Question

What is cosmic radiation? Is it dangerous?

Asked by: George T.

Answer

Cosmic radiation is a collection of many different types of radiation from many different types of sources. When people speak simply of 'cosmic radiation' they are usually referring specifically to the cosmic microwave background radiation. This consists of very, very low energy photons (energy of about 2.78 Kelvin) whose spectrum is peaked in the microwave region and which are remnants from the time when the universe was only about 200,000 years old. There are also very old remnant neutrinos in the cosmic radiation. Neutrinos pass through just about everything with no effect so they are harmless. The photons are too low in energy to be dangerous.

On top of these there are higher energy particles that are being created constantly by all luminous objects in the universe. Photons of all different energies/wavelengths are being created by our sun, other stars, quasi-stellar objects, black-hole accretion disks, gamma-ray bursts and so on. These objects also produce high-energy massive particles such as electrons, muons, protons and anti-protons. These higher energy particles are potentially dangerous, but most of these particles never make it to the earth. They are deflected by magnetic fields between us and the source, or they interact with other particles, or they decay in flight.

The particles that do make it to the earth interact with our atmosphere, which acts as a 'radiation shield.' The high-energy cosmic rays bombard us all the time, but they interact quickly, producing particles of much lower energy which impact the earth harmlessly. If this was dangerous to us, we wouldn't be here to discuss these things! Some particles, like neutrinos and high energy muons, are passing through us all the time, but they interact so weakly that they have no effect on our bodies. Of course, if we were in space without the protection of our atmosphere then we would need some other type of shielding from the radiation (spacesuits and protective covering on our spacecrafts).

The radiation to worry about, of course, is the 'cosmic' radiation produced by our sun. There is only one type of cosmic radiation known to adversely affect us and that's UV radiation from our sun, which causes skin cancer in millions of people every year.. Again, our atmosphere serves as a shield, but ultraviolet photons do make it through -- and without that protective ozone layer which blocks these photons we're all going to need a lot more sunscreen!

Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley

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