104,515 reputation
7143294
bio website motls.blogspot.com
location Czech Republic
age 40
visits member for 3 years, 9 months
seen 12 hours ago

Hi, I am a string theorist and a publicist.


Sep
6
comment How is weight distributed when legs are astride?
Because I don't want to calculate these things with the numbers.
Sep
6
comment How is weight distributed when legs are astride?
You write down the equation that the total forces on all parts of the feet from the floor are equal to the weight of the person; and the total torque $\vec r_i \times \vec F_i$ relatively to the person's center of mass is zero.
Sep
6
answered Why black body radiation is all over the frequency range
Sep
6
answered Why does the state space contain states with negative norm and what would be an example?
Sep
6
comment Gauge covariant derivative in different books
I downvoted the answer because it doesn't contain anything that answers the question. It only adds a relationship to one more question, about another sign, and therefore deepens, not clarifies, the confusion.
Sep
6
answered Dirac's remark that inspired Feynman when formulating path integral
Sep
5
comment What is R-symmetry with supersymmetric theory?
The theory simply has this symmetry. Note that $Sp(2)$ is often just a fancy name for $USp(2)$ which is isomorphic to $SU(2)$. You write down the lagrangian or equations of motion and see that the supercharges may be transformed in the most obvious way by an $SU(2)$ into each other. Each theory has different symmetries in general. So if you ask about an example and you don't really know how to derive it yourself, you won't know how to derive the symmetry group in other theories, anyway. Do you understand what $SU(2)$ means? I don't know what may be hard about the existence of an $SU(2)$.
Sep
5
comment What is R-symmetry with supersymmetric theory?
I noticed that you added a link - and you are aware - of the Wikipedia entry on that. What's wrong with that? I am unhappy to hear it was not useful because I started that Wikipedia page, too. ;-)
Sep
5
answered What is R-symmetry with supersymmetric theory?
Sep
5
revised Lie algebra - basis for adjoint matrix products in SU(N)?
added 194 characters in body
Sep
5
answered Lie algebra - basis for adjoint matrix products in SU(N)?
Sep
4
comment Derivation of formula of potential energy by a conservative force
Because if the kinetic energy increases, then the velocity - an increasing function of energy - increases as well (the velocity was positive, by assumption), and increasing velocity (acceleration to the right side) is achieved by a positive force, too.
Sep
4
comment Why does a wave actually diffract?
Well, waves are somewhat like diffusion, partly very different. They're different equations. Both have the Laplacian for spatial coordinates but the wave equation has the second derivative in time, while the diffusion has the first derivative in time. So the diffusion diffuses while the wave equation tends to preserve the wavelength.
Sep
4
awarded  electrons
Sep
3
comment Electron Charge is 150%?
But it is true that many of these empirical observations may be deduced by purely theoretical arguments, at least when we assume some of the empirical observations. Also, many worlds with wrong values of charges etc. wouldn't admit life so even without direct observations, one could prove many of the statements by the pure "anthropic" reasoning.
Sep
3
comment Electron Charge is 150%?
Well, indeed, helium's nucleus has charge $+2e$ and it attracts double electrons. But because the minimum atom with 1 electron exists, the minimum nucleus - the proton - has $+e$. There exist high-brow proofs that the electric charge ratios can't be irrational - linking things like magnetic monopoles and quantum gravity - but I am afraid that you don't have the background for those. Sometimes we just use the observations - nuclei with $e/2$ don't exist - for example.
Sep
3
revised Quartic terms as two particle interaction?
added 22 characters in body
Sep
3
comment Relative velocity of two cars
"And each time I try, I go with a different approach" - That's an interesting observation. Have you tried to calm down and observe yourself why your reasoning is so chaotic? Some other people learn the art of self-control so every time they try, they may repeatedly use the same correct approach that they previously identified to be correct.
Sep
3
answered Why group velocity represents energy or information transmission? What relation between phase velocity and special relativity
Sep
3
comment Why group velocity represents energy or information transmission? What relation between phase velocity and special relativity
Chris, but only if the whole wave is unphysical. If there are waves of a real physical medium or field that have been properly localized or assigned to the points, the group velocity is the same thing as the speed of the packets and be sure that the packets do spread information.