# cosmology question of the week

CQW is an educational resource for theoretical physics and astrophysics, field theory, relativity and cosmology. we post a new question every wednesday for students to get and for teachers to stay in shape.

## Wednesday, February 22, 2017

### charge asymmetry, part 2

if the charge of electrons was not the exact opposite of the charge of protons, the rings of Saturn would act as a current loop, resulting in a magnetic field. could you estimate the magnitude of the magnetic field (in terms of its dipole moment)?

## Wednesday, February 15, 2017

### gravitational field of the hot sun

could you estimate by how much the gravitational field of the sun is larger because of the high internal energy compared to simply the rest mass? can one write down a correction factor as a function of temperature $T$?

## Wednesday, February 8, 2017

### spin-down of the Crab-pulsar

the Crab-pulsar is a rapidly spinning magnetic neutron star, which induces very large electric fields in the surrounding Crab-nebula, where the resulting electric currents are dissipated by Ohm losses. could you estimate the time-scale of this process and judge if it's a viable model for explaining the spin-down?

## Wednesday, February 1, 2017

### electromagnetic duality

could you provide an argument (both for the equations of motion and the Lagrange-density) why in empty space electrodynamics is invariant under the replacement $\vec{E}\rightarrow\vec{B}$ and $\vec{B}\rightarrow -\vec{E}$?

## Wednesday, January 25, 2017

### gravity and retardation

the field equation of general relativity is invariant under time-reversal (and parity inversion). in what way is retardation of the gravitational field generated by a matter distribution is preferred over advanced potentials?

## Saturday, January 21, 2017

### forces in relativity

relativistic forces are always velocity dependent but velocity dependent forces are not necessarily relativistic... is that true?

this post is in celebration of $10^5$ views!

this post is in celebration of $10^5$ views!

## Wednesday, January 18, 2017

### very heavy and very light

can you explain why the ratio between the Hubble mass (i.e. the mass inside the Hubble volume today for a critical universe) and the Planck mass is about $10^{60}$? why are stars roughly in the middle (on a logarithmic scale)? or even more puzzling: why's the ratio between Hubble and Planck-mass about equal to the ratio between stellar masses and the masses of nucleons?

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