Wednesday, 25 July 2018

string theory - Since when were Loop Quantum Gravity (LQG) and Einstein-Cartan (EC) theories experimentally proven?


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?"



Answer



"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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