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We do not make this statement to insult the human species (which, with certain exceptions, we are rather fond of). Instead, we base it on the fact that humans have had the ability to receive (or send) a radio message across interstellar distances for only a few decades. Compared to the ages of the stars and the Galaxy, this is a mere instant. If there are civilizations out there that are ahead of us in development by even a short time (in the cosmic sense), they are likely to have a technology head start of many, many years.
In other words, we, who have just started, may well be the “youngest” species in the Galaxy with this capability (see the discussion in [link] ). Just as the youngest members of a community are often told to be quiet and listen to their elders for a while before they say something foolish, so may we want to begin our exercise in extraterrestrial communication by listening.
Even restricting our activities to listening, however, leaves us with an array of challenging questions. For example, if an extraterrestrial civilization’s signal is too weak to be detected by our present-day radio telescopes, we will not detect them. In addition, it would be very expensive for an extraterrestrial civilization to broadcast on a huge number of channels. Most likely, they select one or a few channels for their particular message. Communicating on a narrow band of channels also helps distinguish an artificial message from the radio static that comes from natural cosmic processes. But the radio band contains an astronomically large number of possible channels. How can we know in advance which one they have selected, and how they have coded their message into the signal?
[link] summarizes these and other factors that scientists must grapple with when trying to tune in to radio messages from distant civilizations. Because their success depends on either guessing right about so many factors or searching through all the possibilities for each factor, some scientists have compared their quest to looking for a needle in a haystack. Thus, they like to say that the list of factors in [link] defines the cosmic haystack problem .
| The Cosmic Haystack Problem: Some Questions about an Extraterrestrial Message |
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| Factors |
| From which direction (which star) is the message coming? |
| On what channels (or frequencies) is the message being broadcast? |
| How wide in frequency is the channel? |
| How strong is the signal (can our radio telescopes detect it)? |
| Is the signal continuous, or does it shut off at times (as, for example, a lighthouse beam does when it turns away from us)? |
| Does the signal drift (change) in frequency because of the changing relative motion of the source and the receiver? |
| How is the message encoded in the signal (how do we decipher it)? |
| Can we even recognize a message from a completely alien species? Might it take a form we don’t at all expect? |
Although the cosmic haystack problem seems daunting, many other research problems in astronomy also require a large investment of time, equipment, and patient effort. And, of course, if we don’t search, we’re sure not to find anything.
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