Magnetic monopole
A magnetic monopole is an imaginary magnet, which has only one pole, so only the north or the south pole only. After its effects you can buy a single magnetic pole as a end of a long bar magnet (see pole strength ) imagine if the other end is so far away that the forces emanating from there are negligibly small.
In certain solids electronic structures ( quasi-particles ) have been proven to resemble a mixture of exactly the same number of individual and freely moving north and south poles. Although these are referred to as magnetic monopoles, but only in pairs, do not occur individually and do not exist as free particles.
A real magnetic monopole to which there is no antithesis, has not been observed so far. If he existed as particles, he would support a magnetic charge corresponding to the electric charge; magnetic charges would be sources and sinks of the magnetic field (see also monopoly (physics) ). Were discussions about such magnetic monopoles and are available in various areas of theoretical physics. Observed in nature, however, are known previously only magnetic fields of the closed field lines, which do not have sources and sinks.
- 2.1 Dirac's elementary - monopoly 2.1.1 Maxwell equations and symmetry 2.1.1.1 Potential and covariant formulation
- 2.2.1 Design
- 2.2.2 Violation of time-reversal invariance
- 2.2.3 catalysis of proton decay
- 2.2.4 Experimental search for GUT monopoles
- 2.2.5 Expected Frequency
Quasiparticles in solids as the apparent magnetic monopoles
In solids of the type known as spin ice monopoles are observed and researched since 2009. It is monopoly-like quasiparticles in the form of the two ends of long chains " contiguous " oriented electron spins. You can move to gas molecules freely through the solid in analogy and behave in many respects like real single magnetic monopoles. But you can only occur in pairs as North and South Poles. Therefore, one can view them while locally as sources of magnetization, but the magnetic field globally remains free of sources.
After preliminary conceptual work of Castelnovo, Moessner and Sondhi, a team from the Helmholtz Centre Berlin for Materials and Energy in collaboration with colleagues from Dresden, St. Andrews, La Plata and Oxford by neutron diffraction in a dysprosium titanate crystal ( Dy2Ti2O7 ) was the first time such observe so-called monopolies in solid matter.
In October 2010, succeeded at the Paul Scherrer Institute with the help of high-intensity X-ray radiation of the synchrotron light source Switzerland, also map these quasi- monopolies.
2013, researchers discovered the Technical Universities of Dresden and Munich, that the quasi-monopolies in the degradation of Skyrmion - " crystals " can play a role. This effect could be important in any future use of skyrmions in the data storage technology.
2014, researchers led by David Hall ( University of Amherst ) and Mikko Möttönen ( Aalto University ) to replicate virtual monopolies in a ferromagnetic Bose -Einstein condensate.
The Dirac string
A Dirac string ( engl. string = string ) is an imaginary line extending from a magnetic monopole. For an electrically charged particle moving in the magnetic field of the monopole, the phase of the quantum mechanical wave function is singular on this line and the probability is zero. If there is a magnetic monopole, the existence of such a line has to be assumed that particles outside the line otherwise have no clear phase. Such a string would be observable due to the vanishing probability on it and was indeed observed at the quasi- monopolies. In the publications of the quasi-monopolies so was " Dirac monopoles " are mentioned; but it is not about monopolies in the sense of elementary particles, a performance that also goes back to Paul Dirac.
Magnetic monopoles as elementary
Elementary Dirac monopole
By Paul Dirac derived the speculation that there could be the magnetic monopole as the elementary particles, which would be the magnetic counterpart of the electron. Speak two arguments for this idea:
- The strange asymmetry between the otherwise similar phenomena magnetism and electricity - visible, for example in the Maxwell equations - would be resolved.
- It would casually explain why the electric charge always only " quantized ", ie, occurs in integer multiples of the elementary charge.
Both arguments are illustrated in the following sections. Despite intensive efforts, however, has not yet been proved the existence of such a particle.
Maxwell equations and symmetry
Symmetries play a fundamental role in physics. Formulated in the 19th century, Maxwell's equations, which describe the electric and magnetic phenomena exhibit unnatural appearing asymmetry between the vectors of the electric field strength and the magnetic flux density. While the electric charges act as charge density and corresponding current density, are due to the non-existence of magnetic charges, the corresponding magnetic quantities equal to zero:
Assuming, however, the existence of the magnetic charges ( monopoles ), there would also be a non-zero magnetic charge density and magnetic flux density, the equations would become:
Thus, one would obtain a theory which would remain unchanged under the following transformations (so-called symplectic symmetry):
The existence of magnetic monopoles would therefore reduce the differences between electric and magnetic field further, electrical and magnetic phenomena were strictly " dual" to each other.
Potential and covariant formulation
As in the case of Maxwell's equations without magnetic monopoles to potentials can be introduced, which are helpful for the construction of solutions of the respective differential equations and satisfy certain new differential equations. In addition to the electromagnetic four-potential leads to a second, " magnetoelectric " four-potential one. The potentials are chosen so (with a certain arbitrariness is, insofar as it is possible) that the field strength tensor as a differential form
Results, or as in index notation
The Maxwell equations take the following form ( which can be interpreted as four-vectors ):
This will not have the continuity equations:
For the potentials, this means:
This means that just like in the case without magnetic monopoles, regardless of behavior, and fulfill the components of completely analogous in each component the wave equation with magnetic charge and current as inhomogeneity. The theory also has an additional gauge invariance on: For scalar fields, it is not only invariant under the transformation
But also in
The above-mentioned symmetry transformation can be in this notation as a transformation through the Hodge star operator of the field strength tensor on the dual field strength tensor, as well as the transitions and understand.
The quantization of the electric charge
In addition to the angular momentum of the electric charge is quantized, i.e. acts only as an integral multiple of the elementary charge. According to Dirac, the presence of magnetic monopoles would easily explain this circumstance: A moving in the field of a monopoly electron is deflected on a curved path. The associated with the deflection change of the angular momentum can only take place in certain quantized discrete steps, but should be proportional to the electric charge. Therefore, it follows from the Drehimpulsquantisierung together with the existence of the magnetic monopole directly the quantization of electric charge. The reasoning would apply equally well for the magnetic charge. The monopoly would be carriers of the magnetic elementary charge.
Magnetic monopoles as solitons
There may be a reference to the existence of magnetic monopoles from the so-called theories of grand unification (GUT) may result. These theories describe the unification of the electroweak force with the strong force at high energies that existed until about 10-36 seconds after the big bang in our universe. By cooling of the expanding universe, the typical particle energy decreased at this time below a critical value of about 1015 GeV ( which is about 1028 Kelvin). Thus, the symmetry breaking of the unified force was triggered interaction in the separate forces of strong interactions and electroweak. It included performances by stable point-like topological defects of the gauge field, so-called solitons on - the magnetic monopoles. This mechanism is somewhat comparable to the events at solidifying liquids. The crystallization starts simultaneously at different points in space. Now grow two crystals are able to form on the contact surfaces lattice defects. The density of the resulting monopolies can be estimated at the time of emergence to about 1082 m-3. The fact that the particle density is significantly lower today, is seen as a further indication of a strong inflationary phase of the early universe. In these theories the structure and properties of a GUT monopole are described in detail.
Construction
A WELL - monopoly has a mass of about 1016 GeV, a diameter of about 10-15 m and a defined onion-like substructure. Accordingly, located near the center, ie in the range of 10-31 m, a GUT- symmetric vacuum before. This is followed by a bowl of so-called electro- weak unification connects with particles as the gauge bosons of the weak interaction W , W- and Z0. This zone is at about 10-18 m above the confinement shell that is filled with gluons and photons. The outermost shell is formed from fermion - antifermion pairs.
Violation of time-reversal invariance
If we examine the deflection of a charged particle in the vicinity of a monopoly, it is found that such an arrangement, the time-reversal invariance violated. This means that the process is in reverse from the time direction is not in the same manner. This fact spoke for a long time directly against the existence of magnetic monopoles. However, after in 1964 the CP violation could be detected in the decay of K- mesons, it follows from the CPT theorem directly the existence of T- invarianzverletzender processes.
Catalysis of proton decay
Due to the internal structure above GUT monopoles can catalyze the proton and neutron decay. The following reactions of the theories predicted (M stands for monopoly):
The monopoly itself does not decompose at these reactions. He is thus able to influence the stability of this material by decomposition processes.
Experimental search for GUT monopoles
Because of the above very high rest energy of the GUT monopoly - its mass is comparable to that of a bacterium - you can not even create it directly in colliding -beam experiments and evidence. Therefore, existing flux density one is in the search for monopoly on their naturally dependent but which is predicted by the current theories to be very low.
A possible detection of the hypothetical experiment the particle based on the use of superconducting coils. During the passage through such a monopole coil, a ring current is induced by the change of the magnetic flux can be detected. Such a circular current is actually generated only by means of magnetic monopoles and not by that of a conventional dipole magnet field. However, the relatively high susceptibility of such experiments requires careful experimentation.
Further experiments such as Super - Kamiokande ( the Kamiokande experiment successor ), aimed at the detection of the induced proton decay by monopoles described above. These serve as a proton carrier, for example, several (ten ) thousand tons of high-purity water. However, the estimation of the expected decay rate requires the knowledge of the typical cross section of the decomposition reaction.
In a coil experiment in 1982 by Blas Cabrera (Stanford University, USA ) has been observed a single event. However, it can not be ruled out that this is a false signature. Recent experiments always give reason to ceilings of the particle flux, which is currently dependent are, by the method used, approximately in the range of 10-16 s- 1 cm -2. This means converted to an area of 1 m2 at most every 30,000 years, traversed by a monopoly on average.
Expected frequency
Based on the life of galactic magnetic fields, an upper limit for the frequency on earth incident GUT monopoles are determined. This is estimated to average more than one monopole per square meter of the earth's surface and 31.7 thousand years what < 10-16 cm -2s - 1 corresponds to the so-called Parker limit of FM.
Magnetic monopoles in other theories
Also in other gauge theories of the field strength tensor can be split into magnetic and electric components. In these theories can then also exist magnetic monopoles. An example is the quantum chromodynamics or SU (3) - Yang-Mills theory. This so-called chromomagnetische monopolies are associated with the confinement hypothesis. They come for possible explanation scenarios in question, but have so far been purely theoretical.