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487207
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location New York City
age 41
visits member for 3 years, 9 months
seen 6 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. Unfortunately, this seems to be a recurring pattern on such sites--- they grow with promises of open participation, and then shut down in a phase transition of censorious moderatorship. Hopefully physicsoverflow.org will be the first exception to this rule, as the policies there were crafted specifically to avoid this phenomenon.


May
1
comment How do we know that nonperturbative canonical quantum gravity is wrong?
@user1247: Because 2d geometries are classifiable, you can sum over them, and if you look at 3-branes, they don't delocalize the same way, they stay where you put them, so that AdS/CFT on those doesn't have to sum over all topologies of these, rather only over the string exchanges between them, or over the asymptotic 4-d boundary theory in the low-energy limit. Points delocalize, but point-black-hole AdS/CFT gives matrix theory which is even better behaved than strings. The problem just never comes up, which is one reason why strings are so amazing.
Apr
17
comment Why the Principle of Least Action?
@Self-MadeMan: When you don't know the initial conditions, you place a probability distribution $\rho$ on these, then you evolve $\rho$ by evolving the initial conditions according to Newton's laws. Then the information missing in the encoded ignorance of the probability distribution $\rho$, which up to an infinite log-divergent constant (depending on the phase space discretization), $\int \rho\log\rho dx dp$ over phase space, is constant. This is the 19th century law of conservation of entropy in classical reversible mechanics, basically uncovered by Boltzmann/Lorschmidt, Liouville's theorem.
Apr
16
comment How do you start learning physics by yourself?
@OmnipresentAbsence: The Einstein story is not a myth to make incompetent people feel better. It is a fact that Einstein was flunked by Hermann Minkowski in 1902 because he considered Einstein a lazy math student. Einstein was not studying Minkowskian things like rings and abstract algebra, but instead was beginning his revolutionary work on atomic theory. This led Einstein to not get recommendation letters, and he ended up in a patent office. The story is not telling people to be lazy, rather it is warning them that the social order can't appreciate radical work, and will punish them for it.
Apr
15
comment What is lepton number?
@Tim: The issue here is that the neutrino field $\psi$, the chiral field with a single chirality, is not the only field associated with the neutrino. There is also $\bar{\psi}$ of opposite chirality. When the fields are massless, $\psi$ produces neutrino of one helicity, and $\bar\psi$ produces antineutrinos of opposite helicity. When the neutrino is 2-component massive (like in nature), the mass term mixes up the two helicities to one massive particle of no definite helicity, and the massive particle is from a combination of $\psi$ and $\bar\psi$, often also represented as a majorana field.
Apr
15
comment What is lepton number?
@Tim: To explain more properly, the two-component Lagrangian mass term for a chiral field $\psi_i$ is $\psi_i \psi_j \epsilon^{ij} + \bar{\psi_\dot{i}}\bar{\psi_\dot{j}} \epsilon^{\dot{i}\dot{j}}$. The field $\psi$ has one chirality with one helicity when massless, the field $\bar\psi$ transforms as the conjugate of opposite chirality and has opposite helicity, and creates an antineutrino, and the mass term violates lepton number. By chasing a nearly massless neutrino, you reverse direction of motion, and you convert it to an antineutrino, violating Lepton number by boosting.
Apr
15
comment What is lepton number?
@Tim: Oops, I meant helicity, sorry, will fix.
Apr
15
comment What is lepton number?
@Tim: Chirality is only Lorentz invariant for a massless neutrino, it's the spin in the direction of motion. Your comment is incorrect. There is no chirality for a stationary neutrino, there is only a spin. Depending on which way you boost to infinite momentum, you get opposite chirality.
Apr
14
comment Reading the Feynman lectures in 2012
I just did (3d) now, using the most idiotic method. 10 bodies is about right, maybe you can get 20 or 30, I never did it systematically. There are good approaches for blocking up the computation into regions so that you don't have to compute order N^2 forces, rather distant bodies in blocks. You need to deal with drift and the no-control when two bodies collide, you get too-large instantaneous force, and a spurious numerical explosion. You can fix using dynamical timestep that is small enough to ensure energy conservation, or with the two-body in constant+linear background method I described.
Apr
14
comment Reading the Feynman lectures in 2012
@tom: On a recent computer, if you just write the most basic code in C, without fancy classes, just the simplest code, 1000 particles do 10 steps a second. I heard this from a layperson (father of a friend) who showed me his code for gravity clusters and noted that he found systems that stabilized to crazy but non-chaotic orbits long before they reached two particles. He wanted to publish this. There are simple tricks--- use a second order method, and when two particles are about to near-collide, use the exact solution two particle scattering in background field as the first approximation.
Apr
9
comment What's the interpretation of Feynman's picture proof of Noether's Theorem?
@Qmechanic: I expanded my answer to include the simplest formal equivalent of Feyman's argument, and now I see that I ended up unintentionally plagiarizing your answer a little bit. I think I emphasized sufficiently different things to make my answer useful, and since your answer also overlaps my answer in a similar way, I hope you are not annoyed at this.
Apr
8
comment Is it possible to separate the poles of a magnet?
@RetardedPotential: Will answer on physicsoverflow.
Apr
7
comment Landau poles in dimension <4?
@AbdelmalekAbdesselam: It's not a coincidence, but best to ask on physicsoverflow.
Apr
7
comment What's the interpretation of Feynman's picture proof of Noether's Theorem?
The "horizontal line" means perturbing the velocity from $\dot{x}$ to $\dot{x} + \epsilon \delta(t-t_0)$, where the perturbation is thought of as an infinitesimal kick at time $t_0$. This is not mathematically sensible by itself without thinking a bit about regulating the delta-function, but when you do regulate everything and cross the t's and dot the i's, Feynman's proof goes through and produces the shortest niftiest proof of Noether's theorem. It is nowadays standard to use a continuously varying kick $\dot{x} + \epsilon(t)$ instead, to avoid the limits-talk. See my answer.
Apr
7
comment Landau poles in dimension <4?
@AbdelmalekAbdesselam: yes, you are right, it's 4.5 not 1.5, of course, I am sorry for the lapse.
Apr
7
comment Landau poles in dimension <4?
@AbdelmalekAbdesselam: Whoops! The correlation function goes as $1/|x-y|^{1.5}$, not the J! The J powerlaw is fixed by demanding that the equation of motion gives this correlation function as a solution, I'll fix it now. For the $\alpha$, the range of allowed $\alpha$ which produce unitary field theories is precisely the ones for which the Schwinger representation is a sum over Levy flights with a sensible probability exponent, which is why I like to call these "Levy field theories". Generalizing traditional particle Brownian paths to Levy Flights was my path to these, not Speer.
Apr
6
comment Why does a gas get hot when suddenly compressed? What is happening at the molecular level?
@user462437: Each ball would get a smaller increase in speed, and the process of moving the wall in would take longer, the net result is the same (obviously, but work it out if it is confusing).
Apr
6
comment Why does a gas get hot when suddenly compressed? What is happening at the molecular level?
@user4624937: Slow or fast, the gain in energy from the collisions is the same, it is the total work done against the pressure. I was assuming it is adiabatic already.
Apr
4
comment Quantum Mechanics by Dirac
@physicslover: Disclaimer: I learned from Dirac. I made up my own exercises by scrounging undergrad books, writing simple programs, and trying to understand chemistry and so on. Dirac was very good for showing how to use formal methods to guide physical intuition, because Dirac's arguments are extremely formal, guided by mathematical identities, e.g. his clever but nearly physically meaningless derivation of the canonical commutation relations in the early intro chapters. The perturbation theory there is excellent, and you can make up your own exercises easily by perturbing the HO.
Apr
4
comment Quantum Mechanics by Dirac
@physicslover: Young Feynman had issues due to his philosophy that one must rediscover everything (true, but he overdoes it). He eventually redid QM, via the path integral. Dirac's book is best read in parallel with Feynman and Heisenberg's original papers (Dreimannarbeit too). The physical picture can be lost if you don't know the old quantum theory and the stuff that's now on Wikipedia under "Matrix Mechanics". But given this stuff, Feynman's vol III, Wikipedia, Dirac is a very good intro to canonical QM. The only part that is not so great is the QED, but even that covers Dirac gauge well.
Mar
27
comment How can it be that the beginning universe had a high temperature and a low entropy at the same time?
@Sklivvz: The post starts with the rejection of the position, so what you say is clear, I think.