This account is temporarily suspended to cool down. The suspension period ends on Aug 28 '16 at 0:06.
1 reputation
493215
bio website
location New York City
age 41
visits member for 4 years
seen yesterday

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.


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
revised What is lepton number?
fix terminology
Apr
15
comment What is lepton number?
@Tim: Oops, I meant helicity, sorry, will fix.
Apr
15
revised What's the interpretation of Feynman's picture proof of Noether's Theorem?
minor 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
revised Is the EmDrive, or “Relativity Drive” possible?
added 1383 characters in body
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
9
revised What's the interpretation of Feynman's picture proof of Noether's Theorem?
added 7506 characters in body
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
revised Landau poles in dimension <4?
errors
Apr
7
revised Landau poles in dimension <4?
typo
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).