## Guidance: Not by force but by information

[See ref 5 ]

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### Anything that changes from a fast road to a slow one without thinking must obey a 'law of refraction'.

The law of refraction received its name from optics, but it applies much more universally. It makes no difference whether we observe a beam of light, an avalanche of snow, or a herd of cattle that move from one kind of terrain to another where they have to change speed. They all change direction as if they were refracted by the interface. The figure below illustrates the principle.

The yellow lines represent the front of an object (e.g. the wavefront of the beam of light, the front of the avalanche, or the first row of the herd of cattle) approaching the interface between 2 substrates. After the left part of the front has passed the interface, the new substrate slows it down while the rest of the front still proceeds with the original speed. As a result the front turns into a new direction as if is was refracted like a beam of light.

### Cells do not obey a 'law of refraction'.

Whatever mechanical force is supposed to explain the guidance of the cells (e.g. differential adhesion, different local pH which may alter cortex activity, etc.), it will ultimately have to change the speed of the cells. Let us assume, that the guidance we observed on gold surfaces with glass 'roads' scratched into them, is due to e.g. a higher adhesion on gold than on glass. Consequently, cells crossing over from glass to gold should slow down while obeying a law of refraction. Accordingly, their tracks should turn towards the perpendicular line (see figure below). On the other hand, if gold would accelerate the speed of the cells, their track would turn away from the perpendicular line. However, either way it is certain that a cell coming from the lower right quadrant has to advance into the upper left quadrant after crossing the border between gold and glass.

In contrast, I observed cells that acted quite differently. For example, instead if turning towards or away from the perpendicular line, the cell in the figure below moved into a totally unexpected quadrant!.

There are other prediction of the law of refraction that cells disobey. For example, if their angle of incidence is 90° they would have to continue in the same direction, regardless of the difference between the substrates (see figure below).

Again, this is not what I saw actual cells do. In the example below, the cell approached at right angles but instead of continuing its direction, it turned at 90° and followed the direction of the interface.