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How astronomers really distinguishes between the natural and artificial signals? From time to time astronomers detects many signals from outer space and SETI is always trying to find signals that carries the sign of intelligent beings. The Wow signals was such a much hyped signal that we received. So what properties a signal should carry so that we can conclude it is from some alien civilizations?

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    $\begingroup$ The signal was a single detection that was never repeated. It's pretty much impossible to say what it was or where it came from, or even if it was nothing more than an equipment error. The last question you ask seems like an interesting one to get answers to, though. $\endgroup$ – zibadawa timmy Oct 19 '15 at 4:52
  • $\begingroup$ Yes I want to know how to distinguish between the characteristics of signals. $\endgroup$ – Kushal Bhuyan Oct 19 '15 at 4:55
  • $\begingroup$ The only artificiality we know of is what we ourselves do. So if we find ourselves, then we have a perfect match. Ironically, scientific SETI is geocentric. $\endgroup$ – LocalFluff Oct 20 '15 at 1:55
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We could begin to conclude if a signal is from some Extra Terrestrial Intelligence (ETI) if the signals are very difficult or impossible to explain by non-life origins or terrestrial origins. One commonly cited example is with a very narrowband radio signal. In natural radio emission, the frequencies are spread out due to processes like Doppler shift which raise and lower the frequency or radio emission. The Wow signal was so narrowband that it behaved just like one would expect for ETI, but having only one detection that disappeared makes it hard to be sure it wasn't interference or a problem with that particular antenna.

I would suggest that ETI detection should have some of these properties, but feel free to add more:

  • No natural abiotic terrestrial phenomenon that can produce it
  • Verifiability - multiple detection methods, instruments and teams that can confirm the signal is not due to errors in processing, analysis or local interference
  • Evidence for non-random patterns such as prime numbers

KIC 8462852 currently has a feasible solution that dust escaping from a comet or multiple comets can obscure light from its host star. This natural explanation has ways of being confirmed (such as wavelength dependence to the stellar blockage), which will help with the first criterion. Hopefully, the flux dips observed in Kepler's days ~800 and ~1500 will repeat in the future but if they don't, it fails the second criterion. As with definition life, it is hard to come up with a rule book. However, if we see a lot of the properties we associate with ETI, we can start the classification process. As more evidence comes in, like polarization, multi-wavelength and Doppler shift data for KIC 8462852, we can evaluate which criteria are satisfied.

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  • $\begingroup$ Got to know about lots of stuff, beautiful answer. $\endgroup$ – Kushal Bhuyan Oct 20 '15 at 1:45
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    $\begingroup$ Can modulation be a factor? Communication signals got AM/FM modulation can a natural signal be modulated? $\endgroup$ – jean Oct 20 '15 at 13:22
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A signal artificial is a product from no natural radio frequency composition, so ,we principally change Amplitude (AM) or frequency(FM) and combinations, but required a pattern like a time or position, and a common "simple structure of communication", this algorithms are used to detect a signal from Intelligence

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Considering the vastness of space, an alien(artificial) signal will have to be pointed in our direction and when it reaches us the strength will be very very small. Natural signals are more pronounced and 'appear' all the time. To duplicate a natural signal the artificial source have to have lot of energy resources at disposal.

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