Possible alternatives to LQG and ST?

[kye]

You will notice we focus a lot on Loop Quantum Gravity here and String theory out there. Are there other alternatives to these two in case they are found to be false years from now? What are they?

 

[marcus]

You will notice we focus a lot on Loop Quantum Gravity here and String theory out there. Are there other alternatives to these two in case they are found to be false years from now? What are they?

Loop gravity is not a finalized theory. It is a general way of constructing a quantum theory of geometry and of cosmology.
Several loop gravity theories have ALREADY been found to be wrong, and the loop community has modified and come up with improved versions. It is a general technique and theoretical language.

The original loop cosmology model was shown to be wrong (around 2005) and replaced with "improved dynamics" loop cosmology in 2006. The original spinfoam quantum geometry was shown to be wrong by Rovelli in around 2007-2008, and replace by the EPRL spinfoam.

There are currently several versions being worked on either in the full theory or in the application to cosmology. I could probably list half a dozen versions. A lot of researchers like loop and are working on it.

Also ways of TESTING the loop cosmology models by observation (e.g. of the CMB) have been proposed and more are being worked on.

Besides loop and string there are other approaches being pursued (causal sets, energetic causal sets, causal dynamical triangulations, non-commutative geometry, asymptotic safety gravity, Horava gravity, Jacobson's Einstein Aether, Krasnov's approach, Shape Dynamics, group field theory, Rivasseau's "tensor track"...).

the total number of people working on all these (non-loop, non-string) alternatives is comparatively limited and some are in DECLINE, as interest runs out, or steady (confined to a handful of researchers and not attracting young postdocs). But if loop, say, stops gaining interested young researchers and progress stalls, you can be sure that there will be SOME alternative approach that springs up to take its place. It might not be any of the CURRENT alternatives, since the list of alternatives keeps changing as interest in some wanes and others (e.g. Shape) are invented.

 

[kye]

Loop gravity is not a finalized theory. It is a general way of constructing a quantum theory of geometry and of cosmology.
Several loop gravity theories have ALREADY been found to be wrong, and the loop community has modified and come up with improved versions. It is a general technique and theoretical language.

The original loop cosmology model was shown to be wrong (around 2005) and replaced with "improved dynamics" loop cosmology in 2006. The original spinfoam quantum geometry was shown to be wrong by Rovelli in around 2007-2008, and replace by the EPRL spinfoam.

There are currently several versions being worked on either in the full theory or in the application to cosmology. I could probably list half a dozen versions. A lot of researchers like loop and are working on it.

Also ways of TESTING the loop cosmology models by observation (e.g. of the CMB) have been proposed and more are being worked on.

Besides loop and string there are other approaches being pursued (causal sets, energetic causal sets, causal dynamical triangulations, non-commutative geometry, asymptotic safety gravity, Horava gravity, Jacobson's Einstein Aether, Krasnov's approach, Shape Dynamics, group field theory, Rivasseau's "tensor track"...).

the total number of people working on all these (non-loop, non-string) alternatives is comparatively limited and some are in DECLINE, as interest runs out, or steady (confined to a handful of researchers and not attracting young postdocs). But if loop, say, stops gaining interested young researchers and progress stalls, you can be sure that there will be SOME alternative approach that springs up to take its place. It might not be any of the CURRENT alternatives, since the list of alternatives keeps changing as interest in some wanes and others (e.g. Shape) are invented.

What is your opinion about Supersymmetry? The LHC may not have detected them in the limit of visibility. But it is the most elegant way to solve the Hierachy Problem, but Peter Woit said Supersymmetry breaking can produce 100 more parameters on top of the standard model 19 parameters and Supersymmetry's parameter and make it more complicated.

But since Loop Quantum Gravity doesn't have good relationship with Supersymmetry formalism. Hope others not interested in LQG can share their thoughts on Supersymmetry too besides Marcus who is a LQG person.

 

[marcus]

Loop easily accepts SUSY, kye. Occasionally papers are written exploring Loop with SUSY. Also Loop in higher dimensions, in case real extra dimensions are ever found. Thomas Thiemann and his group and Erlangen occasionally research that kind of stuff.

But since it doesn't NEED supersymetry or extra dimensions, and since no evidence has shown up for either, it doesn't make much sense to spend a lot of time speculating what if. They're ready to incorporate those things if and when evidence shows up.

 

[tom.stoer]

In my pointing in is not correct to talk about "alternatives to LQG and ST" simply b/c LQG is not an alternative to ST.

LQG is an incomplete framework to quantize classical theories describing gravity - with GR as the simplest example - w/o unifying other interactions. ST is - in my opinion - an incomplete framework to derive or construct low-energy theories incorporating gauge, SUSY, gravity in a unifying manner (the high-energy completion is the highly speculative M-theory for which no mathematical formulation is known)

 

[kye]

In my pointing in is not correct to talk about "alternatives to LQG and ST" simply b/c LQG is not an alternative to ST.

LQG is an incomplete framework to quantize classical theories describing gravity - with GR as the simplest example - w/o unifying other interactions. ST is - in my opinion - an incomplete framework to derive or construct low-energy theories incorporating gauge, SUSY, gravity in a unifying manner (the high-energy completion is the highly speculative M-theory for which no mathematical formulation is known)

As far as Planck scale is concern.. what is the argument or theorem that something can even exist inside the Planck scale (it's so small).. the search for theory of quantum gravity is how to describe events inside the Planck scale where QM and GR break down. But the Planck is so very small.. maybe it just ceases to exist in principle? If so, there is no need to unify QM and GR? Or is there other reasons like we don't even how quantum particles interact with spacetime or how particles are coupled to spacetime and this is why they have to be unified independent of Planck scale breakdown of QM and GR?

 

[atyy]

As far as Planck scale is concern.. what is the argument or theorem that something can even exist inside the Planck scale (it's so small).. the search for theory of quantum gravity is how to describe events inside the Planck scale where QM and GR break down. But the Planck is so very small.. maybe it just ceases to exist in principle? If so, there is no need to unify QM and GR? Or is there other reasons like we don't even how quantum particles interact with spacetime or how particles are coupled to spacetime and this is why they have to be unified independent of Planck scale breakdown of QM and GR?

There are approaches that consider a fundamental discreteness. None are as developed as string theory or LQG. The most naive forms of fundamental discreteness break Lorentz invariance, but more sophisticated forms like Causal Set Theory don't.

Sabine Hossenfelder's http://arxiv.org/abs/1203.6191 discusses many topics including fundamental discreteness, Lorentz invariance, and Causal Sets. She also discusses how a minimal length may emerge in string theory and canonical LQG, even though those approaches don't have fundamental discreteness.

 

[Ilja]

An alternative approach which predicts all fermions and gauge fields of the Standard Model, together with all charges from a quite simple condensed matter-like model can be found in Foundations of Physics, vol. 39, nr. 1, p. 73 (2009), http://arxiv.org/abs/0908.0591.

There is also a necessity for some massive scalar fields, one for each electroweak pair, but their connection to the SM Higgs field is not clear.

What is also missed are predictions for masses. Some qualitative predictions have been found in Reimer, A. (ed.), Horizons in World Physics, Volume 278, Nova Science Publishers (2012) ISBN: 978-1-61942-538-5, http://arxiv.org/abs/0912.3892 But what is predicted there is not much, The massless group should be U(3) ~ SU(3) + U(1)_em, while weak force should be massive, and neutrinos should have much less mass than the other fermions.

All this is compatible with a corresponding theory of gravity, which generalizes the Lorentz ether to gravity. See Advances in Applied Clifford Algebras 22, 1 (2012), p. 203-242, http://arxiv.org/abs/gr-qc/0205035. The equations of this theory of gravity, derived by simple first principles, have the Einstein equations of GR as a natural limit, while the Einstein equivalence principle holds exactly.

Given this list of results, this would be the dream of string theorists, while LQG does not even try. But the theory is, of course, anathema. It revives, in some sense, the old ether theory: Not only the EM field, but all fields appear to be waves of an ether. So, this theory has no chance in mainstream science, even if it has been published in peer-reviewed journals.

 

[audioloop]

You will notice we focus a lot on Loop Quantum Gravity here and String theory out there. Are there other alternatives to these two in case they are found to be false years from now? What are they?

different aims

"theories such as string theory try to unify gravity with the other fundamental forces, others such as loop quantum gravity make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other forces"
http://en.wikipedia.org/wiki/Quantum_gravity


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