# Loop Quantum Gravity (LQG)

Discussion in 'Physics & Math' started by Fork, Aug 15, 2013.

1. ### ForkBannedBanned

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Wiki has this to say in its section on Loop Quantum Gravity:

Loop Quantum Gravity appears to be agreeable. Definitely agreeable. Reality as a path integral is interesting.

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3. ### ForkBannedBanned

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Feynman made an interesting comment on Einstein's losing sight of the Unified Field Theory:

“Dick [Feynman] fought back against my skepticism, arguing that Einstein had failed because he stopped thinking in concrete physical images and became a manipulator of equations. I had to admit that was true. The great discoveries of Einstein’s earlier years were all based on direct physical intuition. Einstein’s later unified theories failed because they were only sets of equations without physical meaning. Dick’s sum-over-histories theory was in the spirit of the young Einstein, not of the old Einstein. It was solidly rooted in physical reality.”

http://nige.wordpress.com/2010/08/1...-feynmans-path-integral-quantum-field-theory/

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5. ### AlphaNumericFully ionizedRegistered Senior Member

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The notion of physical systems being described by path integrals goes back to the 30s and the origins of quantum field theory, culminating in the Feynmann formalism of quantum electrodynamics, so LQG isn't the only application of such concepts. Similarly string theory views space-time as a seething mass of looped strings flittering in and out of existence, rather than some empty background things happen in. Unlike LQG string theory successfully recovers general relativity when you 'zoom out' of the atomic scale of space-time, despite LQG originally developed via a GR methodology.

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7. ### Farsight

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I got your message, Fork. I find it hard to say much about this. I don't know much about Loop Quantum Gravity. I think quantum gravity is agreeable provided one appreciates that virtual gravitons and virtual photons are two sides of the same coin, but I'm not sure that a spin foam has any underlying reality.

8. ### brucepValued Senior Member

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So that's pretty interesting. LQC doesn't recover GR in the classical domain of applicability? It sounds like the reason is the manifold isn't dynamic? Since this thread is about quantum gravity it would be interesting to read your summation on some of the accomplishments associated with string theory research and where it's at on the path to finalizing a complete theoretical model. The only QM I studied, in a semi-academic way, is QED.

9. ### ForkBannedBanned

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Thanks for you responses guys. I have a little background in Physics but nothing to call home for. I find that Feynman's explanation of the behaviour of virtual particles was one of the biggest insights for me personally. I wouldn't have been able to reach certain understandings without it. It's truly great and I only expect to get better.

10. ### UndefinedBannedBanned

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Hi AlphaNumeric, brucep or anyone else who knows the current up-to-date scuttlebut regarding the following...

Has there been any 'modification' to the QM theory concept/hypothesis of "Graviton" in theoretical circles which has not been made widely known as yet? Or is the 'original' hypothesized "Graviton" still as was thought of from the start? Thanks in advance; will much appreciate any heads up from anyone on that, either way!

Last edited: Aug 16, 2013
11. ### Farsight

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Fork, make sure you read this article on virtual particles by Matt Strassler. He's a physics professor. He says this:

"The best way to approach this concept, I believe, is to forget you ever saw the word “particle” in the term. A virtual particle is not a particle at all. It refers precisely to a disturbance in a field that is not a particle."

A virtual photon isn't a real photon that pops into existence and then disappears, like some kind of magic. Matt says a real particle is a nice, regular ripple in a field, so imagine this field is a grid. Imagine a ripple passes through the grid from left to right. That's a real photon. Now imagine the grid isn't moving, but instead is distorted, a bit like this picture of a vector field. Imagine that this is the electron's electromagnetic field. Any grid element that's twisted is a virtual photon. Hence Matt says "A 'virtual particle', generally, is a disturbance in a field that will never be found on its own, but instead is something that is caused by the presence of other particles, often of other fields". When an electron and a proton are attracted to one another they're said to exchange virtual particles, but they aren't actually throwing real photons at one another. Hydrogen atoms don't twinkle. Instead the distortions in the field between them change.

Undefined: I don't know of any official modification, but once you know about the above along with spin1 and spin2, IMHO it's pretty obvious.

12. ### AlphaNumericFully ionizedRegistered Senior Member

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An electromagnetic field is not a vector field, it is a rank 2 tensor field. What we see as electric and magnetic vector fields are different aspects of the underlying field, and those aspects change when you change frames.

Photons ARE spin 1. Looks like you don't know about them either but then that's hardly surprising given all you have is arm waving and oft-inappropriate pictures.

It handles dynamic space-times but the rules by which the space-time behaves do not become the Einstein field equations in the classical limit. Another example is how LQG can recover the entropy of a black hole is proportional to its area but it cannot compute the proportionality constant. String theory can, both from a semi-classical construction and from a micro-state ensemble point of view using strings.

String theory has within it identified formalisms which arise in pretty much every other area of high energy physics. It can handle inflation models, as seen in cosmology. It can handle dynamical space-time which obeys the Einstein field equations, as seen in GR. It can handle quantised particles, as seen in QFT. It can handle semi-classical field theory such as black hole thermodynamics. It can handle condensed matter physics such as superconductors and Bose-Einstein condensates via gravity/gauge duality. It can even handle consistent quantised gravitational particles, gravitons.

The problem is getting all of these together in a single manageable formalism which is able to be aligned with experiments. The problem isn't so much "Can string theory handle all of these things?" but rather "How do we deal with the fact string theory handles too many systems?". It's a problem of why the universe is as it is when string theory can handle so many possible alternatives just as easily.

The graviton is still viewed as the spin 2 quantisation of the gravitational field whose quantum dynamics should tend to the Einstein field equations in the classical limit. There are different models of it but all of them have that basic definition in mind.

13. ### ForkBannedBanned

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http://en.wikipedia.org/wiki/Quantum_field_theory

Reality is an excitation of reality?

14. ### AlphaNumericFully ionizedRegistered Senior Member

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I think you're 'abusing terminology' a bit there. Particles are excitations within quantum fields, according to quantum field theory. Virtual particles are excitations as well, identical in mathematical structure to normal particles except they don't obey the special relativity condition of $E^{2} = (mc^{2})^{2} + (pc)^{2}$. We never observe such particles directly but their existence can be inferred by the impact they have on particles we do observe, such as corrections to the fine structure constant, g-2 of the electron, Casimir force etc. Pretty much all of the stuff in quantum field theory which isn't in 'standard' quantum theory is in some way a manifestation of virtual particle processes.

And as a general comment, seeing as you're relatively new here, don't take at face value anything Farsight says about any domain of physics, even when he's claiming to be talking about accepted physics. Each and everything he says you should corroborate from a trusted source before accepting it since he has a habit of weaving his own opinion into his descriptions of physics, even mainstream physics. You'll find plenty of threads in the archives where he's shown to be woefully misinformed when it comes to even standard undergrad science. Plus he's practically innumerate so don't trust anything he says about mathematics without corroboration either.

15. ### Farsight

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No. Reality is space with waves running through it, affecting that space. You'll hear about photons being an excitation in the "photon field", but you can't separate this from space. And photons aren't point-particles, they're singleton electromagnetic waves which might have a wavelength of a kilometre or more. It's the same kind of thing for the "electron field". You can diffract electrons, and in atomic orbitals "electrons exist as standing waves". You can't separate the electron field from the photon field, or from space either.

This old Einstein presentation is worth a read: History of Field Theory. See this bit, where he's talking about the electromagnetic field and the gravitational field: "The two types of field are causally linked in this theory, but still not fused to an identity. It can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds". Einstein thought of a field as "a state of space". I can't think of a better way to put it myself.

The question I'd ask if I were you is What's a spin foam made out of? Or the strings Alphanumeric referred to. And as for my views as opposed to his, make sure you read Matt Strassler's article on virtual particles. Alphanumeric thinks they're real particles. They aren't. Hydrogen atoms don't twinkles, and magnets don't shine.

16. ### brucepValued Senior Member

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Thanks for these fundamental details about the theoretical model you and your peers have been working on. This is really interesting for me. So you have a model which can do all the science you mentioned but it has the annoying capacity to predict natural phenomena that doesn't exist in this universe. It's much bigger than a description of this universe. For me that is an exciting extra. A theoretical model which could describe the physics in a different universe from ours. Maybe the physics for each individual universe in the 'multiverse' of Eternal Inflation. Easy for me to say since my opinion has no bearing on the scientific literature. In my opinion it would be disappointing if string theory was only capable of describing the physics of this universe since there's a good chance that this isn't the only universe. Easy for me to say. LOL. Am I right in saying that all universe [vacuua] without a cosmological constant are equivalent and universe with different cosmological constant are different? 'Because it's easy for me to say' why not pick the string theory that describes the natural phenomena in this universe and choose to 'predict' that the complete theory will describe the natural phenomena associated with the inflating 'Multiverse'? There's a reasonable chance the Planck experiment will reveal our universe collided with a different member of the 'multiverse' during inflation. All this is exciting physics from my point of view. It kinda 'pisses me off' when I hear whining about how the search for quantum gravity has been a waste of effort. Kinda what Professor Smolin has done. It's not a wasted effort that leads to such amazing stuff. Thanks again and bring me down to reality if I'm getting carried away. LOL.

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18. ### ForkBannedBanned

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I wasn't talking about separating the electron or photon from their respected fields. Quite the contrary. I was describing reality as a self-excited circuit as explained here:

http://en.wikipedia.org/wiki/Digital_physics

19. ### brucepValued Senior Member

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Really you're going to post that nonsense in this thread? It's an irrelevant view. That type of bullshit seems to pop up all the time so I don't think we can classify it as 'new'.

20. ### river

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Is this because Wheeler views the Universe as a closed system ?

21. ### river

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Who are you to say what is BS ?

22. ### river

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The problem with this is that it is based on geometrical force

It fails though in the case of electro-magnetism

GR fails to account for electro-magnetism because the forces are proportional to charge not to the mass

From the book Electrogravitics Systems , by Thomas Valone , M.A. , P.E.

23. ### brucepValued Senior Member

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I'm saying it's BS because the author says it's modeled on a Euclidean manifold and it can't recover GR in it's domain of applicability. Roundfiled crank nonsense. Irrelevant ether nonsense. Scientifically illiterate crank ether pseudoscience. Because I've studied modern physics I CAN know it's irrelevant bullshit. Since you haven't you can't make a rational evaluation. It's BS roundfiled nonsense. You want to talk about irrelevant crap post it in the section it belongs in.