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bio website motls.blogspot.com
location Czech Republic
age 40
visits member for 3 years, 8 months
seen 7 hours ago

Hi, I am a string theorist and a publicist.


17h
comment What exactly does $S$ represent in the CHSH inequality $-2\leq S\leq 2$?
Dear dk, local realist theories imply that $|S|\leq 2$ is true, i.e. it always hold. The negation of this statement is that $|S|\gt 2$ holds at least sometimes. So it's enough to find one example where $|S|\gt 2$ and all local realist theories are ruled out.
Sep
21
comment Anti-neutrons, anti-quarks, isospin: What is observed and what is derived?
Well, yes, no, even Majorana neutrinos are in principle different from the antineutrinos because the words "neutrinos" and "antineutrions" get correlated with the helicity. It's the same species but if you see a quickly moving particle, you may say whether it's what is called "neutrino" or an "antineutrino" - the Majorana means that the Lorentz-invariant multiplet must contain both of them.
Sep
19
comment what physical quantity do real scalar field operators create/destroy?
It is a quantum number with eigenvalues $+1$ and $-1$, so it's a form of parity, but an unrelated parity to the usual reflection in space. It reflects the sign of $\varphi$. Equivalently, it's $+1$ if the number of the quanta is even, and $-1$ if it is odd. The operator $\varphi$ may create or destroy but whatever it is, it changes an even number to an odd one or vice versa. So the parity of the number of particles always changes by $\varphi$, and if the Hamiltonian is even in $\varphi$ or its derivative, this parity is conserved.
Sep
18
comment Inconsistency between Helmholtz and Gibbs Free Energies
Oops, I think that you are completely right - at equilibrium, G is constant. answers.yahoo.com/question/index?qid=20110806205610AAF8EbM
Sep
18
comment Inconsistency between Helmholtz and Gibbs Free Energies
Hi Tom, well, take a mixture of water and ice at the melting point. It has a well-defined pressure, temperature, and number of molecules, but the fraction the ice may be anything - you have to add one full intensive variable (the percentage of ice) to describe the system. So G can have any value in an interval for those values. This mixture of ice and water is really an optimized replacement of some "forbidden part" of the phase diagram.
Sep
13
comment If water evaporates at below 100 degrees then why not stone?
Phonon, nice, but is the rate of sublimation stictly zero for a rock at room temperature, or just approximately so? Is the rate the exponential of a huge negative number? What is it and why is it large?
Sep
10
comment Showing a fourth rank tensor in $\epsilon$'s reduces to one in the metric $g$
Yes, it is never equal (by equivalent, did you mean equal?). Just substitute $\mu\nu\rho\sigma=1123$ and $x=(0,0,1,1)$, for example. Only $g_{\mu\nu}$ is nonzero among all the metric coefficients, so the first expression is zero while the second is not.
Sep
6
comment Why black body radiation is all over the frequency range
OK, I understood you well but what you wrote is wrong, agreed? You can't write "distribution" if you mean "energy of a particular particle". The distributions are fixed by the temperature and saying that "anything goes" is exactly as wrong as saying that "everything must have one frequency", although these two claims may err on the opposite sides in some sense.
Sep
6
comment Why black body radiation is all over the frequency range
The fact that the temperature is an average doesn't mean that the distributions are not unambiguously determined at thermal equilibrium. They are. Have you heard of Maxwell-Boltzmann distribution for an ideal gas, for example? You may calculate the average even for other distributions but if the distribution doesn't agree with the Maxwell-Boltzmann distribution (e.g. for a gas), then there won't be any equilibrium and you shouldn't say that the average energy you calculate determines a well-defined temperature! Out of equilibrium, the temperature isn't well-defined.
Sep
6
comment Why black body radiation is all over the frequency range
It's not true at all, Anna. Thermal equilibrium dictates all statistical distributions of energy and every other quantity, given a well-defined physical system. All the distributions are ultimately derived from the Boltzmann one. This distribution is nonzero everywhere - not peaked at particular frequencies.
Sep
6
comment Why black body radiation is all over the frequency range
What radiates are not free electrons but bound objects like atoms or - which is better to imagine - harmonic oscillators from springs around the lattice sites. They have different frequencies but each of them has $kT/2$ average energy per degree of freedom. Free electrons, unless they collide with something, are moving by a constant speed and they don't radiate. The speed of the overall motion of a particle (like a free atom or electron) is also dictated by having $kT/2$ per degree of freedom in average but it isn't "exactly" this much for each particle. There is a calculable distribution.
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
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
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
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
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.