Example:chiasma. Modern Latin, from Ancient Greek χίασμα (khíasma), from χίαζειν (khíazein, “to mark with the letter chi”). chiasma (plural chiasmas or chiasmata):
optic chiasm: (neuroanatomy) The part of the brain where the optic nerves partially cross.
- (anatomy) A crossing of two nerves, ligaments etc.
- (genetics, cytology) The contact point between the two chromatids of a chromosome during meiosis.
chiasmus. chiasmus (plural chiasmi or chiasmuses): ()
To stop too fearful, and too faint to go
-- Oliver Goldsmith
haec queritur, stupet haec
(this woman complains, this one gapes)
-- Ovid, Ars Amatoria, 1.124.
, , , ; yawn
, ( ), ( ), ,
∴ ∴ ∴
... ... ...
Peano curve, Mandelbrot set Lorenz attractor.
Conservation Laws: "Momentum, energy and angular momentum cannot be created or destroyed."
The discovery of the second law of thermodynamics by Carnot in the 19th century showed that every physical quantity is not conserved over time, thus disproving the validity of inducing the opposite metaphysical view from Newton's laws. Hence, a "steady-state" worldview based solely on Newton's laws and the conservation laws does not take entropy into account.
The classical formulae for the energy and momentum of electromagnetic radiation can be re-expressed in terms of photon events. For example, the pressure of electromagnetic radiation on an object derives from the transfer of photon momentum per unit time and unit area to that object, since pressure is force per unit area and force is the change in momentum per unit time.
∴ ∴ ∴
In the Standard Model, there are 12 types of elementary fermions: six quarks and six leptons. Fermions are the basic building blocks of all matter. They are classified according to whether they interact via the color force or not.
- A unit with spin number n is called an n-unit and has angular momentum nħ/2, where ħ is the reduced Planck constant. For bosons, such as photons and gluons, n is an even number. For fermions, such as electrons and quarks, n is odd.
Fermions include all quarks and leptons, as well as any composite particle made of an odd number of these, such as all baryons and many atoms and nuclei. Fermions differ from bosons, which obey Bose–Einstein statistics. A fermion can be an elementary particle, such as the electron, or it can be a composite particle, such as the proton. According to the spin-statistics theorem in any reasonable relativistic quantum field theory, particles with integer spin are bosons, while particles with half-integer spin are fermions.
- Leptons are an important part of the Standard Model. Electrons are one of the components of atoms, alongside protons and neutrons. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Exotic atoms with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as positronium. Positronium in high energy states has been predicted to be the dominant form of atomic matter in the universe in the far future, if proton decay is a reality. Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, electromagnetism (excluding neutrinos, which are electrically neutral), and the weak interaction. For every lepton flavor there is a corresponding type of antiparticle, known as an antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. However, according to certain theories, neutrinos may be their own antiparticle, but it is not currently known whether this is the case or not.
- Quarks are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction), as well as the only known particles whose electric charges are not integer multiples of the elementary charge. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. For every quark flavor there is a corresponding type of antiparticle, known as an antiquark, that differs from the quark only in that some of its properties have equal magnitude but opposite sign.
∴ ∴ ∴
In particle physics, proton decay is a hypothetical form of radioactive decay in which the proton decays into lighter subatomic particles. There is currently no experimental evidence that proton decay occurs. According to some such theories, the proton has a half-life of about 1036 years, and decays into a positron and a neutral pion that itself immediately decays into 2 gamma ray photons.
In the Standard Model, protons, a type of baryon, are theoretically stable because baryon number (quark number) is conserved (under normal circumstances; however, see chiral anomaly). Therefore, protons will not decay into other particles on their own, because they are the lightest (and therefore least energetic) baryon.
Some beyond-the-Standard Model grand unified theories (GUTs) explicitly break the baryon number symmetry, allowing protons to decay via the Higgs particle, magnetic monopoles or new X bosons. Proton decay is one of the few unobserved effects of the various proposed GUTs. To date, all attempts to observe these events have failed.
∴ ∴ ∴
In physics, quasiparticles and collective excitations (which are closely related) are emergent phenomena that occur when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in free space. For example, as an electron travels through a semiconductor, its motion is disturbed in a complex way by its interactions with all of the other electrons and nuclei; however it approximately behaves like an electron with a different mass traveling unperturbed through free space. This "electron" with a different mass is called an "electron quasiparticle". In another example, the aggregate motion of electrons in the valence band of a semiconductor is the same as if the semiconductor contained instead positively charged quasiparticles called holes. Other quasiparticles or collective excitations include phonons (particles derived from the vibrations of atoms in a solid), plasmons (particles derived from plasma oscillations), and many others.
These particles are typically called "quasiparticles" if they are related to fermions (like electrons and holes), and called "collective excitations" if they are related to bosons (like phonons and plasmons), although the precise distinction is not universally agreed upon.