Question of the Week Podcast

Question of the Week episode

Sun, 8th May 2016

Could we recognise a message from space?

space signal (c)

This week, Jeff wrote in to ask whether we would be able to recognise and understand an encrypted message from space. Emma Sackville enlisted Dr. Duncan Forgan from the University of St. Andrews to help decode this question...

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Well, it depends. If the encryption is really good, it might not be distinguishable from thermal noise or cosmic microwave background radiation. But for keeping TV signals from nonsubscribers, that is not necessary. You can still have an identifiable carrier frequency. That sort of thing would be easily detected, even if not decipherable.  This assumes, of course, that an alien signal would use principles that we would. That is, however, not necessarily assured, and they could choose some peculiar broadband method, for example, that might not be recognizable by our equipment. Atomic-S, Fri, 8th Apr 2016

Claude Shannon was a pioneer of information theory - how to communicate efficiently.

One result of his analysis is that if aliens wanted to communicate as efficiently as possible, their signal would be indistinguishable from white noise. So we may not even recognize it as an artificial signal.

We can expect that the signal would use some form of coding to improve the ability to extract a clear signal from the random noise and interference with which space is filled. Our methods of coding messages has become much more sophisticated in the past 50 years or so. Current methods use large amounts of memory and computer power that would have been simply uneconomic even 20 years ago. So Aliens with technology even 100 years more advanced than our own may produce messages which cannot be decoded, even if we could recognize it as artificial.   

However, aliens who wanted to be heard would try provide some very clear signal that their potential audience could identify as being notable.

But since we don't know of any aliens, where they are, or what they would be like, or what technology they would use (or even what technology they would expect their audience to be using), then it is pretty hard to guess.

But the most difficult aspect of decoding a message would be to understand its content. If you were played a message from the Inuit people, the message would be incomprehensible. Knowing that it is a human voice does not help much, despite the common context of humans across the planet. What hope would we have understanding aliens with whom we have no common context? evan_au, Fri, 8th Apr 2016

A message to ET has already been sent ...
RD, Tue, 10th May 2016

One way to differentiate a signal with noise versus one with information is to take a histogram of if its "bits". 

If you take any book and went through each sentence and plotted how often each word appears. Say keep a tally how many times the word "the" appears, how many times the word "if" appears, and do this for every word and then order the histogram from largest tally to lowest tally. You'd get a negative sloping curve showing the highest occurring words down to the lowest occurring words.

On the other hand, imagine a random number generator that outputs words.  If you run the generator for a long time and plotted how many times each type of word pops out, like before, you'd get a flat distribution, that is, each word has about the same tally, meaning its just noise and contains no information.  This is one way to differentiate between a noise signal and an information signal. (I think this has already been done to analyze the dolphin's squeaks  ).

But of course this has one immediate problem: how do you know which are the words?  How do you separate one word from the next? ... and so on. 

These problems are interesting. arcmetal, Wed, 11th May 2016

Advanced Aliens would probably communicate by modulating gravity waves so no we wouldn't be able to listen with our primitive radio dishes. d4rr3n, Thu, 12th May 2016

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