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.
Indeed, QFT has been developed by Heisenberg, Pauli, Dirac, and Fock. Feynman used Dirac's canonical formalism. The perturbative expansion has been worked on first by Schwinger, Stueckelberg, Tomonaga, and after by Dyson. All we can credit to Feynman is that he got a really covariant formalism, and was American.
@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".)
Scalar curvature only, no dynamics. Boring. Except... JT gravity is (1+1)-D and has some interest for studies of (overly simple) wormhole topology implications when there is a boundary (AdS).
imagine .....if 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
I think not? Euler was a least action principle guy, so the path integral formulation should have been his cuppa tea, no? The real-time path integral for a region of spacetime cobordism does not have to sum over global spacetime topologies if all the topology is in the local particle phenomenology (spinor and vector fields).
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.
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.
Yes, both theories suck. But the nerdiest theoreticians who vomit at the sight of a condensed matter lab need to be employed so they can feed their kids.
Dishonest statement: at Maxwell's time there was no gauge theory. There was 0 motivation to introduce further interactions, until the beginning of the 20th century. What you are essentially saying is that quantum mechanics was discovered in the 19th century. It was not, although it was in the experimental range and already contained in the canonical formalism. Just no motivation, because no experimental phenomena calling for an explanation. Supersymmetry isn't called for by experiment, it is only one of the numerous possible overspeculations.
I did not claim claim the mathematical formalism of gauge theories were known at Maxwell's time. I was making a hypothetical scenario to make a point. The whole question is, what is logically reasonable to conclude about a claim based on a given experiment. There are two independent ways supersymmetry has reached physics, which are independent of each other. The two hypothesis are (1) There is supersymmetry at low-energies, as it would solve many phenomenological mysteries of the standard model. (2) There is supersymmetry at high-energies (as string theory requires). Note that (2) does not imply (1), as supersymmetry can be broken as lower energies (and it clearly is broken at our energy scales, like most other symmetries we know the standard model does have). Conversely, (1) can be true even if (2) is not.And both can be correct or false at the same time as well. So they are independent claims. Now, if experiments find no evidence of supersymmetry at LOW energies, it is logically reasonable to say it weakens the belief that (1) might be correct. But it does NOT weaken (or strengthen) the belief that (2) is correct, because it does not contradict anything (2) claims nor does it probe the energy scales where that claim can be tested.
@@element4element4 String theory requires nothing, it is but an overspeculation. Supersymmetry was not meant to solve any mystery, it was introduced abstractly purely mathematically. Then it was adorned with all virtues and rationalized. The truth is, no "mystery" of the standard model is addressed by it. At high energy, if anything new is found, the odds are no existing theory will explain it, like it always happened in history. Already we have no explanation for the three generations.
Great lecture, thanks for sharing
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.
Indeed, QFT has been developed by Heisenberg, Pauli, Dirac, and Fock. Feynman used Dirac's canonical formalism. The perturbative expansion has been worked on first by Schwinger, Stueckelberg, Tomonaga, and after by Dyson. All we can credit to Feynman is that he got a really covariant formalism, and was American.
Lo trovo affascinante, qualunque cosa essa sia, non esistono parole che riescano compiutamente a descriverla. Soltanto stupore
@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".)
does not match with GR in the classical limit
What does 2 dimensional gravity look like?
Scalar curvature only, no dynamics. Boring. Except... JT gravity is (1+1)-D and has some interest for studies of (overly simple) wormhole topology implications when there is a boundary (AdS).
Nothing, it doesn't exist.
imagine .....if 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
I think not? Euler was a least action principle guy, so the path integral formulation should have been his cuppa tea, no? The real-time path integral for a region of spacetime cobordism does not have to sum over global spacetime topologies if all the topology is in the local particle phenomenology (spinor and vector fields).
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.
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.
Yes, both theories suck. But the nerdiest theoreticians who vomit at the sight of a condensed matter lab need to be employed so they can feed their kids.
Dishonest statement: at Maxwell's time there was no gauge theory. There was 0 motivation to introduce further interactions, until the beginning of the 20th century. What you are essentially saying is that quantum mechanics was discovered in the 19th century. It was not, although it was in the experimental range and already contained in the canonical formalism. Just no motivation, because no experimental phenomena calling for an explanation. Supersymmetry isn't called for by experiment, it is only one of the numerous possible overspeculations.
I did not claim claim the mathematical formalism of gauge theories were known at Maxwell's time. I was making a hypothetical scenario to make a point.
The whole question is, what is logically reasonable to conclude about a claim based on a given experiment.
There are two independent ways supersymmetry has reached physics, which are independent of each other. The two hypothesis are
(1) There is supersymmetry at low-energies, as it would solve many phenomenological mysteries of the standard model.
(2) There is supersymmetry at high-energies (as string theory requires).
Note that (2) does not imply (1), as supersymmetry can be broken as lower energies (and it clearly is broken at our energy scales, like most other symmetries we know the standard model does have).
Conversely, (1) can be true even if (2) is not.And both can be correct or false at the same time as well. So they are independent claims.
Now, if experiments find no evidence of supersymmetry at LOW energies, it is logically reasonable to say it weakens the belief that (1) might be correct. But it does NOT weaken (or strengthen) the belief that (2) is correct, because it does not contradict anything (2) claims nor does it probe the energy scales where that claim can be tested.
@@element4element4 String theory requires nothing, it is but an overspeculation. Supersymmetry was not meant to solve any mystery, it was introduced abstractly purely mathematically. Then it was adorned with all virtues and rationalized. The truth is, no "mystery" of the standard model is addressed by it. At high energy, if anything new is found, the odds are no existing theory will explain it, like it always happened in history. Already we have no explanation for the three generations.
@@clmasse Was clearly waste of time to engage. You have clearly not studied these topics at any depth nor even engage in the logic I was presenting.