Can humans see in the infrared? By definition, no, but as we will soon see, the definition of “infrared” might be different for different people. When we see objects, what we actually see are photons that are reflected or created at their surface. We call these photons light. Our eyes are only sensitive to some portion of the electromagnetic spectrum. In other words, we can only perceive photons with certain wavelengths, and this is what is called “light”. We cannot see x-rays, and cannot sense radio waves. This small visible band is also perceived in color, ranging from violet (wavelength starting at 390 nm) to red (wavelength ending at 750 nm).
No matter what we do, we cannot see other types of photons with our eyes. In fact, the entire room in which I’m writing this document is beaming with radiation of all different kinds. Every object in it emits infrared rays due to its temperature. The power socket to which this computer is connected has an alternating current oscillating 50 times per second, which creates electromagnetic waves. And in general, the whole room is filled with radio and micro waves from broadcasting stations, cellular antennas, and other such devices. But when I turn off the lamp, I am shrouded in darkness, and cannot see any of this radiation. Much to our misfortune, an entire world of physical phenomena is locked to us, and we can only detect it with man-made tools.
But not all humans are created equal. Some can hear higher tones than others, some have clearer vision, some have higher finger dexterity. There are many traits which are shared by all men, but are not by physical law guaranteed to be exactly equal for the entire population. What do I mean by “physical law”? If a trait, for example, relies solely on a basic physical property of hydrogen, such as absorption lines, then this trait is not subject to change in different people. Simply put, hydrogen does not change from person to person; it is universal. However, if a trait relies on a complex system with many components, each of which depends on different parameters like temperature, it is not at all unlikely that different people will have fluctuations in this trait. Such systems may be very similar, but are not guaranteed to be identical.
An interesting question, therefore, is whether the size of the band which we call “light” is such a trait. Could it be that one person would stop sensing photons at 750 nm, while another at 750.01 nm? In order to say something interesting about the answer to this question, let us examine how we translate photons into electric pulses which our brain understands.
In the back of our eyes, there are millions of cells called photoreceptors. In humans, there are four types of such cells: rods, which are responsible for peripheral vision and cannot indicate color, and three types of cone cells, which are sensitive to color. Each cone cell reacts to a different range of wavelengths, and our brain knows how to combine the input of all of these in order to create the sensation of color.
How are these cells sensitive to different ranges of wavelengths? They all contain a pigment at their ends, which changes its shape when a photon hits it, sparking a chain of events that eventually causes an electrical discharge. The pigment is composed of two items: a form of vitamin A which is called a retinal, and a protein which is called an opsin. The retinal is the same in all photoreceptors; it is the opsin that differs between cones. Hence, it is also the opsin that is responsible for the capture of the photon, and different opsins react to different wavelengths.
How do these proteins capture photons? Proteins are gigantic molecules built out of many parts (specifically, our opsins are made of hundreds of amino acids). The main force which holds acids together are shared electrons (this is known in chemistry as electron bonds). A photon hitting the molecule may “hit” an electron, giving it additional energy and causing it to change its state. The initial types and energy, and the final types and energy of the electrons are determined by qualities of the entire molecule:
1) The chemical structure of the molecule – how electrons are spread throughout it.
2) Oscillations and rotations of the sub-parts of the molecule.
I do not know much about the first of these, but I am pretty confident that at least the second trait is closely affected by the amino acids that make up the protein.
Hence, if the opsin protein was built out of different amino acids, it would react to different wavelengths (and this is indeed what differentiates between different opsins). Luckily for us, we already know the blueprints which tell us which amino acids should build which protein – this is the whole concept of our genetic code, our DNA. Usually our DNA tells us to build the same proteins, but in essence, it could be coded to build anything.
One question therefore remains: is it possible, that a mutation could happen so that the opsin which responds to red light would change its structure in such a way, that it could absorb photons with higher wavelength? To this question, I have no real and concrete answer. However, I keep in mind that our body uses over twenty types of amino acids, and that opsins are hundreds of acids long. The entire structure of the protein depends not only on the acids composing it, but also on their sequential order. It seems likely to me, that such a mutation could occur. Also, although this option was not discussed in the previous paragraphs, it might be that the entire cone-pigment system is sensitive to other parameters, and just like hearing, is more acute in some people than others, resulting in changes in the sensitivity to wavelength. In any case, for the rest of the this article, we will assume that there are people who, due to differences in their red cone cells, have eyes which are sensitive to wavelengths higher than 750 nm (even if the new upper bound is very very close to the original, say, 750.01 nm). **
So there may be people in the world who can see longer wavelengths than others. How do we find them? The experiment is fairly simple. We put a person in a completely dark room, along with a machine that can emit photons of varying wavelengths. Starting from a high wavelength which we are sure that no one can see, we very slowly decrease the wavelength. Eventually, there will be a point in time when the person says, “aha! I see a spot of light!”, in which case we note down the maximum wavelength he was able to see, and move on to the next subject.
Assuming that indeed there are differences between people in this area of vision, we will be able to create an ordered list, starting with the person who can see the highest wavelength (“most infrared”), and ending with the person who has the highest threshold. This last person, when he looks at all the others, must think them a wonder: they can see things that he cannot. In his perspective, they can “see infrared”. Of course, to them, there is nothing wondrous about them – since it is the red photoreceptor that goes off from this radiation, they actually perceive a faint red light. Each person in the list will treat the ones below him as if they could see in the infrared.
Of course, this is just a result of our definition: “infrared” means those wavelengths, that are too high for the human eye to see. However, if, as assumed, the mechanism can have variations between different people, then the term “infrared” is not objective, but subjective. But that is all just terminology; what really matters, is that now we have two people in our hands: one who can only see radiation up to 750 nm, and another who can sense photons up to 751 nm.
We can use this in order to transmit a message to one person, and not the other, while both of them are in the same room. All we need is a projective screen and a filter, the combination of which allows us to emit a two dimensional image in infrared. We set the wavelength of the photons emitted from the screen to 750.9 nm. The first person cannot see this at all, since he is bounded at 750 nm. The second one, though, does register input, however faint. Since this light is near the maximum of his capabilities, his eyes might respond very weakly, but we shouldn’t have a problem amplifying the light source as much as we wish, until he sees a bright image. Effectively, he should see the the image on the screen as red light.
We now have the ability to hold a screen in front of two men, both of whom have no special equipment. One person can see the image on the screen (which appears as a very faint monochrome image, according to what we have just described; however, if our tools are precise enough, we can create “grayscale” [or rather, “redscale”] pictures on the screen). The other cannot. This fact does not change with distance or orientation of the screen, so there is practically very little that the second man can do in order to see the image.
I have described just one application of this difference in vision, if indeed it can exist. Surely there can be many others, and you are welcome to try and conjure some of your own. I was unable to find literature on the subject on the web, so I do not know if someone has already performed the infrared sight detection experiment. Neither do I know if indeed the changes in the opsins are theoretically possible as I have portrayed them. More research on the subject is needed, but comparing to other complex human sensory systems, it is not at all unreasonable, that the findings should turn out positive.
** It is interesting to note that humans are actually well equipped to deal with what we normally call “ultraviolet”. It turns out that the blue-sensitive photoreceptors in our eyes can actually sense ultraviolet, but we do not normally see this, as the eye’s lens blocks it out. For more information see this link.
Aware of this option, I have chosen to discuss infrared in this article due to two reasons: first, shining ultraviolet light in people’s eyes is far less safe than shining infrared. Second, it allows for discussion on traits which are not physically guaranteed to be identical between men.