What are cosmic rays, and where do they come from?

High-energy gamma rays produced by supernovae and detected directly at Earth's surface High-energy particles (mostly protons and helium nuclei, some heavier nuclei and electrons) arriving from space with energies spanning from millions to 10^20 electron volts, originating from supernova remnants at lower energies and AGN at the highest energies Radioactive particles emitted by solar flares that cause auroras when they hit Earth's atmosphere Beams of light from distant quasars that are energetic enough to penetrate Earth's atmosphere

The Oh-My-God particle detected in 1991 had an energy of about 3 x 10^20 eV. To put this in perspective, it had the same kinetic energy as what everyday object?

A car driving at highway speed A falling grain of rice A baseball pitched at about 100 km/h — an entire baseball's kinetic energy concentrated into a single proton A jumbo jet flying at cruising altitude

What happens when a high-energy cosmic ray proton strikes a nucleus in Earth's upper atmosphere?

The proton slows down and comes to rest in the upper atmosphere without producing secondary particles It creates a cascade of secondary particles — pions, muons, electrons, positrons, gamma rays — that shower down through the atmosphere, detectable at Earth's surface The proton heats the atmosphere locally, creating a visible flash of light visible from the ground The proton is absorbed and converted to a neutron, which is then incorporated into an atmospheric nucleus

Why are neutrinos so difficult to detect, and how does the IceCube Observatory manage to observe them?

Neutrinos interact very strongly with ice specifically, making Antarctica an ideal location Neutrinos interact only via the weak force and gravity, passing through almost all matter unaffected — IceCube uses 1 km^3 of Antarctic ice with 5,160 optical sensors to catch the rare flashes of blue light produced when a neutrino does interact Neutrinos are very cold and only come to rest in the ice, where chemical reactions make them detectable IceCube uses a superconducting magnet to deflect neutrinos into detectors at the center

When Supernova 1987A exploded in the Large Magellanic Cloud, neutrinos arrived at Earth several hours before the visible light did. Why?

Neutrinos travel faster than light, so they naturally arrived first The explosion produced neutrinos first and only later produced light when the temperature was high enough Neutrinos interact so weakly with matter that they escaped through the collapsing star almost immediately; light was trapped inside the dense stellar material for hours before breaking through the outer layers Neutrinos were produced closer to Earth in the Large Magellanic Cloud's outer halo

About 65 billion solar neutrinos pass through every square centimeter of your body every second. They come from nuclear reactions in the Sun's core. What does the continuous detection of these neutrinos tell us?

The Sun is slowly burning up its hydrogen — the neutrino flux tells us how much hydrogen remains It gives us a real-time view of nuclear reactions in the Sun's core right now — unlike sunlight, which takes about 100,000 years to escape from the core, neutrinos arrive at Earth about 8 minutes after being produced Neutrinos only come from the Sun's surface layers, so they tell us about solar flares and sunspots The high neutrino flux is evidence that the Sun is about to go supernova within the next million years

Modern astronomy uses four types of cosmic messengers. What are they?

Radio waves, visible light, X-rays, and gamma rays Electromagnetic radiation (all wavelengths), gravitational waves, neutrinos, and cosmic rays (charged particles) Light, sound, gravity, and magnetism Visible light, infrared radiation, dark matter particles, and gravitational waves

The Pierre Auger Observatory in Argentina covers 3,000 km^2 of ground. Why does an observatory for cosmic rays need to be so large?

The cosmic ray particles are so large that only a big detector can capture the whole particle in one piece Ultra-high-energy cosmic rays are extraordinarily rare — at the highest energies, roughly one particle per square kilometer per century strikes Earth, so a huge area is needed to catch even a few events per year Cosmic rays arrive from all directions and a large observatory can better track where they came from The particle showers are extremely narrow and rare, requiring many detectors in a line to catch them

In 2022, IceCube confirmed that our own Milky Way galaxy is a source of high-energy neutrinos. Why is this significant?

It proves that the Milky Way has a much larger supermassive black hole than previously thought It confirms that particle acceleration to very high energies happens within our own galaxy — likely in supernova remnants and other energetic sources in the Milky Way — and opens a new neutrino window on galactic physics It shows that neutrinos are produced only in the Milky Way and cannot escape from other galaxies It means the Milky Way must be colliding with another galaxy that is injecting high-energy particles