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I have made this planet for my creatures and it is constantly dark, night or day.

No it's not a rogue planet, it's dark because of the 1,000 feet tall autotrophs that suck up all the sunlight and let little pass through (think nighttime with a half full moon).

There are also hot vents sticking up out of the ground and giving off a lot of heat which is absorbed by the bottom autotrophs (I won't go into the specific but heat makes the food for them) and the average temperature is 98 degrees Fahrenheit.

And this is a problem because of this: my creatures have superb vision, since they can see from microwaves to UV radiation on the electromagnetic radiation spectrum.

So my question is: what reasons justify a creature having such good vision on such a dark planet?

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    $\begingroup$ If they can see microwaves, how come they are not blinded by the glare of their own body heat? And you must really think about how their eyes work; for example, it is not feasible to have a lens which can focus everything from microwaves to ultraviolet, and even less one which can focus all of it on the same plane. (For people who are not accustomed to the customary furlong-firkin-fortnight system of units used in parts of North America, 98° F is 37° C.) $\endgroup$ – AlexP Apr 28 '18 at 7:52
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    $\begingroup$ @AlexP - "the microwave glare of body heat"? Is that why my belt buckle keeps throwing sparks? $\endgroup$ – Willk Apr 28 '18 at 12:27
  • $\begingroup$ As alex mentioned if it can see microwaves and UV it needs multiple sets of eyes. $\endgroup$ – John Apr 28 '18 at 14:50
  • $\begingroup$ @AlexP Only if you're using diffractive optics. Optical systems which are designed purely with reflecting optics can operate from microwave to UV and indeed up to X-rays. Example $\endgroup$ – user71659 Apr 28 '18 at 22:55
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    $\begingroup$ @AlexP You are completely incorrect. What do you think happens here? In fact with microwaves, you can get away with a cheaper metal mesh instead of a sheet when the holes are much smaller than a wavelength. $\endgroup$ – user71659 Apr 28 '18 at 23:28
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You're asking for a justification for why heightened vision would evolve on a dark world. The answer is, "for the same reasons it would evolve on a brighter world..."

Survival Advantages!

  • Creatures which can see in an environment's available light can hunt better that those which can't.
  • Creatures which can see in an environment's available light can avoid becoming prey better than those which can't.
  • Creatures which can see in an environment's available light can avoid fatal accidents better than those which can't.
  • Creatures which can see in an environment's available light can find mates better than those which can't.
  • Creatures which can see in an environment's available light can keep track of and protect their offspring better than those which can't.

Despite the relative darkness, some branch of your planet's evolutionary tree would eventually try out various eye sizes, shapes and locations; and those lucky species which received the most functional eyes for their current environment would have a spectacular advantage over those species which didn't.

Just like it happened here on our bright blue world.

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Darkness is a concept which has to be applied to specific range of wavelengths.

Your planet is dark, yes, but dark only in a limited spectrum. Let's say it is dark if you look at it in (human) visible light. For the rest, your autotrophs are pretty much transparent, and they let all the rest of radiation pass through their bodies.

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  • $\begingroup$ If the 1000ft tall autotrophs are transparent on a particular spectrum, I'd bet that the much smaller creatures would be as well $\endgroup$ – bendl Apr 28 '18 at 11:43
  • $\begingroup$ @bendl Not necessarily, Living things have lots of types of materials in them. It's possible those autotrophes don't have a material that the living creatures do have, which is non-transparent to it. It means that only part of their bodies is visible, similar to humans under x-ray. However, You can definitely tell a human's a human just from an x-ray pic. $\endgroup$ – Rick M. Apr 28 '18 at 14:15
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Some things make their own light.

deep sea fish with light https://www.pinterest.com/pin/218002438183780158/

A world analogous to yours is the deep sea. It is dark but things see very well. They see because animals can make their own lights. Just as a quiet world let animals communicate with sound, in a dark world, bioluminescence lets animals communicate - finding each other, finding prey, repelling predators and so on.

Your dark world would be full of glowing creatures.

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