Is a New Principle Necessary to Resolve Quantum Gravity and Unify Interactions?

[Fra]

As I think of "Standard Model" it has two parts, the HEP parts (eletroweak & QED), and the low energy part, effectively GR as well as the so based common cosmological models.

So I assume BSM would extend/generalize any of it's part; or all of the parts? ot solve some of the still open questions.

If we forget gravity it's basically the open GUT quest.

The other extreme is I guess the "pure gravity" programs. However, "pure gravity" doesn't make sense to me, as I think matter plays the role of observers, so at minimum you'd have matter at the boundaries or there would be no observations possible. This is why I strongly dislike programs that discuss quantizing gravity and yet seem to have no observers and no matter in it's fundamental building blocks. I simply can't imagine an experiment with pure gravity. At minimum you have matter at boundaries, and there nontrivial things are bound to happen.

/Fredrik

 

[Careful]

I will ask again: Why the heck do all of you identify BSM="lets speak of gravity"? Is it an idea of your own, or does it come from some TV series? It should be pretty obvious: if it does not contain the SM in some limit, it is not BSM.

I agree but (a) you cannot think about everything and (b) you are assuming you can find a ''reason'' for the SM within contemporary mathematical methods/physical theories. I on the other hand, think that these why questions will have very different answers than people try to look for so far.

 

[MTd2]

As for superpartnering, a fermion of spin 1/2 is usually partner of two "sfermions", the s standing for "scalar". Never heard of "bfermions", but it is a good idea if you are not sure if they are spin 0, spin 1 or spin 2

What about the non scalar bosons?

 

[arivero]

Let's do this step by step (I never build such stringy models so I am not going to rush here). You say you build 18 spinless bosons of charge + 1/3, this implies that you consider oriented strings (since the only way to sum up the spin degrees of freedom is by up down - down up), do QCD strings enjoy that property and so yes how does it reflect on the physics?

I think that it is the standard way of QFT, exactly the same that for the pion and the muonium, antisymmetrization of the full wavefunction etc. That is the thing that makes me to choose only one of the irreducible representations of each product decomposition. But I confess that I myself am not very trained on this procedure... it is almost basic chemistry!

Second question for now is, why are 9 spinless bosons of charge 4/3 forbidden?

Ah, I saw you found them too! I took one yeara thinking of ways to forbid it, but at the same time I need to allow the ones of -2/3, and any symmetry rule should apply to both.

Then I though about chirality of the putative partner. The point is, for the other two cases, DD and UD, we can build Dirac supermultiplets, with a Dirac Fermion and four scalars (making two complex scalars if you wish?). But for each colour in the the UU, you can not arrange into Dirac. You could do, at most, one generation of Dirac and one extant chiral spinor. And if you want to do three generations, they should be three different chiral supermultiplets.

So I though, ok, the thing that introduces the chirality in the standard model probably also disposes of the UU beast. And I left it in this way, one year ago: that the message of these states was that the standard model needs be chiral somewhere. Now the discussions on PF the last week have helped me to move a little forward, but the arguments (that I sketched in the last long post) are still baking.

 

[atyy]

The other extreme is I guess the "pure gravity" programs. However, "pure gravity" doesn't make sense to me, as I think matter plays the role of observers, so at minimum you'd have matter at the boundaries or there would be no observations possible. This is why I strongly dislike programs that discuss quantizing gravity and yet seem to have no observers and no matter in it's fundamental building blocks. I simply can't imagine an experiment with pure gravity. At minimum you have matter at boundaries, and there nontrivial things are bound to happen.

I agree. Pure gravity is against the spirit of GR. But I don't think of AS as a pure gravity programme, since even if the UV fixed point exists, we have no guarantee that it will remain once matter (and we don't even know what matter looks like at the relevant scale). I think of it as a property that if it exists, will be important to know about. Like N=8 SUGRA, which has no hope of being a realistic theory, but hopefully understanding why its terms are coming out finite will help someone!

 

[arivero]

What about the non scalar bosons?

Generically in supersymmetry, or in this particular approach?

 

[MTd2]

Generically in supersymmetry, or in this particular approach?

This approach.

 

[arivero]

As I think of "Standard Model" it has two parts, the HEP parts (eletroweak & QED), and the low energy part, effectively GR as well as the so based common cosmological models.

Cosmological models have a "standard model" too, nowadays. But traditionally in physics, "the" standard model is electroweak+qcd+symmetrybreaking.

If we forget gravity it's basically the open GUT quest.

All SM fields are just effective. So we just add the EH term, and we are all right at all energies experimentally accessible thus far. I think electroweak and Higgs will fail way before the EH term does.

Then the question is, do you want to fix all the problems together, or hope some separate out?

BSM is about adding something to the Standard Model to solve their problems. It seems that some of them can be solved by adding "hints" from space time, but not all together, just one each time to see how it fits and what it solves.

Thus, we add the Planck scale as a first argument to the cutoff, and actually this gives a good estimate for the mass scale of the electron (Polchinski explains this point in his book, but it is unrelated to strings!). Or, we add supersymmetry, which is a "square root of spacetime traslations", in order to fix some problems with divergences.

And modernly we do not add random GUT groups, but groups we try to deduce from space time structure, either via Kaluza Klein or via Superstrings. Actually, the ones from Kaluza Klein are, in my opinion, more realistic.

Ah, by the way, edit: the interesting thing of adding, as you ask, a gravitino and a graviton to the supersymmetric effective standard model (with all the fields susy, but putting massive gauge supermultiplets by hand, not by higgs) is that it has 128 bosons and 128 fermions. People does not mention it usually, perhaps they have never even bothered to count them. So N=8 sugra is not so far from the truth :-D

 

[arivero]

This approach.

Each combination of quarks can produce a whole tower of excited states of the QCD string. This is well controlled phenomenologically, under the label of "Regee trajectory", and I do not think that they have a fundamental role. In principle, even the quark states should have the same trajectories, but with a slope controlled by the string constant, but quantisation forbids elementary objects with spin greater than 2, so I do not expect such trajectories, really.

So, forgot excited states.

Gauge bosons: I have no idea of how they appear here. In supersymmetry, a massless gauge boson pairs with a spin 1/2 weyl fermion, and again I have no idea if such partners can be produced via the pairing mechanism. I need to study more of SUSY QCD, to understand the role of these fermions.

What I have learn recently, is about massive gauge bosons in supersymmetry.

I have learn that to add the Jz=0 component, you must to add another spin 1/2 weyl fermion, and then you need to add another scalar to finish the pairing of the new fermion. So when you break a gauge symmetry, you add to each broken vector a pair of spin 0 bosons.

I suspect that in this model these spin 0 bosons are, in disguise, the extra states I found from pairing UU quarks. But again, no clue about the new spin 1/2.

 

[MTd2]

This is getting too complicated and confusing! For example, the top quark is being forgotten in the whole thing.

 

[arivero]

This is getting too complicated and confusing! For example, the top quark is being forgotten in the whole thing.

On the contrary, it is the main piece!. The point is that naturalness allows us to separate two kinds of quarks: those who get mass at the electroweak scale, and those that should be massless from the point of view of the electroweak scale. This is our criteria to choose which quarks we put in the ends of the strings: the theoretically massless (or "symmetry protected", in naturality parlance) ones.

With this idea, we write down the very trivial equations matching the degrees or freedom, and we discover that it implies 3 generations and only one electroweak-massive quark: we predict the number of generations!

 

[MTd2]

Arivero, are these strings an effective theory or more fundamental? I am confused since the strings at one hand are fundamental, on the other hand, you are using a partial input from a very effective theory.

 

[arivero]

This is a question that the rest of the public of the thread could enjoy: Are the so-called effective theories just effective, or something more important? Associated to this, in the seventies, there was the renormalization principle (upps, the guy with the 't name again... shoult I write " 't's principle "?). But Weinberg doctrine about effective theories, combined with the developments in lattice quantum field theory, drove to the perception of low energy theories as meaningless trash, or so it seems to me.

The success of the quark model happened, on other side, because people perceived the maths of the model as an object more important that its actual realization. It was not important if the quarks were mathematical entities or physical bodies, the point is that they allowed to classify the hadrons and to do predictions about them.

 

[arivero]

I am confused since the strings at one hand are fundamental, on the other hand, you are using a partial input from a very effective theory.

Yep, and where is the problem? When I first did it, I also though that it was going to be a crackpot-like theory, mixing fundamental and composite (effective) degrees of freedom. I even dubbed it with a cranky name, sBootstrap. Amusingly, it was not well received in crackpot circles :-DDD I usually say that I suffer "Mowgli syndrome": neither the wolfves nor the humans will perceive me (or my papers) as being of their own kin.. But after rediscovering the history of the 1971 superstring revolution, I saw that the join between qcd strings (pions and gluons) and quarks had been considered by a good bunch of authors, and published in high reputation journals!

This is getting too complicated and confusing!

I appreciate the comment... Usually I believe that people does not follow my posts (and articles) because they are uninteresting to them. You know, it is not about the deep philosophical structure of space time, thing vs no-thing, etc. I didn't noticed that actually the post could be mathematically complicated to follow.

 

[MTd2]

Alright, where does the top quark comes from?

 

[qsa]

Holography today is - in my opinion - like scratching at the surface hiding a fundamental principle still to be fully understood; like Mach's principle was a guideline for Einstein which did not made to a fundamental principle in GR (... he must so to speak throw away the ladder, after he has climbed up on it ...); nevertheless holography is certainly some aspect of reality b/c it shows up in so different approaches so that it's hard to deny that there is something fundamental behind it.

Gravitons (including virtual particles of the gravitational field) on the other hand are mathematical artefacts of perturbation theory; of course some strong-weak dualities allow us to express certain amplitudes in certain regimes using perturbation theory but that doesn't mean that the graviton itself is a fundamental concept; there are too many scenarios where the graviton concept fails completely or is too restricted (just like the virtual gluon concept fails in non-perturbative QCD).

So for me holography is a concept or a guideline pointing towards a fundamental principle, whereas gravitons are a rather limit calculational tool valid only in a rather limited regime.

In that case I want to connect holography (which hint strongly that space-time is emergent) to that other question I had. My guess is that emergence means that space-time can be calculated from the original theory.

One more Question: the ultimate background independent theory is a theory were space and time are emergent, causal set, arkani-hamed's theory(path integral without space-time) and torstens theories come to mind. shouldn't somebody study the connection.


Q1. would you expect a theory were space-time will be emergent to automatically produce the appropriate symmetries for space time.

Q2. Since all these theories hint of space time emergence, can you see any hint of equivalence in some way.

 

[tom.stoer]

"emergence" is only a word; in the AdS/CFT scenario space-time is emergent, too, but due to a conformal, gravity-free SUSY gauge theory livig on the boundary; here "emergence" is more like"duality"; as in AdS/CFT holography is build-in, I agree that there is a connection with emergent gravity, but in that specific case NOT in the sense of Verlinde, causal sets etc.

I don't agree that all theories point towards emergent space-time; unfortunately the situation is far from clear, but looking at twistor theory, LQG like approaches and 4dim exotic smoothness it could be exactly the other way round, namely that matter does emerge from spacetime.

Perhaps these links are interesting ...
http://arxiv.org/abs/1006.2230v1
http://arxiv.org/abs/1003.5506v1

 

[arivero]

Alright, where does the top quark comes from?

Again, generically or in this setup ?

All the information we can extract from this setup is that the partners of the top quark come, jointly with the partners of the up and charm quarks all of them, from the six pairs dd,ds,db,ss,sb,bb and their antiparticles. We do not solve, neither address, the question of the top quark itself. All we know is that their superpartners are part of an intriguing SU(5) symmetry related to the charges of the two flavours of the light quarks.

Even outside of this particular setup, it is clear that whatever the top quark is about, it is related to electroweak symmetry breaking. A yukawa coupling of 0.98 is too near of 1.0 to be considered unique (and actually, 1. is still compatible with the experimental measurement).

 

[Fra]

This is a question that the rest of the public of the thread could enjoy: Are the so-called effective theories just effective, or something more important?

I wouldn't agree that effective theories are less important.

Just to throw back a reflection on this: From the inference perspective I hold, there is actually no way even in theory, to distinguish between an effective theory and fundamental theories. Or put differently, all theories are effective, and the notion of fundamental theory is just a realist remnant.

To understand effective theories as a result of inference processes, IS IMHO important, and that ANY actual theory is ALWAYS the result of an inference process and hences effective. But the saving conjecture in this thinking is that it's the effective laws, that rule the action of the system encoding the theory, not the - unkonwn fundamental theory.

Also, in my view, there is no rational ground in separating law and initial conditions, and hence not knowledge about states in a theory and knowledge of theory itself. The difference is merely that the theory is more unquestionable as it's what happens to be under our feet.

/Fredrik

 

[MTd2]

Again, generically or in this setup ?

This setup! Always this one! :)

 

[arivero]

I wouldn't agree that effective theories are less important.

Just to throw back a reflection on this: From the inference perspective I hold, there is actually no way even in theory, to distinguish between an effective theory and fundamental theories. Or put differently, all theories are effective, and the notion of fundamental theory is just a realist remnant.

To understand effective theories as a result of inference processes, IS IMHO important

I think that a valuable point is the number of free parameters in a theory. We could say that a fundamental theory is what happens in an effective theory when the free parameter disappears.

The gauge bosons in the SM are the prototypical effects. You can see them as effective, with the mass being a free parameter (and the model does not need higgs), or you can see it as fundamental theory, when the mass is generated by the Higgs.

In fact I think that when experimentalists refer to the SM, they still refer to the inferred, effective theory. It is only the theory front, and perhaps even more the science journalist front, who see the SM as the one with the Higgs. It should really be called MSM, in the same way that we call MSSM to the SSM with two higgses.

 

[arivero]

This setup! Always this one! :)

Well, what we have got -besides uniqueness- is to obtain, from this hidden SU(5) symmetry, all the non-gauge bosons of the Superymmetric Standard Model.

Of these, the ones related to the electroweak scale are the partners of the top, and the scalar partners of the W+, W- and Z bosons (more precisely, the scalar partners of the chiral fermion that is absorbed by the gauge bosons to get mass).

All of them come in different ways from the 15 + 15 representation of SU(5), which in turn is extracted from 5x5=15+10. In plain words, they are the set of different pairs you can do with u,d,c,s,b, plus the different pairs you can do with u,d,c,s,b

Of these, six plus six do the partners of Dirac up quarks:

dd,ds,db,ss,sb,bb,dd,ds,db,ss,sb,bb​

six plus six do the partners of Dirac down quarks:

du,dc,su,sc,bu,bc,du,dc,su,sc,bu,bc​

and three plus three do the partners of the Chiral (Weyl?) w+,w+,z companions:

uu,uc,cc,uu,uc,cc​

The Dirac fermions can see colour, so they triplicate for each colour. The Chiral fermions can not see colour (and they can see electroweak charge, but not pure electromagnetic charge), so their partners do not triplicate neither (the mechanism for it, I do not known yet, it implies to use Super-QCD, surely).

All we can expect is that two special combinations of the first (bi)sextet are related to the top, and then its mass before susy breaking is related to the mass of the chiral companions. Within this setup, I do not see any other exploitable feature, and even this one is unclear, as I do not see what combinations we should select. We could put some more group theory into, namely the decomposition of SU(5) into the subgroup SU(3) x SU(2), and we could also pay attention the left and right chiralities.

 

[MTd2]

Even outside of this particular setup, it is clear that whatever the top quark is about, it is related to electroweak symmetry breaking. A yukawa coupling of 0.98 is too near of 1.0 to be considered unique (and actually, 1. is still compatible with the experimental measurement).

So, top=higgs for you, that is, top condensate instead of higgs?

 

[arivero]

So, top=higgs for you, that is, top condensate instead of higgs?

I am agnostic.

If you had some condensation theory predicting also the same sQCD results that post 41 (say, 14 families with a condensate of 7 tops and 10 bottom, or 33 families with a condensate of 22 top and ...) then I would say that top condensation is the way to go

 

[Fra]

I think that a valuable point is the number of free parameters in a theory. We could say that a fundamental theory is what happens in an effective theory when the free parameter disappears.

I see that point, but what you describe is to me more the let's say condensation process where expectations are sufficiently confidence to get turned into unquestionable non-variable elements.

But I think the logic whereby a parameter disappears, and also reappears is itself a physical process. And it's this I seek to understand. It also relates as I see it to the origina of degrees of freedom. I do not personally like when all this is is spoken of as some mathematical renormalization as if it's just a mathematical scaling. The "scaling" here is truly physical and must take the form of an expectations itself, so that it takes another observer to describe the "theory of scaling" - ie. renormalization.

So I still do not think there is a proper distinction between fundamental and effective, because what as you defined "fundamental theory" effectively just means that we "truncate" expectations to become facts. But this doesn't mean they ARE. It just means the doubts are not distinguishable.

I expect the entire RG stuff to alsos be revised (in some way) with a new future understanding.

/Fredrik

 

[arivero]

I expect the entire RG stuff to alsos be revised (in some way) with a new future understanding.

Sometimes, the effective theory seems as fundamental as its high energy theory.

Think in the electroweak part of the SM. The effective theory have three parameters, alpha, MW and MZ. In the RG above the critical point, the masses become zero, and that is all. But as a consequence of it, the effective theory is instead written as a function of g, g' and <v> So Ok, when <v> is zero, we lost a parameter, as said before. But we really need to predict the value of <v> if we want to claim that we have produced a more fundamental theory.

 

[arivero]

Alright, where does the top quark comes from?

By now, I am exhausted and I am sure I have lost any audience left. But I have been thinking hard about an answer to your question, and I could have an sketch of a possibility.

Look to the SU(3) flavour sextet dd,ss,bb,ds,sb,bd.

There is actually only a way to separate this sextet in pairs so that every pair can move to another by a simultaneus action of flavour on a single quark of each component. The pairs are.

dd,sb
ss,bd
bb,ds

And we could thing that these pairs are the partners of the U family.

On other hand, they are three ways to put a SU(2) triplet in this sextet
dd,ds,ss; ss,sb,bb; bb,bd,dd.

You can notice that each of the triplets contains one of the components of the pairs.

But more important, the first of the triples is a mirror of our misterius "electroweak breaking" triplet uu,uc,cc

So I feel inclined to believe that the partner of the top quark is the pair ds,bb. In some way, the breaking of electroweak symmetry puts mass into this ds but it does not put mass to cc nor dd.

Thanks for your attention.

 

[MTd2]

Of these, six plus six do the partners of Dirac up quarks:

dd,ds,db,ss,sb,bb,dd,ds,db,ss,sb,bb​

six plus six do the partners of Dirac down quarks:

du,dc,su,sc,bu,bc,du,dc,su,sc,bu,bc​

So, the partners for the up quark are the dual combinations among d,s,b and its antiparticles
The partners for the down quark are the dual combinations of d,s,b with u,c and its antiparticles.

So, the difference between the partners of up and down, it is that the up is more internal and the down is more external.

Is that it?

 

[arivero]

I have no idea what "external" and "internal" mean. Technically, we are decomposing SU(5) flavour in SU(3) "downflavour" and SU(2) "upflavour". Standard group theory tell us that
5x5=15+10
and that
15= (6,1) + (3,2) + (1,3).

The up partners are in the 6,1: sextet of SU(3) but singlet of SU(2)
The down partners are in 3,2. You can call it "external" but it is just representation theory.

What is important is that inside a SU(3) sextet you can represent a SU(2) triplet, this is the point of #132.

 

[MTd2]

Internal, means you are just dealing with (d,s,b), external means that you are opertating out of this set, to the remaining quarks (u,c).

Oh, why up in SU(3) and down in SU(2)? This is getting confusing, since when you say up and down quarks, I imagine that they don`t have anything special between them. So, what about the companions of the 3 other quarks?

 

[arivero]

Hmm, sorry, I was not willing to add more confusion, I was just pointing out that there is a standard way to name this decomposition of SU(5) into SU(3) and SU(2). SU(5)_flavour is the group that allows to exchange any of the five quarks. SU(3)_"downflavour" is the subgroup that allows to exchange all the s,b,d quarks, and SU(2)_"upflavour" is the group that allows to exchange the u,d quarks. It is pretty obvious notation, but if you are unfamiliar with group theory, just dispose of it.

In this notation, the partners of the dirac UP FAMILIES are named (6,1), and the partners of the diract DOWN FAMILIES are named (3,2). Note that the (6,1) are UP families, but done from pairs of dsb only, so your external and internal labeling is creating confusion too. It does not seem a good notation.

C'mon, this part is VERY elementary. Look at the electric charge of each quark, and add them. They are Dirac quarks, so chiral charges are not in play.

 

[MTd2]

The problem is not really group theory. The only way I am use of thinking of quarks going to other quarks is the CKM matrix. And when I am thinking about SU(3), it about the charges of the gauge theory, for any quark, not as something that labels different kinds of quarks

So, how to distinguish SU(3) for colors from the ones that generates fermions? Are the charges of the SU(3)XSU(2) mapped into the fermions?

 

[arivero]

Amazing. Your are not used to the flavour global symmetries?! Ok, that is a problem. We used to put subindexes to the SUx(N) thing in order to avoid the kind of confusions you have. In any case, forget about group theory.

It is very elementary:
-(-1/3-1/3)= +2/3
-(+2/3-1/3)=-1/3

I expect that at least you have noticed this point! Can you confirm that you noticed it from the start!?

 

[MTd2]

Yes, I noticed.

But the flavour global symmetries will not work here since it broken by the lack of topness.

 

[arivero]

I don't understand your goal. For me, as I have shown, they work perfectly. So you must have a different goal. Mine was just to show that the superpartners have an SU(5) symmetry, and that it is unique.