The Quantum Theory is based on the following concepts:
- Light, radiations, energy, forces, ... occur in discreet packets called
Quanta. The quanta of light is the photon; the quanta of the weak force
are the W- and Z- bosons; the quanta or the strong nuclear force is called
gluon; and the quanta for gravity is the yet undiscovered graviton.
- Matter is represented by point particles, but thee probability to find
a particle is given by a wave.
- Before it is observed an object exists in all possible states and places
simultaneously. By observing the object, the wave function collapses and
the object and the wave function go into a definite state and the object
assumes a definite reality.
The highest formulation of the Quantum Theory is the Standard Model.
- Quantum theory may mean:
- Old quantum theory under the Bohr model.
- Quantum mechanics, an umbrella term sometimes for all of quantum physics,
but sometimes for just non-relativistic theories.
- Quantum field theory, a generic type of relativistic quantum theory, which
includes:
.Quantum electrodynamics
.Quantum chromodynamics
.Electroweak interaction
- Quantum gravity, a general term for theories intended to quantize general
relativity
- Quantum optics
- Quantum chemistry
. Old quantum theory (Bohr model): In atomic physics depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus - similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. The Bohr model is a primitive model of the hydrogen atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory. However, because of its simplicity, and its correct results for selected systems, the Bohr model is still commonly taught to introduce students to quantum mechanics.
. Quantum mechanics: In physics is the study of the relationship between energy quanta (radiation) and matter, in particular that between valence shell electrons and photons. Quantum mechanics is a fundamental branch of physics with wide applications in both experimental and theoretical physics. The effects of quantum mechanics are typically not observable on macroscopic scales, but become evident at the atomic and subatomic level. Quantum theory generalizes all classical theories, including mechanics and electromagnetism (except general relativity), and provides accurate descriptions for many previously unexplained phenomena such as black body radiation and stable electron orbits.
. Quantum field theory (QFT) is a theoretical framework for constructing quantum mechanical models of field-like systems or, equivalently, of many-body systems. It is widely used in particle physics and condensed matter physics. Most theories in modern particle physics, including the Standard Model of elementary particles and their interactions, are formulated as relativistic quantum field theories. In condensed matter physics, quantum field theories are used in many circumstances, especially those where the number of particles is allowed to fluctuate.
. Quantum electrodynamics (QED) is a relativistic quantum field theory of electrodynamics. QED mathematically describes all phenomena involving electrically charged particles interacting by means of exchange of photons.
. Quantum chromodynamics (abbreviated as QCD) is a theory of the strong
interaction, a fundamental force describing the interactions of the quarks
and gluons found in hadrons (such as the proton, neutron or pion).
QCD enjoys two peculiar properties:
Asymptotic freedom, which means that in very high-energy reactions, quarks
and gluons interact very weakly.
Confinement, which means that the force between quarks does not diminish
as they are separated. Because of this, it would take an infinite amount
of energy to separate two quarks; they are forever bound into hadrons. Although
analytically unproven, confinement is widely believed to be true because
it explains the consistent failure of free quark searches.
. Electroweak interaction is the unified description of two of the four fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very different at everyday low energies, the theory models them as two different aspects of the same force. Above the unification energy, on the order of 102 GeV, they would merge into a single electroweak force. Thus if the universe is hot enough (approximately, At the temperature of 1015 K reached shortly after the Big Bang the electromagnetic force and weak force will merge into a combined electroweak force.
. Quantum gravity is the field of theoretical physics attempting to unify quantum mechanics, which describes three of the fundamental forces of nature, with general relativity, it is the theory of the fourth fundamental force: gravity. One ultimate goal hoped to emerge as a result of this is a unified framework for all fundamental forces -called a "theory of everything" (TOE) ", or "Grand Unified Theory" (GUT).
. Quantum optics is a field of research in physics, dealing with the application of quantum mechanics to phenomena involving light and its interactions with matter. Light is made up of particles called photons and hence inherently is "grainy" (quantized); quantum optics is the study of the nature and effects of this. The understanding of the interaction between light and matter following not only formed the basis of quantum optics but also were crucial for the development of quantum mechanics as a whole.
. Quantum chemistry: This is a branch of theoretical chemistry, which applies quantum mechanics and quantum field theory to address issues and problems in chemistry. The description of the electronic behaviour of atoms and molecules as pertaining to their reactivity is one of the applications of quantum chemistry. Quantum chemistry lies on the border between chemistry and physics, and significant contributions have been made by scientists from both fields. It has a strong and active overlap with the field of atomic physics and molecular physics, as well as physical chemistry.
- Bell's theorem: It is a theory that shows that the predictions of quantum
mechanics are not intuitive, and touches upon fundamental philosophical
issues that relate to modern physics. It is the most famous legacy of the
late physicist John S. Bell. Bell's theorem states:
" No physical theory of local hidden variables can ever reproduce all
of the predictions of quantum mechanics. "
- Bra-ket notation or Dirac's Bra-ket Notation: It is the standard notation for describing quantum states in the theory of quantum mechanics. It can also be used to denote abstract vectors and linear functionals in pure mathematics. It is so called because the inner product (or dot product) of two states is denoted by a bracket, , consisting of a left part, , called the bra, and a right part, , called the ket.
- Causal dynamical triangulation: It is an approach to quantum gravity that like loop quantum gravity is background independent. This means that it does not assume any pre-existing arena (dimensional space), but rather attempts to show how the spacetime fabric itself evolves. CDT is a pivotal insight for theorists. It has sparked considerable interest as it appears to have a good semi-classical description. At large scales, it re-creates the familiar 4-dimensional spacetime, but it shows spacetime to be 2-d near the Planck scale, and reveals a fractal structure on slices of constant time.
These interesting results agree with the findings of Lauscher and Reuter, who use an approach called Quantum Einstein Gravity, and with other recent theoretical work
- Dirac sea (Negative energy): It is a theoretical model of the vacuum
as an infinite sea of particles possessing negative energy. It explains
the anomalous negative-energy quantum states predicted by the Dirac equation
for relativistic electrons. The positron, the antimatter counterpart of
the electron, was originally conceived of as a hole in the Dirac sea.
The equation relating energy, mass and momentum in special relativity is:
E2 = p2c2 + m2c4,
In the special case of a particle at rest (i.e. p = 0), the above equation
reduces to E2 = m2c4, which is usually quoted as the familiar E = mc2. However,
this is a simplification because, while x * x = x2, we can also see that
( ? x) * ( ? x) = x2. Therefore, the correct equation to use to relate energy
and mass in the Hamiltonian of the Dirac equation is:
Here the negative solution is antimatter, discovered as the positron.
- Electron degeneracy pressure: It is a consequence of the Pauli Exclusion Principle, which states that two fermions cannot occupy the same quantum state at the same time. The force provided by this pressure sets a limit on how much matter can be squeezed together without it collapsing into a black hole. It is an important factor in stellar physics because it is responsible for the existence of white dwarfs. A material subjected to ever increasing pressure will become ever more compressed, and for electrons within it, the uncertainty in position measurements, ?x, becomes ever smaller. Thus, as dictated by the uncertainty principle, the uncertainty in the momenta of the electrons, ?p, becomes larger. Thus, no matter how low the temperature drops, the electrons must be travelling at this "Heisenberg speed," contributing to the pressure. When the pressure due to the "Heisenberg speed" exceeds that of the pressure from the thermal motions of the electrons, the electrons are referred to as degenerate, and the material is termed degenerate matter. Electron degeneracy pressure will halt the gravitational collapse of a star if its mass is below the Chandrasekhar Limit (1.38 solar masses). This is the pressure that prevents a white dwarf star from collapsing. A star exceeding this limit and without usable nuclear fuel will continue to collapse to form a neutron star or black hole, because the degeneracy pressure provided by the electrons is weaker than the inward pull of gravity.
- 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.
- Feynman diagram: It is a tool invented by American physicist Richard Feynman for performing scattering calculations in quantum field theory. Particles are represented by lines, which can be drawn in various ways depending on the type of particle being depicted. A point where lines connect to other lines is called an interaction vertex, or vertex. Lines fall into three categories: internal lines (which connect two vertices), incoming lines (which extend from "the past" to a vertex and represent the initial non-interacting state) and outgoing lines (which extend from a vertex to "the future" and represent the final non-interacting state). Most commonly the bottom of the diagram represents the past and the top of the diagram represents the future.
- Hamiltonian constraint: In loop quantum gravity, dynamics such as time-evolutions of fields are controlled by the Hamiltonian constraint. The identity of the Hamiltonian constraint is a major open question in quantum gravity, as is extracting of physical observables from any such specific constraint.
- Imaginary time: It is a concept derived from quantum mechanics and is essential in connecting quantum mechanics with statistical mechanics. Imaginary time is obtained from real time via a Wick rotation by : . Another way to see the connection between statistical mechanics and quantum field is to consider the transition amplitude between an initial state I and a final state F. H is the Hamiltonian of the system. Finally by using a Wick rotation one can show that the Euclidean quantum field theory in (D + 1)-dimensional spacetime is nothing but quantum statistical mechanics in D-dimensional space.
- Lamb shift: It is a small difference in energy between two energy levels 2S1 / 2 and 2P1 / 2 of the hydrogen atom in quantum mechanics. According to Dirac and Schrödinger theory, hydrogen states with the same n and j quantum numbers but different l quantum numbers ought to be degenerate.
- Loop quantum cosmology (LQC): It is a finite, symmetry reduced model of loop quantum gravity. Study of LQC has led to many successes, including the emergence of a possible mechanism for cosmic inflation, resolution of gravitational singularities as well as development of effective, semi-classical Hamiltonians.
- Loop quantum gravity (LQG), also known as loop gravity- and quantum geometry, is a proposed quantum theory of space-time which attempts to reconcile the seemingly incompatible theories of quantum mechanics and general relativity. LQG tries to establish a quantum theory of gravity in which the very space in which all other physics occurs becomes quantized. Loop quantum gravity (LQG) preserves many of the important features of general relativity, while at the same time employing quantization of both space and time at the Planck scale in the tradition of quantum mechanics.
- Matrix mechanics: It is a formulation of quantum mechanics. Matrix mechanics was the first complete and correct definition of quantum mechanics. It extended the Bohr Model by describing how the quantum jumps occur. It did so by interpreting the physical properties of particles as matrices which evolve in time. It is equivalent to the Schrödinger wave formulation of quantum mechanics, and is the basis of Dirac's bra-ket notation for the wave function.
- Minimal Supersymmetric Standard Model (MSSM) is the minimal extension to the Standard Model that realizes supersymmetry, although non-minimal extensions do exist. Supersymmetry pairs bosons with fermions, therefore every Standard Model particle has a partner that has yet to be discovered. If the superparticles are found, it is analogous to discovering antimatter and depending on the details of what is found, it could provide evidence for grand unification and might even in principle provide hints as to how string theory describes nature.
- Path integral Theory or formulation of quantum mechanics is a description of quantum theory which generalizes the action principle of classical mechanics. It replaces the classical notion of a single, unique history for a system with a sum, or functional integral, over an infinity of possible histories to compute a quantum amplitude. This formulation provided the basis for the renormalization group which unified quantum field theory with statistical mechanics.
- Probability amplitude, in quantum mechanics, is a complex-valued function that describes an uncertain or unknown quantity. For example, each particle has a probability amplitude describing its position. This amplitude is the wave function, expressed as a function of position. The wave function is a complex-valued function of a continuous variable.
- Quantum decoherence, in quantum mechanics, is the mechanism by which quantum systems interact with their environments to exhibit probabilistically additive behaviour -a feature of classical physics- and give the appearance of wave function collapse. Decoherence occurs when a system interacts with its environment, or any complex external system, in such a thermodynamically irreversible way that ensures different elements in the quantum superposition of the system-environment's wavefunction can no longer interfere with each other. Decoherence does not provide a mechanism for an actual wave function collapse; rather it provides a mechanism for the appearance of wavefunction collapse. The quantum nature of the system is simply "leaked" into the environment so that a total superposition of the wavefunction still exists, but exists beyond the realm of measurement.
- Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated. This leads to correlations between observable physical properties of the systems. For example, it is possible to prepare two particles in a single quantum state such that when one is observed to be spin-up, the other one will always be observed to be spin-down and vice versa, this despite the fact that it is impossible to predict, according to quantum mechanics, which set of measurements will be observed. As a result, measurements performed on one system seem to be instantaneously influencing other systems entangled with it. But quantum entanglement does not enable the transmission of classical information faster than the speed of light.
- Quantum field theory (QFT): It is a theoretical framework for constructing quantum mechanical models of field-like systems or, equivalently, of many-body systems. It is widely used in particle physics and condensed matter physics. Most theories in modern particle physics, including the Standard Model of elementary particles and their interactions, are formulated as relativistic quantum field theories.
- Quantum fluctuation, in quantum physics, is the temporary change in the
amount of energy in a point in space, arising from Werner Heisenberg's uncertainty
principle. A formulation of the principle, energy and time can be related
by the relation
that means that conservation of energy can appear to be violated, but only
for small times. This allows the creation of particle-antiparticle pairs
of virtual particles. Quantum fluctuations may have been very important
in the origin of the structure of the universe: according to the model of
inflation the ones that existed when inflation began were amplified and
formed the seed of all current observed structure.
- Quantum Hall effect (or integer quantum Hall effect) is a quantum-mechanical
version of the Hall Effect, observed in two-dimensional electron systems
subjected to low temperatures and strong magnetic fields, in which the Hall
conductivity ? takes on the quantized values
where e is the elementary charge and h is Planck's constant. In the "ordinary"
quantum Hall effect, known as the integer quantum Hall effect, ? takes on
integer values ( ? = 1, 2, 3, etc.). There is another type of quantum Hall
effect, known as the fractional quantum Hall effect, in which ? can occur
as a fraction ( ? = 2/7, 1/3, 2/5, 3/5, 5/2 etc.)
- Quantum numbers describe values of conserved numbers in the dynamics of the quantum system. They often describe specifically the energies of electrons in atoms, but other possibilities include angular momentum, spin etc. Since any quantum system can have one or more quantum numbers, it is a futile job to list all possible quantum numbers
- Quantum teleportation, or entanglement-assisted teleportation: It is a technique that transfers a quantum state to an arbitrarily distant location using a distributed entangled state and the transmission of some classical information. Quantum teleportation does not transport energy or matter, nor does it allow communication of information at superluminal (faster than light) speed, but is useful to quantum communication and computation.
- Renormalization: It refers to a collection of techniques used to take a continuum limit. When describing space and time as a continuum, certain statistical and quantum mechanical constructions are ill defined. In order to define them, the continuum limit has to be taken carefully. Renormalization determines the relationship between parameters in the theory, when the parameters describing large distance scales differ from the parameters describing small distances. Renormalization was first developed in quantum electrodynamics to make sense of infinite integrals in perturbation theory.
- Schrödinger equation: An equation proposed by Schrödinger in 1926 to describe the space- and time-dependence of quantum mechanical systems. It is of central importance in non-relativistic quantum mechanics, playing a role for microscopic particles analogous to Newton's second law in classical mechanics for macroscopic particles.
- Standard Model: is a grouping of two theories -quantum electroweak and quantum chromodynamics -which, together, provides an internally consistent theory describing the electromagnetic, weak nuclear, and strong nuclear interactions between all experimentally observed particles. It is a quantum field theory based on the gauge group SU(3) SU(2) U(1). All particles found in nature are accounted for by the Standard Model, but, as of yet, it additionally demands the existence of one unobserved particle known as the Higgs Boson, required to dynamically break an internal symmetry of nature.
- Vacuum state (also called the vacuum) is the quantum state with the lowest possible energy. By definition, it contains no physical particles. The term "zero-point field" is sometimes used as a synonym for the vacuum state of an individual quantized field. According to present-day understanding of the vacuum state or the quantum vacuum, it is "by no means a simple empty space" and again: "it is a mistake to think of any physical vacuum as some absolutely empty void." According to quantum mechanics, the vacuum state is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence.
- Wave-particle duality: It is the concept that all matter and energy exhibits
both wave-like and particle-like properties. A central concept of quantum
mechanics, duality addresses the inadequacy of classical concepts like "particle"
and "wave" in fully describing the behaviour of small-scale objects.