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?