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An abstruse concept in the physics of David Bohm, referring to an order that is 'hidden' in the commonsense, physical universe that we assume we live in.

The analogy that stuck in my head, explaining this, was something like the following:

We imagine a cylinder, filled with a perfectly viscous, oily fluid, closed at the bottom, open at the top, with an attached crank handle which will rotate the cylinder wall (but not the base).

Sprinkle a few drops of ink on the surface of the cylinder - the pattern they form is the order that we will render implicate.

Now turn the handle: the viscosity of the fluid will mean that its upper surface will rotate. The speed of the rotation (measured in degrees, not units of length) will vary according to the distance from the inside wall of the cylinder.

Because the drops of ink are not strict points - they have extension in the plane of the liquid's surface - their various parts will be dragged by the rotation at different rates, so as we turn the handle the inkspots will gradually 'smear out', becoming circles centred around the centre of the cylinder. In the analogy, this represents the universe as we see it today.

In the real world, owing to the second law of thermodynamics, that's the end of the matter: the spots are gone forever (brownian motion, diffusion, etc., and the imperfect viscosity of any actual physical medium, will ensure that.)

But in an ideal, perfect, system, where the behaviour is mathematically precise, if we turn the handle the same number of turns in the opposite direction, the smeared out circles would un-smear, and we'd eventually recover the original pattern of spots - the implicate order.

Bohm's physics is a hidden variable model, because he proposed that an analogous implicate order operates, providing a strict determinism, underlying what is normally thought of as 'simply' randomness in quantum behaviour.