- Angular momentum: The angular momentum of an object rotating about some reference point is the measure of the extent to which the object will continue to rotate unless acted upon by an external torque.
- Anti-gravity: It is the idea of creating a place or object that is free from the force of gravity. It does not refer to countering the gravitational force by an opposing force of a different nature, as a helium balloon does; instead, anti-gravity requires that the fundamental causes of the force of gravity be made either not present or not applicable to the place or object through some kind of technological intervention.
- Atom laser: It is a coherent state of propagating atoms. They are created out of a Bose-Einstein condensate of atoms that are output coupled using various techniques. Much like an optical laser, an atom laser is a coherent beam that behaves like a wave.
- Black body: In physics it is an object that absorbs all electromagnetic radiation that falls onto it. No radiation passes through it and none is reflected. These properties make black bodies ideal sources of thermal radiation. That is, the amount and wavelength (colour) of electromagnetic radiation they emit is directly related to their temperature. Black bodies below around 700 K (430 °C) produce very little radiation at visible wavelengths and appear black (hence the name). Black bodies above this temperature however, produce radiation at visible wavelengths starting at red, going through orange, yellow, and white before ending up at blue as the temperature increases.
- Brane, or p-brane, Membrane: In theoretical physics it is a spatially extended, mathematical concept that appears in string theory and its relatives (M-theory and brane cosmology). The variable p refers to the spatial dimension of the brane. That is, a 0-brane is a zero-dimensional pointlike particle, a 1-brane is a string, a 2-brane is a "membrane", etc.
- Bose-Einstein condensate (BEC): it is a state of matter of bosons confined in an external potential and cooled to temperatures very near to absolute zero (0 K or -273.15°C). Under such supercooled conditions, a large fraction of the atoms collapse into the lowest quantum state of the external potential, at which point quantum effects become apparent on a macroscopic scale. The first gaseous condensate was produced by Eric Cornell and Carl Wieman in 1995 using a gas of rubidium atoms cooled to 170 nanokelvin.
- Bosonic string theory: It is the original version of string theory, developed in the late 1960s. Although it has many attractive features, it has a pair of features that render it unattractive as a physical model. Firstly it predicts only the existence of bosons whereas we know many physical particles are fermions. Secondly, it predicts the existence of a particle whose mass is imaginary implying that it travels faster than light. The existence of such a particle, commonly known as a tachyon, would conflict with much of what we know about physics, and such particles have never been observed. Moreover the theory displays inconsistencies due to the conformal anomaly. In a spacetime of 26 dimensions, however, with 25 dimensions of space and one of time, the inconsistencies cancel. Another way to look at this is that in general bosonic string theory predicts unphysical particle states called 'ghosts'. In 26 dimensions the no-ghost theorem predicts that these ghost states have no interaction whatsoever with any other states and hence that they can be ignored leaving a consistent theory. So bosonic string theory predicts a 26 dimensional spacetime. This high dimensionality isn't a problem for bosonic string theory because it can be formulated in such a way that along the 22 excess dimensions, spacetime is folded up to form a small torus. This would leave only the familiar four dimensions of spacetime visible.
- Broken symmetry: It happens when an object breaks either rotational symmetry or translational symmetry. That is, when one can only rotate an object in certain angles or when one is able to tell if the object has been shifted sideways. For example, imagine a jumping bean sitting atop a Mexican hat. It is in a rotationally symmetric state until it inevitably hops and falls down to a lower equilibrium, breaking the rotational symmetry.
- ?erenkov radiation (also spelled Cerenkov or Cherenkov): It is electromagnetic radiation emitted when a charged particle (such as a proton) passes through an insulator at a speed greater than the speed of light in that medium. The characteristic "blue glow" of nuclear reactors is due to ?erenkov radiation.
- D-branes or Dirichlet Branes (or Dirichlet membranes: They are a special class of p-branes. D-branes are typically classified by their dimension, which is indicated by a number written after the D. A D0-brane is a single point, a D1-brane is a line (sometimes called a "D-string"), a D2-brane is a plane, and a D25-brane fills the highest-dimensional space considered in bosonic string theory.
- Doppler effect: named after Christian Doppler, is the change in frequency and wavelength of a wave as perceived by an observer moving relative to the source of the waves. For waves that propagate in a wave medium, such as sound waves, the velocities of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler Effect may therefore result from motion of the source or motion of the observer or motion of the medium. Each of these effects is analysed separately. For waves which do not require a medium, such as light or gravity in special relativity, only the relative difference in velocity between the observer and the source needs to be considered.
- Doppler red shift: In physics and astronomy, redshift occurs when the electromagnetic radiation, usually visible light that is emitted from or reflected off an object is shifted towards the (less energetic) red end of the electromagnetic spectrum. Redshift is defined as an increase in the wavelength of electromagnetic radiation received by a detector compared with the wavelength emitted by the source. Conversely, a decrease in wavelength is called blue shift. Any increase in wavelength is called "redshift", even if it occurs in electromagnetic radiation of non-optical wavelengths, such as gamma rays, x-rays and ultraviolet. This nomenclature might be confusing since, at wavelengths longer than red (e.g., infrared, microwaves, and radio waves), redshifts shift the radiation away from the red wavelengths. An observed redshift due to the Doppler Effect occurs whenever a light source moves away from the observer, corresponding to the Doppler shift that changes the perceived frequency of sound waves.
- False vacuum: it is a metastable sector of a quantum field theory which appears to be a perturbative vacuum but is unstable to instant on effects which tunnel to a lower energy state. This tunnelling can be caused by quantum fluctuations or the creation of high energy particles. Simply put, the false vacuum is a state of a physical theory which is not the lowest energy state, but is nonetheless stable for some time.
- Gravitational constant: Denoted G, it is a physical constant involved
in the calculation of the gravitational attraction between objects with
mass. It appears in Newton's law of universal gravitation and in Einstein's
theory of general relativity. It should not be confused with "little
g" (g), which is the local gravitational field (equivalent to the local
acceleration due to gravity), especially that at the Earth's surface. According
to the law of universal gravitation, the attractive force (F) between two
bodies is proportional to the product of their masses (m1 and m2), and inversely
proportional to the square of the distance (r) between them:
- Gravitational lens: It is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a cluster of galaxies) between the source object and the observer. The process is known as gravitational lensing.
- Gravitational redshift: In physics, light or other forms of electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational field (and which could be said to have climbed "uphill" out of a gravity well) will be found to be of longer wavelength when received by an observer in a region of weaker gravitational field. If applied to optical wave-lengths this manifests itself as a change in the colour of the light as the wavelength is shifted toward the red (making it: less energetic, longer in wavelength, and lower in frequency) part of the spectrum. Light that has passed "downhill" into a region of stronger gravity shows a corresponding increase in energy, and is said to be gravitationally blueshifted.
- Gravitational redshift: In physics, light or other forms of electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational field (and which could be said to have climbed "uphill" out of a gravity well) will be found to be of longer wavelength when received by an observer in a region of weaker gravitational field. If applied to optical wave-lengths this manifests itself as a change in the colour of the light as the wavelength is shifted toward the red (making it: less energetic, longer in wavelength, and lower in frequency) part of the spectrum. Light that has passed "downhill" into a region of stronger gravity shows a corresponding increase in energy, and is said to be gravitationally blueshifted.
- Gravitational wave: It is a fluctuation in the curvature of space-time which propagates as a wave, travelling outward from a moving object or system of objects. Gravitational radiation is the energy transported by these waves. Important examples of systems which emit gravitational waves are binary star systems, where the two stars in the binary are white dwarfs, neutron stars, or black holes. Although gravitational radiation has not yet been directly detected, it has been indirectly shown to exist.
- Helicity: In particle physics it is the projection of the spin onto the
direction of momentum, :
- Heterotic string: It is a peculiar mixture (or hybrid) of the bosonic string and the. In string theory, the left-moving and the right-moving excitations almost do not talk to each other, and it is possible to construct a string theory whose left-moving (counter-clockwise) excitations "think" that they live on a bosonic string propagating in D = 26 dimensions, while the right-moving (clock-wise) excitations "think" that they belong to a superstring in D = 10 dimensions. The mismatched 16 dimensions must be compactified on an even, self-dual lattice (a discrete subgroup of a linear space).
- Hierarchy problem: It occurs when the fundamental parameters (couplings or masses) of some Lagrangian are vastly different (usually larger) than the parameters measured by experiment. This can happen because measured parameters are related to the fundamental parameters by a prescription known as renormalization. Typically the renormalized parameters are closely related to the fundamental parameters, but in some cases, it appears that there has been a delicate cancellation between the fundamental quantity and the quantum corrections to it. Hierarchy problems are related to fine-tuning problems and problems of naturalness.
- Hydrostatic equilibrium: Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. The balance of these two forces is known as the hydrostatic balance.
- Isolated system: In natural science, as contrasted with an open system, it is a physical system that does not interact with its surroundings. It obeys a number of conservation laws: its total energy and mass stay constant. They cannot enter or exit, but can only move around inside.
- Lorentz covariance: It is a property of space-time that follows from
the special theory of relativity, where it applies globally. Local Lorentz
covariance refers to Lorentz covariance applying only locally in an infinitesimal
region of space-time at every point, which follows from general relativity.
Lorentz covariance has two distinct, but closely related meanings.
1. A physical quantity is said to be Lorentz covariant if it transforms
under a given representation of the Lorentz group. According to the representation
theory of the Lorentz group, these quantities are built out of scalars,
four-vectors, four-tensors, and spinors. In particular, a scalar (e.g. the
space-time interval) remains the same under Lorentz transformations and
is said to be a Lorentz invariant
2. An equation is said to be Lorentz covariant if it can be written in terms
of Lorentz covariant quantities. The key properties of such equations are
that if they hold in one inertial frame, then they hold in any inertial
frame. This condition is a requirement according to the principle of relativity,
i.e. all non-gravitational laws must make the same predictions for identical
experiments taking place at the same space-time event in two different inertial
frames of reference.
- Lorentz transformation: converts between two different observers' measurements of space and time, where one observer is in constant motion with respect to the other. In classical physics (Galilean relativity), the only conversion necessary was x' = x ? vt, describing how the origin of one observer's coordinate system slides through space with respect to the other's, at speed v and along the x-axis of each frame. Special relativity shows that this is a good approximation at speeds lower than the speed of light. In homogeneous space the Lorentz transformation must be a linear transformation. Since relativity postulates that the speed of light is the same for all observers, it must preserve the space-time interval between any two events in Minkowski space. They form the mathematical basis for Albert Einstein's theory of special relativity. The Lorentz transformations remove contradictions between the theories of electromagnetism and classical mechanics. In 1905 Einstein derived them under the assumptions of Lorentz covariance and the constancy of the speed of light in any inertial reference frame.
- Many worlds quantum mechanics: The Many-worlds interpretation of Quantum mechanics predicts that the universe will not end this way -provided many worlds is true-. Each time a quantum event happens which causes the universe to decay from false vacuum to true Vacuum state the universe splits. In one or more new worlds the universe decays. In the other new world or worlds the universe continues as before.
- Orbits: In physics, an orbit is the path that an object makes around another object while under the influence of a source of centripetal force such as gravity.
- Plasma: In physics, it means an ionized gas where the protons in the atom are separate from the nucleus, it is the fourth state of matter apart from gases, because of its unique properties. Ionized refers to presence of one or more free electrons, which are not bound to an atom or molecule. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma takes the form of neutral gas-like clouds (e.g. stars) or charged ion beams, but may also include dust and grains (called dusty plasmas). They are formed by heating and ionizing a gas, stripping electrons away from atoms enabling the positive and negative charges to move more freely.
- Quantum (plural: quanta): It is an indivisible entity of a quantity that has the same units as the Planck constant and is related to both energy and momentum of elementary particles of matter (called fermions) and of photons and other bosons. Behind this, one finds the fundamental notion that a physical property may be "quantized", referred to as "quantization". This means that the magnitude can take on only certain discrete numerical values, rather than any value, at least within a range.
- Redshift: It occurs when the electromagnetic radiation, usually visible light that is emitted from or reflected off an object is shifted towards the (less energetic) red end of the electromagnetic spectrum. More generally, redshift is defined as an increase in the wavelength of electromagnetic radiation received by a detector compared with the wavelength emitted by the source. This increase in wavelength corresponds to a decrease in the frequency of the electromagnetic radiation. Conversely, a decrease in wavelength is called blue shift. Any increase in wavelength is called "redshift", even if it occurs in electromagnetic radiation of non-optical wavelengths, such as gamma rays, x-rays and ultraviolet.
- Scattering matrix (S-matrix): It relates the initial state and the final state for an interaction of particles. It is used in quantum mechanics, scattering theory and quantum field theory. More formally, the S-matrix is defined as the unitary matrix connecting asymptotic particle states in the Hilbert space of physical states (scattering channels). While the S-matrix may be defined for any background (space-time) that is asymptotically solvable and has no horizons, it has a simple form in the case of the Minkowski space. In this special case, the Hilbert space is a space of irreducible unitary representations of the inhomogeneous Lorentz group; the S-matrix is the evolution operator between time equal to minus infinity, and time equal to plus infinity.
- Soliton: It is a self-reinforcing solitary wave (a wave packet or pulse) that maintains its shape while it travels at constant speed; solitons are caused by a cancellation of nonlinear and dispersive effects in the medium. Solitons are found in many physical phenomena, as they arise as the solutions of a widespread class of weakly nonlinear dispersive partial differential equations describing physical systems.
- Speed of light in a vacuum: This is an important physical constant denoted by the letter c. It is the speed of all electromagnetic radiation, including visible light, in a vacuum. More generally, it is the speed of anything having zero rest mass. The speed of light is exactly 299,792,458 metres per second, 670,616,629.2 miles per hour or 983,571,056 feet per second, which is about 186,282.397 miles per second, or roughly one foot per nanosecond. The speed of light when it passes through a transparent or translucent material medium, like glass or air, is slower than its speed in a vacuum. The ratio of c to the observed phase velocity is called the refractive index of the medium. General relativity explains how a gravitational potential can affect the apparent speed of distant light in a vacuum, but locally light in a vacuum always passes an observer at a rate of c.
- Spin: may refer to:
- Rotation, rotating on an axis, spin around, spinning top, etc.
- Spin (physics), (or "particle spin") a fundamental property
of elementary particles.
- Stress-energy tensor or stress-energy-momentum tensor: It is a tensor quantity that describes the density and flux of energy and momentum in spacetime, generalizing the stress tensor of Newtonian physics. It is the source of the gravitational field in general relativity, just as mass is the source of such a field in Newtonian gravity. The stress-energy tensor has important applications, especially in the Einstein field equations.
- Sublimation (chemistry): This term describe the change from solid to gas, while at no point becoming a liquid.
- Superfluidity is a phase of matter or description of heat capacity in which "unusual" effects are observed when liquids, typically of helium-4 or helium-3, overcome friction by surface interaction when at a stage, known as the "lambda point" for helium-4, at which the liquid's viscosity becomes zero. Also known as a major facet in the study of quantum hydrodynamics.
- Supermembrane: With the development of M-theory, an extra dimension appeared and the fundamental string of string theory became a 2-dimensional membrane called an M2-brane (or Supermembrane). Its magnetical dual is an M5-brane. The various branes of string theory are thought to be related to these higher dimensional M5-branes wrapped on various cycles. The M-theory states that strings of energy could grow into larger membranes or branes up to even the size of the universe. It also goes on to state that these strings need to vibrate in more than three dimensions (six) plus another dimension - time.
- Ultraviolet catastrophe (also called the Rayleigh-Jeans catastrophe):
It was a prediction of early 20th century classical physics that an ideal
black body at thermal equilibrium will emit radiation with infinite power.
According to classical electromagnetism, the number of electromagnetic modes
in a 3-dimensional cavity, per unit frequency, is proportional to the square
of the frequency. This therefore implies that the radiated power per unit
frequency should be proportional to frequency squared. Thus, both the power
at a given frequency and the total radiated power approach infinity as higher
and higher frequencies are considered: this is clearly an impossibility.
Einstein pointed out that the difficulty could be avoided by Max Planck's
theory that electromagnetic energy did not follow the classical description,
but could only oscillate or be emitted in discrete packets of energy proportional
to the frequency. The radiated power eventually goes to zero at infinite
frequencies, and the total predicted power is finite.