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Quantum field theory
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In physics, spontaneous symmetry breaking occurs when a system that is symmetric with respect to some symmetry group goes into a vacuum state that is not symmetric. When that happens, the system no longer appears to behave in a symmetric manner. It is a phenomenon that naturally occurs in many situations.

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 common example to help explain this phenomenon is a ball sitting on top of a hill. This ball is in a completely symmetric state. However, its state is unstable: the slightest perturbing force will cause the ball to roll down the hill in some particular direction. At that point, symmetry has been broken because the direction in which the ball rolled has a feature that distinguishes it from all other directions.

Contents

Mathematical example: the Mexican hat potential

Graph of a Mexican hat potential function.

In the simplest example, the spontaneously broken field is described by a scalar field theory. In physics, one way of seeing spontaneous symmetry breaking is through the use of Lagrangians. Lagrangians, which essentially dictate how a system will behave, can be split up into kinetic and potential terms

(1) \qquad \mathcal{L} = \partial^\mu \phi \partial_\mu \phi - V(\phi).

It is in this potential term (V(φ)) that the action of symmetry breaking occurs. An example of a potential is illustrated in the graph at the right.

(2) \qquad V(\phi) = -10|\phi|^2 + |\phi|^4 \,

This potential has many possible minima (vacuum states) given by

(3) \qquad \phi = \sqrt{5} e^{i\theta}

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 (corresponding to a choice of θ) this symmetry will be lost or spontaneously broken.

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Higgs mechanism

In the Standard Model, spontaneous symmetry breaking is accomplished by using the Higgs boson and is responsible for the masses of the W and Z bosons. A slightly more technical presentation of this mechanism is given in the article on the Yukawa interaction, where it is shown how spontaneous symmetry breaking can be used to give mass to fermions.

Broader concept

More generally, we can have spontaneous symmetry breaking in nonvacuum situations and for systems not described by actions. The crucial concept here 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.

If a vacuum state obeys the initial symmetry then the system is said to be in the Wigner mode, otherwise it is in the Goldstone mode.

Examples

  • For ferromagnetic materials, the laws describing it 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 (in the idealized situation where we have full equilibrium; otherwise, translational symmetry gets broken as well) nonzero value which points in a certain direction. The residual rotational symmetries which leaves the orientation of this vector invariant remain unbroken but the other rotations get 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.
  • The laws of physics are spatially invariant, but here on the surface of the Earth, we have a background gravitational field (which plays the role of the order parameter here) which points downwards, breaking the full rotational symmetry. This explains why up, down and the vertical directions are all "different" but all the horizontal directions are still isotropic.
  • General relativity has a Lorenz gauge 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.
  • Here on Earth, Galilean invariance (in the nonrelativistic approximation) is broken by the velocity field of the Earth/atmosphere, which acts as the order parameter here. This explains why people thought moving bodies tend towards rest before Galileo. We tend not to be aware of broken symmetries.
  • For the electroweak model, as explained earlier, the Higgs field acts as 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 explains why it took so long for us to discover electroweak unification.
  • For superconductors, there is a collective condensed matter field ψ which acts as the order parameter breaking the electromagnetic gauge symmetry.
  • In general relativity, diffeomorphism covariance is broken by the nonzero order parameter, the metric tensor field.
  • Take a flat plastic ruler which is identical on both sides and push both ends together. Before buckling, the system is symmetric under the reflection about the plane of the ruler. But after buckling, it either buckles upwards or downwards.
  • 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 must choose between the two new equilibrium points, thus breaking symmetry. Note: This can easily be tried at home with an electric drill, a marble, and a pot cover, (or any other combination you can think of) and is the two-dimensional, mechanical analogue of the symmetry breaking that occurs in the Higgs Boson field.

Nobel Prize

On October 7, 2008, the Royal Swedish Academy of Sciences awarded the 2008 Nobel Prize in Physics to two Japanese citizens and a Japanese-born American for their work in subatomic physics. American Yoichiro Nambu, 87, of the University of Chicago, won half of the prize for the discovery of the mechanism of spontaneous broken symmetry. Japanese physicists Makoto Kobayashi and Toshihide Maskawa shared the other half of the prize for discovering the origin of the broken symmetry.[1] The trio shared the 10 million kronor (1.25 million USD) purse, as well as a diploma and an invitation to the prize ceremonies in Stockholm on December 10, 2008.

See also

Notes

  1. ^ The Nobel Foundation. "The Nobel Prize in Physics 2008". nobelprize.org. http://nobelprize.org/nobel_prizes/physics/laureates/2008/index.html. Retrieved January 15 2008.  

External links


Simple English

Spontaneous Symmetry Breaking is a way that scientists start off with something completely symmetrical and end up (without creating an outside force) with something non-symmetrical. Spontaneous means sudden or unexpected. Symmetry (Latin sym- meaning united, metric- meaning measure) refers to the fact that rules (known as symmetries) of physics that are changed. Breaking refers to the change of the symmetry. Spontaneous Symmetry Breaking commonly happens in the theoretical Higgs effect.

Uses for Spontaneous Symmetry Breaking

Spontaneous Symmetry Breaking can create a theoretical particle called a Higgs Boson. This is a particle which is predicted to be able to give mass to certain particles called bosons, like a photon. Also, many scientists believe in the Higgs Effect (which is very similar to Spontaneous Symmetry Breaking) to answer questions that are not answered in the Standard model of physics. The Standard model predicts that certain types of quarks should have a mass of zero, while in reality they have a non-zero mass value. Some scientists believe that Spontaneous Symmetry Breaking is the answer.

Method of Spontaneous Symmetry Breaking

For Spontaneous Symmetry Breaking to happen, you need an environment which is completely symmetrical, and has at least two outcomes that are equally likely. Spontaneous Symmetry Breaking starts with two particles that are completely equal; their spin is equal, too. Mathematically, Spontaneous Symmetry Breaking can be extremely puzzling, since you start out with two identical things and end up with two non-identical things. However, in practice, it is not so puzzling. If you have two particles moving at each other with equal speed, it would seem impossible for either of them to do anything but be symmetrical. However, if each particle has an equal 50-50 chance to be spinning one way or another, it is possible–in theory and in practice–to have this symmetry broken. It begins with symmetry to start with because the particles have an equal and symmetrical 50-50 chance of spinning one way or another.

Scientists have been able to use Spontaneous Symmetry Breaking. However, it has not proven (or disproven) the theory of the Higgs Boson. The energy required to generate a Higgs Boson is simply too powerful for the particle accelerators that we have available. However, the future will probably reveal the existence or nonexistence of the Higgs Boson.

Particles in Physics
Elementary: Fermions: Quarks: up - down - strange - charm - bottom - top
Leptons: electron - muon - tau - neutrinos
Bosons: Gauge bosons: photon - W and Z bosons - gluons
Composite: Hadrons: Baryons: proton - neutron - hyperon
Mesons: pion - kaon - J/ψ
Atomic nuclei - Atoms - Molecules
Hypothetical: Higgs boson - Graviton - Tachyon

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