A civilization detects another in a nearby star system. They send an electromagnetic signal to establish contact. In this setting, I'm working from these assumptions:

  • The emitter civilization has our current level of technology.
  • The distance between the two planets can range from 4 to 20 light years (to be established)
  • Both civilizations can communicate by text or by speech, can produce very similar vocalizations, but naturally have very different languages

I'd like to know about the technology needed:

  • How sensitive/big would have to be a radio receiver to accurately register the signal? (the emitter civilization will probably send the signal multiple times, and work with our current power cap, but I have not found what that is)
  • Would computers be indispensable for the task of deciphering the message? Could analogic radios/CRT TVs showcase audio or images without the need for a common protocol? (if I could get the signal to be pictorial, it would be pretty neat)
  • Could the receiving civilization accurately deduce the direction the signal is coming from, just with the radio antenna? What about the distance that separates them from the emitters?

This is my very first question, so I'm open to observations in how to better contribute. Thanks for the help!

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    $\begingroup$ Welcome to World building Cristian. This is an interesting question but the problems might be profound. For instance I think even analogue signals would need some form of protocol such as how many lines etc. To play the devil’s advocate let’s say the aliens build webs. They might assume the "obvious" protocol would be start in the middle and spiral out rather than start at the top and scan down one line at a time... $\endgroup$ – Slarty Oct 8 '17 at 3:23
  • $\begingroup$ Thanks, Slarty. You raise a very interesting point, and I guess they could't instantly get a meaningful message unscrambled by their devices. Still, they could relise that an unusual signal is being picked, record it, and then run it through their devises using a different ordering each time. Off course, if they tried this method with a video message, using a radio, it would never make sense, but there is a realistic chance. $\endgroup$ – Cristian Camilo Garcia Barrera Oct 8 '17 at 3:43
  • $\begingroup$ @Slarty If you get a message consisting of a number of on or off pulses which is the product of two prime numbers, it’s not hard to guess that you should try displaying it as a rectangular grid. $\endgroup$ – Mike Scott Oct 8 '17 at 5:10
  • $\begingroup$ Ever heard of quantum communication? Nigh unhackable! $\endgroup$ – user6760 Oct 8 '17 at 8:42
  • $\begingroup$ @Mike Scott If your human absolutely or at least someone would think of it at some point. But if your an alien its hard to be definite about anything as there as so many unknowns. I think the best that can be said is probably $\endgroup$ – Slarty Oct 8 '17 at 8:44

Receiving the signal

There is no fixed limit on how powerful a signal could be sent by a civilisation like ours. We use klystrons to send very powerful radio signals, with deep space communication being one application; the Wikipedia page mentions a 1MW klystron used at the Arecibo observatory.

For a simplified estimate, let's say your transmitter is a point source with power 1x108W (100MW). The signal expands in all directions, so by the time it reaches a receiver 5 light years (4.7x1016m) away, its power will be distributed over a surface area of 3.3x1034m2. This gives an intensity of 3x10-28 watts per square meter (Wm-2), which is extremely weak. However, if the transmitting civilisation knows where they're transmitting to, they don't need to broadcast in all directions. If they focus the signal into a 1-degree beam, that will increase the intensity at the target planet by a factor of about 100,000, giving 3x10-23Wm-2.

If your receiving civilisation has an antenna one square kilometer in area, it will therefore receive 3x10-17W of power. That's slightly more than a billionth of a billionth of one watt, which is very weak. But it's much stronger than the signals NASA receives from the Voyager space probes, for example.

Are computers required?

...However, NASA uses a lot of signal-processing tricks to isolate those very faint signals from the background noise, and these tricks pretty much require a digital signal. I believe you would need at least a 1970s level of computer technology to even detect the signal, and your listeners would have to figure out the coding techniques before they could look at the actual content.

Once that's done, because it's a digital signal, they'd see the message as a string of numbers. The numbers could be anything – prime numbers, text, a JPEG file – but I think if you wanted to send a message, the best bet would be an audio waveform, like you'd find on a CD.

Obviously, this will be a problem if the receiving civilisation doesn't have ears. But if they do, it's probably the easiest thing for them to figure out; if they've seen a sound wave before, they'll recognise what it is. Any kind of image or video would involve another layer of complexity and assumptions, and would require more advanced computers.

But as blind people know, you can communicate a lot with sound. Provided the receivers can get this far, they don't necessarily need computers to figure out the rest of the message. You could start by playing sounds that they'd likely understand – a metal bell, running water – and gradually build up a language from that. It'd be a painstaking exercise, but it's interesting to think about.

Signal direction

This one is no problem. They're hearing you through some kind of radio telescope, and they will certainly be able to figure out where you're broadcasting from. In fact, they won't hear you at all unless they're pointing their dishes in roughly the right direction...

  • $\begingroup$ Thanks for your answer, bobtato. It is really helppful and just what I needed. $\endgroup$ – Cristian Camilo Garcia Barrera Oct 15 '17 at 5:08

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