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Radio Signals

The search for intelligent life outside our solar system concentrates on transmissions from a technological civilization. The primary emphasis has been on looking for radio signals, because these are the sort of signal that we ourselves are able to send and detect.

If you are not familiar with electromagnetic waves and different varieties of light, we highly recommend NASA's introduction to the topic. If you already know about different parts of the electromagnetic spectrum, you can continue on below.

As it turns out, the universe is packed full of radio signals from natural sources. Our own sun and other stars produce radio waves. Particles caught in Jupiter's magnetic storms do as well. The center of our galaxy produces radio waves, and some galaxies produce very large amounts of radio waves. Take a look at this picture showing a view of our galaxy in radio wavelengths (top) and in visible light (bottom):

Because each star creates radio waves, and life evolves on planets near stars, we cannot simply look for sources of radio waves. Every star would be a candidate. We must instead look at the content of those waves, and look for signals that indicate an intelligent presence. Some of this we can hear for ourselves, by "listening" to the radio signals. Jupiter produces static "noise storms" that can sound like crashing waves, and a pulsar produces regular beats, but a signal like morse code is easily distinguished as a more complex signal that carries information. You can hear many of these by following the first link at the bottom of this page.

Hearing alone is an inaccurate and inefficient way to detect information-bearing signals. Modern searches for extraterrestrial intelligence use mathematical techniques, such as a Fast Fourier Transform. These techniques pick out signals with repeated patterns that change over time. Signals that are too fast for our ears to distinguish as anything but static can be recognized and analyzed much more easily in this manner.

Recently SETI has begun to branch out from radio observations to the visible spectrum - specifically, visible-light lasers. Visible light, with its higher frequency, can encode information at a much higher rate than low-frequency radio transmissions. Laser light has two more advantages when it comes to SETI. First, it is easily distinguished from ordinary light. Second, unlike with radio signals, there are no natural sources of laser light. This means that a laser signal from space could only be the result of an intentional transmission from intelligent, technologically-savvy life.

Unfortunately, lasers are also much more directional than radio transmissions. Radio transmissions spread out in all directions; lasers travel in a straight beam. We would only detect a SETI signal in visible light if it were aimed directly toward us, or if we were in the path of a transmission aimed past us. It's worse than looking for a needle in a haystack; it's more like looking for a needle that happens to be pointing directly north. On the plus side, if we find such a signal, it will be unambiguous evidence for technologically advanced aliens.

Солнечная система и ее тайны