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

[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 einstein 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