## Wednesday, April 26, 2017

### quantify curvature

the curvature can be quantified with the invariant curvature $R_{\mu\nu\rho\sigma}R^{\mu\nu\rho\sigma}$ or with the Ricci-scalar $g^{\mu\nu}g^{\rho\sigma}R_{\rho\mu\sigma\nu}$. do they carry the same information? can you think of situations where the Ricci-scalar is zero but the invariant curvature is not?

## Sunday, April 23, 2017

### duality and the propagation of waves

the propagation direction $\vec{k}$, the electric field $\vec{E}$ and the magnetic field $\vec{B}$ of an electromagnetic wave form a right-handed system: is the same true if one applies duality, i.e. if one interchanges $\vec{E}\rightarrow \vec{B}$ and $\vec{B}\rightarrow -\vec{E}$?

this post is in celebration of $2^{17}$ views.

## Wednesday, April 19, 2017

### dark energy in the lab

would it be possible to determine properties of cosmological fluids in a laboratory experiment?

## Wednesday, April 12, 2017

### Rydberg satellites

would it be necessary to impose angular momentum quantisation for the orbits of satellites around Earth? what would be the radial distance between successive orbits?

## Wednesday, April 5, 2017

### entropy density

can you estimate the entropy density of the universe and compare to the energy density?

## Wednesday, March 29, 2017

### measurement of the Planck-constant

could one use naturally occuring Planck-spectra for determining the Planck-constant? what would be a viable experiment and would the Sun or the CMB provide a better measurement?

## Wednesday, March 22, 2017

### 40, 20, 10

imagine that one measures the 40 components of the Christoffel-symbol by observing geodesic motion: in what way are the 20 components of curvature fixed? and how does that determine the 10 components of the metric?