Approaches to Quantum Gravity (now in Cambridge Press catalog)

[marcus]

This may be an odd suggestion, but do you suppose we could ask him? (Gerard 't Hooft) From his website he seems approachable enough. (I also happen to study at Utrecht University, though my major is CS, not Physics) This book seems very exciting :)

I think that is an excellent idea. He may have a draft of the chapter which he would be willing to share with others. Or he might reply that his contribution to the book is along the same lines as his essay in the December 2005 Physicsworld:
http://physicsworld.com/cws/article/print/23668
Any response would be helpful.

As a student at his university, you seem like a good person to write him an email asking about this. I think you are right about his website---it shows him wanting to educate and encourage students. I expect he would like to hear from a young person who is interested in what he has to say on that general topic (approaches to quantum gravity, investigating the fundamental nature of space and time.)

I don't believe he would be offended by being asked. Whether he says yes or no, I think he would be glad to be asked.

 

[Fra]

't Hooft ideas

I skimmed 't Hooft's

(1) The Free-Will Postulate in Quantum Mechanics
-- http://arxiv.org/abs/quant-ph/0701097

(2) The mathematical basis for deterministic quantum mechanics
-- http://arxiv.org/abs/quant-ph/0604008v1

(3) Emergent Quantum Mechanics and Emergent Symmetries
-- http://arxiv.org/abs/0707.4568

Unlike many others, 't Hooft sticks out by acknowledging some fundamental issues in QM.

From source (1) he writes

"An essential element of superstring theory is the assertion that it should be an allembracing
theory, a “theory of everything”. If so, then it should also, in a natural manner, answer some of the questions concerning quantum mechanics itself. What is Quantum Mechanics? Should a “theory of everything” not also explain (at least in principle) how all those fluctuations in space and time could have emerged, what their origin might be, in terms of rigorous equations of motion, how exactly galaxies, stars and planets came into being?..."

"...Superstring theorists thus-far have turned their backs to the issues raised by those who try to understand Quantum Mechanics at a deeper level. It is time to face these questions."

Regardless of the answers I like the way he doesn't hide from this issues! Considering that he seems to be among the exceptions to ask such questions, that alone motivates me to learn more about his thinking.

Further down in his papers I find some interesting things but I'm not sure I understand exactly what he has in mind. It seems he has some desire to restore a higher level of determinisim in QM. I am not sure how to interpret that, but after skimming those papers again, I suspect that he mainly means that there is a yet uncovered logic that will - not predict each outcome instead of probabilities - but that will EXPLAIN the emergence of the QM formalism. If that is so, I think it is great and I'm all in on that.

He also expresses in (1), reflecting over things like quantum logic, that

"We insist that there exists only one kind of logic, even if the observed phenomena are difficult to interpret."

I am not sure I know what he means with that. I guess he means that there should be a unified logical framework in which the two are related?? Ie. there should be a logical connection between classical and quantum? That makes sense.

He is also talking about the notion of unconstrained initial state and wether there are any constraines on which measurements a given observer "chooses" to do. Maybe I didn't understand it but there seems to be a lot left to do. But I got some good associations to this and it could relate to the general learning problem of "given our latest answers, what questions to ask next"

In a certain sense, we are "free to ask" any question we can come up with, but that doesn't mean that we, as seen from a third observer, will fire random questions. How can both be true?

So we have the "choice" to ask any question, but not all questions are of equal utility. Some choices are more beneficial than others. So it is somehow related to the question of "we have the choice to live or die", but most of us choose to live, why? ;-)

This might fit well into a evolutionary perspective. To me this might be keys to a "constructive logic".

I'm curious too see if you guys can get any drafts from Hooft that would be great.
I will stay tuned.

/Fredrik

 

[marcus]

A closer look at part of the 't Hooft Physicsworld essay

I skimmed 't Hooft's
...

(3) Emergent Quantum Mechanics and Emergent Symmetries
-- http://arxiv.org/abs/0707.4568

Unlike many others, 't Hooft sticks out by acknowledging some fundamental issues in QM.
...

I appreciate that you are trying to see what 't Hooft's essential train of thought is---that will probably come out in his chapter of Oriti's book about the fundamental nature of space and time. And so you are looking at past writing and abstracting ideas from it. I will try to do this too. Here is part of the essential message of that Physicsworld essay I quoted earlier.

...what previous 't Hooft writing will seem to us closest to the topics and reasoning in his new essay The fundamental nature of space and time which we have not seen.

... Just a quick guess...essay from December 2005 which he wrote for a magazine Physicsworld

http://physicsworld.com/cws/article/print/23668==exerpt==

...Asking both questions in quantum gravity does not appear to make sense. At distance scales small compared with the Planck scale, some 10-33 cm, there seems to be no such thing as a space-time continuum. That is because gravity causes space-time to be highly curved at very small distances. And at small distance scales, this curvature exceeds all bounds. But what exactly does this mean? Are space and time discrete? What then do concepts such as causality and locality mean? Without proper answers to such questions, there is no logically consistent formalism, not even a quantum-mechanical one.
...
...[THESIS]Quantum mechanics could well relate to micro-physics the same way that thermodynamics relates to molecular physics: it is formally correct, but it may well be possible to devise deterministic laws at the micro scale.

[ANTITHESIS]However, many researchers say that the mathematical nature of quantum mechanics does not allow this - a claim deduced from what are known as "Bell inequalities". In 1964 John Bell showed that a deterministic theory should, under all circumstances, obey mathematical inequalities that are actually violated by the quantum laws.

[SYNTHESIS]This contradiction, however, arises if one assumes that the particles ...are real, existing entities.

But if we assume that objects are only real if they have been precisely defined, including all oscillations as small as the Planck scale - and that only our measurements of the properties of particles are real - then there is no blatant contradiction. One might assume that all macroscopic phenomena, such as particle positions, momenta, spins and energies, relate to microscopic variables in the same way thermodynamic concepts such as entropy and temperature relate to local, mechanical variables. Particles, and their properties, are not (or not entirely) real in the ontological sense. The only realities in this theory are the things that happen at the Planck scale. The things we call particles are chaotic oscillations of these Planckian quantities. What exactly these Planckian degrees of freedom are, however, remains a mystery.
...
...
==endquote==

This essay contains a number of different exciting ideas. One of them is what I have exerpted here. It is a simple "hegel dialectic" type argument.

A. because of Heisenberg, spacetime is uncontrolled chaotic at small scale so it isn't even a smooth continuum. quantum mechanics and gen rel are only regular APPEARANCES that arise by averaging out, at larger scale. there are probably underlying micro degrees of freedom obeying deterministic laws-----like the deterministic micro picture that underlies thermodynamics.

B. but Bell-type arguments appear to say that there cannot be deterministic micro dynamics underlying quantum mechanics

C. however the Bell-type reasoning only works if particles exist at fundamental level.
suppose that when you get down to Planck scale, particles are just tangles or oscillations in what is really there. then we get around the Bell objection and we can try to construct some micro deterministic degrees of freedom from which the appearance of space time and matter arise by smoothing and averaging at larger scale (like the gas law arises from molecular dynamics, like pressure arises from collisions)

 

[Fra]

About the the part of 't Hooft's writing that Marucs exerpted. For the sake of discussion, let's assume that his line of reasoning so far are good. What's the next step?

He says himself
"What exactly these Planckian degrees of freedom are, however, remains a mystery."

So what is the physical basis for and the logic of these degrees of freedom and what is their observational connection? And how does the answer to that merge with the general information theoretic and relational information picture with the holographic ideas?

I don't know what you think but I always associate a sensible notion of degree of freedom with the notion of distinguishability. The degrees of freedom are associated to the number of - relative to a particular observer or logic - distinguishable (micro)states.

This on one hand, seems to suggest that if we consider all arbitrary observers, the notion of degree of freedom would be different?

But then what is the microstructure whose elementa correspond to these Planckian degrees of freedom, which would provide an (at least effective) universal microstructure?

Considering hooft's idea on the holographic principle one might suspect that he has interesting ideas on this. Maybe, Planck degrees of freedom somehow refers to communication channels, so that it's "communicated" degrees of freedom so that spacetime is more to be seen as a communication channel which develops self-organising structures.

Then what is it that is beeing communicated? And why?

But as I read Hooft there is no clear observer or relational notions in his reasoning. But he talkes about local determinism, and I wonder how to interpret local in this context.

Could he mean local in some space of observers? So that this could be interpreted in terms of relational logic? So that to a local observer, there is a possibly deterministic logic to the actions, but that this logic (the mecahnics) is not distinguishable to a remote observer.

Edit: what I mean here is like the assoication of a network of players trying to win, where the rules of the game are always in moution and the strategy of the other players aren't known. Yet it may seem like the best rational choice, to suspec that the other players also behave rationally. But then the rationality may be hidden since this isn't known. So that each player may "act rationally", but the actions of all other player "appear" irrational to a single player. Could Hooft be after something like that? so that

local determinism ~ local logic ~ local rationality

?

Because if space is a communication network, then it seems natural to suggest that local objects are like transceivers, or communicating observers? And that from each local point of view, there is a clear logic, but it is not possible to see the clear logic of a remote information processing node?

It might suggest that for a realistic simulation, each node would have to be intelligent, and somehow independently running simulation??

This could be a bit of a stretched interpretation of his writing though.

What other possibilities are there to the Planckian degrees of freedom? And WHO is supposed to answer that question? ( I don't mean which of us formum members :) I mean relative to what system/observer is the question formulated, or is that not a relevant question)

/Fredrik

 

[Emanuel]

Sorry about the delay guys, here's the e-mail I just sent to Gerard 't Hooft:
(Dutch)
"Nu bekend is dat dit boek (Approaches to Quantum Gravity) in maart volgend jaar uit komt en de definitieve index online beschikbaar is, loopt op Physics Forums een discussie over de bijdragen van de verschillende wetenschappers die eraan meegewerkt hebben. We hebben al een ruwe versie van zo'n twintig hoofdstukken gevonden, maar we zijn erg benieuwd naar wat u te zeggen heeft in 'The fundamental nature of space and time'.
Aangezien ik toch aan de UU studeer (hoewel in een andere faculteit) leek het mij een goed idee om het u gewoon direct te vragen: kunt u een samenvatting geven van wat u in uw hoofdstuk bespreekt? (of anders een artikel of publicatie waarin u hetzelfde overbrengt)
Hoe dan ook ben ik zeer benieuwd naar dit boek in zijn uiteindelijke vorm."

(English)
"Now that we know that this book (Approaches to Quantum Gravity) will be released in March next year and the definitive table of contents is available online, there's a discussion on Physics Forums about the contributions of the various scientists involved. Already we've gathered drafts of some twenty chapters, but we're quite curious about what you have to say in 'The fundamental nature of space and time'.
As I study at the University of Utrecht - though in a different faculty - I thought it would be a good idea to simply ask you directly: could you give a summary of what you discuss in your chapter? (alternatively perhaps an article or a paper in which you convey the same)
Either way, I'm greatly looking forward to seeing this book in published form."

If the first line seems awkward, the subject line was 'Approaches to Quantum Gravity' :) (for other oddities, blame difficulty in translating from Dutch to English)

 

[marcus]

I think that is outstanding. It is clear and direct. It is courteous and says exactly what we would like. I am so glad you wrote, Emanuel. Thanks!

We cannot do more than this. If he chooses to answer, he will. If not, we can wait for the appearance of the book.

 

[Emanuel]

Thank you for the kind words. No reply yet, but then it's the holidays so we should probably give it some time. Classes start again in September, but hopefully I'll have received a reply by then.

 

[marcus]

Approaches to Quantum Gravity ( Cambridge Press catalog says Feb 2009)

The publisher's date for availability has been advanced again.

Two months ago, when I started this thread, the target date for publication was May 2009.
... in the past two months, for what it's worth, this book's publication date has ADVANCED and is now earlier.
Now it is listed as March 2009.
...

Now the catalog says available as of February 2009.

It looks like an advance copy of Gerard 't Hooft's essay on the fundamental nature of space and time will NOT be obtainable. I am grateful to Emanuel for trying.
I think the best guess, to give an idea of what it might say, is his piece in the December 2005 Physicsworld:
http://physicsworld.com/cws/article/print/23668

==========================
for me one of the most intriguing features is that after each of the five Parts of the book there is a discussion among all the authors called Questions and Answers.
The five parts (with some paraphrasing by me) are:
Part I Fundamental Ideas
Part II [String]
Part III [Loop and allied approaches]
Part IV Discrete
Part V Effective [and How to Test]

After each part there is a free-for-all where any author, not limited to one featured in that section, can ask questions about stuff in that section. I think that is potentially interesting and constructive.

for ready reference, here is the TOC again, just so you don't have to scroll back.
Keep in mind that we have links to preprint for several of the chapters. I you see one that interests you, you can scroll back and see if there is a preprint link.

Preface;
Part I. Fundamental Ideas and General Formalisms:
1. Unfinished revolution C. Rovelli;
2. The fundamental nature of space and time G. ‘t Hooft;
3. Does locality fail at intermediate length scales R. Sorkin;
4. Prolegomena to any future quantum gravity J. Stachel;
5. Spacetime symmetries in histories canonical gravity N. Savvidou;
6. Categorical geometry and the mathematical foundations of quantum gravity L. Crane;
7. Emergent relativity O. Dreyer;
8. Asymptotic safety R. Percacci;
9. New directions in background independent quantum gravity F. Markopoulou;
Questions and answers;

Part II:
10. Gauge/gravity duality G. Horowitz and J. Polchinski;
11. String theory, holography and quantum gravity T. Banks;
12. String field theory W. Taylor;
Questions and answers;

Part III:
13. Loop Quantum Gravity T. Thiemann;
14. Covariant loop quantum gravity? E. Livine;
15. The spin foam representation of loop quantum gravity A. Perez;
16. 3-dimensional spin foam quantum gravity L. Freidel;
17. The group field theory approach to quantum gravity D. Oriti;
Questions and answers;

Part IV. Discrete Quantum Gravity:
18. Quantum gravity: the art of building spacetime J. Ambjørn, J. Jurkiewicz and R. Loll;
19. Quantum Regge calculations R. Williams;
20. Consistent discretizations as a road to quantum gravity R. Gambini and J. Pullin;
21. The causal set approach to quantum gravity J. Henson;
Questions and answers;

Part V. Effective Models and Quantum Gravity Phenomenology:
22. Quantum gravity phenomenology G. Amelino-Camelia;
23. Quantum gravity and precision tests C. Burgess;
24. Algebraic approach to quantum gravity II: non-commutative spacetime S. Majid;
25. Doubly special relativity J. Kowalski-Glikman;
26. From quantum reference frames to deformed special relativity F. Girelli;
27. Lorentz invariance violation and its role in quantum gravity phenomenology J. Collins, A. Perez and D. Sudarsky;
28. Generic predictions of quantum theories of gravity L. Smolin;
Questions and answers;

Index
=========================
Here is the link to the Cambridge Press page on the book:
http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521860451

 

[SuperStringboy]

Murcus, nice job! Thanks

 

[Emanuel]

I just finished the preface after receiving my copy from Cambridge Press earlier today. Has anyone else on here received their copy yet? I wanted to bump this topic before I start the book proper to bring it back into the public eye, and because I'll probably get lost in its pages ;) I hope you'll be able to help me out if I do.

 

[marcus]

I just finished the preface after receiving my copy from Cambridge Press earlier today. Has anyone else on here received their copy yet? I wanted to bump this topic before I start the book proper to bring it back into the public eye, and because I'll probably get lost in its pages ;) I hope you'll be able to help me out if I do.

Emanuel, it's good to hear from you. I remember you wrote an excellent email to 't Hooft inquiring about his Nature of Space and Time essay, which we were all wondering about.
Being myself by nature a frugal person, and given that so much of the book (40 or 50 percent?) is available online in draft form, I have not yet taken the plunge and ordered a copy. At the moment, you are the only person I know who owns the book.

What I'm especially curious about are the Q and A sections where the authors of the different chapters can ask questions and respond to each other. Dialog (as Galileo discovered) is a great format for communicating science. So right now it is this Q and A dialog that is primarily tempting me to obtain the book. And also a lot of the chapters have probably been revised, updated, from their original draft versions on arxiv.

BTW at first the book was available directly from Cambridge University Press but not from Amazon. But I just checked the UK amazon and they not only have it in stock, but have almost sold out!

http://www.amazon.com/dp/0521860458/?tag=pfamazon01-20

 

[Emanuel]

Yeah, this is an expensive book! I actually got it as a birthday present; imagine what my parents must think :) If there's anything you'd like to know about the book, let me know.

 

[marcus]

Yeah, this is an expensive book! I actually got it as a birthday present; imagine what my parents must think :) If there's anything you'd like to know about the book, let me know.

Whew! I'm relieved to hear that, Emanuel. HAPPY BIRTHDAY!

Well I am interested in several things actually. One for example is what is your general impression of the Q&A at the end of Part I?

Part I covers a lot of topics and I wonder how many of them get touched on in the Q&A.
For example Part I has Rovelli's perspective piece, which raises among other things some potentially controversial issues by comparing string and background independent approaches. And it also has Percacci's Asymptotic Safety essay. I saw some Q&A regarding that and I wonder if it made it into the final draft. It actually was very helpful, seeing someone else's questions get answered helps me.
And I was not satisfied with the draft I saw of 't Hooft's essay, it left me wondering about a lot of things. 't Hooft is a kind of lighthouse figure that one navigates by, and I was left with a feeling of wanting more clarification about what he was saying.
So I am wondering what was the Part I Q&A like, whose essays got discussed? Rovelli is remarkably adept in dialog, I hope he got asked a few questions.

 

[weburbia]

In case anyone is still looking for 't Hooft's contribution online, there is a version here:
http://www.phys.uu.nl/~thooft/gthpub/QuantumGrav_06.pdf

 

[marcus]

In case anyone is still looking for 't Hooft's contribution online, there is a version here:
http://www.phys.uu.nl/~thooft/gthpub/QuantumGrav_06.pdf

Thanks! I had just seen it once and had been unable to retrieve it. I want to study it more carefully.

 

[Fra]

Thanks weburbia, I was also waiting for this! and I just skimmed it.

Somehow my impression that I expressed in some previous posts in this thread still stands - based on 't Hooft's idea of "deterministic QM", in the sense that there is a yet unknown deterministic model at the Planck scale, in which picture QM is emergent on the large scale. (Since most particle and atomic physics, while beeing the currently smallest explored scale, is still "large scale" as compare to the Planck scale). In this way, he also imagines to escape the usual Bell-type argumentas against hidden variable theories.

I like some of his reasoning, but now it seems to me that what he is leaning towards, is not something I believe in. But it's still possible that his ideas can be developed in a plausible direction.

Somehow the general idea that quantum logic is emergent, is good I think. But I think that I'm moderatley attraced to his idea of his description of this emergent process. At best, I can connect to this idea, if this Planck scale microstructure, rather than beeing classical in the realist sense, is a limit case of where the observer grows and ultimately coverse the universe. I do see how what he says makes sense in that sense, but I fail to see the physical relevance of that picture. Ie. the physical basis and uniquess of this auxiliary structure as he calls it.

One additional way this can make sense, is that one observer (a big observer) can understand the emergence of the "quantum mechanical interactions" WITHIN a smaller system (say particles making up an atom). In THIS sense, I would probably fully agree with his idea that from the point of view of an external large observer, HE can understand the logic of ther emergence of quantum mechanical interactions within the atom.

But, I fail to see how this can make sense if you extend this idea of cosmological scale models, and in general where the approximation of a external observer completely fails. This objection is also as I understnand it the general argument behind Smolins evolving law.

So, in short I think that 't hooft's reasoning could work in an hierachial model where the Planck scal microstructure itself is evolving! Then I think we are close. But as I read 't Hooft this is not quite how he pictures it.

Does anyone have a different impression? Maybe I am missing som depth due to seeing all this from my own perspective.

/Fredrik