And so nearly all of a supernova's energy is emitted as neutrinos. Despite the fact that only a pitiful fraction of a percent of a supernova's energy goes into the burst of light, that light, which rivals an entire galaxy, makes supernovas relatively easy to spot. This was the case on a cold clear February night in 1987, when an astronomer on a mountaintop in Chile stepped outside and looked up at the Large Magellanic Cloud, a cluster of stars just outside our galaxy. The news quickly spread to the rest of the world. Instead they had been look ing for the end of the Universe in the form of proton de cay.
Protons are an essential and apparently stable build ing block of matter.
In some reasonable but as-yet unproven theories, however, protons are predicted to de cay. If a proton decayed, it would fall apart with a great deal of energy, emitting smaller particles at high veloci ties approaching the speed of light in vacuum. In fact, they would be so energetic that they would surpass the speed of light in water. At that speed, a particle builds up an electromag netic shockwave, not too dissimilar to a sonic boom, and emits a cone of blue light, known as Cerenkov radiation. Cerenkov radiation is a useful phenomenon commonly used by high energy physicists to detect their prey and to gauge its speed.