The Third Road to Quantum Gravity

[Careful]

**In an elementary topos, a proposition is understood in terms of its interpretation in terms of truth values. This is an axiomatic setting outside of ordinary set theory. This is simply a fact. **

I did not mean to define what a proposition is, I wanted to speak about a set of propositions such as : ``the moon is there, or the Earth is round, etc... ´´

**No. The point is that categories can do more subtle geometry than this. If all we were going to do was work with ordinary manifolds then I would agree: categories would not be enough. This is, however, very far from being the case.**

I KNOW that, but (a) it is always GOOD to give an example which is understandable for everyone (you have to learn to communicate an idea intuitively, and many mathematicians often can't - they are stuck in their details) (b) you seem not to appreciate my comments that deviating from manifoldness too violently highly likely leads to nonrenormalizable theories.


**One does not begin with a model of spacetime, which is clearly a highly derived concept. And yes, when I say scale I am thinking of energy scales, but then again even this is an entirely classical concept. **


So what do you start with (we need to know what we are talking about!)?? The use of energy scales in a fundamental theory is IMO highly anti relativistic, but ok, the high energy community would back you up here.

**Physically, energies are no different to quantum numbers: they need to be looked at in the context of the experiment.**

So, you stick to the reduction postulate in QM? Right?


**So, as I often say here on PF, the question what is scale is by no means trivial, and I will certainly not be answering it in a few lines. One does not work in a simple 1-dimensional category.**

That is a difficult question REGARDLESS of categorical considerations.

**Hence the question what category do you restrict to is completely meaningless.**

It is not ! If you do not do that, it is impossible for you to define a controllable dynamics! I would appreciate it if you would comment on my other remarks too and not only select those which are specifically category theory oriented.

 

[Kea]

I did not mean to define what a proposition is, I wanted to speak about a set of propositions such as: "the moon is there" or "the Earth is round" etc...

Yes, but the idea that physical propositions can form a fixed set is IMHO exactly the problem. The notion of a fixed classical reality is tantamount to the inclusion of a universal observer in one's framework. In the reduction of the general picture to ordinary QM one must of course recover some such description, but I believe it is a prejudice that needs doing away with.

...it is always GOOD to give an example which is understandable for everyone.

OK. Fair enough.

...you seem not to appreciate my comments that deviating from manifoldness too violently highly likely leads to nonrenormalizable theories.

Except that, to my knowledge, the best understanding of renormalisation that we have comes from the current Connes, Marcolli, Kreimer et al work, which is beginning to use motivic cohomology and such things rather heavily...

So what do you start with (we need to know what we are talking about!)?

I guess what you want to know is: what higher dimensional categories should one operate into be able to do either (a) GR or (b) the standard model. For the latter it would seem a suitable choice of Gray categories (with the ability to do quantum logic), such as a rich form of Vect, would do the trick.

So, you stick to the reduction postulate in QM? Right?

Well, not exactly. Although the only way I know how to speak about observables is in a language that sounds like QM, it is quite certain that the topos-like axiomatics are naturally written in a form that only reduces to the usual case on the choice of a particular model.

The interplay between theories and models is an important aspect of the logical point of view, and one of the main reasons that I make the claim that categories are much more than an organisational tool.

 

[Careful]

**Yes, but the idea that physical propositions can form a fixed set is IMHO exactly the problem. The notion of a fixed classical reality is tantamount to the inclusion of a universal observer in one's framework. **

AH, but that depends upon what you want to do with QM (and that is the whole crux of the story). I mentioned previously that for a classical theory, there is no problem, the set of physical propositions we can make is DYNAMICAL (I think I explicitely mentioned this already in post 97). Example: put some differential equation in your computer with exotic intial conditions (possibly with boundary conditions). Suddenly you notice that very complex patterns form in time for this particular solution mankind has never seen before. At that moment you learn something more and your set of propositions gets enlarged. All you need for that is a classical theory of the brain which allows for pattern recognition and an (classical) arrow of time.

Now, in ordinary QM you are screwed, the propositions are merely put in by the experimentator and not dynamically generated at all. So, this leaves a few possibilities :

(a) you introduce a double world picture, both with its own dynamics and a principle which interrelates them. This is for example done in the work of Aerts et al (based upon work of Piron). He considers quantum words embedded in different classical entities (the problem is that there is no dynamics yet). The kinematical setting of this work is very abstract but concrete (they work in a specific category as you would call it)
(b) Everything is quantum but then my guess is that you have to make QM nonlinear to make sense out of the macroworld (and as such out of a dynamical set of propositions)
(c) Everything is classical and the Schroedinger equation is nothing but a statistical divise containing macroscopic parameters (like mass and charge) to compute outcomes of experiments. Therefore, we have to look for an underlying deterministic mechanics (my preferred approach)

If you have something to add, please go. You see: you can adress this problem within a fixed ``master´´ category (you just have to choose it big enough so that it suits your desires).

**Except that, to my knowledge, the best understanding of renormalisation that we have comes from the current Connes, Marcolli, Kreimer et al work, which is beginning to use motivic cohomology and such things rather heavily...**

I don't know that and I would like to hear what NEW insight is gained vis a vis the methods physicists normally use.

**I guess what you want to know is: what higher dimensional categories should one operate into be able to do either (a) GR or (b) the standard model. For the latter it would seem a suitable choice of Gray categories (with the ability to do quantum logic), such as a rich form of Vect, would do the trick. **

Sure, everyone would like to know this ! So, I suggest that you concentrate on explaining the virtues of these two categories.


**The interplay between theories and models is an important aspect of the logical point of view, and one of the main reasons that I make the claim that categories are much more than an organisational tool. **

Hmmm, but it are in the end only the models which form the *physical* theory. So your level of abstraction is good for mathematics, but in the end physics concentrates itself on a specific kinematics endowed with a specific DYNAMICS.

Cheers,

Careful

 

[Kea]

So, this leaves a few possibilities :
(a) you introduce a double world picture, both with its own dynamics and a principle which interrelates them. This is for example done in the work of Aerts et al (based upon work of Piron). He considers quantum words embedded in different classical entities (the problem is that there is no dynamics yet). The kinematical setting of this work is very abstract but concrete (they work in a specific category as you would call it)
(b) Everything is quantum but then my guess is that you have to make QM nonlinear to make sense out of the macroworld (and as such out of a dynamical set of propositions)
(c) Everything is classical and the Schroedinger equation is nothing but a statistical divise containing macroscopic parameters (like mass and charge) to compute outcomes of experiments. Therefore, we have to look for an underlying deterministic mechanics (my preferred approach)

My preference is for a mixture of (a) and (b). My work is probably most closely related to (b). Piron's ideas have led many people to topos theory. Quite frankly, as a physicist, I don't see how you can take (c) seriously.

I don't know that and I would like to hear what NEW insight is gained vis a vis the methods physicists normally use.

The renormalisation studies are about the methods physicists normally use. Sure, it's very mathematical, and I can well understand why any physicist would be reluctant to touch it...but I reserve the right to burden myself with any mathematics that I have good reason to think supports this approach.

So, I suggest that you concentrate on explaining the virtues of these two categories.

That is, naturally, the long term goal of this and some other threads. If I could do this in the next 5 minutes, I would be busy calculating things and not wasting my time here. Moreover, PF has no facility for drawing lots of diagrams.

Hmmm, but it are in the end only the models which form the *physical* theory.

Well, time will tell, won't it? Careful, you have not given us a single solid argument to deter people from this approach. Once again, I urge you to begin another thread, introducing us to your own ideas.

 

[Careful]

**My preference is for a mixture of (a) and (b). My work is probably most closely related to (b). Piron's ideas have led many people to topos theory. Quite frankly, as a physicist, I don't see how you can take (c) seriously.***


My God, you are completely unaware of the fact that the best thinkers in physics have mostly taken (c) seriously (and continue to do so). To name a few: Einstein, de Broglie, 't Hooft, A.O Barut (partly) etcetera... . ¨I want to stress that (c) is a logical possibiltiy which has NOT been experimentally refuted. Moreover, (a) as I said is really *inexistant* in the sense that there is no DYNAMICS yet (although this approach goes along for some twenty years too no?) (b) has been the subject of intense research without any conclusive outcome so far. Considering (c) is usually frowned upon albeit there exist already some partial results in that direction (SED, Self field approach etcetera)! These theories partly reproduce in a (semi) classical way quantum results (so they left the kinematical stage already for a some 25 years) and have a clear (classical) ontology.


**The renormalisation studies are about the methods physicists normally use. Sure, it's very mathematical, and I can well understand why any physicist would be reluctant to touch it...but I reserve the right to burden myself with any mathematics that I have good reason to think ssupports this approach. **

So, you cannot tell us why it brings us anything extra while you used this argument to ``counter´´ my objection that if you make the category too wild, the dynamics you are interested in is probably nonrenormalizable. There is serious evidence for this claim: check out the literature in dynamical triangulations to start with. Moroever such debating tricks are not serious : you should consider going into politics since you clearly do not like to answer real objections.


**That is, naturally, the long term goal of this and some other threads. If I could do this in the next 5 minutes, I would be busy calculating things and not wasting my time here. Moreover, PF has no facility for drawing lots of diagrams. **

You can still explain in words...

**Well, time will tell, won't it? Careful, you have not given us a single solid argument to deter people from this approach. Once again, I urge you to begin another thread, introducing us to your own ideas.
**

O yeh, I have given pleanty of them: (a) it did not bring anything new yet (b) I did not hear yet any argument why it should contribute one day to physics (on the contrary, there is evidence that you have to keep your kinematical structure under control - but of course you are completely ignoring these facts or are simply not aware of them) (c) the examples you gave us are entirely *kinematical* and avoid as such the real problem: that is the dynamics (d) the good inventions in physics NEVER emerged from a twisted desire to abstraction, but always were the result of deep, albeit relatively simple ideas and calculations.

So, IMO you are a mathematician who is on a promotional tour trying to link her business to ``physics´´. Perhaps I am wrong and you can still present us some real physics = kinematics + dynamics.

 

[Kea]

Careful

You have voiced your objections and are no longer adding value to this thread. The first points above would be more appropriate on a thread that discusses your own, clearly superior, ideas.

 

[Careful]

Careful
You have voiced your objections and are no longer adding value to this thread. The first points above would be more appropriate on a thread that discusses your own, clearly superior, ideas.

Your answer clearly expresses that there is nothing to it yet otherwise you could give a clear and unambiguous answer. :zzz: BTW, some of my ideas concering QM were already discussed on the quantum mechanics forum. I shall not bother you anymore in your self promotion activities. It is clearly a waste of my time.

 

[setAI]

Oh yeh, I did, but you started off bad. You referred:
``The Computational Universe: Quantum gravity from quantum computation
Seth Lloyd
http://arxiv.org/abs/quant-ph/0501135´´
If you look up the word CRACKPOTISM 2005, this paper should be in the top ten. It is not only utterly naive, but it contains elementary mistakes as well.

the quantum computational approach is currently the most fruitful and promissing thing to happen to cosmology since the discovery of the Big Bang- you stand practically alone amongst all professional scientists in your disreguard of Seth's Thesis-

here is a section of my recent "IT from qubit" article:

...given the nature of Quantum Mechanics and Quantum Computers the next logical step was obviously the search for a theory Quantum Gravity through Quantum Computational Cosmology- in essence to recognize that quantum computers are harnessing Nature’s own processes and that the Cosmos itself should be described as a quantum computation!



Seth Lloyd and others in the field of Quantum Computational Cosmology conjecture that the 'correct' theory-of everything for our universe will be solved through universal quantum computation: “Computational universality allows a quantum computer to give rise to all possible computable structures... such a universal quantum computer that computes all possible results, weighted by their algorithmic amplitude, preferentially produces simple dynamical laws. Our own universe apparently obeys simple dynamical laws, and could plausibly be produced by such a universal quantum computer.” –Seth Lloyd- obviously the implications of this transcend even the most fantastical Virtual Reality and Time–travel concepts-



Lloyd conjectures that these physical laws will be a form of Quantum Gravity emergent from the computation of a Quantum Cellular Automaton embedded in a lattice of quantum logic gates with 4 connections [2 inputs/ 2 outputs]- this automatically establishes by it's inherent structure a 4D spacetime metric with 1 degree of freedom establishing a causal hierarchy between universe-states corresponding to the causal structure of spacetime which emerges as Time/Causality/ sets the covariant entropy bound/ and establishes the information bound that shapes the structure/ rules/ evolution of physical systems and begins to resolve the mystery of why/how time/the quantum geometric limit/ and the entropy/information bounds are complementarily interrelated!


which means the old idea of a theory-of-everything that you can print on a T-shirt is within view! the correct theory of QG [or whatever the correct theory[ies] is/are] could easily be expressed as the eigenvalues of a Quantum Cellular Automaton formulated as an algorithm for a 2input/2output quantum logic-gate matrix-



some more research in Quantum Computational Cosmology:



http://www.arxiv.org/abs/quant-ph/0506113
http://arxiv.org/abs/quant-ph/0502166
http://arxiv.org/abs/gr-qc/0403057
http://www.arxiv.org/abs/gr-qc/0503073
http://arxiv.org/abs/quant-ph/0505064
http://www.arxiv.org/abs/gr-qc/0510052
http://www.arxiv.org/abs/gr-qc/0412076

Crackpotism indead! within a decade or two we will be able to run quantum algorithmic versions of all of our cosmological theories on a quantum computer and examine their hilbert spaces to determine if spacetimes like ours are a possible result- if we run versions of String Theory and LQG and nothing like our world results- then those theories can immediately be tossed and forgotton! decades of conjecture settled and disgarded in a lazy afternoon-

this technology is actually making theory obsolete- or rather it is transorfming theoretical physics into reverse engineering! the future of theoretical science will be very different when anyone can check their theories against the Universe itself!

 

[Kea]

Thank you, setAI. It was my opinion that we should probably just ignore this empty tirade, rather than encourage it.

 

[Careful]

Thank you, setAI. It was my opinion that we should probably just ignore this empty tirade, rather than encourage it.

I actually *know* that I am far from being alone in the professional society with my opinion here. That the quantum computing community wants to self promote some own ``ideas´´ (because there really is no theory) concerning QG is only natural (and can indeed lead to such proposals). Again, I actually read the paper *in detail* (which was hard to do because he uses obscure constructions); before you SetAI make such quotations, you might (actually should) do the same effort (in the same way I guess Kea did not read it either).

Cheers,

Careful

 

[vanesch]

I would like to propose something to the different antagonists here which - I hope - will calm down spirits (or attract me a lot of trouble :-). I'm working in much more applied domains and I have to say that I cannot even follow technically several discussions around here. Nevertheless, I notice something that I consider a fundamental problem in attitude (which you do not find in other branches of science to my knowledge).

I think it is fair to say that all attempts to unify general relativity and quantum theory are more or less embryonic attempts at formulating a possible frame for a future, speculative theory. No real *theory* exists, which clearly leads to the quantum theory we know of on one hand, and to general relativity on the other, without ambiguities ; and even if such a theory one day would emerge, one should keep in mind that it is still a totally speculative endeavour.

Given the situation, two points - I think - can be made:

1) all good ideas are welcome! Nevertheless, one shouldn't forget what Feynman set out as his attitude in scientific endevour: if you have an idea, you should try, by all means, to show that it DOESN'T work. You shouldn't try to show that it works, but you should rather try to find why it doesn't work. It is only when you've tried that, honestly, in all possible ways, and you *do not find a single way to show that your idea is flawed*, that you can show it to others, who are then supposed to do exactly the same.
It is my personal impression that a lot of people working on different ideas on the unification between general relativity and quantum theory do not play that game, but rather the opposite ; that they try to convince others that their idea WILL OVERCOME all eventual troubles, and they seem to be hostile to the "normal" attitude of other scientists, which is: trying to find why the idea cannot work. I cannot undo myself from the impression that the ideas in this field are usually *strongly* oversold ; I think more self-critique is due when presenting ideas.

2) one should keep an open mind. Although it requires of course some "faith" in the idea you're working on to keep being motivated, it should not turn into a religious war. One shouldn't feel "attacked" by any other scientist who tries to point out where your idea has a problem ; in fact, normally you should have thought of it yourself and have the answer ready, and if not, you should be grateful that somebody is pointing it out for you. The attitude I have often seen has been rather to close oneself up into different "sects" where the only thing that counts is the *promotion* of the dogma at hand, and the denial of the validity of others' critiques or approaches. Although psychologically understandable, I think it leads to uselessly nasty and unproductive discussions.

So let's get back to "normal" scientific discussion which consists in:
1) pointing out where one sees a serious problem in the other person's ideas and where
2) that other person has already considered the objection and knows the answer to it (in which case the first author of the critique learns something) OR where the one pointing out a new idea is confronted with a critique he didn't think of, which sends him back to the drawing board.

Amen

 

[Careful]

I would like to propose something to the different antagonists here which - I hope - will calm down spirits (or attract me a lot of trouble :-). I'm working in much more applied domains and I have to say that I cannot even follow technically several discussions around here. Nevertheless, I notice something that I consider a fundamental problem in attitude (which you do not find in other branches of science to my knowledge).
Amen

I fully agree : let me refer people to a discussion I had with Vanesch on the philosophy forum concering consciousness in the MWI to QM. Although it is clear that I think that it stinks, I still acknowledge that it is a logical and consistent approach (although not complete) which does the trick (and in the mean time we had fun driving each other nuts ). To add a further comment to what Patrick says here: I think he and I agree that it is of extreme importance to have a *physical* theory which produces falsifiable results in a reasonable amount of time (say 5 years). Actually, I would add that the production of an experimentally refuted theory which is based upon clear physical principles actually teaches us *more* than talking about a theory which either does not exist, makes no predictions at all or is not logically well founded. As a string non believer, I must nevertheless admit that string theory is the most logical game in town we have so far (a weakness is that it did not produce any physical result so far of course , but none of the nonperturbative approaches did either.). I would like to invite Kea to do her best and try to give one detailed example with dynamics (if possible) which makes her *believe* in what she does. It is clear that I dismissed the category approach some time ago for good reasons based upon concrete calculations within what you would expect to be the most ``reasonable kinematical category´´ on which to define a dynamics. However, I would not object consideration of an honest example without the addition of buzz words.

Cheers,

Careful

 

[Kea]

I must nevertheless admit that string theory is the most logical game in town we have so far...

This is String theory.

 

[Kea]

Chronos!

On Not Even Wrong, Woit has mentioned this beautiful article by the mathematician Mazur
http://www.math.harvard.edu/~mazur/preprints/when_is_one.pdf

I think it is just for you.

 

[vanesch]

Chronos!
On Not Even Wrong, Woit has mentioned this beautiful article by the mathematician Mazur
http://www.math.harvard.edu/~mazur/preprints/when_is_one.pdf
I think it is just for you.

It wasn't meant for me, but it turns out to be helpful for me !

Thanks, Kea

 

[Kea]

Reference:
The Anthropic Landscape of String Theory
L. Susskind
http://arxiv.org/abs/hep-th/0302219

Perhaps it is about time that, in regard to the Third Road approach to String theory, we thought about some of the physical aspects of the landscape.

When a physicist of the calibre of Weinberg accepts the notion of an anthropic principle, one should take the science behind the idea seriously. The question is: how might one retain the landscape and yet do away with the ludicrous notion of anthropicity? Is this possible at all?

The Third Road would say that the notion that the multitude of vacua exists, in any objective sense, is erroneous. What we observe depends very much on the context of the experiment. So, until we are capable of devising experiments that step outside the conditions under which we are used to operating, we should not be at all surprised that reality has the appearance of magically fitting parameters that could, according to the standard picture, take on many other values.

There is a big challenge here: to rigorously derive the landscape in a category theoretic framework, and hence show that it is possible to place constraints upon it which give it predictive power.

Looking at Susskind's article:

This change in viewpoint is demanded by two facts, one observational and one theoretical. The first is that the expansion of the universe is accelerating...The second fact is that some recent progress has been made in exploring the landscape.

This statement is followed by a definition of the landscape. Let the landscape be parameterised by a collection of fields such that the value of V(\mathbf{\phi}) at a vacuum point gives the cosmological constant \Lambda for the vacuum. Special SUSY vacua with V \equiv 0 form a flat subspace within the hilly landscape, replete with great cliffs (domain walls) and mountain passes.

Hopefully the reader can see that already at this point the landscape picture is beginning to make all sorts of assumptions about the nature of a multitude of universes as if they actually exist. Moreover, these assumptions have been made within the context of standard M-theory, ie. with the addition of no new physical principles.

Susskind then goes on to discuss how vacua may be parameterised by
sets of integers which arise from different ways of wrapping things around compactified dimensions. It turns out to be possible to recover the Standard Model (!) with a small positive \Lambda by exploring this landscape.

OK. So to do away with the landscape rigorously we just have to begin by recovering the SM from a fully working M-theory... Sigh. It always comes down to that. :zzz:

 

[Chronos]

Thanks for the plug, Kea. Mazur has greatly influenced my admittedly naive ideas. I've never actually mentioned that on PF, and I am, well, shocked you made the connection [is it not a truly brilliant treatise?] You scare me sometimes.

Careful, you make good points. But, your criticism of Kea was disturbing and misinformed, in my opinion. She is one of the most tolerant and forgiving people on PF, along with SelfAdjoint. That aside, I am a huge fan of QIT. Not sure if that is the third road, or the fourth road, but, I'm convinced it is the right road.

ps. I forgot to mention the excellent contributions SetAI and Vanesch made to this discussion.

 

[Careful]

**
Careful, you make good points. But, your criticism of Kea was disturbing and misinformed, in my opinion. She is one of the most tolerant and forgiving people on PF, along with SelfAdjoint. That aside, I am a huge fan of QIT. Not sure if that is the third road, or the fourth road, but, I'm convinced it is the right road. **

I do not think so, I shall read the Masur paper (but at first glance it seems like an introduction to the playground of category theory accompanied with some philosophical remarks towards physics). My comments are still very adequate since I come up with CONCRETE evidence that all these generalizations probably have very little applicability (but again, nobody cares).

 

[Kea]

...but at first glance it seems like an introduction to the playground of category theory accompanied with some philosophical remarks towards physics...

Indeed! We confess.

 

[Chronos]

Looks like hand-waving to me. I see more mud than cement.

 

[Careful]

Looks like hand-waving to me. I see more mud than cement.

Dear Chronos,

I would kindly request you to take a look at the Ambjorn - Durhuus book about ``quantum gravity´´ (especially the Euclidean part is very useful) and then read up a bit more on the Lorentzian ``quantum gravity´´ formulation within dynamical triangulations. You might also want to study the Sorkin-Rideout classical dynamics for causal sets. Then, you might notice that only in CDT, the *dimension* comes out right (even a ridiculous observable like dimension poses great difficulties - and so far there are not more substantial results). In the spin foam, I am not even aware that *any* such results exists (not even a negative one). Next, you might want to gain some understanding of WHY this is so difficult and then see if your opinion deviates from mine.

To repeat myself: category theory is a beautiful abstraction (I was once seduced by medusa too ), but anyone who is announcing that it could very well be successful is either ignorant of these results, or is not willing to consider them. Ignorance is not bad at all (happens to me every day), however unwillingness (to listen) is stupid. I am still willing to listen if Kea comes up with a concrete example (she basically just said: ``string theory´´).

Cheers,

Careful

 

[Tonko]

the quantum computational approach is currently the most fruitful and promissing thing to happen to cosmology since the discovery of the Big Bang

Absolutely fascinating reading! Thank you so much for this list of articles. Excuse my lazyness, if there already is one and I haven't spotted it, but is there a thread dedicated to quantum computation? If not, can somebody qualified start it with a decent introduction for nonexperts?

I have just listened to a few lectures from the Perimeter Institute and I am in awe of what has develeped out of EPR and quantum entanglement.

It is not that I have completely missed on developments, I read about the quantum teleportation, for example, but I have never connected the dots.

I find Seth Lloyd's approach very appealing. Time to forget those almost century old images of clocks and rods in space, with spaceship explosions as the events

Tony

 

[Careful]

**
I have just listened to a few lectures from the Perimeter Institute and I am in awe of what has develeped out of EPR and quantum entanglement. **
NOTHING PRACTICAL has been developped so far (that is main critique people have about the research on entanglement - quantum computers are a dream until now). Microsoft reported recently some sucess AFAIR, but I am not at all convinced that this is QUANTUM (people tend to forget that classical correlations can be as high (as quantum ones) on sales of 1/10'th of a meter where you cannot exclude spacetime locality for realistic measurements).


** I find Seth Lloyd's approach very appealing. **

Read the section about spacetime reconstruction from the quantum computing process and tell me wether this is mathematically correct or not (I could equally ask you how many mistakes it contains).


**Time to forget those almost century old images of clocks and rods in space, with spaceship explosions as the events
Tony **

Really What do we have to believe in then? In infinite dimensional ghost spaces where god is playing dice but only allows us to see a four dimensional projection of this russian roulette ?

 

[Kea]

The String Coffee Table http://golem.ph.utexas.edu/string/index.shtml is a blog (maintained these days mainly by Urs Schreiber) that discusses the categorical aspects of String Theory. It is a great resource for those interested in the technical side of this story.

 

[Kea]

What do we have to believe in then? In infinite dimensional ghost spaces where god is playing dice but only allows us to see a four dimensional projection of this russian roulette?

Not a bad way of putting it! But a little better: in what we can measure and in what we can compute.

 

[Careful]

Not a bad way of putting it! But a little better: in what we can measure and in what we can compute.

Sorry, but a local realist would agree with that as well (so that is not a good characterization of QM) ! The question is whether the framework *underlying* the ``operational´´ machinery (i.e. the wavefunction in QM) as well as the measurement itself (the reduction in QM) is locally causal in spacetime or not. I was simply making a bit of fun of how huge Plato's quantum mechanical cave is in comparison to seize of the shadowworld.

 

[jarek]

NOTHING PRACTICAL has been developped so far (that is main critique people have about the research on entanglement - quantum computers are a dream until now).

Putting aside already aviable commercial quantum crypthographical devices (IDQuantique of N. Gisin and MagiQ), if you consider knowledge alone to be practical (at least in principle) then that's a bit of b-s what you are saying here, Careful. During the study of quantum information and entanglement people did learn quite a few new things about quantum mechanics: there is something in quantum states that bears some signs of "reality" and, for the lack of a better name, is labeled quantum information. I can understand that the language they speak in QIT sounds weird, much like engineering (my background is in GR)- convex sums, entropies, information content,etc - but no theory is born already in its final form. Also, I don't claim quantum info is *the way* (I fully agree with your view on Lloyd's paper but he is pretty isolated in his aspirations).
As for classical realism and stuff, my personal view on LHV and Bell-like theorems is that it simply states that quantum statistics cannot be simulated by local classical statistics. That's it. People try to inferr too much from this fact. I referr to Werner & Wolf's excellent paper on Bell stuff: quant-ph/0107093.
-jarek

 

[Careful]

**Putting aside already aviable commercial quantum crypthographical devices (IDQuantique of N. Gisin and MagiQ), if you consider knowledge alone to be practical (at least in principle) then that's a bit of b-s what you are saying here, Careful. **

Quantum cryptography is not equal to quantum computing, and again the question is wether this is QUANTUM or classical (and that is not clear at all).


**During the study of quantum information and entanglement people did learn quite a few new things about quantum mechanics: there is something in quantum states that bears some signs of "reality" and, for the lack of a better name, is labeled quantum information.**
I can understand that the language they speak in QIT sounds weird, much like engineering (my background is in GR)- convex sums, entropies, information content,etc - but no theory is born already in its final form. **

No, this language is not weird at all (I have been doing some quantum entropy myself - in my masters time - before I went to do real physics, ie. GR ). I deliberatly reacted so sharp because people seem to already have decided that the correlations obtained in such processes cannot be obtained by any *classical* means (on the appropriate distance scales). So, the question wether this quantum information is really so quantum in the sense that no underlying realistic theory can account for the correlations (or wether these correlations really can be obtained in practical situations) remains to be seen. Nevertheless, I acknowledge that gaining further insight is valuable when you later apply it to the relevant distance scales.


**Also, I don't claim quantum info is *the way* (I fully agree with your view on Lloyd's paper but he is pretty isolated in his aspirations). **

Good, that is sensible.

**As for classical realism and stuff, my personal view on LHV and Bell-like theorems is that it simply states that quantum statistics cannot be simulated by local classical statistics. **

Well, that depends upon the distance scales you consider. For example local classical statistics can be exactly the same as quantum statistics on those distance scales where the separability assumption cannot be made. That is one reason why I have good hopes for local realism.

Cheers,

Careful

 

[jarek]

Quantum cryptography is not equal to quantum computing, and again the question is wether this is QUANTUM or classical (and that is not clear at all).

Not equal, but cryptography (in the modern incarnation) *heavily* uses entanglement (like evesdropping detectiion, NOT POSSIBLE classically) and your objection concerned practical benefits from studying entanglement. It is QUANTUM ...

No, this language is not weird at all (I have been doing some quantum entropy myself - in my masters time - before I went to do real physics, ie. GR ).

I moved in the other direction

I deliberatly reacted so sharp because people seem to already have decided that the correlations obtained in such processes cannot be obtained by any *classical* means (on the appropriate distance scales). So, the question wether this quantum information is really so quantum in the sense that no underlying realistic theory can account for the correlations (or wether these correlations really can be obtained in practical situations) remains to be seen.

Well, that depends upon the distance scales you consider. For example local classical statistics can be exactly the same as quantum statistics on those distance scales where the separability assumption cannot be made. That is one reason why I have good hopes for local realism.
Cheers,
Careful

Not really understand what do you mean by your "scales" and "separability assumption", but if you referr to Bell-type experiments then let me just comment that they have been succesively closing the remaining locality and efficiency loopholes (ions give 95+% eff. and they change setups fast enough to exclude causal interaction). The subject seems to be still quite active though...but that belongs to another thread.

-jarek

 

[Careful]

**Not equal, but cryptography (in the modern incarnation) *heavily* uses entanglement (like evesdropping detectiion, NOT POSSIBLE classically) and your objection concerned practical benefits from studying entanglement. It is QUANTUM ... **

In theory, yes, but in practice probably not

**
I moved in the other direction
**

Ah, mistakes are there to be forgiven

**
Not really understand what do you mean by your "scales" and "separability assumption", **

The separability assumption indeed means that no causal contact is possible and that both ``particles´´ can be assumed to move independently which is not possible in experiments where both detectors are less than 1/10'th of a metre apart (because detection times are like 3 nanoseconds).


**but if you referr to Bell-type experiments then let me just comment that they have been succesively closing the remaining locality and efficiency loopholes (ions give 95+% eff. and they change setups fast enough to exclude causal interaction).**

And that in ONE experiment ?? If that were true then there would be no reason for Bell type experiments anymore so I guess it is false. In the other case, please give me the reference which must necessarily date from 2005 :-) ).

Cheers,

Careful

 

[jarek]

Ah, mistakes are there to be forgiven

The time will show

And that in ONE experiment ?? If that were true then there would be no reason for Bell type experiments anymore so I guess it is false. In the other case, please give me the reference which must necessarily date from 2005 :-) ).

I didn't say that in one experiment, that's why the field is still active. There has been a proposal of Fry to do such an experiment (with Hg+ ions), but as far as I know he hasn't done it yet (I heard him in 2001, but I think the idea is much older).
There are also another experimental methods of detecting entanglement (if we speak just of entanglement and not realism stuff), like entanglement witnesses. Wineland and Blatt groups have been generating multi-ion GHZ- and W-states (the record is 8-ion W) with 70%+ fidelities. But of course producing entanglement and experimental breaking Bell ineq. are two different things.


But there are also indirect evidences for genuine QUANTUM corellations (I'm not talking about realism now). One of them is precise spectrometry, see e.g. Leibfried at al, Science 304, 1476 (2004). The idea is that they get sensitivity beyond the one predicted by in an ideal case with non-entangled particles. It is also practical, Careful, as people are working on application of entanglement enhanced spectrometers to gravity wave detectors what an irony, eh?.


Geee...I have never written so much on experiments since my high school (puke)
-jarek

 

[Careful]

**
I didn't say that in one experiment, that's why the field is still active. There has been a proposal of Fry to do such an experiment (with Hg+ ions), but as far as I know he hasn't done it yet (I heard him in 2001, but I think the idea is much older).**

Perhaps it has been done already, but the result might not have been what one hoped...

**There are also another experimental methods of detecting entanglement (if we speak just of entanglement and not realism stuff), like entanglement witnesses. Wineland and Blatt groups have been generating multi-ion GHZ- and W-states (the record is 8-ion W) with 70%+ fidelities. But of course producing entanglement and experimental breaking Bell ineq. are two different things.**

Indeed...

**
But there are also indirect evidences for genuine QUANTUM corellations (I'm not talking about realism now). One of them is precise spectrometry, see e.g. Leibfried at al, Science 304, 1476 (2004). The idea is that they get sensitivity beyond the one predicted by in an ideal case with non-entangled particles. **

Thanks, I shall figure that out, but I feel pretty confident that other explanations are possible Let me stress that I agree that *standard* QM needs entanglement since measurement is supposed to be something instantaneous; but I am afraid people want to push it to distance scales where this consideration is not appropriate anymore.

**It is also practical, Careful, as people are working on application of entanglement enhanced spectrometers to gravity wave detectors what an irony, eh?. **

Same comment, don't worry I like some good irony

**
Geee...I have never written so much on experiments since my high school (puke) **

Nah, it is good to do real physics from time to time . I am a theorist myself but it is my sacred duty to figure out what those lab people are messing around with. Let's quit this discussion about entanglement: the outcome is always the same (make a local realist theory !). Like our friend Vanesch, I am of the opinion that one should not look for a case against local realism (it is extremely difficult and probably impossible to rule out), rather local realists should look for a unified theory (stepwise of course).

One can only learn from this.

Cheers,

Careful

 

[vanesch]

What do we have to believe in then? In infinite dimensional ghost spaces where god is playing dice but only allows us to see a four dimensional projection of this russian roulette ?

Admit at least, that *this* at last, has some panache ! Physics finally relieved of its century-long dusty boredom
I really hope it stays that way !

 

[Careful]

Admit at least, that *this* at last, has some panache ! Physics finally relieved of its century-long dusty boredom
I really hope it stays that way !

Bah, you just like too much science fiction and conscious ants It is clear you never calculated Feynman diagrams :-) By the way, the local realist picture is also pretty exiting (involves fast spinning clouds and so on); exit point particles, hello microscopic explosions.

Grandpa

 

[vanesch]

Bah, you just like too much science fiction and conscious ants It is clear you never calculated Feynman diagrams :-) By the way, the local realist picture is also pretty exiting (involves fast spinning clouds and so on); exit point particles, hello microscopic explosions.
Grandpa

Pouh! Fast spinning clouds against conscious ants in superposition... What do you think will sell best ? Your stuff is way not crazy enough.

 

[Kea]

There appears to be a rush of papers before the holidays, such as this little one:

Gerbes and Heisenberg's Uncertainty Principle
J.M. Isidro
http://www.arxiv.org/abs/hep-th/0512241

I thought this might appeal to Careful as it looks at a classical characterisation of QM, although of course it does this via a kind of categorification.
Must go! Lots to do.

 

[Kea]

This change in viewpoint is demanded by two facts, one observational and one theoretical. The first is that the expansion of the universe is accelerating...The second fact is that some recent progress has been made in exploring the landscape.

Now if one were forced to question the validity of the first 'fact', would the landscape be so compelling?

 

[selfAdjoint]

Via Woit's comments I found this paper:

A Garrett Lisi

Clifford bundle formulation of BF gravity generalized to the Standard Model

http://arxiv.org/gr-qc/0511120


Classical gravity plus standard model, with an opening toward LQG. Looks good to me!

 

[marcus]

Via Woit's comments I found this paper:
A Garrett Lisi
Clifford bundle formulation of BF gravity generalized to the Standard Model
http://arxiv.org/gr-qc/0511120
Classical gravity plus standard model, with an opening toward LQG. Looks good to me!

Our PF thread started 21 November about this paper
https://www.physicsforums.com/showthread.php?t=100984

we began talking about Garrett's but at post #7 Torsten Helge was introduced and we veered off onto that.

However Garrett's paper was the OP topic, so we could resume it if you want

 

[selfAdjoint]

Thanks for the heads-up, Marcus. Sorry about the mixup. I'm away from home, and won't have an opportunity to digest the paper till next week. I'll post something on the thread then.

 

[Kea]

John Baez is currently speaking at a conference on the Geometry of Computation http://iml.univ-mrs.fr/geocal06/ on

Universal Algebra and Diagrammatic Reasoning
http://math.ucr.edu/home/baez/universal/

These slides end with the quote: ... to get closer to reality we should climb the n-categorical ladder, and learn to love the quantum universe.


P.S. I had a wonderful time at a conference on the Poincare conjecture, which turned into a conference mostly on TFTs and quantum computation (the talk titles were not advertised so this took some people by surprise!).

 

[Kea]

Category Theory Blog

What a pleasant surprise to discover yet another Category Theory blog:
David Corfield's
Philosophy of Real Mathematics
http://www.dcorfield.pwp.blueyonder.co.uk/blog.html

From today's comments:
...it wouldn't surprise me if much of the category theoretic 'metaphysics' gets done by computer science people and physicists.

What a nice mathematician! He might be right about the computer scientists but I'm not so sure about the physicists. A philosopher once sighed and said to me that philosophers were about 100 years behind the physicists, but even though he meant it I can't help feeling that deep down he knew that the opposite was true. After all, physicists have to worry about formulating the concepts in some very concrete way and can't spend too much time trying to express their most elusive thoughts. Quite an excuse, heh?

 

[Kea]

Over at The String Coffee Table http://golem.ph.utexas.edu/string/index.shtml
Urs Schreiber has started talking about renormalisation and operads!

 

[Chronos]

You're on the cutting edge, Kea, and don't even know it yet! [or maybe you do and don't want to show your hand]. Treating the universe as a quantum computer is a very sound approach, IMO.

 

[Kea]

Treating the universe as a quantum computer is a very sound approach, IMO.

 

[GOsuchessplayer]

Wow i just find all of this so interesting. I hate the fact that my school (high school) does not offer AP physics. But hey these forums are teaching me alot. Since i know most of the basics. Anyways thanks guys keep posting.

 

[marcus]

Where is setAI when we need him?

Once Kea thus bewailed the absence of setAI---when another wildman It-from-Bit spacetime-is-a-computer theory made its appearance.

this one also seems choice and right down the alley of some here:

http://arxiv.org/abs/astro-ph/0603084
On the similarity of Information Energy to Dark Energy
M.P.Gough, T.D.Carozzi, A.M.Buckley
5 pages, no figures, no tables

"Information energy is shown here to have properties similar to those of dark energy. The energy associated with each information bit of the universe is found to be defined identically to the characteristic energy of a cosmological constant and the universe information content of 10^90 bits provides a total energy comparable with the high dark energy value. Information energy is also found to have a significantly negative equation of state parameter, w < -0.4, and thus exerts a negative pressure, similar to dark energy."

It cites several Seth Lloyd papers, and with their help finds the information density of the universe. Then DARK ENERGY turns out to be comparable to the energy associated with the estimated information.

 

[setAI]

Once Kea thus bewailed the absence of setAI---when another wildman It-from-Bit spacetime-is-a-computer theory made its appearance.

I am always lurking in the shadows- and read all the new papers

BTW- only one week to go until https://www.amazon.com/gp/product/1400040922/?tag=pfamazon01-20 ]Seth Lloyd's book [/url] comes out! have you pre-oprdered yet?

 

[marcus]

I am always lurking in the shadows- and read all the new papers

that is reassuring to know

BTW- only one week to go until [u rl=[URL]https://www.amazon.com/gp/product/1400040922/?tag=pfamazon01-20[/URL] ]Seth Lloyd's book [/u rl] comes out! have you pre-ordered yet?

no, I have not pre-ordered----I am naturally frugal and extremely patient. do you have any other links that might provide sample chapters or other clues as to what's in the book?

 

[setAI]

...do you have any other links that might provide sample chapters or other clues as to what's in the book?

no- but I would assume it is an expansion of his original paper on the subject: http://arxiv.org/abs/quant-ph/0501135

there are some things I am hoping for- in the paper Seth wrote:

This paper showed that any computation, including, for example, one that calculates
the digits of π, corresponds to a class of spacetimes that obeys the Einstein-Regge equations.
Which computation corresponds to the world that we see around us? As noted,
quantum cellular automata and random computations are both reasonable candidates for
the ‘universal’ computation. Cellular automata possesses a built-in regular structure which
simplifies the analysis of their behavior. A quantum cellular automaton is a natural choice
for a computational substrate...

I am hoping there has been some work on this- perhaps some ideas or examples of quantum cellular automata that could simulate different approaches to Quantum Gravity-

edit: on a personal note- you may know that I am an experimental/abstract computer musician- lately I have been playing around with the idea of somehow utilizing classical simulations of very simple quantum logic gates as sound-modulation sources and/or for algorithmic music composition- I am hoping that the book might contain more detailed quantum logic gate descriptions and quantum algorithms that I might be able to kludge for my work-