What are you touching right now? Probably a set of clothes, a pair of shoes, perhaps a computer mouse. Maybe one or two other things.
Before we can explore the thought-provoking ideas I want to touch on, here’s a bit of high school review. Tactile sensation is as familiar to us as any of the other traditional senses, and with good reason. It’s effective for obtaining information about your environment, picking up and manipulating objects, and if you include pain and temperature, it’s also a valuable mechanism of protection. Like too many other physiological marvels, tactile sensation and perception are easy to take for granted. But what does it really mean to touch something?
From a physiological point of view, your experience of touch – let’s say in the hand – could begin with a distortion of the skin on your fingertips. Pretty much any disturbance you can nominate activates a unique receptor or group of receptors. It can be a stretch, poke, pressure, vibration, or other occurrence that activates a corresponding receptor. In fact, even your limb position has a special class of receptor devoted to its detection, called proprioceptors. Try it right now: without looking down, visualize the position and angle of every joint in your body.
After a receptor is activated, the neuron connecting to it depolarizes (or “fires”) and the signal shoots up into your spinal cord, whereupon it takes one of several vertical paths to your brain. In your brain the signal is propagated by one or more neurons and ultimately gets projected onto your brain’s sensory cortex.
But hold on.
What causes that distortion of the skin, anyway? It happens when you, for example, press your fingertip against some object such that your skin and the object can’t occupy the same space, right? That means that 1) one of them changes shape until 2) the forces equalize and you can’t keep pushing into the object. But why can’t your finger and the object occupy the same space? After all, recall that matter is mostly empty space. If the nucleus of an atom were 2 centimetres across, the nearest electron would be about 100 meters away. Do the math and you’ll see that only about 1/1012 (one trillionth) of the volume to the nearest electron is actually occupied by mass. That’s 0.0000000001%, much less than a thousandth of a pixel on your whole monitor.
And the electrons are basically delocalized fluff around the comparatively massive nucleus. Matter is mostly empty space.
The reason you can’t push right through something with your finger is because the electron clouds surrounding every atom clash once the atoms get within a certain distance of one another. Electrons are negative, and negative charges repel other negative charges. This is called the electromagnetic force and it can be observed when you push two magnets into one another with the poles that repel. If they’re strong enough magnets, you cannot get them to touch no matter how hard you push.
You might have heard that gravity is the weakest of the four fundamental forces of our universe. It’s hard to believe when gravity is the one force you experience most often. But keep in mind that the gravitational field you observe is the result of 6×1021 (six sextillion) metric tonnes worth of planet beneath your feet. And by rubbing a balloon against your hair for a few seconds, you can easily overcome it with electrostatic forces as you attract and lift a feather. The bottom line is that no matter is touching when you see two objects touching. No matter is contacting other matter when you grasp a doorknob in your hand and feel it turn. None of your atoms are touching any of your other atoms. Ever.
Remember the last time you fell to the ground. Maybe you slipped on ice or lost your footing while climbing over a barbed wire fence for some reason. It probably hurt. Your brain told you that because events in your spinal cord indicated injury. That signal came from ascending neurons in your feet or rear as you hit the ground. Those neurons were activated when some of your tissues were squished and distorted by one another or by the pavement beneath you. Irrespective of where you fall from – your own height or the cruising altitude of a 747 – the electromagnetic force won’t let you pass through the ground unharmed. It’s committed to fighting the gravitational force at all costs and will keep you (or what used to be you) above ground, no matter what.
So keep this in mind next time you stub your toe or bump your head. Imagine the pathway of the pain signal and remember that ultimately…
The electrons did it.
Alex Cojocaru is a fourth-year student at the University of Alberta, entering his first year in Medicine.