Forces are invisible, odourless, tasteless, they have no texture, they don’t make any sound and yet, even though our senses cannot detect them, we can feel them. Isn’t that fascinating? We can’t draw them or take a picture of what they look like but yet they exist. We know they do because if they didn’t we wouldn’t be here to talk about them.
Perhaps the most common of forces is the one we all fall for: gravity. Interestingly, this is the weakest of all forces that exist. There are four fundamental forces in the universe. By that we mean that if all forces which we encounter everyday were decomposed to their bare minimum, if we were to investigate the source of these forces, we would find out that they all sprout from just four types of forces. We cannot rank those forces in order of importance or usefulness but we can do so by their relative magnitude.
Starting with the strongest one, we have the force called the strong force. Yes, not very creative when it comes to naming them. Truth be told, the best names are sometimes the simplest and most descriptive ones. Think of naming something like this: deoxyribonucleic. It’s 16 letters long, it’s meaning is obscure, it’s hard to pronounce and we won’t necessarily remember it that easily. This is why we often refer to this acid as ‘DNA’. DNA is catchy and sounds cool but its meaning is not obvious. Even after spelling it out in full, deoxyribonucleic acid, it does not convey any more information than its contracted counterpart. The strong force, however, is exactly what it says it is: strong. So, what is this strong force then?
All of matter is made up of tiny particles called atoms. Atoms are indivisible elements of stuff – well, that is what we used to think. Atoms can be further broken down or split into smaller particles called protons, neutrons and electrons. Uncontrolled splitting of atoms causes a chain reaction which eventually leads to an explosion. This is the principle behind the atomic bomb. Devastating consequences are expected. Controlling these reactions are not easy but they are possible and are what drives the production of electricity in nuclear power plants. Consequences can be bad but not necessarily as lethal as bombs. Chernobyl and Fukushima incidents are examples of such controllable explosions gone out of control. So, inside atoms we have those tiny particles called protons and neutrons which form the nucleus. The electrons, like satellites orbiting planets, swoosh around the nucleus at tremendous speeds.
Protons and neutrons, it turns out, can also be split into even smaller units. These units are known as quarks and, as far as we can tell, they cannot be broken down any further. Just like the electrons, they seem to be the smallest units of matter. For this reason, we call them fundamental particles. Protons are positively charged particles whereas electrons are negatively charged. Neutrons carry no net charges and are thus electrically neutral – hence their name. The reason they carry no charge is because inside them, there are 3 quarks whose electric charges cancel each other out. Neutrons, like protons, are each composed of 3 quarks. Neutrons are made up of the following kinds of quarks: 1 up and 2 down quarks. Protons are 2 up and 1 down quarks put together. The up quark has a charge of 2/3 while the down quark has a charge of -1/3. Adding them up according to their respective proportions result into the net positive unit charge in a proton and zero charge in a neutron.
We’ve learnt at school that like charges repel and unlike charges attract. Somewhat like magnets where like poles repel, unlike poles attract. A north pole faced with another north pole would try to push each other away. A north pole put near a south pole would find it hard to resist the mutual attraction. This is the typical behaviour. Two positively charged particles would, in principle, repel each other. In contrast, a positive and a negative charge would attract each other. Just like the moon and the earth are mutually attracted to one another, electrons and protons are attracted to one another.
How come, therefore, that 2 positively charged quarks, the 2 up quarks inside the proton, can resist the repulsive forces between them? They should, in principle, repel each other just like two north poles pushing off each other. If you try to stick two magnets together you might find it hard to do so if their north poles are facing each other. You can feel the resistance in forcing those two like poles together. So how is it that 2 like charges are able to overcome this resistive force, this mutual repulsion, and remain bounded inside the proton? Well, this is where the strong force comes in. The strong force is responsible in keeping those quarks bound together. If it weren’t for the strong force, the 2 up quarks would repel another and they would never be able to combine to form protons and without protons you cannot have atoms, without atoms there is no matter, no stuff and all of existence and life would not be possible. All the universe would be composed of is a cloud of individual fundamental particles, quarks and electrons left to their own devices, unable to merge, unable to combine to make up reality as we know it. It would be a very, very different universe, to say the least. We owe everything, absolutely everything, to the strong force, ultimately.
But that doesn’t quite explain why we call it the strong force. It needs to be strong enough, that is for sure, but just how strong is it? Let’s take the force of gravity, for comparison. We are all very familiar with gravity. In fact, some of us even worry about its effect on us. What we experience as the force of gravity on us is none other than this thing we call ‘weight’. Our weight is simply how the force of the Earth’s gravity manifests itself. The Moon has a much weaker gravity and the same person would weigh much less on the Moon as they would on Earth. So, strictly speaking, so-called ‘weight-watchers’ should be watching out for their mass as opposed to their weight. If they wanted to lose weight then we could “easily” send them to the Moon where they would weigh about 6 times less than on Earth. They should perhaps call themselves ‘mass-watchers’ instead. What they really want to achieve is to have less mass, regardless of whether they are on Earth or on the Moon. The same mass on Earth would weigh less on the Moon than it would on Earth. So, obese or not, we are all familiar with this force we call gravity.
The force of gravity attracting two objects together depends on their respective masses and the distance between them. The more massive they are, the stronger the force of attraction. The shorter the distance between them, the stronger the force. We can work out the force of attraction between the Earth and the Moon just as we can work out the force of attraction between two cars, for example. In principle, anything that has mass will exert a force of gravity attracting other masses towards it while being attracted to everything else that has mass. Now, let’s consider the 2 up quarks inside a proton. We know they should repel each other but yet they stick together as though bound by the strongest glue. There must be a force keeping them that way. They must also be attracting each other with their mutual gravity, since they have mass. If the force of gravity between them is, say, 1 unit then the strong force will be about 10 to the power of 38 units! That’s right. The strong force is about 100 billion billion billion billion times stronger than the force of gravity! An inconceivably strong force indeed! And it very well deserves its name, we might emphasise.
Strong it might be but it cannot boast about a large territory. Gravity, on the other hand, can spread its pulling power all over the universe. The force of gravity which the Earth exerts on the Moon also acts on Sirius, the brightest star in the night sky. But the force with which Earth is pulling on Sirius is far less than that pulling on the Moon because, even though Sirius is much more massive than the Moon (nearly 60 million times more), it is quite a distance away (some 25 million times farther). Regardless of how far apart two bodies are in space, whether it’s just a millimetre or the whole span of the universe, they will feel the mutual force of gravity acting. This is not so for the strong force. You see, its range is no match to its might. The strong force is only felt within the protons and neutrons themselves and only residual of the force transgress to just within the nuclear confines of the atom. To have an idea of how big or, rather, should I say, how small that can be, let’s consider the size of a proton. It is of the order of 10 to the power of minus 15 metre. That is 0.00,000,000,000,000,1 metre. Which, quite literally, is close to nothing. A human hair is more than a billion times thicker than that! You can’t get a pair more contrasting in their characteristics than the strong force and the force of gravity. Where one wins in might the other wins on range. Where one is weak the other only manifests itself in the tiniest of regions of space.
So, what about the other two remaining fundamental forces? The other force which is ubiquitous, like the force of gravity, is the electromagnetic force. It is the one responsible for the force acting between electrically charged particles. Just as gravity acts between two masses, the electromagnetic force acts between two charges. Whereas gravity is always attractive, electromagnetic force can be either repulsive or attractive depending on what type of charges are involved. As we’ve already mentioned, like charges attract, unlike repel. It is what keeps the negatively charged electrons bound to the positively charged nucleus in atoms. It is what keeps atoms and molecules huddled together. It was what keeps all of matter from disintegrating. The weak force of gravity is not enough to keep molecules joined together to form more complex ones. If it weren’t for the electromagnetic force, there wouldn’t be any complex molecules and without complex molecules, you can rule out the chemical reactions necessary to bring about life. We can easily take this force for granted as we go about our daily lives. Just like with the strong force, we can easily go by living without the knowledge of its existence. The same cannot be said about the force of gravity.
For tens of thousand of years we have lived on this planet without the knowledge of the existence of the four fundamental forces. Even now there are those who do not know of the fundamental forces other than the force of gravity. But gravity, because of its immediate influence on us, because of its seemingly instantaneous effect on us, because of its straightforward implications, is a more intuitive force for us than the other three. Even if we did not know what the mathematics behind its workings are, even if we did not know why it exists, even if we had no name for it, gravity would still make sense to us in some very rudimentary and, shall we say, down to earth way. We throw something up, it comes down. As simple as that. We aim at an antelope and throw a spear, just like in our long gone hunting days in the savannah plains of Tanzania, we expect the spear to travel a certain way and not just fall flat to the ground or rise to the sky. We almost master the force for our everyday purposes. We learn to overcome it when we take our first steps as a toddler. We learn to use it in our advantage when we drop a heavy rock from a cliff onto an ambushed prey below. We did not know what it was but we knew how to use it, how to work with it.
And so, for the most part of our existence as Homo sapiens, we have lived with the force of gravity as being the most relevant force to us. Yet, more important than that are the strong force and the electromagnetic force. Because, like I mentioned earlier, without them there wouldn’t be a atoms, let along life, in this universe. It was until the 1950s or so that we started to delve even deeper into the workings of the atom and its nuclei. The strong force only came to light around 40 years ago. The electromagnetic force was initially considered to be two separate forces: electric and magnetic. It was about 140 years ago that we finally came to realise that the electric and magnetic force are but the two faces of the same coin, thanks to the work accomplished by James Clerk Maxwell. Thanks to Sir Isaac Newton, it was about 340 years ago that we came to a thorough, mathematical understanding of gravity. Almost a century ago, Albert Einstein gave us the most accurate description of gravity to date. And in the history of fundamental forces, it was about 80 years ago that Enrico Fermi postulated the existence of the weak force to explain radioactivity. Radioactivity is the spontaneous breakdown of heavy chemical elements into lighter chemical elements. The heavy elements tend to be unstable and therefore ‘decay’ into more stable ones. As they break down into smaller, lighter, more stable elements, they emit particles and radiations. Hence the term ‘radioactivity’. They are active in the sense that they change their state ‘on their own accord’; just by being too unstable and heavy. And they emit radiations. The amount and intensity of radiation they emit can be harmful depending on how long we are exposed to them. When I say that they are active ‘on their own accord’, what I mean is that there are no external agents causing them to decay in that way. It is not as though you have to heat them or that you have to dissolve them in water for them to start emitting radiation. It is the weak force that determines this decaying process. It is this force which dictates, if you will, how the heavy elements will break down into lighter ones. One way to think of it is as follows: Imagine a broken vase with dozens of pieces of vase splattered on the floor. Very patiently and diligently you piece all of them together to reconstruct the vase as accurately and carefully as you can. Now, if you had used a super powerful glue then it is very likely that the pieces will remain stuck together for a very long time and you would have your vase back. Had you used a weaker glue, then those pieces might gradually become unstuck and soon your vase would change from this singular, unstable piece of pottery into a cluster of smaller, lighter and more stable parts. What kind of glue you have will determine if and how your vase will ‘decay’. This is a very banal analogy but it gives us an idea of how the weak force controls radioactivity.
The weak force, therefore, completes the set of four fundamental forces in the universe. It is called weak force but it is by no means weaker than gravity. In terms of strength, the forces rank as follows: strong, electromagnetic, weak and gravity. And like the strong force, the weak force has a very short range, limited to what goes on in the nucleus of atoms. If we were to give gravity a relative strength of 1 unit, weak force would be about 10 million billion billion units, electromagnetic would be about 1 billion billion billion billion units and, finally, strong force would be about 100 billion billion billion billion units. As you can see, the disparity in terms of magnitude of strength in the forces is very apparent between gravity and the other 3 forces. The three forces which deal with the microscopic universe are vastly more significant than the force dealing with the macroscopic universe. That said, when it comes to the large scale of the universe, when it comes to dealing with everyday stuff, those three high-ranking fundamental forces are less dominant than the force of gravity. When we are dealing with atoms and electrons, gravity plays not significant role compared to the other three forces. Such is the order of things in the universe. No wonder, therefore, that until we came to understand the atomic world, the force most relevant and apparent to us was gravity. It is still the case for most people, I have to admit. But now, at least, we have an understanding of what forces govern our universe.
The four fundamental forces were not always here. When the universe came into existence about 13.7 billion years ago, through this amazing event we call the Big Bang, there was only 1 force driving the universe. As the universe expanded and the physical conditions of temperature and pressure changed in the universe, the force split into 2. One of them was gravity and the other was some sort of unified force called electronuclear force. As time went by, the electronuclear force split into 2 giving us the strong force and the electroweak force. The universe then had 3 fundamental forces: strong, electroweak and gravity. Eventually the electroweak force split into two, giving us the weak force and the electromagnetic force. We therefore end up with the four fundamental forces as we know them today. It is difficult to recreate the initial conditions of the birth of the universe in order to verify that state the forces were in at that time. However, the best theory so far tells us that those forces should have come from a single one.
Attempts to understand exactly how these forces, in particular gravity and electronuclear forces, work together have failed even the most brilliant minds in physics and mathematics. It is a known fact that even Einstein could not find a way to merge the two forces together. We need someone mightier than that to be able to achieve this great unification of forces. This does not mean, however, that we have reached the limit of our understanding of how the universe works. On the contrary, we can but anticipate what more we are about to discover from the unification of forces. Now wonder physicists call this theory of unified forces the Theory of Everything. It will indeed give us the most complete description of the universe. This will undoubtedly open up more avenues to explore and surely raise more questions about what we already know. The understanding of the fundamental forces is itself a powerful intellectual endeavour. It will reinforce our notion that the universe is a truly amazing thing; not that we needed this confirmation in the first place. But if anyone doubted the power and beauty of physics, understanding forces will give them a glimpse into our most elegant universe.