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We feel there is a need for an interim forum for LLLT information. A good study can take a year to complete and another year or so of waiting before it is accepted for publication. Then, a quick look at the results could be valuable. And further, many qualified clinicians do not have the time, money or patience to make scientific studies.
But still they have a lot of expertice which they would be willing to share with others, should there be a suitable forum for it. And from the perspective of the therapist: Who can afford to subscribe to several scienticis magazines? So where can I find out about new research and new clinical observations without spending a fortune on subscriptions and congresses?

Well, this is precicely the "void" LaserWorld Guest Editorial wants to fill! Several persons have been invited to send their contribution. But we certainly accept unsolicited contributions, but reserve the right to edit them (in cooperation with the author) or to refuse publication.

   Extraordinary biological action of low intensity laser radiation is well known and widely used in laser therapy. In spite of a great success in this field the primary mechanism of laser stimulating effect remains disputable. The main question to be answered is: whether the observed effects are caused by absorption of light by some photoreceptors, the excitation of which starts some chains of biochemical events, or we encounter here with some other mechanism of interaction of light with biological matter. If the first is true, i.e. primary interaction of light with biological object is of pure photochemical nature, then we must find those receptors, study their properties and look for the light source, which emission is best overlapped with the absorption band of the receptors. In this case coherence and polarization of light can not be of importance.
   Indeed high coherence of light (i.e. when the phase of oscillations in electromagnetic wave remains unchanged for a long time) may be important if the phase of electron oscillations in a substance, excited by light, is also kept unchanged long enough. Large biomolecules contain tremendous number of atoms, interaction between which leads to very fast loss of the electron oscillations phase (during 10-13 sec or less). Because of this reason high coherence of light is not needed, and photochemical effects produced by coherent and noncoherent light will be indistinguishable.
   One may use a concept of spectral properties of light and substance instead of coherence and phase memory. Then the above statement can be expressed in the following form: as the spectral absorption bands of organic substances are quite broad (which means that the phase memory is short) one does not need for exciting a photochemical reaction the light with very narrow spectral line (coherent light).
   Polarization of light normally is also not important in photochemical process. It could be important if all (or majority of) absorbing molecules were oriented in space such a way that their transition moments happened to be aligned along some preferential direction. But there is absolutely no reason why the mentioned above hypothetical photoreceptors would be aligned in a body along one direction. And for random orientation of photoreceptors in a body the polarized and nonpolarized light will produce the same effect.
   So we may conclude that if the biological effect of light is based on photochemical processes (i.e. on resonance interaction of light with absorbing molecules) the coherence and polarization of light do not play any important role. From this point of view there is no special reason for using lasers in therapy instead of photodiodes or other noncoherent light sources. In case of noncoherent light source there is also no sense in using polarized light.
   However there are enough of data today indicating importance of coherence (superior qualities of lasers) and polarization at application of light in therapy [1]. From this follows that there must exist some nonresonant mechanisms of light interaction with biological matter, which determine universal action of laser light on a human body. Of course, one can not deny existence of specific photochemical interactions of light with some substances in a body (superoxideolis-mutase, catalase, ceruloplasmin, cytochromes a and a3, cytochromoxidase, endogenic porphyrins, oxygen). But those specific interactions can hardly explain universal stimulating biological effect of laser light observed for very wide range of laser wavelengths.
   In case of nonabsorbing medium the photochemical mechanisms are excluded. Nevertheless light may influence the properties of such transparent medium. In case of inhomogeneous medium like a biological one the considerable effect might be caused even at low intensities of light. Nonresonant action of light on transparent dielectric objects (for example cells) is based on the interaction of light induced dipole moments of these objects between themselves or with the incident light field. Two cases must be distinguished: interaction with noncoherent and coherent light.

   I. Noncoherent light (importance of polarization). Illumination of particles (cells) with light induces in them oscillations of electrons which are synchronous for all particles. This effect takes place for both laser and non-laser sources (lamps, photodiodes). If particles are packed closely enough to each other (as cells in a body) the dipole moments induced in them interact as is shown in the Fig. 1. As dipole moments in all particles oscillate synchronously their polarities are kept identical in respect to each other all time. The result of interaction strongly depends on light polarization. Both attraction (Fig. 1a) and repulsion of particles to each other (Fig. 1b) can be realized depending on orientation of electrical vector of electromagnetic wave relatively the pair of interacting particles. For linearly polarized light the orientation of electric field oscillations remains constant which provides preservation of the type of interaction in time (attraction for the case of Fig.1a and repulsion in the case of Fig.1b). For circularly polarized light or nonpolarized light orientation of electrical oscillations in respect to the particles position changes very fast (from the case of Fig 1a to the case of Fig1b and back). As a result repulsion and attraction replace each other very quickly and on the average the force of interaction between particles becomes equal to zero.

   So we may make the first conclusion: noncoherent light may influence ensemble of particles (biological system) through light induced dipole-dipole interaction.
The effect is realized only for linearly polarized light and is absent if the incident light is circularly polarized or nonpolarized.
   There is another possibility of nonresonant action of light on particles – optical Kerr effect. If polarizability of a particle is anisotropic then the direction of a dipole moment induced by light in the particle may not coincide with the direction of electrical vector of light (as is shown in the Fig.2a). Due to that a torque appears which causes rotation of a particle until its dipole moment becomes aligned along the electrical vector of the field (Fig.2b). Linear polarization of the incident light is also necessary to produce this effect.

   In comparison with the dipole-dipole interaction the optical Kerr effect is less probable at interaction of low intensity light with biological objects. Because of low intensity of light, required by biological safety, considerable torque may appear only for particles of micron scale size. Such large particles contain large number of electrons whose oscillations are summarized producing the light induced dipole moment of the whole particle. If the particle is not a crystal but a biological cell or an organelle then it is difficult to expect essential anisotropy over the whole volume of a particle.
   So the second conclusion is that in principle noncoherent light may also act on particles via optical Kerr effect causing some torque acting on them. The effect also requires linearly polarized light. But this mechanism of light action on biological system is less probable in comparison with dipole-dipole interaction.

   Coherent light.
   Both above mechanisms may also act in case of coherent light. But in addition to that there is a specific physical mechanism which distinguishes action of laser light on a biological system from the action of noncoherent one.
   The specific feature of illumination of an object with coherent light is formation of the so called speckle structure. This is a strong micron-scale modulation of light intensity, which unavoidably appears at the surface and in a bulk of optically inhomogeneous medium even at ideally uniform illumination. Such modulation is a result of the interference of micro-beams reflected and scattered by heterogeneous medium (Fig.3).

It is formed only at illumination of an object with coherent light. The higher degree of the coherence the higher degree of the intensity modulation in a speckle structure is observed. At illumination of the same object with noncoherent light from lamps, photodiodes or so like the speckle structure disappears and the object appears illuminated uniformly.
   Interaction of the speckled light field with particles leads to appearance of so called gradient forces. The gradient forces have electric nature and can be easily understood with help of the Fig. 4.

   Let us assume that some dielectric particle is placed in the homogeneous electric field. Under the action of the field the particle becomes polarized and two Coulomb forces acts on separated charges in opposite directions. If the field is homogeneous the two forces compensate each other, and the resulting force acting on the particle is equal to zero (Fig. 4a). But if the electric field is non-uniform and has a gradient along some direction then Coulomb forces, acting on the particle in the opposite directions, do not compensate each other any more. In this case the resulting force (which is called gradient force) is not equal to zero, and drives the particle towards the stronger field (see Fig. 4b). This is also true for alternative electric component of electromagnetic wave. The electrons in the particle follow synchronously oscillations of electric component of the electromagnetic wave. This is why the resulting force does not change its direction, in spite of oscillations of electric field in a light wave, and always points towards the region of the higher field strength (compare of Figs. 4b and 4c).
   From above considerations follows that if a light field with modulation of intensity in space (gradient field) is applied to particles, they will be subjected to action of gradient forces pulling them to the region of maximal intensity. Such forces are well known in optics and employed in laser tweezers.
   Calculations show that at application of low intensity lasers to a medium at room temperature the gradient forces are quite important for comparatively large particles whose sizes are of a micron scale. For small particles like molecules the energy of interaction connected with gradient forces is much smaller than the thermal energy of molecules at room temperature, and the action of those forces can be ignored.
   Now we can make another important conclusion: there is a specific influence of coherent light on a system of particles (cells in a biological system) which is based on the action of gradient forces originated by speckle structure of laser field inside the illuminated object. Polarization of light is also a factor of importance for this mechanism of action: effect of gradient forces is stronger for linearly polarized light because in this case the speckle structure of laser field has a higher contrast.
   Theoretical estimation shows that at typical conditions of illumination of a body with low intensity lasers the gradient forces plays major role in comparison with the dipole-dipole interactions.
So finally the above considerations lead us to the following conclusions:

1. Apart from a photochemical mechanism of light action on biological systems there exist more universal physical mechanisms based on interactions of light induced dipole moments of particles between themselves and with electrical component of light field. Both laser and non-laser light can produce biological effect via these mechanisms.
   
2. Laser light is the most efficient at action on biological systems. At that linearly polarized laser light is expected to produce better effect than nonpolarized or circularly polarized one.
   
3. Non-laser light can also produce biological action but of smaller efficiency than the laser one and only in case if it is linearly polarized.

   The action of gradient laser fields on particle ensembles and biological systems was considered in [2]. In particular it was shown that action of gradient laser fields on micron scale particles (cells, organelles large biomolecules) may influence biological processes through the following processes:

• change of local concentration and spatial orientation of particles;
• change of composition of particles of different type;
• selective increase of the partial temperature for larger particles;
• small reversible distortions of particles structure (cellular massage);
• stimulation of conformational changes in enzymes and other
  structures.

Fig. 5 illustrates action of gradient forces on 6 ? plastic particles in water illuminated by the interference field of He-Ne laser. One can see that under the action of gradient laser field all particles are gathered at the maximums of laser intensity. So the laser field here causes change of local concentration of particles.

Fig. 6 shows effect of spatial orientation of particles by a gradient field of Ar-laser. Two erythrocytes which at the first moment happened to be oriented by their long sides across the interference fringes (created by interference of two laser beams) some minutes later became oriented along the fringes.

Fig. 7 demonstrates splitting of erythrocyte rouleau by the interference field of Ar-laser. Uniform illumination of an erythrocyte rouleau with a single beam of the laser does not cause any change in its structure. But as soon as the interference structure is created (by switching on the second laser beam) the rouleau is immediately split into separate red blood cells.
There are also other experiments described in [2] which prove essential influence of gradient laser fields on some fundamental processes in a cell.

References
1. J Tunér and L Hode, "Laser Therapy. Clinical; practice and scientific background", Prima Books AB, 2002
2. A.N. Rubinov, J. Phys D: Appl. Phys. 36 (2003) 2317-2330.

Captions to figures

Fig.1. Interaction of dipole moments induced by light in neighboring particles.
a) – particles attract each other when electrical vector of light E is parallel to the line connecting the particles; b) - particles repel each other when electrical vector of light E is perpendicular to the line connecting the particles;

Fig.2. Rotation of the particle characterized by anisotropic polarizability under action of linearly polarized light. a) – the torque acts on a particle because the light induced dipole moment P does not coincide with the direction of the electrical vector E of the light field. b) – particle is aligned with its dipole moment P along the electrical vector E under action of the torque.

Fig.3. Interaction of reflected and scattered micro-beams in an inhomogeneous medium, which causes appearance of speckles of a laser field in the medium.

Fig.4. Polarization of a dielectric particle under action of uniform (a) and not uniform (b, c) electric field; b) and c) show that at change of the electric field sign the direction of the gradient force does not change.

Fig.5. Trapping of 6 ? plastic particles in water under action of gradient forces in the interference field of a He-Ne laser; a) – random distribution of particles at illumination by a single laser beam. b) – distribution of the particles in the fringes of the two beam interference laser field.

Fig.6. Aligning of erythrocytes under action of gradient forces in the interference fringes of Ar laser; a) – the first moment after the interference field has been switched on; b) – several minutes later – both erythrocytes have oriented themselves along the interference fringes.

Fig.7. Splitting of the erythrocyte rouleau under the action of gradient forces of the Ar-laser interference field; a) an erythrocyte rouleau illuminated by a single laser beam; b) – splitting of the rouleau into separate erythrocytes at switching on the interference field.

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