Monday, 16 January 2012

What Are The Subatomic Particles?

So in school you learn atoms are the smallest particles, but then you find out that this can be broken down into protons, neutrons and electrons. However even those can be broken down further, but to what?

We live in an age where development in particle physics is powering along. Facilities like the Large Hadron Collider in CERN, Switzerland, are, as I write, colliding Protons together and analyzing the particles that are produced. So what are the looking for? Well firstly lets get a sense of scale. In the full stop at the end of this sentence, there are 7.5 trillion atoms. 99.9999999999999% of atoms are just empty space, so the particles we are going to be looking at can be 10-20 meters small, so small in fact that we cannot even imagine being able to see them individually in a microscope for years. So now that we know that lets move onto the particles:
Source AAAS
This list may not mean very much to you at the moment, except if you have read our post on Neutrinos. However, by the end of this post, I hope you will know what every one of these particles are. So let me start by explaining the main properties that they can have.

When reading about these particles, you will come across the words spin and charge, so what do they mean? Well lets start with charge, also called electrical charge. Charge is a property that changes the way that the object will interact with other charged objects. They produce a force. There are two types of charge, positive and negative. When two particles with the same charge meet, they repel each other, when two particles with different charges, on the other hand, meet, they will attract each other. Charge can be measured in Coulombs but when looking at particles the coulomb is too large a unit and therefore we will use the unit e. A Proton has a positive charge of one e. Electricity is the movement of charged particles, specifically Electrons.

The other property is spin. Spin is a measure of angular momentum in a particle. However it is not angular momentum in the way that we think of it. The angular momentum we usually see is in the form of an object's rotation at a certain rate. At an atomic level it doesn't work in this way. When you run a current through a loop or wire, or take a charged object and spin it around, a magnetic field is formed, like you find around a bar magnet. Using the object's speed, charge and magnetic field you can work out the speed it is spinning at. The magnetic field of an electron, though, is much too larger than it should be, if it was spinning. When calculating the speed it must spin at it has to be larger than the speed of light, which is not possible. However, the fields are there and therefore it must have the angular momentum, this is what we call spin. More information on spin here and here. Now onto the particles:

There are two major sets of particles, Fermions and Bosons. Firstly I would like to talk about Fermions. Fermions are the main building blocks of matter. All atoms consist of them and there are only 12 (as well as antimatter), split into two categories, quarks and leptons. Every lepton has half integer spin, no matter which category they are in. So what is a quark? Quarks are the building blocks of Hadrons, a particle like a Proton or Neutron. There are 6 'flavors' of them, the up, charm and top quarks, or up-type quarks, and the down, strange and bottom quarks, or down-type quarks. Although it can be said that there are 12 quarks, as each of the previously mentioned ones have their own antiparticle. These quarks stick together due to the strong force to form the particles, or hadrons.  Hadrons can only be formed by triplets of quarks, baryons, or a quark and anti-quark, mesons. Only the up and down quarks are stable. Protons are made from two up quarks and a down quark, whilst neutrons are two down quarks and an up quark. So how does a Proton have a charge but a neutron does not? Well the quarks have weird charges, all of the down type quarks have a charge of -12e and up type quarks have a charge of +23e. Another strange ability of  quarks are that, in certain conditions, they can change flavors. So, sometimes, a Neutron emit a W boson and become a proton, electron and electron anti-neutrino. Read more about quarks here.

There are six Leptons too. Three electron-type Leptons, the electron, muon and tau, and three neutrinos, the electron, muon and tau Neutrinos. The only difference between electrons, muons and taus are their size, the electron being the smallest and the tau being the largest. These electron-type particles are what gives almost every substance in our universe it's physical properties. All chemical reactions are due to the electrons in each of the substance's atoms and how they are arranged. Most of the importance of these electron-type particles is because they have a charge of one -e, the opposite of a proton. In fact, an electrical current is just the flow of electrons around a loop of wire; a lightning bolt a lot of electrons traveling from the clouds to earth and a static shock just electrons traveling to or from your finger. I may do a post on electricity soon. Neutrinos I have already done a post about so I will let you look there for more information, but essentially they are very similar to the Electron-type particles, but without charge, and they can pass through matter. However there have been test showing that Neutrinos can go faster than the speed of light, something that shouldn't be possible due to Einstein's theory of relativity. I talk about this in the previously mentioned post about Neutrinos.

and if you are more interested in quarks and leptons, here is another video:

The other set of particles are called Bosons. These Bosons are what are known as force carriers. You see, in the universe their are four fundamental forces in physics, these being the electromagnetic force, the weak force, the strong force and the gravitational force, the Higgs boson does not relate to any of these, but more on that later. Electromagnetism is the previously mentioned force that acts between charged particle. Leading to effects such as friction, electrical current and even rainbows. It is a very strong force. Photons are the carriers of the electromagnetic forces. You may recognize it as the particle of light, this is because light is just how we see electromagnetic radiation, or just how we see photons.  They don't have a mass and can travel infinite distances, this is why we can see stars that are light years away. This is also the reason why light can travel to infinite distance and we can see stars that are far away.
The strong force is the force that holds quarks together to form Hadrons, as up-type quarks should repel other up-type quarks and down-type quarks should repel other down-type quarks due to their similar charges. However the strong force carrying particles, Gluons, act as a glue, hence the name, and stick the quarks together. They have no mass and no charge, but act with charge-like properties in order to stick the quarks together.
The weak force causes the decay of particles. This is through the emitting and absorption of the weak force carrying particles, the Z and W bosons. Referring to the example I used about quarks changing flavor, Neutrons can become Protons but only if one of their down quarks becomes an up quark through emitting a W boson. This W boson then becomes a Anti-electron neutrino and an Electron.  The Z and W Bosons are the only Bosons that have mass (other than the Higgs) and that means that the the range of the weak force is very small.
The final force is gravity. I have talked about gravity and its effects on time before, but never gravity itself. Gravity was described by Einstein's theory of relativity, and he discovered that it was twisted spacetime. However there is no way to describe it in the standard model. A theoretical particle called the Graviton was theorized in the 1930s and tries to describe gravity in terms of quantum mechanics, however they would be very difficult to spot Gravitons. Wikipedia gives a good example, saying that "A detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions." This shows just how rare they are. Gravity is very weak, the electromagnetic forces that repel two jugs of 1 gallon of water put 1 meter from each other are stronger than the gravitational force of a planet the same mass as ours would way on our planet. However the repelling and attracting forces cancel out in the case of the water jugs so they do not move. If you move beyond the standard model, there are more theories as to how gravity could fit into quantum theory but that isn't relevant at the moment.

So those are the four fundamental forces, and the four force carrying Bosons. But what about the most famous boson that has caused such a  fuss in the news at the moment? I am, of course talking about the Higgs boson. It is the last predicted particle in the standard model that had not yet been prove to exist. Before we start talking about the Higgs, first we have to talk about virtual particles. These particles are mainly bosons that flash in and out of existence by temporally borrowing energy from other particles. When these particles flash into existence they form a sort of field, the gluons that stick quarks to each other in hadrons are virtual particles. So these Higgs bosons form the Higgs field, the way particles interact with this field governs their mass. A top quark, the most massive particle, interacts a lot with the Higgs field, whilst photons, which have no mass, pass straight through it without interacting. When I say massive, I do not mean they are larger in size, in fact all these particles don't really have a size, instead massive means it has a large mass. So, how do we find all these virtual particles? They cannot be detected when they are virtual particles because they are in existence for a tiny amount of time, instead you have to make them real particles. The way to do this is to give them huge amounts of energy, this is what the large hadron collider is trying to do. It has already detected gluons and has been searching for the Higgs for a while now. The other problem with finding the Higgs is that even as a normal particle it decays very quickly. So instead they have to look for what the Higgs decays into, two lighter particles (more information in a video bellow). However they have been finding the more and more likely masses the Higgs will have, so they are looking within that mass range, and expect to find, or discover it doesn't exist, later this year.

More information of what the Higgs actually is:

More information on finding Higgs Bosons:

and a good diagram here, about all the particles and their interaction.

I really hope you enjoyed this post. There is a lot more to learn about everything I said in here, and I tried to include as many useful sources as I could so that, if you like, you can read and watch some more. Please follow us, @theaftermatter, or email us at We really like hearing your feedback or just talking about the posts or other physics and maths.

Check out our last two posts: 
What is Time? - It is one of the most debated phenomenon of the universe so, what is time? Is time travel possible? Does time even exist?
The Irregularity of Time [2/2] -  Why is time moving slower for us on earth than for someone is space? Part of a series (the others are linked within this one) looking a relativity and how time is not as constant as we like to think.

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