Suppose there is a planet where intelligent life (with intelligence approximately same as that of a being of the human species, say) exists. However, due to certain conditions (such as, for example, a sun which gives out light mostly in the IR spectrum), these beings have eyes that see "heat" (in the form of rays of infrared radiation), but not colour (i.e., they can't "see" visible light, ironic as that may sound).

How would the fundamental design of everything be changed?

For example, a door handle may have to be warmed slightly, so as to be able to distinguish it from the surrounding door, assuming the surrounding door is at the same temperature.

Update: Let's ignore this example I provided. Unfortunately, the fact that it somehow did not strike me that IR rays could be reflected seems to have become the focal point of the question at present. In all fairness, putting in an example of a glowing handle was not, well, the brightest of ideas. Apologies.

  • $\begingroup$ Some of the effects on life could be interesting...predators may have the advantage of hunting glowing creatures, but a natural defense camouflage of controlling body-heat might arise. Our eyes picked up the ability to detect red (theory) due to younger and healthier leaves (more nutritious) had a red colour to them. I'd be curious if a herbavore would pick up an infrared version of detecting the good to eat greens instead. $\endgroup$
    – Twelfth
    Oct 28, 2014 at 20:54
  • 8
    $\begingroup$ This isn't exactly an answer, but I think it's an important point to consider so I'm putting it here. If these beings' eyes pick up radiation in the same spectrum that their own bodies radiate it then you have an interesting conundrum, because the very molecules and cells and organs that are supposedly receiving the light, are also emitting it! Imagine if our own eyes glowed. We wouldn't be able to see anything. I think the eyes would actually have to be very cold in order to function. They would need to be separate appendages or in some other way cooled in order to function. $\endgroup$ Oct 28, 2014 at 22:33
  • $\begingroup$ The Martian Chronicles by Ray Bradbury covers this, to an extent. $\endgroup$
    – Almo
    Oct 29, 2014 at 2:18
  • 3
    $\begingroup$ Note that eyes would need to be significantly larger in order to focus the longer wavelengths of IR light. $\endgroup$
    – Tim B
    Oct 29, 2014 at 10:11

7 Answers 7


A lot of people have covered sight, but one factor people have missed is the size of the eye.

Insects see ultraviolet wavelengths because UV-light has a much shorter wavelength. For a cell to detect a vibration in the electromagnetic spectrum, it would needs be be at least as large as the vibration. Hence why smaller animals see smaller wavelengths. Therefore, to have an eye that detects much larger wavelengths, the cells would need to be larger, which would mean the eyes would probably be bigger too.

Addendum on eye size.

Sadly, I can't find a specific reference and I'm not a biologist, but I did read the wikipedia article of photoreceptors and I found a good article on the evolution of vision. As I understand it light detection in cells requires the cell to be excited by a photon of the appropriate wavelength. This excitation is based on the energy of the photon and the number of photons reaching the eye - the more photons, the more stimulus. To maximise the stimulus, you want to catch as many photons as possible, which means the cell needs to be at least as large as the photon wavelength (otherwise the photon travels around you). For example, some radio waves have a wavelength of several meters, so the chance of a radio wave hitting you in the eye (let alone a specific cell is quite low).

If we assume cell size has to increase to catch the right wavelengths of light, if a creature wanted to have similar visual resolution to humans, the number of cells would remain constant, thus the eye would need to increase.

Additionally, such a species would likely not have evolved in water. Primarily because the visible spectrum as we know it is spectrum of light that most easily penetrates water. For infrared vision to be evolutionarily advantageous, it would have had to be beneficial outside of the water. Therefore, the eye structure would probably not have an aqueous solution like our does.

enter image description here

Thus the eye would not look like a human eye, and more like that of an insect where the light detecting cells are directly on the surface. Consequently, because the nerves would be behind the cells, they wouldn't have a blind spot.

enter image description here

As an additional, camouflage would be based on heat differentials, not colour. So for example, this polar bear would be nearly invisible. This is due less to the "colour" of the bear, but due to the fact that they have very good heat insulation - not for evading Predators (of the movie variety) but as an adaptation to keep them warm in their cold environment.

enter image description here

  • 1
    $\begingroup$ Why does the cell need to be as big as the wavelength? That sounds contrived as you freely go from 'eye size' to 'cell size', both of which are purely biological constructs and don't have any physical meaning to a photon. The protein rhodopsin is actually what absorbs and detects light in animals, and it's barely 15 nanometers long. $\endgroup$
    – Nick T
    Oct 30, 2014 at 5:24
  • $\begingroup$ @NickT I've added some links and more explanation in. Obviously this is all speculation and guess work. But thats the basis of this site I'm guessing? $\endgroup$
    – user2547
    Oct 30, 2014 at 5:48
  • $\begingroup$ If you're ignoring physics in your speculation, it might be prudent to point that out. Absorption (the start of any detection) generally happens because a given material is able to absorb that photon to excite an electron up some specific energy levels, vibrate a molecule, etc. You're notion of photons "travelling around" things I don't understand. $\endgroup$
    – Nick T
    Oct 30, 2014 at 6:00
  • $\begingroup$ I've added in a link that has some explaination on why we can't see radio waves that covers the physics. Badically radio waves are very low energy and have a very low likelihood to collide with an eye. This is why radio telescopes need to be so big. To maximise the chance they will recieve enoigh energy of the right wavelength... i think. $\endgroup$
    – user2547
    Oct 30, 2014 at 6:03
  • $\begingroup$ No, they don't get picked up because most radio waves will not excite an electron, a bond, etc. On the other hand, water can pick up 12.2 cm radio waves because that energy can be captured by some mode (forgot which between stretching, vibrating, rotating, etc) of water molecules. $\endgroup$
    – Nick T
    Oct 30, 2014 at 6:22

Civilization may be very similar to ours or very different. The exact wavelenghts seen by people of that species are not so decisive.

What we call visible light is only a small amount of wavelenghts, which we can detect with four different receptors in our eyes (three for daylight colour vision and another one for intensity-only nighttime gray vision). We call these three wavelenghts red, green and blue, and a certain mixture of them white light (precisely a good approximation of our Sun's light that can reach us through our atmosphere).

If there were a different set of wavelenghts that our cells could detect, those would be the "real" colours, and what we now call "visible light" would be in the infra-something or in the ultra-something ranges. If these three colours were "foo", "bar" and "baz" in the infrared, our visible light would be part of the "ultrabaz", invisible light, while most of our infrared would be "infrafoo".

It happens that the range of temperatures on which water is liquid (0C-100C, 273K-373K) are those which we consider suitable for life (particularly those in which proteins are stable, thus reducing the set to about 0C-35C (273K-308K). At these temperatures, all bodies radiate1 with peaks between 10.6µm and 9.4µm (what we call the near side of far infrared).

Infrared distribution

If the species has their cells tuned to near infrared or medium infrared, there would be no difference with our vision: objects still would not glow. It just happens that the temperature at which they start glowing (red hot metal) is lower than it is for us.

On the other hand, if the species has their cells tuned to the far infrared, objects will naturally glow. Artificial light is not necessary (unless going to very cold places), but fire still would be need for cooking, ore processing, etc. Since fire and wheel are the same, and most technology would develop from that two main inventions, they could be very similar to us.

(This paragraph added after kaine's comment) Another important issue is that of transparency. The set of transparent materials is quite different from what we think is transparent for "visible light". Air will continue being transparent, as well as glass, but water is not. Heatened gases will not only disturb vision due to blur, but since they glow as well, they'll be like having colour smoke in front of you. On the other hand, lightweight clothes would be far more exposing than they are for us.

Wildlife does not necessary be different from ours, but it can be. Please note that most animals do not actually see the same wavelenghts as we do. Dogs do not detect red light, bees use ultraviolet, and some bats see infrared. It is up to you to question if wildlife for that species share their vision or is as wide as ours.

Sociologically, there are wider implications. Sexual arousal and lies would be more difficult to hide, since they cause an increase of body's temperature, which would be quite visible for this species.

  • $\begingroup$ You are the only one talking about the sociological aspects it would change, and this would certainly be one of the major disparities in this case. We know now the heat repartition in the body for each emotion, and with such eyes, everything involving tactics or hiding informations would be totally different (politics, chess, poker, relationships, secrets, military,..) $\endgroup$
    – Shunwoo
    Oct 29, 2014 at 8:17
  • 2
    $\begingroup$ Is there a case for arguing that the emotionally-induced changes in body temperature are small enough that only perceptive people would detect them? An equivalent to micro-expressions, if you will. People's eye pupils may contract or expand visually, but most observers won't notice, at least not consciously. $\endgroup$
    – lea
    Oct 29, 2014 at 8:35
  • 1
    $\begingroup$ Good answer but there are many things ignored here but the most important seems to be that what is transparent to them and us would be alot different. Glass and water are largely opaque while silicon is transparent. Things that look the same to us look distinguishable to them. physics.stackexchange.com/questions/3750/… $\endgroup$
    – kaine
    Oct 29, 2014 at 13:29
  • $\begingroup$ @Shunwoo Please elaborate on sociological implications. That is not my main expertise field. I'll be happy to quote you. $\endgroup$
    – Envite
    Oct 29, 2014 at 20:53
  • 1
    $\begingroup$ Something that I recall from the last point you make about various signals being harder to hide... liquid crystal pants $\endgroup$
    – user487
    Oct 29, 2014 at 22:57

Seeing 'heat' is really just viewing a different section of the electromagnetic spectrum. Similar to how many insects (especially bees) tend to see well into the ultra-violet side of things.

Different items absorb and release heat differently, while it is unlikely that a infrared vision would have the detail of 'visible' light you would still get stuff. Look up images of infrared. you can see gradients and intensities, (granted they are usually painted with colors in our visible spectrum but it still gives an idea.

Heat will not be absorbed or released evenly on any object or room, though a room where everything is exactly the same temperature would be very limiting, kind of like putting us in a dark room or say a bright white one where everything is the same color as the ambient light, very difficult to move around in.

  • $\begingroup$ This is the correct answer. +1. I've added a bit about colour and heat vs wavelength in a supplementary answer. $\endgroup$
    – abcdefg
    Oct 28, 2014 at 19:24

Assuming the hypothetical being (HB) had 2+ eyes or reasonable spatial visualization, the HB would see as if the entire world were monochromatic (black/grey/white) but could still discern three-dimensional objects just as you can watch a black and white television show and realize that part of a wall is, in fact, a door.

Additionally, there is a significant enough chunk of spectrum in the IR band that the HB could possibly see "colors" in a completely different (but adjacent) spectrum than what humans consider "visible light". Think of what was shown as the predator's vision from the movie of the same name.

In either of these cases, warming the door handle would be equivalent to making the door handle glow and cast its own light, reflections, and shadows.

  • $\begingroup$ A very valid point. It had somehow escaped me that objects reflect light, and don't need to glow to be visible. $\endgroup$ Oct 28, 2014 at 18:52
  • 1
    $\begingroup$ I think it is inaccurate to assume they would see as though things were monochromatic- we don't even know how other humans perceive colour internally, but it is reasonable to assume a well designed brain would perceive available light as a spectrum if that provided the organism with an evolutionary advantage. $\endgroup$
    – glenatron
    Oct 29, 2014 at 10:29

User @bowlturner makes the fundamental point.

There is additionally the issue of colour.

Our eyes have evolved to "see" that narrow range of electromagnetic radiation we call optical light. Within this range, we differentiate between (essentially) three different sub-ranges that we call red, green, and blue. These colours help our brains to fill in the details when we look at our environment. However, these colours only exist in our mind, they do not exist in reality.

We see an object as red if that object reflects the red spectrum but absorbs the green and blue spectrum. Mixed colours like brown come from a mixture of red and blue (?). Etc...

If our eyes had evolved to detect different wavelengths, then we would most likely have evolved colour sense in a similar fashion.

Finally, "heat" covers the widest of spectra. The sun produces highly energetic (short wavelength) radiation in the form of heat, while the friction between my fingers and my keyboard produces only a low energy (long wavelength) heat.

EDIT Elaborating a bit on my final point. To see heat across the full spectrum of electromagnetic radiation would mean seeing everything : Gamma rays, X-rays, ultra-violet, optical, infra-red, microwave, and radio waves. I may have missed some.

EDIT 2 What I should have said in the first place is that all electromagnetic radiation has a property which we call heat. In this sense, what we see with our eyes is heat, namely that heat which is present in a narrow range of wavelengths from the electromagnetic spectrum.

  • 1
    $\begingroup$ We may not have enough letters for the new ROYGBIV that you seem to be suggesting =D $\endgroup$
    – Culyx
    Oct 28, 2014 at 20:00
  • $\begingroup$ @Culyx That's for sure. We would be (metaphorically) blinded each time we opened our eyes. $\endgroup$
    – abcdefg
    Oct 28, 2014 at 20:08
  • $\begingroup$ I'd replace the brown example with purple. If you look at where brown(s) are oin a RBY grid, you realize brown(s) are "dark red" and "dark yellow" and "dark orange". (I've been told that this is more common knowledge amongst females due to the study of hair) $\endgroup$ Oct 29, 2014 at 20:56
  • $\begingroup$ @MooingDuck Thanks. I was too lazy to wiki it at the time. Purple seems obvious now you mention it. If my father is anything to go by, I had better start studying hair before its too late! $\endgroup$
    – abcdefg
    Oct 29, 2014 at 22:23

It is very likely that their body language would incorporate temperature patterns and signals. For example a warm smile could be literally a warm smile. An agressive action might be accentuated with a flash of heat. You will see things of this nature in squid, octopi, and cuttlefish where visual information on the skin accentuate posture in conveying information.


"Infrared is heat" is a simplification. At a given temperature a particular spectrum of light which excludes any frequencies above a particular limit, quickly ramps up to a peak, and then trails off through the long frequencies.

The visible frequencies are the peak at temperatures in the thousands of Kelvins.

We can think of humans as having a "visible" range of temperatures as well. Those temperatures where we might have some sort of meaningful direct experience of them. Those temperatures are in the rough vicinity of 300 Kelvins.

So we have a mismatch. By the time we start seeing any visible light, we are well into the range of ridiculously hot things that will cause immediate, significant damage to us if we touch them, starting with things like the red of stove heating elements and going up from there.

Infrared is a wide swath of the spectrum. Much wider than the visible spectrum. Temperature wise, it starts down below the range we're comfortable with, all the way up to where your stove starts to glow red.

Since the lowest frequency infrared overlaps our range of "normal temperatures" we call it thermal infrared. Higher frequencies are far infrared, then mid infrared, and finally the highest frequencies are near infrared.

So we already see heat in the same way thermal imaging does. We just see much hotter light. Light from the sun. Of course we can see light that doesn't come from heat as well like fluorescent light bulbs, and thermal IR can be produced by other means than incandescence as well.

Now it's important to note that if you are are about the same temperature as the things whose thermal infrared you want to see, you are going to be generating a great deal of light. Any focusing optics that are part of your body will be emitting light you can see. Even the very photoreceptors would be emitting light that would trigger themselves. This is why early Thermal Imaging systems, had to be cooled to below the temperature range of the heat they need to see. Now, as long as we know the amount of interference, we can filter it out electronically, but cooled sensors are still better.

So, sensing heat in your own body heat range is a highly improbably evolutionary development. There's no benefit without a filter, and no reason to develop the filter without the capability. There may be some especially odd path where some other capability was re-purposed like an ability to actively cool the surface, although I can't think of any use for that either.

Life built around a solvent with a lower freezing temperature than water like ammonia or methane might well end up seeing in what we consider thermal infrared. They would probably regard us the way we would regard aliens that live in molten metal and would not think of their own vision as being 'heat vision' To them, "normal hot things" would emit even lower infrared, or high frequency microwaves.

If things with a significantly higher temperature than you are important, you can get some level of radiant heat sensor. Pit Vipers for instance have this ability for detecting birds and mammals, although the capability is at best "something warm is in front of me" rather than vision.

  • $\begingroup$ I would like to point at the a lot evolutionary things don't have good predecessors. Even the flagella on cells and bacteria doesn't have a good precursor. So just because you can't see an evolutionary precursor doesn't mean it can't exist. In fact, this is one of the main arguments against evolution, the lack of a lot of precusors. $\endgroup$
    – DonyorM
    Oct 30, 2014 at 5:59

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .