Crying your pardon
Firstly, an apology. I’ve been completely inactive on my website for the past month. This is partly because I was preparing for the Institute of Physics annual public Science lecture (which I delivered on November 22nd to a gracious and patient audience).
Last year I was able to transcribe and summarise the lecture in a couple of blogs but this year I’m afraid that wouldn’t be possible. The main topic covered was the Standard Model of Particle Physics and that's not easy to describe in an essay. Personally, I foam at the mouth with excitement when the whole topic of particle behaviour is discussed, but apparently some plebs don’t share my excitement. As a result, there was a lot of stuff I had to edit out of my lecture...stuff I’m now going to subject you to.
Full disclosure: this will be a self-indulgent blog that will bore many of my readers. I’m doing it anyway, because it’s my website and I freaking love this stuff. How dost thou like them apples?
Seventeen is a Magic Number...Apparently
I once recorded a barbershop-quartet song I wrote about the standard model of particle physics (cos I’m just that awesome) but if you’ve not come across it before, the standard model is usually depicted like the grid above or sometimes in a wheel like this:
These are the seventeen fundamental particles which can’t be broken down into anything smaller. As strange as it sounds, you can’t chop these particles in half because there literally is no half for them to be chopped into.
What’s more, we’re fairly confident this really is the bottom rung of the ladder. We have lots of reasons to suspect that these particles are the true building blocks of the Universe, which means every object or process you can think of is the result of interactions between the particles listed above. With the exception of gravity (which doesn’t play nicely) what you see is the alphabet of reality. Well...almost.
In truth, the seventeen particles of the standard model are not the whole story. Nature is rarely so considerate or simple. In fact, she seems to have a complete disregard for what humans will find intuitive and tends to prefer intricate complexity wherever possible. It’s almost like our brains evolved for the purposes of hunting and breeding rather than conceptualising the quantum-mechanical nature of reality.
Divide and Describe
Asking how many types of particle there are is like asking how many types of human there are. If you speak to a contact-lens designer they might say there are five: brown-eyed, blue-eyed, grey-eyed, green-eyed and hazel-eyed humans. That’s not wrong of course but it would be useless information to a hemotologist. They might say there are eight types of human based on the blood groups A+, A-, B+, B-, AB+, AB-, O+ and O-.
To give a full picture that includes both properties, we might therefore say there are really forty types of human: blue-eyed people for each of the eight blood groups, brown-eyed people for each of the eight blood groups and so on. But we could always subdivide again based on something like hair colour for instance – blonde, brunette and ginger – to yield 120 types of human.
We could categorise and cross-categorise the human population according to gender, sex, sexuality, skin-colour, language, dietary habits or whatever else we felt like. The sheer number of possible “human particles” is staggering because there are so many different properties available. A similar complication arises when we want to describe particles of the Universe which is a much better way to spend the time. After all, putting humans into categories is sort of frowned upon these days.
Let’s Just Say There Are Four
For the sake of clarity I’m going to say there are four properties/available characteristics a particle can have. This isn’t really true, but a lot of particle properties aren't independent of each other so we count them as one.
For example, a tiger has the property of orange stripes and also the property of black stripes. Those are two distinct properties but logically they must occur together. We can group both properties into one and say that tigers have the property of being "stripey".
In the same way, you might have heard of many properties particles can have, but I'm going to ignore a lot of them because they make no difference. If you don’t like it, see above comment about apples. Here are the four properties...
Mass: This property means roughly the same thing as it does in our everyday life: it's a measure of how heavy a particle is or how reluctant it is to change trajectory. For particles it can take any value from zero to 0.02 milligrams (anything above that is impossible).
Charge: This property means how willing a particle is to be around other particles with the same property. It comes in two varieties called positive and negative. Particles with opposite charges will attract, while particles with identical charges will repel. Particles that have zero charge are unaffected. This one’s also easy to visualise because it’s similar to our notion of magnetism with like poles repelling and opposite poles attracting.
Colour: This one is a little harder to visualise because it doesn’t compare to anything in our everyday world. The name is also misleading because it doesn’t refer to the appearance of a particle (it’s just a word we use to describe it) it actually refers to whether a particle can be separated from particles with corresponding properties. Particles with zero "colour" are able to move around on their own but particles with "colour" must clump together in specific arrangements. The interactions and types of "colour" available are quite complicated so I won’t go into it here, but my recent Instagram post (@timjamesScience) explains the basics if you're curious.
Helicity: This property is by far the strangest. Whole books have been written about it, so if the explanation I give here seems a little incomplete, that’s because it is.
Particles have a property called "spin"; a name as misleading as "colour". It doesn’t refer to whether a particle is literally rotating in space, but the mathematics of rotating objects happen to match the behaviour of this property, so we use the same vocabulary.
Spin values can either be whole numbers or half-numbers. Particles with whole-number spin are able to occupy the same physical location as each other without interacting (think of two beams of light overlapping) and we call these particles bosons. Those with half-number spin will stack against each other (like your body and the chair you are sitting on) and we call these particles fermions.
As well as coming in different numerical values, spin can also come in two varieties which we usually call up-spin and down-spin, but I’m going to break with convention and use the words clockwise and anti-clockwise to describe the two types. My reason is that "spin", just like literally spinning objects, can appear different depending on your frame of reference.
A particle moving toward you can be spinning clockwise, but when it passes and you start watching it move away the same particle can suddenly appear to be spinning anti-clockwise. "Spin" behaves in a similar way. The spin of a particle can be measured differently depending on what is around it and how you look at it.
This makes spin a confusing thing to talk about, but there is another aspect to spin - a complication which actually ends up making things easier to manage. The spin of a particle can "point" in a specific direction as if particles were like spinning tops with pointy ends. If the pointy end is facing the direction in which the particle is moving then we call it “right-handed” but if it points in the opposite direction it is “left-handed”. It is also possible for the spin to be pointing at right-angles to the direction of travel, giving us the possibility of “neither-handedness”.
This property of being left, right or neither-handed is called helicity. So although it is really to do with the particle’s spin, the handedness of particles is what allows us to distinguish them. Clockwise and anti-clockwise particles are practically the same, but left and right-handed particles are not.
From these four properties we can describe all the different types of particle and the varieties which arise among them. On the plus side, there aren't many different properties to worry about. The problem of course is that it's hard to visualise what some of these properties actually are. At least when we were talking about human particles we could always get our heads around them. Human characteristics and behaviour are never beyond explanation.
Photons The simplest particles. They have no mass, no charge, no colour and only come in two helicities - left and right handed - each with a spin of 1.
Z’s – Z particles have mass, but no colour or charge. They can have any of the three helicities (left, right or neither) all with a spin of 1, giving us three possible types.
W’s – W’s are similar to Z’s. They have mass, spin of 1 and three helicities, but they also come in two different charge varieties, +1 or -1, meaning there are six W particles.
Gluons – Gluons have no mass or charge but they do have colour in eight versions (see my Instagram post) and helicity coming in left or right, giving us a total of sixteen gluons.
Higgs – The Higgs boson particle has mass but no colour or charge. It has a spin of 0, meaning there is only one helicity possible (neither-handedness) and therefore only one type of Higgs exists.
Fermions – spin 1/2
Quarks – These particles have all four properties. There are six different quark masses, each with a different name: up, down, charm, strange, top and bottom. Up, charm and top quarks have a charge of +2/3 whereas down, strange and bottom quarks have a charge of -1/3.
Quarks also possess one of three colours named red, green and blue, giving 18 types so far. Quarks can also come in charge/colour-reversed versions called anti-quarks. Anti-up, anti-charm and anti-top quarks have charges of -2/3 while anti-down, anti-strange and anti-bottom have charges of +1/3 (the reverse of the “ordinary” quarks). Anti-quarks also possess the colours: anti-red, anti-green and anti-blue. This gives us 36 types of quark.
Then, because quarks have spin, they come in either left or right-handed helicity, giving us a grand total of 72 possible quarks.
Leptons – These fermions possess no colour. There are three with mass, called the electron, the muon and the tau. They also have a charge of either -1 or +1 (the anti-versions). Then there are left and right handed helicities, giving 12 massive leptons.
The remaining leptons have no charge and possibly no mass (they sort of do have mass, but for now let’s say they don’t) and they are called neutrinos. Neutrinos come in three flavours: electron-neutrinos, muon-neutrinos and tau-neutrinos. There are also anti-neutrinos for each, but because neutrinos have a charge of zero you can almost think of anti-neutrinos as having a charge of anti-zero. I mean, you probably shouldn't think of it like that...I'm just saying you could.
What’s really strange (apart from the fact that they sort of have mass and sort of have anti-zero charge) is that all neutrinos are right-handed and all anti-neutrinos are left-handed. This gives us six types of neutrino in total, making for 18 leptons.
The Grand Total
Photons x 2
Z's x 3
W's x 6
Gluons x 16
Higgs's x 1
Quarks x 72
Leptons x 18
118 different types of particle
Is That It?
In all honesty we don’t know. These are the 118 types of particle which definitely exist and which can’t be broken down but there is absolutely the possibility, in fact the likelihood, of there being more. For instance, left-handed neutrinos and right-handed anti-neutrinos have never been discovered but it seems reasonable to suggest they exist somewhere.
There are lots of other hypothesised particles which many physicists believe may be real but haven’t been discovered. Things like the graviton (the particle responsible for causing gravity), the inflaton (the particle which played a key role in the early expansion of the Universe) or the Majorana fermion (which does things).
Then there are the so called quasi-particles, some of which don’t exist independently but merge with already existent particles (like Goldstone bosons) some of which aren’t exactly self-contained units but act as if they are (like phonons) and some of which don’t exist at all but we act as if they do to make the equations neater (like Popov-ghosts). In short, the Universe is awfully big and awfully complicated. The standard model we have now is probably only a glimpse of what nature has in store.
Standard Model Grid: businessinsider
Jerry Smith: pinimg
Standard Model Wheel: Symmetrymagazine
Nick Griffin: guim
Insane Clown Posse: wennermedia
I love science, let me tell you why.