An extraterrestrial civilization may use technology to communicate at the fastest speed possible, the speed of light, just like we do. All of our broadcast signals—radio, television, microwave, radar, and light—travel at the speed of light. So they are the fastest and most effective way to send messages through space.
How fast is that? About 300,000 kilometers per second (9,500 billion km per year), the ultimate speed limit in the universe. But the universe is a very big place. Light from Proxima Centauri, the nearest star to our sun, takes 4.2 years to reach us. In comparison, a probe would take 75,000 years to travel the same distance!
These signals can be visible, like the light waves we see every day. They can also be nonvisible, like x-rays and radio waves. What kind of signals would an extraterrestrial civilization be most likely to use?
 Visible, like a giant searchlight? |
 Nonvisible, such as radio waves? |
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