Is there a quantum theory of other entities besides particles and fields?

[ephen wilb]

QM or quantum mechanics is quantum theory of particles
QFT or quantum field theory is quantum theory of fields (containing QED, QCD, and maybe QG)

are there none beside particles or fields.. why is there no quantum theory of other stuff?

 

[Vanadium 50]

What "other stuff" are you talking about?

 

[ephen wilb]

What "other stuff" are you talking about?

No. I was just asking if there are just particles and fields... what category does particle and field fall under.. what else in the category and why only the two present?

 

[jtbell]

Fields (described by QFT) and particles (described by QM) are not different kinds of "stuff." They are different descriptions of the same "stuff." The description that uses fields and QFT is more fundamental and applies under more circumstances than the description that uses particles and QM. Under some circumstances the fields behave in ways that we label as "particles" that follow the rules of QM, which are easier to work with.

 

[atyy]

There are also quantum theories of strings and membranes, spins stuck to a lattice and lattice gauge theory.

 

[ephen wilb]

There are also quantum theories of strings and membranes, spins stuck to a lattice and lattice gauge theory.

In jtbell context... particles, fields, strings, membranes are "different descriptions of the same "stuff.""? so fields are literally strings and vice versa?

 

[atyy]

In jtbell context... particles, fields, strings, membranes are "different descriptions of the same "stuff.""? so fields are literally strings and vice versa?

Yes and no. Heuristically, fields can be thought of as quantum theories of many particles. Also, fields and lattice gauge theory are heuristically the same things. It is also conjectured that fields in a lower dimension can be equivalent to strings in a higher dimension. Of the preceding heuristics, the most rigourous one is the notional equivalence of fields and lattice gauge theory. Thinking of quantum field theories as quantum theories of many particles is less rigourous, but most quantum field theory books do things this way. The relationship between fields and strings is an area of active research.

 

[ephen wilb]

Yes and no. Heuristically, fields can be thought of as quantum theories of many particles. Also, fields and lattice gauge theory are heuristically the same things. It is also conjectured that fields in a lower dimension can be equivalent to strings in a higher dimension. Of the preceding heuristics, the most rigourous one is the notional equivalence of fields and lattice gauge theory. Thinking of quantum field theories as quantum theories of many particles is less rigourous, but most quantum field theory books do things this way. The relationship between fields and strings is an area of active research.

Here's something I'm getting my head into. Do all fields have to be quantized? For example. If fields cause wave function collapse.. does it has to be quantum mechanized? I presume being quantized and quantum mechanized is the same.. But gravity as field doesn't necessarily mean it's quantized.. so how do you tell whether a field can remain classical or should be quantized (quantum mechanized)?

 

[atyy]

Here's something I'm getting my head into. Do all fields have to be quantized? For example. If fields cause wave function collapse.. does it has to be quantum mechanized? I presume being quantized and quantum mechanized is the same.. But gravity as field doesn't necessarily mean it's quantized.. so how do you tell whether a field can remain classical or should be quantized (quantum mechanized)?

In the traditional "Copenhagen" interpretation of quantum mechanics, there is a practical division of the universe into a classical part and a quantum part. It is the interaction of the classical part with the quantum part to produce a measurement result that causes collapse. The division of the universe into a classical part and a quantum part is subjective, so each observer can do it differently (or even the same observer can do it in several different ways). In this traditional interpretation, the "whole universe" is not necessarily quantum. However, it should be possible to formulate a quantum theory of any part of the universe that we can observe - so in that sense "everything" is quantum.

The basic philosophy here is that we have an intuitive idea as to what a measurement result is, and quantum mechanics is not necessarily about a "story of reality that is continually unfolding", but rather a way to predict probabilities for what an observer observes.

 

[ephen wilb]

In the traditional "Copenhagen" interpretation of quantum mechanics, there is a practical division of the universe into a classical part and a quantum part. It is the interaction of the classical part with the quantum part to produce a measurement result that causes collapse. The division of the universe into a classical part and a quantum part is subjective, so each observer can do it differently (or even the same observer can do it in several different ways). In this traditional interpretation, the "whole universe" is not necessarily quantum. However, it should be possible to formulate a quantum theory of any part of the universe that we can observe - so in that sense "everything" is quantum.

The basic philosophy here is that we have an intuitive idea as to what a measurement result is, and quantum mechanics is not necessarily about a "story of reality that is continually unfolding", but rather a way to predict probabilities for what an observer observes.

I understand what is the Wigner Friend all about. It's anti realism of QM due to conflicting accounts with Many Worlds as only alternative. Have you come across any daily life analogy of the Wigner Friend.. such as for example if all wear uniform or certain weather, there is some so and so conflict (just an example of daiy life). I'd like to know if the Wigner friend scenario is a common one or rare one and what's it correlates in other field or topic.

 

[atyy]

I understand what is the Wigner Friend all about. It's anti realism of QM due to conflicting accounts with Many Worlds as only alternative. Have you come across any daily life analogy of the Wigner Friend.. such as for example if all wear uniform or certain weather, there is some so and so conflict (just an example of daiy life). I'd like to know if the Wigner friend scenario is a common one or rare one and what's it correlates in other field or topic.

Yes, it is common throughout QM. But the response need not be anti-realism or Many Worlds. The response could be that QM is a practical theory, and in practice we always know how to divide the universe into a classical part and a quantum part. Wigner's friend and associated problems may simply indicate that QM is incomplete. This was Dirac's own view. Since Bohm's work, we are able to write down explicitly for non-relativistic QM some possibilities for these more complete theories without the Wigner's friend problem. Whether and how we might do so for relativistic QM is still being researched.

 

[ShayanJ]

Yes, it is common throughout QM. But the response need not be anti-realism or Many Worlds. The response could be that QM is a practical theory, and in practice we always know how to divide the universe into a classical part and a quantum part. Wigner's friend and associated problems may simply indicate that QM is incomplete. This was Dirac's own view. Since Bohm's work, we are able to write down explicitly for non-relativistic QM some possibilities for these more complete theories without the Wigner's friend problem. Whether and how we might do so for relativistic QM is still being researched.

I think paradoxes like Schrodinger's cat and Wigner's friend can be solved using decoherence. Also decoherence tells us that we don't need the Heisenberg cut any more.

 

[ephen wilb]

Yes, it is common throughout QM. But the response need not be anti-realism or Many Worlds. The response could be that QM is a practical theory, and in practice we always know how to divide the universe into a classical part and a quantum part. Wigner's friend and associated problems may simply indicate that QM is incomplete. This was Dirac's own view. Since Bohm's work, we are able to write down explicitly for non-relativistic QM some possibilities for these more complete theories without the Wigner's friend problem. Whether and how we might do so for relativistic QM is still being researched.

I was asking if you have analogy of Wigner Friend in other subjects for example, economics, bathrooms, lego objects, toys.. I can't give an example that is why I was asking.

 

[bhobba]

I was asking if you have analogy of Wigner Friend in other subjects for example, economics, bathrooms, lego objects, toys.. I can't give an example that is why I was asking.

Well Wigner's Friend represents a bit of a misunderstanding of QM anyway. It relates to the misunderstanding Von Neumann made in his conciousness causes collapse interpretation that he and Wigner held to. Later Wigner woke up to the issue and did a 180% about face. So no - the same misunderstanding doesn't crop up in other fields so its pretty useless outside QM - and these days within QM as well.

Thanks
Bill

 

[ephen wilb]

In the traditional "Copenhagen" interpretation of quantum mechanics, there is a practical division of the universe into a classical part and a quantum part. It is the interaction of the classical part with the quantum part to produce a measurement result that causes collapse. The division of the universe into a classical part and a quantum part is subjective, so each observer can do it differently (or even the same observer can do it in several different ways). In this traditional interpretation, the "whole universe" is not necessarily quantum. However, it should be possible to formulate a quantum theory of any part of the universe that we can observe - so in that sense "everything" is quantum.

The basic philosophy here is that we have an intuitive idea as to what a measurement result is, and quantum mechanics is not necessarily about a "story of reality that is continually unfolding", but rather a way to predict probabilities for what an observer observes.

Going back to this. In special relativity, objects can time dilate and length contract with respect to one another.. this shows reality is malleable.. so what is wrong with the more literal "story of reality that is continually unfolding" in QM? If it can happen in relativity, why not in QM?

 

[bhobba]

Going back to this. In special relativity, objects can time dilate and length contract with respect to one another.. this shows reality is malleable

Its shows space-time is described by Minkowskian geometry - not reality is mailable - whatever that means.

Thanks
Bill

 

[ephen wilb]

Its shows space-time is described by Minkowskian geometry - not reality is mailable - whatever that means.

Thanks
Bill

Minkowski geometry is flexible. it bends, expands, contracts etc. So it is not difficult to make minkowski geometry and the fields in it appear and vanish in instances related to wave function collapse. Since relativity is relative.. then the answer to Wigner Friend is maybe QM has relativity of observations too akin to SR?

 

[bhobba]

Minkowski geometry is flexible. it bends, expands, contracts etc.

Really - then how come the metric is constant?

You are thinking about the pseudo Riemannian geometry of GR - but flexible is not the appropriate analogy there.

The key to GR is space-time geometry is dynamical - not 'flexible' which has connotations not appropriate to what's going on.

Thanks
Bill

 

[atyy]

I was asking if you have analogy of Wigner Friend in other subjects for example, economics, bathrooms, lego objects, toys.. I can't give an example that is why I was asking.

Hmmm, good question. I don't know. I have a different way of stating the question, have started a thread on it: https://www.physicsforums.com/threa...in-classical-probability.817405/#post-5130948.

 

[ephen wilb]

Really - then how come the metric is constant?

You are thinking about the pseudo Riemannian geometry of GR - but flexible is not the appropriate analogy there.

The key to GR is space-time geometry is dynamical - not 'flexible' which has connotations not appropriate to what's going on.

Thanks
Bill

Do you know of any function or device in mathematics that can bind discrete and continuous (QM and GR) that can transform into each other and even make them emergent of this more primary aspect?

 

[bhobba]

Do you know of any function or device in mathematics that can bind discrete and continuous (QM and GR) that can transform into each other and even make them emergent of this more primary aspect?

I have no idea what you are asking.

So let's take it step by step.

What do you mean by 'bind discrete and continuous (QM and GR)'

Thanks
Bill

 

[ephen wilb]

I have no idea what you are asking.

So let's take it step by step.

What do you mean by 'bind discrete and continuous (QM and GR)'

Thanks
Bill

GR (continuous) and QM (discrete) can't be united because they are opposite (continuous and discrete). So there may be transforms that can unite them. I'm thinking of the possibility that both GR and QM are emergence from something else.

 

[bhobba]

GR (continuous) and QM (discrete)

Where do you get the idea QM is discreet?

Thanks
Bill

 

[ephen wilb]

Where do you get the idea QM is discreet?

Thanks
Bill

QM is random, jumpy.. quantized.. at least during wave function collapse.
you are saying QM is continuous before wave function collapse? that it is compatible with GR before wave function collapse?

 

[bhobba]

QM is random, jumpy.. quantized.. at least during wave function collapse.
you are saying QM is continuous before wave function collapse? that it is compatible with GR before wave function collapse?

You are getting confused about a number of concepts.

First wave-function collapse is only part of some interpretations - its not part of the QM formalism.

In combining QM and GR we use Quantum Field Theory, which is a continuous field theory, exactly like GR. You can make GR into a QFT exactly like one does for any classical field theory - there is no issue at all. The issue comes when you try to extract answers from the theory - it blows up with infinity. There is a technique for handling that called renormalisation. The problem is it doesn't work here.

It took many years of hard work to sort out what was going on. I recently wrote an insights paper explaining, at a basic level, what renormalisation is:
https://www.physicsforums.com/threads/renormalisation-made-easy-comments.813025/

Along with this better understanding came a new view of renormalisation, called the Effective Field Theory approach. It turns out by applying that to the quantum theory of gravity you can get finite answers - but the resulting theory is only valid up to the Plank scale:
http://arxiv.org/pdf/1209.3511v1.pdf

'Effective field theory shows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective field theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective field theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be. Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability. The usual marriage of general relativity and quantum mechanics is fine at ordinary energies, but we now seek to uncover the modifications that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated view of the past.'

Thanks
Bill

 

[ephen wilb]

You are getting confused about a number of concepts.

First wave-function collapse is only part of some interpretations - its not part of the QM formalism.

In combining QM and GR we use Quantum Field Theory, which is a continuous field theory, exactly like GR. You can make GR into a QFT exactly like one does for any classical field theory - there is no issue at all. The issue comes when you try to extract answers from the theory - it blows up with infinity. There is a technique for handling that called renormalisation. The problem is it doesn't work here.

It took many years of hard work to sort out what was going on. I recently wrote an insights paper explaining, at a basic level, what renormalisation is:
https://www.physicsforums.com/threads/renormalisation-made-easy-comments.813025/

Along with this better understanding came a new view of renormalisation, called the Effective Field Theory approach. It turns out by applying that to the quantum theory of gravity you can get finite answers - but the resulting theory is only valid up to the Plank scale:
http://arxiv.org/pdf/1209.3511v1.pdf

Thanks
Bill

If Planck constant or H bar were higher in value.. would this make you get finite answers only at much lower energy say at 5 times less than the Planck scale? what dictate why the resulting theory is only valid up the Planck scale.. is it due primarily to the value of c, the strength of gravity and Planck constant?

 

[TrickyDicky]

First wave-function collapse is only part of some interpretations - its not part of the QM formalism.

It is part of the formalism in the explicitly time-dependent(sequential measurements) case.

In combining QM and GR we use Quantum Field Theory, which is a continuous field theory, exactly like GR.

This is true only for the free QFT. The effective interacting QFT space has finite particles Fock space.

 

[julcab12]

what dictate why the resulting theory is only valid up the Planck scale.. is it due primarily to the value of c, the strength of gravity and Planck constant?

... Yes. It is called constant bec it has consistent value in relation to the other constants G, C. Renormalization/cutoffs stops to make sense beyond that scale. In this sense we need a new physics. Well, We can only say at the moment that plank constant is the smallest meaningful derivation- e.g, there's nothing in the universe smaller than a plank. So that new unit would be meaningless because there's nothing to measure that hold consistent to our present physics/experiment etc.

 

[ephen wilb]

Bill and fellas. If the Planck scale doesn't have *inside* because it is a whole thing like superstring. Are you saying there is no fundamental incompability at lower energy far from Planck scale for interacting QFT and gravity? Or would the problem remain? If such is the case, then QFT and gravity is fundamentally incompatible.

 

[bhobba]

If the Planck scale doesn't have *inside* because it is a whole thing like superstring..

I am saying no one knows anything about the Planck scale so your questions can't be answered. Even if I could figure out what they mean eg what you mean by an 'inside' I have no idea.

Why does the effective field theory of gravity have a cut-off at the Planck scale? Its because the theory predicts that's where it falls apart and new physics emerges - but to understand the details you need to grind through the math - its associated with something called the plank mass in the theory. The interesting thing about effective field theories is they also tell you where they break down.

Thanks
Bill

 

[ephen wilb]

I am saying no one knows anything about the Planck scale so your questions can't be answered. Even if I could figure out what they mean eg what you mean by an 'inside' I have no idea.

Why does the effective field theory of gravity have a cut-off at the Planck scale? Its because the theory predicts that's where it falls apart and new physics emerges - but to understand the details you need to grind through the math - its associated with something called the plank mass in the theory. The interesting thing about effective field theories is they also tell you where they break down.

Thanks
Bill

What I was asking in this thread is we don't exactly know how matter is *binded* or *attached* to spacetime at all orders of energies. Effective field theory just pretend its binded to solve certain calculations. Is this correct? Einstein tried to model matter as geometry to bind geometry to geometry.. so how they are bind is still a big mystery even at low energies.

 

[bhobba]

What I was asking in this thread is we don't exactly know how matter is *binded* or *attached* to spacetime at all orders of energies.

I have noticed in a number of your posts you are using words in a confusing way - I have no idea what you mean by matter in binded or attached to space-time. Matter is contained in space-time. And it has nothing to do with energy.

No - EFT does not work that way, nor is that what Einstein did or tried to do.

He dis however make an interesting extension to GR where he showed the EFE's were all by themselves enough to determine a particles motion without the geodesic idea. If you don't know what geodesic means - look it up.

Thanks
Bill

 

[ephen wilb]

I have noticed in a number of your posts you are using words in a confusing way - I have no idea what you mean by matter in binded or attached to space-time. Matter is contained in space-time. And it has nothing to do with energy.

No - EFT does not work that way, nor is that what Einstein did or tried to do.

He dis however make an interesting extension to GR where he showed the EFE's were all by themselves enough to determine a particles motion without the geodesic idea. If you don't know what geodesic means - look it up.

Thanks
Bill

It came from you in your old messages in sci.physics... I read about how you said it is unknown to all orders and degrees how matter is coupled to spacetime.. one is a manifold.. the other is discrete or quantum.. how the manifold holds matter in the microscopic level we don't know.. i think the coupling is done in the Planck scale.. no? are you denying this? I will have to search your old messages I forgot the exact subject.

 

[bhobba]

It came from you in your old messages in sci.physics... I read about how you said it is unknown to all orders and degrees how matter is coupled to spacetime

I think you aren't recalling correctly what I may have said. It may have to do with gravity gravitating - but that's just a guess.

We do not know how the stress energy tensor is coupled to space-time ie why gravity exists - GR simply says it does. Its tied up with the why no prior geometry - which is the rock bottom essence of GR. Its very reasonable - but it just a fact we have to accept - at least at this stage.

However if that interests you, it may be better to pursue in on the GR sub-forum - we discuss QM here.

Thanks
Bill

 

[ephen wilb]

I think you aren't recalling correctly what I may have said. It may have to do with gravity gravitating - but that's just a guess.

We do not know how the stress energy tensor is coupled to space-time ie why gravity exists - GR simply says it does. Its tied up with the why no prior geometry - which is the rock bottom essence of GR. Its very reasonable - but it just a fact we have to accept - at least at this stage.

However if that interests you, it may be better to pursue in on the GR sub-forum - we discuss QM here.

Thanks
Bill

Yes. We do not know how the stress energy tensor (which stands for matter) is coupled to space-time.
This is not difference than saying we do not know how matter is binded or attached to space-time. My words now makes sense, correct?

 

[julcab12]

What I was asking in this thread is we don't exactly know how matter is *binded* or *attached* to spacetime at all orders of energies.

..

.. Not really. We can identify a proper description of how natures behave and interact. QFT does well with all this thing except for gravity(exemption of N=8 Supergravity which is sketchy with the includsion of 11D). Realizations of QFT can go further. For instance, phenomena of electromagnetism are described accurately by QED, the strong interaction that describes how nuclei behave is covered by QCD. Together with the theory of weak interactions, these two theories form the Standard Model of Particle Physics. Apart from particle physics, there also exist QFT approaches towards condensed matter physics. We can also go further by quantization or introducing a different structure QG such as we can built model such as LQG(spin networks/foams/minimal structure) and string (well, strings) which is a future development and hopefully experimentally.

Actually the binding is not the problem -- which is covered a large portion on the dynamical part(physics). It is more on the question of how to picture the physics to some physical structure. ..

 

[julcab12]

Yes. We do not know how the stress energy tensor (which stands for matter) is coupled to space-time.
This is not difference than saying we do not know how matter is binded or attached to space-time. My words now makes sense, correct?

... No .We do know. As an observer we interpret and put constraints. We know what is happening-- GR (to some extent). We can formulate relationship and redefine parameters and so on(Math). Some stems from our imagination following the spirit of successful theories extended like how Einstien did with blocked space-time. The simple answer is we do know -- From taking that dynamic picture spacetime/mathematical structure 'attached' (for simplification's sake) to matter/physical object. We can reflect on our observation. More to that make future prediction out of it.

 

[bhobba]

Yes. We do not know how the stress energy tensor (which stands for matter) is coupled to space-time.
This is not difference than saying we do not know how matter is binded or attached to space-time. My words now makes sense, correct?

Yes. But now can you please clearly state what your question is?

Thanks
Bill

 

[ephen wilb]

Yes. But now can you please clearly state what your question is?

Thanks
Bill

Yes. But now can you please clearly state what your question is?

Thanks
Bill

You know Uncle Al who is one of the genuine physicists that you have discussed at sci.physics. He wrote the following and many others that indicate even far from Planck scale, gravity and quantum field theory is inherently incompatible:

Uncle Al said

"
c=infinity G=G h=0 Newton
c=c G=0 h=0 Special Relativity
c=c G=G h=0 General Relativity
c=infinity G=0 h=h quantum mechanics
c=c G=0 h=h quantum field theory
c=c G=G h=h quantum gravitation (utter failure)

Physics exists in two incompatible parts. Gravity is geometrically
modeled by General Relativity, everything else is grabbed by either
classical physics (the smoothed middle ground - Correspondence
Principle) or quantum mechanics on the other end. The three vital
constants are Big G, Planck's constant h, and lightspeed c. Various
models of reality set one or more of them to their proper values and
(tacitly or otherwise) assume for the remainder big is infinity and
small is zero. This has been posted and threaded here and in
sci.physics.research. See if you can find the posts in
http://www.google.com/grphp?hl=en
Draw (the cube for) the eight combinatorial possibilities and see if
you can match the asignments with the physical theories. Here's a
start: SR(G=0,h=0,c=c), GR(G=G,h=0,c=c), QM(G=0,h=h,c=infinity).
The pisser is that the fundamental structure of spacetime at Planck
lengths requires both GR and QM. Nobody can get them to fuse
explicitly or with an end-around run.

 

[bhobba]

You know Uncle Al who is one of the genuine physicists that you have discussed at sci.physics.

Actually he was a chemist.

The last bit is wrong - however, if you read the paper I linked to, you will see its a very common misconception because people have not kept up with the advances in Effective Field Theory.

Please read it and give it a bit of thought before positing again. Please:
http://arxiv.org/pdf/1209.3511v1.pdf

Actually I will post the key bit:
'Effective field theory has added something important to the understanding of quantum gravity. One can find thousands of statements in the literature to the effect that “general relativity and quantum mechanics are incompatible”. These are completely outdated and no longer relevant. Effective field theory shows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective field theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective field theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be. Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability. The usual marriage of general relativity and quantum mechanics is fine at ordinary energies, but we now seek to uncover the modifications that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated view of the past.'

Thanks
Bill

 

[ephen wilb]

Actually he was a chemist.

The last bit is wrong - however, if you read the paper I linked to, you will see its a very common misconception because people have not kept up with the advances in Effective Field Theory.

Please read it and give it a bit of thought before positing again. Please:
http://arxiv.org/pdf/1209.3511v1.pdf

Actually I will post the key bit:
'Effective field theory has added something important to the understanding of quantum gravity. One can find thousands of statements in the literature to the effect that “general relativity and quantum mechanics are incompatible”. These are completely outdated and no longer relevant. Effective field theory shows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective field theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective field theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be. Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability. The usual marriage of general relativity and quantum mechanics is fine at ordinary energies, but we now seek to uncover the modifications that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated view of the past.'

Thanks
Bill

I have read the paper and given it some thoughts. Is the following analogy valid. Consider a maglev train moving on the ground. The relationship of the train with respect to Earth (like velocity acceleration etc) can be considered the effective field theory while how the maglev is connected to the ground is the unknown Planck scale physics.. so the occupants who never know what propel the train can still effectively compute for the velocity with respect to Earth (effective theory) but not whether the train uses magnetic levitation, or wheels or even jets (unknown Planck scale physics).. is this analogy right?

 

[bhobba]

I have read the paper and given it some thoughts. Is the following analogy valid. Consider a maglev train moving on the ground. The relationship of the train with respect to Earth (like velocity acceleration etc) can be considered the effective field theory while how the maglev is connected to the ground is the unknown Planck scale physics.. so the occupants who never know what propel the train can still effectively compute for the velocity with respect to Earth (effective theory) but not whether the train uses magnetic levitation, or wheels or even jets (unknown Planck scale physics).. is this analogy right?

No.

Here is something closer to what's going on.

Read my paper on re-normalisation I gave previously. See how it gets rid of infinity by imposing a cut-off and then using renormalisation. What Effective Field Theory does is come up with a theory that up to a cut-off is the same as gravity and that trick will work. That's not quite it - and there are some things I am not up to speed with - its quite advanced - but it gives you an idea.

The following may also help:
http://en.wikipedia.org/wiki/Effective_field_theory

Thanks
Bill

 

[ephen wilb]

No.

Here is something closer to what's going on.

Read my paper on re-normalisation I gave previously. See how it gets rid of infinity by imposing a cut-off and then using renormalisation. What Effective Field Theory does is come up with a theory that up to a cut-off is the same as gravity and that trick will work. That's not quite it - and there are some things I am not up to speed with - its quite advanced - but it gives you an idea.

The following may also help:
http://en.wikipedia.org/wiki/Effective_field_theory

Thanks
Bill

Yes I totally got the idea. But note it is just a temporary fix. For example the above web mentions "General relativity itself is expected to be the low energy effective field theory of a full theory of quantum gravity, such as string theory.". We must aim for the full theory valid at highest energies.

Anyway. When we finally understand quantum gravity or quantum spacetime. Would or should it answer how the stress energy tensor is coupled to spacetime?

 

[bhobba]

Would or should it answer how the stress energy tensor is coupled to spacetime?

Who knows - maybe yes - maybe no. Its hard to predict future progress.

But I have to ask why it worries you? That GR basically follows from no prior geometry is very very beautiful and elegant. It's also very obvious. Sure its an assumption - but every theory has assumptions. The most you can ask is, is the assumption reasonable - and in this case it most assuredly is - in fact many, including me, think it is the most beautiful of all physical theories:
http://www.ias.ac.in/jarch/jaa/5/3-11.pdf

That's the problem with the standard model, and the EFT treatment of gravity. It contains parts of dazzling beauty - and other parts - a kludge (EFT Gravity is most definitely a kludge). What we want is everything with the beauty of GR. Mathematicians and physicists smell that nature should be like that - not a kludge. But exactly how that will be resolved - who knows. It may even be that nature is a kludge - that would be very disheartening - but nature is as nature is.

Thanks
Bill