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I am working on a world which is very dark due to an opaque gas layer in the hight atmosphere; but it is opaque mostly in the terahertz range. The creatures in this world therefore rely very little on visible light. Some creatures have evolved sensitivity to radio waves (inspired by a handful of questions here in WB).

This answer to Best place on an humanoid body to develop biological radio? gives an example of a low-power, active radar device for short range that consumes 20 watts of power, which is about 0.86 kcal/hour. This may be prohibitive for smaller creatures (in the 1~5 kg of body mass, I think), so I wonder if passive radar could be used by them.

Could radio emissions from the planet's parent star, and gas giants in the same star system or other radio emission sources be reflected by the environment and perceived by such creatures, akin to how we can see by starlight?

I am not worried about the resolution of this kind of sight. I am more worried about whether radio emissions from space have enough energy to excite my aliens' radio-sensitive organs as much as starlight excites our retinal rods.

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    $\begingroup$ I suspect that there would be enough radiation, but that to use it effectively the aliens would need really large eyes, which is why at least with the conditions on earth it has been more efficient to use visible light. $\endgroup$ – trevorKirkby May 4 at 18:05
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I can't speak to the biological aspects, because we really don't have a good sense of how high of fidelity a biological radar can be. Signal quality is everything.

However, I can point to several very important things which suggest its plausibility:

  • There are creatures which navigate by sound alone. In fact, you are capable of it... though you don't use the ability much while your eyes are open. Everybody hears the echoes of their shoes and map the world using the echos. Sound isn't the same as radar, but shares a lot of concepts.
  • We have technologically derived missiles which use passive radar sensors, such as HARM. So there is at least some prior art for using passive radar sensing as your only sense.
  • There is something called Synthetic Aperture Radar (SAR), which lets one image using radar, the way a human eye or a camera images. So imaging can be done in the radar realm. Of course, SAR requires an active radar, so...
  • For my final point, I'd just like to provide a picture. This picture was created using Very Long Baseline Interferomoetry (VLBI). It used passive radio sensors to image an object over 50 million light years away. So there... it can be done, in glorious manner. You all should recognize the image by now:

M-87

If they can image that, I think your biological creature stands a chance of imaging something closer!

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    $\begingroup$ Just a slight correction: That black hole picture is of the center of Messier 87, which is, according to Wikipedia, 53.5 million light years away. I think you may have left out the word "million" in your answer when giving the distance. $\endgroup$ – Andon May 4 at 16:55
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    $\begingroup$ @Anson. Thanks! Fixed! 50 light years away just isn't quite as impressive. $\endgroup$ – Cort Ammon - Reinstate Monica May 4 at 18:58
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    $\begingroup$ It is actually 50 blocks away, and is the sign on the all-night donut place as seen without glasses. $\endgroup$ – Willk May 4 at 21:19
  • $\begingroup$ @CortAmmon Let's be honest, 50 light years is still impressive $\endgroup$ – Andon May 5 at 19:04

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