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
Cells have organelles (nuclei, mitochondria,
chloroplasts, etc.) as their consituents, whose
free relatives are the bacteria, so their study is basically
Bacteria are made up of molecules,
related to the inorganic molecules that exist freely, which are the
domain of chemistry.
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...
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 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 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.
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
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
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.