Shorter days for Saturn


Using the change of radiation as Saturn's magnetic field rotates, the Voyager spacecraft measured almost 30 years ago that a day of Saturn lasts 10 hours, 39 minutes, and 24 seconds. However, the same measurements by the Cassini spacecraft suggests that it lasts 10 hours, 47 minutes, and 6 seconds, over seven minutes longer. Such a significant change in the bulk rotation rate of Saturn probably means that Saturn's magnetic field does not quite rotate in lockstep with the planet itself. Another method might be needed to measure the length of a Saturnian day.

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Cloaking from earthquakes

Remember the possibility of creating invisibility cloaks using metamaterials? While perfect invisibility can theoretically be achieved, one big problem is that light cannot reach inside the cloaking device, preventing anyone from seeing the outside. No surprise there, since it works by diverting the incoming light waves away from the inside and then putting them back on their original course.

However, it turns out that blocking out the waves can itself be a very useful thing. Not so much for light, where a cheap box would do. Instead, someone remembered that light is not the only thing that is made up of waves. Earthquakes cause seismic waves through the ground which shake things up and even destroy buildings, and an "invisibility cloak" that worked on seismic waves would protect the inside from earthquakes. The weakness of an invisibility cloak can end up being its strength! And since cloaking would not be the goal here, there is no need to even attempt perfect invisibility in seismic waves and should be a lot easier to do.

Researchers from the University of Liverpool figured out how such an invisibility cloak for earthquakes can be constructed out of concentric rings of plastic. It is only a theoretical design so far, but hopefully it could be applied in the real world in the not too distant future. Even if it never becomes practical, it is still pretty neat how the idea of an invisibility cloak can be turned on its head.

Positrons from normal, not dark, matter

Gamma-rays from electron-positron annihilations in center of Milky Way
Gamma-rays from electron-positron annihilations in center of Milky Way

The ESA space observatory Integral had observed gamma-rays from the center of the galaxy, which indicated the presence of positrons distributed in a way that couldn't quite be explained with known phenomenon. Hence some physicists speculated that the positrons may have been the result of dark matter annihilation. Not only did the distribution of positrons within our galaxy turn out to be lopsided, arguing against dark matter annihilation as the source, but it has now been explained how supernovae could be responsible for the distribution of positrons.

Some had thought that supernovae could not be the source of most of the positrons because it was assumed that they would all annihilate very close to their origin, which would not match the observed distribution of positrons. But it turns out that the positrons from supernovae, which are the result of the decay of heavy elements from the stellar explosion, travel nearly at the speed of light and can travel for thousands of light-years before slowing down and annihilating with an electron. By considering how electrons move in galactic magnetic fields, they were able to model how positrons would travel before being annihilated, and the results seem to be consistent with the Integral observations.

This deals a blow to the hypothesis that dark matter annihilation may be responsible for the positron distribution. I wonder if the same implication can be inferred for the PAMELA observations?