Are There Viable Alternatives to Quantum Field Theory and Second Quantization?

[negru]

Most recently Witten is working on infrared issues of scattering amplitudes in string theory

 

[suprised]

Why is no one sharing on Supestrings in this forum?

There have been threads on this, you may need to invest a few minutes of search. But at any rate, this forum is by no means representative of what is actually happening in science. You should listen to actual scientists. It is the wrong place the get a fair impression of the field, as are those books.

 

[waterfall]

There never was a problem. Smolin was never able to prove that starting from actual LQG you could derive that, mathematically/logically.
And around 2005 he got several people interested in trying to prove it, by 2007 the main guy had given up. A talented mathematical physicist named Jerzy Kowalski-Glikman.
Basically Smolin had an intuitive feeling---sometimes he said the higher energy would travel faster. Sometimes other people said they would travel slower. But based on LQG they could never prove that the theory predicted any such thing.

So there was no problem that anyone needed to make go away. Just a logical void, some intuition, and talk. However the research did have a nice spin-off in some other directions (some work by Bee Hossenfelder and some by Laurent Freidel and others.)

There was, and still is, another theory called DSR (doubly special relativity) in some version of which you can get results like that, I believe, but it is not derivable from LQG. Separate theory.

I found the entire Lee Smolin article "The Atom of Space and Time" here:

http://www.phys.lsu.edu/faculty/pullin/sciam.pdf

Go to page 74. It is mentioned "In the past several years, however, a few imaginative young researchers have thought up new ways to test the predictions of loop quantum gravity that can be done now. These methods depend on the propagation of light across the universe. When light moves through a medium, its wavelength suffers some distortions, leading to effects such as bending in water and the separation of different wavelengths, or colors. These effects also occur for light and particles moving through the discrete space described by a spin network."

It is based on the idea of light traveling in a medium with wavelength distortion. How do you (or Rovelli counter this?)

Continuing: "A gamma-ray burst spews out photons in a range of energies in a very brief
explosion. Calculations in loop quantum gravity, by Rodolfo Gambini of the University of the Republic in Uruguay, Jorge Pullin of Louisiana State University and others, predict that photons of different energies should travel at slightly different speeds and therefore arrive at slightly different times [see illustration above]. We can look for this effect in data from satellite observations of gamma-ray bursts. So far the precision is about a factor of 1,000 below what is needed, but a new satellite observatory called GLAST, planned for 2006, will have the precision required."

 

[marcus]

I found the Gambini and Pullin reference for you, in case you are interested:
http://arxiv.org/abs/gr-qc/9809038
It's impressive how much the theory has evolved since 1998.

 

[marcus]

WF, you seem intelligent and motivated enough so that you could, I think, make a big improvement in quality by quoting only from peer-reviewed sources. And preferably ones that are reasonably current, at least from the past 5 or 6 years.

Personally I have a real high regard for Gambini and Pullin, and I'd guess that other people do as well (but I don't want to speak for others). Their 1998 paper was peer-reviewed and published in Physical Review series D.

They carefully qualified their calculation, making clear what assumptions it was based on. A formulation of LQG which, as it happens, was not covariant and I haven't heard much about for many years. And a proposed Hamiltonian dynamics which was already being criticised by the Loop researchers in 1998. Its author had abandoned it by 2003 or so and gone back to the drawing board. If you are interested in Loop history, as you seem to be, you can follow up some of the references in the Gambini Pullin article.

My feeling is that life is too short to go back and study a 1998 calculation based on now obsolete formulations. But you may be historically inclined and less interested in the situation NOW. It's a matter of taste.

If on the other hand you want to learn about the situation now then I'll suggest a peer-reviewed source that I think is accessible. The two people who come closest to speaking for the field as a whole, these days, are Ashtekar and Rovelli. When a journal wants a review article surveying the status, results, open problems, they get asked. When a conference or school is organized they typically get asked to give the introduction or main lectures.

(A conference will have some brilliant younger people to talk about specific research initiatives and results. It's like any area of physics. Every few years there are comprehensive survey articles written by recognized leaders. Most of the active younger researchers in the field have been PhD students and or postdocs working with a few central figures.)

So if you are interested in knowing something about the actual state of Lqg theory, and want help finding accessible peer-referreed overview stuff, or Loops conference videos that you can understand, just ask.

 

[waterfall]

A recent paper:
http://arxiv.org/abs/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle
(Submitted on 10 Jan 2012)
Spin-foam models are hoped to provide a dynamics for loop quantum gravity. All 4-d spin-foam models of gravity start from the Plebanski formulation, in which gravity is recovered from a topological field theory, BF theory, by the imposition of constraints, which, however, select not only the gravitational sector, but also unphysical sectors. We show that this is the root cause for terms beyond the required Feynman-prescribed exponential of i times the action in the semiclassical limit of the EPRL spin-foam vertex. By quantizing a condition isolating the gravitational sector, we modify the EPRL vertex, yielding what we call the proper EPRL vertex amplitude. This provides at last a vertex amplitude for loop quantum gravity with the correct semiclassical limit.
Comments: 4 pages

Some other recent papers:
https://www.physicsforums.com/showthread.php?p=3755045#post3755045

Has anyone actually read the above paper entirely? I can't understand the arguments. But based on your reading and by other experts, Is it agreed now by the majority that the semiclassical limit recovering general relativity has been shown to exist categorically in LQG? If so. How come I didn't hear this in the news that "LQG spin foam has recovered general relativity in the classical limit!". This is almost the same as saying LQG is on the right path! Can anyone find any weakness in any arguments in the paper that prevent it from making headlines just like Lisa Randall RS1 and RS2 papers which became talk of the town?

 

[marcus]

Has anyone actually read the above paper entirely? I can't understand the arguments. But based on your reading and by other experts, Is it agreed now by the majority that the semiclassical limit recovering general relativity has been shown to exist categorically in LQG? If so. How come I didn't hear this in the news that "LQG spin foam has recovered general relativity in the classical limit!". This is almost the same as saying LQG is on the right path! Can anyone find any weakness in any arguments in the paper that prevent it from making headlines just like Lisa Randall RS1 and RS2 papers which became talk of the town?

This is an example of why I suggest you read a careful balanced peer-reviewed summary, rather than rely on paraphrase and scuttlebut.
Read this. It will put things into perspective for you.
LQG the first 25 years (December 2010)
Rovelli is good at laying out the gaps and unresolved issues. Avoids hype. describes limitations and unfinished business. But also knows what progress has been made in the past 3 or 4 years.
This is an invited review article by a scholarly journal.

Google "loop first twenty-five years"
or go directly to http://arxiv.org/abs/1012.4707

 

[waterfall]

This is an example of why I suggest you read a careful balanced peer-reviewed summary, rather than rely on paraphrase and scuttlebut.
Read this. It will put things into perspective for you.
LQG the first 25 years (December 2010)
Rovelli is good at laying out the gaps and unresolved issues

Ok Thanks. http://arxiv.org/pdf/1012.4707.pdf (updated Jan. 31, 2012)

"Classical limit. There is substantial circumstantial evidence that the large distance limit of the theory is correctly general relativity, from asymptotic analysis and from large distance calculations of npoint functions and in spinfoam cosmology; and there are open directions of investigations to reinforce this evidence. The degrees of freedom are correct and the theory is generally covariant: the low-energy limit is not likely to be much else than general relativity. But there is no solid proof yet."

So there is no solid proof yet. Let's just wait for the breakthrough paper that will put it in the level of Einstein's "On the Electrodynamics of Moving Bodies."

I wonder if anyone here has read or own this book:

https://www.amazon.com/dp/0199590753/?tag=pfamazon01-20

"A First Course in Loop Quantum Gravity"

Please review it those who have read it esp as it talks about QFT too. Thanks.

 

[waterfall]

It's basic to the current formulation of LQG that geometry is a quantum field. The spin networks are quantum states of this field. So LQG is accord with the actual quote itself. But one can find that same message in many other places both antedating Dreyer and also more recent. I would separate that important idea from the 2004 article by Olaf Dreyer, and focus on the idea expressed in the quote, that the basic thing is the field.

Spacetime as such (e.g. some mathematical continuum, or differential manifold, some x,y,z,t space construction) does not appear in the current LQG theory. There is no mathematical object in the theory which you can point to and say that is space, or that is spacetime. It is not needed.

The fundamental object is the field---the quantum states of that field---namely geometry.
The basic philosophy is that a quantum theory is not about what nature IS but more accurately how nature responds to measurement.

In the case of geometry this means the network of geometrical measurements, including ones about which one may have only an expectation or a a probability amplitude. There may be indefiniteness.

In that sense in LQG (see http://arxiv.org/abs/1102.3660) "spacetime" is purely emergent. The focus is on the web of interrelated geometrical measurements (distances, angles, areas, volumes, durations) that specify quantum states of geometry.

Mind you I'm not talking about the various miscellaneous earlier LQG formulations or what somebody said in 2004 or 2006. There is too much variation to keep track of or generalize about including a kind of revolution that started around 2008. I'm just talking about the current formulation (in the Zakopane Lectures I linked to) which many people seem to think encompasses the Loop mainstream of the past 3 or 4 years.

Thanks for attributing expertise, waterfall I watch the current QG research scene with active interest, but am not an authority. I also follow cosmology and AsymSafe QG research, not only Loop, but can't claim to be an expert!

Marcus, I don't really have a good mathematical background only knowing calculus concepts by words only and don't do any calculations. Therefore I can't understand any of the paper at its core. This is my weakness. Therefore I just want to understand something. Many of us laymen just want a bird eye view without necessary having to take 7 years course or spent that much to master the rigorous math.

What I want to know is this. In Superstrings, the gravitons produce effects similar to the curvature of General Relativity. So the curvature is not really there in 4D spacetime but only effects brought about by gravitons. As way of illustration. For example. If you have 4D vision, you would see the spacetime curvature in General Relativity. But with the same 4D vision aiming at universe with Superstrings, you won't see any curvature of space and time but only see gravitons making it appear like there is curvature. Now aiming the same 4D vision at Loop Quantum Gravity. Is it supposed to recreate the metric of General Relativity as in really there in 4D spacetime, or do the dynamics of the spin foams recreate the effects of gravity just like gravitons without necessarily having to actually create the curvature in 4D spacetime? This is what I'm not sure about.

We just need this concept for now so as to appreciate the difference and the aim of LQG. Again, I and general laymen didn't have the math training to even understand any of Rovelli paper at its core so a bird eye view is enough to introduce us what's the programme all about in the metric level.

 

[marcus]

Marcus, I don't really have a good mathematical background only knowing calculus concepts by words only and don't do any calculations. Therefore I can't understand any of the paper at its core. This is my weakness. Therefore I just want to understand something. Many of us laymen just want a bird eye view without necessary having to take 7 years course or spent that much to master the rigorous math.

What I want to know is this. In Superstrings, the gravitons produce effects similar to the curvature of General Relativity. So the curvature is not really there in 4D spacetime but only effects brought about by gravitons. As way of illustration. For example. If you have 4D vision, you would see the spacetime curvature in General Relativity. But with the same 4D vision aiming at universe with Superstrings, you won't see any curvature of space and time but only see gravitons making it appear like there is curvature. Now aiming the same 4D vision at Loop Quantum Gravity. Is it supposed to recreate the metric of General Relativity as in really there in 4D spacetime, or do the dynamics of the spin foams recreate the effects of gravity just like gravitons without necessarily having to actually create the curvature in 4D spacetime? This is what I'm not sure about.

We just need this concept for now so as to appreciate the difference and the aim of LQG. Again, I and general laymen didn't have the math training to even understand any of Rovelli paper at its core so a bird eye view is enough to introduce us what's the programme all about in the metric level.

Strange idea! Who told you that? Do the gravitons also make it appear that space is expanding? And expanding at different rates at different times and places? Do they make the expansion appear accelerate by various amounts, but it isn't really accelerating?

Sounds like someone sold you a load of bunkum, WF.

 

[marcus]

WF are you saying you can't read 1012.4707?

That paper has slightly less than two equation per page. It is by far mostly words, and very much written for the audience of non-specialists (not Loop community) to understand.

Maybe by mistake you were looking at 1102.3660. Try the other again. I am convinced you can get a fair amount of it. Skip the equations, most of the message is in plain English.

So as not to have to remember the number, my way is to google "loop first twenty-five"

 

[waterfall]

WF are you saying you can't read 1012.4707?

That paper has slightly less than two equation per page. It is by far mostly words, and very much written for the audience of non-specialists (not Loop community) to understand.

Maybe by mistake you were looking at 1102.3660. Try the other again. I am convinced you can get a fair amount of it. Skip the equations, most of the message is in plain English.

So as not to have to remember the number, my way is to google "loop first twenty-five"

Ok. I don't understand any math other than how to multiple or divide or add in groceries. I don't know how to do calculus so all those equations even simple ones are foreign to me. So we normal laymen of these nature are hopeless to understand them?

We understand by concepts and analogies and words. This is why hundreds of QM books for laymen don't have equations. Well. We know General Relativity as 4D spacetime is just a model.. a model of our world. What perflexed me is how come Loop Quantum Gravity is trying to emulate the model... GR is just a model. What if our world is described by other nature. Then LQG will be at loss for trying to emulate a non-existent GR manifold world. Get the point? This is why I'm asking if LQG is like this where if one uses a 4D vision, one would see a 4 dimensional spacetime.. or none.

 

[marcus]

what I'm saying is there is an awful lot there you can read and understand without any math.
Only two equations per page. And those are paraphrased in words for people who like that, and included in symbols for people who like that.

as far as geometry goes I think you are misinformed if you think string is not done in curved spacetimes. Spacetime curvature is at the heart of the graviton idea. The graviton is the quantum of a certain field, and that field is the (curved, rippling, expan. contra...etc) geometry.

all approaches to QG (that I know of anyway) involve some representation of geometry, some pretty novel! like "causal sets" and "quantum graphity", and the geometrical relations are dynamical in some fashion, in all of them. That's basically what it's about.

there is a kind of pragmatism about that, because the big puzzles that confront us are puzzles of geometry, the accelerating expansion, the funny distorted picture of the early U that we see in the sky, the strange fun-house lensing by invisible matter that we only see because of how it distorts what we look at through it.
when you want to solve geometry puzzles, and explain curious geometric stuff going on, the natural straightforward thing to do is to construct theories of geometry and try them out.

 

[waterfall]

what I'm saying is there is an awful lot there you can read and understand without any math.
Only two equations per page. And those are paraphrased in words for people who like that, and included in symbols for people who like that.

I also don't know the math of GR so can't know what certain paragraphs are talking about. Anyway, the following is simply what I want to know.

as far as geometry goes I think you are misinformed if you think string is not done in curved spacetimes. Spacetime curvature is at the heart of the graviton idea. The graviton is the quantum of a certain field, and that field is the (curved, rippling, expan. contra...etc) geometry.

Superstrings are in certain fixed spacetime or unknown spacetime but the gravitons can approximate the curvature of spacetime in General Relativity. We who are on the surface can't know the difference because we would only feel the gravity and not directly the gravitons or curvature. Supposed just for sake of discussion we use a 4D vision to perceive it. We can know the difference because if we don't see any curvature inspite of the 4D vision, then there is no curvature and the gravitons are like just the photons in electromagnetic wave transfering the forces and in the former case the force of gravity. Note the dualism between electromagnetic wave and photon, they can't be merged together. Hence the gravitons are not equal to the curvature although the results can be the same for those who don't have 4D vision..

GR is like a map to a territory. GR is not the territory. Now what perflexed me about LQG is it seems to be trying to re-create the map. Why doesn't it directly deal with the territory?? A map is just a model of the terrority, the map or model could be wrong. So why does LQG have to follow the map and not directly the terrority.. or maybe it does? This is what I'm not sure about hence asking.

 

[waterfall]

Marcus,

I saw the arguments from Bill Hobba in sci.physics (he is also here so maybe he can clear it up. Someone asked him there "But in string theory, spacetime still has curvature." Bill Hobba replied all the following:

"No it doesn't. It emerges as a limit - but the underlying geometry of space-time - if it has one - is not known.|

"As Steve Carlip once explained, it is experimentally impossible to tell a theory formulated in flat space-time that makes rulers and clocks behave as if it was curved from a curved one, so the question is basically meaningless at our current level of knowledge."

"Up to about the plank scale the assumption it is flat is fine, with gravitons making it behave like it had curvature or actually giving it curvature (we can't determine which) works quite well. "

"Gravitons interact with all matter-energy. They interact in such a way as to make rulers and clocks behave as if space-time had curvature. It is a semantic issue of zero scientific value if space-time is thus curved or just appears curved. At this stage their is no way to experimentally distinguish between the two views."

Got it Marcus? Hence when I mentioned using 4D vision, it can determine whether it's curved or not. Hence in Superstrings, intrinsic spacetime curvature is not definite.

Now going to LQG. Is it trying to recreate the 4D spacetime curvature as really there existing when one uses a 4D vision or is it like in Strings (in the arguments above), where it makes spacetime appears curves by emulating it by other dynamics (which if we would use 4D vision wouldn't see any intrinsic spacetime curvature)?

 

[marcus]

WF it seems to me that you do not a one "map" and a "territory", you have two maps. One is GR, which has been tested to exquisite accuracy in a lot of subtly different ways and fits nature remarkably well. The other map is something you (or Bill Hobba? don't know of him) have made up---it does not correspond to string theory or anything else I know. In this map, things called "gravitons" are responsible for all the geometric effects including those I mentioned. Expansion, inflation, accelerated expansion, black hole collapse, the gravitational field outside the BH horizon. I mentioned some others...

Your second map, that you call "territory" would have to be formulated exactly in order to be tested and would have to be tested (as GR has been) and my guess is would turn out to be a dud. Everything the whole universe, would be happening in some fixed eternal Euclidean space, and everything includes BH collapse. Your theory would then have to explain how a "graviton" gets from the heart of a black hole out past the horizon to exert a "pull" on somebody orbiting the BH. And all the stuff about how the clock on the mountain top runs faster than the one in the valley. I guess because the "gravitons" slow clocks down.

Basically I'm skeptical of your second map. Ask Hobba to give you a reference to the paper by Steve Carlip and see exactly what Carlip said. I doubt a Euclidean cosmology (with "gravitons") has ever been formulated in a way that comes near matching what we observe. But I think it is probably dear to your heart and you are not going to change your ideas. So AFAICS we have to agree to disagree on that. Agreed?

 

[atyy]

What distinguishes strings and LQG are not gravitons. Any theory of quantum gravity must have gravitons. Gravitons are quantized excitations of the gravitational field at low energies. The difference between strings and LQG is whether the gravitational field still exists at high energies. Strings says no, canonical LQG tries to say yes. In this respect, canonical LQG is similar to Asymptotic Safety.

 

[waterfall]

WF it seems to me that you do not a one "map" and a "territory", you have two maps. One is GR, which has been tested to exquisite accuracy in a lot of subtly different ways and fits nature remarkably well. The other map is something you have made up---it does not correspond to string theory or anything else I know. In this map things called "gravitons" are responsible for all the geometric effects including those I mentioned. Expansion, acceleration, black hole collapse, the gravitational field outside the BH horizon.

Your second map, that you call "territory" would have to be formulated exactly in order to be tested and would have to be tested (as GR has been) and my guess is would turn out to be bunk. Everything would be happening in some fixed eternal Euclidean space, and everything includes BH collapse. Your theory would then have to explain how a "graviton" gets from the heart of a black hole out past the horizon to exert a "pull" on somebody orbiting the BH. And all the stuff about how the clock on the mountain top runs faster than the one in the valley. I guess because the "gravitons" slow clocks down.

Basically I'm skeptical of your second map. I doubt it has ever been formulated in a way that comes near matching what we observe. But I think it is probably dear to your heart and you are not going to change your ideas. So AFAICS we have to agree to disagree on that. Agreed?

Of course I didn't make up the second map. It's from Bill Hobba as detailed in post #105 above. And he didn't invent it either.. but got it from Steve Carlip. So basically.

1. We have pure GR
2. Gravitons in Superstrings can cause curvature (whether intrinsic we don't know as per Carlip or Hobba)
3. In LQG.. is it trying to re-create GR? or is the effect like in the second where it is emulating the curvature (when there may not even be intrinsic curvature)? This is what I simply want to know. I'm not taking any sides. Just want to know what is really the case. I can't get this specific answer from any Rovelli papers so hope someone familiar with it can directly address this whole thing.

 

[marcus]

See my post #106, when I heard you were channeling sci.physics and someone named Hobba *paraphrasing* Steve Carlip I made corrections in #106.

Atyy's post is also relevant. Get the Carlip exact page reference if you want. I like Carlip, heard him give a seminar talk here at Berkeley a year or two ago. He's written many good QG papers. One recent one I liked comparing Loop, Triangulations, Safe, Horava, and even classical. My guess is he would have said the two models are indistinguishable ONLY in an approximate limited local sense, not globally. Reservations often get lost in paraphrase (esp. when the paraphraser is not giving links to references.) So if you want to pursue it with Hobba, get his online Carlip link with a page reference, so you don't have to go thru reams of stuff.

I'll leave you to work this out with whoever is interested in representing the cosmos (and its wonders) in a fixed flat eternal Euclidean space. Good luck

 

[waterfall]

Marcus, first of all. I'm unbiased. I'm only interested in what the evidence or theoretical evidences lead.

Second I still can't understand exactly what Bill Hobba is saying. He uses textbooks as referenced (mentioned below) so it's not his own creation but actually from the string theories themselves. For example the following conversations at sci.physics in the thread "Non-geometric approach to gravity impossible?"?

Hobba said and with reference:

"Gravity in flat space-time , otherwise known as linaerised gravity, is easily constructed based on EM - See Ohanian and Ruffini - Gravitation and Space-time. Trouble it it contains the seeds of its own destrcution. It can be shown that particles moves as is space-time had an infinitesimal curvature and its gauge invarience is infinitesimal coordinate transformation. The obvious consequence leads immediately to GR."

"It has long been known that a quantum theory of gravity as spin two particles in a flat space-time leads to GR eg the link I seem to have to give over and over:

http://arxiv.org/abs/gr-qc/9512024 "

"I suggest you think a bit clearer. A membrane as a continuum and treated by the methods of continuum mechanics emerges as a limit from the atomic structure of an actual membrane - yet does not imply it is a continuum at the level of individual atoms. The same with GR. Gravity as space-time curvature emerges from spin two gravitons when the underlying geometrical background is not known, but usually assumed to be Minkowskian flat, so the methods on QFT theory can be applied."

Someone asked Bill:

> How do the gravitons of the entire Earth conspire and coordinate
> in such a way that gravitational mass is the same as inertial
> mass as well as forming consistent geometry.

Bill replied: "It is a prediction of the model, the same way as Euclid's fifth conspires to
ensure the angles of a triangle add up to 180%, or even the rules of arithmetic conspire to ensure the amount of your bank balance is really the number of dollars and cents you actually have. Absolutely foundational and fundamental understanding of science and scientific modelling.

As previously explained, the specifics in this case, are when you mathematically analyse the linear equation, it shows, even though you assume space-time is flat, particles move as if it had an infinitesimal curvature. Also its gauge symmetry is infinitesimal coordinate transformations. In developing the linear equations an assumption was made - namely since gravity interacts with all mass-energy, and gravity itself has energy, it must interact with itself - this means the equations are non linear. So the assumption of the linear equations is gravity is weak enough that its interaction with itself can be ignored. To remove that restriction, the very reasonable hypothesis is made, that space-time is curved, and the equations are invariant - exactly as the analysis of the linear equations
suggest. When this is done the mathematics shows that GR inevitably results - exactly as Euclid's fifth forces the angles of a triangle to add up to 180%. It is this assumption that does the 'conspiring' you refer to. It is required for the equations to make reasonable sense. If you can not comprehend that a theories logical consistency allows a theory to make predicitons that seem almost like magic, just like Euclidian geometry does, then you have not grasped what 10 year olds who are taught Euclidean geometry are able to grasp, and I am afraid physics is beyond you.


The details can be found in Gravitation and Space-time by Ohanian and
Ruffini
https://www.amazon.com/dp/0393965015/?tag=pfamazon01-20


Bill"

This are all standard Superstring concepts, no? Bill Hobba is a member of physicsforums so hope he can clarify.

 

[waterfall]

I was able to track where Steve Carlip stated it. It's in the thread sci.physics.research which is moderated like physicsforums.

http://groups.google.com/group/sci....Einstein+Field+Equations+and+Flat+Space+Time#

> Do you have a reference that goes into this in greater detail? I am
> taking a break from work and have a bit of time to check into some
> things that have been on my mind. I will be going through Feynmans
> Lectures on Gravitation


That's a good place to start. Two standard papers are by Deser,
Gen. Rel. Grav. 1 (1970) 9 and Class. Quant. Grav. 4 (1987) L99,
which deal with classical calculations. You might also look at a
paper by Boulware and Deser, Ann. Phys. 89 (1975) 193, for a more
quantum field theoretical argument (based on earlier work by
Weinberg). You might also look at two articles, by Duff and Deser,
in the book _Quantum Gravity: An Oxford Symposium_ (edited
by Isham, Penrose, and Sciama, Clarendon Press, 1975).

Steve Carlip "

Now I'm confused enough to maybe start a thread at the relativity forum. But if you see this Bill Hobba. Please comment on all this like how many string theories take this view as well as latest from Steve Carlip.

 

[waterfall]

Looking at this matter further. I found out it was not even original claim by Steve Carlip but direct from Misner, Thorne, & Wheeler's book "Gravitation". I saw the following in Physicsforums:

https://www.physicsforums.com/showthread.php?t=278874

"Is spacetime really curved? Embedded somewhere?

Message #4:

"There's a fascinating analysis due to Deser ["Self-interaction and
gauge invariance", General Relativity & Gravitation 1 (1970), 9-18;
see also his later paper "Gravity from self-interaction in a curved
background", Classical and Quantum Gravity 4 (1997), L99-L105],
summarized in part 5 of box 17.2 of Misner, Thorne, & Wheeler's book.

Quoting from that latter summary:

"The Einstein equations may be derived nongeometrically by
noting that the free, massless, spin-2 field equations
[[for a field $\phi$]]
[[...]]
whose source is the matter stress-tensor $T_{\mu\nu}$, must
actually be coupled to the \emph{total} stress-tensor,
including that of the $\phi$-field itself.
[[...]]
Consistency has therefore led us to universal coupling, which
implies the equivalence principle. It is at this point that
the geometric interpretation of general relativity arises,
since \emph{all} matter now moves in an effective Riemann space
of metric $\mathcal{g}^{\mu\nu} = \eta^{\mu\nu} + h^{\mu\nu}$.
... [The] initial flat `background' space is no longer observable."

In other words, if you start off with a spin-2 field which lives on a
flat "background" spacetime, and say that its source term should include
the field energy, you wind up with the original "background" spacetime
being *unobservable in principle*, i.e. no possible observation can
detect it. Rather, *all* observations will now detect the effective
Riemannian space (which is what the usual geometric interpretation of
general relativity posits from the beginning)."

Comment?

 

[waterfall]

Check out the full arguments here in Misner, Thorne, Wheeler "Gravitation":

http://www.scribd.com/doc/81449908/Flat-spacetime-Gravitons

See the starting lines at :
5. Einstein's geometrodynamics viewed as the standard field theory for a field of spin 2 in an "unobservable flat spacetime" background

(body of arguments)

ending at
"
...[The] initial flat 'background' space is no longer observable." In other words, this approach to Einstein's field equation can be summarized as "curvature without curvature" or - equally well - as "flat spacetime without flat spacetime"!"

Marcus and other Quantum Gravity fellows. This is not my idea nor Hobba nor Carlip but from the grand textbook on Gravitation therefore please address it. I need to how LQG make use of the concept (if at all)... or rather I still don't know how to tie it to LQG.

 

[atyy]

Check out the full arguments here in Misner, Thorne, Wheeler "Gravitation":

http://www.scribd.com/doc/81449908/Flat-spacetime-Gravitons

See the starting lines at :
5. Einstein's geometrodynamics viewed as the standard field theory for a field of spin 2 in an "unobservable flat spacetime" background

See my post #107.

 

[marcus]

Check out the full arguments here in Misner, Thorne, Wheeler "Gravitation":

http://www.scribd.com/doc/81449908/Flat-spacetime-Gravitons

See the starting lines at :
5. Einstein's geometrodynamics viewed as the standard field theory for a field of spin 2 in an "unobservable flat spacetime" background

(body of arguments)

ending at
"
...[The] initial flat 'background' space is no longer observable." In other words, this approach to Einstein's field equation can be summarized as "curvature without curvature" or - equally well - as "flat spacetime without flat spacetime"!"

Marcus and other Quantum Gravity fellows. This is not my idea nor Hobba nor Carlip but from the grand textbook on Gravitation therefore please address it. I need to how LQG make use of the concept (if at all)... or rather I still don't know how to tie it to LQG.

Someone should express appreciation for the *scholarship*. In your past 3 or 4 posts you have provided links so one can see where you are coming from! I found the discussion in the PHYSICSFORUMS thread which you link to here helpful. But you only quote post #4. Read on thru to the end of the thread.

One of the guys is making the distinction between local and global. There are derivations and equivalences you can establish in a local neighborhood which do not necessarily extend over the whole. Topological considerations enter---the difference between an infinite plane, a sphere, and a donut. And so on.
The Einstein Field Equation of GR is local. It's only part of the GR picture that comes from studying global solutions. One or more of the people in that Physicsforums thread brought out that distinction and mentioned socalled FRW solutions.

I think maybe it was Poincaré who said something like no mathematics is TRUE it's only CONVENIENT. If no experimental observation can distinguish between two models then it is meaningless to ask which is true. You simply use the one that is more convenient.

In this case one of the approaches has the inconvenience that it applies locally but not necessarily globally (I mentioned some things earlier like collapse to a black hole.) From what I've seen of Steve Carlip's papers, or for that matter Wheeler, Misner etc, the approach where you have geometry is found to be more convenient.

I tend to think of the other way as a mathematical curiosity pointed out by some people in the 1970s which might occasionally be used in "effective" field theory (I don't know that it is but certainly could be.) It would necessarily be used in limited local situations, I should think. Maybe some other posters know of instances.

Looking at this matter further. I found out it was not even original claim by Steve Carlip but direct from Misner, Thorne, & Wheeler's book "Gravitation". I saw the following in Physicsforums:

https://www.physicsforums.com/showthread.php?t=278874

"Is spacetime really curved? Embedded somewhere?

Message #4:

"There's a fascinating analysis due to Deser ["Self-interaction and
gauge invariance", General Relativity & Gravitation 1 (1970), 9-18;
see also his later paper "Gravity from self-interaction in a curved
background", Classical and Quantum Gravity 4 (1997), L99-L105],
summarized in part 5 of box 17.2 of Misner, Thorne, & Wheeler's book.

Quoting from that latter summary:

"The Einstein equations may be derived nongeometrically by
noting that the free, massless, spin-2 field equations
[[for a field $\phi$]]
[[...]]
whose source is the matter stress-tensor $T_{\mu\nu}$, must
actually be coupled to the \emph{total} stress-tensor,
including that of the $\phi$-field itself.
[[...]]
Consistency has therefore led us to universal coupling, which
implies the equivalence principle. It is at this point that
the geometric interpretation of general relativity arises,
since \emph{all} matter now moves in an effective Riemann space
of metric $\mathcal{g}^{\mu\nu} = \eta^{\mu\nu} + h^{\mu\nu}$.
... [The] initial flat `background' space is no longer observable."

In other words, if you start off with a spin-2 field which lives on a
flat "background" spacetime, and say that its source term should include
the field energy, you wind up with the original "background" spacetime
being *unobservable in principle*, i.e. no possible observation can
detect it. Rather, *all* observations will now detect the effective
Riemannian space (which is what the usual geometric interpretation of
general relativity posits from the beginning)."

Comment?

I think some relevant comment is contained in the posts that follow #4. By starting with a flat background you rule out big bang and black hole stuff. Also rule out one of the more common spatially finite versions of standard cosmology. As I recall someone in the thread was pointing that out. Basically it is inconvenient, one could say crippling, to start out that way but you can recover a sector of the geometric theory, at least locally.

I'd say no QG approach has to explicitly deal with this special flat model because it empirically indistinguishable where it applies. (and since it doesn't cover all the cases it would be a bother---so people normally use the full theory.) but mathematically interesting certainly.

You've got great search technique! You are supplying great links to sources. Congratulations and thumbs up for that. Thanks for taking trouble.

Hopefully others will comment.

 

[martinbn]

Any theory of quantum gravity must have gravitons.

I've heard something like that many times, why is it?

 

[atyy]

One of the beautiful things about gravity as spin 2 on flat spacetime is that you can derive the equivalence principle. In the curved spacetime view, this has to be postulated.

http://phys.columbia.edu/~nicolis/GR_from_LI_2.pdf
http://arxiv.org/abs/1007.0435v3 (section 2.2.2 and Appendix A)

 

[waterfall]

Someone should express appreciation for the *scholarship*. In your past 3 or 4 posts you have provided links so one can see where you are coming from! I found the discussion in the PHYSICSFORUMS thread which you link to here helpful. But you only quote post #4. Read on thru to the end of the thread.

What's weird is that beginning post #4, they were not originally discussed here at physicsforums but the moderators somehow acquired them and appended here from the sci.physics.research thread called:

"Einstein Field Equations and Flat Space Time Options"

See the original thread here including the replies of Steve Carlip which the above cut out:

http://groups.google.com/group/sci....equations+may+be+derived+nongeometrically+by#

One of the guys is making the distinction between local and global. There are derivations and equivalences you can establish in a local neighborhood which do not necessarily extend over the whole. Topological considerations enter---the difference between an infinite plane, a sphere, and a donut. And so on.
...
Hopefully others will comment.

Well. I had this misconception for the past 5 years. I thought it was standard in string theory and know now it is not. Thanks for pointing that out. It will be a new chapter of understanding for me.

 

[martinbn]

One of the beautiful things about gravity as spin 2 on flat spacetime is that you can derive the equivalence principle. In the curved spacetime view, this has to be postulated.

http://phys.columbia.edu/~nicolis/GR_from_LI_2.pdf
http://arxiv.org/abs/1007.0435v3 (section 2.2.2 and Appendix A)

Atyy, was that a response to my question? I should probably explain. I've heard that any quantum field theory, which contains massless particle of spin 2 contains gravity. i would be interested to see that too, but to me it seems that it applies only to quantum field theries, at least as stated, and it doesn't say anything about other type of theories. For example string theory, it is not exactly quantum field theory, right? And often it is said that it is a theory of quantum gravity, but why? Does the statement apply here?

 

[atyy]

Atyy, was that a response to my question? I should probably explain. I've heard that any quantum field theory, which contains massless particle of spin 2 contains gravity. i would be interested to see that too, but to me it seems that it applies only to quantum field theries, at least as stated, and it doesn't say anything about other type of theories. For example string theory, it is not exactly quantum field theory, right? And often it is said that it is a theory of quantum gravity, but why? Does the statement apply here?

No, that wasn't a reply. Basically, gravity as spin 2 already works at low energies. Any new theory must reduce to a working old theory in the appropriate regime. Carlip has some references in here http://arxiv.org/abs/gr-qc/0108040, try searching for "Donoghue".

Hence, one way to see if LQG works is to see if it reproduces the graviton propagator http://arxiv.org/abs/0905.4082.