I don’t normally balloon, but when I do, I prefer it with an electrometer

During the height of the Heroic Age of Antarctic Exploration, around 1912, part of the scientific community was busy back in Europe investigating radiation. At the time, it was known that there was radiation all around us, which could be measured using an electrometer. But, the source of that radiation wasn’t known.

Victor Hess investigated from a balloon in eastern Germany, his experiments revealed that the amount of radiation detected by an electrometer increased with altitude. Around the same time, Domenico Pacini observed that there was less radiation under water than at the surface.

These results implied that (some of) the radiation observed was coming from space! Robert Millikan made up the term “cosmic ray” in the 1920s to describe this radiation from space, which he thought were high energy photons of a mystical origin. Millikan’s ideas about cosmic rays were disproven, but the name stuck. Hess won a Nobel prize for his balloon-based experimental work in 1936.

Now, a hundred years after Hess’ experiments, we still don’t know where cosmic rays come from. We do know that cosmic rays aren’t radiation at all, but are instead electrically charged particles (mostly atomic nuclei and protons) that are moving really fast. Cosmic rays are moving so fast that they couldn’t be produced by anything in space that we’re currently aware of. But, there definitely are cosmic rays. So, what is producing them?

Since cosmic rays are charged particles, their paths are bent by magnetic and electric fields as they travel through space, and there are lots of these fields in space. Unfortunately, that means a cosmic ray telescope – if we built one – wouldn’t be able to look up and see anything like the original source of the cosmic rays.

I don’t really understand it, but there is some physics which says that whatever is producing these cosmic rays is also producing neutrinos. From the last hand-waving-science post, we know that neutrinos aren’t affected by magnetic or electric fields!

This means that if we can tell what direction a neutrino came from, it should point back at a source of cosmic rays. What we need is a neutrino telescope, just like IceCube.