Recent content by Orbb

Anyone an idea? Sorry for bumping this.

Hello Physicsforum, I am trying to compute the following double sum: \sum_{j\in\mathbb{N}_0/2}\sum_{m=-j}^j\frac{x^{j+m}}{(j+m)!(j-m)!}e^{-\kappa^2j(j+1)/s} where x, kappa and s are parameters. It is possible with e.g. Mathemtatica to carry out the sum over m explicitly, which yields...

One thing I didn't get in verlindes illustration in terms of matrix models is how can he actually disentangle between what's on the diagonal and what's not? To my understanding, matrix model lagrangians usually exhibit a symmetry acting on the matrices as a rotation in matrix space of some sort...

It's the parameter space of vacuum (lowest-energy) configurations of a given theory. In that sense, the different String Theories correspond to picking specific points in the full parameter space of vacua of M-Theory (correct me if I'm wrong).

You might want to check out http://arxiv.org/abs/1006.0199 and references therein. It's about the relation of twistor geometry to the classical phase space of loop gravity.

Yes, I agree, my comment wasn't really a response to your post. I was more suggesting that actually the tetrad formalism is necessary to couple spinors to gravity. Also in that sense one might want to take seriously the extra solutions corresponding to degenerate metrics that have no analogue in...

How would you deal with coupling spinors without introducing a tetrad?

One thing I have always wondered in this context is what happens when one talks about the scattering of D0-branes, which are pointlike objects. In that case, no smearing out of the interaction vertices occurs, as it does for extended objects like strings. So doesn't this bring short-distance...

At low energies, M Theory is described by branes embedded into a 11 dimensional Minkowski space. A p-dimensional brane traces out a p+1-dimensional worldvolume during its time evolution, just as a point particle traces out a one-dimensional worldline. It is therefore the bulk already that...

This is not quite a deep thing, and definitely not singling out 4D over anything else, but what's peculiar about 4D is that the hodge dual of any 2-form is again a 2-form. In GR, this is what makes possible the switch from the palatini action to the holst action, which is equivalent on a...

Really no one with a hint? Is the question somehow ill-posed? Or should I add more detail about the specific calculation I am attempting?

The argument for invoking sampling theory is that the bandlimitation immediately follows from the presence of a minimal length. Wether there actually is a minimum physically relevant distance in nature is of course unsettled, but arguments for its existence follow pretty straigthforwardly from...

Hi, I have a question about the relation between the propagator of a scalar field and the heat kernel. I'm not sure wether I should rather put this question into the math section: Given a Laplacian D on some manifold M, what I mean by heat kernel is just K(x,y;s) = \langle x | \exp(-sD)...

But the idea equally applies to AdS/CFT and its relatives (if they exist). If, for example, a working bulk theory for QCD is developed, which talks about gravity and strings in one more dimension and is entirely equivalent, there is no reason to believe that one description has more implications...

bcrowell, thanks for your insightful words about what is observable and what is gauge. It appears the clash is really that while GR and E&M can be expressed in a very similar language, the same mathematical objects in the respective theories are 'shifted' by one derivative with respect to one...