World within Planck Scale & QM/GR Unification

[rogerl]

After reading more about Loop Quantum Gravity. I realized they tried to solve quantum gravity by literally sweeping Planck scale under the rug. That is. The smallest volume and area of the spin networks in LQG is Planck size. In String Theory, they make the string Planck size too. They are just avoiding the Planck scale by literally blurring it by making their smaller *object* Planck size. This is not good mode of thinking. It's avoidance of the problem and ugly and both String Theory and Loop Quantum Gravity would likely fail (I'd bet off my Ipad they would fail).

Anyone knows of any QM/GR unification programme that explores and models what is inside Planck scale which can make GR and QM as its lower limit? This Third Theory of the dynamics inside the Planck Scale may be what can unite the QM/GR. In other words. Having both general relativity and quantum theory emerge from a theory very different from both. Planck scale may not contain normal spacetime. But maybe some kind of space just like what we had before the Big Bang expanded its spacetime. Only an exploration of this may give us the real Quantum Gravity and Unification of all forces. Or it is impossible or already refuted that inside Planck scale is some stuff that can have both general relativity and quantum theory emerge from it? How is this refuted if it has been refuted already? If not yet refuted. What is the best approach to explore the world inside Planck Scale? What below Planck scale models/theories already exist that have not been refuted yet?

 

[rogerl]

Isn't it that the size of a black hole is just it's event horizon... all the mass are concentrated inside the Planck length. The amount of the mass can determine how strong it can curve spacetime and how big is the extend of the event horizon.. where light can't escape, that is why black holes are black. This means there are contents inside the Planck length where it can affect outside. But how come string theory and loop quantum gravity just blur the Planck length by making their string and spin network the same size as the Planck length. Unless you are saying it is possible the mass of the black holes are not inside the Planck length but much bigger like perhaps a ping pong ball or larger depending on the solar mass imploded to the black hole and there is reall nothing below the Planck length?

 

[tom.stoer]

My expectation regarding LQG is different. I guess that something like Kadanoff-like block spin renormalization will become important.

First what looks like a single intertwiner will consist of an entire spin network at a smaller scale; so the intertwiner is "effective" (this applies especially to "large" intertwiners)
Second once "zooming into" this intertwiner coupling constants and Immirzi parameter will be rescaled
Third the length scale i.e. the spectrum of certain operators is affected.

So there will not be any fixed length scale in the theory but the scale emerges just like coupling strength and scale in ordinary QFT do.

 

[rogerl]

My expectation regarding LQG is different. I guess that something like Kadanoff-like block spin renormalization will become important.

First what looks like a single intertwiner will consist of an entire spin network at a smaller scale; so the intertwiner is "effective" (this applies especially to "large" intertwiners)
Second once "zooming into" this intertwiner coupling constants and Immirzi parameter will be rescaled
Third the length scale i.e. the spectrum of certain operators is affected.

So there will not be any fixed length scale in the theory but the scale emerges just like coupling strength and scale in ordinary QFT do.

General Relativistic spacetime has zero minimum distance. Quantum Mechanics has minimum Planck length.

Must Gravity yield to quantum? Or must Quantum yield to gravity?

According to the following paper. Quantum must yield to gravity:

http://arxiv.org/abs/gr-qc/9810078
"Why the Quantum Must Yield to Gravity"

Anyway. I wonder why String Theory and Loop Quantum Gravity just blur what is inside the Planck length.. because they think gravity must yield to the quantum? But what if the quantum must yield to gravity. Then perhaps there is something inside the Planck length?

What other reasons why physicists don't want to explore what is inside the Planck length and instead make it as the minimum in String theory and LQG?

 

[tom.stoer]

LQG did something different: they quantized gravity and found a discrete spectrum; discreteness was a result, not an input (look at angular momentum: a continuous symmetry produces a discrete spectrum).

 

[rogerl]

LQG did something different: they quantized gravity and found a discrete spectrum; discreteness was a result, not an input (look at angular momentum: a continuous symmetry produces a discrete spectrum).

LQG is nice but it looks kinda rigid. Anyway. Is there no possibility for the spin networks to quantum tunnel and form entanglement at distances away and be friendly to Bell's Theorem?

 

[tom.stoer]

I am not so sure.

What one can have is the following. Let's construct a two-dim. network of vertices and links. This network defines something like "neighbourhood" and "locality". Two vertices are "closed" to each other if they are connected via a link. Now select two vertices which are (according to this definition of "locality") far away from each other (i.e. with very many vertices in between). Now connect these two vertices via a new link.

The two vertices are (according to the first definition of "locality") far away from each other, but due to the new link they are closed to each other (nearest neighbours). That's why a spin network changes what we call "locality". There is micro-locality simply defined by the vertices and links, and there is macro-locality defined via some smoothing or classical limit of the network. The latter one is emergent, so the two definitions of locality need not coincide. What we expect is that macro-locality is not violated on macrscopic scales - simply b/c we do not observe it.

Of course one could speculate that quantum non-locality could be produced via such a mechanism. Smolin had the idea that the mismatch of micro- and macro-locality produces an effect which mimics the cosmological constant. I don't think that this is ore than just that - speculation.

 

[rogerl]

I am not so sure.

What one can have is the following. Let's construct a two-dim. network of vertices and links. This network defines something like "neighbourhood" and "locality". Two vertices are "closed" to each other if they are connected via a link. Now select two vertices which are (according to this definition of "locality") far away from each other (i.e. with very many vertices in between). Now connect these two vertices via a new link.

The two vertices are (according to the first definition of "locality") far away from each other, but due to the new link they are closed to each other (nearest neighbours). That's why a spin network changes what we call "locality". There is micro-locality simply defined by the vertices and links, and there is macro-locality defined via some smoothing or classical limit of the network. The latter one is emergent, so the two definitions of locality need not coincide. What we expect is that macro-locality is not violated on macrscopic scales - simply b/c we do not observe it.

Of course one could speculate that quantum non-locality could be produced via such a mechanism. Smolin had the idea that the mismatch of micro- and macro-locality produces an effect which mimics the cosmological constant. I don't think that this is ore than just that - speculation.

In the sci-am article. Lee Smolin mentioned:

"An important test is whether one can derive calssical general relativity as an approximation to loop quantum gravity. In other words, if the spin networks are like the threads woven into a piece of cloth, this is analogous to asking whether twe can compute the right elastic properties for a sheet of the material by averaging over thousands of threads. Similarly, when averaged over many Planck lengths, do spin networks describe the geomery of space and its evolution in a way that agrees roughly with the "smooth cloth" of Einstein's classical theory?"

Maybe the right elastic properties for a sheet of material can only math Einstein's GR if they have to take into account the experimental finding of Bell's Theorem and adjust more parameters of the elastic properties. By ignoring Bell's Theorem, it is hard for LQG to succeed because they are ignoring a part of reality.

Btw.. I wonder how String Theory can encompass Bell's Theorem. Maybe M-Theory would involve a central conductor (like in orchestra) that can create/annihilate the strings like in QFT creation/annihilation operators. This can make them truly non-local.

 

[Haelfix]

A little nomenclature time out. Bell's theorem is a classical statement about set theory that is violated in the real world by quantum mechanics (the inequalities are violated). This violation is essentially isomorphic to the statement that 'quantum mechanics empirically works and classical mechanics does not'.

All theories hitherto presented satisfy quantum mechanics or they would be automatically ruled out.

It is thus mostly irrelevant for discussion about quantum gravity.

 

[tom.stoer]

All what I wanted to say is that there might emerge an element of "non-locality" from LQG.

 

[rogerl]

A little nomenclature time out. Bell's theorem is a classical statement about set theory that is violated in the real world by quantum mechanics (the inequalities are violated). This violation is essentially isomorphic to the statement that 'quantum mechanics empirically works and classical mechanics does not'.

All theories hitherto presented satisfy quantum mechanics or they would be automatically ruled out.

It is thus mostly irrelevant for discussion about quantum gravity.

But quantum gravity is not just about gravity, it is about spacetime which is supposed to be a fabric that is continuous. Even if the smaller scale is discrete, the whole manifold is still continuous. Objects need the manifold to move. So Bell's Theorem at say 1 million light years entanglement is not compatible with the continuous fabric of spacetime. It's like water in the ocean, a microscopic portion of it is discrete (composed of H20), but the water in Japan is not entangled with the water in the United States. There is a huge separation.

Unless quantum objects don't necessarily use the manifold and correlerations can be explained by the fact it exits the fabric of spacetime and there is a bigger manifold that is discontinuous where it makes use of?

Another thing. There is this contextuality issue in quantum mechanics. People believe that in quantum entanglement or Bell's Theorem, there is no communication between the two locality because we are told that quantum properties like position doesn't exist before measurement, so the entangled electron at 1 million light years separation are not exactly there before measurement... when you measure particle A, particle B doesn't necessarily exist. Now here is the question, if it doesn't exist and is not coupled to spacetime. Where is particle B exactly? Maybe outside of the fabric of spacetime?

 

[rogerl]

I read in Elegant Universe that a point particle with zero volume is just idealization of years past. And the reason General Relativity is in conflict with Quantum Mechanics is because of the quantum fluctuations inside the Planck length that can make spacetime fluctuate so much forming quantum foam. So string theory is created to smear the Planck length. Supposed String theory is right and there are strings. Does it mean General Relativity and Quantum Mechanics is each completely correct? I'm asking this because I heard many saying that one of them could be wrong because they can't be combined in the Planck scale. But if there are strings, then they can both be completely right?