Q.M. AND PRINCIPLES so it must be extra-physical .
    This is fact not prejudice.

    1. We don’t know that. It is prejudice. It may turn out someday to be wrong; it may be that it is not necessary to be extra-physical to generate quantum mechanics. In fact Einstein spent 30 years trying to find just such a theory.

  19. MATT.
    Why you call prejudice as something very bad ?
    Strings , branes , extra D. , susy , ……etcetcetc. are all some kind of prejudice , even the higgs was 30 years ago a kind of prejudice that there MUST be something of certain properties …….is not this the scientific method ?
    If after long research one reaches a conviction , it is not prejudice any more….
    prejudice is declaring that man descended from the apes….that IS prejudice
    prejudice is declaring that chance created the universe…..that IS prejudice
    prejudice is declaring with such trust that GOD does not exist ……that IS prejudice…
    all of the A/M were declared by scientists chatting over a beer.

    1. prejudice often leads to error; it is simply important to keep track of what is knowledge and what is prejudice.

      As for humans being descended from apes; that is not prejudice. There is extremely powerful DNA data that is profound and convincing. You should read more about it.

  20. Do GUTs or string models easily or generally give non-minimal SUSY? I mean, what looks like the simplest alternative from the bottom up might not be so realistic from the to down perspective.

    1. Models arising within string theory generally give non-minimal SUSY, but whether it looks approximately minimal by the time you get down to LHC energies is something you can’t calculate. The simplest GUTS (grand unified theories — theories that unify the non-gravitational forces) easily can give non-minimal SUSY but don’t have to.

  21. perhaps there is some unified essence connected to the formation of this universe but perhaps that essence is something altogether separate – perhaps this universe was the result of a broken symmetry in the first place so the particles of this universe cannot account for the whole from which it came – maybe that could be why there are more particles than antiparticles

    1. perhaps. maybe. perhaps not. maybe not. to do physics, you have to turn speculations into mathematics, and from there to predictions. otherwise it is just chat over a beer.

      1. thing is – if we cannot really know for sure how the universe came about then ultimate physics, like string theory is supposed to be, seems, for the most part, relegated to speculation

        1. Well – I don’t see why this comment is relevant to this webpage. It seems like some sort of political statement. But I will address it, because it’s wrong.

          A theory may answer questions like “how do particles and forces work” (which string theorists hope their theory will answer) without addressing questions like “how did the universe get started” (which string theory does not necessarily answer.) The first question is a “how do things work” question, the second is a “what exactly happened” question. In principle, string theory could be shown to be the correct answer to the first question without being able to or being required to answer the second question. Any theory that could answer the first question would be considered a huge success. And it would mean the theory would not be relegated to speculation.

          Meanwhile, what is “ultimate physics”? The answer to every possible question? In my view, not all questions can or will ever be answered, no matter what theory we have, and some things will always be relegated to speculation. So the fact that string theory — and any theory of any sort — won’t answer every question I can possibly think of doesn’t bother me.

          1. political? hardly – by ultimate physics i just meant a GUT or TOE and wondering how much of physics is now more speculative than not because of lack of knowledge about origins – it’s just that how it all began must have some relevance to how things unfolded and how things work – so how do we know if we get things right if we can’t know whether it can be shown to proceed from an origin – string theory posits all kinds of things but experimentation is not possible, and the math is so complex involving so many calculations that super computers can’t be trusted to get them right, also there are those convoluted Calabi-Yau concoctions of which there are a near infinite variety that somehow need to be sorted through to find just the right one – seems to me we’re just flying blind if we don’t know what it all needs to connect to on the other side of the big bang

            1. It’s not more or less speculative than before. You say :

              “it’s just that how it all began must have some relevance to how things unfolded and how things work – so how do we know if we get things right if we can’t know whether it can be shown to proceed from an origin”

              There’s a huge logical fallacy there. There are many examples where I can most definitely learn how things work without knowing how they started. In fact, that’s what physicists have done — with enormous success — for centuries.

              ” string theory posits all kinds of things but experimentation is not possible”

              This is irrelevant to your point — experimentation is only impossible in practice, not in principle. That may change over time, as technology improves and/or calculational techniques improve. And why is all your focus on string theory? That may not be the right theory.

              “the math is so complex involving so many calculations that super computers can’t be trusted to get them right, also there are those convoluted Calabi-Yau concoctions of which there are a near infinite variety that somehow need to be sorted through to find just the right one”

              Again, irrelevant; this is about the technical details of string theory. What does this have to do with the world and what we know about it? Are you assuming that string theory is correct, and therefore what we know or don’t know about string theory is in one-to-one correspondence to what we know or don’t know about the world?

              “seems to me we’re just flying blind if we don’t know what it all needs to connect to on the other side of the big bang”

              And are biologists unable to understand biology if they don’t understand exactly, in every detail, how humans evolved?

              The notion that presently operating physical mechanisms cannot be understood without a complete understanding of the past is simply not defensible. You’re assuming this is true; but I’m pointing out it was never true and it isn’t any more true now than it was in the past. The whole point of doing controlled experiments is that they DON’T require a complete understanding of the past.

        2. TJ,
          your rants against ST are quite off topic here, so could you please stop this?
          You are disrupting the nice physics discussions.


          1. rants against ST???
            actually just curious about what chance it has to ever be verified – but i see that is a very touchy subject here – like i am casting doubt on someone’s belief system – anyway it has been extremely revealing and i will leave you to your nice discussion

            1. TJ — this is a bizarre reply. You’re the one who brought us a belief system: that a successful theory has to explain the past if it is to explain the present. For you to read the replies you’ve received as someone here having a belief system suggests you’re not even bothering to read my replies carefully. You’re the one who focused on string theory and verifying it; nowhere in this article, nor in my replies to your comments, have I suggested that anyone ought to believe specifically in string theory as a “theory of everything”. It’s a speculative theory without basis in experiment.

              Moreover, you now say you are “curious what chance string theory has to be verified”; well, that’s not what you asked previously, so that’s not the question I answered. (And moreover, it’s not the appropriate place for this discussion, since this article has nothing to do with string theory at all.) If that’s what you are curious about, however, then the answer is: maybe; and the point is: it can be verified as a theory of how things work without it being shown that it is a theory of how the universe came to be. And the former will surely require fewer technological advances than the latter.

          2. Ok sorry right, that was not very mindful indeed …

            I was just annoyed because I noted too, that TJ is not in the slightest interested in the answers he gets, but rather tried to start an off topic, not constructive, and not so nice discussion :-/
            I know what such people are up to and where they come from …

  22. The reasoning seems to be related to the anthropic principle. In short, the earth happened (“accidentally” in human thinking) to fall into a “Goldilocks” region of the universe that was conducive to the evolution of intelligent life. Therefore, humans are here to realize (and be amazed at) how hospitable and “fine-tuned” the earthly situation is. On the other hand, there may be billions of other planets in regions of the universe NOT conducive to the evolution of life, and therefore intelligent life forms would NOT be there to realize how inhospitable the conditions are.

    1. You’re referring to split supersymmetry here? Indeed I specifically mentioned the word “anthropic” in my text (though that’s not the only principle that can be at work.) More generally this kind of effect is known as a “selection bias”.

      1. Fascinating stuff, as always! – concerning the “finetuned” Higgsfield: I always thought for a field to have some effect it should have a gradient as if it were completely flat it’s effect would cancel itself out from all directions and it therefore may just as well be nonexistent – therefore in my naive view: could the Higgsfield itself have a gradient and we just happen to be “accidentally” lucky to find ourselves at its 246 GeV value

        1. Actually,your impression isn’t right. A field need not have a “gradient” (i.e. a changing value over space) to have an effect. The Higgs field is the only example of an elementary field that has this property, but we know of many composite fields that have this property. For example, there is a somewhat similar field inside a superconducting metal; its effect is to change the electric and magnetic properties of the material. And even the density of air can be thought of as a field which need have no gradient to have an effect. Even if the density of air is constant, it still creates all sorts of effects, such as pressure and buoyancy.

          It is true that the electric field is the gradient of a field called the “electric potential”. But it’s the electric field, not the electric potential, which has physical importance. That’s a subtle point though… A constant electric field certainly will make charged particles move.

          1. Yes for sure but to measure the effect of pressure or boyancy you have to introduce a device which contains a different pressure or boyancy – otherwise you get no effect, so in essence you have to create a artificial local gradient to see or measure these properties

        2. It is possible to measure air pressure via air resistance as well as a few other methods that do not rely on an instrument having a differing pressure. Consider also fields like humidity or even, locally speaking, gravity (Which at human scales is nearly constant at the earth’s surface.)

  23. Uncertainty is a strict requirement for our cosmos as we know it as part of Quantum world , but it is not as such for all levels of possible existence ,

    1. Whether or not uncertainty applies to various aspects of our universe is a very profound topic indeed. If our current situation is indeed an ‘accident’, the result of a process governed chaotically, then the answer to ‘Why is the universe like it is?’ is one of chance.

      On the other hand it is possible that there was no alternative but for a universe to be like the one we observe, that the exact state of our universe is specified in some manner in some theory. I think this would be preferred by physicists, it’s certainly preferred by me.

      1. “that the exact state of our universe is specified in some manner in some theory… I think … would be preferred by physicists,”

        It is preferred by some physicists. What I prefer, personally, is to get things right, no matter what they are. Truth before prejudice, if you will permit me.

  24. Yes , that is true , BUT , with respect to US , not in-principle ipso facto , as you know , my stand is ; there exist a level of information from which even uncertainty in physics is not valid……uncertainty is a result of non-zero value of h , so in principle there could exist levels of information where h = zero and every thing is certain to any degree.
    Science can never prove that as false.

  25. Word of honor to MATT.:
    I really – and very honest – find in your writings what i miss in ALL other ones….

  26. So my statement is not false since if we know the precise initial conditions we ca calculate it , every thing can be calculated from first principles , accidents = ignorance .

    1. I agree it isn’t strictly false (and I revised my comment around the time you sent this last one.) But be careful; because of classical chaos and because of quantum uncertainty, sufficiently precise initial conditions are sometimes impossible to obtain *in principle*, not just in practice. So I don’t think accidents = reducible ignorance; rather, in many cases, accidents = irreducible first-principle ignorance.

  27. On what bases we decree something to be natural or un-natural ? every thing in nature IS natural , it is our theories that may be un-natural.

    1. your statement “any accident based solution is no solution at-all” misses the point.

      We live on the earth, a rock 93 million miles from the sun. Why is it 93 million and not 97 million or 89 million? That’s an accident; you’d have to know the precise initial conditions of the disk out of which the sun and planets formed to calculate it. The precise height of Mt Everest? An accident. The area of the island of Sri Lanka? An accident.

      Recognizing which quantities are accidents (or partial accidents), and which quantities have to be calculated from first principles, is a crucial step in doing science.

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