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Authors: Dr. K. N. Prasanna Kumar, Prof. B. S. Kiranagi, Prof. C. S. Bagewadi


     The observed particles can be classified into two main classes, according to whether they are affected by nuclear ("strong") forces: Hadrons are, leptons aren't. There is a second, independent way of dividing up the particles, according to spin or statistics: Bosons can occupy the same space, and have integral spin (0, 1...), while fermions can't, and have half-integral spin (1/2, 3/2...). On a less technical level, fermions can be thought of as "matter", while bosons are the "energy" that mediates the interactions between the fermions. For example, an atom consists of a nucleus made up of baryons (the fermionic hadrons), namely the nucleons (protons and neutrons), and also electrons (a kind of fermionic lepton); the nucleons are held together by mesons (the bosonic hadrons), mostly pions, while the electrons are held in to the nucleus by photons (a kind of bosonic lepton). Hadrons can be treated as made up of yet other particles, which haven't been observed freely: Three quarks (fermions) are held together by gluons (bosons) to form a baryon, while two quarks (really a quark and an antiquark) plus gluons make a meson. The reason why hadrons are treated this way is that, unlike the (known) leptons, they resemble atoms in that they appear in related forms that differ only by being "excited" to different energy levels. However, unlike atoms, which fall apart if you hit them hard enough (very hard if you want to break the nucleus), quarks and gluons have never been broken off of hadrons (except as parts of newly created hadrons). The interpretation is then that the potential well describing the force between the quarks, unlike that for electrons in atoms (or bodies in the Earth's gravitational pull), rises infinitely high on the sides, so the quarks can never escape no matter how fast or far they travel. This quark-gluon theory of hadrons is called "(quantum) chromodynamics" (QCD). Nuclei are usually thought of as bound states of nucleons, but they can probably be described better as bounds states of many quarks and gluons, which at high temperature can form a "quark-gluon plasma". For example, an isolated neutron decays, but inside a helium nucleus it is stable: The helium nucleus is thus more of a particle than the neutron is. Details of the observed "energy levels" of hadrons show they have the same form as those for the vibrational modes of a string.

     The QCD interpretation is that gluons, because they interact not only with quarks but also with themselves (unlike photons), tend to condense into tubes of flux, which act like strings. The string is also a relatively simple model, both calculation ally and interpretation ally, and so is a useful approximation to any finite-size generalization of particles. In particular, the simplest string models automatically have a graviton (the boson responsible for gravity), and solve some of the problems found when attempting to describe the graviton as just a particle. Consequently string theory is now the most popular method for describing quantum gravity. Another consequence of string theory, although it is also a feature of some particle theories, is that it unifies all the known particles by treating them as different varieties of the same particle. In particular, string theory implies super symmetry, which relates bosons to fermions.

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