added physics content -- readmore -- initial commit

readmore/antimatter.html

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+Know’s
+Antimatter sounds like an exotic and mysterious substance that could destroy the world in no time at all. In fact, it is the same as everything else, just with a different charge[ref]. Antiparticles have all the same properties[ref] as particles, with the same mass and the same stability, and so are not more than everyday matter. Every particle has a corresponding antiparticle, with all of the same properties, except with an opposite charge. So an electron[ref] which is positively charged[ref], will have  a negatively charged[ref] antiparticle called a positron.
+The problem comes when matter and antimatter come together. Because there is a lot of matter in our universe, antimatter is quite likely to bump into its corresponding matter partner. When an equivalent matter and antimatter pair collide they annihilate, which means that they disappear, releasing a huge amount of energy. Antiparticles can only annihilate with their particle pair, so a antiproton will not annihilate with an electron. However, because there are lots of protons around in our universe, it is likely that the antiproton will quickly find one to annihilate with.
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+Don’t Know’s
+There is a lot that we still don’t know for certain about antiparticles. We do not know why there are so many particles and not many antiparticles. It is obvious that antiparticles will annihilate quickly which explains why there is not much antimatter left in our universe, but if all antimatter has been annihilated by matter, then there should be no matter left at all. As it is, we  have lots of matter, so there must have been a lot more matter created at the time of the big bang than antimatter. We are currently trying to work out why that is.
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+We create antiparticles in exactly the same quantity as particles when we collide particles. When the particles hit a target at a very high energy, many different particles will come out. Particles will be produced in pairs, with each particle also having their antiparticle. 
+Outlook
+We’re almost done. You were probabily already waiting for the last chapter, the one on the Higgs boson. Here it comes!

readmore/the_electromagnetic_interaction.html

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+If will take a closer look at the Hydrogen atom, we already know that it is made of a proton with electric charge +1 and an electron with charge -1. Particles with opposite electric charge attract each other just like magnets. This is an example of the electromagnetic force, or “the electromagnetic interaction”. 
+What is interaction?
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+How do the proton and the electron know about each other? How do they “interact”, and what do we mean by “interaction”? Particles interact with each other by exchanging other particles. How does this work?
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+Two astronauts in space, being outside of their spaceship, repairing their spacecraft. Both are floating around, and as it happens, are closing up. One of the two would now throw a tool, that the other would eventually catch. What happened? The throwing astronaut transferred momentum onto the tool he threw. He is not fixed to the spacecraft, but floating in space freely. The momentum he will achieve from throwing the tool will carry him in the opposite direction of the tool. The other astronaut, catching the tool, will receive the tools momentum, carrying him in the direction of flight. What happened? The two astronauts are now floating away from each other, whereas they were approaching each other before. They exchanged a tool, and transmitted momentum from one another, a process particle physicists call “scattering”. Sadly, this example only works for a repelling interaction. However, there is a similar example, that also works for an attractive interaction, which is a little more complicated.
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+[READ MORE: Here is another analogy: Two people are sitting in small boats on a huge and quiet lake. Both boats are approaching each other slowly. If one of the two people now throws a ball to the other person, who will catch it. If the ball is very heavy, both boats will then move away from each other, because the inertia of the ball transmitted momentum to both boats. The one boat gained momentum by the person throwing the ball, and the other picked up momentum by the person catching the ball. What happened? The boats changed direction, they exchanged momentum by exchanging a basket ball. The boats interacted with each other, by exchanging a ball as their messenger.
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+The above analogy is useful to understand repulsion of particles by exchange of some kind of “messenger”, for example two electrons, due to their electric charge. The same analogy is possible for attraction, only requiring the change of the basketball for a boomerang and the directions that it is thrown and caught. If the person throwing the boomerang throws it away from the other boat, and it loops around so that the person catching it is facing away from the first boat, the exchanged momentum is just opposite to that for the analogy for repulsion.]
+Electromagnetism and Light
+What is that object being thrown between the proton and the electron exchange in order to communicate their attraction to each other? The answer is simple, it’s the photon. These photons are the same particles that make up the light that comes to us from the sun and enables us to see the world around us. But the photons we can see are only a small part of the full spectrum of the photons that are out there. Visible light, X-rays, microwaves and radio waves are all photons. The only difference is that they have a higher or lower energy than the light we can see - much like the force that keeps us on the surface of our planet and the force that binds our planet to the sun is the same - only that there is a different amount of energy involved. 
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+[http://en.wikipedia.org/wiki/File:EM_spectrum.svg -GFDL Version 2]
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+[The most energetic photon particle discovered is in the Tev range. So if the range of the visible photon our eyes can detect is scaled to the width of a human hair, the range of the electromagnetic spectrum is the distance between the earth and the moon!] 
+[The photons that are described in the figure are characterized by a wavelength, meaning that photons are waves. But why did we say that the photon is a particle before? The reason for this is that the physical laws we are familiar with (i.e. Newtonian mechanics) cannot be applied at very small scales. When at these small scales, quantum mechanics apply. One of those rules of quantum mechanics is that all particles have both particle and wave properties. This is referred to as the wave-particle duality.]
+What is a photon?
+First of all a photon is an elementary particle, which means that - as far as we know - it doesn’t have any substructure. It is not a composite object, and not divisible into smaller building blocks. The photon belongs to a group of particles that we call bosons - we will encounter other members of that group later on. Bosons are characterised by the fact that they have integer spin, where spin is an intrinsic property of particles that can take different values, but never changes, much the like charge we have encountered earlier. Other particles, such as the electron and the quarks, have spin ½ and are called fermions (all particles with half-integer spin are fermions).
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+\ID card
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+Like all other bosons, photons can - given that sufficient energy is available - be produced out of nowhere and can also vanish into nothingness, leaving behind nothing but energy. Therefore, the electrons and protons in the atoms - as opposed to the persons in the boats we used as an analogy earlier - do not need to carry around their messengers (in our case photons), as they can simply be produced by all particles that carry charge. 
+Outlook
+The exchange of photons between particles is what we call “electromagnetic interaction”, The photon can create either attraction or repulsion between particles and transport momentum as well as energy. Now we know what binds together the electron and the nucleus to form atoms, such as the Hydrogen atom or the Helium atom that we used as examples - it is the electromagnetic interaction, the exchange of photons, communicating the attraction between the particles due to their opposite charge. In the next chapter, we will learn what glues together the quarks to form protons and neutrons.

readmore/the_higgs.html

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+The Higgs Mechanism
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+When you are in a room filled with jelly, every movement will be difficult - just like if you were much heavier than you really are. In a similar way, something called the Higgs field fills the universe, making every particle feel massive through the interaction with the Higgs field.
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+This way, the particles will acquire masses, but why are they are different from one another? Much the same way a large person will “feel” the jelly stronger than a small person will, the different particle types will feel the Higgs field differently. Switching to physicists speech: The strength of the interactions between the particles and the Higgs field are different. In a way, this does not answer the question, because it only relates the masses of the particles to their couplings to the Higgs field - but it is the best we can do!
+What about the Higgs boson?
+In a room filled with jelly, not only the jelly affects you, but also you affect the jelly. If you punch the jelly, it will move. And if you punch it in the right way, there will be waves running through the jelly. 
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+We know already that photons can be viewed as particles and waves at the same point. In a way, the Higgs Boson is a wave in the Higgs field - a wave in the jelly, that we can try to create by shaking and punching the jelly in the right way.
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+Experimental particle physicists have been trying to do exactly this with the LHC: To collide particles in a way that will create an excitation (or wave, if you want) in the Higgs field - because measuring the wave in the field is the only way for us to know that the field is actually there.
+What now?
+The journey of particle physics is not over. There are still a lot of things we do not fully understand - some of them we have already mentioned, some others (like the mystery of Dark Matter) you might have heard about.
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+If you read everything up to now - congratulations! We also have exciting quizzes, where you can test your knowledge about the Standard Model, and additional chapters on Feynman diagrams - the little sketches physicists love to use to explain what they are doing. Hang on!

readmore/the_strong_interaction.html

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+What is color?
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+Color has the nice property that you need three different colors: red, green and blue, to get back something neutral, white, - which is very similar to what we observe in the proton, where we also need three different “colors” to form something that is going to be neutral in the end: A red quark, a green quark and a blue quark.  
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+[Picture: Intersecting Color plains]
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+[READ MORE: In fact, protons and neutrons are not really white, or color-neutral. The nuclei of atoms consist solely of  positively charged protons and sometimes also neutrally charged neutrons. Hence the electromagnetic interactions cannot hold the nucleus together, but would tear them apart instead. Thus, there must be some very strong force to keep them together, which is where the name of the strong force came from in the first place. The strong force affects both the neutrons and the protons, because they are so close to each other that they will not only “see” the charge of the others as a whole, but also the partial color-charges of the quarks.]
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+The mediators of the strong force are called the gluons (because they bind particles together very tightly, just like some kind of super-glue). But the strong force is very different from the electromagnetic one in various aspects. Speaking-of the mediators instead of the forces, this means that the gluons are different from the photons in a very fundamental way: they carry color, unlike photons, which aren’t electrically charged themselves. This means that gluons can participate in their own interaction, which makes the strong interaction very special.
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+The strong force is special!
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+One consequence of this is that the strong interaction does not decrease with distance. Two electrically opposite charged objects will attract each other less when their distance increases. The strong interaction of two particles of different color, on the other hand, will increase rapidly as you tear them apart, and eventually grows so strong that the energy you needed to remove the particle any further would be sufficient to create new colored particles instead (due to Einstein’s famous relation E=mc², allowing us to transform energy into mass).
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+[Cool animation showing color string snapping and quark pair creation]
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+This process is the reason why we have up to now never observed an isolated quark - they only appear in groups of three or two (as in the animation, although we have not explained how this happens for pairs yet, and this will have to wait until we explain antimatter). The property of quarks to stick together so tightly and to never show up alone is called “confinement”.
+Outlook
+We have come pretty far. We know how the protons and neutrons are made from quarks, and we know what holds together the protons and electrons to form atoms. This is a great achievement, as it allows us to explain about everything that holds together the building blocks of natures. But it is not the whole story.
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+Sometimes, thinks break apart - and it was only in the 20th century that radioactivity was discovered - the breaking apart of nuclei, emitting highly energetic and dangerous radiation. We cannot explain this kind of thing happening until now, and this will be what the next chapter is about.

readmore/the_weak_interaction.html

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+Weak Interaction and nuclear decay
+There are more particles after the decay than just the proton, there is an electron and a new particle that has not been introduced yet. This new particle is the neutrino (which is italian and means “the small neutral one”). As its name suggests, it is electrically neutral. It has no color charge either, so it does not interact strongly. It only interacts with this new force, the weak force, and because the weak force is so weak, it is extremely difficult to detect. It has been detected, for the first time in 1956, 26 years after it had been predicted. It also differs from the quarks and the electron in that its mass has not been measured, it is so light that there are only upper limits on its mass. The best, up do date limit is about 1/250 of the mass of the electron.
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+[FIGURE OF THE NEUTRON DECAY, n -> p + nu + e?]
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+This figure shows what happens in the beta decays, the neutron turns into a proton, an electron and this new particle, the neutrino. For now the interaction is a “black box”, we know what comes in and what comes out, but what happens in between?
+Charged Messengers
+The other forces had messenger particles, the photon[ref] for electromagnetism and the gluons[ref] for strong interactions, this is also true for the weak force, and like for the other two forces, the messenger is a boson[ref]. There is an important difference though, the messenger particles for the weak force are very heavy, and there are three of them. Their weight is the reason that this force is so weak. This sounds counter-intuitive, right? We won’t dive into this, so if you want to know more, head over here->[READ MORE?]. The messenger particles are called the W bosons, there are two of them, one with positive electric charge and one with negative charge, and there is also a neutral one, which is called Z and which will be explained later.
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+What actually happens in this “black box”, is that the neutron turns into a proton and a [W-] boson, and the [W-] boson decays into an electron and a neutrino. The W boson is very heavy, its mass is about 80 times that of the proton, and it will only exist for an extremely short time (we cannot explain this with what we have explained so far, but you might want to keep reading until we explain the second and third generations of matter to understand the short lifetime of the W). As we know neither the proton nor the neutron is fundamental, so we can still look closer at the neutron becoming the proton and the [W-] boson. The neutron is made up of one up-quark and two down-quarks, and what happens is that one of the down-quarks turns into an up-quark and the [W-] boson. This is the extent to which the Standard Model is describing this weak interaction, the down-quark turning into an up-quark and a [W-] boson and the decay of the [W-]. These two interactions are “black boxes”, perhaps there is something more fundamental occurring if one just looks closely enough, nobody knows that as of today, again this is for someone (maybe you) to find out! 
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+[FIGURE OF THE ENTIRE INTERACTION?]
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+So far the matter particles that have been introduced are the electron, the up and down-quarks and the neutrino. The quarks are connected in that they can interact strongly with each other, and they are connected to the electron by the electromagnetic force. But the matter particles are grouped into quarks and leptons, the leptons (so far) being the electron and the neutrino[[, muon, tau and their respective neutrino]]. The weak force really gives the connection that motivates giving the electron and the neutrino this common name, leptons, as it allows electrons to turn into neutrinos and the other way around. It also connects the two types of quarks in the same way, allowing the up-quark to turn into the down-quark as mentioned.
+Neutral messenger and discovery
+The third messenger of the weak force is the Z boson, which is about as massive as the W bosons, 90 times as massive as the proton. But unlike the W bosons, it is not charged. Both the Z and the W bosons were discovered at CERN, and their discovery is one of the major accomplishments of both CERN and the Standard Model. It is remarkable because the masses of the particles were predicted, and the Z boson, unlike the W bosons, was not involved in the weak interactions that were known, [but was a consequence of the theory and no experiment had seen any indication of its existence when it was predicted, can you explain this one]. Both the W bosons and the Z boson were discovered in 1983, and the two persons responsible for the experiments, Rubbia and van der Meer, got the Nobel prize for this just one year after the discovery, in 1984. 
+Outlook
+Is this it? For what we know, yes. What we know by now is sufficient to explain everything that surrounds us, from the atoms and nuclei being formed, quarks glued together, down to things as rare and obscure as nuclear decay and radiation. Is this the whole picture? Not quite. We have not explained antimatter, and then there are some really strange particles turning up in cosmic rays. And, in the end, you might want to learn about the Higgs as well. Read on!

readmore/three_generations.html

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+So now you know about almost every particle in the standard model. All that is left is the Higgs boson, which you will see in the last chapter.
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+Why do we not see any of these new, heavier particles? 
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+Why are there no atoms made of charm quarks, strange quarks and muons? The answer is that the heavy particles are very unstable, and quickly transform into their lighter brothers. Heavy particles are produced in very high energy collisions, such as in particle accelerator experiments. They are also found in cosmic rays – very energetic particles that fall to Earth from space, and that are produced in places as exotic like the core of the sun or even a supernova explosion.
+Outlook
+Why are there so many particles? Why do three generations exist rather than just one? The answer is simple: nobody knows!
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+And there is more to come: You have probably heard of antimatter, haven’t you? This is what the next chapter will explain.

readmore/what_is_the_world_made_of.html

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+Atoms actually consist of protons, neutrons and electrons, and can be torn apart by raising the temperature of the atom so high that the forces are not strong enough to hold them together. Once we break apart the atom, what happens? How do these smaller particles interact?
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+We can understand the atom at follows:
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+[Picture of a Hydrogen Atom]
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+A Hydrogen Atom, the simplest possible atom, is made of one proton and one electron. The proton, which is much heavier than the electron (in fact about 2000 times heavier) sits in the center, with the electron orbiting around the proton. The electron is negatively charged[ref], while the proton is positively charged[ref], so they are attracted to each other. The electron stays in orbit around the proton, just like the Earth stays in orbit around the Sun.
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+Is this as small as it gets?
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+Are these particles the smallest particles, or can we split them, just like we split the atom?
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+We believe the electron is indeed fundamental. After trying to split it apart in many different ways, physicists have concluded that it is probably not possible, meaning that electrons are not made of anything smaller.
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+However, we know that the proton is made up of other smaller particles, which we call quarks. The proton is made up of two kinds of quarks, called "up" and "down". [We can imagine]?? the proton is made up of three quarks: two up quarks and one down quark.
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+[Picture of the Proton]
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+What about other atoms?
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+This simple model can be expanded to describe more complicated atoms. For example, the Helium atom consists of two protons and two neutrons in the center, forming the nucleus [ref], with two electrons orbiting them. The proton, made of quarks is very similar to the neutron, while the electron is indivisible.  With a similar mass[ref], they have very similar properties[ref], apart from their different electric charge [ref] (the proton is positively charged[ref], the neutron is neutral and has no electric charge[ref]). This is because they are also made up of quarks, just in a different combination. While the proton has two up quarks and one down quark, the neutron consists of one up quark and two down quarks.
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+[Picture of the neutron and proton]
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+What about their charges?
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+How can it be that the proton and neutron have a different charge, if they are made of the same building blocks? 
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+We know that the proton is up, up, down and charged +1 and the neutron is up, down, down and charged 0. The only possible way of resolving this is that the up quark is charged +2/3 and the down quark is charged -1/3. The fractions only appear here because we discovered the electron before we discovered the quarks, and called the electron charge "-1". If we had called the  charge of the electron "-3", the quark charges would be whole numbers.
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+Questions we still need to answer
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+What are charges? Why do we call them "+" and "-"? What holds together the Hydrogen atom? And what holds together the nucleus? What about radioactivity? Why do physicists always draw funny graphs when explaining this? And finally, what is this "Higgs boson" everyone is talking about? Continue to answer these questions.