Recent content by Goddar

Right, i get that. With large enough "universe", expansion will work to make a signal never reach back.. Probably time to wrap-up this post and dig more into the literature. Thanks, guys!

Thanks for the input, guys. Ok. The other, more obvious interpretation would be that we happen to sit on a more or less central region of an expanding-ball like universe but i understand this is discarded on the grounds that we have no reason to feel special... still possible though, since...

Thank you, Orodruin. (and others, sorry i hadn't seen yet!) Yes, i always understood that this is what was observed (i mean on average: i know they can also cross paths, collide or even orbit each other at times..); My understanding (so far) was also that the interpretation of space itself...

Yep, indeed it's starting to sound like black magic... I'm not going to insist because i hate to be annoying but indulge me one last bit, please. So to be clear we're talking about a universe that doesn't necessarily require extra-dimensions and where the metric is nearly flat; Let's imagine...

Thanks, Phinds. I understand your point; it seems though that we can still determine an effective "center" (i.e. the center-of-momentum) at any time? That would seem to make the isotropy principle only valid with respect to this point (and i don't mean in some absolute space, but in a purely...

Thanks for the reply. Ok. Then what's the right picture (in Big Bang model, i mean)? I meant an "edge" in the sense that a Big Bang model seems to suppose a finite amount of matter (maybe locally? there could be many local Big Bangs..), and this matter would be spread within a finite volume of...

Hi there. I'm having a hard time understanding the precise meaning of the so called "cosmological principle": My understanding of the general Big-Bang model is that far enough back in time the observable universe came down to something very small (compared to now), very dense, very hot... Ok, i...

Hi. The most useful formula i know for the Maxwell stress tensor is (in Gaussian units) the matrix form: T = (1/4π)[EE + BB – ½I(E2+B2)], Where I is the identity matrix and , for example: EE = (Exi +Eyy +Ezk )2 = (Ex)2ii + (Ey)2jj + (Ez)2kk + ExEy(ij + ji) + ExEz(ik + ki) + EyEz(jk + kj)...

Hi. You can't possibly do that without imposing more constraints on your vector: In tensor language, the vectors are contracted on both sides so you can't "solve" for b. This may be confusing because a⋅b looks like a vector expression but it's really a scalar; if you want to solve for a vector...

Hi. This is pretty straightforward, there are no tricks involved with wires and such. In the first part, start from Biot-Savart law: - express the current density J in cylindrical coordinates in terms of the current I (with its vector-direction) and delta/theta-functions; - plug this into...

Hi. I think you're taking a detour here... You obtained the full wave function in momentum space: Φ(k,t ) = Φ(k,0)⋅exp(–ik2t/2m) (remember in momentum space p-hat is just p, not a differential operator) So to get the spatial wave function, just take the inverse Fourier transform.

Hi. This all seems a bit confusing, partly because of notation and possible typos... (What's the correct r(t) you are given, by the way?) Now regardless of that, you seem to have taken a wrong track in the beginning. So to avoid the hats I'm going to define: r-hat = er ; θ-hat = eθ , and...

Seems like you're right here (disregard my last comment about denominator of A): you indeed get only one power of sin(θ) in the denominator of B so at least up to a constant depending on your units, this should be right. (by the way, B simplifies further since k = 2π/d) I won't have time to be...

Ok, first: 1–exp(2πi cosθ) = –2i sin(πcosθ)*exp(iπ cosθ), So as i said earlier the difference between the two integrals is just a phase (a factor of magnitude 1 that doesn't depend on r) and won't affect the total power but to see this in detail you have to look at the final integral over dP =...

The curl is not that simple on the derivatives, especially since the z unit-vector must be converted in spherical coordinates... The whole thing is not very complicated though, just don't mix up notations because that would throw all your results away. For E, if you derived this relation from...