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 $T$?

# 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 15, 2017

## 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?

## Wednesday, January 11, 2017

### forces on dust particles

can you compute the ratio between radiation pressure and gravity acting on a dust particle in the solar system? could one relate the two forces to the mass of the Sun and establish a minimal mass from which on there would be no dust and hence no planetary system?

we wish all CQW-readers a successful year 2017 full of cosmological discoveries.

we wish all CQW-readers a successful year 2017 full of cosmological discoveries.

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