On this page, several concepts mentioned in the next few pages are elaborated. One of these is the taxonomy of the particles that physicists have unearthed from various particle accelerators like the one in Fermilab. There are many ways to classify these particles, but this section only goes into the classification that is used for supersymmetry, and consequently, superstrings.
One of the most common ways of classifying particles is by spin. "Spin" is merely a name given to a quantum mechanical property of a particle, its angular momentum. If you like, you can think of it as actual "spinning", it's not terribly important right now.
Here is a classification chart. (The larger marbles are categories, and the smaller marbles are names of actual particles. "etc." means there are more, but they are not mentioned).
A word about anti-particles: Anti-particles are not to be confused with the superpartners discussed later on in the next section. For instance, when an anti-particle and a particle collide, they cancel each other out in a large burst of energy. The anti-particle of an electron is a positron. They have the same mass, but a different electrical charge. When they collide, they produce a photon. One final thing to remember is that anti-particles have been proven to exist. The positron was found by Carl Anderson in 1932.
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When physicists saw this division of fermions and bosons, they naturally wondered why the division was there. Some of them went on to suggest that maybe it was because fermions and bosons were two halves of the same whole. This was when a theory was concocted that particles existed in dimensions beyond the space-time we're familiar with. These particles would have "arrows", and when they pointed in one direction, they'd have certain properties. Pointing in another direction, they'd have a different set of properties.
A few problems arose from this theory. One of the main predictions of supersymmetry was that for each particle, there would be a supersymmetric partner, or "superpartner". A superpartner of a fermion is a boson of the same mass, and the superpartner of a boson is a fermion of the same mass (note the difference between superpartner and anti-particle). One problem is that certain superpartners are simply not allowed to exist in this universe. For instance, if the superpartner of the electron existed, it would clog up the lower orbitals of the atom because it doesn't follow the Pauli Exclusion Principle like fermions do (only a certain number of electrons are allowed to orbit on a certain energy level). Another problem is that none of the particles found by physicists so far are superpartners of each other. From this perspective, the theory looks a little bleak.
Physicists, however, showing a remarkable resistance to discouragement, added something else to their theory to keep it together. They suggested that symmetry exists, but that it is "broken". This is name given to an abstract mathematical operation that allows different varieties of particles to be produced. This topic will be expanded on in grad school, if you're interested. This mathematical fiddling, however, does not convince everybody of the stability of this theory, but it convinced enough people to have a new particle definition based on strings to be produced.
In the 1970's, Glasgow, Salam, and Weinberg showed that electromagnetism and the weak force could be united in the quantum field theory mathematical framework. The strong force has been attempted, but it's still too theoretical for certainty. Grand Unification, in fact, is the search for this unification of the electroweak and the strong interactions. Several theories have cropped up, but none of them are in a position to be tested since the energy required to test them is literally a trillion times our present capabilities.
Force is thought to be caused by the swapping of messenger particles that tell the other particles how to react. For example, both the attraction and repulsion of the electromagnetic force is caused by the exchange of photons. It makes sense that gravity could also be described this way, via a particle called the graviton that has not been observed experimentally. It was hoped that all the forces could be shown to come from one, unified force that has roots in the Big Bang. When physicists graphed the forces at different energies, at energies close to the Big Bang (really, really hot), the forces almost, but not quite, united. One of the reasons sypersymmetry is so appealing (besides its obvious aesthetic value) is the fact that it also allows for the forces to be neatly united. The graphic below demonstrates this (SUSY means SUperSYmmetry):
"Is the End of Theoretical Physics in Sight?" was the question posed by Stephen Hawking in 1979. That speech was about the merits of a new theory (at the time) that was an extension of supersymmetry. It was also an extension of Grand Unification because it predicted the particle of the last remaining force. It predicts the graviton, a particle that has no mass, travels at the speed of light, and has a spin of 2. Gravitons are the gravity "ripples" previously discussed in general relativity. In accordance with supersymmetry, it also predicts a superpartner to the graviton called the gravitino.
Supergravity is the zero-limit of string theory. It is a theory, in other words, of string theory when a string has zero length. 11-dimensional supergravity, however, is the zero-limit of M-Theory, the amalgamation of all string theories and the above-mentioned supergravity.
It seemed to solve the problems of the incompatibility between relativity and quantum mechanics because the infinities caused by gravitons could be cancelled out by the negative infinities caused by the gravitinos. The problem was that although this worked well enough for calculations involving one or maybe two gravitons, anything more complicated quickly got out of hand. Physicists, again, chose not to despair.
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