Wednesday, April 27, 2016

divergence-free tensors

can you find apart from the Einstein-tensor and the metric tensor new tensors whose covariant divergence vanishes? would they be viable terms for the field equation?

Wednesday, April 20, 2016

eternal sunshine

can you estimate the life-time of the Sun with the result from previous week, assuming that the source of energy is fusion? What about Jules Verne's idea that the Sun was powered by meteors hitting the surface or by burning coal (to be found in "Round the Moon")?

Wednesday, April 13, 2016

more power, R2!

can you estimate the power density of the Sun, i.e. the amount of power produced by fusion in each cubic meter? what measurements would you need to carry out, and what assumptions do you make?

Wednesday, April 6, 2016


can you use mechanical similarity to explain why astronauts on the moon appear to move in slow motion or as if they were under water?

Wednesday, March 30, 2016

Zeno and cosmology

can you give an intuitive argument why in a FLRW-cosmology with only a cosmological constant $\Lambda$ distant objects appear at a constant angular size irrespective of their redshift?

Wednesday, March 23, 2016


if gravity is equivalent to an inertial force, are then all inertial forces gravitational? if so, where does the bulging of the Earth come from, which is usual attributed to centrifugal acceleration due to its rotation?

Wednesday, March 16, 2016

$\pi$ in the sky

one needs the value of $\pi$ for quite a number of computations in FLRW-cosmologies: can you list them and say to what accuracy $\pi$ is actually needed?

bonus question: which of the approximations of $\pi$ would work for these particular purposes?

(this post in in celebration of 14.Mar.2016 just 2 days ago)