
#1
Feb1512, 12:42 AM

P: 381

Does any of our quantum gravity theories like String Theory or Loop Quantum Gravity (what else?) answer "why" mass/stress/energy curve Spacetime? Or do they just describe it a priori?
Note I'm not asking why mass/stress/energy curve Spacetime. I just want to know if there is a Quantum Gravity theory that describe why. Thanks. 



#2
Feb1512, 08:15 AM

Astronomy
Sci Advisor
PF Gold
P: 22,809

Personally I can't think of any QG theory that explains the interaction of matter with geometry. I've often wondered about that. There might be some underlying description of the world in which matter and geometry are fundamentally the same thing. Different aspects of the same mathematical model. Then their interaction might be understandable simply as how that single unified math entity behaves. You've probably heard about some of the stalled attempts. BilsonThompson et al (matter as braids in spin nets) was one. I haven't seen much in the way of papers for several years now, in braid matter. I'll try to think of others. Fascinating question. Several people here may know of ideas along these lines. Mitchell might. One approach would be to explore the possibility in 3D gravity (spatial 2D plus time) where things are simpler and try to get a toy model. Then the hope would be to jack that up to 4D. I think back in 2005 and 2006 there was some work by Laurent Freidel in 3D gravity where one got a glimpse of matter occurring as topological defects in geometry. If I get some time later today I will try to find a reference. Or somebody else may already know one. Have to do some other stuff now. 



#3
Feb1512, 10:10 AM

P: 216

Marcus wrote:
"I've often wondered about that. There might be some underlying description of the world in which matter and geometry are fundamentally the same thing. Different aspects of the same mathematical model. Then their interaction might be understandable simply as how that single unified math entity behaves." I think, it is very important and may be we have to search it. I would like to test an Information Universe where the space time is created by the interaction between the quantum information and we observe the geometry and the matter. Each interaction creates one quantum event and Planck time dilation, so we observe the flow of time and inertia. http://www.calphysics.org/articles/gravity_arxiv.pdf 



#4
Feb1512, 05:01 PM

P: 381

Why mass/stress/energy curve Spacetime?Searcing this archive. I found the BilsonThompson et al paper. But it doesn't mention about LQG yet you seem to say LQG can encompass it. Why is the paper a stalled attempt? Please elaborate. Thanks. http://www.physicsforums.com/showthread.php?t=225876 "http://arxiv.org/abs/0804.0037 Particle Identifications from Symmetries of Braided Ribbon Network Invariants Sundance BilsonThompson, Jonathan Hackett, Lou Kauffman, Lee Smolin 9 pages, 7 figures (Submitted on 1 Apr 2008) "We develop the idea that the particles of the standard model may arise from excitations of quantum geometry. A previously proposed topological model of preons is developed so that it incorporates an unbounded number of generations. A condition is also found on quantum gravity dynamics necessary for the interactions of the standard model to emerge." Then you commented in message #2 there: "So in a sense LQG contains the Standard Model particles as crisscross complications in the network that describes the quantum state of geometry. There has been some progress: braidmatter contains all three observed generations of particles. Indeed, the LQG particle model actually predicts an infinite series of generations (by increasingly complicated twisting and braiding.) Thus it would be favorable to LQG if the Large Hadron Collider were to detect evidence that the number of generations is not limited to three. the basic idea is that (the quantum state of) space is like a WEBa fine network of geometric relationships. And in the fabric of that web there can be little snaggies. And these little snaggly tanglets can propagate and interact with each other and make other snaggles. The way two knots can interweave with each other and make a third knot. The whole web evolves by local MOVES which reconnect neighbor nodes in different ways. The passage of time is realized by the constant progress of these local moves going on everywhere in the network. These local reconnectionmoves take care of the motion and interaction of particles, as well as the evolution of geometry as required by General Relativity. That's the idea. It is an ongoing program. You can read the paper and see how far they have come and what the next issues are that they want to investigate. __________________" Rovelli not working on it? Know any latest about it? Theory or model that would explain the coupling between matter and geometry is the real quantum gravity, the rest are imitations. 



#5
Feb1512, 05:33 PM

P: 381

I actually posted this thread at the relativity forum but the mods transferred it here.
I think String theory doesn't answer why mass/stress/energy couple to spacetime curvature nor does it say how the coupling is done. String Theory is just like QFT but using strings and some dynamics. I'm not sure about Loop Quantum Gravity whether it describes it. Whatever, a goal of quantum theory of gravity must not just be a theory to calculate the dynamics of a center of black hole or big bang. It must explain why mass/stress/energy couple to spacetime and how the coupling is done. Then when we know. We would become master of space and time and matter. Also I'm reading Brian Greene The Elegant Universe. It says: "To get a sense of scale, if we were to magnify an atom to the size of the known universe, the Planck length would barely expand to the height of an average tree." I have a question that I'd rather not start a new thread. Do you know how extremely tiny is the planck scale . Maybe by just assuming spacetime no longer exist there. Then no infinities would occur in calculations and no need to propose strings nor loops or spin networks. It's like asking if air can still occur in the atoms. It no longer exist at some point. Therefore maybe the simplest and instant quantum gravity is simply to assume spacetime ceases to exist at the planck scale. And things are solved. (?) Why is this not a solution? 



#6
Feb1512, 05:47 PM

Astronomy
Sci Advisor
PF Gold
P: 22,809

The two people that are most often invited to give overview talks or write current status /overview papers didn't pick up on this braid matter business. It was a highrisk gamble. I admire the courage of people who undertake and explore new stuff like this. But also value the judgement of more cautious deliberate ones. Most new ideas don't work out. I can't see the future so I can't say it won't suddenly come to life but right now it seems stalled.




#7
Feb1512, 05:55 PM

P: 381





#8
Feb2212, 04:08 AM

P: 381

The Renormalization Group suggests the Landau pole is due to some finite length and not infinite point and strings are being suggested. LQG is about spacetime being discrete. So is there no possibility that strings occur in the backdrop of LQG spacetime (since there is no clear coupling between quantum matter and geometry so both ends can be anything like strings and LQG spacetime)?




#9
Feb2312, 04:13 PM

P: 73

I will try to give an answer to your question. But remember, my interpretation of mathematical derivation of GRT used usual pictures to visualize it. So I want to simplify it (hopefully not to much)
At first lets start with string theory: Here you have a virbrating string forming a world surface, i.e. a 2dimensional manifold. This manifold is embedded into a background space (in the critical dimension of the string, the dimension is 26 or in the superstring case 10). The fact, that the string is embedded into the background space, gives a relation between the metric of the world surface and the metric of the background manifold. Both objects are not independent of each other. The effect can be claculated for small changes of the world surface generating a change of the background. In normal coordinates, this change is proportional (at first order) to the Ricci tensor. That answers your question because is also formed by the vibration of the string (as source of the world surface change). (I omit the QFT argumentation using the beta function) Now LQG: It is similar to string theory. The state is described by a spin network representing the possible curvature (equivalent to the holonomies by using the AmbroseSinger theorem). The spin network is a graph which must be embedded into some space. Matter(better fermions) is attached to the vertices of the spin network (which changes also the edges). A change of the matter changes the spin network and especially the edges between the vertices. But the edges are nothing then the holonomy expressing the curvature. 



#10
Feb2412, 05:07 AM

P: 23

mm, that's interesting claculation.
Considering the warp as worldsurface phenomena emenating from specific string vibrations, then theoretically it may be possible to induce the same effects through manipulation of the strings via the creation of mini blackholes. By experimentally inducing certain stringeffects upon the worldsurface paradigm, it is not out of the question that we may begin to establish artifical warps in SpaceTime. Upon verification of the above hypothesis, it may be possible to induce specific worldsurface warps by affecting peripheral strings with a perpetual mini blackhole. If we could contain the hypothetical blackhole on the forwardhelm of a space ship, then we could take advantage of the effects of special relativity (and gravitational time dilation,) to achieve space travel capacities. within the solarsystem, of course. 



#11
Feb2412, 05:12 AM

P: 23

*AF classified TS per DD214, 11/7/09; PEA7



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