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Can this template at Wikipedia be true? It seems to suggest that Einstein-Cartan theory, Gauge theory gravity, Teleparalleism and Euclidean Quantum Gravity are fully compatible with observation!

It also suggests that Loop Quantum Gravity and BEC Vacuum Theory among others, are experimentally constrained whereas string theory/M theory are disputed!

What I understand by "Fully compatible with observation" is that all its predictions are confirmed by experiments and it has been found to be more accurate than General Relativity. Has such evidence really been found? Or am I misinterpreting "Fully compatible with observation"? Maybe it means it has been tested only when it reduces to General Relativity? But if that where the case, shouldn't M theory/String theory also be listed under "Fully Compatible" since their predictions also go down to Classical General Relativity at the low-energy, classical limit, if all other forces (other than gravity?) are gotten rid off?

What I understand by "Experimentally constrained" is that it is true given certain modifications. However, as far as I know, Loop Quantum Gravity violates Lorentz symmetry and has thus been experimentally "excluded" while BEC Vacuum theory isn't even mainstream?

What I understand by "Developmental/Disputed" is that it is still undergoing development OR it has almost been experimentally proven wrong but it is still not settled in mainstream physics. If LQG doesn't go to the excluded section, it should at least come here? Since the violation of Lorentz symmetry has been disproven according to this.

So my question is "Is this template really reliable?"

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Without knowing how exactly Wikipedia defines "fully compatible with observation," "experimentally constrained," and so on, I'm not sure we can answer this question. (But you are using an odd definition of "fully compatible with observation.") –  David Z Jun 25 '13 at 7:12
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"Fully compatible with observation" seems to mean, at minimum "no observations currently contradict it". This would, at least on the surface of it, apply to string theory. –  Ehryk Jun 25 '13 at 7:18
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I think you are right and this wikipedia template looks at least very strange up to nonsensical and badly informed about the current stage of different quantum gravity approaches to me. –  Dilaton Jun 25 '13 at 7:23
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If you look at the template's edit history, these classifications (and this system of categories) were introduced by one user, "Teply", who thought the previous version was too wishy-washy, and who (on the talk page) asked people to correct it, if it contained errors. But no-one has done so. –  Mitchell Porter Jun 25 '13 at 8:42
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@Dilaton I would class F-theory as a part of Type IIB. Unlike the extra dimension of M-theory, the extra two dimensions of F-theory never become large and physical. For now it's just a formalism. Also F and M are connected by dualities. –  Mitchell Porter Jun 25 '13 at 13:17

2 Answers 2

up vote 3 down vote accepted

"Fully compatible with observations" is a rather vague statement. Actually, two aspects of adequacy to reality have to be distinguished when a new theory reaches a degree of explicitation. These are

  • compatibility with older theories, in domains where the new theory is not supposed to bring more than a new formulation. For instance, special relativity is compatible with newtonian mechanics when velocities are small compared with c. Since older theories taken in reference have been usually thoroughly tested (otherwise you don't take them as reference), this is a good first check for your new theory.

  • compatibility with new phenomena. Indeed what makes a new theory interesting is the change of insight that it might bring on reality. And this means that beyond proposing a new description of reality, it shall predict new observable features which older theories don't account for.

As far as LQG is concerned, my understanding is that the first aspect has been addressed in the sense that right from the outset, conpatibility with GR has been used as a guide to develop the theory. For the second aspect, this one of the topics which focuses a good part of the efforts of the LQG community. This means finding new observable features that survive going from the Planck scale to the scales that are accessible to us in experiments or astrophysical observations. It's tricky but not impossible.

So as far as the statement "fully compatible with observations", I would advise to replace it with "compatible with previous observation-tested theories, but still expecting genuine experimental predictions for testing".

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LQG, unlike string theory, has not addressed the first issue. Obtaining a continuum at large scale has not been addressed. And for the second issue, LQG violates Lorentz invariance, and that has already been falsified, as far as I know. String theory does agree with GR at long distances though (you can easily obtain the EH action as a low-energy effective action for gravity only) ,. –  Dimensio1n0 Jun 29 '13 at 3:51
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Still, I'll accept your answer because other than the third paragraph, everything is fine. –  Dimensio1n0 Jun 29 '13 at 7:25

What I understand by "Fully compatible with observation" is that all its predictions are confirmed by experiments and it has been found to be more accurate than General Relativity.

No, the standard interpretation of this phrase would be that it's not contradicted by any observation.

Re Einstein-Cartan, Trautman 2006 has some relevant remarks at p. 6, with a numerical estimate showing why the theory gives the same results as GR under conditions we have access to.

However, as far as I know, Loop Quantum Gravity violates Lorentz symmetry and has thus been experimentally "excluded"

No. As far as I know, it is still an open question whether LQG's semiclassical limit is GR. People working on it certainly hope that it is. If it is, then it is consistent with Lorentz invariance under currently accessible experimental conditions. There was some hope a while back that it could be tested through searches for dispersion of the vacuum, but it turns out that that was Smolin's incorrect interpretation of the theory.

Trautman, "Einstein-Cartan Theory," http://arxiv.org/abs/gr-qc/0606062

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LQG does not have GR as a semiclassical limit. Ashoke Sen recently showed this in a way that even LQG advocates should admit settles the issue on their own terms: arxiv.org/abs/arXiv:1205.0971. –  Matt Reece Jun 29 '13 at 5:12
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@MattReece: Interesting. I've started a separate question about the Sen result: physics.stackexchange.com/questions/69656/… –  Ben Crowell Jul 1 '13 at 10:13

protected by Qmechanic Jun 29 '13 at 6:37

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