Soliton

Source: Wikipedia, the free encyclopedia.
For other uses, see Soliton (disambiguation).
wave channel

In

nonlinear and dispersive effects in the medium. (Dispersive effects are a property of certain systems where the speed of a wave depends on its frequency.) Solitons were subsequently found to provide stable solutions of a wide class of weakly nonlinear dispersive partial differential equations
describing physical systems.

The soliton phenomenon was first described in 1834 by

Korteweg–de Vries equation, which models waves of the type seen by Russell. The name was meant to characterize the solitary nature of the waves, with the 'on' suffix recalling the usage for particles such as electrons, baryons or hadrons, reflecting their observed particle-like behaviour.[1]

Definition

A single, consensus definition of a soliton is difficult to find. Drazin & Johnson (1989, p. 15) ascribe three properties to solitons:

  1. They are of permanent form;
  2. They are localized within a region;
  3. They can interact with other solitons, and emerge from the collision unchanged, except for a phase shift.

More formal definitions exist, but they require substantial mathematics. Moreover, some scientists use the term soliton for phenomena that do not quite have these three properties (for instance, the 'light bullets' of nonlinear optics are often called solitons despite losing energy during interaction).[2]

Explanation

carrier signal, while the red line is the envelope
soliton.

nonlinearity can interact to produce permanent and localized wave forms. Consider a pulse of light traveling in glass. This pulse can be thought of as consisting of light of several different frequencies. Since glass shows dispersion, these different frequencies travel at different speeds and the shape of the pulse therefore changes over time. However, also the nonlinear Kerr effect occurs; the refractive index of a material at a given frequency depends on the light's amplitude or strength. If the pulse has just the right shape, the Kerr effect exactly cancels the dispersion effect and the pulse's shape does not change over time. Thus, the pulse is a soliton. See soliton (optics)
for a more detailed description.

Many

Korteweg–de Vries equation, the nonlinear Schrödinger equation, the coupled nonlinear Schrödinger equation, and the sine-Gordon equation. The soliton solutions are typically obtained by means of the inverse scattering transform, and owe their stability to the integrability
of the field equations. The mathematical theory of these equations is a broad and very active field of mathematical research.

Some types of

soliton model in neuroscience proposes to explain the signal conduction within neurons
as pressure solitons.

A

homotopy classes
.

No continuous transformation maps a solution in one homotopy class to another. The solutions are truly distinct, and maintain their integrity, even in the face of extremely powerful forces. Examples of topological solitons include the

crystalline lattice, the Dirac string and the magnetic monopole in electromagnetism, the Skyrmion and the Wess–Zumino–Witten model in quantum field theory, the magnetic skyrmion in condensed matter physics, and cosmic strings and domain walls in cosmology
.

History

A plaque marking the workshop of John Scott Russell at 8 Stafford Street in Edinburgh

In 1834,

wave of translation.[nb 1] The discovery is described here in Scott Russell's own words:[nb 2]

I was observing the motion of a boat which was rapidly drawn along a narrow channel by a pair of horses, when the boat suddenly stopped – not so the mass of water in the channel which it had put in motion; it accumulated round the prow of the vessel in a state of violent agitation, then suddenly leaving it behind, rolled forward with great velocity, assuming the form of a large solitary elevation, a rounded, smooth and well-defined heap of water, which continued its course along the channel apparently without change of form or diminution of speed. I followed it on horseback, and overtook it still rolling on at a rate of some eight or nine miles an hour, preserving its original figure some thirty feet long and a foot to a foot and a half in height. Its height gradually diminished, and after a chase of one or two miles I lost it in the windings of the channel. Such, in the month of August 1834, was my first chance interview with that singular and beautiful phenomenon which I have called the Wave of Translation.[3]

Scott Russell spent some time making practical and theoretical investigations of these waves. He built wave tanks at his home and noticed some key properties:

  • The waves are stable, and can travel over very large distances (normal waves would tend to either flatten out, or steepen and topple over)
  • The speed depends on the size of the wave, and its width on the depth of water.
  • Unlike normal waves they will never merge – so a small wave is overtaken by a large one, rather than the two combining.
  • If a wave is too big for the depth of water, it splits into two, one big and one small.

Scott Russell's experimental work seemed at odds with

Korteweg–de Vries equation, including solitary wave and periodic cnoidal wave solutions.[6][nb 4]

surface gravity waves. The wave heights of the solitary waves are 1.2 and 0.6, respectively, and their velocities are 1.4 and 1.2.
The upper graph is for a frame of reference moving with the average velocity of the solitary waves.
The lower graph (with a different vertical scale and in a stationary frame of reference) shows the oscillatory tail produced by the interaction.[7]
Thus, the solitary wave solutions of the BBM equation are not solitons.

In 1965

Korteweg–de Vries equation (KdV equation) in a computational investigation using a finite difference approach. They also showed how this behavior explained the puzzling earlier work of Fermi, Pasta, Ulam, and Tsingou.[1]

In 1967, Gardner, Greene, Kruskal and Miura discovered an inverse scattering transform enabling analytical solution of the KdV equation.[8] The work of Peter Lax on Lax pairs and the Lax equation has since extended this to solution of many related soliton-generating systems.

Solitons are, by definition, unaltered in shape and speed by a collision with other solitons.[9] So solitary waves on a water surface are near-solitons, but not exactly – after the interaction of two (colliding or overtaking) solitary waves, they have changed a bit in amplitude and an oscillatory residual is left behind.[10]

Solitons are also studied in quantum mechanics, thanks to the fact that they could provide a new foundation of it through

light quanta, to all the particles of matter. The observation of accelerating surface gravity water wave soliton using an external hydrodynamic linear potential was demonstrated in 2019. This experiment also demonstrated the ability to excite and measure the phases of ballistic solitons.[11]

In fiber optics

See also: Soliton (optics)

Much experimentation has been done using solitons in fiber optics applications. Solitons in a fiber optic system are described by the

Manakov equations
. Solitons' inherent stability make long-distance transmission possible without the use of repeaters, and could potentially double transmission capacity as well.[12]

Year Discovery
1973
telecommunications
.
1987
dark soliton
, in an optical fiber.
1988
Sir C. V. Raman who first described it in the 1920s, to provide optical gain
in the fiber.
1991 A Bell Labs research team transmitted solitons error-free at 2.5 gigabits per second over more than 14,000 kilometers, using erbium optical fiber amplifiers (spliced-in segments of optical fiber containing the rare earth element erbium). Pump lasers, coupled to the optical amplifiers, activate the erbium, which energizes the light pulses.
1998 Thierry Georges and his team at
France Telecom R&D Center, combining optical solitons of different wavelengths (wavelength-division multiplexing), demonstrated a composite data transmission of 1 terabit
per second (1,000,000,000,000 units of information per second), not to be confused with Terabit-Ethernet.

The above impressive experiments have not translated to actual commercial soliton system deployments however, in either terrestrial or submarine systems, chiefly due to the Gordon–Haus (GH) jitter. The GH jitter requires sophisticated, expensive compensatory solutions that ultimately makes dense wavelength-division multiplexing (DWDM) soliton transmission in the field unattractive, compared to the conventional non-return-to-zero/return-to-zero paradigm. Further, the likely future adoption of the more spectrally efficient phase-shift-keyed/QAM formats makes soliton transmission even less viable, due to the Gordon–Mollenauer effect. Consequently, the long-haul fiberoptic transmission soliton has remained a laboratory curiosity.

2000
semiconductor saturable absorber mirror (SESAM). The polarization state of such a vector soliton could either be rotating or locked depending on the cavity parameters.[14]
2008 D. Y. Tang et al. observed a novel form of higher-order vector soliton from the perspectives of experiments and numerical simulations. Different types of vector solitons and the polarization state of vector solitons have been investigated by his group.[15]

In biology

Solitons may occur in proteins[16] and DNA.[17] Solitons are related to the low-frequency collective motion in proteins and DNA.[18]

A recently developed model in neuroscience proposes that signals, in the form of density waves, are conducted within neurons in the form of solitons.[19][20][21] Solitons can be described as almost lossless energy transfer in biomolecular chains or lattices as wave-like propagations of coupled conformational and electronic disturbances.[22]

In material physics

Solitons can occur in materials, such as

ferroelectrics, in the form of domain walls. Ferroelectric materials exhibit spontaneous polarization, or electric dipoles, which are coupled to configurations of the material structure. Domains of oppositely poled polarizations can be present within a single material as the structural configurations corresponding to opposing polarizations are equally favorable with no presence of external forces. The domain boundaries, or “walls”, that separate these local structural configurations are regions of lattice dislocations.[23] The domain walls can propagate as the polarizations, and thus, the local structural configurations can switch within a domain with applied forces such as electric bias or mechanical stress. Consequently, the domain walls can be described as solitons, discrete regions of dislocations that are able to slip or propagate and maintain their shape in width and length.[24][25][26]
 

In recent literature, ferroelectricity has been observed in twisted bilayers of van der Waal materials such as molybdenum disulfide and graphene.[23][27][28] The moiré superlattice that arises from the relative twist angle between the van der Waal monolayers generates regions of different stacking orders of the atoms within the layers. These regions exhibit inversion symmetry breaking structural configurations that enable ferroelectricity at the interface of these monolayers. The domain walls that separate these regions are composed of partial dislocations where different types of stresses, and thus, strains are experienced by the lattice. It has been observed that soliton or domain wall propagation across a moderate length of the sample (order of nanometers to micrometers) can be initiated with applied stress from an AFM tip on a fixed region. The soliton propagation carries the mechanical perturbation with little loss in energy across the material, which enables domain switching in a domino-like fashion.[25]

It has also been observed that the type of dislocations found at the walls can affect propagation parameters such as direction. For instance, STM measurements showed four types of strains of varying degrees of shear, compression, and tension at domain walls depending on the type of localized stacking order in twisted bilayer graphene. Different slip directions of the walls are achieved with different types of strains found at the domains, influencing the direction of the soliton network propagation.[25]

Nonidealities such as disruptions to the soliton network and surface impurities can influence soliton propagation as well. Domain walls can meet at nodes and get effectively pinned, forming triangular domains, which have been readily observed in various ferroelectric twisted bilayer systems.[23] In addition, closed loops of domain walls enclosing multiple polarization domains can inhibit soliton propagation and thus, switching of polarizations across it.[25] Also, domain walls can propagate and meet at wrinkles and surface inhomogeneities within the van der Waal layers, which can act as obstacles obstructing the propagation.[25]

In magnets

In magnets, there also exist different types of solitons and other nonlinear waves.[29] These magnetic solitons are an exact solution of classical nonlinear differential equations — magnetic equations, e.g. the Landau–Lifshitz equation, continuum Heisenberg model, Ishimori equation, nonlinear Schrödinger equation and others.

In nuclear physics

Atomic nuclei may exhibit solitonic behavior.[30] Here the whole nuclear wave function is predicted to exist as a soliton under certain conditions of temperature and energy. Such conditions are suggested to exist in the cores of some stars in which the nuclei would not react but pass through each other unchanged, retaining their soliton waves through a collision between nuclei.

The Skyrme Model is a model of nuclei in which each nucleus is considered to be a topologically stable soliton solution of a field theory with conserved baryon number.

Bions

The bound state of two solitons is known as a bion,[31][32][33][34] or in systems where the bound state periodically oscillates, a breather. The interference-type forces between solitons could be used in making bions.[35] However, these forces are very sensitive to their relative phases. Alternatively, the bound state of solitons could be formed by dressing atoms with highly excited Rydberg levels.[34] The resulting self-generated potential profile[34] features an inner attractive soft-core supporting the 3D self-trapped soliton, an intermediate repulsive shell (barrier) preventing solitons’ fusion, and an outer attractive layer (well) used for completing the bound state resulting in giant stable soliton molecules. In this scheme, the distance and size of the individual solitons in the molecule can be controlled dynamically with the laser adjustment.

In field theory bion usually refers to the solution of the Born–Infeld model. The name appears to have been coined by G. W. Gibbons in order to distinguish this solution from the conventional soliton, understood as a regular, finite-energy (and usually stable) solution of a differential equation describing some physical system.[36] The word regular means a smooth solution carrying no sources at all. However, the solution of the Born–Infeld model still carries a source in the form of a Dirac-delta function at the origin. As a consequence it displays a singularity in this point (although the electric field is everywhere regular). In some physical contexts (for instance string theory) this feature can be important, which motivated the introduction of a special name for this class of solitons.

On the other hand, when gravity is added (i.e. when considering the coupling of the Born–Infeld model to general relativity) the corresponding solution is called EBIon, where "E" stands for Einstein.

Alcubierre drive

Erik Lentz, a physicist at the University of Göttingen, has theorized that solitons could allow for the generation of Alcubierre warp bubbles in spacetime without the need for exotic matter, i.e., matter with negative mass.[37]

See also

Notes

  1. ^ "Translation" here means that there is real mass transport, although it is not the same water which is transported from one end of the canal to the other end by this "Wave of Translation". Rather, a fluid parcel acquires momentum during the passage of the solitary wave, and comes to rest again after the passage of the wave. But the fluid parcel has been displaced substantially forward during the process – by Stokes drift in the wave propagation direction. And a net mass transport is the result. Usually there is little mass transport from one side to another side for ordinary waves.
  2. ^ This passage has been repeated in many papers and books on soliton theory.
  3. Joseph Boussinesq
    mentioned Russell's name in his 1871 paper. Thus Scott Russell's observations on solitons were accepted as true by some prominent scientists within his own lifetime of 1808–1882.
  4. ^ Korteweg and de Vries did not mention John Scott Russell's name at all in their 1895 paper but they did quote Boussinesq's paper of 1871 and Lord Rayleigh's paper of 1876. The paper by Korteweg and de Vries in 1895 was not the first theoretical treatment of this subject but it was a very important milestone in the history of the development of soliton theory.

References

  1. ^ a b Zabusky & Kruskal (1965)
  2. ^ "Light bullets".
  3. ^ Scott Russell, J. (1845). Report on Waves: Made to the Meetings of the British Association in 1842–43.
  4. ^ Bazin, Henry (1862). "Expériences sur les ondes et la propagation des remous". Comptes Rendus des Séances de l'Académie des Sciences (in French). 55: 353–357.
  5. C. R. Acad. Sci. Paris
    . 72.
  6. doi:10.1080/14786449508620739
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  7. doi:10.1063/1.863011
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  8. doi:10.1103/PhysRevLett.19.1095
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  9. ISBN 9783540659198
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  10. doi:10.1063/1.2205916
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  11. S2CID 219506298.{{cite journal}}: CS1 maint: multiple names: authors list (link
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  12. ^ "Photons advance on two fronts". EETimes.com. October 24, 2005. Archived from the original on July 28, 2012. Retrieved 2011-02-15.
  13. ^ Fred Tappert (January 29, 1998). "Reminiscences on Optical Soliton Research with Akira Hasegawa" (PDF).
  14. hdl:10261/54313
    .
  15. S2CID 35230072
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  16. ISBN 978-0-7923-1029-7
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  17. ISBN 978-3-527-40417-9
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  18. PMID 16516929
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  19. PMID 15994235.{{cite journal}}: CS1 maint: multiple names: authors list (link
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  21. S2CID 2218193.{{cite journal}}: CS1 maint: multiple names: authors list (link)[dead link
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  22. ISBN 0-444-70283-0
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  23. ^
    PMID 35210566
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  24. PMID 23798395
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  25. ^
    S2CID 248303403
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  26. S2CID 205567456
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  27. S2CID 216407866
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  28. PMID 27533089
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  29. S2CID 121609608. Archived from the original
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  30. S2CID 55223766
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  31. S2CID 250768449
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  32. S2CID 119333950
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  33. S2CID 54184500
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  34. ^
    S2CID 220301701
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  35. S2CID 85461409
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  36. S2CID 119331128
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  37. ^ Physics World: Astronomy and Space. Spacecraft in a 'warp bubble' could travel faster than light, claims physicist. March 19, 2021. https://physicsworld.com/a/spacecraft-in-a-warp-bubble-could-travel-faster-than-light-claims-physicist/<accessed on June 29, 2021>
  38. ^ Powell, Devin (20 May 2011). "Rogue Waves Captured". Science News. Retrieved 24 May 2011.

Further reading

External links

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