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Supposing a world like the Underdark, full of civilizations that developed and live their lives completely away from the light of the sun. In the typical D&D type setting, these creatures have dark-vision or infra-vision, which I recall is typically explained by sight that works in the infrared band.

Upon thinking more about this, sight in the infrared band seems like it would come with several problems. This vision would be relying on the radiant emitted by blackbody radiation, according to their temperature. But, the frequency band of this emission relies on the temperature of the object, so two different objects at different temperatures should look....the same?

What is the science-based, biologically plausible basis for vision in the pitch-black of the Underdark, that meets the following requirements:

  • Operates in the infrared band

  • If you are walking down a curved passageway of uniform, smooth stone, you must be able to distinguish the curve in the passageway.

  • If you are standing in a cavern with a lake in it, you must be able to distinguish the surface of the lake from a particularly smooth patch of floor.

  • If you are hunting from underground mushrooms, you must be able to distinguish by sight between Ripened Feast mushroom, which tastes like cheddar cheese, and a Pustulent Death mushroom, which liquifies your skin. These mushrooms smell exactly alike, and have the same exact shape, and can only be distinguished by their 'color'.

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    $\begingroup$ Last time I used an infrared camera for thermographic inspections I didn't have trouble distinguishing what I was looking at. Can you clarify? $\endgroup$ – L.Dutch - Reinstate Monica Nov 15 '18 at 16:05
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    $\begingroup$ @L.Dutch I have used such cameras as well. Are you sure the camera only sees infra-red? Because the one I used (to inspect insulation on high voltage electrical junction boxes) adds visual spectrum light in to make a clearer image. $\endgroup$ – kingledion Nov 15 '18 at 16:08
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    $\begingroup$ @Giter I'm trying to get to a reasonable explanation of the Drow eye. Unfortunately, Drow don't glow in the dark. $\endgroup$ – kingledion Nov 15 '18 at 16:32
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    $\begingroup$ Apparently organics can be used to detect IR: laserfocusworld.com/articles/2010/03/ir-detectors-low-cost.html $\endgroup$ – nzaman Nov 15 '18 at 17:01
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    $\begingroup$ Ah hah! I knew I had answered a similar question before. I just couldn't find it. That one was more pathologically dark, though. Your question leaves open more options. $\endgroup$ – Cort Ammon - Reinstate Monica Nov 16 '18 at 16:04
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Let me address your second paragraph:

Upon thinking more about this, sight in the infrared band seems like it would come with several problems. This vision would be relying on the radiant emitted by blackbody radiation, according to their temperature. But, the frequency band of this emission relies on the temperature of the object, so two different objects at different temperatures should look....the same?

We have two key equations to consider: Wien's displacement law and the Stefan-Boltzmann law: $$\lambda_{\text{max}}=\frac{b}{T},\quad F=\varepsilon\sigma T^4$$ which give us the maximum wavelength at which an object emits radiation, and the flux from that object. Notice that $\lambda_{\text{max}}$ has a much weaker temperature dependence than $F$. As an example, say we have a box with one end at room temperature (293 K) and the other at the boiling point of water (373 K). The difference in $\lambda_{\text{max}}=2\mu\text{m}$ - a small enough difference given the size of the infrared band. I would argue that in most cases, your creatures could have an eye sensitive to objects with peak emission at wavelengths from $0.8\mu\text{m}$ to perhaps $8\mu\text{m}$, covering temperatures from a bit below the freezing point of water to several hundred degrees Fahrenheit.

Now, thermographic imaging works because hotter objects are more luminous - recall the Stefan-Boltzmann law. Even in a range of about $1\mu\text{m}$, the changes between the two ends will be drastic. The takeaway here is that yes, you will be able to distinguish objects at different temperatures if your infra-vision depends on the net flux from an object in the infrared band - in other words, if you use the thermographic imaging method.

What you need are proteins analogous to photopsins, the photoreceptors used by humans to detect visible light. The three photopsins act sort of like the filters in telescopes that allow astronomers to observe an object at select wavelengths (see photometric systems, such as the Johnson-Cousins filters). The three photopsins peak at about 420, 534 and 564 nm - too short for us.

However, related proteins have peak sensitivities at other bands - including non-visible bands. The protein phytochrome peaks at far-red wavelengths (750-800 nm); it's not too far out of the question to imagine a protein peaking at $\sim1\mu\text{m}$. Indeed, research on some species of fish has shown sensitivity in the near-infrared portion of the spectrum (and the sensitivity has indeed been traced back to the eyes), indicating that opsins of some sort are indeed active at wavelengths of $\sim0.8\text{-}0.9\mu\text{m}$.

In short, we want a protein with a sensitivity curve roughly like the purple one below. This is a modified version of a figure based on Fig. 1 of Bowmaker & Dartnall 1980. I've extended the axis and added in a qualitative example of how the opsin sensitivity should behave in the $0.7\text{-}0.8\mu\text{m}$ range:

Modified cone response graph
Original image by Wikipedia user Maxim Razin under the Creative Commons Attribution-Share Alike 3.0 Unported license. Image modified to include qualitative behavior of the hypothetical opsin protein.

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Infrared has a lot more to it than just temperature. Temperature is a major aspect, because every object emits in the infra-red spectrum. Of course, not all objects are at the same temperature. Unless your Underdark is completely still, devoid of heat sources or cyclical heating patterns (like day/night), objects are going to cool off at different rates. Large high-heat-capacity objects will be at a different temperature from small low-capacity highly-conductive objects.

Your lake example is quite easy in this respect. Lakes are almost always cooler than their surroundings due to evaporation.

There can also be color in the infrared spectrum. While blackbody clearly provides a fundamental limit as to what these colors can be, there's no reason one could not have a phosopholuminescent effect in the IR spectrum. Indeed, I think it might be easier than doing so in the visible spectrum, because the energies are low. You also might be able to pull of some interesting tricks with frequency doubling/halfing. You could reflect IR (like a mirror does), but half some frequencies with non-linear optics to create colors where the reflection isn't quite blackbody.

If I were making a civilization underground, and wanted to build curved tunnels, I would almost certainly decorate the walls in a way which provides "color" so that I could distinguish where it was going.

As for the mushrooms, I may have to throw that one back at you. First off, you put "color" in scare quotes, which gives great freedom in definitions. Perhaps these mushrooms do any one of these non-linear or bioluminescent effects I describe. If not, then we're into the arms race of coming up with ideas which meet the concept of "color," and then redefining color to exclude them.

However, I'll point out that 100% of sentient species have figured out how to not die when eating. They might ignore the mushrooms and not eat them, just as we are recommended to avoid mushrooms unless we know what we are doing. They may also use tools. Perhaps bring a bio-luminescent light-source along to help ID the mushrooms. Once you have a light source, it's easy to see "color."

Or perhaps they find other ways to tell the difference. Maybe you split the mushroom open and put a small piece of another fungus into it. If that other fungus is liquified, you probably shouldn't eat that mushroom. People are really clever when it comes with finding ways to not die.

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    $\begingroup$ Yes, or always carry a bit of leather or animal skin with you and see how it reacts to the mushroom. Maybe even your own toenail clippings or dandruff. $\endgroup$ – chasly from UK Nov 16 '18 at 0:21
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The "infra-red band" is not a singular frequency.

The way light detection works in the human eye is that we have detectors which trigger at certain frequencies. We happen to have some which trigger at roughly cyan, magenta and yellow.

Say instead we had sensors in the near IR, medium IR and far IR, and we called those colours hot (think reddish), warm (think greenish) and tepid (think bluish).

Looking at a person in normal light, if they are silhouetted, then yes, they do look "the same?" - but you can still see their outline. Normally, though, you can see them closer, and in the IR you'd see their warmer areas having different temperatures, and different intensities of those temperatures.

Underground, in a cave, the temperature below the frost line is fairly even and depends on latitude, between about 45 and 70F in areas you're likely to be interested in. So let's call it 60F.

The air, though, will generally not be the same temperature. So edges will be slightly different colors as they change towards the temperature of the air.

Further, some surfaces will give off more, or less, infra red, as they are rougher, "darker", etc, in the same way that the color that you paint a radiator determines how much heat it gives off. They will also give off different amounts of different wavelengths.

Some surfaces will be nearer or further away.

A wall with the same smoothness will look somewhat akin to a white wall with visible light. You can still see it, you can see it recede into the distance, it's just... white.

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I would recommend looking at a perfectly viable example that's been done in the real world: snakes. Many snakes (pitvipers, for instance, or rattlesnakes) have special pit organs in their heads, which for your purposes can be considered a set of eyes that view the far infrared spectrum instead of the visual spectrum.

Operates in the infrared band

Obviously, this is covered, although it needs to be emphasized that eyes seeing the visual spectrum are incapable of also seeing into the far infrared spectrum: the necessary structure required is too different. No, this doesn't rule out seeing the visual spectrum as well, provided you have eyes together with those pit organs: those pitvipers I mentioned have more ordinary eyes as well, and they work just fine.

Incidentally, the snakes in question have a thermal sensitivity estimated at >0.001C, so even minor temperature differences are easily detectable. Rattlesnakes actually use this vision to aim for their prey's weak points.

If you are walking down a curved passageway of uniform, smooth stone, you must be able to distinguish the curve in the passageway

I believe the result will be comparable to walking down a city street at night, with the lights out: dark, yes, perhaps difficult to see at a distance, but not outright indistinguishable. You'll have minor thermal fluctuations in the stone, enough to give notice that something is there. Besides, people tend to be pretty good about not walking into walls even without having to touch them.

If you are standing in a cavern with a lake in it, you must be able to distinguish the surface of the lake from a particularly smooth patch of floor.

At a distance, probably not, unless the lake is home to life of some sort. At close range, though, you should be readily able to separate the two before you actively step in the water.

If you are hunting from underground mushrooms, you must be able to distinguish by sight between Ripened Feast mushroom, which tastes like cheddar cheese, and a Pustulent Death mushroom, which liquifies your skin. These mushrooms smell exactly alike, and have the same exact shape, and can only be distinguished by their 'color'.

This isn't fully answerable without a more detailed description of the differences. However, if the two mushrooms in question maintain different temperatures (for instance, if one tends to 22C and the other 18C), this would be trivial.

Now, it is important to note a significant caveat to far-infrared vision. Snakes cannot see very far with this vision, and even then their visual resolution is poor. If you don't want badly nearsighted cave people with fuzzy vision, the pit organs will need to be larger than in snakes (larger than human eyes, in other words, if you want anywhere near the same precision), and/or possibly more than two of those sensory organs.

Cultural interpretation of how your people might work with such organs is an exercise for the reader, but I suspect that blows to the face will be taken much more seriously (as posing a risk for blindness, even in the absence of active attempts at eye-gouging), probably with some societal taboo against touching another's face without permission, much less in the way of facial cosmetics, etc. I wish you the best of luck in figuring out other potential implications (read: further worldbuilding, possible points of interest from which stories can be launched, etc.).

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