| 2. Theories of Everything |
A model is a way of explaining something difficult in simple terms. The further we probe into matter, the more detailed our models have to be. We explain things in different ways:
The picture shows us the staircase that leads us to the heart of matter.
The Standard Model of Particle Physics identifies
There are two main groups:
Each family has generations. The table below shows the quarks:
| 1st Generation | 2nd Generation | 3rd Generation | Charge | |||
| Up | u | Charm | c | Top | t | +2/3 e |
| Down | d | Strange | s | Bottom | b | -1/3 e |
The names may seem rather whimsical, but nothing better has replaced them. Notice that they come in contrasting pairs, for example "top" and "bottom". The strange particle was called that for its unexpected behaviour. The charm quark was called that because it "worked like a charm" to fit in neatly with the strange quark.
More importantly note their charge:
The top quark is the quark with the heaviest mass. It has 200 times the mass of a proton, making it heavier than many molecules.
Quarks are never found on their own; they are bound in pairs or threesomes.
Other particles are unstable and decay to particles of lower mass. The Delta plus (D+) is made up of three up quarks, the Delta minus (D-) is made up of three down quarks. Both last about 10-25 s. An isolated neutron lasts about 10 minutes before it decays by beta minus decay to a proton.
The table shows the leptons:
| 1st Generation | 2nd Generation | 3rd Generation | Charge | |||
| Electron | e- | Muon | m | Tau | t | -1 e |
| electron neutrino | ne | muon neutrino | nm | tau neutrino | nt | 0 |
Unlike quarks, single leptons can be found in nature. The table shows where:
| Particle | Where it's found |
|
Electron |
Atoms Electric currents Beta-minus radioactive decay |
|
Muon |
Upper atmosphere, produced by cosmic rays |
|
Tau |
Only in high energy physics experiments |
| Electron neutrino |
Beta-plus decay Atomic reactors Nuclear reactions in stars |
| Muon neutrino |
Atomic reactors Upper atmosphere by cosmic rays Nuclear reactions in stars |
| Tau neutrino |
Only in high energy physics experiments |
All the forces we experience in physics can be explained in terms of the fundamental forces.
| Force | Range | Relative Strength | Acts between |
| Gravity | Infinite |
10-34 |
all objects |
| Electromagnetic | Infinite |
1 |
charged objects |
| Strong force | 10-15 m |
103 |
quarks |
| Weak force | 10-18 m |
10-10 |
fundamental particles |
We know how weight is a force resulting from the gravitational attraction on a mass. Forces like friction are the result of the electromagnetic force, as it's due to atoms rubbing together. The other two forces are confined to nuclei.
The strong force binds quarks into groups called hadrons, of which there are two kinds:
Baryons - made up of three quarks. Baryon means "heavyweight".
Mesons - made up of a quark and an antiquark (more of which later). Meson means "middleweight".
One important effect of the strong force is that it binds the nucleons in the nucleus together. Otherwise the repulsive electromagnetic forces between protons would ensure that the nucleus would fly apart.
The strong force does not act on leptons; the weak force does. The weak force acting on quarks is completely hidden by the strong force.
For every particle there is an antiparticle. Each antiparticle has the same mass as its particle, but opposite charge. Here are the anti-quarks:
| 1st Generation | 2nd Generation | 3rd Generation | Charge | |||
|
Anti-Up |
_ u |
Anti-Charm |
_ c |
Anti-Top |
_ t |
-2/3 e |
|
Anti-Down |
_ d |
Anti-Strange |
_ s |
Anti-Bottom |
_ b |
+1/3 e |
The symbol with bar over it tells us that it's an antiparticle. "Anti-up" is written ū, "u-bar".
We can therefore make up an anti-baryon such as an anti-proton. Since a meson is made up of a quark and an anti-quark, we can't really talk of anti-mesons.
The table shows anti-leptons:
| 1st Generation | 2nd Generation | 3rd Generation | Charge | |||
| Positron | e+ | Anti-Muon | m | Anti-Tau | t | +1 e |
| electron antineutrino |
__ ne |
muon antineutrino |
__ nm |
tau antineutrino |
__ nt |
0 |
In this family we can see that it is possible to have a positively charged anti-electron called a positron. It is, in theory, possible to make up anti-atoms. For example, a hydrogen anti-atom has been made, with a positron orbiting an antiproton. If a hydrogen anti-atom meets a hydrogen atom, the two annihilate in a puff of electromagnetic energy. We will look at this later.
All of this shows the idea of symmetry in the model:
top - bottom;
electron - positron.
Mesons are short-lived particles that are always made up of a quark and an anti-quark. Any combination is possible. The table below shows the family of mesons called p-mesons or pions, which are made up from the lightest of the quarks and anti-quarks:
| Quarks | Charge | Meson | Symbol |
|
_ u u |
0 | pi zero | p0 |
|
_ d d |
0 | pi zero | p0 |
|
_ u d |
+1e | pi plus | p+ |
|
_ d u |
-1e | pi minus | p- |
You may see in some books the muon being referred to as a mu-meson. Although it's as big as many mesons, the muon is a lepton.
When a particle and an anti-particle of the same type interact, they cancel each other out in a flash of electromagnetic radiation, usually a gamma ray photon. For example:
electron + positron ® 2 gamma ray photons
e- + e+ ® g + g
The reverse can happen:
g + g ® e- + e+
If the gamma photon energy is high enough we can get:
g + g ® m- + m+
Or even:
g + g ® t- + t+
The process of producing particles and anti-particles from photons is called pair production.
One of the most surprising things about the above statements is that mass can be turned into energy and vice versa. At this level, mass and energy are interchangeable, which helps us to explain a number of things that we see at the fundamental level.
Mass and energy are related by the famous Einstein equation:
E = mc2
where E is the energy in J, m the mass in kg, and c is the speed of light in a vacuum (3 × 108 m/s).
The more energetic the photons, the more massive the particles that are produced.
Particles can only interact if the following conditions are met:
The total charge is conserved. Charge before = charge after. If the charge is not conserved in the interaction, the interaction cannot go ahead.
Other quantum numbers are conserved, for example, baryon number and lepton number. Again, if there is a change from start to finish, the interaction will not happen.
For example, consider this interaction:
p + p- ® n + p0
Charge:
+1 + -1 = 0 + 0
Charge is conserved.
Baryon number (if it's a baryon, its baryon number is 1):
1 + 0 = 1 + 0
Baryon number is conserved.
The lepton number is 0 for all the particles, since none of them are leptons. Since all these are conserved, the interaction will go ahead.
There is quite a lot more about the interactions between particles in particle physics, but this is all we need to know at this point.
Immediately after the big bang, the universe was hot, dense, and small. Its matter consisted of quarks, hadrons, and leptons. Now it is big, cold, with matter spread thinly. The matter is in the form of atoms, molecules, and morons. Physicists are fascinated to know how it got from one state to the other. The study of particle physics has recreated the conditions that are believed to have occurred. Physicists have worked out what happened with 10-35 seconds after the Big Bang, which is not an awfully long time.
The history of the universe is summed up in this log - log graph (i.e. both scales are logarithmic). The log log graph enables to huge scales to be compressed into something more manageable.
The universe's history can be summed up in phases:
Inflation in which the universe started from a single point and grew to 1050 times bigger. This took 10-35 s
Between 10-35 s and 10-18 s the universe was full of matter, antimatter, and radiation.
Between 10-18 s and 10-12 s there was a soup of quarks, leptons, and exchange particles for the fundamental forces.
Between 10-12 s and 180 s hadrons, mostly protons and neutrons, were predominant.
From 3 to 13 minutes the universe had cooled down to enable the simplest nuclei to form, the nucleosynthesis era.
For the next 3000 years it was still hot enough for the universe to exist as a plasma, completely ionised gas.
After that, matter cooled sufficiently to allow electrons to join up with nuclei to form atoms. From this point its was possible for material to gather to from stars and galaxies.
It is believed that cosmic background radiation and the current temperature of the universe (2 K) also point towards the Big Bang, as well as the fact that the universe is still expanding. That's the best theory, although no-one was there to witness the start, and would have ended up as pork scratchings if they had.
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