NOT EVEN ONE PERCENT

This sounds funny, but the reality we have grown accustomed to, the enticing smell of the first batch of our morning espresso, its frothy structure and the solid lingering taste, forms only a tiny sliver of a Brobdingnagian, grander and more exquisite reality. Our known reality is undeniably only a construct of our five principal senses. According to quantum mechanics, our very act of seeing causes a thing to exist; in that sense, when we are not looking at the moon, the moon does not exist! We have come to a bizarre paradox. Set aside, let us begin this article with something simple. Our eyes are remarkable indeed. However, most people are unaware that the eyes see only a tiny fraction of all that surrounds us, which we will discuss in the following paragraphs. 

Once again, our known reality, the world with which we interact around the clock, is a marvellous cook-up of our five principal senses, sight, sound, touch, taste and smell. Speaking of touch, we have seen in one of our previous articles titled A Universe Full of Hovering Butts and Why It Is Impossible To Kiss Someone that touching someone/something is not entirely defined. Of course, from our perspective, we can pick up our cup of morning coffee or give that cat its daily dose of belly rubs, but as discussed in the said article, if we think of atoms, then this whole act of touching becomes a shady business. Atoms are not like billiard balls. They are characterised by their quantum mechanical wavefunction. Accordingly, atoms or microscopic (essentially quantum) particles exist as a fuzzy probability wave, described by an abstract mathematical function, which makes no intuitive sense at first look. But it works beautifully. So speaking from a quantum mechanical perspective (QM for short), when two atoms come close, they do not touch like the way macroscopic objects touch. Theoretically, no object, macro or microscopic, can come into close contact so as to fuse into one singular entity because of Coulombic repulsion and the Pauli exclusion. Objects interact. Following this line of thought, we are not actually picking up our coffee cups though that is what it seems to be. Only the cup's wave function interacts with us, and the net result is a cup in our hand followed by a happy feeling of getting our first batch of morning espresso. However, this discussion is only analogical, and the actual thing is quite complex. Since QM is applicable only for microscopic systems, we are exempt from choosing whether to drink or not(pun intended). Hopefully, we will come to this at some other convenient time (another pun; QM is punny). 

Let us come to our main topic, which is sight. Human vision or vision, in general, is made possible because of photosensitive rod and cone cells at the back of the retina that can respond to particular frequencies (and corresponding wavelengths) of the incident electromagnetic radiations (EM for short) as they enter the eye and strike the retina. The photosensitive cells of most organisms with certain exceptions have evolved in a way to absorb only those EM frequencies ranging from 400 THz (750 nm wavelength) to 790 THz (380 nm wavelength) which we perceive as the visible spectrum, i.e., the VIBGYOR or the ROYGBIV (same thing in descending order of wavelength) spectrum seen in a rainbow. This spectrum is known as the visible spectrum because it encompasses only those particular frequencies of EM radiations that carry sufficient energy to cause molecular excitations of the photosensitive opsin pigments in the rod and cone cells. These molecular excitations are then transduced into electrochemical signals, which the brain interprets by associating a distinct colour to each band of visible frequencies. Hence the Violet-Indigo-Blue-Green-Yellow-Orange-Red spectrum with increasing order of wavelength and decreasing frequency. It is the brain's way of reading photon energies as they excite the retinal pigments. When this visible light bounces off of objects and enters our eyes, we have what is called visual perception, i.e., we attain the ability to see. It is not that we are seeing anything, but what we know by vision is our brain's way of giving us a mental image of the world around us. And this works as long as the light is entering our eyes. If we close them, the brain has no light to read or information to process, and consequently, we see nothing. 

Perhaps the Eyes Don't Have It. 
Image Credits: publicdomainpictures.net

On average, a typical human eye has about 92-120 million photosensitive rod cells, responsible for low-light vision, night vision, and grey-scale monochromatic vision also known as scotopic vision. The second class of photosensitive cells, the cone cells, are fewer in number, about 6 million and function only under bright light conditions, thereby resulting in what is known as photopic vision. In addition to rods and cones, the retina contains a third class of photoreceptors called ganglion cells. Most humans have what can be best described as RGB colour vision or trichromacy/trichromatic vision, resulting from three distinct classes of opsin pigments. The human eye possesses three types of cones, the short, the medium and the long, whose peak spectral sensitivity peaks at around 420, 530 and 560 nm, respectively. The overall spectral sensitivity of the human eye peaks at a wavelength of 555 nm during daylight conditions when the cone cells are most active, while at night it shifts to a wavelength of 507 nm at night. Henceforth we can infer that humans can see green better than any other colour. 

♪ Green is the colour of her kind ♪
Image Credits: Photo by Pixabay, Public Domain

The eye is just a window that lets in the EM waves to strike the retinal pigments. It is the brain that grants us our ability to see. Without the brain, we would have no notion of colour, because colour in itself, does not exist. No object possesses any colour. An apple is not inherently red. It so happens that the molecules present on an apple's surface can effectively absorb all frequencies of visible white radiation except the red portion. An apple strongly reflects the red wavelengths when illuminated under visible light, daylight or white light. When this reflected (red) light strikes the retina, we see the apple to be red. If illuminated under UV light or low-pressure sodium lamps, the same apple will look a darker, violet or greyer shade. So instead of saying that the apple is red, we should really be saying that under the optical (visible) spectrum, the apple looks red to us. Much like this, what is blue to us may not be blue to some other organisms, such as the Australian sea lion. The sea lion has monochromatic vision, i.e., it can only detect the presence or absence of light, the gradation of light intensity and perceives no colour. So a blue flower means nothing to the sea lion. It has no conception of blue. A hoard of examples can be cited in this regard, such as the tetrachromacy in some women who possesses an extra cone cell with peak sensitivity in the near-ultraviolet region which comes from a specific mutation in their X-chromosomes. As such, their visual spectrum extends beyond the standard VIBGYOR spectrum and is thus able to see more colours than the general population. Most birds and fishes are supposed to be tetrachromats, while dogs are dichromats having blue and yellow cones only. Our whole act of seeing is the brain's way of interpreting the class of EM frequencies striking the retina. 

The eye and the brain together form a very remarkable pair of natural architecture, yet the former can respond to only a very tiny fraction of the entire EM spectrum. The EM spectrum encompasses a seemingly large array of frequencies, of which the eyes are sensitive only to the so-called visible bands of frequencies. Since the upper and the lower bounds of the EM spectrum are not theoretically defined, except the shortest wavelength (highest frequency) can not be less than the Planck length while the longest wavelength can not exceed the diameter of the observable universe, it is hard to provide an exact figure regarding how much of the spectrum do we see. But assuredly, whatever our eyes see is many orders less than a unit percentage. There is a reason why Earth-based lifeforms have evolved to have a vision sensitive only in the 380-750 nm range or in the extended region of near-ultraviolet to near-infrared. This is primarily because life on Earth has evolved with respect to the energy output of the Sun. The latter's maximum energy output corresponds to the ultraviolet-visible-infrared bands on the spectrum. This section also contains enough energy to sustain the growth and development of all life. Radio waves carry very little energy to serve any purpose, just like x-rays or gamma rays carry too high an energy that can hamper the very progress of life. Also, Earth is brilliantly shielded from high-energy radiations. Now here we can ask what about alternate lifeforms in extremely high energy regions, such as on some planet in the vicinity of a neutron star or a black hole? Could life in such an environment evolve to benefit from X-rays? That is a question for another day. It is a good intellectual exercise to speculate how the world would look if we could gaze across the whole length of the EM spectrum, but we should bear in mind the distinction between science and fiction. 

Plot of Earth's atmospheric opacity to various wavelengths of EM radiations. This shows what fraction of the EM radiations reaches the surface and how much of the incident radiation is blocked off by the atmosphere. 
Image Credits: Wikimedia Commons

Upon hearing the word light, we readily think of the visible spectrum, almost forgetting that radio waves and X-rays are also light, meaning they are all photons, except the difference lies in their frequencies along with their corresponding wavelengths, energies, penetrability, ionizing power, etc. If we take a red light photon whose wavelength is 650 nm (frequency ∼ 460 THz) say, we find that it carries an energy of 1.9 eV. Similarly, with a 30 MHz VHF radio wave, we calculate that it has an energy of 124 neV (nano electron volts). Thus a VHF radio photon carries about 10⁻⁹ orders of magnitude less energy than a visible red light photon. Much like this, a gamma ray photon with a frequency of 300 EHz or 300×10¹⁸ Hz carries an exorbitant energy of 1.2 MeV, which is 10⁶ times more energetic than our red photon in consideration. Let us now imagine how our world would have looked if we had radio vision. First and foremost, our eyes and brains would have evolved differently, giving us a completely unfamiliar colour spectrum. Secondly even the highest-frequency radio waves carry very little energy. So to gather all that energy and focus into a tight spot, we would need eyes the size of standard radio dishes. Keep aside, for argument's sake, if we could suddenly turn our eyes into little radio detectors via some Frankenstein-like engineering, we would be instantly overwhelmed by the jungle of radio waves around us. In fact, the whole universe is awash in radio waves. With the sudden introduction of radio vision, every single human-made radio source, every naturally occurring and astrophysical radio source, would immediately light up like Christmas lights. As Earth's atmosphere is transparent to radio waves, every celestial object in our solar system, interstellar objects, distant quasars, black holes, neutron stars, stellar nurseries, etc., would be plainly visible across the night sky with their characteristic radio spectrum. Since our bodies are also transparent to radio waves, we would not be able to see our friends, for they will have become transparent, and we will all go bumping in the night. We can go on speculating about what would happen if we suddenly get ourselves radio vision. But for an organism to evolve into having radio vision, it would require eyes of impractical dimensions. It is indeed an interesting question if an organism can develop radio vision or some novel means of detecting radio oscillations. No organism has been discovered capable of this, at least on Earth. Regarding alien biology, this is an open question. 

To see microwaves, our eyes would have to be as large as these radio dishes (in pic) mounted on a typical communications tower. 
Image Credits: Wikimedia Commons

With the sudden introduction of microwave vision, all our communication equipment, broadcast towers, and radio antennas, much like in radio vision, would shine bright like high-mast lighting posts. And so will our microwave ovens and every astrophysical/natural source of microwaves. What is more interesting, with microwave vision, we will see the Cosmic Microwave Background Radiation, i.e., the relic (remnant) radiation left behind from an early era in our universe's evolutionary history after the Big Bang when the universe cooled enough to allow the formation of neutral atoms. The CMBR fills all of space uniformly in all directions. So the whole sky, and simultaneously, the universe will look equally bright with regions of local microwave activity. 

With infrared vision, everything around us would glow vigorously because every speck of matter in our known universe radiates unrestrictedly in infrared or what is also called heat radiation. Having an infrared vision will grant us some necessary advantages that can be understood easily. However, with infrared vision, sleeping would be truly uncomfortable since biological organisms are themselves strong emitters of infrared radiation originating from cell metabolism.

Thermogram of a snake held by a human hand. Snakes are cold-blooded animals. So they appear darker in this image than the human hand, which is glowing brightly. Through this, we can approximate a world under infrared vision.
Image Credits: Wikimedia Commons

In the case of ultraviolet vision, our familiar blacklights will shine a whole lot brighter than the standard violet-to-blue fluorescence. Also, our concept of human beauty would drastically change, for all of us would look significantly darker, bringing out wrinkles and dark spots, highlighting areas of melanin production, which absorbs the UV rays and gives protection to the underlying cells. Our eyes block almost all of the UV rays that reach the Earth's surface, and as a result, a very tiny fraction enters the eyes. But due to the lack of necessary sensors, we can not see in UV. However, certain animals, such as bees and avians, have UV vision. Having X-ray or Gamma-ray vision would serve no biological purpose, as the Earth's atmosphere acts as an effective shield against ionizing radiations. However, we can understand what would happen if we had such vision abilities. 

This article has one way beyond its intended length, so I have no other option but to end it here. With a good grasp of the fundamentals, one can speculate about hypothetical/alternate scenarios. This is the beauty of science. It allows us to question whether exotic life can evolve in grossly un-Earth-like environments and what is the theoretical possibility of such beings existing somewhere across the universe. At this length, I hope I have been able to show that there exists a reality embedded within our familiar reality. Through technology, we have learned to use the entire EM spectrum and even transduce gravity waves into acoustic vibrations so that we can feast upon the sound of colliding black holes. But what if we wish to see actual microwaves emanating from our cell phones or feel the squeezing of the planet in the wake of a passing gravity wave? How would that be? 

Bibliography: 

1. https://www.scientificamerican.com/article/touching-illusions-2008-05/
2. https://en.wikipedia.org/wiki/Intrinsically_photosensitive_retinal_ganglion_cell
3. https://www.bbc.com/future/article/20150727-what-are-the-limits-of-human-vision
4. https://www.britannica.com/story/are-dogs-really-color-blind
5. https://www.bbc.com/future/article/20160316-i-can-see-colours-you-cannot-perceive-or-imagine
6. https://www.bbc.com/future/article/20150727-what-are-the-limits-of-human-vision
7. http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html#:~:text=The%20different%20parts%20of%20the,human%20body%20is%20quite%20transparent.
8. https://education.nationalgeographic.org/resource/infrared-vision
9. https://edition.cnn.com/2017/06/05/health/colorscope-green-environment-calm/index.html#:~:text=Green%2C%20the%20mixture%20of%20blue,any%20color%20in%20the%20spectrum.