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The ultimate building blocks (no, not lego)
I was looking through the comments on the entry I originally made about writing about physics to see what people were wanting me to write about, and one of the requests was particles. I like this request, because the thing that I would like to say about particles is fairly easy and straightforward to write about.

Particle physics is about trying to find the deep, underlying constituent material that makes up the universe. I remember when I was young, maybe about 7 or 8 or so, having a conversation with my brother, which went something like this:

HIM: So inside of us, we have all these different organs that make up our body
ME: And what are they made out of?
HIM: They're made out of tissues
ME: And them?
HIM: Cells
ME: And what makes up the cells?
HIM: Molecules. And the molecules are made of atoms.
ME: And the atoms?
HIM: Are made up of protons and neutrons
ME: And them?
HIM: They're made up of quarks
ME: And them?

At this point, my brother was somewhat stuck for an answer, seeing as I'd reached the limit of human knowledge, so he made up the answer that quarks are made up of smaller quarks, and if you keep splitting them up then all you get is a gooey mess of ever smaller quarks. This explanation lacked the virtue of truthfulness, but did have the greater virtue of getting me to stop asking annoying questions.

A particle physicist is like a petulant child, always wanting to go one level deeper and know just what the most basic building blocks of the universe really are.

I think that the best way to approach the subject is to look at the history of this search. This history reminds me of nothing so much as the Bahá'í teaching of progressive revelation (which I won't even try to explain since I'd undoubtedly get it wrong, but the links are there if you're curious). It's a history of being told "All this stuff that you thought you knew? Well, it's not exactly wrong, but it's incomplete. Here's a totally different way of looking at things."

Putting aside things like the classical Greek division of all matter into fire, earth, air and water, the first real step along this path came with theory of elements -- that there are certain chemicals that cannot be broken down further. For instance, water can be split up into hydrogen and oxygen, but hydrogen and oxygen cannot be broken down further by chemical means. This was then supplemented by the atomic theory (from the Greek a tomos, not divisible) which said that matter wasn't just a smooth, continuous sea of stuff, but that it was made up of really tiny little building blocks. Analogy time: if you look at sand then you might notice that you can do things like pour it, or mould it into shapes, and you might conclude that there's just a continuous mass of sand of sand, with no constituent building blocks. You don't need to look very hard, however, to observe that sand is actually made up of lots and lots of tiny individual grain, which are small enough to make the sand appear to be continuous for most circumstances. Now imagine you're looking at water. You get the same things. You can pour it. You can make it take shapes. There doesn't seem to be any sort of internal structure. There's just water. Turns out though that there is a building block that makes up the water, but it's only about a billionth of a metre across, so we can never actually see it.

By 1869, about 60 or 70 different elements were known (we're now up to about 110), and it was then that Dmitri Mendeleyev first announced his periodic table. This took the known elements, arranged them in order of atomic mass, and noted that the chemical properties seemed to repeat periodically. For instance, if you consider lithium, then eight elements further on you have sodium, which has similar chemical properties. Then if you go another eight elements forward you get to potassium which has similar properties again (and no, the whole thing isn't nearly that simple, but this gets the point across). And the best thing about it was that it enabled him to predict the properties of some elements that hadn't been discovered at the time.

With hindsight, the clues seem to be mounting that atoms weren't the fundamental building blocks after all. If you have a group of 70 or so things which follow a regular pattern, it's not too big a stretch of the imagination to presume that there might be some sort of internal structure.

And this is how things turned out. As every high school chemistry student knows, atoms are made up of protons, neutrons and electrons. The electron was discovered in 1897, the proton in 1918, and the neutron in 1932. The Bohr model of the atom (whereby atoms look a little like the solar system, with electrons orbiting the nucleus) was proposed in 1913 (though clearly at that time it wasn't known that the nucleus was made of protons and neutrons). Around about this time, research was also been done into radioactivity, fission, fusion and such topics. Not only were atoms not indivisible, but elements could change into other elements.

The old theories were being washed away, but the new ones seemed so promising. They had it all. They knew the fundamental building blocks of all matter. Protons, neutrons, and electrons, and that was it. Well, and there were photons (particles of light) as well. And you had to have neutrinos as well to make sure that momentum was conserved in beta decay. And there'd have to be some particles exchanged between the protons and neutrons to make sure they stayed together in the nucleus (we now know these to be pions (though my spellchecker is insisting that they’re actually pianos, which amuses me)). But that was definitely it. Nothing else to know.

And then in 1936 someone had to go and discover the muon. Oops. "Who ordered that?" was Isidor Rabi's famous response. And then more and more different particles kept being detected. There was the kaon, and the pion, and the eta and the lambda and -- my personal favourite of all hadrons -- the rho. Now, all these newly discovered particles were rare and only existed due to either cosmic rays or particle accelerators, so chemistry, for instance, was quite happy just sticking with protons, neutrons and electrons. Particle physicists were always wanting to dig deeper and find out what was at the bottom of everything.

More and more elementary particles started coming in, and physicists being physicists, studied their properties, observed similarities and came up with classifications and theories as to how it all fit together. And patterns and trends started to emerge. And in 1964, a new theory was proposed independently by Gell-Mann and Zweig which could explain almost all of the known, existing particles (the photon was an exception, as was the electron, the neutrino, and ironically enough, the muon) as being made up of only three different constituent particles, known as quarks. These three quarks are known as the up quark, the down quark, and the strange quark.

Great, just three quarks to worry about. Much better than the vast quantities of chemical elements and subatomic particles that we'd had before. Of course, since then the number of quarks has crept up to 6. In order we've added charm, bottom and top (the latter two names regrettably emerging on top of the other proposed names of beauty and truth). We now have the "standard model" which proposes that everything that there is, all matter, and all forces, is made from the following particles:

Fermions (matter particles)

First generationdown (d)
up (u)
electron (e)
electron neutrino (νe)
Second generationstrange (s)
charm (c)
muon (μ)
muon neutrino (νμ)
Third generationbottom (b)
top (t)
tau (τ)
tau neutrino (ντ)

(the particles in each generation tend to have similar properties to the corresponding particles form the other generations)

Gauge Bosons (force carrying particles)

Stronggluons (g)
ElectromagneticPhoton (γ)
Weak W+, W-, Z0

And then there's the Higgs boson, which is a curious little thing which makes mass exist. And apparently, that's it. All the fermions also have an associated anti-particle, all the quarks come in three different "colours" (not that they're actually coloured, mind, that's just the name), and there are 8 different types of gluon, but that's meant to be it.

The standard model is incredibly successful. It can be used to make predictions which match experimental results with great accuracy. But it has its own problems. The Higgs boson and the graviton have never been observed. It doesn't really explain how gravity works properly anyway. Nobody knows why there are three generations of fermions. The values for things like the particle masses aren't predicted by the theory, but are determined empirically from experiment. The original model had neutrinos with 0 mass, which has since been demonstrated to be impossible. The standard model predicts that matter and antimatter should be equally abundant. The list goes on.

The general consensus amongst particle physicists is that we haven't yet found the "ultimate truth" as to what lies at the root of all matter, but nobody really knows what comes next. There have been and are various possible candidates, but they all have problems of their own. Many of them give predictions which are contradicted by reality, which is always a bad sign.

There's also superstring theory which is widely publicised as The Next Big Thing but which I have to confess that I don't like. String theory, essentially, moves away from the idea of particles altogether and posits that everything can be explained in terms of vibrations of strings and loops. There are two big problems with this:

1. It doesn't make any predictions. this puts it in the same "not science" category as the flying spaghetti monster or Aristotelian doctrine.

2. It's horrendously complicated. Now there's nothing wrong with complexity, but when it doesn't seem to add anything beyond what we already have, it would seem to fall afoul of Occam's Razor.

Now, this isn't to say I don't think it's worth studying, since it might end up turning into something useful, but as of yet, it hasn't reached that stage.

The other concern, at this stage, is the concept of reality. We're well into the realm of the very small and things that aren't directly observable. Do things we can't observe actually exist? Quarks do not exist in isolation, only in groups. Do individual quarks actually exist? I've written about this before when I talked about the nature of reality (friends only, I'm afraid) so I won't go into it further here. Suffice is to say that the deeper we dig, the less firmly anchored in reality we will become (whoops, mixed metaphor).

Personally, I happen to believe that there is no ultimate truth at the bottom of everything and that we'll never be able to say "OK, that's physics finished up with. Let's pack up and go home." As far as I'm concerned, it's turtles all the way down.

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I knew a good deal of this (at some point, some of it I continued to know, so this was a nice refresher for the bits I'd stopped thinking about) and I really liked the way you described it and stuff.

String theory is interesting and I do have the advantage of being taught by some of the (then) leading lights in string theory research.

String theory isn't a single theory as such, it is a general name for a class of theories that take as their base premise the existence of multidimensional strings. Some formulations of the theory hint at the unphysical (empty universe, universe with the density of lead, etc). Other formulations look tantalisingly close to the universe as we observe it.

The string theories are not 4 dimensional creations, they are ugly and obtuse in less than their natural (10, 11 or 26) dimensions. There is some handwaving to explain where these other dimensions are, and there are some testable hints (predictions is probably too loaded a word) at quantum gravity which may explain where the dimensions are. Some formulations of the theory putting detectable quantum gravity and extra dimensional effects within the energy reachable by the LHC at CERN.

Of course, if these effects are not seen then that is not the end of QG or string theory, it is just the end for that particular set of formulations.

The most promising formulation so far is M-theory, which seems to contain the standard model and super-symmetry, one significant failing it does have is that it makes no predictions for the masses of particles or any other of the variables in the SM and SS.

I tend to believe that there is a fundamental layer to the universe, quite probably at the scale covered by string theories, however I'm not convinced that we'll ever be able to fully understand and explain it. (This is actually a rather poor explanation of what I think on this, I should probably develop it further for a post of my own)

I sort-of knew all of that about string theory, but I'm not nearly familiar enough with it to want to try to explain it to other people, so thanks for doing so. I'm not trying to say that string theories aren't worthy of research time, effort and money, or even that I think that they aren't right. There's just nothing (that I've seen) that makes me think that its time has truly come, if that makes sense.

And I will confess that I am probably prejudiced against it due to knowing someone who is completely obsessed with it, and will keep bringing it up at the most inappropriate times.

I sort-of knew all of that about string theory, but I'm not nearly familiar enough with it to want to try to explain it to other people, so thanks for doing so.

I guessed you probably knew more about it than you had written, most of my understanding of it comes from a short, 5-week course I took as an undergrad and stuff I've read since then.

I'm not trying to say that string theories aren't worthy of research time, effort and money, or even that I think that they aren't right. There's just nothing (that I've seen) that makes me think that its time has truly come, if that makes sense.

I know what you mean here, my opinion of string theory as it currently stands is that it is a bunch of really neat mathematics looking for a universe to describe. It is something to keep an eye on, but I'd not let it get in the way of other avenues of research.

I really hope something 'interesting' is seen at LHC, though I think any results from the upper energy range of LHC will give theorists something to chew on for a while.

Even if we only get things we're expecting, then at the very least it's either going to uncover the Higgs or not uncover the Higgs, either of which would be pretty big. And I'd be highly surprised if there wasn't at least something interesting and unexpected to come out of it.

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