You’ve probably heard this story before, likely from Brian Greene.
In string theory, the fundamental particles of nature are actually short lengths of string. These strings can vibrate, and like a string on a violin, that vibration is arranged into harmonics. The more energy in the string, the more complex the vibration. In string theory, each of these vibrations corresponds to a different particle, explaining how the zoo of particles we observe can come out of a single type of fundamental string.
Particles. Probably.
It’s a nice story. It’s even partly true. But it gives a completely wrong idea of where the particles we’re used to come from.
Making a string vibrate takes energy, and that energy is determined by the tension of the string. It’s a lot harder to wiggle a thick rubber band than a thin one, if you’re holding both tightly.
String theory’s strings are under a lot of tension, so it takes a lot of energy to make them vibrate. From our perspective, that energy looks like mass, so the more complicated harmonics on a string correspond to extremely massive particles, close to the Planck mass!
Those aren’t the particles you’re used to. They’re not electrons, they’re not dark matter. They’re particles we haven’t observed, and may never observe. They’re not how string theory explains the fundamental particles of nature.
So how does string theory go from one fundamental type of string to all of the particles in the universe, if not through these vibrations? As it turns out, there are several different ways it can happen, tricks that allow the lightest and simplest vibrations to give us all the particles we’ve observed.* I’ll describe a few.
The first and most important trick here is supersymmetry. Supersymmetry relates different types of particles to each other. In string theory, it means that along with vibrations that go higher and higher, there are also low-energy vibrations that behave like different sorts of particles. In a sense, string theory sticks a quantum field theory inside another quantum field theory, in a way that would make Xzibit proud.
Even with supersymmetry, string theory doesn’t give rise to all of the right sorts of particles. You need something else, like compactifications or branes.
The strings of string theory live in ten dimensions, it’s the only place they’re mathematically consistent. Since our world looks four-dimensional, something has to happen to the other six dimensions. They have to be curled up, in a process called compactification. There are lots and lots (and lots) of ways to do this compactification, and different ways of curling up the extra dimensions give different places for strings to move. These new options make the strings look different in our four-dimensional world: a string curled around a donut hole looks very different from one that moves freely. Each new way the string can move or vibrate can give rise to a new particle.
Another option to introduce diversity in particles is to use branes. Branes (short for membranes) are surfaces that strings can end on. If two strings end on the same brane, those ends can meet up and interact. If they end on different branes though, then they can’t. By cleverly arranging branes, then, you can have different sets of strings that interact with each other in different ways, reproducing the different interactions of the particles we’re familiar with.
In string theory, the particles we’re used to aren’t just higher harmonics, or vibrations with more and more energy. They come from supersymmetry, from compactifications and from branes. The higher harmonics are still important: there are theorems that you can’t fix quantum gravity with a finite number of extra particles, so the infinite tower of vibrations allows string theory to exploit a key loophole. They just don’t happen to be how string theory gets the particles of the Standard Model. The idea that every particle is just a higher vibration is a common misconception, and I hope I’ve given you a better idea of how string theory actually works.
*But aren’t these lightest vibrations still close to the Planck mass? Nope! See the discussion with TE in the comments for details.
4 gravitons 
So vibrations of the string don’t account for standard model particles? So if a quark, or an electron, is ultimately a string (according to string “theory”), then it’s a non-vibrating string? I get the impression that because it’s so difficult to vibrate a string (due to its large tension), any vibrating string would have a mass nearing the Planck mass. So, such things can’t describe Standard Model particles, which have vastly smaller masses. Does string “theory” have an explanation for these particles?
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The lowest vibrations of strings (and generally only the lowest) account for standard model particles. These generally have zero (stringy) mass, and get their mass from interactions with other particles as they would in the standard model. Only the higher modes have energies near the Planck mass.
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4gravitons, to be sure, I agree with all the claims you made in this answer to Rick. He also asked: “Does string “theory” have an explanation for these particles?”
The answer is that string theory has an explanation. These particles are simply new predictions of string theory. These predictions aren’t easy to verify in practice but they’re verifiable in principle and the existence of the whole tower of the massive states – probably not far from the Planck scale – is needed for the consistency of the whole theory. (There are many string vacua – so there are many ways to add the tower that produces a consistent theory. But the qualitative lesson that a tower has to exist is right in general, as discussed below.)
If the coupling is weak, many of these excited energy levels may be trusted and the corresponding heavy particles are metastable. Their number increases exponentially with the mass. This exponential increase is very close to the exponential increase of the black hole microstates (as a function of the increasing black hole mass). The detailed form of the exponent is different but when one continuously increases the string coupling, a part of the Hagedorn string spectrum literally becomes the black hole eigenstates for small enough black holes.
So another, detailed explanation of these heavy states is that these states simply have to be there in a consistent theory of quantum gravity because they’re the states that become the black hole microstates when the coupling is adiabatically increased. I think it’s a damn good and important justification. It’s actually a general consistency constraint in quantum gravity. If you have a theory where some particles are much lighter than the string scale and interact with some couplings much smaller than one, and if this theory coupled to gravity produces some “fuzzy” mass scale parametrically smaller than the Planck scale, the theory should better produce a lot of (an exponentially high number) of states lighter than the Planck scale because there exist black hole microstates that must belong to this mass range and the sub-Planckian, “gravity-free” theory must know about them. In this sense, “something like the stringy” extended structure of the particles at a parameterically small coupling – a feature of string theory – is a derivable condition that holds for any consistent theory of quantum gravity with some extra dimensionless couplings.
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(My comment disappeared…I assume because comments are under moderation.)
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Yes, all comments are moderated, and most appear after a brief wait.
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I always wondered what role time plays in string theory. In music, just a tone, a vibrating something (single sound) basically means nothing. Music evolves from the interplay of different self similar combinations of sounds that has a certain feature, that can be (and are) called “swing”. Swing basically means that – over time! – a certain “drive/groove” (or call it mass) evolves, when there are stresses on the closing symmetry in each subdivision within the self similar series of sound events.
I suppose, string theory doesn’t have an eye on that kind of evolution over time, or does it?
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The main difference between music and string theory is that in music you have someone playing the strings. That person can add energy to the system, and change which vibrations happen. In physics, those changes have to come from interactions between different vibrations/different strings, which means they stabilize over time without some sort of external driving force.
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My point is indeed, that “swing” evolves/stabilizes over time, in the “listener system”, without any direct connection to the “player system’s” added energy. It seems to be part of music itself, so to speak
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Another part of the question was about time. To my understanding time doesn’t play a constituent role in classical physics as part of the description of a system.
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Understanding you better, I think a key difference here is that in physics, these sorts of things stabilize extremely quickly.
In a sense there is a vague analogue to what you’re talking about, not just in string theory but in field theory more broadly: you start out with certain fundamental fields, but when they interact new combinations can be emphasized and end up seeming fundamental. In a sense, that’s how the Higgs field works. Particle masses are the “swing” that results when for example an electron “note” is played alongside a Higgs “note”. But because this occurs extremely quickly due to the energy scales involved, it doesn’t really make sense to think about it happening over time: the “swing” end state is all we observe.
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Man, thank you, you got me. But that “swing end state” maybe can be understood better when looking at the analogies happening in music. Stress on “closing symmetries”, … maybe that one is too cryptical.
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I don’t understand how you could conclude that “particles aren’t vibrations of strings”. In perturbative string theory, they surely are. The facts that realistic models have complicated compactififcations or branes; or that SUSY is a symmetry of all realistic vacua, don’t contradict the statement that particles are vibrations of strings.
Moreover, at the beginning, you implicitly criticize the picture painted by folks like Brian Greene. But Greene very carefully and accurately explained the compactifications, SUSY, and branes in his books and films and lectures, too. So this criticism is just unjustified – he was much more accurate than you are, I think.
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Note the parenthetical in the title (and the fact that “you” in a science popularization title rarely refers to Lubos Motl).
I’ve run into a surprisingly large number of people who think that phenomenological particles are the higher harmonics of strings. That’s pretty straightforwardly wrong, and the only place I can imagine they’re getting it is from that cute animation near the beginning of Elegant Universe.
Brian Greene does indeed cover the whole story eventually. I don’t know whether he could have presented things better in order to clarify that no, those higher harmonics have other purposes, they’re not different SM particles. It’s quite possible he was as diligent as he could have possibly been. Nonetheless the misapprehension persists.
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Dear 4graviton, first, I’ve never used my name. This is not about me. At most, I mentioned Brian Greene. But this is about the right communication of the results of string theory to the interested lay audiences.
What you wrote about the harmonics; that’s not “wrong”. The particles are music of the strings. They are vibrations. On general Calabi-Yau manifolds and with the fermionic vibration degrees of freedom, the spectrum may be complicated, just like the tons going from complicated enough musical instruments may be complicated (even if the wood of the guitar only gives some “flavor” to the underlying vibrations of a string).
But there are also many phenomenologically interesting models that do reduce to the the free strings with the simple spectrum at g=0 given by the higher harmonics. In particular, that’s the case for the whole class of heterotic strings in the free fermionic description. All the fields on the world sheet are free and the one-string theory is solvable in terms of harmonic oscillator. At the string scale, all the squared masses may be computed by adding the integers (or their fractions like 1/4) from the simple higher harmonics.
At the tree level, all the spectrum we know from experiments comes from the strictly massless part of the spectrum – all massive states end up being at the string scale which is usually assume close to the GUT scale etc. The more refined spectrum of masses is then generated just like in quantum field theory, e.g. via the Higgs mechanism, the Higgs couplings to other particles, and so on (and also by confinement).
But these Higgs-like interactions may be said to be interactions between different particles or vibrating strings. It’s still true that the elementary objects entering these interactions are vibrating strings – and in this case, exactly with the special, simple, frequency=N harmonic-oscillator-based spectrum.
The lesson of these simple models is quite general. Even in heterotic strings on Calabi-Yaus and other models, it’s still true that the observed experimental part of the spectrum comes from the strictly massless states of string theory in some approximation – and all the (small) masses we know from the experiments are obtained by some “subleading” interactions such as QFT-like interactions of the states at low energies.
I just don’t think it’s justified for you to fight against the idea that the spectrum comes from the higher harmonics. It’s “morally” the case – the qualitative lessons are correct – and there are whole classes of example realistic vacua where the assumption is strictly true. The higher harmonics indeed are what perturbative string theory is largely all about and why the name is so appropriate. If some laymen make this simplification while you believe that the higher harmonics have nothing to do with perturbative string theory, well, I would argue that they have understood string theory more correctly than you have.
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One of the “qualitative lessons” I’ve seen people get from the higher harmonics picture is that the infinite tower of string excitations are all phenomenologically accessible particles, and hence that string theory predicts an infinite tower of such particles, rather than understanding that the infinite tower is off at the Planck length. Another “qualitative lesson” I’ve seen people get is that each harmonic is another SM particle, and thus string theory ought to be able to predict the masses of the SM particles. (This last admittedly comes with not just a misunderstanding of the higher harmonics, but a misunderstanding of how compactifications work.) It’s simply not true that someone who thinks that electrons, protons, and neutrons are all just higher modes has gotten the right qualiative picture.
Now, in only describing the use of higher harmonics for eliminating UV divergences, I admit I’m failing to mention phenomenological models in which their existence has an important effect on the low-energy spectrum. Compactifications &c only work because we’re dealing with strings, not particles, hence why you can’t get phenomenologically viable models out of N=8 SUGRA. I’ll have to think about how to work that into the post cleanly. There’s also the issue of misunderstandings like “Rick”‘s earlier in the comments, but I don’t think he’s misunderstanding what your model of laymen would suggest he would.
That, by the way, is the relevance to pointing out that the “you” in the title doesn’t refer to Lubos Motl. I feel like you have a tendency to model the average layman as just a younger version of yourself, and that has a distinct tendency to backfire.
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Dear 4gravitons, I don’t understand in what sense this debate may have something to do with “my imagining that the reader acts like a younger counterpart of myself” – which he or she surely should (and yes, people who may be approximated in this way are certainly the “most important idealized readers” of all complex enough posts and comments I am writing – because I know how I would have appreciated if similar things were available when I was 17 or so etc. – if something backfires, it’s because many readers fail to act this sensibly but it’s not my fault), by the way. It’s you who is trying to make things harder than they are here, not me!
The general compactification doesn’t have masses that are simply an integer in some units. But there are compactifications where the whole stringy spectrum has squared masses that are integer multiples of a basic “quantum” – the quantum is a numerical multiple of the string tension.
With a finite number of exceptions that are massless, all these states are very heavy – the string scale which will probably be inaccessible at the colliders forever. But even this isn’t necessarily the case. There are stringy braneworlds whose spectrum is still simple (masses are integer multiples of something) and the string scale is a few TeV.
So even when you add the adjective “phenomenological”, I am still not so sure that your fight against these conclusions is justified. The conclusions seem basically right to me. This heavy tower of string states is qualitatively a tower of combinations of higher harmonics (a single string has many vibrations at the same moment, so 1 higher harmonic is not 1 particle; instead, 1 string with integer occupation numbers for each harmonic gives a particle because many waves coexist on one closed/open string i.e. on one particle). And these are damn real states, they are what is making the theory string theory, what distinguishes it from point-like particle-based quantum field theory.
If the string coupling is weak, lots of these excited string states are lighter than the Planck scale, so they may be viewed as long-lived particles with the properties indicated by the perturbative string theory you are trying to undermine.
I am just not getting why you are trying to assault these general conclusions – I think that they’re indeed the right “layperson’s” sketch of the very point of (perturbative) string theory. It seems that you criticize some details but you are almost certainly throwing the baby out with the bath water.
Also, you wrote: “…and thus string theory ought to be able to predict the masses of the SM particles… It’s simply not true that someone who thinks that electrons, protons, and neutrons are all just higher modes has gotten the right qualiative picture. ”
Sorry but the misunderstanding you just described has nothing to do with a minunderstanding of string theory – the person misunderstands the Standard Model because the protons and neutrons are not elementary particles of the Standard Model. Moreover, the person who seems to misunderstand these basics is you – or you have created a straw man. Realistic string vacua describe – in terms of one strings – elementary particles of the SM i.e. gauge bosons, Higgs, leptons, and quarks. And these do come from the vibrations of strings and may come from a model where everything boils down to linear fields on the worldsheet, although in almost all these models, all the SM spectrum comes from the strictly massless states in this tower.
But the rest of the stringy tower is equally real and equally important, and the properties of all these states – at the massless level and all the massive levels – are equally derivable consequences of a string vacuum. The people whom you criticize are right if they get this lesson.
Concerning your “and thus string theory ought to be able to predict the masses of the SM particles”, this statement is also right. Every (stabilized) string vacuum predicts the masses of all the particles – it’s a question whether people know how to do all the calculations in a given case. But a fully understood string vacuum surely has this property. The non-expert person who says that “string theory can calculate all the particles’ masses – including the SM elementary particles’ masses” – almost certainly can’t do such a thing. And he would have naive ideas about “how simple” calculations are enough to determine the masses. But his qualitative opinion that string theory can compute all the masses is certainly correct and important. I just don’t know why you would try to bring this person into doubts. This person hasn’t said anything wrong – it’s you who seems to be saying something wrong (namely that string theory can’t do it, it surely can).
I will look at your exchange with Rick separately.
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You really don’t have to explain the basics of string theory to me, and I don’t understand why you spend the majority of this comment outlining them. In particular, I’m really not sure why you keep bringing up the existence of compactifications where all squared masses are integer multiple of the string tension, so I suspect there some sort of sustained miscommunication here.
One thing I wasn’t aware of is that there were braneworld models where some of the tower of heavy string states are TeV scale, unless I’m misunderstanding you and you’re referring to KK modes in which case I’m confused why you’re even bringing it up. Regardless it still seems rather a corner case in this context.
Literally the only point my post is making is that the tower of massive string states does not correspond to the spectrum of known or phenomenologically accessible particles. Not that the tower is irrelevant (indeed I emphasize its relevance to quantum gravity, if not in any detail), nor that the higher harmonics metaphor is bad for describing it. Just that the low-energy spectrum we observe (usually) comes from the massless states of the theory.
People do misunderstand that, even if you’ve never seen them do it. They probably aren’t your ideal, diligent readers. Many of them are educated older folks, who aren’t going to spend time reading a long fairly technical post but are still interested in getting a rough idea of how other fields work.
If you think that in clearing up that misconception I’m introducing new ones, then we can figure out what about my language is doing that and fix it. But if you just think that there’s no point to clearing up that misconception as long as people have an otherwise correct “qualtiative understanding” then I’m not sure you get the point of this blog.
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“You really don’t have to explain the basics of string theory to me, and I don’t understand why you spend the majority of this comment outlining them.”
If you really don’t understand why, let me explain the trivial point: it’s because your blog post above – which defines the topic – is about particles’ being vibrations. That’s surely about basics of string theory, isn’t it? How can you claim that these comments are off-topic? I simply wouldn’t agree with the title and many other sentences and they are about basics of string theory, so it’s obvious that I have to talk about basics of string theory if I want to show in more detail why I believe that you are not right.
There are models with the string scale near a TeV. Open scholar dot google dot com and search for “TeV string scale”. It won’t give you all the papers with low-string-scale braneworlds, not at all (there are at least hundreds of them), but surely enough to get dozens.
“Literally the only point my post is making is that the tower of massive string states does not correspond to the spectrum of known or phenomenologically accessible particles.”
Sorry, with powerful enough experiments, it does correspond.
“Just that the low-energy spectrum we observe (usually) comes from the massless states of the theory.” – The states are only massless in some approximation, when the (string-string) interactions and/or virtual strings are neglected. More precisely, these important states in the string tower are just light. If you fix this inaccuracy in your sentence, you will get “low energy states are light states” which is just a vacuous tautology.
But string theory implies much more than vacuous tautologies. Every string vacuum predicts the whole mass spectrum (and interaction strengths) of all observable particles from the analysis of vibrations on strings and these strings splitting and joining. All these particles that are allowed in Nature form a tower, exactly as you claim the things not to be.
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Again, I’m not denying that the tower of higher string modes are ever particles. If you actually thought my post was intended to say that, then it’s more confusingly written than I had thought.
I’m denying that higher string modes are the particles that the “you” in the title thinks they are. In particular, as I go right out and say in my post, I’m saying:
“Those aren’t the particles you’re used to. They’re not electrons, they’re not dark matter. They’re particles we haven’t observed, and may never observe.”
With experiments that access the Planck scale, or in corner cases of models where the string scale is around a TeV, sure, we could observe the particles corresponding to higher modes. But in context, the distinction I’m drawing is pretty clear. It’s disigenuous of you to pretend to not understand it, but I’ll rephrase it in more precise terms:
What I’m saying is that the particles that the typical layman has heard of don’t come from anything above the lowest level of the tower of massive string states. They come from the lowest-lying states, from the states that would be massless in the free theory (but yes, gain masses due to interactions).
The key phrase here is the particles that the typical layman has heard of. Hence the sentence about electrons and dark matter, hence the “you” in the title.
In a model with a TeV string scale, sure, that’s not necessarily true. That wasn’t something on my mind when I wrote this post, and it seems like enough of a corner case that it isn’t worth bringing up in this context. In any case, it doesn’t seem like the core of what’s bothering you.
What I can’t tell is whether you honestly believe that I’m trying to argue that the tower of massive string states don’t have corresponding particles, or whether you simply think it’s a way someone might misunderstand my post. If it’s the latter, we can figure out where the post is misleading and adjust it. If it’s the former, and you still believe that at this point, then I don’t think this discussion is going to be very productive.
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Sorry, but the string tower does include the elementary particles that all the laymen have heard about, too. They’re just a part of the truth, a subset of the list – a subset near the low values of the masses.
And then there are particles that the laymen haven’t heard of – and no one has seen them, either. A particular string vacuum makes a lot of detailed predictions about those. There is no real difference between the knowledge of the list of particles according to a layman and according to an expert. Empirically, we just only know the particles up to the Standard Model so far (with the possible exception of the 750 GeV hint etc.) so your repetitive claims about the meaning of the word “you” have absolutely nothing whatever to do with the reason why I think that your claims are incorrect.
But there are other particle species beyond those we have detected experimentally and a main purpose of string theory as a BSM theory of physics is to say something about this list, the pieces we’re so far largely ignorant about.
The laymen are capable of learning a “list of particles” just fine – and understanding that a list is incomplete. It’s completely missing the point if you think that this is the part of physics that needs to be “simplified”. It’s just a list of some entries which have a name. Most kids in the kindergarten can deal with such lists. Even if the meaning of “you” mattered, your suggestion that no one who thinks similarly to me ever reads your blog post has been experimentally refuted as well, hasn’t it?
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I had exactly the misunderstandings of the heuristic connection between string theory and SM particles from sometime in the 1980s until maybe four or five years ago when I started paying a bit more attention to the issue in the course of physics blogging. And, I dare say, I feel more than a little aggrieved and misled by the layman oriented presentations that said so which dramatically overstated what string theory was capable of doing. I appreciate the clarification.
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Once all those branes got involved, I dunno… something about String Theory screams “epicycles” to me. Surely nature can’t be that complicated?
(Yeah, I know, I know. Nature wills as nature will. 🙂 )
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Branes aren’t epicycles in the usual sense. They’re not something added after the fact that string theory needs to be consistent with the real world. Rather, they’re something implied by string theory itself, an inescapable part of the theory.
The sort of complexity going on here probably shouldn’t bother you as much as it does. Think about it like the solar system: nine planets seems rather excessive, but the important type of complexity isn’t the number and diversity of planets, but the complexity of the laws governing them.
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I do understand and by “epicycles” just meant an explanation that seems to work but which depends on complicated mechanics. (And which turns out to be not real. 🙂 )
Actually, I find the solar system quite simple, although complexity emerges from the interactions. But it’s just a big analog computer with each piece following a beautifully simple equation (GR) that describes an elegantly simple picture.
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The theory of epicycles was not the correct one, but unlike the string hypothesis, they made testable predictions regarding reality.
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Dear 4Graviton,
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The massless states are still vibrations, yeah. They’re just the lowest-lying ones, the first line in the violin-string picture.
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I believe this post is misinformed.
The non-vibrating configuration of the string always describes a tachyon. In supersymmetric string theories, this tachyon mode does not consistently couple to the other vibrational modes of the string, and so the tachyon is excluded from the physical spectrum.
The massless modes of the string, describing gauge bosons and the graviton, represent the first excited state of string. This is not surprising, since gauge bosons and the metric have spin, and there must be some internal motion of the string to account for this spin.
Of course, to say that the first exited state of the string is “vibrating” requires some literary license. Would you say that the first excited state of the quantum harmonic oscillator is “vibrating?” Technically no, since it is an eigenstate of the Hamiltonian and does not move at all.
Anyway, this has nothing to do with supersymmetry or compactification. The massless particle states of the string really do reflect some kind of quantum internal motion of the string.
Probably Lubos made this point somewhere above, but I have not read through all of his comments.
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As far as I could tell Lubos neglected to make that point above, though he does in his response post. (If he had made it above, we would probably have had a much more civil conversation.)
For the lay reader, what TE is bringing up here is that, (as I say in my reply to Adel above), the lowest vibrations of the string still are vibrations, and that the reason why is due to states called tachyons.
When Peter Woit summarized my post, he said that the states which correspond to known particles in string theory aren’t like music playing on guitar strings, but more like carrying a guitar around. This isn’t quite true, though. The metaphor breaks down, and it’s because guitars don’t have tachyons.
I’ve talked about tachyons here before. They’re things that naively look like particles with negative mass-squared, but that really indicate a potential problem in your theory.
Superstring theory solves that potential problem by making sure that the tachyons never actually survive as free particles. However, they still do have an impact, in that they can combine with other vibrations. Because they have negative mass-squared, they can combine with states that would otherwise be extremely massive, making the combination massless.
What this means is that, while a guitar string’s simplest physical vibration involves no motion at all, a superstring still does have motion in its simplest physical state.
Note, though, that there isn’t an infinite tower of tachyons that can lower the infinite set of vibrations. There’s just one level worth of them, one “basement floor” to string theory’s infinite tower. You’re still dealing with a finite number of lowest states, and generally these lowest states have to be distinguished by supersymmetry/compactifications/branes/etc. to give rise to the diversity of observed particles.
So back to you, TE. As I say, I’m not trying to say that the lowest physical string states are not themselves vibrations. There’s a bit of a double meaning in the title, in that “not the ones you think” applies to both vibrations and to particles. I now see that double meaning was a bit too subtle for some readers, so I’ll try to reword part of the post to make that clearer.
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Dear 4graviton
Looks like you are aware of the basic point I was making.
I still think people might get the wrong idea though. If the vibrationless mode of the string was responsible for low energy physics, we wouldn’t see anything in our universe but plain scalar fields. No amount of compactification or branes would turn it into something interesting. So the vibrations of the string are very much supposed to be responsible for richness of low energy physics.
But the relevant vibration is only the first harmonic. With only the first harmonic you can’t make very interesting music. So breathless musical analogies are probably not very appropriate, at least far as the particle physics coming from string theory is concerned.
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I’ve added language to the post that should make things clearer, as well as a footnote pointing people to this discussion.
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“I’ve talked about tachyons here before. They’re things that naively look like particles with negative mass-squared, but that really indicate a potential problem in your theory.
Superstring theory solves that potential problem by making sure that the tachyons never actually survive as free particles. However, they still do have an impact, in that they can combine with other vibrations. Because they have negative mass-squared, they can combine with states that would otherwise be extremely massive, making the combination massless.”
Holy Shit. I think the above sentences have just solved years of wondering what Brian Greene* and others meant when they said that “quantum jitters” add negative contributions to the giant, Planck-scale mass’s that particles would get from the massive “tension” on the strings. But please let me know if this isn’t quite correct!
As a side note, might this mean that physicists are a bit too quick to negate any ontological commitments to tachyons? I realize that you mentioned they are “suppressed” in a certain sense, but they seem like indispensable posits of the theory itself. Then again I understand if you don’t necessarily want to go down the murky roads of metaphysics. Forgive me, one of my majors was philosophy so I love this stuff.
*(https://books.google.com/books?id=jYHtp6kx8qgC&pg=PA151&lpg=PA151&dq=brian+greene+planck+mass+quantum+jitters&source=bl&ots=tNfzHGosXD&sig=1rcR-d2KDD82CWAxjLtz6_DNE6M&hl=en&sa=X&ved=0ahUKEwjpw_iK1enMAhUE5GMKHXMFCBEQ6AEIKDAC#v=onepage&q=brian%20greene%20planck%20mass%20quantum%20jitters&f=false)
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Yes, tachyons are exactly the “quantum jitters” Brian Greene was talking about.
The thing about tachyons is, as long as they’re free to exist as independent particles, you’ve got the wrong theory (I talk about that in the linked post). So you can’t really give them the same ontological status as free particles. Whether there’s still room for them depends on what you’re doing with your ontology, and since I tend to be skeptical of most attempts to assign an ontology to quantum field theory I’m afraid I can’t help you there. 😉
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OK! You have all these criticisms against string theory. But what is your alternative? I understand LQG has severe problems with Lorentz invariance. So it looks like ST may be the only game in town!
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I’m not a critic of string theory, I’m a string theorist.
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Thanks 4graviton for your reply. If I were you, I would write on Peter Woit’s blog what you told me that you are a string theorist, not that I have any business advising you!
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Believe me, Woit knows.
(That said, his readers might not, so some sort of clarification might be a good idea yes.)
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In my view, it is not important the issue of conceiving particles as vibrating bodies. The growing credit of string theory will be more related with the possibility of establishing new links between quanutm and gravitation (as, e.g., Maldacena’s link between entanglement and wormholes).
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