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{{Quantum field theory}}
'''Spontaneous symmetry breaking'''<ref>''Dynamical Symmetry Breaking in Quantum Field Theories''. By [[Vladimir A. Miranskij]]. Pg [http://books.google.com/books?id=UOpY9kmL3YQC&pg=PA15 15].</ref><ref>Patterns of Symmetry Breaking. Edited by Henryk Arodz, Jacek Dziarmaga, Wojciech Hubert Zurek. Pg  [http://books.google.com/books?id=uF1P7cFFsNMC&pg=PA141 141].</ref><ref>Bubbles, Voids and Bumps in Time: The New Cosmology. Edited by James Cornell. Pg [http://books.google.com/books?id=vRIs8mY1yZcC&pg=PA125 125].</ref> is a mode of realization of [[symmetry breaking]] in a physical system, where the underlying laws are [[Invariant (physics) | invariant]] under a [[Symmetry (physics)|symmetry]] transformation, but the system as a whole changes under such transformations, in contrast to [[explicit symmetry breaking]]. It is a [[spontaneous process]] by which a system in a [[Symmetry (physics)|symmetrical]] state ends up in an asymmetrical state. It thus describes systems where the equations of motion or the Lagrangian obey certain symmetries, but the lowest energy solutions do not exhibit that [[Symmetry (physics)|symmetry]].
 
Consider the bottom of an empty wine bottle, a symmetrical upward dome with a trough for sediment. If a ball is put in a particular position at the peak of the dome, the circumstances are symmetrical with respect to rotating the wine bottle. But the ball may ''spontaneously break'' this symmetry and move into the trough, a point of lowest energy. The bottle and the ball continue to have symmetry, but the system does not.<ref>Gerald M. Edelman, Bright Air, Brilliant Fire: On the Matter of the Mind (New York: BasicBooks, 1992) [http://books.google.com/books?id=NGIIGZmarOAC&pg=PA203 203].</ref>
 
Most simple phases of matter and phase-transitions, like crystals, magnets, and conventional superconductors can be simply understood from the viewpoint of spontaneous symmetry breaking. Notable exceptions include topological phases of matter like the [[fractional quantum Hall effect]].
 
==Spontaneous symmetry breaking in physics==
[[File:Spontaneous symmetry breaking (explanatory diagram).png|thumb|right|250px|'''''Spontaneous symmetry breaking simplified''''': - At high energy levels ''(left)'' the ball settles in the center, and the result is symmetrical. At lower energy levels ''(right)'', the overall "rules" remain symmetrical, but the "[[Mexican hat]]" potential comes into effect: [[local property|"local" symmetry]] is inevitably broken since eventually the ball must roll one way (at random) and not another.]]
 
===Particle physics===
In [[particle physics]] the [[force carrier]] particles are normally specified by field equations with [[gauge symmetry]]; their equations predict that certain measurements will be the same at any point in the field. For instance, field equations might predict that the mass of two quarks is constant. Solving the equations to find the mass of each quark might give two solutions. In one solution, quark A is heavier than quark B. In the second solution, quark B is heavier than quark A ''by the same amount''. The symmetry of the equations is not reflected by the individual solutions, but it is reflected by the range of solutions. An actual measurement reflects only one solution, representing a breakdown in the symmetry of the underlying theory. "Hidden" is perhaps a better term than "broken" because the symmetry is always there in these equations. This phenomenon is called ''spontaneous'' symmetry breaking because ''nothing'' (that we know) breaks the symmetry in the equations.<ref name="Weinberg2011">{{cite book|author=Steven Weinberg|title=Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature|url=http://books.google.com/books?id=Rsg3PE_9_ccC|date=20 April 2011|publisher=Knopf Doubleday Publishing Group|isbn=978-0-307-78786-6}}</ref>{{rp|194-195}}
 
====Chiral symmetry====
{{Main|Chiral symmetry breaking}}
 
Chiral symmetry breaking is an example of spontaneous symmetry breaking affecting the [[chiral symmetry]] of the [[strong interactions]] in particle physics. It is a property of [[quantum chromodynamics]], the [[quantum field theory]] describing these interactions, and is responsible for the bulk of the mass (over 99%) of the [[nucleons]], and thus of all common matter, as it converts very light bound [[quarks]] into 100 times heavier constituents of [[baryons]]. The approximate [[Nambu–Goldstone boson]]s in this spontaneous symmetry breaking process are the [[pions]], whose mass is an order of magnitude lighter than the mass of the nucleons. It served as the prototype and significant ingredient of the Higgs mechanism underlying the electroweak symmetry breaking.
 
====Higgs mechanism====
{{Main|Brout-Englert-Higgs mechanism|Yukawa interaction}}
 
The strong, weak, and electromagnetic forces can all be understood as arising from [[gauge symmetry|gauge symmetries]]. The [[Higgs mechanism]], the spontaneous-symmetry breaking of gauge symmetries, is an important component in understanding the [[superconductivity]] of metals and the origin of particle masses in the standard model of particle-physics. One important consequence of the distinction between true symmetries and ''gauge symmetries'', is that the spontaneous breaking of a gauge symmetry does not give rise to characteristic massless Nambu-Goldstone modes, but only massive modes, like the plasma-mode in a superconductor, or the Higgs mode observed in particle physics.
 
In the standard model of particle physics, spontaneous symmetry breaking of the SU(2)×U(1) gauge-symmetry associated with the electro-weak force generates masses for several particles, and separates the electromagnetic and weak forces. The [[W and Z bosons]] are the elementary particles that mediate the [[weak interaction]], while the [[photon]] mediates the [[electromagnetic interaction]]. At energies much greater than 100 GeV all these particles behave in a similar manner. The [[Unified_field_theory#Modern_progress|Weinberg–Salam theory]] predicts that, at lower energies, this symmetry is broken so that the photon and the massive ''W'' and ''Z'' bosons emerge.<ref>A Brief History of Time, Stephen Hawking, Bantam; 10th anniversary edition (September 1, 1998). Page 73-74.</ref> In addition, fermions develop mass consistently.
 
Without spontaneous symmetry breaking, the [[Standard Model]] of elementary particle interactions requires the existence of a number of particles. However, some particles (the [[W and Z bosons]]) would then be predicted to be massless, when, in reality, they are observed to have mass. To overcome this, spontaneous symmetry breaking is augmented by the [[Higgs mechanism]] to give these particles mass. It also suggests the presence of a new particle, the [[Higgs boson]], reported as possibly identifiable with a boson detected in 2012. (If the Higgs boson were not confirmed to have been found, it would mean that the simplest implementation of the Higgs mechanism and spontaneous symmetry breaking ''as they are currently formulated'' require modification.)
 
[[Superconductivity]] of metals is a condensed-matter analog of the Higgs phenomena, in which a condensate of Cooper pairs of electrons spontaneously breaks the U(1) gauge "symmetry" associated with light and electromagnetism.
 
===Condensed matter physics===
Most phases of matter can be understood through the lens of spontaneous symmetry-breaking. For example, crystals are periodic arrays of atoms that are not invariant under all translations (only under a small sub-set of translations by a lattice vector). Magnets have north and south poles that are oriented in a specific direction, breaking [[rotational symmetry]]. In addition to these examples, there are a whole host of other symmetry-breaking phases of matter including nematic phases of liquid crystals, charge- and spin- density waves, superfluids and many others.
 
There are several known examples of matter that cannot be described by spontaneous symmetry breaking, including: topologically ordered phases of matter like fractional quantum Hall liquids, and spin-liquids. These states do not break any symmetry, but are distinct phases of matter. Unlike the case of spontaneous symmetry breaking, there is not a general framework for describing such states.
 
====Continuous symmetry====
The ferromagnet is the canonical system which spontaneously breaks the continuous symmetry of the spins below the [[Curie temperature]] and at <math>h=0</math>, where <math>h</math> is the external magnetic field. Below the [[Curie temperature]] the energy of the system is invariant under inversion of the magnetization <math>m(\mathbf{x})</math> such that <math>m(\mathbf{x})=-m(-\mathbf{x})</math>. The symmetry is spontaneously broken as <math>h\rightarrow 0</math> when the Hamiltonian becomes invariant under the inversion transformation, but the expectation value is not invariant.
 
Spontaneously, symmetry broken phases of matter are characterized by an order-parameter that describes the quantity which breaks the symmetry under consideration. For example, in a magnet, the order parameter is the local magnetization.
 
Spontaneously breaking of a continuous symmetry is inevitably accompanied by gapless (meaning that these modes do not cost any energy to excite) Nambu-Goldstone modes associated with slow long-wavelength fluctuations of the order-parameter. For example, vibrational modes in a crystal, known as phonons, are associated with slow density fluctuations of the crystal's atoms. The associated Goldstone mode for magnets are oscillating waves of spin known as spin-waves. For symmetry-breaking  states, whose order parameter is not a conserved quantity, Nambu-Goldstone modes are typically massless and propagate at a constant velocity.
 
An important theorem, due to Mermin and Wagner, states that, at finite temperature, thermally activated fluctuations of Nambu-Goldstone modes destroy the long-range order, and prevent spontaneous symmetry breaking in one- and two-dimensional systems. Similarly, quantum fluctuations of the order parameter prevent most types of continuous symmetry breaking in one-dimensional systems even at zero-temperature (an important exception is ferromagnets, whose order parameter, magnetization, is an exactly conserved quantitity and does not have any quantum fluctuations).
 
Other long-range interacting systems such as cylindrical curved surfaces interacting via the [[Coulomb potential]] or [[Yukawa potential]] has been shown to break translational and rotational symmetries.<ref>
{{cite journal
|last=Kohlstedt |first=K.L.
|last2=Vernizzi |first2=G.
|last3=Solis |first3=F.J.
|last4=Olvera de la Cruz |first4=M.
|year=2007
|title=Spontaneous Chirality via Long-range Electrostatic Forces
|journal=[[Physical Review Letters]]
|volume=99 |pages=030602
|doi=10.1103/PhysRevLett.99.030602
|arxiv = 0704.3435 |bibcode = 2007PhRvL..99c0602K }}</ref> It was shown, in the presence of a symmetric Hamiltonian, and in the limit of infinite volume, the system spontaneously adopts a chiral configuration, i.e. breaks [[mirror plane]] [[symmetry]].
 
==Generalisation and technical usage==
For spontaneous symmetry breaking to occur, there must be a system in which there are several equally likely outcomes. The system as a whole is therefore [[Symmetry (physics)|symmetric]] with respect to these outcomes. (If we consider any two outcomes, the probability is the same. This contrasts sharply to [[explicit symmetry breaking]].) However, if the system is sampled (i.e. if the system is actually used or interacted with in any way), a specific outcome must occur. Though the system as a whole is symmetric, it is never encountered with this symmetry, but only in one specific asymmetric state. Hence, the symmetry is said to be spontaneously broken in that theory. Nevertheless, the fact that each outcome is equally likely is a reflection of the underlying symmetry, which is thus often dubbed "hidden symmetry", and has crucial formal consequences. (See the article on the [[Nambu–Goldstone boson|Goldstone boson]]).
 
When a theory is symmetric with respect to a [[symmetry group]], but requires that one element of the group be distinct, then spontaneous symmetry breaking has occurred. The theory must not dictate ''which'' member is distinct, only that ''one is''. From this point on, the theory can be treated as if this element actually is distinct, with the proviso that any results found in this way must be resymmetrized, by taking the average of each of the elements of the group being the distinct one.
 
The crucial concept in physics theories is the [[order parameter]]. If there is a field (often a background field) which acquires an expectation value (not necessarily a [[vacuum expectation value|''vacuum'' expectation value]]) which is not invariant under the symmetry in question, we say that the system is in the [[ordered phase]], and the symmetry is spontaneously broken. This is because other subsystems interact with the order parameter which forms a "frame of reference" to be measured against, so to speak. In that case, the [[vacuum state]] does not obey the initial symmetry (which would put it in the '''[[Wigner mode]]'''), and, instead has the (hidden) symmetry implemented in the '''Nambu–Goldstone mode'''. Normally, in the absence of the Higgs mechanism, massless [[Goldstone boson]]s arise.
 
The symmetry group can be discrete, such as the [[space group]] of a crystal, or continuous (e.g., a [[Lie group]]), such as the rotational symmetry of space. However, if the system contains only a single spatial dimension, then only discrete symmetries may be broken in a [[vacuum state]] of the full [[Quantum mechanics|quantum theory]], although a classical solution may break a continuous symmetry.
 
==A pedagogical example: the Mexican hat potential==
[[Image:Mexican hat potential polar.svg|270px|thumb|Graph of Goldstone's "Mexican hat" potential function ''V'' versus ''φ''.]]
 
In the simplest idealized relativistic model, the spontaneously broken symmetry is summarized through an illustrative [[scalar field theory]]. The relevant [[Lagrangian]], which essentially dictates how a system behaves, can be split up into kinetic and potential terms,
:{{NumBlk|:|<math>\mathcal{L} = \partial^\mu \phi \partial_\mu \phi - V(\phi).</math>|{{EquationRef|1}}}}
 
It is in this potential term ''V''(''Φ'') that the symmetry breaking is triggered. An example of a potential, due to [[Jeffrey Goldstone]]<ref>{{cite doi|10.1007/BF02812722|noedit}}</ref> is illustrated in the graph at the right.
:{{NumBlk|:|<math>V(\phi) = -10|\phi|^2 + |\phi|^4 \,</math>.|{{EquationRef|2}}}}
 
This potential has an infinite number of possible [[minimum|minima]] (vacuum states) given by
:{{NumBlk|:|<math>\phi = \sqrt{5} e^{i\theta} </math>.|{{EquationRef|3}}}}
for any real ''θ'' between 0 and 2''π''. The system also has an unstable vacuum state corresponding to ''Φ'' = 0. This state has a [[Unitary group|U(1)]] symmetry. However, once the system falls into a specific stable vacuum state (amounting to a choice of ''θ''), this symmetry will appear to be lost, or "spontaneously broken".  
 
In fact, any other choice of ''θ'' would have exactly the same energy, implying the existence of a massless [[Goldstone boson|Nambu–Goldstone boson]], the mode running around the circle at the minimum of this potential, and indicating there is some memory of the original symmetry in the Lagrangian.
 
==Other examples==
* For [[ferromagnet]]ic materials, the underlying laws are invariant under spatial rotations. Here, the order parameter is the [[magnetization]], which measures the magnetic dipole density. Above the [[Curie temperature]], the order parameter is zero, which is spatially invariant, and there is no symmetry breaking. Below the Curie temperature, however, the magnetization acquires a constant nonvanishing value, which points in a certain direction (in the idealized situation where we have full equilibrium; otherwise, translational symmetry gets broken as well). The residual rotational symmetries which leave the orientation of this vector invariant remain unbroken, unlike the other rotations which do not and are thus spontaneously broken.
* The laws describing a solid are invariant under the full [[Euclidean group]], but the solid itself spontaneously breaks this group down to a [[space group]]. The displacement and the orientation are the order parameters.
* General relativity has a Lorentz symmetry, but in [[Friedmann–Lemaître–Robertson–Walker metric|FRW cosmological models]], the mean 4-velocity field defined by averaging over the velocities of the galaxies (the galaxies act like gas particles at cosmological scales) acts as an order parameter breaking this symmetry. Similar comments can be made about the cosmic microwave background.
* For the [[electroweak]] model, as explained earlier, a component of the Higgs field provides the order parameter breaking the electroweak gauge symmetry to the electromagnetic gauge symmetry. Like the ferromagnetic example, there is a phase transition at the electroweak temperature. The same comment about us not tending to notice broken symmetries suggests why it took so long for us to discover electroweak unification.
* In superconductors, there is a condensed-matter collective field ψ, which acts as the order parameter breaking the electromagnetic gauge symmetry.
* Take a thin cylindrical plastic rod and push both ends together. Before buckling, the system is symmetric under rotation, and so visibly cylindrically symmetric. But after buckling, it looks different, and asymmetric. Nevertheless, features of the cylindrical symmetry are still there: ignoring friction, it would take no force to freely spin the rod around, displacing the ground state in time, and amounting to an oscillation of vanishing frequency, unlike the radial oscillations in the direction of the buckle. This spinning mode is effectively the requisite [[Goldstone boson|Nambu–Goldstone boson]].
* Consider a uniform layer of [[fluid]] over an infinite horizontal plane. This system has all the symmetries of the Euclidean plane. But now heat the bottom surface uniformly so that it becomes much hotter than the upper surface. When the temperature gradient becomes large enough, [[convection cell]]s will form, breaking the Euclidean symmetry.
* Consider a bead on a circular hoop that is rotated about a vertical [[diameter]]. As the [[rotational velocity]] is increased gradually from rest, the bead will initially stay at its initial [[equilibrium point]] at the bottom of the hoop (intuitively stable, lowest [[gravitational potential]]). At a certain critical rotational velocity, this point will become unstable and the bead will jump to one of two other newly created equilibria, [[equidistant]] from the center. Initially, the system is symmetric with respect to the diameter, yet after passing the critical velocity, the bead ends up in one of the two new equilibrium points, thus breaking the symmetry.
 
==Nobel Prize==
On October 7, 2008, the [[Royal Swedish Academy of Sciences]] awarded the 2008 [[Nobel Prize in Physics]] to three scientists for their work in subatomic physics symmetry breaking. [[Yoichiro Nambu]], of the [[University of Chicago]], won half of the prize for the discovery of the mechanism of spontaneous broken symmetry in the context of the strong interactions, specifically [[chiral symmetry breaking]]. Physicists [[Makoto Kobayashi (physicist)|Makoto Kobayashi]] and [[Toshihide Maskawa]] shared the other half of the prize for discovering the origin of the [[Explicit symmetry breaking|explicit breaking]] of CP symmetry in the weak interactions.<ref>{{cite web|author=The Nobel Foundation|title=The Nobel Prize in Physics 2008|url=http://nobelprize.org/nobel_prizes/physics/laureates/2008/index.html|work=nobelprize.org|accessdate=January 15, 2008}}</ref> This origin is ultimately reliant on the Higgs mechanism, but, so far understood as a "just so" feature of Higgs couplings, not a spontaneously broken symmetry phenomenon.
 
==See also==
{{div col|colwidth=24em}}
* [[Autocatalytic reactions and order creation]]
* [[Catastrophe theory]]
* [[Chiral symmetry breaking]]
* [[CP-violation]]
* [[Dynamical symmetry breaking]]
* [[Explicit symmetry breaking]]
* [[Gauge gravitation theory]]
* [[Goldstone boson]]
* [[Grand unified theory]]
* [[Higgs mechanism]]
* [[Higgs boson]]
* [[Higgs field (classical)]]
* [[Irreversibility]]
* [[Magnetic catalysis]] of chiral symmetry breaking
* [[Mermin-Wagner theorem]]
* [[Quantum fluctuation]]
* [[Sakurai Prize|Sakurai Prize for Theoretical Particle Physics]]
* [[Second-order phase transition]]
* [[Symmetry breaking]]
* [[Tachyon condensation]]
* [[Tachyonic field]]
* [[Wheeler-Feynman absorber theory]]
* [[1964 PRL symmetry breaking papers]]
{{div col end}}
 
==Notes==
;Citations
{{Reflist}}
 
==External links==
* [http://hyperphysics.phy-astr.gsu.edu/hbase/forces/unify.html#c2 Spontaneous symmetry breaking]
* [http://prl.aps.org/50years/milestones#1964 Physical Review Letters - 50th Anniversary Milestone Papers]
* [http://cerncourier.com/cws/article/cern/32522 In CERN Courier, Steven Weinberg reflects on spontaneous symmetry breaking]
* [http://www.scholarpedia.org/article/Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia]
* [http://www.scholarpedia.org/article/Englert-Brout-Higgs-Guralnik-Hagen-Kibble_mechanism_%28history%29 History of Englert-Brout-Higgs-Guralnik-Hagen-Kibble Mechanism on Scholarpedia]
* [http://arxiv.org/abs/0907.3466 The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles]
* [http://www.worldscinet.com/ijmpa/24/2414/S0217751X09045431.html International Journal of Modern Physics A: The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles]
* [http://www.datafilehost.com/download-7d512618.html Guralnik, G S; Hagen, C R and Kibble, T W B (1967). Broken Symmetries and the Goldstone Theorem. Advances in Physics, vol. 2 Interscience Publishers, New York. pp. 567-708 ISBN 0-470-17057-3]
* [http://lanl.arxiv.org/abs/hep-th/9802142 Spontaneous Symmetry Breaking in Gauge Theories: a Historical Survey]
 
{{DEFAULTSORT:Spontaneous Symmetry Breaking}}
[[Category:Theoretical physics]]
[[Category:Quantum field theory]]
[[Category:Standard Model]]
[[Category:Quantum chromodynamics]]
[[Category:Symmetry]]
[[Category:Quantum phases]]

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In Yr 2013, c ommercial retails, shoebox residences and mass market properties continued to be the celebs of the property market. Units are snapped up in document time and at document breaking prices. Developers are having fun with overwhelming demand and consumers want more. We feel that these segments of the property market are booming is a repercussion of the property cooling measures no.6 and no. 7. With additional buyer's stamp duty imposed on residential properties, traders switch their focus to business and industrial properties. I consider every property purchasers need their property investment to appreciate in worth. Spontaneous symmetry breaking[1][2][3] is a mode of realization of symmetry breaking in a physical system, where the underlying laws are invariant under a symmetry transformation, but the system as a whole changes under such transformations, in contrast to explicit symmetry breaking. It is a spontaneous process by which a system in a symmetrical state ends up in an asymmetrical state. It thus describes systems where the equations of motion or the Lagrangian obey certain symmetries, but the lowest energy solutions do not exhibit that symmetry.

Consider the bottom of an empty wine bottle, a symmetrical upward dome with a trough for sediment. If a ball is put in a particular position at the peak of the dome, the circumstances are symmetrical with respect to rotating the wine bottle. But the ball may spontaneously break this symmetry and move into the trough, a point of lowest energy. The bottle and the ball continue to have symmetry, but the system does not.[4]

Most simple phases of matter and phase-transitions, like crystals, magnets, and conventional superconductors can be simply understood from the viewpoint of spontaneous symmetry breaking. Notable exceptions include topological phases of matter like the fractional quantum Hall effect.

Spontaneous symmetry breaking in physics

Spontaneous symmetry breaking simplified: - At high energy levels (left) the ball settles in the center, and the result is symmetrical. At lower energy levels (right), the overall "rules" remain symmetrical, but the "Mexican hat" potential comes into effect: "local" symmetry is inevitably broken since eventually the ball must roll one way (at random) and not another.

Particle physics

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Chiral symmetry

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Chiral symmetry breaking is an example of spontaneous symmetry breaking affecting the chiral symmetry of the strong interactions in particle physics. It is a property of quantum chromodynamics, the quantum field theory describing these interactions, and is responsible for the bulk of the mass (over 99%) of the nucleons, and thus of all common matter, as it converts very light bound quarks into 100 times heavier constituents of baryons. The approximate Nambu–Goldstone bosons in this spontaneous symmetry breaking process are the pions, whose mass is an order of magnitude lighter than the mass of the nucleons. It served as the prototype and significant ingredient of the Higgs mechanism underlying the electroweak symmetry breaking.

Higgs mechanism

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The strong, weak, and electromagnetic forces can all be understood as arising from gauge symmetries. The Higgs mechanism, the spontaneous-symmetry breaking of gauge symmetries, is an important component in understanding the superconductivity of metals and the origin of particle masses in the standard model of particle-physics. One important consequence of the distinction between true symmetries and gauge symmetries, is that the spontaneous breaking of a gauge symmetry does not give rise to characteristic massless Nambu-Goldstone modes, but only massive modes, like the plasma-mode in a superconductor, or the Higgs mode observed in particle physics.

In the standard model of particle physics, spontaneous symmetry breaking of the SU(2)×U(1) gauge-symmetry associated with the electro-weak force generates masses for several particles, and separates the electromagnetic and weak forces. The W and Z bosons are the elementary particles that mediate the weak interaction, while the photon mediates the electromagnetic interaction. At energies much greater than 100 GeV all these particles behave in a similar manner. The Weinberg–Salam theory predicts that, at lower energies, this symmetry is broken so that the photon and the massive W and Z bosons emerge.[6] In addition, fermions develop mass consistently.

Without spontaneous symmetry breaking, the Standard Model of elementary particle interactions requires the existence of a number of particles. However, some particles (the W and Z bosons) would then be predicted to be massless, when, in reality, they are observed to have mass. To overcome this, spontaneous symmetry breaking is augmented by the Higgs mechanism to give these particles mass. It also suggests the presence of a new particle, the Higgs boson, reported as possibly identifiable with a boson detected in 2012. (If the Higgs boson were not confirmed to have been found, it would mean that the simplest implementation of the Higgs mechanism and spontaneous symmetry breaking as they are currently formulated require modification.)

Superconductivity of metals is a condensed-matter analog of the Higgs phenomena, in which a condensate of Cooper pairs of electrons spontaneously breaks the U(1) gauge "symmetry" associated with light and electromagnetism.

Condensed matter physics

Most phases of matter can be understood through the lens of spontaneous symmetry-breaking. For example, crystals are periodic arrays of atoms that are not invariant under all translations (only under a small sub-set of translations by a lattice vector). Magnets have north and south poles that are oriented in a specific direction, breaking rotational symmetry. In addition to these examples, there are a whole host of other symmetry-breaking phases of matter including nematic phases of liquid crystals, charge- and spin- density waves, superfluids and many others.

There are several known examples of matter that cannot be described by spontaneous symmetry breaking, including: topologically ordered phases of matter like fractional quantum Hall liquids, and spin-liquids. These states do not break any symmetry, but are distinct phases of matter. Unlike the case of spontaneous symmetry breaking, there is not a general framework for describing such states.

Continuous symmetry

The ferromagnet is the canonical system which spontaneously breaks the continuous symmetry of the spins below the Curie temperature and at , where is the external magnetic field. Below the Curie temperature the energy of the system is invariant under inversion of the magnetization such that . The symmetry is spontaneously broken as when the Hamiltonian becomes invariant under the inversion transformation, but the expectation value is not invariant.

Spontaneously, symmetry broken phases of matter are characterized by an order-parameter that describes the quantity which breaks the symmetry under consideration. For example, in a magnet, the order parameter is the local magnetization.

Spontaneously breaking of a continuous symmetry is inevitably accompanied by gapless (meaning that these modes do not cost any energy to excite) Nambu-Goldstone modes associated with slow long-wavelength fluctuations of the order-parameter. For example, vibrational modes in a crystal, known as phonons, are associated with slow density fluctuations of the crystal's atoms. The associated Goldstone mode for magnets are oscillating waves of spin known as spin-waves. For symmetry-breaking states, whose order parameter is not a conserved quantity, Nambu-Goldstone modes are typically massless and propagate at a constant velocity.

An important theorem, due to Mermin and Wagner, states that, at finite temperature, thermally activated fluctuations of Nambu-Goldstone modes destroy the long-range order, and prevent spontaneous symmetry breaking in one- and two-dimensional systems. Similarly, quantum fluctuations of the order parameter prevent most types of continuous symmetry breaking in one-dimensional systems even at zero-temperature (an important exception is ferromagnets, whose order parameter, magnetization, is an exactly conserved quantitity and does not have any quantum fluctuations).

Other long-range interacting systems such as cylindrical curved surfaces interacting via the Coulomb potential or Yukawa potential has been shown to break translational and rotational symmetries.[7] It was shown, in the presence of a symmetric Hamiltonian, and in the limit of infinite volume, the system spontaneously adopts a chiral configuration, i.e. breaks mirror plane symmetry.

Generalisation and technical usage

For spontaneous symmetry breaking to occur, there must be a system in which there are several equally likely outcomes. The system as a whole is therefore symmetric with respect to these outcomes. (If we consider any two outcomes, the probability is the same. This contrasts sharply to explicit symmetry breaking.) However, if the system is sampled (i.e. if the system is actually used or interacted with in any way), a specific outcome must occur. Though the system as a whole is symmetric, it is never encountered with this symmetry, but only in one specific asymmetric state. Hence, the symmetry is said to be spontaneously broken in that theory. Nevertheless, the fact that each outcome is equally likely is a reflection of the underlying symmetry, which is thus often dubbed "hidden symmetry", and has crucial formal consequences. (See the article on the Goldstone boson).

When a theory is symmetric with respect to a symmetry group, but requires that one element of the group be distinct, then spontaneous symmetry breaking has occurred. The theory must not dictate which member is distinct, only that one is. From this point on, the theory can be treated as if this element actually is distinct, with the proviso that any results found in this way must be resymmetrized, by taking the average of each of the elements of the group being the distinct one.

The crucial concept in physics theories is the order parameter. If there is a field (often a background field) which acquires an expectation value (not necessarily a vacuum expectation value) which is not invariant under the symmetry in question, we say that the system is in the ordered phase, and the symmetry is spontaneously broken. This is because other subsystems interact with the order parameter which forms a "frame of reference" to be measured against, so to speak. In that case, the vacuum state does not obey the initial symmetry (which would put it in the Wigner mode), and, instead has the (hidden) symmetry implemented in the Nambu–Goldstone mode. Normally, in the absence of the Higgs mechanism, massless Goldstone bosons arise.

The symmetry group can be discrete, such as the space group of a crystal, or continuous (e.g., a Lie group), such as the rotational symmetry of space. However, if the system contains only a single spatial dimension, then only discrete symmetries may be broken in a vacuum state of the full quantum theory, although a classical solution may break a continuous symmetry.

A pedagogical example: the Mexican hat potential

Graph of Goldstone's "Mexican hat" potential function V versus φ.

In the simplest idealized relativistic model, the spontaneously broken symmetry is summarized through an illustrative scalar field theory. The relevant Lagrangian, which essentially dictates how a system behaves, can be split up into kinetic and potential terms,

Template:NumBlk

It is in this potential term V(Φ) that the symmetry breaking is triggered. An example of a potential, due to Jeffrey Goldstone[8] is illustrated in the graph at the right.

Template:NumBlk

This potential has an infinite number of possible minima (vacuum states) given by

Template:NumBlk

for any real θ between 0 and 2π. The system also has an unstable vacuum state corresponding to Φ = 0. This state has a U(1) symmetry. However, once the system falls into a specific stable vacuum state (amounting to a choice of θ), this symmetry will appear to be lost, or "spontaneously broken".

In fact, any other choice of θ would have exactly the same energy, implying the existence of a massless Nambu–Goldstone boson, the mode running around the circle at the minimum of this potential, and indicating there is some memory of the original symmetry in the Lagrangian.

Other examples

  • For ferromagnetic materials, the underlying laws are invariant under spatial rotations. Here, the order parameter is the magnetization, which measures the magnetic dipole density. Above the Curie temperature, the order parameter is zero, which is spatially invariant, and there is no symmetry breaking. Below the Curie temperature, however, the magnetization acquires a constant nonvanishing value, which points in a certain direction (in the idealized situation where we have full equilibrium; otherwise, translational symmetry gets broken as well). The residual rotational symmetries which leave the orientation of this vector invariant remain unbroken, unlike the other rotations which do not and are thus spontaneously broken.
  • The laws describing a solid are invariant under the full Euclidean group, but the solid itself spontaneously breaks this group down to a space group. The displacement and the orientation are the order parameters.
  • General relativity has a Lorentz symmetry, but in FRW cosmological models, the mean 4-velocity field defined by averaging over the velocities of the galaxies (the galaxies act like gas particles at cosmological scales) acts as an order parameter breaking this symmetry. Similar comments can be made about the cosmic microwave background.
  • For the electroweak model, as explained earlier, a component of the Higgs field provides the order parameter breaking the electroweak gauge symmetry to the electromagnetic gauge symmetry. Like the ferromagnetic example, there is a phase transition at the electroweak temperature. The same comment about us not tending to notice broken symmetries suggests why it took so long for us to discover electroweak unification.
  • In superconductors, there is a condensed-matter collective field ψ, which acts as the order parameter breaking the electromagnetic gauge symmetry.
  • Take a thin cylindrical plastic rod and push both ends together. Before buckling, the system is symmetric under rotation, and so visibly cylindrically symmetric. But after buckling, it looks different, and asymmetric. Nevertheless, features of the cylindrical symmetry are still there: ignoring friction, it would take no force to freely spin the rod around, displacing the ground state in time, and amounting to an oscillation of vanishing frequency, unlike the radial oscillations in the direction of the buckle. This spinning mode is effectively the requisite Nambu–Goldstone boson.
  • Consider a uniform layer of fluid over an infinite horizontal plane. This system has all the symmetries of the Euclidean plane. But now heat the bottom surface uniformly so that it becomes much hotter than the upper surface. When the temperature gradient becomes large enough, convection cells will form, breaking the Euclidean symmetry.
  • Consider a bead on a circular hoop that is rotated about a vertical diameter. As the rotational velocity is increased gradually from rest, the bead will initially stay at its initial equilibrium point at the bottom of the hoop (intuitively stable, lowest gravitational potential). At a certain critical rotational velocity, this point will become unstable and the bead will jump to one of two other newly created equilibria, equidistant from the center. Initially, the system is symmetric with respect to the diameter, yet after passing the critical velocity, the bead ends up in one of the two new equilibrium points, thus breaking the symmetry.

Nobel Prize

On October 7, 2008, the Royal Swedish Academy of Sciences awarded the 2008 Nobel Prize in Physics to three scientists for their work in subatomic physics symmetry breaking. Yoichiro Nambu, of the University of Chicago, won half of the prize for the discovery of the mechanism of spontaneous broken symmetry in the context of the strong interactions, specifically chiral symmetry breaking. Physicists Makoto Kobayashi and Toshihide Maskawa shared the other half of the prize for discovering the origin of the explicit breaking of CP symmetry in the weak interactions.[9] This origin is ultimately reliant on the Higgs mechanism, but, so far understood as a "just so" feature of Higgs couplings, not a spontaneously broken symmetry phenomenon.

See also

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Notes

Citations

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External links

  1. ↑ Dynamical Symmetry Breaking in Quantum Field Theories. By Vladimir A. Miranskij. Pg 15.
  2. ↑ Patterns of Symmetry Breaking. Edited by Henryk Arodz, Jacek Dziarmaga, Wojciech Hubert Zurek. Pg 141.
  3. ↑ Bubbles, Voids and Bumps in Time: The New Cosmology. Edited by James Cornell. Pg 125.
  4. ↑ Gerald M. Edelman, Bright Air, Brilliant Fire: On the Matter of the Mind (New York: BasicBooks, 1992) 203.
  5. ↑ 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

    My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
  6. ↑ A Brief History of Time, Stephen Hawking, Bantam; 10th anniversary edition (September 1, 1998). Page 73-74.
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  8. ↑ Template:Cite doi
  9. ↑ Template:Cite web
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