# Search results

1. ### Hello!May I ask a few questions, I am trying to understand how the

Hello! May I ask a few questions, I am trying to understand how the metric tensors work in general relativity. In particular, why you would want to make either the radial or transverse part look Euclidean? In what situations is that useful? I am happy that in a very general form, the...
2. ### Acoustics - Critical Bandwidths ?

Hello! I'm getting a bit confused with how to deal with sound waves that are within one critical bandwidth of each other. I do not fully understand how you are meant to combine the intensities of sound waves when they lie within one critical bandwidth?! I would really appreciate some...
3. ### The Shape of the Universe, and General Relativity

Hello! I'm trying to get my head around general relativity at the moment...(!), and there's one aspect of it that's really causing me a lot of kerfuffle. I understand that in an appropriately sized local inertial frame, the laws of special relativity occur. On those scales the curvature...
4. ### The Affine Connection components

Hello! I have a few questions about how the Affine connection works. I know the geodesic equation; \Gamma^{\lambda}_{\mu \nu} = \frac{\partial x^{\lambda}}{\partial \xi^{\alpha}} \frac{\partial^{2} \xi^{\alpha}}{\partial x^{\nu} \partial x^{\mu}} So, for example, if I had the...
5. ### Mean Energy Fluctuations

Hello! I'm getting muddled with the notation in my notes, in which I have \overline{(E-\overline{E})(P-\overline{P})} From which you can get \overline{EP} - \overline{E} \ \overline{P} I can see where these come from, but not where the \overline{E \overline{P}} - \overline{\overline{E}...
6. ### The Wave Zone Definition

The "Wave Zone" Definition Hello! We are trying to figure out exactly what the "Wave Zone" is when considering potentials. We know that a moving charge will generate a "disturbance" in the surrounding fields, which propagates outwards at light speed. This means that for great distances...
7. ### Help with differentiating summations

Hello! I'm getting confused when differentiating summations. I understand that if you differentiate an expression and it gives a kroneker delta, that then sums over the appropriate summation and it disappears. But in my notes it has \frac{\partial}{\partial p_{i}} [-k \sum_{i=1}^{r} p_{i} ln...
8. ### Differentiating Tensors

Hello! I am very VERY confused! Would anyone please be kind enough to point me in the right direction. I read that, in general, the derivative of a tensor is not a tensor. What do you find when you differentiate a tensor, then? I thought that you wanted to find the acceleration, to...
9. ### Acoustics Wave Equation

Hello! When considering the acoustics wave equation \frac{\partial^{2}P}{\partial t^{2}} = c^{2} \nabla^{2} P I don't really understand why you can say that the applicability of this equation varies for different sound pressure levels. I don't see why this shouldn't hold for all...
10. ### IDL and arrays.help!

IDL and arrays.........help! Hello! I have 2 .fits files which I wish to read in to IDL. Then I want to convert these into arrays with i columns and j rows. I am going a bit mad trying to 'define' i and j and would really appreciate any help. Say I had 2 files, A.fits and...
11. ### Luminosity Distances

Hey I'm getting very muddled with my units, and would really appreciate some clarity :-) I have angular distances between galaxies at some redshift, in arcseconds I want to calculate the distance in parsecs, taking into account the luminosity distance. In the equation; r =...
12. ### I've been thinking about blackbodies, and have become confused with

I've been thinking about blackbodies, and have become confused with regards to their temperature. Is the temperature constant throughout in a blackbody, or is there a temperature gradient dependant on which side is closest to the source of heat? Would it matter?
13. ### Treating fluids classically

Hey Just wondering why atoms in liquids can be considered classically If you compare the deBroglie wavelength with the atomic spacing, I understand that the deBroglie wavelength should be much smaller than the spacing. But how could you show that? If \lambda = \frac{h}{mv} How...
14. ### Are all of the quantum numbers of anti-particles opposite

Hey Just wondering- Are all of the quantum numbers of anti-particles opposite to their corresponding particle quantum numbers? I know that lepton numbers are opposite for particles and anti particles As is charge..... But what about the quantum numbers like strangeness, spin, parity...
15. ### Conservation of Lepton Number

Hello there! May I ask: I am learning about the conservation of lepton number. I understand that the lepton number is composed of different sorts of lepton number, L_{\mu} and L_{\tau} and L_{e}. And these are conserved in all interactions. I am confused however about the...
16. ### Calculating the Redshift at which the Universe was dominated by different components

Homework Statement Hey :-) I want to calculate the redshift at which the Universe became vacuum dominated, with given values for the energy densities of Omega_{m} and Omega_{Lambda}. Homework Equations I know that the scale factors rho_{m} is proportional to a^{-3} rho_{r} is...
17. ### Using the RW Line element

Hello! :-) I've been thinking about the RW line element (c^2 d tau^2), and I understand that there is a term which describes the geometry of the Universe (d sigma^2) and then a term corresponding to the cosmic time (c^2 dt^2). The resulting (c^2 d tau^2 ) term would be the line measured by...
18. ### Confusion with Uncertainties

Hey :-) I want to establish the uncertainty in my fitted model to my data I can evaluate the variance for the predicted model, but I'm getting very confused as to how the variance can then give me the uncertainty in my model :S I know that the standard error of the mean may be taken as the...
19. ### The formation of Galaxy Clusters from density fluctuations in the early Universe

Hello!:wink: May I ask for a little assistance in filling in the gaps of my understanding? :S I understand that the current large scale structure of the Universe is thought to be the result of early fluctuations in density which have been stretched out in the expansion of the Universe. I...
20. ### Converting units for Mass Estimates of Astronomical bodies

Hello! I'm new so I hope I've posted in the right place One quickie please, if anyone can help me :-) I hope to get my mass estimates in units of h70-1 x 10{blah} Msun However! I have found some values in units of just x10{blah} Msun (*) Where they have used h50 in their calculations...
21. ### Confusion with conversion for the Hubble Constant

Hello! I want to get my data in terms of h70 -1 Sometimes the estimates are in terms of h50 or h70, and I know how to convert those just fine. However, sometimes they give H0 = 100 h Mpc-1 kms-1 instead..... My question is, what is this in terms of h? I know that the relation is...