One of the biggest mysteries in astrophysics is the origin of high energy cosmic rays. These were first discovered using an electrometer at the top of the Eiffel Tower in 1909 by Theodor Wulf. He showed that the radiation measured at the top of the tower (then the World’s tallest building) was greater than at the base. Subsequent work by Victor Hess in 1912, using a balloon to go to an altitude of 5,300 metres, confirmed the existence of these high energy charged particles.
We now know that cosmic rays are some 89% high energy protons, some 10% are high energy Helium nuclei (alpha-particles), and some 1% high-energy electrons (beta-particles). When these high energy charged particles enter out atmosphere they cause a cascade of particles to be created, in exactly the same way as we create subatomic particles in particle accelerators like the LHC at CERN.
With increasing investigations of radioactive beta decay, scientists noticed the beta particles were ejected from the atomic nuclei with various amounts of kinetic energy. In 1930 Wolfgang Pauli suggested the electron’s energy was being shared with an unseen particle, which he called the neutrino (it was first detected experimentally in 1956). Neutrinos were, of course, in the news a lot a few months ago when an experiment using neutrinos generated at CERN were found to be arriving at a neutrino detector at Gran Sasso, Italy, in a time suggesting they were travelling faster than the speed of light (this claim has since been withdrawn after errors were found in the experiment).
Because neutrinos are neutral particles, they are not affected by electric or magnetic fields, and so travel in straight lines from their source. Although they interact very weakly with matter (billions created in the Sun are passing through your body every second), in the last 10 years scientists have built increasingly sensitive neutrino detectors. One of the latest is IceCube, which has been built near the South Pole.
The problem in determining from where these cosmic rays are coming is that they do not travel in straight lines. Unlike EM waves, which essentially travel in straight lines through space (EM waves are bent by gravity – so called “gravitational lensing”, but in most cases this is not a large effect), cosmic rays get bent because they are charged. Any electrical or magnetic field in space will deviate them, so as the cartoon below illustrates, the cosmic rays from a particular source will almost certainly miss the Earth even though EM waves from the same source arrive at Earth.
Astronomers have thus been using IceCube to see if they can detect neutrinos from the sort of high-energy astrophysical events that we suspect produce cosmic rays to see if they can detect neutrinos. The argument is that if neutrinos come from such events, then even if we do not detect cosmic rays from the same source, we can assume they are being produced, but are not reaching us as they are diverted from their path, unlike the undeviated neutrinos.
So far, only sources of gamma-ray bursts (GRBs) have been studied. GRBs are the highest energy astrophysical events we have so far found, believed to be due to such events as the merging of two black holes, or the merging of a neutron star and a black hole. IceCube has been looking for neutrinos arriving at its detectors which coincide in time to detections of GRBs. The results are that no neutrinos have been detected from these GRB sources. This has been a surprise and disappointment, and astronomers felt GRBs were the best candidate to explain the ultra high energy cosmic rays (UHECRs) arriving at Earth.
But astronomers have not given up on this line of enquiry. The plan is to next study X-ray bursts, less energetic events than gamma ray bursts, but still feasibly the source of the UHECRs. X-ray bursts come mainly from the nuclei of some galaxies, so called Active Galactic Nuclei sources (AGNs). Should AGNs also prove not to be the source of UHERCs, it may then be time to panic, but not yet!