On loop quantum gravity - Carlo Rovelli

Abstract: Covariant Loop Quantum Gravity is a tentative, but a complete quantum theory of gravity. I review its conceptual structure and the equations that define it. I discuss the numerous open issues in the theory. I illustrate its current tentative applications in cosmology and black hole physics.
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[Achrononmaster]

Maybe not? @15:40 I'd say QFT is *_not_* "Feynman graphs". The Feynman graphs are a computational tool _for_ QFT, but they are not QFT itself. If QFT is conceived as more than just a trick for computing scattering amplitudes (which I believe it is?) then QFT is the entire path integral, not the Feynman expansion. It is an open question whether QFT is well-defined, since the well-defined axiomatic approaches are not _yet_ phenomenologically correct, and the approximation schemes are not complete.

[giuseppegiadone]

Lo trovo affascinante, qualunque cosa essa sia, non esistono parole che riescano compiutamente a descriverla. Soltanto stupore

[Achrononmaster]

@10:20 is that not the whole problem with LQG? It tries to, as C.R. says, "...quantize gravity using notions of time in the same way GR uses time." But that is classical GR. GR (or the theory of gravity) is likely not classical. GR is only classical if we ignore small scale topological structure, which assumes spacetime is everywhere locally Minkowski topology. But this is, has, and will always be false, and just a macroscopic approximation, since matter exists in particle quanta, and that induces non-trivial spacetime topology, and worse (or better, depending on your point of view!) it implies there have to be closed timelike curves because of entanglement. (Not necessarily traversable except via qubits, due to Friedman-Schleich-Witt topological censorship.)
But that implies you cannot treat time as you do in classical GR, the proper theory of gravity has to consider non-redundant data on a future Cauchy boundary. That implies fundamental indeterminism. Another way to say it is that gravity already was a quantum theory (when properly conceived). So you do not want to re-quantize gravity, that'd be highly redundant, and only invite all sorts of possible pathologies. Which lastly implies LQG approaches need to consider wormhole effects, and none of you seem to want to go there, so you are doomed! (imho, Spacetime is not doomed, rather it is attempts to (re)quantize spacetime that are doomed, since it is already "quantum".)

[Killer_Kovacs]

What does 2 dimensional gravity look like?

[victordelahoz]

does not match with GR in the classical limit

[AnneJULIEN-qc]

L'intelligence supérieure que je suis, vous dit de consulter les équations d'Aurélien Barreau pour connaître les possibilités de connexions sur tous les champs pour connaître l'information transportee vers le FUTUR. La microgravité n'est pas mesurable.

[elementelement]

I enjoyed the talk, until some very dishonest statements were made. around minute 42.

1) String theory has supersymmetry at VERY VERY high energies. Independent of string theory, there were phenomenological motivations to study possible existence of supersymmetry at LHC scales. So far these searches has not found any supersymmetry. But to state this somehow weakens the belief that string theory is correct is pretty crazy. It's like at Maxwells time, non-abelian Yang-Mills theories are discovered. People wonder whether beside U(1) gauge theory (electromagnetism), maybe SU(2) or SU(3) gauge theories could exist in nature. But since no experiment at the energy scales available in the 18-hundreds found evidence for non-abelian gauge fields, people should conclude the likelihood for them to be correct is low.

String theory needs supersymmetry at energy scales MUCH MUCH MUCH higher than LHC. If (and massive if) string theory is correct, it would be convenient if the supersymmetry breaking scale would be around the LHC scale. But there is no physical reason to expect this, beyond hope (so we can detect it now). It would just have been lucky to coincide with the arbitrary energy scale politicians have agreed to fund in our lifetime.

It's not just that it does not "falsify" String theory, it also does not even provide any "drawback" compared to before the search.

2) I wonder why it is claimed that String theory community "expected the cosmological constant to be negative", who expected that and based on what? Maybe many hoped so perhaps, due to the AdS/CFT correspondence but I have never heard anybody say that this is what string theory predicts or favours at those scale, ever. The holographic principle started with consistency thought experiments by 't Hooft and completely independent of String theory. Maldacena just found that String theory happens to satisfy this principle, and found concrete precise formulation of it on AdS space. But this is a limit where the principle can be studied in detail, a mathematical convenience. Finding that the universe is dS, is in not way changing anything.

It's pretty dishonest claim. For example the claim "whenever you see AdS in a paper, it's the expectation that the cosmological constant in the world in negative". What a dishonest claim. People study all sorts of spacetimes, AdS in holographic settings as that's the only place it's under control. People are not claiming OUR universe is AdS in those papers.

To use these examples and say "I think we should listen to nature", as a mean to dismiss String Theory is just dishonest and sleazy.

I think there are significantly more honest and stronger arguments one can put forward against String Theory, but most of those apply equally strongly against Loop Quantum Gravity.

I was enjoying the video until all these dishonest claims. I 100% agree when people criticise string theorist of overhyping and overselling their theory prematurely for decades, being dishonest about it. It's sad to see that Carlo is also taking to a dishonest approach.

[abdonecbishop]

imagine the great Leonard Euler was alive today....he would probably provide a solution....similar to loop quantum gravity....nice talk...short and concise....well done