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location New York City
age 41
visits member for 3 years, 2 months
seen 4 hours ago

I do not participate on this site any longer, except to respond to comments regarding my own text, if that text is unavailable in another form. I do not accept the political moderation atmosphere here, it is not compatible with open science. I participate at physicsoverflow.org.


Oct
19
comment Are gravitomagnetic monopoles hypothesized?
@BenCrowell: Weak energy is never violated in the classical limit, and strong energy is only violated by scalar VEVs.
Oct
18
comment Why does kinetic energy increase quadratically, not linearly, with speed?
@bobie: I don't participate on this site, but in your case, the question is no good. The "force of inertia" is just stuff people said in Latin (Latin == stupid) before they understood Newtonian mechanics. A grounding in Newton removes any confusion. The concept of force is momentum flow from x to y, inertia is conservation of momentum. In non-inertial frames, you could say the fictitious force is "force of inertia", but that's a bad name. While I hate the moderation here, for other reasons, namely suppressing hostility, in this case, they are removing a question which is pure history.
Oct
11
comment What is a non linear $\sigma$ model?
@New_new_newbie: Sure, all is well in the sense that you have an approximate theory. It will have infinitely many parameters though that you need to fix, these ultimately should come from QCD, but if you fix them phenomenologically order by order, you get by.
Oct
11
comment What is a non linear $\sigma$ model?
@New_new_newbie: Renormalizability is not important today, but 1960 is well before QCD, and Gell-Mann and Levy didn't yet know about quarks.
Oct
11
comment What is a non linear $\sigma$ model?
@New_new_newbie: I don't know a reference, I worked it out for myself by considering what would happen if you simulated the nonlinear sigma model on a lattice. The natural way to do this is to use "x,y,z,w" coordinates with the lattice constraint that the sum of the squares is constant, and then doing a block-renormalization step, averaging the four fields, you just get back the usual scalar field model with O(3) symmetry, because the block renormalization doesn't preserve the hard constraint. It's the same argument as for the Ising model turning into $\phi^4$, and this is shown in Polyakov.
Oct
9
comment Why singularity in a black hole, and not just “very dense”?
@pabouk: Regarding 'settled state', the end point for the black hole is always a Kerr-Neumann static solution, the major issue is what happens to the interior matter in the case that it misses the singularity, i.e. when you have a charged/rotating collapse. My feeling on this is that it is partially ejected, and the black hole is only produced to the extent that mass-energy is destroyed at the singularity, and this is only massless stuff, for highly charged or highly rotating black holes. I suspect the full answer requires string theory, the classical theory has nonsense extra universes.
Oct
9
comment Why singularity in a black hole, and not just “very dense”?
@pabouk: Not clear. The singularity is timelike if the black hole is spinning or charged, and in this case, we don't have exact collapse solutions. It should be possible to figure out the behavior of exact charged collapsing dust, but it hasn't been done. The singularity theorem has been misinterpreted to mean that there is a spacelike singularity which swallows all matter, and this is only true for spherically symmetric collapse, it fails when the singularity is timelike. My own opinion about what happens in this case is that there is a partial explosion, but this is not what others think.
Sep
30
awarded  Explainer
Sep
24
awarded  Autobiographer
Sep
22
awarded  Nice Answer
Sep
17
comment Is the universe a quantum computer - is light speed barrier a computational constraint
@celtschk: here universe==the observable universe. The number of bits is assumed by the number of Planck areas on the cosmological horizon. The U atom with electrons has 350 particles, which, with even a coarse quantum description swamps the size you can simulate with a classical computer that size. But of course, for ground states, or low excited states there are better methods like DFT.
Sep
16
awarded  Nice Answer
Sep
13
awarded  Nice Answer
Sep
11
awarded  Nice Answer
Sep
5
comment An electromagnetic space elevator?
@Floris: You don't need to do work to get the thing off the ground, you supply the current once, it's next to no work, then you cool it to superconducting and it traps the current in there. Then you let it go, and it rises up, the Earth just slows down as it lifts the thing. The magnetic field of the Earth is ultimately because the core is rotating. The cryostat doesn't need to be heavy compared to the wire, the wire can be a bundle of wires 1m in radius, the cooling is at the boundary. This is perfectly feasable.
Aug
29
comment When we define the S-matrix, what are “in” and “out” states?
@A.Zerkof: The asymptotic plane-wave states are already smeared out over infinite volume and are "noninteracting" in the sense that the scattering is a subleading correction to their behavior. This is why the Fock space is already in the theory.
Aug
29
comment How Exactly Does Linear Regge Trajectories Imply Stability?
@annav: This kind of string theory is now known as AdS/QCD, and nobody disagrees that it is predictive for hadron resonances, although the degree of quantitative agreement can be disputed. This stuff predicts things like hidden local symmetry (Hidden Regge theory in the 80s), topological baryons (Large N physics of the 80s), and Polchinski-Tan Pomerons (BFKL pomerons in string theory, early 2000s). There's a ton of work, this should give good handles, but AdS/QCD is most recent and most direct. This is not the same as predictions of fundamental string theory, the results are not controversial.
Aug
27
comment Does a Weak Energy Condition Violation Typically Lead to Causality Violation?
No, I didn't ask for an example of a spacetime that violates weak energy and doesn't have a CTC (reread the question). The negative mass Schwartschild solution is such an example. I asked for a spacetime which violates weak energy where you can't use the violation to produce a different spacetime, which is necessarily present in the theory if the first is, which does contain a CTC.
Aug
27
awarded  homework
Aug
26
comment Does a Weak Energy Condition Violation Typically Lead to Causality Violation?
I don't see how to model shrinking black holes classically in any way that doesn't have negative focusing somewhere. Regarding M<0 Vaidya spacetime, there is no CTC in one of these, but if you put together two boosted copies of these moving relative to each other very fast, you get CTCs.