Xi (pronounced /ʃi/) is the name used by æalogists to refer to the cosmos which contains the plane of Tamamna and, therefore, the planet Earth (or at least True Earth, though alternate Earths may exist in other cosmoi). Xi is notable for the high degree of symmetry in its physics; unlike those of some other cosmoi, its physical laws seem invariant under translation, rotation, and a large host of other transformations.
The physics of Xi includes four fundamental forces, known as gravity, electromagnetism, and the strong and weak nuclear forces. Of these, the last two are important only on the microscopic scale; the first two are the forces most easily detectable in everyday life. Both gravity and the nuclear force obey the inverse square law, becoming weaker in proportion to the square of the distance between the two objects. The electromagnetic force can be attractive or repulsive, but only applies to objects that have an electric charge, which by no means all objects do. Gravity seems to be only an attractive force, and applies to any two objects with mass.
While the classical physics at everyday scales has been well understood for centuries, only recently has it been discovered just how much and how unexpectedly things vary at scales far removed from quotidian experience. New theories have been developed that apply to these scales, with classical physics retaining some validity as an approximation under the appropriate circumstances.
At very large scales or very high speeds or when very large masses are involved, the theory of relativity applies. (There is no relation to the coincidentally similarly named æalogical theory of ontological relativity.) Essentially, this theory states that there is no such thing as absolute position or even absolute time, and covers how things appear different at different reference frames. The broader theory of general relativity even incorporates gravity and acceleration, and in fact identifies the two as different aspects of essentially the same phenomenon. One well known consequence of relativity, as it is currently understood, is that nothing can move faster than the speed of light—or, more accurately, nothing moving slower than light can ever reach or surpass light speed (since accelerating to the speed of light would take infinite energy); the theory does not necessarily preclude particles that move faster than light as long as they always move faster than light. Such particles—though entirely hypothetical according to modern-day physics—are known as tachyons.
Conversely, on the very small scales, another theory, called quantum mechanics, applies. Quantum mechanics gets its name from the concept that most quantities—time, velocity, energy—do not actually vary continuously, but in discrete quanta, though under normal circumstances those quanta are far too small to be detectable in everyday life. The unusual implications of quantum mechanics go well beyond that, however; another notorious result is that of the principle of uncertainty, which, in one formulation, states roughly that a particle's position and momentum cannot both be accurately defined simultaneously. Sometimes this principle is misunderstood to imply only that the act of measuring the particle's momentum disturbs its position, and vice versa, to the extent that they can't both be measured simultaneously, but the real implications are deeper: it's not just a matter of measurement; the principle of uncertainty states that a particle can't simultaneously have a well-defined position and momentum. Actually, in fact, even speaking of a particle quantum mechanically is problematic, since anything that behaves as a particle also behaves as a wave (and vice versa); sometimes the word "wavicle" is used to describe such quantum-mechanical objects, though this has not become really standard. Again, many characteristics of a particle or wave are actually undefined until observed, or rather are only defined in terms of a probability distribution rather than possessing a unique quantity, though, contrary to a frequent misunderstanding that leads to false claims that quantum mechanics proves the possibility of telekinesis or other paranormal powers, "observed" in this context does not necessarily imply a conscious observer or that mind affects reality.
The "Theory of Everything"
While relativity and quantum mechanics are both well-established theories that have made many predictions that have proven accurate, and that have shown themselves to be entirely reliable within the situations when they each apply, they don't tell the whole story. There are some situations in which they both are applicable—such as in the surroundings of a small black hole. And in these situations, some of their predictions disagree. This does not, of course, mean that they are wrong, just that at least one of them, if not both, is actually an approximation under the proper circumstances to a more general theory. This so far unknown (or at least unverified) theory is sometimes known as a Theory of Everything, though this is something of a misleading name, and sometimes leads the scientific layman to drastically overestimate how much the formulation of such a theory will simplify scientific predictions. The best known current candidate for the "theory of everything" is string theory, but so far no one working on the theory has come up with a way the theory can be tested using current technology, so it remains speculative. However, the possible existence of such a broader theory may (or may not) imply that, under special circumstances, some of the apparent limitations of quantum mechanics and relativity may be circumventable.
With the exception of very hot plasmas such as the substance of stars (admittedly a very large exception, since the stars contain the vast majority of the visible mass in the known universe), most matter in the cosmos of Xi is made up of units called atoms, which in turn are made up of smaller units called protons, neutrons, and electrons. These latter particles are the subject of particle physics, but they have an important impact on chemistry as well; for one thing, the number of protons in an atom is the main factor determining its behavior, and in fact it's by the number of protons that atoms are classified: each number of protons corresponds to a different chemical element. The number or protons is called the atom's (or element's) atomic number. An atom with a single proton is an atom of the element hydrogen, up to an atom with ninety-two protons, corresponding to the element uranium; this is the element of the highest atomic number to have been found in natural occurrence (though unstable atoms with higher atomic numbers have been created in laboratory settings).
A neutral atom contains the same number of electrons as protons. Some atoms, however, may contain more electrons than protons, or vice versa, in which case they will carry an electrical charge; such charged atoms are called ions. Atoms (and ions) combine together into molecules via "chemical bonds" brought about by sharing of electrons, or by the attraction of two oppositely charged ions. Although there are exceptions, generally atoms combine together in fairly predictable ways, and each element can be thought of as having a certain number of "valence electrons" (which, except for the first two atoms, is smaller than its actual number of electrons) which determine what other elements it will bond with, and how. The elements are often arranged in a periodic table that groups elements of similar characteristics. The element with atomic number six, carbon, is notable because it can bond in complex ways and give rise to especially intricate molecules; it is because of this that these carbon molecules are the basis of all known life of natural origin in Xi (or at least in most alternate versions of Xi). The atom with atomic number fourteen, silicon, is second to carbon in this regard, such that there has been much speculation about the possibility of silicon-based life; however, it's a very distant second, and such silicon-based life may not be possible.
The genetic basis for most naturally occurring life on Xi is a molecule called nucleic acid. This consists of a long, spiraling string made up of several different "bases", often paired up with another parallel strand to form a so-called "double helix". The sequence of bases on a strand generally codes not for the structure of the body directly, or for any other such macroscopic feature, but for the creation of large carbon-based molecules called proteins, made up of units called amino acids. The segment of a nucleic acid molecule that codes for a particular amino acid is called a codon; the exact encoding is largely arbitrary and is different for each empire of life that arose independently. Generally, a codon comprises three bases, this being the minimum number necessary to allow for every possible amino acid to be encoded (though with some redundancy); however, rare cases of inefficient four-codon encodings do exist. All life with a nucleic acid basis is assigned to the fœdus Spirilex.
Xi does not inherently have any magic "built into" its physics or structure. However, in at least some alternate versions of the cosmos, magic systems have been created, apparently through advanced technological means. The best known and perhaps the most widespread of these is known as Narlovian magic, and comprises fourteen subarcana, known as "arts". Aside from spellcasting, other common applications of Narlovi magic include the taking of magical oaths, and the production of magical potions through alchemy. One interesting feature of Narlovian magic, however, is that it is impossible for someone to learn it who has not first been exposed to it; someone who has been exposed to magic is said to have been immagiated. Inhabitants of worlds where magic is very common may be considered to be immagiated from birth, but on worlds where magic is rare—including many magical Earths—immagiation may be an important factor.
The worlds where Narlovian magic is in common use are collectively referred to as "Narlovia". Other magic systems may exist in other versions of the cosmos, however, or may coexist with Narlovi locally.
The only well known naturally occurring planes in the cosmos of Xi are a collection of universes known as Yacun, which may or may not have slightly different physical laws. The particular universe to which Earth belongs (and the one which certainly has the physical laws described above) is called Tamamna. In versions of Xi where magic exists, however, other planes may also have been created; Narlovia, for instance, includes the nether planes, the elemental planes, and the divine planes, among others.