|A simulation of what the Earth would look like if we could see only neutrinos. The Earth is transparent because neutrinos pass through it easily, and the spots on the surface are nuclear reactors. (Data source: Atomic Energy Agency. View a version with continental outlines.)|
On Feb. 23, 1987, neutrino detectors in Ohio, Japan and Russia observed a burst of neutrinos. This type of experiment usually only sees neutrinos produced in the sun and the far more energetic neutrinos produced in the Earth’s atmosphere. These neutrinos trickle in at about 10 per day. In 13 seconds, however, the detectors saw 24 neutrinos. Hours later, astronomers witnessed the brightest supernova seen since the invention of telescopes. When the core of a star known as GSC 09162-00821 collapsed, 99 percent of its energy was radiated as neutrinos; the remaining 1 percent became a bright flash of light hours later.
Neutrinos are barely detectable particles produced in weak-force interactions, much as photons are particles of light produced in electromagnetic interactions. Unlike photons, neutrinos are so weakly interacting that they could pass through light-years of lead without much attenuation. If we could see the neutrinos, the universe would look quite different. We would be able to look directly at the core of the sun, where the nuclear reactions take place, rather than its relatively cool surface. The spinning Earth would look like the animation above, with a diffuse glow from natural radioactive elements in the Earth’s crust and bright spots emanating from nuclear power plants, easily visible through the planet. If we could also selectively see neutrinos of different energies, we could focus on neutrinos from particle accelerators, which are typically much higher in energy than solar and supernova neutrinos and more consistent than the sparkle of neutrinos produced in the atmosphere by cosmic rays. I sometimes wonder if that would be the most conspicuous evidence of human civilization to faraway observers: high-energy neutrinos such as those from NuMI at Fermilab, revolving every 24 hours like a lighthouse beam.
However, just as there are extragalactic sources of ultra-high-energy cosmic rays, there may be ultra-high-energy neutrinos coming from active galactic nuclei, gamma-ray bursts and starburst galaxies, and they might also be formed when cosmic rays collide with photons. They may even come from decays of dark matter, if dark matter is not stable. Until last year, none of these ultra-high-energy neutrinos had been observed because so few neutrinos interact in a detectable way per cubic foot of ordinary matter. IceCube, an enormous detector that uses a billion tons of Antarctic ice as its detection medium, observed two ultra-high-energy neutrinos last year — each with about 100 times as much energy as an LHC collision. They may be the first extragalactic neutrinos seen since Supernova 1987A. If the signal is real and points back to a small region of the sky, we could be looking at a cosmic accelerator with neutrino eyes.
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