The fundamental Building blocks of the Universe
What's the smallest possible thing you can think of? A tiny bacterium? A nanoparticle? An individual atom? The nucleus of an atom? You get the idea: where does it stop?
Well, we actually have a model, referred to as the Standard Model of Particle Physics, which lists the fundamental building blocks of our known universe. Here we say a particle is fundamental if, as far as we know, it can't be broken down into smaller constituent pieces. An electron, for example, is considered to be fundamental, but a proton isn't because it's made up of smaller pieces called quarks & gluons (more on that later).
So the fundamental particles we're about to list can interact and be combined to make pretty much most things you can think of: you, me, everything you can see or have ever seen on earth or out in outer space for that matter, stars, other galaxies, even light itself! You might have heard about so-called dark matter and dark energy – these are different, but let's ignore them for now.
Absolutely everything else can be made of these particles. First there are the bosons:
Some of these particles (which, again, we've called bosons) are the so-called force carriers for three of the fundamental forces we know of in the universe: the electromagnetic force, and the strong and weak nuclear forces. For an example of what I mean by a force carrier, remember how 'like charges repel, opposite charges attract'? You probably learned about that when you were discovering static electricity. Well, broadly speaking, two charged electrons, for example, know that they repel one another by exchanging virtual photons (photons are the yellow guys in the table above). So the photons 'carry' or 'transmit' the information about the force, in this case the electromagnetic force. It's a somewhat similar idea for the strong and weak nuclear forces (and if you've never heard of those, don't worry about it).
You might have also heard of a particle called the graviton, related to (as you might guess) the force of gravity. The graviton isn't in the table though. Why? Well even though we do understand gravity very well, it hasn't yet been incorporated into the framework of the Standard Model to make a more general, unified, theory of how the universe works. People are working on it but we're not there yet. Again, this doesn't mean we don't understand gravity very well – we do! – it just means we treat the force of gravity differently from the other forces.
Getting back to the particles, there are also the fermions or matter particles:
So there you have it, we've listed them all in two pretty tables: all of the known fundamental building blocks of the universe.
Well, the Standard Model does far more than simply enumerate the basic building blocks of nature. It ties these particles together, through its equations, into a kind of cosmic fabric. It governs how the particles interact with one another. It lays the theoretical foundations on which calculations can be made, so that physicists can make predictions and then test those predictions through experiments.
We actually know a ton about all of these particles. There's a site here where you can see a vast amount of information about these particles and summary tables listing some of the experimental measurements that have been made about their various properties. Some of these properties (such as spin, mass and charge) you can see are even listed in my colourful tables above.
You might be surprised to hear that few of these particles were likely discovered during your lifetime – the top quark in 1995, the tau neutrino in 2000, the Higgs boson as recently as 2012!
Another point: the fundamental particles have been depicted above as colourful circles, but try to keep in mind that we truly think of them as point particles – not spheres, not little marble-like balls, just infinitely dense points in space with no volume but with a finite mass. It's recognizably a bit of a strange concept, but an important one.
According to the Standard Model the particles are inextricably linked to one another. The image below is an attempt to depict the various interactions between particles – how certain types of particles 'speak' to other types through the known fundamental forces of nature. You might notice that some lines seem to be missing, so not all particles seem to interact with one another. They're not missing of course – the choice was deliberate as not all particles are 'allowed' (as dictated by the universe) to interact with one another.
Overall the Standard Model represents an amazingly successful theory. We are continually searching for something the model gets just plain wrong but so far have been unable to. That said, we know it to represent an incomplete theory – the Standard Model simply doesn't explain all of the things we observe.
Eventually we'd like to have a theory that works for everything – a Theory of Everything – so for all four forces and in any given regime (for objects that are big or small, or that are extremely fast- or slow-moving) but for now we don't. Again, it's being worked on.
If this is confusing, here's another example to illustrate what I mean: Newton's laws which govern planetary motion don't disagree with Einstein's special and general relativity, provided you're dealing with low speeds and you're far away from large gravitational bodies. In other words, Newton's laws are an approximation. Einstein's theory is also some sort of approximation but we know that it's closer to the full picture than Newton's theory. So Newton's theory isn't incorrect, it's just incomplete. If you were to, say, use Einstein's equations for relativity but make certain approximations (such as you're far enough away from the sun or a black hole, and you're travelling at a low enough speed), then the answer you'd get would be the same as what Newton's equations would give you!
Same idea with the Standard Model: it's not wrong per se, but it can't be the whole picture.
Let's go back and focus on the particles listed in the tables. Some will probably be familiar to you...
...while others may not.
Matter & Anti-matter
So there are also some hidden particles in the table, in the sense that for each of the fermions (the 2nd table) there's an associated anti-particle. They're implied but not explicitly shown. We just don't write them in order to save space. Now don't be fooled – they're called anti-particles, but they're definitely still particles!
Typically we put a solid line above a particle's symbol to indicate that we're talking about an anti-particle. The electron's different, but its anti-particle could also be written with a bar. Anyway, here are a few examples so you can get the idea:
Get a particle and its anti-particle too close together and bam!, they annihilate in a burst of energy. It's one of the aspects of particle physics that's captured the imaginations of science fiction writers for years (that particle-antiparticle annihilation energy is what makes interstellar travel possible in Star Trek!), but crucially it's real science, not science fiction. It's also the primary reason it's so hard to contain anti-matter when we produce it! We can't simply store anti-electrons (or positrons as they're often referred to) in a regular container, because there are regular electrons everywhere in the walls of that container. Before you could know what was happening all of the precious positrons you worked so hard to create and collect would be gone, and in their place there would be a ton of extra energy released, mostly in the form of high-energy photons (which are actually tiny little 'packets' or 'quanta' of light).
So got it? Particles and anti-particles. Matter and anti-matter.
You can then start combining some of the particles in order to make other composite particles:
They're called composite to highlight that they're composed of smaller pieces.
Ok so don't worry if you've never heard of the pion. The point here is that there are a ton of these types of objects – composite particles made up of quarks. We actually call these composite particles hadrons, but that's just a name. Even the proton, if we were to shake it up and 'excite' the quarks inside it to higher energy levels, isn't even a proton anymore (by which I mean we don't call it a proton). It even has a different mass. Because of this and all of the ways you could combine the quarks, we end up with a veritable particle zoo.
But don't worry: the list of fundamental particles (which you saw above) remains reasonably short.
The top quark is the only quark that doesn't form the composite particles or hadrons – that's just one of the things that makes it special (but I'm just saying that to get you interested in my thesis topic since my research focused on the top quark).
That same site I put a link to earlier also has huge tables listing some of the measured properties of all of these composite particles or hadrons (only the ones that we've observed so far of course). You can put 3 quarks or anti-quarks together to make what's called a baryon (such as the proton), or you can put a quark and an anti-quark together to make a meson (such as the pion). So all baryons and mesons are collectively referred to as hadrons.
Again, these are nothing more than names to differentiate the groups, but you can see the tables for both mesons and baryons at the same link here (just so you can get a sense of how many there are).
Last thing about this: in 2015 (very recent!) a potential discovery of a 5-quark state (called a pentaquark) was even made by another experiment at CERN (see the paper here if you're feeling particularly intrepid), so things get stranger still! And yet another collaboration using data from the Tevatron accelerator near Chicago claims to have observed a tetraquark (four-quark state). Again, here you can find the paper if you're curious.
There are anti-matter analogues to many of the composite particles too, so whereas the proton is made of two up-quarks and one down-quark (more or less...there's some other stuff in there which we'll learn about later), the anti-proton is made of two anti-up quarks and one anti-down quark.
Let's go one step further and start to build up the periodic table. A hydrogen atom for example is just a single proton with an electron 'cloud' around it. Why do I say cloud? Ok, so you might be used to seeing a single electron orbiting around the proton, but that gives the illusion it acts like a star-planet system and that's not really a great analogy... Better to think of it as one electron (that part's still fine) zooming around the proton with a certain probability of being in a particular range, but it's not orbiting the way a planet would around a star.
The point: one proton together with one electron makes a hydrogen atom. Now we move on to heavier elements. The nuclei of the heavier atoms (such as heilum shown below) are formed from both protons and neutrons. But it's the number of protons that tell you what element it is; take away one neutron from the helium atom and it's still considered helium. Of course a neutral helium atom like the one shown below has four electrons in its electron cloud, not just a single electron. The clouds also have structure, but we won't get into that – leave it for the chemists.
Next, atoms can combine to form molecules, and so on and so forth, until you get to macroscopic objects like the ones you see around you. Here's a picture:
The Abundance of lighter particles
There's a reason you're more familiar with the electron as compared to, say, the muon or the tau particle (even though they otherwise look pretty similar in the table way earlier): in nature things generally go from higher potential energy to lower energy:
Objects up in the sky (high potential energy) inevitably fall to the ground (lower potential energy).
Two positively charged particles placed close to one another (high potential energy) eventually move as far as possible away from one another (lower potential energy).
In effect this same basic principle makes heavier particles decay to lighter particles, whenever the laws of physics allow the decay to happen. Heavier particles are therefore often said to be unstable. So we've got more of the stable stuff around because the heavier, less-stable stuff has already decayed!
Consider a W boson – it is one of the fundamental particles I just introduced you to, but it's a very heavy particle (it's about 160,000 times heavier than an electron!). It can decay into several different things (sometimes it decays one way, sometimes it decays another – it's a 'roll of the dice' sort of thing), but rest assured: if you produce one it will decay and very quickly. When the dust settles you'll be left with only the far lighter particles wondering what happened.
Here's an example (again, this is just one possible 'decay mode' as we call it):
And here's another example showing the ways in which the heavier quarks (such as the top and bottom) decay to the lighter quarks. Don't worry about the details: the key point is the concept of the decay 'ladder' – particles decay their way down the ladder (but with only certain paths allowed and some more probable than others!) until they can't decay anymore.
We'll talk about the funny diagrams on the right in Part III, but pictorially it's probably at least somewhat straightforward what's going on. For now just know that when a quark decays from a heavier type to a lighter type, it does so by 'emitting' a charged W boson (this is part of the interesting behaviour of the so-called weak nuclear force).
So we have a huge number of up and down quarks (which make up protons and neutrons) in our present day universe, and essentially no top or bottom quarks just sitting around, aside from when they pop in and out of existence for exceedingly short periods of time. More on that later.
In the earlier universe though, when things were much hotter, some of the heavier particles would have been far more prevalent.
But in the (comparatively) cool universe we live in today, we're left mostly with the lighter, stable stuff: electrons, up & down quarks (which can give us any of the elements we need), neutrinos, and light (photons). In fact just those few pieces alone are all we really need to make most stuff you're familiar with.
Now, if we add energy to the mix, that's a different story: it's more likely we can make the same process go the other way around, i.e. we can start with lighter particles and produce the heavier, unstable particles. Of course, they end up decaying on us anyway and we're back where we started.
Ridiculous: why do it at all if we're back where we started, right? Well, for a brief instant we really were able to produce the heavier, more interesting particles, and crucially we are not back exactly where we started! Often the reverse reaction gives us different particles, and even if it doesn't, there is clear evidence that the heavier particle existed! For a flicker of an instant the heavier particle was there, and the fact that we produced it gives rise to differences in what we observe. And those differences give us insight into the inner workings of the universe!
One way we probe nature then (that is to say, one way we do it as collider physicists) is by producing the heavier, rarer particles in the tables above. Adding energy to the lighter particles (in the form of the energy of motion) is the key ingredient we need in order to make the production of heavier particles possible. And that energy comes from enormous particle accelerators.
At the Large Hadron Collider (LHC), which you'll hear about in the next section, we collide oppositely directed beams of protons travelling at just under the speed of light, and this allows for the production of heavier particles – yes, the ones in the table (and this teaches us more about them) but hopefully also as-of-yet-undiscovered particles which could bring us entirely new insight into the peculiar ways that nature behaves at the smallest scale.
So there we have it! We’ve introduced the key players. Next up we’ll talk about particle accelerators – the LHC in particular – and what we need them for.