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

[qsa]

One more Question: the ultimate background independent theory is a theory were space and time are emergent, causal set, arkani-hamed's theory and torstens theories come to mind. shouldn't somebody study the connection.

 

[qsa]

I disagree. The thread started badly, going straight againts its own tittle, menacing to focus in gravity, and avoiding any mention of the problems of BSM. After that, we got to do some post on principles; I myself invoked naturalness, and some other were mentioned. And we did also some calculations. Fine enough for a PF thread.

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.

QG stands for quantum gravity after all, but I did not imply that SM should not be considered. but only to have some themes to consolidate to reach some insight.

 

[PAllen]

I disagree. The thread started badly, going straight againts its own tittle, menacing to focus in gravity, and avoiding any mention of the problems of BSM. After that, we got to do some post on principles; I myself invoked naturalness, and some other were mentioned. And we did also some calculations. Fine enough for a PF thread.

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.

Well these forums seem to define this subject category as anything beyond GR+SM. It might be clearer to have separate subject areas (quantum gravity, BSM), with string ideas appearing in both, depending on the emphasis.

 

[atyy]

Sure, likewise we do not know whether little angles are not pushing the planets so that they follow their orbits Why don't you go and devise a theory of that ? :zzz:
Seriously, let me give an elementary course in what are good ideas in physics and what are bad ideas:
(a) a good idea always gives instantaneous pay-back. You give something up which makes life a bit more complicated, but you get rewarded by piles of gold. Giving up the continuum does not satisfy this criterion and for sure does not giving up Lorentz invariance.
(b) a bad idea is physically unmotivated, but merely stems from mathematical masturbation excercises such as : (i) help QFT has infinities, we have to cut these out! (ii) let us apply the Heisenberg uncertainty principle where we shouldn't ''we will apply it to space-time coordinates! (which have no operational meaning)'' or (iii) euh the vacuum energy diverges, we can correct this if we modify the dispersion relations (unguidedly), let's do that and proclaim that we magically turned infinity into a finite number (not that it would solve any phyiscal problem).
(i) applies to causal sets, all of them apply to the rest (and I can easily figure out some more of them).

Careful

Does the Poisson sprinkling in causal sets not bother you?

 

[tom.stoer]

do you think holography is inconsistent with the virtual particle-antiparticle picture of forces or graviton.

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.

 

[MTd2]

Arivero, let me try to think now qualitatively. Each string is a mix of heterotic and Type I strings, that is, open and closed strings but there are bosonic fields on this story, formed by bound fermions. And also, fermion fields are superpartners of boson fermions. Is that it?

 

[tom.stoer]

Why the heck do all of you identify BSM="lets speak of gravity"?

BSM is of course not QG exclusively but something like SM+QG = ? So in order to talk about ? One should talk about QG as well.

Of course there may be some intermediate steps beyond SM - like SUSY (w/o QG; just like e.g. LQG is QG omitting the SM).

 

[arivero]

Well these forums seem to define this subject category as anything beyond GR+SM. It might be clearer to have separate subject areas (quantum gravity, BSM), with string ideas appearing in both, depending on the emphasis.

Indeed. And the subject of the topic should indicate the emphasis. Perhaps it was a mistake of the OP poster, but I understood that the topic of the thread was about new principles and ideas for particle theory beyond the standard model. QG and String theory are valid in this topic if they are used to construct models whose limit in some aspect is the standard model.

It is OK for me if people wants to speak of "beyond standard quantum gravity". Just create a thread with such title. But do not come to rant that a thread about Standard Model has gone offtopic because it is not addressing quantum gravity.

BSM is of course not QG exclusively but something like SM+QG = ? So in order to talk about ? One should talk about QG as well.

No! BSM is something as SM + XXX = YYY. The either the XXX or the YYY could be QG, but they could be any other thing, and we should be on topic. But if SM is not in the equation, we are off-topic.

(w/o QG; just like e.g. LQG is QG omitting the SM).

My point, indeed.

 

[atyy]

SM does include gravity!

So to say it cannot be BSM without including SM would rule out all BSM that does not "solve" QG.

 

[arivero]

Arivero, let me try to think now qualitatively. Each string is a mix of heterotic and Type I strings, that is, open and closed strings but there are bosonic fields on this story, formed by bound fermions. And also, fermion fields are superpartners of boson fermions. Is that it?

I am not sure if I parse adequately your description. Your first phrase seems an attempt to describe the different kinds of critical strings, and indeed you can have gauge and gravity multiplets, but it needs more detail.

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

 

[Careful]

What I build was different: I built 18 (=3x(3x2)) particles of charge +2/3, 18 of charge -2/3, 18 of charge -1/3 and 18 of charge +1/3 by putting quarks at the extremes of the QCD string. Again, sorry the confusion.

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

 

[arivero]

SM does include gravity!

Sorry? Say again.

 

[atyy]

Sorry? Say again.

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?

 

[Careful]

Does the Poisson sprinkling in causal sets not bother you?

It is mathematically rigorously defined. If you ask me whether I think the Poisson sprinkling will have anything to do with the final theory, the answer is no. The way I think about it is as a guideline to construct realistic theories.

Careful

 

[atyy]

It is mathematically rigorously defined. If you ask me whether I think the Poisson sprinkling will have anything to do with the final theory, the answer is no. The way I think about it is as a guideline to construct realistic theories.

Careful

Well, mathematically rigourous but I feel conceptually klugey (that's subjective, of course). But how can the Poisson sprinkling not be in the final version if Lorentz invariance is to be maintained? I prefer LLI not be maintained, but I think LLI was one of causal sets motivations?

 

[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?