Light A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) get separated Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is visible to the human eye and is responsible for the sense of sight. Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10−9 m, to 700 nanometres – between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths). Often, infrared and ultraviolet are also called light. The main source of light on Earth is the Sun. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. Electromagnetic spectrum and visible light The behaviour of EMR depends on its wavelength. Speed of light Optics Refraction where
Libertarianism (metaphysics) The term "libertarianism" in a metaphysical or philosophical sense was first used by late Enlightenment free-thinkers to refer to those who believed in free will, as opposed to determinism. The first recorded use was in 1789 by William Belsham in a discussion of free will and in opposition to "necessitarian" (or determinist) views. Metaphysical and philosophical contrasts between philosophies of necessity and libertarianism continued in the early 19th century. Explanations of libertarianism that do not involve dispensing with physicalism require physical indeterminism, such as probabilistic subatomic particle behavior – theory unknown to many of the early writers on free will. Physical determinism, under the assumption of physicalism, implies there is only one possible future and is therefore not compatible with libertarian free will. Nozick puts forward an indeterministic theory of free will in Philosophical Explanations.
4-manifold In mathematics, 4-manifold is a 4-dimensional topological manifold. A smooth 4-manifold is a 4-manifold with a smooth structure. In dimension four, in marked contrast with lower dimensions, topological and smooth manifolds are quite different. There exist some topological 4-manifolds which admit no smooth structure and even if there exists a smooth structure it need not be unique (i.e. there are smooth 4-manifolds which are homeomorphic but not diffeomorphic). 4-manifolds are of importance in physics because, in General Relativity, spacetime is modeled as a pseudo-Riemannian 4-manifold. Topological 4-manifolds Examples: Freedman's classification can be extended to some cases when the fundamental group is not too complicated; for example, when it is Z there is a classification similar to the one above using Hermitian forms over the group ring of Z. For any finitely presented group it is easy to construct a (smooth) compact 4-manifold with it as its fundamental group. See also
Wave–particle duality Origin of theory The idea of duality originated in a debate over the nature of light and matter that dates back to the 17th century, when Christiaan Huygens and Isaac Newton proposed competing theories of light: light was thought either to consist of waves (Huygens) or of particles (Newton). Through the work of Max Planck, Albert Einstein, Louis de Broglie, Arthur Compton, Niels Bohr, and many others, current scientific theory holds that all particles also have a wave nature (and vice versa). This phenomenon has been verified not only for elementary particles, but also for compound particles like atoms and even molecules. For macroscopic particles, because of their extremely short wavelengths, wave properties usually cannot be detected. Brief history of wave and particle viewpoints Thomas Young's sketch of two-slit diffraction of waves, 1803 Particle impacts make visible the interference pattern of waves. A quantum particle is represented by a wave packet.
Ultimate fate of the universe The ultimate fate of the universe is a topic in physical cosmology. Many possible fates are predicted by rival scientific theories, including futures of both finite and infinite duration. Once the notion that the universe started with a rapid inflation nicknamed the Big Bang became accepted by the majority of scientists, the ultimate fate of the universe became a valid cosmological question, one depending upon the physical properties of the mass/energy in the universe, its average density, and the rate of expansion. There is a growing consensus among cosmologists that the universe is flat and will continue to expand forever. The ultimate fate of the universe is dependent on the shape of the universe and what role dark energy will play as the universe ages. Emerging scientific basis Theory The theoretical scientific exploration of the ultimate fate of the universe became possible with Albert Einstein's 1916 theory of general relativity. Observation Big Rip
Quaternion Graphical representation of quaternion units product as 90°-rotation in 4D-space, ij = k, ji = −k, ij = −ji History Quaternion plaque on Brougham (Broom) Bridge, Dublin, which says: Here as he walked by on the 16th of October 1843 Sir William Rowan Hamilton in a flash of genius discovered the fundamental formula for quaternion multiplicationi2 = j2 = k2 = ijk = −1 & cut it on a stone of this bridge Quaternion algebra was introduced by Hamilton in 1843. Important precursors to this work included Euler's four-square identity (1748) and Olinde Rodrigues' parameterization of general rotations by four parameters (1840), but neither of these writers treated the four-parameter rotations as an algebra. Carl Friedrich Gauss had also discovered quaternions in 1819, but this work was not published until 1900. i2 = j2 = k2 = ijk = −1, into the stone of Brougham Bridge as he paused on it. On the following day, Hamilton wrote a letter to his friend and fellow mathematician, John T.
Photon Nomenclature In 1900, Max Planck was working on black-body radiation and suggested that the energy in electromagnetic waves could only be released in "packets" of energy. In his 1901 article  in Annalen der Physik he called these packets "energy elements". Physical properties The cone shows possible values of wave 4-vector of a photon. A photon is massless,[Note 2] has no electric charge, and is stable. Photons are emitted in many natural processes. The energy and momentum of a photon depend only on its frequency (ν) or inversely, its wavelength (λ): where k is the wave vector (where the wave number k = |k| = 2π/λ), ω = 2πν is the angular frequency, and ħ = h/2π is the reduced Planck constant. Since p points in the direction of the photon's propagation, the magnitude of the momentum is The classical formulae for the energy and momentum of electromagnetic radiation can be re-expressed in terms of photon events. Experimental checks on photon mass
Philosophy of physics Centuries ago, the study of causality, and of the fundamental nature of space, time, matter, and the universe were part of metaphysics. Today the philosophy of physics is essentially a part of the philosophy of science. Physicists utilize the scientific method to delineate the universals and constants governing physical phenomena, and the philosophy of physics reflects on the results of this empirical research. Purpose of physics According to Niels Bohr, the purpose of physics is: not to disclose the real essence of phenomena but only to trackdown (...) relations between the manifold aspects of experience. Many, particularly realists, find this minimal formulation an inadequate formulation of the purpose of physics, which they view as providing, in addition, a deeper world picture. Philosophy of space and time Time Time, in many philosophies, is seen as change. Time travel A second, similar type of time travel is permitted by general relativity. Space Elsewhere:
Convex regular polychoron The tesseract is one of 6 convex regular polychora In mathematics, a convex regular polychoron is a polychoron (4-polytope) that is both regular and convex. These are the four-dimensional analogs of the Platonic solids (in three dimensions) and the regular polygons (in two dimensions). These polychora were first described by the Swiss mathematician Ludwig Schläfli in the mid-19th century. Schläfli discovered that there are precisely six such figures. Five of these may be thought of as higher-dimensional analogs of the Platonic solids. Properties Since the boundaries of each of these figures is topologically equivalent to a 3-sphere, whose Euler characteristic is zero, we have the 4-dimensional analog of Euler's polyhedral formula: where Nk denotes the number of k-faces in the polytope (a vertex is a 0-face, an edge is a 1-face, etc.). Visualizations The following table shows some 2-dimensional projections of these polychora. See also References External links
Thermal radiation This diagram shows how the peak wavelength and total radiated amount vary with temperature according to Wien's displacement law. Although this plot shows relatively high temperatures, the same relationships hold true for any temperature down to absolute zero. Visible light is between 380 and 750 nm. Thermal radiation in visible light can be seen on this hot metalwork. Its emission in the infrared is invisible to the human eye and the camera the image was taken with, but an infrared camera could show it (See Thermography). Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. Examples of thermal radiation include the visible light and infrared light emitted by an incandescent light bulb, the infrared radiation emitted by animals and detectable with an infrared camera, and the cosmic microwave background radiation. Thermal radiation is one of the fundamental mechanisms of heat transfer. Overview Surface effects Here,
Determinism Determinism is the philosophical position that for every event, including human action, there exist conditions that could cause no other event. "There are many determinisms, depending upon what pre-conditions are considered to be determinative of an event." Deterministic theories throughout the history of philosophy have sprung from diverse and sometimes overlapping motives and considerations. Other debates often concern the scope of determined systems, with some maintaining that the entire universe is a single determinate system and others identifying other more limited determinate systems (or multiverse). Varieties Below appear some of the more common viewpoints meant by, or confused with "determinism". Many philosophical theories of determinism frame themselves with the idea that reality follows a sort of predetermined path Philosophical connections With nature/nurture controversy Nature and nurture interact in humans. With particular factors With the soul