Recent content by Finbar

In his review talk at loops 13 entitled "Loop Quantum Cosmology: A Eulogy" Martin Bojowald has declared that loop quantum cosmology has failed. http://www.perimeterinstitute.ca/videos/quantum-cosmology-1

Good question. I suppose the scale at which you expect this to happen is quite large though(?). The energy scale at which the fine structure constant approaches one. It is interesting that the reason we consider only relevant operators has changed since the 70's. Originally is was supposed that...

I want to make some comments on the Hamber-Toriumi but haven't got much time at the moment. I think in the end in the effective average action approach only the limits k->0 and k-> infinity really contain universal information which is not dependent on the RG scheme used. Also Hamber-Toriumi are...

Why does a global foliation of space-time not allow for closed time-like curves? Surely one can just identify two space-like hypersurfaces at times t=0 and t=T so its periodic in time. Isn't the point more about the topology of spacetime? Classical general relativity is not a dynamical theory...

So the on metric on the world sheet is not physical right? It's an auxiliary field introduced to make one's life easier. Then the choice of a flat metric is arbitrary too. Is there then a deeper reason that 2-d conformal field theory plays such an important role in string theory?

SUSY is not going anywhere as a theory in general. But if it is ruled out as a solution to the hierarchy problem then it loses the motivation for it to be at the EW scale. SUSY is very popular for many reasons. One of the major reasons is that it is within many physicists confort zones. I...

A mixed state is not a superposition. Because any superposition of pure states is itself a pure state. A mixed state has an uncertainty beyond that of a quantum mechanical superposition. A mixed state requires a probability distribution over pure states. So one is formally doing quantum...

Pure states are states that can represented as state vectors in the Hilbert space.

Isn't it rather that the universe at the big bang can be viewed as being in a pure state since all the matter was in causal contact. Then once the universe evolves we can only consider "observable" universes which are subsystems of the whole system. Theses observable universes are described as...

The difference has nothing to do with the presence of some surrounding space that doesn't collapse. The difference is in the symmetries and reduction to a finite number of degrees of freedom in LQC.

I think we are missing the essential point here. The idea of LQC with a bounce is that the universe at some point collapsed under it's own gravity bounced and formed a big bang. Is this right? On the other hand when matter collapses from some generic initial conditions we expect it will form a...

I agree. But reversing the 2nd law and requiring fine tuning seems worse than simply the fine tuning. Nothing is trivial here. LQC works with much symmetry, it's just a toy model.Toy models can be useful but they're not reality.

The second law of thermodynamics has an explanation from statistical physics. I can't understand how you could explain this flip? To create some state before the big bang that created the special low entropy state at the big bang would require some fine tuned pre-big bang state. It can't begin...

Shouldn't we take more care in distinguishing entanglement entropy obtained by a partial trace over some pure state and a the entropy of a genuine mixed state? My guess would be that within quantum gravity these two entropies are somehow unified. However from our current point of view, with...

You seem to suggest the existence of a creator? :-p Thermodynamics isn't time symmetric so you can't have the universe as a thermal state the decreases in entropy before the big bang started. The ccc models I guess are some kind of loop hole in the 2nd law like mapping states of high entropy...