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

[tom.stoer]

It is sometimes stated that new principles are required to solve the puzzle of quantizing gravity and harmonizing all interactions.

Let me first list some known principles
- quantization of classical theories; not really a principle, but a "program"
- equivalence principle in GR
- gauge principle
- holography
- ...

In constructing different theories on is using such principles or at least one is being inspired by such principles; but
- string theory doesn't introduce new principles for its construction (I wouldn't call the world sheet a "principle")
- LQG does not use any new principle; it just uses a different quantization method
- for NCG I am not sure if there's a new principle
- asymptotic safety isn't a principle at all; it's just applying the renormalization group
- the holographic principle isn't "used" for constructing a theory; but it shows up in some approaches

So why should a new principle be missing? and how should it look like? should it be a physical principle, or should it be a new mathematical tool?

 

[arivero]

I am amazed that most people seems to consider BSM=Gravity. Of course gravity is BSM, but BSM is a lot of problems not solved by the SM.

 

[tom.stoer]

sorry if this was missleading; QG is one special topic BSM simply because a consistent quantization seems to be missing (we have candidates, but we are either not sure if they are consistent or we are not sure if they are true); but of course there are additional topics, that's why I am listing "quantization" as one principle that might be questioned.

 

[marcus]

For me the "home base" or anchor for perspective on BSM is the 6 July 2009 CERN talk by Steven Weinberg where he spent over an hour developing the idea that we might not need anything new.
He is a QFT guy, and a cosmology guy, and a HISTORY guy. He is old (although still very sharp) and he has seen many more ideas come and go than most of us. He has been in the pit with a lot of different movements, revolutions, fads etc. So I pay attention especially.

The first 50 minutes of the talk was HISTORY of quantum field theory, say for 80 years 1930-2010 roughly. He graphed its stepwise progress like a flight of stairs. A rise, and then a period when people thought it wouldn't work and it stagnated, and then another rise, and then another plateau or stagnant slacking off, and then another rise...
He talked about the history and details of each stage.

He analysed the immediate past, like 1985-2005 as one of these slack periods when people think that QFT won't work as is, and think some new principle is needed. And then with a question mark he plotted another sharp rise in the fortunes of QFT.

Then around minute 52 of the talk he began conjecturing that with the help of AsymSafe gravity it might be that "good old QFT" was all there was. That might be how the world is, he said several times.

And he spent the last 12 minutes (the talk was about 70 minutes with questions) talking about AS gravity, particularly the numerical results---Percacci, Codello, Rahmede, and such.
===================

Personally I have no desire or reason to favor the idea that we are good all the way to Planck scale with what we've got. And surely some improvement/perfections of the Standard Model are in order. It's hardly my place to say.
But I think the DEFAULT assumption, barring some remarkable evidence, is that no new principles are needed.

The alternative, I think, is some kind of millennialist crisis-monger mentality. Where folks say to themselves "Oh dear! we have a terrible crisis! We need some great new conceptual revolution to save us! What could this radical salvation idea possibly be? Let's look for something as outlandish as possible!"

And maybe what Steven Weinberg said in his CERN talk is not RIGHT, but it is plausible enough (and he is wise enough) so it can be used as a homebase or default assumption.

 

[Careful]

So why should a new principle be missing? and how should it look like? should it be a physical principle, or should it be a new mathematical tool?

For reasons why, people like Isham, Sorkin, Finkelstein, Hartle, Penrose, 't Hooft, Durr, Piron, Feynman just to name a few have written about that. You can all find that in the standard literature. And who says it will be something as simple as one principle ? Why not, say, five ? And new physics always needs ''new'' mathematics, so that is obvious. What? Find it out ! You may start by giving a rigorous nonperturbative formulation of interacting QFT, that might help.

To Marcus: Weinberg is right in the sense that you need to look conservatively, you don't have to be wild at all. But sometimes a new idea which is not so far from what we know to be true can have devastating consequences.

Careful

 

[marcus]

Speaking of Penrose, he gave a great talk at the Abhay Ashtekar 60th birthday confence (June 2009) at Institute for Gravitation and Cosmology. It is very much an overview talk---his perspective on physics and related mathematics.
This is some 10 YouTube segments. I've seen other versions overlapping with this talk---some of the same slides. But this version seems especially interesting and complete. Penrose critique of string theory starts with #7 if you want to jump to that.

1.
2.
3. http://www.youtube.com/watch?v=4BtYn2Wde4o&NR=1
4. http://www.youtube.com/watch?v=zVTCaaP5Y0s&feature=related
5. http://www.youtube.com/watch?v=bQNUwHnQzvA&feature=related
6. http://www.youtube.com/watch?v=DvfRYzRgqWE&feature=related
7. http://www.youtube.com/watch?v=afsd3_PJais&feature=related
8. http://www.youtube.com/watch?v=UMi_QB0vWC0&feature=related
9. http://www.youtube.com/watch?v=tiRy07yweA0&feature=related
10. http://www.youtube.com/watch?v=5jXj1TwiFys&feature=related

http://gravity.psu.edu/events/abhayfest/

The title was "Fashion, Faith, and Fantasy in the New Science of the Universe"

The thing is he has the balance of a highly successful old hand. He considers new ideas openmindedly, but also weighs them critically.

 

[Careful]

To give a nice story about Pauli in this regard: Pauli was visiting Bryce de Witt and de Witt told him that he was working on the problem of quantum gravity. Pauli started to shake his head knodding yes and no alternatively. Finally he said : ''this is indeed an important problem, but it will take someone really smart to solve it''.

 

[Careful]

Speaking of Penrose, he gave a great talk at the Abhay Ashtekar 60th birthday confence (June 2009) at Institute for Gravitation and Cosmology. It is very much an overview talk---his perspective on physics and related mathematics.

Penrose is still amazing, he is almost 80 now, is still creative and forthcoming; as far as I remember he has a young kid even. You do that when you should be a grandfather !

Well he is definately open minded, but he is not positive at all to most programs (including string and LQG).

 

[marcus]

...
Well he is definately open minded, but he is not positive at all to most programs (including string and LQG).

His criticism of String is long and many-sided. I could not summarize it. It goes on through the video segment #7-#9.

His criticism of Loop---I mean mainstream Loop cosmology as practiced by his friend Ashtekar---is based on the second law. This is a serious issue. In Loop cosmology the bounce finding is generic---they try all the variations they can think of and they still get a bounce. So a pre-bang universe collapses to a critical density and then triggers a period of inflation.

Penrose does not like the conventional inflation scenarios and he sees violation of the second law in the bounce. He explains all this wonderfully with his handdrawn slides. Even if one already understands without watching the video, one could watch just for fun.

 

[atyy]

But, but how can one not like strings now that it includes HIS twistors?

Strings indicates that we do need a new principle - how can we formulate it non-perturbatively?

AS indicates we need a new principle since it's really not clear (to me - does this background field method really work) what sort of coarse graining should be done (should that be fine graining, since we are going backwards in AS)?

So even conservatively from the best working directions, we do need a new principle.

 

[Careful]

His criticism of Loop---I mean mainstream Loop cosmology as practiced by his friend Ashtekar---is based on the second law. This is a serious issue. In Loop cosmology the bounce finding is generic---they try all the variations they can think of and they still get a bounce. So a pre-bang universe collapses to a critical density and then triggers a period of inflation.

His criticism of loop is much deeper and much more elaborated than the second law. PS: I don't care so much about the validity of the second law in those scenario's. You cannot test it anyway.

 

[Careful]

But, but how can one not like strings now that it includes HIS twistors?

Strings indicates that we do need a new principle - how can we formulate it non-perturbatively?

AS indicates we need a new principle since it's really not clear (to me - does this background field method really work) what sort of coarse graining should be done (should that be fine graining, since we are going backwards in AS)?

So even conservatively from the best working directions, we do need a new principle.

If you would say principleS, I would totally agree with you (on all points).

 

[tom.stoer]

If Weinberg is right, and no new physics is needed, still new insigts are required.
- why is spacetime four-dimensional
- why just U(1)*SU(2)*SU(3) * SO(3,1)
- if no strings are needed, there's still a landscape problem
- what about initial conditions, pre-big bang etc.
I think that these issues cannot be addressed within the current framework

@Careful: any hints for reading (Isham, Sorkin, Finkelstein, Hartle, Penrose, 't Hooft, Durr, Piron, Feynman)

 

[Fra]

So why should a new principle be missing? and how should it look like? should it be a physical principle, or should it be a new mathematical tool?

IMHO, the inference perspective of law suggests design principles which are both physical and mathematical. The suggestion is that physical interactions may follow a rational action; and the description of this necessarily requires a new formulation and foundation of probability theory, possibly that doesn't make use of real number for quantification.

There are today, several reconstructions of probability theory. The measure theoretic which is essentially axiomatic and the least "physical", then we have Cox and Jaynes derivation of probability theory as following from rules of inference describing degrees of belief. Then Knuths quite interesting idea (but which is realted) to consider probability as resulting from consistent valuations of lattices/posets. All these are illustrative, but I think we are still awating for another reconstruction, that may be needed for a proper implementation of the new inference ideas. The common denominators of Jaynes and Knuth, is the the degree of plausability IS an ordering relations on the set of possibilities. But also that Knuth s well as JAynes without even questioning it for a second, assumes that degrees of plausabilities as well as valuations of the order are real numbers.

I think what is missing, is a proper PHYSICAL intrisic counting model. But depending on the approach, this has many names. Counting, rating degrees of belief, valuating ordered sets, counting in the PI. What is the physical constraints on this mathematics?

This is what I expect, we need to develop, along with the new physical principles.

I have personllay made some associations so several of the large programs (ST - the string action is a possible unified action; but the selection principle is missing... so new principles are needed, but I fear that once that's added, string theory as it's known will not make sense, so I don't think it would be string theory anymore). I've also associated to LQG, but there are other issues... Rovelli completely avoids discussing these counting procedures, which he admits when avoiding discussing meaning of probability. Actually string theory avoids this as well.

/Fredrik

 

[arivero]

- why is spacetime four-dimensional
- why just U(1)*SU(2)*SU(3) * SO(3,1)
...

@Careful: any hints for reading

Lets try to do a list of academical articles actually predicting U(1)*SU(2)*SU(3), and check what inputs they use. Same for SO(3,1), or four-dimensionality. Please, let's try hard. I should be very surpresed if the only one were Witten's "Realistic Kaluza Klein Theories".

 

[tom.stoer]

Honestly, I haven't seen any approach that is able to single out some of the above mentioned structures. Afaik Connes uses a new approach but it is not restricted to the SM; similar constructions for other models should be possible. The only structure that seemed to unique were twistors, but afaik they became more complicated when taking into account non-self dual gravity and mass.

So from my side: I do not know about any idea that could serve as a guideline, selection principleor whatever.

 

[arivero]

Indeed because of that I am asking a simpler question: just a paper with "restrictions of the landscape" so that only a few solutions, including 4D space and/or SM gauge group, appear. Let's see how many papers we found and we can think about it. If not, we are just chit-chatting. I hate lazy talking.

 

[suprised]

If Weinberg is right, and no new physics is needed, still new insigts are required.
- why is spacetime four-dimensional
- why just U(1)*SU(2)*SU(3) * SO(3,1)
- if no strings are needed, there's still a landscape problem
- what about initial conditions, pre-big bang etc.
I think that these issues cannot be addressed within the current framework

New physics and perhaps principles are needed in any case for addressing these questions. Even if gravity would be asymptotic safe, which is totally unclear, the question about why the Standard Model is as we find it, remains.

But I don't see the situation as pessimistic. A new principle, the holographic one, has been discovered and its consequences are still being explored. It is seen as a fundamental principle underlying QG and as such must be obeyed by any consistent theory such as strings. While it does not directly address the physics of the SM, it is very important for cosmology and other foundational questions eg in the context of black holes. It points to a deeper relation between gravity, thermodynamics, information theory and emergence. It may well be that understanding these matters will prove essential for understanding the above questions, and will ultimatly have a bearing on the questions of vacuum selection, landscape, etc. It’s a long and difficult process and we shouldn’t be too impatient.

Twistors are not a principle per se, they are a rewriting/reparametrizing of known theories. Similarly, approaches that rely on ad-hoc choices, such as NCG approaches, KK compactifications etc, LQG, won't be really able to explain things, ie, able to answer these questions. This is part of the landscape problem, perhaps made worse as even the consistency of these models is not clear.

 

[Alder3]

For me the "home base" or anchor for perspective on BSM is the 6 July 2009 CERN talk by Steven Weinberg where he spent over an hour developing the idea that we might not need anything new.
He is a QFT guy, and a cosmology guy, and a HISTORY guy. He is old (although still very sharp) and he has seen many more ideas,http://www.ashisoft.com" come and go than most of us. He has been in the pit with a lot of different movements, revolutions, fads etc. So I pay attention especially.

The first 50 minutes of the talk was HISTORY of quantum field theory, say for 80 years 1930-2010 roughly. He graphed its stepwise progress like a flight of stairs. A rise, and then a period when people thought it wouldn't work and it stagnated, and then another rise, and then another plateau or stagnant slacking off, and then another rise...
He talked about the history and details of each stage.

He analysed the immediate past, like 1985-2005 as one of these slack periods when people think that QFT won't work as is, and think some new principle is needed. And then with a question mark he plotted another sharp rise in the fortunes of QFT.

Then around minute 52 of the talk he began conjecturing that with the help of AsymSafe gravity it might be that "good old QFT" was all there was. That might be how the world is, he said several times.

And he spent the last 12 minutes (the talk was about 70 minutes with questions) talking about AS gravity, particularly the numerical results---Percacci, Codello, Rahmede, and such.
===================

Personally I have no desire or reason to favor the idea that we are good all the way to Planck scale with what we've got. And surely some improvement/perfections of the Standard Model are in order. It's hardly my place to say.
But I think the DEFAULT assumption, barring some remarkable evidence, is that no new principles are needed.

The alternative, I think, is some kind of millennialist crisis-monger mentality. Where folks say to themselves "Oh dear! we have a terrible crisis! We need some great new conceptual revolution to save us! What could this radical salvation idea possibly be? Let's look for something as outlandish as possible!"

And maybe what Steven Weinberg said in his CERN talk is not RIGHT, but it is plausible enough (and he is wise enough) so it can be used as a homebase or default assumption.

Ya I Too have terrible crisis! But Your Post Helped Me This Matter So Thanks!

 

[tom.stoer]

New physics and perhaps principles are needed in any case for addressing these questions. Even if gravity would be asymptotic safe, which is totally unclear, the question about why the Standard Model is as we find it, remains.

Agreed.

But A new principle, the holographic one, has been discovered and its consequences are still being explored. It is seen as a fundamental principle underlying QG and as such must be obeyed by any consistent theory such as strings. While it does not directly address the physics of the SM, it is very important for cosmology and other foundational questions eg in the context of black holes. It points to a deeper relation between gravity, thermodynamics, information theory and emergence. It may well be that understanding these matters will prove essential for understanding the above questions, and will ultimatly have a bearing on the questions of vacuum selection, landscape, etc. It’s a long and difficult process and we shouldn’t be too impatient.

Agreed.

Twistors are not a principle per se, they are a rewriting/reparametrizing of known theories.

What is interesting is that twistors seem to abandon the ideas of fundamental space and time; space and time are replaced by twistor space. This replacement lacks a fundamental principle, it is a purely mathematical procedure. However it seems that there is something missing b/c as soon as one goes to the real world (gauge symmetries coupled to gravity + mass) all the nice features of twistir theory fade away. So perhaps there IS a deeper principle behind twistors, but we do not know it (compare it to SUSY: as soon as you have to break SUSY the whole picture becomes ugly; the hope is that string s provide a deeper principle such that SUSY breaking becomes emergent rather than explicitly engineered).

I liked twistors as they seemed to single out 4-dim. space-time.

 

[Careful]

If Weinberg is right, and no new physics is needed, still new insigts are required.
- why is spacetime four-dimensional
- why just U(1)*SU(2)*SU(3) * SO(3,1)
- if no strings are needed, there's still a landscape problem
- what about initial conditions, pre-big bang etc.
I think that these issues cannot be addressed within the current framework

These are just some superficial problems, they are not the real issues. But even, if you admit these issues cannot be adressed within current framework, doesn't this logically imply that you will have new physics. You will have to give up at least one cherished assumption before you can proceed.

@Careful: any hints for reading (Isham, Sorkin, Finkelstein, Hartle, Penrose, 't Hooft, Durr, Piron, Feynman)

There is the paper I have quoted to you before: as a starter there is the excellent book Quo Vadis quantum mechanics in which most of these prominent figures talk about QM. This is a good place to start.

Careful

 

[Careful]

Indeed because of that I am asking a simpler question: just a paper with "restrictions of the landscape" so that only a few solutions, including 4D space and/or SM gauge group, appear. Let's see how many papers we found and we can think about it. If not, we are just chit-chatting. I hate lazy talking.

I don't know about papers, but the idea I favor most in this regard comes from Clifford algebra.

Careful

 

[suprised]

What is interesting is that twistors seem to abandon the ideas of fundamental space and time; space and time are replaced by twistor space. This replacement lacks a fundamental principle, it is a purely mathematical procedure.

I would think that it has not much more significance than eg going from coordinate to momentum space, by Fourier transformation. Twistors use the fact that a vector index is group theoretically equivalent to a pair of spinor indices, so this is again just a reparametrization.

Indeed it's often difficult to distinguish parametrization or formalism from actual physical content. Sometimes a particular parametrization makes some features more manifest which are hidden in other parametrizations. So a non-standard parametrization can well have its merits.

On related matters, the issues that arise in the recent investigations of loop amplitudes (dual conf symmetries, dual representations of scattering amplitudes, the Grassmannian story) are extremely interesting and provide new insights, but so far these are just discoveries of hidden features of known theories. Whether this is just technical progress or will lead to some new conceptual insights (eg emergence of space-time) remains to be seen.

 

[atyy]

If you would say principleS, I would totally agree with you (on all points).

I could agree to principles, but never principleS. :tongue2:

 

[Careful]

I could agree to principles, but never principleS. :tongue2:

Hehe :rofl::rofl:

 

[arivero]

I don't know about papers, but the idea I favor most in this regard comes from Clifford algebra.

Careful

Indeed, we have a fuzzy set of relationships between Clifford algebra, Bott periodicity, Hoft fiber bundles, octonions, division algebras, triality, supersymmetry... and they seem to include the SM gauge group in the conspiration. But, taking the work of Witten appart, all the other attempts seem to be out of mainstream, or even out of academia... and even in this case I can only think of two or three authors.

Lacking more proposals, I'd say that Witten observation, that D=11 has a narrow set of compactifications where the ones with the standard model are prominent, is the only one where the gauge group is not introduced completely by hand.

 

[Careful]

Lacking more proposals, I'd say that Witten observation, that D=11 has a narrow set of compactifications where the ones with the standard model are prominent, is the only one where the gauge group is not introduced completely by hand.

But doesn't Witten's idea have a myriad of problems which are typically associated to dynamical (in) stability of Kaluza Klein compactifications?
I don't know his idea precisely, but just thinking from what I know.

 

[arivero]

Other favorite idea of me is "Dual quark-gluon model of hadrons", by J.H. Schwarz, Phys.Lett.B37:315-319,1971. There he proposes to consider supersymmetry between quarks and the QCD string, instead of a whole set of new particles.

 

[Careful]

Other favorite idea of me is "Dual quark-gluon model of hadrons", by J.H. Schwarz, Phys.Lett.B37:315-319,1971. There he proposes to consider supersymmetry between quarks and the QCD string, instead of a whole set of new particles.

I don't know about this one, but how do the degrees of freedom match? You have 12 quarks (anti-quarks included), normally you have 8 gluons, so you have 4 degrees of freedom too much. What kind of new physics do these guys give?

Careful

 

[ConradDJ]

So why should a new principle be missing? and how should it look like? should it be a physical principle, or should it be a new mathematical tool?

So from my side: I do not know about any idea that could serve as a guideline, selection principle or whatever.

To my mind, the most striking thing is that we have a basic principle of great generality, that we don’t know how to make use of in constructing a fundamental theory.

QM provides plenty of empirical evidence that determinate facts about things only exist in a context in which those facts are physically measured. On the face of it, this implies that any fundamental physical theory has to include a theory of measurement-contexts. That is, for every physical parameter in the theory, it should give the structure of physical interaction that’s needed to make that parameter measurable.

Instead, we seem to have swept “the measurement problem” aside, as something weird about QM that we just have to accept, that has nothing to do with the way the universe is built.

But surely the most obvious fact about our universe is that it’s observable. For every physical parameter, there are certain specific physical contexts that make it empirically meaningful. Yet all theoretical models I know of just assume their basic structures and parameters without any attempt to show how, within that model, they could actually be observed.

Yes, this is how physics has always been done. And physics is about equations, so it’s easy to assume we’ve incorporated QM into our theory by including superpositions and phase-factors. But QM is not just the equations – it includes a definite prescription about how the equations are to be used. Without specifying any measurement-context, the equations are meaningless.

So really, should we expect to get the right foundational theory by ignoring this key aspect of QM? Is it really a trivial fact about the universe that all of its parameters are physically observable?

 

[Careful]

So really, should we expect to get the right foundational theory by ignoring this key aspect of QM? Is it really a trivial fact about the universe that all of its parameters are physically observable?

I hope you are not implying here that every variable/parameter should have a direct operational meaning, because then I sharply disagree. For the rest, your text is beautifully written.

It is a part of the problem. But I wouldn't say we have to solve it right away, but merely that we should invent preliminary structures allowing for a genuine solution to this issue. And QM as it stands is much too simple for this.

Actually, if you think about it, you will see that the ultimate theory has to be self-dual in a sense that the equations should be defined in terms of the equations. That's precisely the kind of thing which lead to Russel's paradox. It implies we have to move beyond classical logic (denying the axiom of restricted comprehension in traditional Zermelo Freankel, which was added precisely to avoid Russel's paradox). Such extensions of logic are done by Sorkin, Isham (Topos theory) and in general in category theory. But as I said, this problem is too difficult to tackle directly, there are other pressing, more mundaine issues.

Careful

 

[Haelfix]

These are just some superficial problems, they are not the real issues. But even, if you admit these issues cannot be adressed within current framework, doesn't this logically imply that you will have new physics. You will have to give up at least one cherished assumption before you can proceed.

It is pretty clear that nature cannot just be the standard model + Einstein gravity and nothing else. Even if you assume asymptotic safety is correct, something else has to resolve the laundry list of problems (mathematical, physical and experimental) with beyond the standard model physics.

Dark matter, the hierarchy problem, the strong CP problem, matter/antimatter asymmetry, all the standard model 'why' questions (chief amongst them, why is it so arbitrary and adhoc), cosmology and structure formation issues...

And then we get into pure gravity problems perse, including the information loss paradox, the positive cosmological constant problem, the exact nature of inflation, as well as the annoying landscape issues.

New physics will have to enter the picture at some scale and in some manner.

 

[Careful]

It is pretty clear that nature cannot just be the standard model + Einstein gravity and nothing else. Even if you assume asymptotic safety is correct, something else has to resolve the laundry list of problems (mathematical, physical and experimental) with beyond the standard model physics.

Dark matter, the hierarchy problem, the strong CP problem, matter/antimatter asymmetry, all the standard model 'why' questions (chief amongst them, why is it so arbitrary and adhoc), cosmology and structure formation issues...

And then we get into pure gravity problems perse, including the information loss paradox, the positive cosmological constant problem, the exact nature of inflation, as well as the annoying landscape issues.

New physics will have to enter the picture at some scale and in some manner.

This is already a more extensive laundry list. But the question now is, in which logical order are you going to place these issues? Is even one of them a pressing issue or can they all be derived from something which is beyond imagination (in either 99,999%) of most people currently?

Especially, the strong CP problem, the CC problem and even the value of the fine structure constant are ''existential'' issues in some sense.

I don't know if the landscape is a problem, I think I have a logical way out of it, but I have no idea how to make the idea quantitative so far.

Careful

 

[qsa]

This is already a more extensive laundry list. But the question now is, in which logical order are you going to place these issues? Is even one of them a pressing issue or can they all be derived from something which is beyond imagination (in either 99,999%) of most people currently?

Especially, the strong CP problem, the CC problem and even the value of the fine structure constant are ''existential'' issues in some sense.

I don't know if the landscape is a problem, I think I have a logical way out of it, but I have no idea how to make the idea quantitative so far.

Careful

Unify EM plus gravity in a natural way and all the puzzle pieces will start fitting perfectly, that is all what is needed.

 

[Careful]

Unify EM plus gravity in a natural way and all the puzzle pieces will start fitting perfectly, that is all what is needed.

:rofl::rofl::rofl: You were joking right ?