Recent content by berra

I am asking how you go from what you see with your eyes or a measurement device that is to my understanding the best representation of a time-foliation of spacetime to making a mathematical model of it in spacetime. I will read the wiki page now and see if it addresses my question. Nope, I...

Say you have a box with a round marble on it. Classically I would have no problem describing that, but how is it done in general relativity? Do one just assume that the marble is round and that the box is ... boxy... in the 3D subspace at a certain foil of time? (Foliation is the correct term...

I am wondering how big the local coordinate charts in a coordinate atlas of General Relativity can be. Is it just a few nanometres or all the way to the furthest stars? Also I wonder how the speed of light is defined in GR. Is the velocity defined as the differential with respect to coordinate...

Thermodynamical computations. Gibbs free energy requires temperature and pressure constant, for example. But the temperature is certainly not constant when heating in a microwave.

Hi, I just microwaved some food in a plastic container and noticed that it started to buckle inwards. So I wonder what thermodynamic variables were constant? Obviously not the volume V. The number of particles N was probably approximately constant. What more than N was constant?? What ensemble...

The approach I have seen is to write the free charges as \rho_f = \langle\sum_{g\in G}(\sum_{k\in g} q_k) \delta(\mathbf{x}-\mathbf{x}_g)\rangle and free current as J_f = \langle\sum_{g\in G}(\sum_{k\in g} q_k)\frac{d \mathbf{x}_g}{d t} \delta(\mathbf{x}-\mathbf{x}_g)\rangle where the partition...

I thought the plane wave approach to the field equations were only a partial solution. Okay I guess the wave equations for the particles in quantum physics is a smooth function. I was reading Classical multipole theory by Raab and Lange and they did not mention this in the non quantum chapter...

If he doesn't do spatial averaging, his equations are not smooth and thus curl is not defined? Curl only works on smooth differential manifolds? Something feels amiss. Also, why does he only include polarisation and not the higher order moments? Thanks anyway... Guess I'll have to search even...

I mean elements of the group, not atoms! Like electrons or protons. I don't believe you about the D and H fields, without charged particles Maxwells equations do not apply. Like in Russakoff, there is always free charge (it can have charge 0) that is composed of bound charges. The "bound...

Hi, I just need to ventilate and see your opinions. Charge can be partitioned into groups of particles of one or more elements. If they have more than one element it is a molecule, and its position is represented by for example the center of mass, seeing each particle as carrying both the...

It seems to me you have written your jacobi matrix wrong. The i'th row <=> coordinate function x_i, the j'th column <=> partial derivative with respect to argument j.

I want to explore the phase shift from gas to liquid along the wall of a container (its cold outside it). I guess I would have to have a semiopen system then, since the liquid only arises on the wall. But then what is the volume of the system so I can look at the phase diagram and see if it has...

I am confused over what the volume V stands for in thermodynamics. Is it the smallest possible region of space containing all particles being studied? If so does it have to be connected? Is it the volume enclosed by the surroundings of the system? If so, i thought the system was supposed to...

Thanks for all answers, I feel like I should explain my worries maybe hehe. The reason I asked was that I read a huge black hole is 300 million lightyears away. So let's say it takes 100 generations until its possible to evacuate earth, we wouldn't need to worry then until it is a couple of...

I wonder what the minimum safe distance would be for a black hole with mass of a couple of billion solar masses. With that I mean at what distance would the gravitational pull be negligible, I guess. I am sorry if this is the wrong kind of question to ask in this forum.