High-energy cosmic rays: You better duck

Cosmic Rays have a mysterious-sounding name, and live up to it. Even their history is intriguing.

Their presence was first suspected over a century ago, when physicists noticed that electrically charged laboratory objects slowly lost their charge for no apparent reason. Some electrical entity was apparently sneaking into the lab and neutralizing these critters. That it came from the sky was not at all obvious until 1912, when Austrian physicist Victor Hess carried a charge-measuring instrument aloft in a balloon, and found that charges changed as he ascended. The culprit was assumed to be some kind of invisible light from outer space, and thus was born the name Cosmic Ray.

It took until 1950 to prove that these are solid particles, even though the original “ray” name remains. Further studies showed that 89 percent of them are ordinary protons, while ten percent are alpha particles: packages of two protons and two neutrons – weighty stuff. The remaining one percent are electrons. All carry an electric charge, and are thus influenced by magnetism.

A proton is a hydrogen atom’s nucleus. An alpha particle is a helium atom’s nucleus. Their cosmic-ray ratio matches the relative abundance of hydrogen and helium in the cosmos. It’s as if pieces of the universe are leaking in here. What doesn’t make sense is why electrons – more abundant than the others – are so underrepresented in cosmic rays. But that’s the least of the mysteries.

A cosmic ray’s energy depends on its speed, since a faster-moving object does more damage than a slow one. Cosmic rays (CRs) arrive in an amazing range of speeds, whose power is expressed in electron volts or EVs. The commonest, slower ones are created in the Sun and present little mystery. The higher-energy CRs come from deep space. Then there are ultra-high-energy cosmic rays, or UHECRs, which are incredibly powerful and utterly baffling.

We on Earth are happily protected from nearly all cosmic rays by our atmosphere, and to a much lesser extent our planet’s magnetic field. Still, enough CRs reach your body to deliver about 26 millirems of radiation exposure annually. You get an extra five millirems for every 1,000 feet higher your home is located. CRs really crank up their intensity in the upper troposphere at 35,000 to 42,000 feet, which is why you receive an extra millirem for each thousand miles you travel by plane. It’s Frequent Flyer radiation. Thanks to their extended time in that high-CR environment, airline crews have a one percent higher lifetime cancer incidence: 23 cases in 100, instead of 22.

Astronauts – especially those leaving our protective magnetic field to venture to the Moon, or maybe someday to Mars – face a fearsome cosmic-ray environment. In space, 5,000 cosmic rays tear through the body each second. During a multiyear mission, this creates an enormously elevated risk of cancer and the wholesale destruction of brain neurons.

Shielding is problematical. To achieve the same cosmic-ray blockage we get from our atmosphere, you’d need to huddle beneath 16 feet of water, or something with its mass equivalent.

While most cosmic rays have energies between ten million and ten billion electron volts, cosmic rays of more than 100 million trillion EVs are periodically detected. These are 40 million times more powerful than anything that we can create in a particle accelerator. A single such cosmic-ray particle can deliver a wallop equal to a tennis ball hitting you at 100 miles an hour. They’re probably protons traveling at just under the speed of light.

How does a proton with its substantial mass get accelerated that crazily? No known process can do it. For years, the leading candidates for such UHECRs have been supernova remnants, but even these can’t explain truly ultra-high-energy particles. Recently, colliding galaxies have gained favor, but there are problems with this theory too. Today, the leading candidates are AGNs: active galactic nuclei like the ones inside ten percent of all galaxies. It’s assumed that their supermassive black holes slingshot these bullets to their fantastic speed and power.

Perhaps the most intriguing idea is that UHECRs materialize when theoretical dark matter particles hypothetically decay into high-speed proton pairs, one of which falls into a black hole while the other is shot across the cosmos. It’s a case where desperate, baffled astronomers are using the bizarre as evidence for the exotic.