Science & theoretical high energy physics

The sciences

One way to define the sciences is in terms of the objects being studied. They can be classified by their scale in terms of complexity, and not necessarily size. Objects at each level are a heterogeneous composition of objects at the next simpler level. The 4 more complex levels are associated with life, the 4 simpler ones are not.

• At the most complex level there is sociology, which studies the workings of societies, such as humanity or insect colonies. (This level can be subdivided according to the evolution of a society by personality types.)
• The next level is biology, concerned with the organisms that can make up societies, and related organisms (animals, plants, and maybe fungi). (Societies are made up of organisms in an inhomogeneous way, with different organisms playing different roles; the same goes for all the following levels of complexity.)
• Organisms in turn are made up of cells (of related, but not the same types), who have free-living relatives (protozoa, algae, etc.); for lack of a better name, we can call that study "cytology".
• Cells have organelles (nuclei, mitochondria, chloroplasts, etc.) as their consituents, whose free relatives are the bacteria, so their study is basically bacteriology.
• Bacteria are made up of molecules, related to the inorganic molecules that exist freely, which are the domain of chemistry.
• Atomic physics studies the atoms that make up molecules or live freely (and condensed matter physics studies atoms appearing in large numbers of the same type).
• "Particles" (nucleons, leptons, photons, etc.) appear in atoms and elsewhere. Nuclei in some sense can themselves be considered particles (see below), and this area studies primarily the strong and weak nuclear forces, so I'll call this "nuclear physics" to distinguish it from the rest of "particle physics". (Astronomy is hard to classify, since it combines the nuclear and atomic levels of physics, and since it is not experimental in the usual sense, because stars are hard to manipulate.)
• The most fundamental level of structure of the universe is described by unobserved objects that are also called particles, although this is really a misnomer -- probably "partons" would be better. According to quantum mechanics, interactions at smaller distances involve higher energies. High-energy physics describes the constituents of observed particles: "Fermionic leptons" (see below) are composites of fundamental fermions and "Higgs bosons", "vector bosons" (photon, W, Z) are composites of Higgs bosons and fundamental vectors, "hadrons" are composites of "quarks" and "gluons". Probably even the "graviton" (particle of gravity) can be described only as a composite. My area of research falls under this heading.

The science of...	studies...
sociology	societies
biology	organisms
cytology	cells
bacteriology	bacteria
chemistry	molecules
atomic physics	atoms
nuclear physics	particles
high-energy physics	partons

These objects can also be arranged by their time of origin: There was a kind of "evolution" from simple to complex, even before life. The universe began with the Big Bang. Shorter distances are associated not only with higher energies, but with higher temperatures: At the Big Bang, temperatures were at the order of 100,000,000,000,000,000,000,000,000,000,000's of degrees, and the universe consisted of a "parton plasma". As the universe expanded, it quickly cooled to temperatures of 10,000,000,000,000's of degrees, and the partons condensed into particles. This state was the usual "plasma". (The 4 states of matter are plasma, gas, liquid, and solid. These were identified by the ancient Greeks as "fire, air, water, and earth", respectively; they called them "elements", but that term has a different usage now.) The plasma then condensed into atoms (first as a gas) at temperatures of 100,000's of degrees.

The particles

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).

PARTICLES	bosons (energy/force)	fermions (matter)
leptons (weak)	gravitons (spin 2), photons, W, Z (spin 1), Higgs (spin 0)	electrons, muons, tau, neutrinos
hadrons (strong)	mesons	baryons

Hadrons

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.

Strings

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 calculationally and interpretationally, 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 supersymmetry, which relates bosons to fermions.

---

Glossary

• Experiment: examining nature to test theories and help find new ones.
• Feynman diagram: in quantum field theory, a pictorial representation, with a numerical interpretation, of the scattering of particles or waves, consisting of intersecting line segments each of which illustrates the path of a particle or wave between collisions at the intersections.
• Field: the quantity that describes waves, by which propagate forces (interactions), such as gravity, electromagnetism, and nuclear forces.
• High energy physics: the study of the fundamental building blocks of nature and the fundamental forces.
• Mass: amount of inertia; energy not due to linear motion or external force; related to gravitational weight.
• Mechanics: the description of matter -- particles, strings, etc.
• Particle: a point-like object that makes up matter, such as the submicroscopic protons, neutrons, and electrons that make up atoms.
• Quantum: an object that acts like both a wave and a particle (or string).
• Quantum field theory: the formalism that unifies mechanics with field theory, treating the building blocks on the same footing as the forces through which they interact.
• Quantum gravity: quantum field theory of gravity.
• String: the generalization of a particle to an extended object, whose vibrational modes of different energies appear as particles of different masses.
• Supergravity: gravity plus matter, with supersymmetry relating them.
• Superstring: string with supersymmetry.
• Supersymmetry: the principle that treats all types of particles of the same mass as different varieties of the same superparticle.
• Symmetry: a principle that relates things that could have differed, such as the bilateral symmetry between the left and right sides of most living things.
• Theory: law of nature (proposed or confirmed).