Role of philosophy in QG (not too abstract)

[marcus]

I assume there is no single right philosophy-of-physics (p-o-p) doctrine. No pretended philosophical "high ground". PoP is an academic subject--there are conferences and workshops. There is a fat two-volume Handbook of Phil. of Phys. published by North Holland Press. Courses are taught, seminars are held, books are written, and no doubt there are many scholarly disagreements.

My attitude towards PoP as it applies to QG is empirical and pragmatic. Philosophy and theory go hand in hand---they guide each other reciprocally. Every physics theory embodies concepts--and therefore philosophy. Philosophy here is simply the analysis and improvement of concepts used in mathematically modeling nature. The conscious refinement of language. What is space?

I consider philosophy to be empirically refutable or falsifiable. If you have a carefully considered conceptual frame, and develop a physical theory within that frame, and the theory proves wrong (empirically) then maybe the concepts were wrong.

It's probably a good idea to make at least a modest effort to be clear about one's concepts.
For me the prime example of someone who advanced physics by carefully scrutinizing basic concepts like space and time and observation was A.E. We shouldn't work so hard, just make a modest effort to be clear. Not spend much time on concepts. But at least devote some.

Also I don't claim expertise. Please correct me if I am wrong. I will try to start the discussion by jumping in headfirst, and may make naive or incorrect statements

Please don't get too abstract. I will tell you if I think you are getting too abstract.

 

[marcus]

A spacetime continuum can not exist

A spacetime cannot have physical existence any more than the continuous trajectory of a particle has physical existence.

Quantum mechanics does not allow a particle to follow a classical trajectory.
And a spacetime, represented by a 4D manifold with metric geometry, is very closely analogous to a classical particle trajectory.

So any quantum geometry theory which requires a 4D manifold can not be physical, can not be fundamental. Can only be an approximate idealization. It talks about something which is impossible to exist.

Please say, if you disagree.

 

[Fra]

Although the corroboration is unquestionably in the pudding as they say ;) I certainly agree that philosophy or to just have a conceptual guide is important, because it's the only guide you have to making the pudding, if you by "philosophy" include everything that in fact is philosophy, such as the scientific method. All writings about the scientific method are in effect philosophical papers. How would you falsify the scientific method? Maybe by it's inability to be constructive, but that's a fuzzy enough requirement to require further analysis.

To speak for myself, to invest too much time and energy into a research program witout having confidence in it's basic building blocks and conceptual foundations would potentially feel like a waste of time.

To my knowledge I am not sufficiently satisfied with the foundations of any of the major research QG programs to commit to it. This is why I find it more rational to try to find a new path and discuss the founding ideas and pick the good parts of the existing programs, instead of discussing details that have meaning only within certain programs.

A spacetime cannot have physical existence any more than the continuous trajectory of a particle has physical existence.

Quantum mechanics does not allow a particle to follow a classical trajectory. And a spacetime, represented by a 4D manifold with metric geometry, is very closely analogous to a classical particle trajectory.

If we by spacetime means the usual 4D continuous manifold then I agree.

So any quantum geometry theory which requires a 4D manifold can not be physical, can not be fundamental. Can only be an approximate idealization. It talks about something which is impossible to exist.

Please say, if you disagree.

To elaborate this, the first questons that your note induces the needs to be answered to make progress is that we need to define "fundamental" and "physical" means. I don't mean we need length philosophical discussions of meaning of words but just to clarify what we talk about.

Some people seem use "physical" synonymous to ontologically real things, in a context of realism. In essence this is the idea of some non-provable objectivity. This form of physical is however obscured and incomprehensible for me.

Another meaning of "physical", that I prefer is that it refers to something that refers to what's measureable and representable by a real observer. But this form of physical reality is inherently subjective or observer dependent. Wether relations between the subjective views exists in a form of structural realism and this are physical in the first meaning, is debatable. I personally think it doesn't. I think the first form of physicaly reality is only emergent and effective.

It seems there is a relation between fundamental and physical. If fundamental means fixed, eternal and objective as opposed to subjective and evolving then I don't there is anything that's fundamental and a way that scientifically verifiable (since all we could ever conclude is a ongoing corroboration; which is fine but we shouldn't confuse it with eternal truths).

This really touches what Rovelli discusses in his RQM papers, the papers on partial vs complete observables etc. Ie what is observable and what's not. This is complex and I think it's well worth contemplating as this may set the foundations for the constructing principles of a new understanding.

For me one of the prime quests is: To produce/computer and expectation of the future, given incomplete information about the present (=including PRESENT RECORDS of the past) so as to guide further action. But also, to revise the action strategy based on any new evidence (the ongoingly revised and updated "present" moments).

The above is cast relative to an observer - an inside player. Call it what we want but whatever encodes and executes the above is a subsystem. A particle, a human, a planet or a galaxy. It's just different scalings of complexity.

/Fredrik

 

[Demystifier]

Quantum mechanics does not allow a particle to follow a classical trajectory.

Please say, if you disagree.

If by a classical trajectory you mean a trajectory satisfying the classical (say, Newton) equation, then I agree.
But if by a classical trajectory you mean any function of the form x_i(t), i=1,2,3, then I certainly disagree. Bohm has shown in 1952 that a "classical" trajectory in this second sense is possible within quantum mechanics.

The main objection against this Bohmian interpretation of nonrelativistic QM is that it makes the same measurable predictions as standard "Copenhagen" QM, so that it is not testable. That's why it perfectly fits in this "philosophy" thread.

And a spacetime, represented by a 4D manifold with metric geometry, is very closely analogous to a classical particle trajectory.

True. That analogy has been exploited to construct a Bohmian variant of quantum gravity. See e.g.
http://xxx.lanl.gov/abs/hep-th/0407228 [Eur.Phys.J.C42:365-374,2005]
and references therein.

 

[Fra]

If by a classical trajectory you mean a trajectory satisfying the classical (say, Newton) equation, then I agree.
But if by a classical trajectory you mean any function of the form x_i(t), i=1,2,3, then I certainly disagree. Bohm has shown in 1952 that a "classical" trajectory in this second sense is possible within quantum mechanics.

I think one can extend the discussion, depending on what the purpose is.

I'm not sure if Marucs meant that spacetime was the main focus or just an example of what's physical and what's not.

From the general point of view, the objective existence in the realist sense is not impossible per see, I just object to that way of thinking simply because I do not see how something that is not known to me, would influence my expectations and thus actions - and thus in the extension how any system would be influenced by things to which be definition the system is unaffected by. It's a form of nonlocality in information space, where a decision would depend on information that isn't at hand. It doesn't fit in any coherent view for me.

About the specifics of spacetime, I personally think the physical basis of what we call spacetime in the classical limit, is simply part of the event index structure that each observer encodes. This is IMO as real as it gets. However, I think of it as observer dependent. The only objectivity that I see constructable is where you have a group of interacting systems/observers that come to a consensus / agreement / equilibrium of certain quasi-universal things that doesn't depend on the details not known.

My personal vision is that if you picture scaling down the observer to say Planck scale or something equivalent (details not irrelevant) then the spacetime part of the event index probably becomes indistinguishable from the event index referring to othre things related to ew and stongg forces.

So probably what we think of as the locally observer independent equivalence class of spacetimes and the corresponding transformations emerge only as we consider more complex observers (lower the energy scale in common language). At some point prior to that, I don't think there will be a clear distinction between external and internal degrees of freedom.

In that sense I fully agree that the 4D manifold and in particular the equivalence classes of them, is something that should be explained in terms of an outcome of interactions/negotiations of the simplest possible observers, as the complexity scale increases.

There seems to exists a clear duality here in that the high energy limit (looking at interactions between ssmaller and smaller parts) can only be described relative to a low energy observer (a very complex observer), simply due to the encoding requirements. This is what I think the very complex standarmodel (as judged from our low energy perspective) probalby would be surprisingly simple if view from the point of view of one of the participating parties.

/Fredrik

 

[Demystifier]

Fra, I think marcus wants to know what is FUNDAMENTAL and what is not. On the other hand, you want to know what can be known and used by an OBSERVER. I don't think that marcus would agree that observers are fundamental, but let us leave him to speak for himself.

 

[Fra]

Fra, I think marcus wants to know what is FUNDAMENTAL and what is not. On the other hand, you want to know what can be known and used by an OBSERVER. I don't think that marcus would agree that observers are fundamental, but let us leave him to speak for himself.

Yes, we'll see what he meant. I assumed he was looking for some discussion around the topic.

Just to add, I certainly wouldn't call observers fundamental either, in the sense of what I said in my first post. I just see it as an unescapable context.

The reason why I want to form any abstraction relative to an observer context, is because I think it's somehow seems to be how nature is devised. So I think understanding nature to the extent possible suggests trying to understand the relation between observer, environment and how knowledge is encoded and used for decisions. This is why I defined what I see as the key question. They question for me isn't to find what fundamental in the sense noted. But the fact that our disagreements can be traced back to the questions we ask, are quite instructive IMHO.

/Fredrik

 

[Demystifier]

But the fact that our disagreements can be traced back to the questions we ask, are quite instructive IMHO.

Yes, definitely.

 

[ConradDJ]

12/29/10

A spacetime cannot have physical existence any more than the continuous trajectory of a particle has physical existence.

Quantum mechanics does not allow a particle to follow a classical trajectory.
And a spacetime, represented by a 4D manifold with metric geometry, is very closely analogous to a classical particle trajectory.

So any quantum geometry theory which requires a 4D manifold can not be physical, can not be fundamental.

Marcus – Yes, I agree with you. If we take QM as a guide to what’s fundamental in physics, it doesn’t make sense to treat spacetime structure in classical terms... i.e. as an inherently well-defined geometric continuum.

Of course that raises a problem with QM itself, to the extent it assumes such a background – even if we leave gravity out of the picture. Since we don’t really understand yet what QM by itself is telling us about space and time, it’s not surprising that attempts to unify it with GR often feel like groping in the dark.

From a philosophical standpoint, I think it’s easy to identify the problem. There’s an aspect of the physical world that physics knows very well how to deal with, because since Newton’s time physics has been all about inventing mathematical models of reality and testing them against experiment. Which of course worked extremely well, right through the completion of the Standard Model in the ‘80s.

If this never came close to giving us a viable picture of physical foundations, that’s because we know from QM that there’s another aspect to the world, that has to do with “the role of the observer.” At bottom we have no good reason to believe that the world has the kind of precise, inherent determinacy that mathematical models do. A vast body of experiment supports the quantum principle that determinacy – whether of particle properties, particle paths or spacetime structure – arises out of observation, i.e. physical processes that both define and communicate information.

Now QM shows us how to model indeterminacy in mathematics, i.e. as a superposition of all possible measurement-results in a given physical context. But Quantum Gravity theories, so far as I can tell, are based on superpositions of geometric or topological structures, which is something very different, and completely bypasses the physics of observation.

Of course, there’s always the possibility that we’ll learn something important from this sort of mathematical model-building – if and when we can compare it with experiment. But to me there’s nothing philosophically compelling here. It seems the key lesson of QM has not been taken seriously – though Rovelli has made some notable efforts in that direction.

On the other hand, Relativity as well as QM began by making “the role of the observer” fundamental. Since it was only observer’s point of view in space and time that was relevant to Relativity, it was easy to forget all about the observer and focus on mathematical models of spacetime, as if it were a “real object” in itself. But the point is, if what we’re looking to understand is the underlying relationship between Relativity and QM, we’re most likely to find it by asking why “the observer” shows up in the basis of both theories.

I don’t mean to ignore the other posts here on “the observer”, but I have to get to work... will check back in tomorrow.

Conrad

 

[atyy]

Reality exists. There is only one reality.

Should it be a particular solution of a theory with multiple solutions, as it has been ever since Newton?

After all, the other solutions don't exist. So why do we need the theory?

However, observers who can learn patterns at certain intermediate scales (eg. humans who have theories like Newtonian physics, Maxwell's equations etc) exist.

Can a universe that is not a particular solution of a theory (ie. a universe that obeys no rules) give rise to such observers (TOE does not exist, E exists)?

Or must a universe with such observers be a particular solution of a theory (TOE exists)?

Weinberg's search is for E or TOE (E/TOE), usually TOE, since E would be hard to discover even if it is true.

Perhaps Fra is pointing out that the constraint on E/TOE is double: (i) the intrinsic properties of the observers and (ii) the theories they devise.

However, typically we only use (ii) to constrain E/TOE. The anthropic principle includes (i), but as we know it is at present too vaguely formulated to be of use.

 

[friend]

So any quantum geometry theory which requires a 4D manifold can not be physical, can not be fundamental. Can only be an approximate idealization. It talks about something which is impossible to exist.

Please say, if you disagree.

Or, more fundamentally, does the continuum (of spacetime) exist in reality? I think we have to assume a contiuum, because we could never observe points not in existences. To say that we went from one point to the next with nothing in between seems to assert the existence of non-existent space in between. How would information propagate from one point to the next across nothing?

 

[Fra]

Can a universe that is not a particular solution of a theory (ie. a universe that obeys no rules) give rise to such observers (TOE does not exist, E exists)?

Or must a universe with such observers be a particular solution of a theory (TOE exists)?

Weinberg's search is for E or TOE (E/TOE), usually TOE, since E would be hard to discover even if it is true.

Perhaps Fra is pointing out that the constraint on E/TOE is double: (i) the intrinsic properties of the observers and (ii) the theories they devise.

However, typically we only use (ii) to constrain E/TOE. The anthropic principle includes (i), but as we know it is at present too vaguely formulated to be of use.

I'm not sure if that's what I meant to point out, What is E? I don't think I understand what you said.

An unrelated note: I hope it's clear what the distinction is between silly antrophic principles, and evolutionary principles, right?

I don't think it makes sense to make up an theory space and then rule out any theories that according to the same made up logic doesn't allow human life. That's silly. What I envision is a much deeper mechanism, that is more like darwinian evolution applied to inference systems - so it's not that antrophic selection is something that we apply to a multiverse, it's rather that real evolution takes place in our ONE universe.

I find the distinction clear, but maybe not everyone agrees?

This tangents to the reasoning of Smolin/Unger that has been mentioned before.

/Fredrik

 

[atyy]

I'm not sure if that's what I meant to point out, What is E? I don't think I understand what you said.

An unrelated note: I hope it's clear what the distinction is between silly antrophic principles, and evolutionary principles, right?

I don't think it makes sense to make up an theory space and then rule out any theories that according to the same made up logic doesn't allow human life. That's silly. What I envision is a much deeper mechanism, that is more like darwinian evolution applied to inference systems - so it's not that antrophic selection is something that we apply to a multiverse, it's rather that real evolution takes place in our ONE universe.

I find the distinction clear, but maybe not everyone agrees?

This tangents to the reasoning of Smolin/Unger that has been mentioned before.

/Fredrik

How about the law governing both the observers and the evolving law they observe. Do you conceive of that evolving?

 

[friend]

What I envision is a much deeper mechanism, that is more like darwinian evolution applied to inference systems

Propositional logic and Predicate logic are complete. There IS nothing that can be added to it. There is no evolution of inference systems. And do you suppose that there is something in reality that does not complete with these logic systems?

 

[inflector]

I guess I get to be profoundly naive since I'm the neophyte here. But it seems to me that if one wants to get away from under the proverbial streetlight to find some fresh ground for the search, then perhaps it makes sense to revisit some cherished ideas and see what it might mean if they don't hold.

Time is one of those cherished notions—enshrined in our psyches and perception systems certainly—that is also written in the stone of our most revered physical laws: general relativity and QM.

One of the great simplifications of 20th century physics was the idea of spacetime and the 3 + 1 dimensions of Minkowski's formulation for space. But perhaps this is just a mathematical convenience that elevates time improperly to the fore as one of the most important core elements of physics. What if this is wrong? What if it is a simplification that diverts our minds from the deeper reality?

Time has won, and no one seems to challenge the idea. But perhaps if it is a misleading fundament, then a large number of potentially fruitful ideas won't even get started because they won't seem possible.

The idea of spacetime, and the way in which general relativity uses curvature in spacetime to model accelerations as forceless phenomena may have been only local minima in the search for higher physical law. Perhaps those minima might take us higher than what existed before but in order to get to the actual summit we must go down and take another route.

The elevation of time to a preferred position as a full-fledged, albeit sign-reversed, dimension, along with the success of general relativity's spacetime curvature, has cut off the debate over which is fundamental.

It seems to me that what we can actually observe and measure is relative movement. Most of which shows up as oscillation, a confined version of relative movement. We perceive time, but what if relative movement and oscillation; or the underlying driver of that movement; or something that gives the fabric of space elasticity; is fundamental? What if acceleration or velocity is more fundamental than time?

A spacetime cannot have physical existence any more than the continuous trajectory of a particle has physical existence.

With that said, I think I can agree with this statement, a spacetime cannot have physical existence.

I should note, however, that perhaps a space alone can have a physical existence if we don't include time. There may be some configuration of space that allows for a representation of all the myriad fields we now consider unphysical abstractions out of which time might develop emergently.

 

[Fra]

How about the law governing both the observers and the evolving law they observe. Do you conceive of that evolving?

No I don't. That's not darwinian evolution, that is just normal deterministic dynamical evolution. Two different things.

My point is that once you analyze the situation, where en INSIDE observer are to INFER the laws the predicts it's own evolution, the conclusion is that such a law is always out of reach. This that law isn't known - this is why it evolves in the darwinian sense.

Sure, I can "imagine" that "there exists" out there a law that determines the evolution, it's just that I'm unaware of it. But I think that's a realist fallacy to think that such unkonwn thinkg would influence the observer.

Did you ever listen to any of Smolins talk about evolving law? If so what do you think if his arguments? I don't think are best, but it's a start.

/Fredrik

 

[Fra]

Propositional logic and Predicate logic are complete. There IS nothing that can be added to it. There is no evolution of inference systems. And do you suppose that there is something in reality that does not complete with these logic systems?

No evolution of inference systems? I couldn't disagree more. That's obviously wrong. Note that I am not talking about deductive inference, I'm talking about inductive type inference.

How about the evolution of life?
How about evolution of human brain?
How about the de facto evolution of human knowledge of physical law during history of science? This has influenced human actions.

The point is that there are always questions we can not answer, this is what forces the evolution of the inference systems. Analogy with life: there is not ultimate organism, it needs to adapt to the environment to survive. This is evolution. Also sometimes we detect inconsistencies in our own way of thinking, then we also need to revise and evolve.

There are plenty of evidence that the strategy of starting from eternally true and fixed axioms and then move forward by deductions, simply is an INVIABLE approch. WE don't see that in nature. Neither in animals, humans and I think also not in matter (except of course I can't say for sure, since there are too many open questions yet).

/Fredrik

 

[oldman]

A spacetime cannot have physical existence any more than the continuous trajectory of a particle has physical existence.

Quantum mechanics does not allow a particle to follow a classical trajectory.
And a spacetime, represented by a 4D manifold with metric geometry, is very closely analogous to a classical particle trajectory.

So any quantum geometry theory which requires a 4D manifold can not be physical, can not be fundamental. Can only be an approximate idealization. It talks about something which .

Please say, if you disagree.

Please forgive me for commenting that I think you're running before knowing how to walk here, Marcus. You are questioning whether a "quantum geometry theory" is "fundamental" or only an "approximate idealization" treating something that "is impossible to exist".

For someone like myself, who is still at a crawling stage, to decide such questions it is helpful to first consider how physics treats theoretically simpler and more tangible situations. Say like a solid lump of crystalline metal --- whose existence philosophers might doubt, but most physicists would accept.

A description of how the lump's constituent atoms are arranged while swimming in a sea of itinerent electrons is sufficiently fundamental for many practical purposes. But even at this simple level it is convenient to "talk about something which is impossible to exist", namely an abstract reciprocal space (which exists only in the minds of physicists) and, for the lump's electrons, an equally imaginary sea-surface in the same abstract space; the Fermi surface.

This makes it clear that physicists use quite complicated layered treatments even in relatively simple situations. In complicated and as yet unverified Quantum Gravity there are more layers. I think that to get down to a fundamental level one first needs to bring observers into the picture, as Fra often suggests, especially the nature of observers. I've said so in another recent (Dec 3) thread in this Forum, namely "Physics and Story-Telling".

 

[czes]

Wheeler said that XXI century will be a physics of the quantum information. We see it. Quantum computation is possible and developed.
The Holographic Principle suggests the information is fundamental.
I am imagining a computer program modeling a vacuum as a set of the points of the interaction between the quantum information and each this interaction has encoded a time dilation of the Plack time. We get then just the General Relativity of Einstein, very simple.
I am under impression that such a simple computer program creates a spacetime where the concetration of the Vacuum (information interaction) creates an arbitrary virtual reality.
You may call the increasing entropy forwards time and decreasing entropy backwards time (just a convention).
It is a speculation unless the Holographic Principle is true.