Yes, light is electromagnetic radiation, just like gamma rays and x-rays and microwaves and things like that! And light can be harmful; ultraviolet radiation is the number one cause of skin cancer in humans (and probably other animals too). There is other radiation as well, though -- charged particles. These aren't photons. They're naked subatomic particles. Neutrinos are harmless, but some subatomic particles can cause harm. The Apollo astronauts reported seeing strange flashes of light from time to time. These were the impacts of alpha particles on their retinas. Most alpha particles were passing straight through them unimpeded, but a few would hit their retinas.<br /><br />The reason radiation is so harmful, besides the fact that in certain circumstances it can cook you (as radio waves are used to heat food in your microwave), is that it can disrupt the delicate structure of your DNA. This can lead to cancer for the individual, but for the species it causes mutations. These mutations can be beneficial; that's how evolution works. But they can also cause problems. If the DNA is getting corrupted more quickly than it can be repaired (through internal repair processes in the individual, or through breeding out of bad mutations in the long run), it will lead to declining fertility and declining survival rates and eventually the species will die out. If the mutations are sufficiently severe, reproduction won't even be possible -- the genes will be too badly corrupted to produce viable offspring.<br /><br />There are ways organisms can cope with it, though. One possibility is extremely robust genes. Perhaps an alien race might use something less prone to radiation damage than deoxyribonucleic acid. Another possibility is rigorous error checking in the genetic material. Compare it to the Galileo space probe. Your computer wouldn't work at all in the vicinity of Jupiter. It'd even have a tough time in Earth orbit, because radiation would be inducing spurio <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>