A sense is a physiological systems that provides information for perception. It is how a brain-body finds out what’s going on inside and outside itself. Most people think of the usual 5 senses that feed information directly to the brain, but there are an indefinite number of others when you consider other systems in the body, some of which bypass the brain altogether.
The first matter that needs to be cleared up is the difference between sensing and perceiving. Perception is sensory input that has been processed. In normal usage perception takes place in the brain, but again we need to expand the definition to include all processing of sense data, no matter where it occurs or what effects it has.
Although most perception is not conscious, it always involves some kind of transformation of the sensed information. For example, the pattern of photons that strike the eye are transformed and processed even before any nerve signals leave the retina. Furthermore, it is possible (indeed common) to perceive things without involving the senses, in dreaming, hallucinating and imagining. An amputee can for example experience pain in a limb that is no longer there (phantom limb syndrome).
My approach is to sort through the various environmental phenomena that do and do not have an effect on the brain-body, and also to see what those effects are. The hidden agenda is to argue why it is highly unlikely that a brain-body can sense certain things that some of my friends and relatives (and many, many other people) think it can. These fall into the general category of “paranormal phenomena,” where the Greek root “para” in this context means “beyond.”
As a card-carrying materialist, I discount paranormal phenomena on the grounds that such a thing cannot exist (if it did, it wouldn’t be paranormal). In this respect, I differ from most skeptics, who twist themselves in knots supporting the notion that science must always be open-minded about the possibility of something new coming along. A common way to express this idea is that science cannot prove something correct, but only prove something wrong. I respectfully disagree, but the argument is long and I may be mistaken. It’s certainly not something that philosophers agree about.
I will go through my list of environmental phenomena that could possibly have an effect on any living thing, with special emphasis on the living things we most admire: us.
Visible light and radiated heat are forms of electromagnetic radiation. Technically, it means energy that is transmitted by massless photons. We have to accept that electromagnetic radiation can be either waves or particles, because in practice they “really” are mathematical expressions. “Waves” and “particles” are merely convenient ways of turning these expressions into something we can manipulate. It is useful to think of short wave-length radiation as photons, but not very useful to think of radio waves in terms of photons. Here’s why.
Different wavelengths of electromagnetic radiation have different effects on organisms. They range from extremely short wavelength gamma rays to extremely long wavelength radio waves, with visible light in the middle. The frequency (rate of vibration) of an electromagnetic wave is the inverse of its wavelength, so gamma rays are high frequency, and radio waves are low frequency.
The energy carried by a photon goes up dramatically as the wavelength gets shorter, and falls off dramatically as it gets longer. Thus a single visible light photon has enough energy to trigger a retinal cell to fire, whereas a typical radio wave photon has such low energy that there may be a trillion of them in a single wave several feet long.
Of particular interest are the specialized sense organs that are “tuned” to certain wavelengths. This naturally implies a complex organism, unlike the vast majority of organisms in this world (bacteria and “archaea”) neither of which have organs.
The many kinds of eyes that have evolved time and again in nature are receptors for radiation that is named visible light because it is, well, visible. Some animal eyes can detect “near ultraviolet” light, which has wavelengths just too short for us to see, and some animal eyes can detect “near infrared” light, with wavelengths just too long for us to see. Each kind of eye has its own spectrum of sensed radiation. Birds for example have eyes that respond to color very much as ours do, whereas dogs do not.
Luckily, the molecules in our atmosphere block harmful forms of radiation, which is why we are alive. It is transparent to a wide range of radio waves down to the wavelength that might cook us, where it becomes opaque. Just below that, a window opens to let in heat and light, and just enough ultraviolet to give us sunburns or fade fabrics. The degradation of the ozone layer in the upper atmosphere has allowed more ultraviolet to get through, which is not good for us or other living things. The diagram shows the frequencies that are blocked by the atmosphere.
Around 40- 45 percent of the sun’s radiated energy that reaches the earth is in the infrared, depending on sky conditions. Beyond infrared are microwaves, with just the right wavelengths to cause water molecules to boil, which is how they cook food, so they can cook us as well. Luckily, microwaves of the cooking frequencies are blocked by our atmosphere or the dots on the microwave oven window.
The next-shorter waves than ultraviolet light are X-rays, which pass through different tissues in different amounts, helping us to see inside our bodies They cause cancer if you get too much of them. And gamma rays, the shortest ones, are destructive to living tissue, which is one reason you don’t spend much time around highly radioactive materials.
Summarizing, we have a specialized organ for detecting one bit of the electromagnetic spectrum, visible light. Short, destructive frequencies from outer space are blocked by our atmosphere, but we are vulnerable to man-made radiation in those wavelengths.
We are bathed in long-wavelength radio waves, with too little energy to harm us, even right next to a transmitter. In the worst case, the risk is less than leakage from a microwave oven, which is pretty small. We have a lot of other things to worry about, believe me. Note: electric fields from power lines are something else, discussed below.
Physical Media and Objects
Air, water and solid objects transmit sound and other pressure waves such as wind or the shock waves from supersonic jets and explosions. We sense sound with our ears, and other pressure waves affect various sense organs including the balance sensing machinery in our ears (the vestibular system), pressure (touch) sensors in our skin and sensors that detect motion at joints. We also feel air motion with the touch receptors of our skins. Special receptors register coolness when water evaporates from our skin, when the wind blows on us, or when we are immersed in a cold liquid. Yet others sense heat directly from air or hot objects, and from infrared radiation (from the sun or hot source such as a fire).
Heat and cold also trigger signals from pain receptors. When you touch a hot or cold object, the pain signal shoots quickly to the brain to cause you to let go, and only later do you know whether it was hot or cold. Colliding with or handling objects can trigger a whole battery of touch, pain, joint position and balance machinery. Whether animals can detect earthquakes before they happen is controversial, except immediately before, when they detect the preliminary effects before we do.
Our mouths and noses have taste, odor and pain receptors that help us eat and drink safe and nourishing things and avoid toxic things. Our sense of smell detects all sorts of chemicals in the air, an ability that other creatures such as dogs and rodents use much more than we do. Just for comparison, we typically have about 6 million odor receptors, while a dog might have 300 million, with a proportional amount of the brain devoted to odor perception.
Odor receptors are interesting because they are direct extensions of neurons from the brain, unlike vision where the retinal cells are many steps removed from the final brain cells that register what we are seeing. Our sense of taste detects only 5 distinct classes chemicals (sweet, sour, bitter, salt, and umami); most of the “taste” of food comes from odor, texture, pain and temperature sensors. Many creatures in water sense various chemicals, and even the gradient of a chemical. We can taste water just as we can taste food.
Our various senses involved in eating and smelling can detect chemicals that were present and important when our senses evolved. Today there are tens of thousands of man-made chemicals in our environment, some of which are toxic to varying degrees, alone or in combination. We can directly sense some of them, but not most of them. This makes us very nervous. Luckily, we can learn certain chemicals to seek or avoid because our brains continue to develop from the moment of conception to maturity. Unluckily, we are built to crave tastes that were in short supply when the senses evolved, but now are very cheap, notably sugar, salt and fat, with predictable results.
Each type of odor or taste sensor is specially designed to glom onto a specific type of molecules. When it does, it changes shape, sends a signal to the brain, lets the molecule go, changes back to its original shape and waits for another one.
If you stretch the definition of “sense,” you can argue that our immune system senses invading bacteria, viruses and parasites. The information is not sent directly to the brain, but is used by various other cells to fight off the infection. Sooner or later the brain gets an indirect message about the battle because you feel crappy.
Fast Molecules and Particles
Various kinds of molecules and elementary particles whiz about and either strike us or pass through us. Particles are distinct from the photons that carry the electromagnetic force in that they have mass, while photons don’t (one or two other particles are or may be massless, but they are not relevant to this discussion). They are distinct from the molecules we detect with our noses and mouths because they are moving very fast and are minuscule by comparison.
Radioactive materials emit Beta particles (fast electrons) and Alpha particles (helium nuclei) that can harm living tissues. Powerful cosmic rays (high-energy protons and atomic nuclei, from unknown sources) luckily don’t normally strike us because we are protected by the earth’s magnetic field, but a few get through and collide with molecules in the atmosphere to produce a shower of particles that can damage tissue. Neutrinos, probably the most common particles in the universe, pass through almost all atoms (which are almost entirely empty space) without interacting with them, which is why it is extremely difficult to detect neutrinos. Trillions of neutrinos pass through your body every second.
If you become part of an electrical circuit, electrons will stream through your body, causing a shock or convulsions. This occurs because our nerves operate with very small electrical currents, and when you overload them, they react violently. Positrons (positively charged electrons) can be created and focused on the brain to illuminate brain processes. Radioactive materials are also used for various medical purposes such a treating cancer, tracing metabolism or tracking blood flow. But no sensations are involved, only direct effects on tissues.
We are immersed in various kinds of fields. Fields can only be understood mathematically, and I don’t have the math and most likely neither do you. You can get a sense of a magnetic field by scattering iron filings on a piece of paper over a permanent magnet, as many do in high school experiments. Fields transmit forces. For example; the magnetic field transmits a force that causes a compass needle to swing, while the gravitational field transmits the force of gravity.
The most important field is earth’s gravitational field. Our bodies are designed for the amount of gravity we experience on the earth’s surface, and without that force acting on us we may get a form of motion sickness, which is why astronauts need a few days to acclimate themselves. On the moon, you can leap very high; on a bigger planet, you would have to crawl because you couldn’t support yourself. We don’t have a true “gravity” sense; instead we have a sense of balance that helps us cope with sideways forces as well as gravity. We also have pressure sensors in our joints and our skin that directly register the effects of gravity, as it presses us against objects.
We cannot detect the earth’s magnetic field, but some birds, insects and other animals can, and use it to navigate – no one is quite sure exactly how, but they are close to finding out. Gravitational waves are so weak by the time they get to us that only recently (2016) have they been detected, triumphantly supporting Einstein’s general theory of relativity. Electrical fields in the brain can be detected, and magnetic fields that make temporary changes in the brain allow us to detect and locate brain activity.
Electrical fields are sensed by certain fish that live in the dark of the deep sea. They can actually “see” prey and mates with these fields. Electrical fields can affect us, even if we can’t sense them directly. Powerful electrical fields can disorient our brains during electro-convulsive shock therapy, and any powerful electrical field can hurt or kill us, as when lightning strikes nearby. It is unlikely that the electrical fields from power lines or cell phones affect us, but they might – it is hard to tell because the effects are so small. Again, we have a lot more important things to worry about.
In any case, we don’t “sense” these because we don’t have special nerves dedicated to receiving information from electrical fields. The molecules in our bodies directly react to electrical fields, just as they do to Gamma rays or X-rays, without their being sensed by a sense organ.
On the cutting edge there is some question about whether we can detect and use quantum fields. If so, it is very likely that the process will be entirely internal.
I hope this gives you some sense of what “sense” is all about. It goes without saying that science has found zero evidence for any other senses, and that if any new senses are discovered, they will have to detect one or the other of the outside forces I have described above. For example, it is possible, although unlikely, that we have a weak ability to extract information from certain kinds of electrical or magnetic fields. Extremely sensitive equipment has not yet detected any such thing so far.
There is no last word in science, but the options become more and more restricted the more we know, which means any future senses we discover will be extremely subtle. Those who believe in paranormal senses pounce on this to claim that only special people (usually themselves or someone they know) are able to sense these imaginary things. Thousands of careful tests have failed, over and over, but believers, who operate with faith and hope, are not deterred by scientific evidence.
I discuss elsewhere the relation between those who believe in non-physical phenomena (such as souls) and those who like me believe only in physical phenomena. Conflicts arise when those who believe in non-physical things get anxious about their beliefs and misuse the scientific belief system for support. What’s wrong with just believing in non-physical phenomena? Why pretend to support them with science?
In any case, if there is a subtle sense we have not detected, it is likely many people will have it, or it would not have remained in the gene pool. It is highly unlikely that anything as complex as a sense organ could evolve spontaneously in one person. Such things take time, and thus affect many people.
Once again, there is a huge difference between what we sense (information that hits the sense organs) and what we perceive (information from sense organs or internal processes that has been transformed in some way). Perception transforms raw sense data into forms that trigger actions, memories, emotions, imagery and the like. Perception can result from brain processes that imitate sense data, as in hallucinations, dreams and imagination. It is very easy to perceive something that doesn’t exist outside the brain that invents it.
Bottom line, if you know of any new phenomenon in nature that could be detected by a sense organ, please contact your nearest physicist, who will be anxious to hear about it.
April 20, 2016