’t Hooft on the Foundations of Superstring Theory

[tom.stoer]

This paper may be intersting:

http://www.springerlink.com/content/d3182t263w74267g/?MUD=MP
On the Foundations of Superstring Theory
Gerard ’t Hooft

Abstract: Superstring theory is an extension of conventional quantum field theory that allows for stringlike and branelike material objects besides pointlike particles. The basic foundations on which the theory is built are amazingly shaky, and, equally amazingly, it seems to be this lack of solid foundations to which the theory owes its strength. We emphasize that such a situation is legitimate only in the development phases of a new doctrine. Eventually, a more solidly founded structure must be sought.
Although it is advertised as a “candidate theory of quantum gravity”, we claim that string theory may not be exactly that. Rather, just like quantum field theory itself, it is a general mathematical framework for a class of theories. Its major flaw could be that it still embraces a Copenhagen view on the relation between quantum mechanics and reality, while any “theory of everything”, that is, a theory for the entire cosmos, should do better than that.

(free access)

 

[Demystifier]

People interested in that paper might also be interested in those:
http://arxiv.org/abs/hep-th/0702060
http://arxiv.org/abs/hep-th/0605250
http://arxiv.org/abs/hep-th/0512186

 

[atyy]

Two thoughts.

1) There's a partial derivation of the Born rule by Zurek, so the question is whether string theory can derive his assumptions.

2) Since string theory seems to provide a complete theory of quantum gravity in AdS/CFT, so it may provide a concrete playground for the above aim. I don't think anyone's done that yet, but stuff like http://arxiv.org/abs/1102.2910 and http://arxiv.org/abs/1201.3666 seem like a start.

 

[Demystifier]

1) There's a partial derivation of the Born rule by Zurek, so the question is whether string theory can derive his assumptions.

I am quite confident that string theory cannot do that any better than other quantum theories.

 

[tom.stoer]

Up to now string theory does have to say anything about its foundations, not about quantum theory and not even about gravity (in the most general sense).

 

[Quantumental]

1) There's a partial derivation of the Born rule by Zurek, so the question is whether string theory can derive his assumptions.

Actually Zurek's derivation has been rebutted by quite a few people, among them Jacques Mallah who has this to say:

W. Zurek attempted to derive the Born Rule using symmetries that he called 'envariance' or enviroment-assisted invariance. While interesting, his assumptions are not justified. The most important assumption is that all parts of a branch, and all observers in a branch, have the same "probability". Albert's fatness rule provides an obvious counterexample. I also note that a substate with no observers in it can not meaningfully be assigned any effective probability.

He uses this, together with another unjustified assumption that is similar to locality of probabilities, to obtain what Wallace called 'equivalence' and then the Born Rule from that. Because the latter part of Zurek's derivation is similar to the DW approach, the two approaches are sometimes considered similar, although Zurek does not invoke decision theory.

 

[atyy]

I am quite confident that string theory cannot do that any better than other quantum theories.

Is doing no better good or bad?

 

[Demystifier]

I don't see how could that be good.

 

[Quantumental]

Why is there virtually no interest whatsoever when one of the greatest physicsts of all time is speaking his mind and providing several technical papers about his ideas?!

 

[atyy]

Why is there virtually no interest whatsoever when one of the greatest physicsts of all time is speaking his mind and providing several technical papers about his ideas?!

Because the issue is very difficult.

 

[Quantumental]

Because the issue is very difficult.

I am painfully aware of this, but so is most of the hypotheses discussed here. Why isn't his work getting any shine here?

 

[RUTA]

I am painfully aware of this, but so is most of the hypotheses discussed here. Why isn't his work getting any shine here?

Who are you referring to?

 

[marcus]

Why is there virtually no interest whatsoever when one of the greatest physicsts of all time is speaking his mind and providing several technical papers about his ideas?!

Because the issue is very difficult.

I am painfully aware of this, but so is most of the hypotheses discussed here. Why isn't his work getting any shine here?

That sounds like a reasonable question to ask, but the situation is complex. Obviously we are talking about Gerard 't Hooft and the "Foundations" paper that Tom linked in post#1 and the related July 2012 paper about Cell'r Automata (CA) that he cites in support of the more surprising assertions. That July 2012 paper thanks "M. Porter" in the acknowledgments. So if that is Mitchell Porter we would naturally expect him to let us know and to give us a little coaching or an intuitive leg-up.

It's normal here for the people who understand something better than the others to help by explaining it to the others.

But this for some reason must be a complicated situation, everyone is being unusually circumspect and, I would say, tight-lipped. People whom I expect have a better than average grasp of this paper are not being forthcoming about it. So people like myself (less familiar with the subject matter) conclude that there must be something delicate about the situation that they don't understand, and hold back.

But please don't conclude that we're not interested. I would like very much if someone would say in simple terms what 't Hooft is driving at in that July 2012 paper. It seems to be a ONE DIMENSIONAL QUANTUM FIELD THEORY realized deterministically by a series of finite state automata. In other words the whole thing sounds too good to be true, and nutty, at first hearing. I will get the abstracts.

First there was the May paper:
http://arxiv.org/abs/1205.4107
Duality between a deterministic cellular automaton and a bosonic quantum field theory in 1+1 dimensions
Gerard 't Hooft

Then the July paper:
http://arxiv.org/abs/1207.3612
Discreteness and Determinism in Superstrings
Gerard 't Hooft
(Submitted on 16 Jul 2012 (v1), last revised 15 Sep 2012 (this version, v2))
Ideas presented in two earlier papers are applied to string theory. It had been found that a deterministic cellular automaton in one space- and one time dimension can be mapped onto a bosonic quantum field theory on a 1+1 dimensional lattice. We now also show that a cellular automaton in 1+1 dimensions that processes only ones and zeros, can be mapped onto a fermionic quantum field theory in a similar way. The natural system to apply all of this to is superstring theory, and we find that all classical states of a classical, deterministic string propagating in a rectangular, D dimensional space-time lattice, with some boolean variables on it, can be mapped onto the elements of a specially chosen basis for a (quantized) D dimensional superstring. This string is moderated ("regularized") by a 1+1 dimensional lattice on its world sheet, which may subsequently be sent to the continuum limit. The space-time lattice in target space is not sent to the continuum, while this does not seem to reduce its physically desirable features, including Lorentz invariance. We claim that our observations add a new twist to discussions concerning the interpretation of quantum mechanics, which we call the cellular automaton (CA) interpretation. Detailed discussions of this interpretation, and in particular its relation to the Bell inequalities, are now included.
30 pages

 

[Quantumental]

That July 2012 paper thanks "M. Porter" in the acknowledgments. So if that is Mitchell Porter we would naturally expect him to let us know and to give us a little coaching or an intuitive leg-up.

I should probably have included that I am the one who got Gerard 't Hooft to participate over at Physics.Stackexchange about his ideas in an attempt to spread his ideas. Which worked partially and he ended up in a interesting debate with Mitchell Porter and Ron Maimon over there. But since then it's been dead silent.
If you check the citations of his papers, there are nearly zero, which is very weird when even the most crackpottery papers usually is cited a few times.

 

[mitchell porter]

I would like very much if someone would say in simple terms what 't Hooft is driving at in that July 2012 paper. It seems to be a ONE DIMENSIONAL QUANTUM FIELD THEORY realized deterministically by a series of finite state automata. In other words the whole thing sounds too good to be true, and nutty, at first hearing.

At the bottom of this is the observation that a harmonic oscillator can be viewed as a system cycling through four states. If the extreme displacements are "-r" and "+r", then the four states are "at 0 and heading in + direction"; "at +r, resting, and about to head back towards 0"; "at 0 and heading in - direction"; "at -r, resting, and about to head back towards 0".

Earlier in the year, 't Hooft constructed a discrete dynamical system encoding the analogous behavior in a quantum harmonic oscillator. A quantum field is a set of coupled local harmonic oscillators, and what he did was to construct a QFT such that time evolution applied to some special basis states can similarly be represented as timesteps between particular states of the oscillators. It's a free field theory, which is how he avoids problems of entanglement.

 

[martinbn]

Why is there virtually no interest whatsoever when one of the greatest physicsts of all time is speaking his mind and providing several technical papers about his ideas?!

Why should there be any interest!? He is trying to build a hidden variable theory, and I think many people (if not most) are skeptical about whether it is possible, so why should this be interesting. Well, there is one good reason, the author, but other than that it is just an attempt for a hidden variable theory. Even if it is successful I would find it a very poor explanation. It is supposed to be an alternative of the quantum mechanical explanation, but it cannot explain without QM. If I understand correctly the explanation is that the dynamics is given by a CA but the set up is such as to give outcomes that conform with QM predictions. It looks like the claim that the truth is that Earth is motionless and planets move in such a way so that from the point of view of the sun they orbit in a simple way, while we have general relativity to explain gravity.

 

[Quantumental]

Why should there be any interest!? He is trying to build a hidden variable theory, and I think many people (if not most) are skeptical about whether it is possible, so why should this be interesting. Well, there is one good reason, the author, but other than that it is just an attempt for a hidden variable theory. Even if it is successful I would find it a very poor explanation. It is supposed to be an alternative of the quantum mechanical explanation, but it cannot explain without QM. If I understand correctly the explanation is that the dynamics is given by a CA but the set up is such as to give outcomes that conform with QM predictions. It looks like the claim that the truth is that Earth is motionless and planets move in such a way so that from the point of view of the sun they orbit in a simple way, while we have general relativity to explain gravity.

Wtf how could you ever make such a far fetched comparison?
You do not have a quantum theory to explain quantum phenomena. All you've got is math.
You do not have a way of reconciling QM with GR, all you have is math that ends up not working.

Unless you can explain exactly what is going on in QM, you should really ask yourself why you dismiss 't Hoofts attempts at finding the true answer.

 

[negru]

So what do you want instead of math, literature? Math is the only thing you'll ever get.

If there's any issue with reconciling QM and GR, it's that GR needs a modification. Not QM.

So what are the unresolved issues in QM that need extra understanding? Non philosophical ones please, just ones that make sense.

 

[Quantumental]

So what do you want instead of math, literature? Math is the only thing you'll ever get.

If there's any issue with reconciling QM and GR, it's that GR needs a modification. Not QM.

So what are the unresolved issues in QM that need extra understanding? Non philosophical ones please, just ones that make sense.

Not really sure how you can be so certain about that?
What makes you so certain that QM is correct and GR wrong?

Also, ofcourse the unresolved issues will stem back to the fact that either you have to close your eyes and accept magic (indeterminism) or you have to come up with a deeper theory

 

[negru]

So what if I said that determinism is magic? You're just stating your opinion. There's no single observation, experiment or even theoretical reason to suspect that there should be something else behind the theory. It's just like saying "oh i don't like differential geometry and the way it explains GR, there has to be something more like algebra there". Why, or how do you reach such a conclusion?

QM is by far the most complete, tested and accurate theory in physics. It doesn't fail anywhere, whereas classical mechanics does fail in various places. Why assume then that determinism is more fundamental, or preferable?

I'm not saying that GR is wrong, it' just incomplete, at higher energies it needs new degrees of freedom. It's just more plausible that we only need to modify GR, than the whole Standard Model which is based on QM. The SM makes incredibly accurate predictions, all of them verified. There's just no reason to look for stuff that doesn't need to be there. Looking for hidden variable QM is just like looking for pink unicorns eating lava in the center of the earth. Sure they might be there, but what makes you think that? Just because it's possible doesn't mean it's at all worth investigating.

 

[Berlin]

I think it is very important what 't Hooft is doing, and in fact -maybe subjective- very very interesting. First of all he makes a connection between classical and quantum models. He is not alone in this. Wetterich makes a interesting connection between a classical Ising model and the quantum physics of fermions, Hossenfelder takes Plancks constant and G to zero at high energies, giving nature a classical as well as a quantum phase. Futhermore, there is a big trend of studying information theory and thermodynamics in Gravity describing it as a thermodynamic equation of state, still allowing for underlying classical theory for the microscopic degrees of freedom (as I understand it...). To me it does not seem crack-pottery at all.

't Hooft does not make the claim (yet) that he is describing models of nature, but he has clearly described the path to reach that:

"We set out to follow the following strategy: turn a simple, exactly soluble cellular automaton into an exactly soluble quantum field theory, derive its hamiltonian, and subsequently add interaction terms. The hamiltonian will receive small, local corrections so that now a cellular automaton can be mapped onto an interacting quantum field theory. From there, find our way to the Standard Model."

The fact that he is using a discrete space, gives a classical description of superstringtheory and gets around the Bell's theorem should wake us all up. Stay tuned!

berlin

 

[mitchell porter]

This thread would be a good place to mention another recent idea of 't Hooft's that isn't getting any attention, what I call his http://physics.stackexchange.com/questions/34554/t-hoofts-landscape-of-conformally-constrained-qfts. Reality according to 't Hooft seems to be: cellular automaton -> strings -> "conformally constrained QFT" -> standard model.

 

[Quantumental]

So what if I said that determinism is magic? You're just stating your opinion. There's no single observation, experiment or even theoretical reason to suspect that there should be something else behind the theory. It's just like saying "oh i don't like differential geometry and the way it explains GR, there has to be something more like algebra there". Why, or how do you reach such a conclusion?

I guess we just have a irreconcilable difference of opinion when it comes to metaphysics.
I consider indeterminism to be unscientific and in the same realm that I place solipsism.
Please explain to me how it is that a particle is indeterministic, yet it follows the Born Rule? Why would it, why wouldn't it rather be completely impossible to calculate the probability?
Think about it, you're claiming that a particle is indeterministic but that there is a limit to it's indeterminism so that it always follows the Born Rule and I ask WHY? Why does it follow the Born Rule. It just SCREAMS: "hidden cause."

If indeterminism *could* even in principle be true, then the Universe should not be possible to model at any scale. What do we find? Determinism. ALL THROUGHOUT the history of scientific inquiry we found a new layer and a new layer of determinism the deeper we peeled back and now because we have hit a barrier we should conclude "Oh yeah indeterminism is true" ? I don't think so, but you are free to.

If that is your position then go ahead, since you think it's worthless to look for such causes I guess all I can say is that I appreciate the input, but it won't help this thread to discuss this further with you.

 

[RUTA]

There are many physicists in the foundations community who believe the study of foundational issues, e.g., measurement problem, EPR-Bell phenomena, Born rule, etc., will bear on the unification/reconciliation of GR and the Standard Model. Smolin and ‘t Hooft are two of the most famous examples, but having attended several conferences in foundations of physics, I can tell you there are many more. The physics community has worked on this problem for decades without finding a solution, so why not welcome new approaches? You don’t have to participate, nor even keep abreast of them, but I don’t see why you would revile them.

As for why ‘t Hooft’s idea isn’t getting more attention here, I don’t think there’s a conspiracy Many technical approaches to foundational issues with implications for unification have been published. They don’t get a lot of air time in forums such as this because they require committed study to fully appreciate/understand and haven’t produced results commensurate with such an investment. When someone produces a new theory that formally unifies GR and the SM, and yields new physics, then the community will invest the time and energy necessary to learn it. At that point, enough people will understand and appreciate it that it will be discussed extensively on Physics Forums.

 

[Quantumental]

There are many physicists in the foundations community who believe the study of foundational issues, e.g., measurement problem, EPR-Bell phenomena, Born rule, etc., will bear on the unification/reconciliation of GR and the Standard Model. Smolin and ‘t Hooft are two of the most famous examples, but having attended several conferences in foundations of physics, I can tell you there are many more.

But who/where/what?
Everytime I find a new paper discussing some of these issues they usually just assume "standard QM" and aren't really looking for a deeper answer with a few exceptions, like you mentioned.
But were are the "many more" people?

As for why ‘t Hooft’s idea isn’t getting more attention here, I don’t think there’s a conspiracy Many technical approaches to foundational issues with implications for unification have been published. They don’t get a lot of air time in forums such as this because they require committed study to fully appreciate/understand and haven’t produced results commensurate with such an investment. When someone produces a new theory that formally unifies GR and the SM, and yields new physics, then the community will invest the time and energy necessary to learn it. At that point, enough people will understand and appreciate it that it will be discussed extensively on Physics Forums.

I understand this and don't think there is any conspiracy. It's just a bit depressing to see the reception a potentially revolutionary idea like the one he got in mind gets when we take into consideration that there are really no other satisfactory answers at the moment.

 

[negru]

I guess we just have a irreconcilable difference of opinion when it comes to metaphysics.
I consider indeterminism to be unscientific and in the same realm that I place solipsism.
Please explain to me how it is that a particle is indeterministic, yet it follows the Born Rule? Why would it, why wouldn't it rather be completely impossible to calculate the probability?
Think about it, you're claiming that a particle is indeterministic but that there is a limit to it's indeterminism so that it always follows the Born Rule and I ask WHY? Why does it follow the Born Rule. It just SCREAMS: "hidden cause."
.

Physics has never in its entire history answered a question of "why", only "how", or "how much". "why" is purely philosophical, and will never get a scientific answer. Until you can relate to experiment, observation, etc, arguing between determinism and indeterminism has no connection (or relevance) to science.

Find just one situation where indeterminism fails and everyone will agree QM is lacking something.

 

[marcus]

I think it's fair to be skeptical of dogma and doctrine in science when it gets too rigid and also when reputable people start trying ways to jump out of the rut. I really liked what Berlin said:

I think it is very important what 't Hooft is doing, and in fact -maybe subjective- very very interesting. First of all he makes a connection between classical and quantum models. He is not alone in this. Wetterich makes a interesting connection between a classical Ising model and the quantum physics of fermions, Hossenfelder takes Plancks constant and G to zero at high energies, giving nature a classical as well as a quantum phase. Futhermore, there is a big trend of studying information theory and thermodynamics in Gravity describing it as a thermodynamic equation of state, still allowing for underlying classical theory for the microscopic degrees of freedom (as I understand it...). To me it does not seem crack-pottery at all.

't Hooft does not make the claim (yet) that he is describing models of nature, but he has clearly described the [a possible, I would say] path to reach that:
...
berlin

There could have been dogmatic conservatives who scolded and harumphed when it was first proposed that physical constants could RUN. Because then they wouldn't be...well...constant.

I like it that Berlin mentions Hossenfelder's recent paper where hbar -> 0. If hbar runs to zero at extremely high energy then what happens to uncertainty? Universe could be classical at high energy.

Hossenfelder is no dummy and she just organized a 4-week workshop on Fundamental Cosmology to be held next month in Stockholm. It has an interesting list of participants. Google "fundamental cosmology hossenfelder" to get the workshop webpages.

 

[RUTA]

But who/where/what?
Everytime I find a new paper discussing some of these issues they usually just assume "standard QM" and aren't really looking for a deeper answer with a few exceptions, like you mentioned.
But were are the "many more" people?

A good journal is Foundations of Physics ('t Hooft is editor) and a good online community is http://www.fqxi.org/community.

I understand this and don't think there is any conspiracy. It's just a bit depressing to see the reception a potentially revolutionary idea like the one he got in mind gets when we take into consideration that there are really no other satisfactory answers at the moment.

You could join the effort, but if you're depressed as an outsider, I should warn you it'll be much worse as an insider

 

[negru]

People should be extremely conservative with regard to old theories until strong evidence shows up to prove them wrong. Otherwise what, every time some guy comes with a new theory we start jumping up and down?

 

[RUTA]

Physics has never in its entire history answered a question of "why", only "how", or "how much". "why" is purely philosophical, and will never get a scientific answer. Until you can relate to experiment, observation, etc, arguing between determinism and indeterminism has no connection (or relevance) to science.

Find just one situation where indeterminism fails and everyone will agree QM is lacking something.

I agree that physics provides "how" not "why," as I would define the terms anyway. In the video "Creation of the Universe" (produced by Northstar, 1985, written by Timothy Ferris) Hawking says (paraphrasing), "From the age of 13 or 14 I've wanted to know how the universe works and why it works the way it does. I now understand how it works, but do not yet know why."

However, I suspect that some people use the word "why" to mean "an ontological story." And, I have seen physicists motivated by ontological questions even though ontology is not uniquely specified by theory. So, in that sense, "why" can be important to progress in physics.

 

[Quantumental]

Physics has never in its entire history answered a question of "why", only "how", or "how much". "why" is purely philosophical, and will never get a scientific answer. Until you can relate to experiment, observation, etc, arguing between determinism and indeterminism has no connection (or relevance) to science.

Find just one situation where indeterminism fails and everyone will agree QM is lacking something.

I am not asking WHY, I am asking HOW, how does the Universe obey the Born Rule if there are no causes that makes it?

 

[tom.stoer]

"why" was definately an important question during the development of quantum mechanics; read Heisenberg, Schrödinger, Weitzsäcker et al. But I agree that even if "why" is interesting, only the "how" can be answered (partially) by physics.

Just as an example: there is no satisfactory answer to "why are people buying Hawking's books?", whereas "how they are buying them?" can be answered quite easily.

 

[Quantumental]

"why" was definately an important question during the development of quantum mechanics; read Heisenberg, Schrödinger, Weitzsäcker et al. But I agree that even if "why" is interesting, only the "how" can be answered (partially) by physics.

Just as an example: there is no satisfactory answer to "why are people buying Hawking's books?", whereas "how they are buying them?" can be answered quite easily.

I don't think it's that hard to answer why people are buying Hawking books. He is one of the most prolific scientists of our time due to the media.

I'm not even sure if you can really make a firm distinction between how and why in some of these cases.
Why are quantum particles obeying Born rule? vs. How are quantum particles obeying Born Rule? Doesn't seem to be an important distinction

 

[marcus]

Just as an example: there is no satisfactory answer to "why are people buying Hawking's books?", whereas "how they are buying them?" can be answered quite easily.

 

[martinbn]

Wtf how could you ever make such a far fetched comparison?
You do not have a quantum theory to explain quantum phenomena. All you've got is math.
You do not have a way of reconciling QM with GR, all you have is math that ends up not working.

Unless you can explain exactly what is going on in QM, you should really ask yourself why you dismiss 't Hoofts attempts at finding the true answer.

You misunderstood me. I don't dismiss anything. All I am saying is that even if he can get things to work, his alternative to QM will be a bad explanation because the explanation it provides is that QM is not complete, there are hidden variable, and things are set up in such a way so that the outcomes of any experiment to be so that it appears that QM is right even though it is not. How can you consider this a satisfactory answer to your favorite "why" question!

 

[Quantumental]

You misunderstood me. I don't dismiss anything. All I am saying is that even if he can get things to work, his alternative to QM will be a bad explanation because the explanation it provides is that QM is not complete, there are hidden variable, and things are set up in such a way so that the outcomes of any experiment to be so that it appears that QM is right even though it is not. How can you consider this a satisfactory answer to your favorite "why" question!

Because he'd be solving the GR + QM problems at the same time. This is afterall his motivation. He didn't sit down like "**** that Schroedinger Cat, I need to find a solution", this was just a side effect of his work.

Yeah it'll be "hidden variables" or rather a fundamental theory that is sub-quantum. Nothing wrong with that, it'll solve the problems of QM and be a complete theory (if it works), how is this bad?
Yeah the theory will make QM *seem* right, but only approximately. How is this a problem? This is the story of all theories. QM make classical physics seem right too, until you take a deeper look. It would be the same here.

 

[Haelfix]

The reason people are skeptical, is that his CA model is a little bit hokey. The dynamics are decidedly classical (alla Bohm), but the states are essentially quantum. Eg he puts Quantum mechanics in by hand, including the projection postulate.

Further he does tremendous violence to the Hilbert space, and requires highly non local conspiracies to take place in order to bypass Bells theorem and similar thought experiments. These conspiracies in his model seems to me to not be between typical QM correlations but actual classical objects, which almost surely implies a violation of Lorentz invariance as well.

There is a reason QM has survived decades of precision tests and thousands of thought experiments done by skeptical physicists... It's structure is very rigid and doesn't seem amenable to change. Even changing it at the smallest of scales very quickly leads even the greatest theorists into a world of pain, which is what 'T Hooft is struggling with at the moment.

 

[Demystifier]

Why is there virtually no interest whatsoever when one of the greatest physicsts of all time is speaking his mind and providing several technical papers about his ideas?!

Because Witten didn't say that these stringy papers are cool.

 

[negru]

Because he'd be solving the GR + QM problems at the same time.

It's not immediately obvious to me that a new theory of QM will solve these problems.

Identical problems appear for other theories as well, like chiral perturbation theory, not just GR. They are all solved by realizing the theory is not complete and adding new degrees of freedom.

If a new theory of QM will solve the GR issues in some other way, I think it is safe to assume it will just be wrong.

 

[martinbn]

Because he'd be solving the GR + QM problems at the same time. This is afterall his motivation. He didn't sit down like "**** that Schroedinger Cat, I need to find a solution", this was just a side effect of his work.

Yeah it'll be "hidden variables" or rather a fundamental theory that is sub-quantum. Nothing wrong with that, it'll solve the problems of QM and be a complete theory (if it works), how is this bad?
Yeah the theory will make QM *seem* right, but only approximately. How is this a problem? This is the story of all theories. QM make classical physics seem right too, until you take a deeper look. It would be the same here.

I am not sure if it is clear that he will be solving QM+GR, but what is clear is that his motivation is completely different. He has said that he is discussed by the many world interpretation and the Bohmian mechanics, I guess he doesn't like the other interpretations very much either, and that's his motivation.

You still misunderstand me. I am not against people trying to do what he is attempting. I am for it, if nothing else a no-go theorem may come out of it. I am only saying that it is not a good explanation. If you have a hidden variable theory that explains better than QM it should explain the apparent quantumness of the world, and as far as I can see his does not.

 

[mitchell porter]

So what are the unresolved issues in QM that need extra understanding? Non philosophical ones please, just ones that make sense.

The deficiency of QM, that I think ought to be most accessible to someone reared in the Copenhagen interpretation, is that it doesn't tell you which observables are the ones that actually take definite values. This is most familiar from consistent histories, where you have infinitely many choices about how to coarse-grain. Quantum theory gives you a choice of observables, but they don't commute, and the user of QM just chooses which subset of observables they care about on a given occasion.

It might seem that the physical analysis of measurement processes explains this. The microscopic observables we care about on any given occasion, are the ones which get magnified into macroscopically observable quantities, and the physics of the measurement interaction explains why and how it's one particular set of microscopic observables, rather than another, which is tracked by the macroscopic apparatus.

However, this is only an explanation at the level of wavefunctions. You still have the usual freedom to say which microscopic observables actually take values, and the usual limitation that they can't all take values at the same time (and still obey the rules of QM). State preparation is a macroscopic procedure, reading out measurements is also a macroscopic procedure, and the correlations between these macroscopic observables will still be the same, regardless of how you microscopically coarse-grain what happens between one and the other.

Here I have assumed the usual Copenhagen view, that observables are what's real, not wavefunctions. If you want to reify wavefunctions, that's a different choice and leads to different problems. But I think the usual Copenhagen view gives you the clearest perspective on quantum mechanics, so long as you add that quantum mechanics is obviously incomplete. It's the attempt to rationalize quantum mechanics as a complete description of reality, which makes people tie themselves in philosophical knots.

This is different from saying that quantum mechanics is the best predictive algorithm possible. That may be true. But you can still ask what model of reality best explains quantum mechanics. One way to ease into that question, is to consider maximally coarse-grained consistent histories, something that Gell-Mann and others studied in the late 1980s I believe. That line of inquiry foundered because there were too many possibilities and no criterion for favoring one coarse-graining over another. There's also the more subtle "problem" that you still need the wavefunction of the universe to explain or make sense of the coarse-graining, which runs against the wavefunction antirealism of Copenhagen.

What would be interesting, is a maximal coarse-graining (or at least, a relatively fine-grained one) where the sequence of values taken by observables could be motivated by a dynamical rule that didn't require the wavefunction as an independent causal agent, the way it normally is e.g. in Bohmian mechanics. This is one reason why 't Hooft's new work is interesting, because the "histories" have a simple causal rule and it's even deterministic.

I agree substantially with what Haelfix said; in particular, I think 't Hooft is just wrong thinking that he can get around Bell's theorem somehow. If you're going to get QM from local determinism, that can't be locality in phenomenological space-time, it would have to be locality in some other "space" related to this one by a nontrivial transformation. But 't Hooft's 2012 work is nonetheless a step in a new direction for subquantum realism, potentially a step towards reality, and definitely noteworthy for anyone keeping track of possible realist explanations of QM.

 

[John86]

you could also say that deterministic theories explain classical world best.
But maybe if there is ever to be a next theory it will transcend Quantum physics in indeternism.
Like the relative locality framework

i don't see our future theories to be in any way more classical

 

[atyy]

People interested in that paper might also be interested in those:
http://arxiv.org/abs/hep-th/0702060
http://arxiv.org/abs/hep-th/0605250
http://arxiv.org/abs/hep-th/0512186

From your Bohmian QFT stuff, can you reconstruct QM in the bulk from Bohmian boundary QFT, analogous to what these guys are trying for bulk observables?
http://arxiv.org/abs/1102.2910
http://arxiv.org/abs/1201.3666

BTW, I see a bunch of papers by Struyve about Bohmian QFT too. But you don't cite him in your recent review (the chapter for Oriols and Mompart's book), nor does he cite you. Are the approaches completely different?

 

[Demystifier]

From your Bohmian QFT stuff, can you reconstruct QM in the bulk from Bohmian boundary QFT, analogous to what these guys are trying for bulk observables?
http://arxiv.org/abs/1102.2910
http://arxiv.org/abs/1201.3666

I don't know, essentially because my understanding of AdS/CFT is quite deficient.

BTW, I see a bunch of papers by Struyve about Bohmian QFT too. But you don't cite him in your recent review (the chapter for Oriols and Mompart's book), nor does he cite you. Are the approaches completely different?

Yes, they are very different. In my review I study particle beables, while he studies field beables. Roughly, this is like comparing perturbative string theory with string field theory; they have different sets of starting assumptions and deal with different types of technical problems.

 

[atyy]

I don't know, essentially because my understanding of AdS/CFT is quite deficient.Yes, they are very different. In my review I study particle beables, while he studies field beables. Roughly, this is like comparing perturbative string theory with string field theory; they have different sets of starting assumptions and deal with different types of technical problems.

Can BM handle condensed matter models like those in http://arxiv.org/abs/1210.1281? They are non-relativistic spins on a lattice, but they produce emergent relativistic QFTs.

There is some effort, which I find very interesting, to connect these with string theory via AdS/CFT.
http://arxiv.org/abs/0905.1317
http://arxiv.org/abs/1209.3304

 

[Demystifier]

Can BM handle condensed matter models like those in http://arxiv.org/abs/1210.1281? They are non-relativistic spins on a lattice, but they produce emergent relativistic QFTs.

Yes it can. If that does not convince you, then you would help me to give a better answer by explaining why exactly do you think that it might not?

 

[unusualname]

I am not asking WHY, I am asking HOW, how does the Universe obey the Born Rule if there are no causes that makes it?

Suppose you have a single particle (with no internal structure) in the entire universe. What would it do? You'd probably say nothing, but let's describe it by exp(ix), where x is real, and allow x to spontaneously change (otherwise nothing ever happens)

We could perhaps just describe it by x, and suppose that spontaneously changes instead, but that may not describe nature (see below)

Now suppose a second particle is added to the universe. What happens? Shall we allow both particles to spontaneously change in an unlawful manner? That wouldn't be a very interesting universe. INSTEAD, let's ASSUME that when one of the particles SPONTANEOUSLY changes the universe evolves to a new state according to a rule connecting both particles, eg a 2x2 matrix equation.

Now add ~10^80 particles with similar rules.

The Born rule falls out as conserved quadratic form (which is why we need to represent states by complex numbers - real representations don't match our observations) - and experiment shows it's not a quartic or higher power.

't Hooft just wants a deterministic system - but at the individual/tiniest particle level what possible deterministic law can there be - I mean, what's an individual particle meant to do? Hence we assume no causality at the level of individual particles/cells/states - and wow, you just need the Schrödinger Equation and a Hamiltonian matrix to make it all work.

That's QM, formulated ~1927

 

[Quantumental]

Suppose you have a single particle (with no internal structure) in the entire universe. What would it do? You'd probably say nothing, but let's describe it by exp(ix), where x is real, and allow x to spontaneously change (otherwise nothing ever happens)

1st stop right here: so.. we are going to assume that it spontaneously changes, because otherwise nothing happens?
So you look at the universe and see that things happen, so therefore you conclude that change occurs, but how can you say that it is spontaneous?

Now suppose a second particle is added to the universe. What happens? Shall we allow both particles to spontaneously change in an unlawful manner? That wouldn't be a very interesting universe. INSTEAD, let's ASSUME that when one of the particles SPONTANEOUSLY changes the universe evolves to a new state according to a rule connecting both particles, eg a 2x2 matrix equation.

2nd stop: again why do you assume this?
This is my entire argument: if they are "random/indeterministic/spontaneous" they should not follow laws. Laws need explanation.
The law of gravity means there is this thing called gravity affecting the universe... The Born Rule cannot just *be* without actually *being*.

't Hooft just wants a deterministic system - but at the individual/tiniest particle level what possible deterministic law can there be - I mean, what's an individual particle meant to do? Hence we assume no causality at the level of individual particles/cells/states - and wow, you just need the Schrödinger Equation and a Hamiltonian matrix to make it all work.

Gerard 't Hooft and others think that at the Planck scale / or somewhere around that length a cellular automata is taking place. Then we suddenly have a mechanism (whatever rule the CA is playing) that is deterministic which everything in the universe emerges from. Just like Game of Life

Hell you could even say de-Broglie came before 1927 and formulated his pilot wave theory, it's not hard to assume some deterministic law.

 

[unusualname]

Gerard 't Hooft and others think that at the Planck scale / or somewhere around that length a cellular automata is taking place. Then we suddenly have a mechanism (whatever rule the CA is playing) that is deterministic which everything in the universe emerges from. Just like Game of Life

Hell you could even say de-Broglie came before 1927 and formulated his pilot wave theory, it's not hard to assume some deterministic law.

The problem is, what stops the universe happening "all at once"? With a deterministic CA you have to make an additional assumption that each iteration takes some finite time-step. (Wolfram's computational speed limit of the universe idea)

Whereas if you have spontaneous "jumps" seeding each evolution step you already have a mechanism which prevents the universe all happening instantaneously. ie Nothing happens until there is a spontaneous jump, then the Universe updates every other particle/state/cell in response to whatever the spontaneous change was (via schrodinger evolution of the universe wave function)

That way you have quantum superpositions without MWI splitting, and you also have a natural delay to the universe's evolution.

However, I agree that's rather easy speculation, just pointing out that determistic CAs have to do extra work to get superpositions and a cosmic speed limit.

 

[audioloop]

Wtf how could you ever make such a far fetched comparison?
You do not have a quantum to explain quantum phenomena. All you've got is math.
You do not have a way of reconciling QM with GR, all you have is math that ends up not working.

Unless you can explain exactly what is going on in QM, you should really ask yourself why you dismiss 't Hoofts attempts at finding the answer.

agree, not dismiss for dismiss.