Daniel

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bio	website	about.me/danieldf
	location	Providence, RI
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Research Faculty at Brown University, Theoretical Physics (hep-th, math-ph, hep-lat, gr-qc).

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	2,568 reputation	bio	website	about.me/danieldf	visits	member for	2 years, 6 months
416 badges		location	Providence, RI		seen	Feb 22 at 21:12

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Nov 16	answered	Type of stationary point in Hamilton's principle
Nov 16	comment	How does gravity escape a black hole? @Marek: not to play devil's advocate... but, still, even under String Theory, the "Information Paradox" is not fully resolved (even if some claim otherwise, and sell books based on it ;-).
Nov 16	answered	How does gravity escape a black hole?
Nov 16	answered	Noticing that Newtonian gravity and electrostatics are equivalent, is there also a relationship between the general relativity and electrodynamics?
Nov 15	comment	What are some approaches to discrete space-time used in modern physics? @Amir: i perfectly understood your question (5). If you check the link i sent, you'll see that things such as "Planck Volume" and "Planck Time" exist and are well defined (and have a numerical value, as per your wish). So, in this sense, a notion of "spacetime atom" does exist, even if it's vague. To make it precise, it will of course depend on the particular theory you have at hand.
Nov 15	answered	What categorical mathematical structure(s) best describe the space of “localized events” in “relational quantum mechanics”?
Nov 15	answered	What are some approaches to discrete space-time used in modern physics?
Nov 15	comment	Is a “third quantization” possible? (continuing…) This geometrical meaning is given by noting that Curvature is really the relevant quantity in this game. The question, then, is the following: What would you get if you did what you want? Fine, you go ahead and quantize again… what kinds of structures do you get? What do they represent? I hinted at this in my answer above…
Nov 15	comment	Is a “third quantization” possible? @Tobias: But why would you do that? More importantly, which object would you get if you did that? Let me try to make a long story short, amputated: the Jacobi Metric is given by $\tilde{g}_E = \sqrt{2 (E - V(q))}$, where $V$ is the Potential energy for your system (be it particles, fields, etc). Once you re-write your Lagrangian in terms of Jacobi's metric, you map the Hamiltonian flow into the Geodesic one. The bottom line is that the eqs of motion, now, have a very clear geometrical meaning. (continues…)
Nov 14	comment	Suggested reading for renormalization (not only in QFT) Just a couple of more links that i think can come in pretty handy, given the above comments: <a href="dx.doi.org/10.1016/0003-4916(81)90072-5">Dimensional Analysis in field theory</a> and Renormalization as Dimensional Analysis — worth checking out. ;-)
Nov 13	answered	Suggested reading for renormalization (not only in QFT)
Nov 13	comment	Are the physical laws scale-dependent? @j.c.: you're correct, on both counts: i did have something else in mind; and the $\epsilon$-expansion is very good thing to keep in mind. ;-)
Nov 13	comment	Are the physical laws scale-dependent? It's very hard to find non-trivial solutions to the Renormalization Group eqs; in the end of the day, they are the ones that end up determining how a certain phenomenon in a given scale will behave in another. But, i'm not sure i'd go as far as saying "never"… but it is pretty hard.
Nov 13	comment	Are the physical laws scale-dependent? @Sklivvz: this is far from being a matter of opinion: there are macroscopic quantum effects: Bose-Einsein condensates being a good example.
Nov 13	comment	Are the physical laws scale-dependent? @Sklivvz: it's not true that wavefunction collapse is related to scale; see quantum decoherence. For more on this, see my answer below.
Nov 13	comment	Are the physical laws scale-dependent? @mbq: show me a single "Newtonian dynamics" effect that predicts the phenomena of Radiation decay, e.g., X-Ray radiation. Yet, its effects are very "macroscopic": you can get your X-Ray slide with you and show it to your doctor. Radiation decay is a purely quantum effect.
Nov 13	answered	Are the physical laws scale-dependent?
Nov 13	comment	Are the physical laws scale-dependent? Sklivvz: the statement of "scale independence" in Physics is usually associated to Conformal symmetry, ie, whether or not the theory in have in hand has conformal symmetry. In this sense, GR is scale dependent, for not all solutions of Einstein's Field Equations are conformally symmetric.
Nov 13	answered	Is a “third quantization” possible?
Nov 13	answered	Monte Carlo use