Fundamental Forces.


Two years before the outbreak of World War Two the Japanese introduced a new high-level diplomatic cypher that the US named Purple. Purple was a cutting-edge cryptosystem that proved fiendishly difficult to break, using machine-generated cyphertext with a similar level of complexity to the Enigma devices — but unlike Enigma the US were unable to capture any working Purple devices to give them clues as to the design of the system. All they had to go on was underlying patterns in the cyphertext and the cribs (or operational errors) that represented the few chinks in Purple’s armour. Nevertheless, by 1941 the SIS had constructed an analogous device that successfully decoded Purple messages based on just this information, in effect making a perfect working replica of the Purple device without ever having seen one themselves.

The reason I’m telling you about this little historical curio is because it’s a very good analogy for our current models of particle physics. We have directly or indirectly observed most of the particles predicted by the Standard Model now, much like the US cryptographers reading the Japanese cyphertext. However, using that particle bestiary to construct a model of how things work at subatomic scales is a different matter entirely. Historically all we’ve been able to do is observe the aftereffects of a particle interaction and try to infer the exact mechanism from that alone, and this is why reading up on our current models of particle physics starts to sound like a foray into the mind of the more tinfoil-hatted among us1. It’s also why the subsequent discovery of particles predicted by these models using high-energy particle accelerators is considered to be such a success story, since this validates the model in the same way that the analogue Purple machine producing word-perfect decrypts demonstrated that the US analysis and reproduction of the encyphering process was sound. It will be useful to keep this in mind during the next couple of posts; while the Standard Model is very counterintuitive in places it both makes accurate predictions about particle behaviour and has had the proposed mechanisms which explain that behaviour confirmed experimentally. It’s as solid a piece of work as you’ll ever find in science.


This particular post is going to be on what are called the fundamental forces: gravity, electromagnetism, strong nuclear and weak nuclear. I’ve mentioned them offhandedly during previous posts but have never gone into what they mean in detail, largely because I couldn’t remember if they actually taught this stuff in school or not. Then I realised that if I couldn’t remember learning it there it’s highly unlikely anyone else does, taught or not, and so it’s going to be useful to pin down exactly how each force works and what they affect.

All matter — even the weird stuff, like dark matter — is made up of a class of particles called fermions. Fermions are defined by their inability to occupy the same quantum space as another fermion; this gives matter its reassuring habit of forming rigid structures and not collapsing into an amorphous mess of quantum goo. A “force” (in the sense that we’re using the word here; it’s becoming trendier to refer to them as the fundamental interactions instead to avoid confusion) is the physical representation of one fermionic particle exerting influence on another. In order to exert influence there has to be some sort of transmission of information between the two fermions, and this is achieved via a “virtual”2 carrier particle of a different type called a boson. Unlike fermions, the defining feature of a boson is that it can occupy the same quantum state as another particle; this makes them incapable of forming solid structures like fermions but it also gives them the ability to permeate everything in the form of a quantum field3 — electromagnetic, gravitational, you name it. Exactly what field you get will depend on the type of boson involved, as three of the four fundamental forces has a specific boson associated that gives it its particular characteristics.


The four fundamental forces are:

Strong nuclear4: Transmitted by gluons. The strong nuclear force is the strongest of the fundamental forces by a couple of orders of magnitude — hence its name — but its other defining characteristic is its absolutely tiny range of influence, which only stretches out to about the thickness of an atomic nucleus. The strong nuclear force is responsible for binding quarks into protons and neutrons, and protons and neutrons into atomic nucleii. The strength of the strong nuclear force makes it very difficult to split atomic nucleii up, but its miniscule range ensures that your hands do not suddenly glue themselves inseparably to your computer keyboard.

Weak nuclear: Transmitted by W and Z bosons. About 100,000,000,000,000 times weaker than the strong nuclear force and with an even shorter range; this is because the W and Z bosons are particularly massive bosons which decay very quickly and can’t travel far, barely making it beyond a few particle lengths before fizzling out. Exactly what the weak nuclear force does is a little complex and requires some understanding of advanced particle physics5, but basically it makes it possible for neutrons to decay into protons and thus enables the whole radioactivity shindig.

Electromagnetism6: Where it starts to get interesting. You’re already familiar with the carrier particle for electromagnetism: it is the humble photon, billions of which are emitted from our Sun every second to light up the Solar System, but which are also exchanged in a virtual form between particles exerting an electromagnetic force on each other. Electromagnetism is only about a hundred times weaker than the strong nuclear force (vastly stronger than gravity) and appears to have an effectively infinite range; however things aren’t flying around the universe propelled by electromagnetism because it’s a force which both repels and attracts in equal measure, and when measured over a large system (i.e. human scale objects) things tend to have equal quantities of positive and negative charge. It’s only on the chemical/atomic scale that electromagnetism becomes dominant. For large systems, the dominant force is…

Gravity: By far the weakest of the four fundamental forces (the strong nuclear force is *counts on fingers* a hundred million billion billion billion times more powerful), it nevertheless dominates large systems because like electromagnetism gravity has an infinite range, but unlike electromagnetism gravity always attracts, which means it trumps all three of the other forces over large distances. Curiously there’s not currently any room in the Standard Model for a boson associated with gravity (one is proposed — the graviton — but it’s still wildy theoretical) making it the only force without a carrier particle, and the exact mechanism by which gravity exerts its influence on an atomic scale is still a mystery to us7.

So those are the forces and what they do. But why are they important? Well, you might recall a couple of weeks back I mentioned elementary particles like quarks being the basic building blocks of all other particles; they’re the smallest things we know of in the universe and if you manipulate them right you can use them to build a universe. Fundamental forces are similar to this in that these four forces — strong nuclear, weak nuclear, gravity and electromagnetism — are in theory responsible for everything that happens in the universe. Think of a particular phenomenon, no matter how exotic, and when you pare it down to its most basic physical causes you should identify one or more of these four forces as the culprit.

Thus it should be possible to explain everything in the universe using these forces. However, even though we’ve narrowed it down to just four forces that’s not enough for physicists. Much like string theorists are trying to “simplify” the Standard Model even further by positing that all particles are made up of tiny vibrating strings, it’s also possible that these four fundamental forces all spring from a common source, which is why one of the current scientific holy grails is unifying all four forces into a Theory of Everything that describes… well, everything. We’ve already managed to combine electricity and magnetism into electromagnetism, and then electromagnetism and the weak nuclear force into electroweak theory8, but reconciling the other forces — and especially gravity — is going to be another of these great scientific challenges for the next century I keep talking about. If we can do it, though, it’s going to lead to a revolution in scientific thinking that’ll probably exceed the eventual advances made through quantum theory. It’s a tantalising thought if nothing else.


  1. Like literally everything we can perceive — particles, atoms, stars, bricks, cats — being specific excitations of quantum fields.
  2. I don’t pretend to fully understand this, but: bosons are real particles that have been found in particle accelerators. However, the thing about particle accelerators is that they generate these bosons using extravagant quantities of energy not usually found outside of e.g nuclear reactors or  the core of a star. The observed particle form of the force carrier boson is therefore a special excitation of the already existing force field which is not usually found in nature because they exist for a very, very short period of time. These particles are referred to as “virtual” not because they are not real — they are, although this is a part of physics that begins to bleed into philosophy as it starts to call into question exactly what we mean by “real” — but because of their transient nature. This is why I related the story of the Purple cypher machine; it’s difficult to understand why the hell this approach of transitory particles should not only work but also predict the behaviour of actual real particles without a more grounded analogy to fix your point of reference regarding the power of analysis to predict outcome.
  3. Wave particle duality rears its ugly head here; we speak of bosons as particles but it’s easier to understand their influence as a field if we think of them as waves.
  4. These days described via the theory of quantum chromodynamics (QCD).
  5. It’s the one force which can violate symmetry by changing the flavour of particles, and neutrons cannot decay into protons unless one of their component quarks has its flavour changed from up to down.
  6. And this was rewritten into the more accurate and quantum-compatible quantum electrodynamics (QED) back in the 70s.
  7. This is one of the reasons why it’s such a pain in the ass to reconcile general relativity with quantum mechanics.
  8. Although only at very high energy levels. The goal of the Theory of Everything isn’t to unify the forces as they are now, but as they would have existed just after the creation of the universe during the Big Bang, since this (presumably) is the time at which they did operate as a single unified force, which then later split into separate forces as things cooled down and the universe stabilised.
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3 thoughts on “Fundamental Forces.

  1. Gap Gen says:

    Interesting analogy at the start. And it’s true that all problem-solving works in more or less the same way – you come up with ideas to explain why things aren’t working like they should, test them, and if it works then that’s probably it.

    As for gravity – I’ve never seen a paper (as far as I can remember) that explored the effects of a non-instantaneous gravitational force on large scale structure. I think everyone assumes that it basically makes no difference, but I’ve never seen (or bothered to make for myself) estimates for why this should be.

    • Hentzau says:

      I’ve always been slightly hazy on the meaning of the word “instantaneous” when applied to gravity. I always assumed it meant it acted at the speed of light just like everything else, and that this was down to the speed of light being fundamental to the structure of the spacetime on which general relativity is based. What that means on a quantum level, I have no idea. I guess we’ll have to wait for quantum gravity to tell us, and by the looks of things we could be waiting a very long time indeed.

      • Gap Gen says:

        Right, that’s what I meant – I assume gravity is limited to the speed of light like everything else, but I haven’t seen anything that explores this on intergalactic or cosmological scales. Someone smart has probably done it, though.

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