[ExI] Reversible computing (Was: Re: ET Emergence (Was Re: Uploads as a group of AI agents))

[John Clark]

On Wed, Apr 1, 2026 at 9:00 AM Adrian Tymes via extropy-chat <
extropy-chat at lists.extropy.org> wrote:

* >> With careful design the Casimir force can be shaped, for example if
>> you use a dielectric material instead of a conductive metal the attraction
>> is greatly reduced,  and if you put a fluid with carefully designed
>> dielectric properties between the plates you can even get a force of
>> repulsion instead of attraction. You could use Casimir to keep two parts in
>> your nanomachine precisely aligned without contact, and then you could make
>> a no contact no wear bearing that would operate similar to the way magnetic
>> levitation does but by using quantum vacuum forces instead of magnetism.*
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> * > How would this work?  If you just had two surfaces near each other,
> they would attract.  If you had a part balanced between two others, if it
> got out of alignment, it would be exponentially more attracted toward the
> one it was nearer - but for alignment, wouldn't you need it to be more
> attracted to the one it is further away from, so it is most (net) attracted
> to the equilibrium point?*


*First of all I should emphasize that a reversible perfectly frictionless
Casimir Bearing would be nice to have but would not be absolutely necessary
for Drexler style Nanotechnology, instead you could use something that
works in a way similar to the way a bacterial flagellar motor works, it
would be robust reliable and much easier to build, but it would not be as
efficient or be reversible because it would not be perfectly frictionless.
But it works well enough to do a lot of interesting things as bacteria can
attest.  *

*Purely attractive forces in 3-D will always be unstable, but if you
pattern the surface in just the right way you can cause the Casimir force
to depend on lateral alignment and even become repulsive. The pattering
creates a metamaterial that produces a periodic vacuum energy landscape
with preferred angular positions. The  teeth of a gear also produce
preferred angular positions  but metamaterials can  do it without any
physical contact, all they need is a vacuum. *

*I asked Mr.GPT to give me a design for a **Casimir Force Bearing and this
is what he came up with: *
*===*

*A **repulsive Casimir bearing is the closest thing to a true “quantum
levitation” device at the nanoscale. I’ll give you a concrete design, then
walk through the physics and numbers so you can see how it could actually
work.*
------------------------------
Concept: a levitated cylindrical rotor

*We want to reverse the usual attractive Casimir effect so that:*

   - * The rotor is pushed away from the stator *
   - * A stable equilibrium gap forms (say ~50 nm) *
   - * The rotor “floats” without contact *

The key trick: a three-material system

*Repulsive Casimir forces don’t normally occur in vacuum between two
solids. But they can occur if you insert a medium with the right optical
properties.*
Lifshitz condition (qualitative)

*You can get repulsion if, over relevant frequencies:*
*ε1 <εmedium <ε2 *

*So the design is:*

   - * Rotor: high-permittivity material *
   - * Gap: carefully chosen fluid *
   - * Stator: lower-permittivity material*

A concrete design Geometry

   - * Rotor radius: R=1 μm *
   - * Length: L=10 μm *
   - * Target gap: d=50 nm *

------------------------------
Materials (example set)

   - * Rotor: gold (high permittivity at relevant frequencies) *
   - * Medium: bromobenzene (commonly used in experiments) *
   - * Stator: silica (SiO₂) *

*This combination has actually been shown experimentally to produce
repulsive Casimir forces.*
What the force looks like

*The pressure still scales roughly like:*

*P∼d41 *

*But now it’s positive (repulsive instead of attractive).*

*At ~50 nm:*

   - * Typical magnitude: on the order of 1–10 Pa *

*So very similar strength as before—but reversed.*
------------------------------
Levitation behavior

*Now the key difference:*
Inward vs outward balance

   - * If the rotor gets too close to the stator → repulsion increases
   sharply *
   - * If it moves away → repulsion drops rapidly *

*This creates a stable equilibrium gap*
------------------------------
Effective stiffness

*Using the same geometry as before:*

*Surface area:*

*A≈6×10−11 m2 *

*Force:*

*F∼(1–10)Pa×A∼10−10 to 10−9 N *

*Effective spring constant:*

*k∼dF∼10−3 to 10−2 N/m *

*So:*

   - * Nanometer displacements → piconewton restoring forces *
   - * Enough for stable confinement *

------------------------------
Why this is *much* better than the attractive case

*In the previous (attractive) design:*

   - * Any instability → collapse → sticking *

*Here:*

   - * Instability → increased repulsion → self-correction *

*So this behaves like:*

*A true non-contact bearing with a restoring gap*

Rotation and friction

*The rotor can spin because:*

   - * The Casimir force is mostly normal (radial) *
   - * It does not strongly oppose tangential motion *

*Remaining losses come from:*

   - * Fluid viscosity (important!) *
   - * Fluctuation-induced electromagnetic drag (tiny) *

*So the dominant dissipation is actually:*

*the fluid you needed to create repulsion*

------------------------------
The big tradeoff

*This design solves one problem but introduces another:*
Pros

   - * Stable levitation *
   - * No stiction *
   - * Passive centering *
   - * Potentially reversible (low dissipation) *

Cons

   - * Requires a liquid medium, not vacuum *
   - * Fluid introduces: *
      - * Viscous drag *
      - * Thermal noise *
      - * Possible chemical complications *

------------------------------

Where metamaterials could improve this

*Metamaterials could, in principle:*

   - * Reproduce the “effective medium” condition without a liquid *
   - * Create: *
      - * Vacuum-compatible repulsion *
      - * Tunable equilibrium gaps *
      - * Reduced dissipation *

*That’s still mostly experimental, but it’s a major research direction.*
------------------------------
Stability check (important insight)

*This system is stable because:*

   - * The Casimir force is strongly distance-dependent *
   - * Repulsion increases faster than any destabilizing drift *

*So you get:*

*A potential well centered at a finite gap, not at contact*

*That’s exactly what you want for a bearing.*
------------------------------
Intuition

*The rotor isn’t “floating freely” like in air.*

*It’s more like:*

*Sitting in a very steep, invisible cushion of quantum pressure that keeps
it from touching the walls.*


*How far are we?*

*We already have:*


   - * Measurements of Casimir forces in structured geometries *
   - * Demonstrations of lateral Casimir forces and torques *
   - * Early work on Casimir engineering with metamaterials *

*But we do not yet have:*


   - * A fully repulsive vacuum Casimir system *
   - * A working Casimir bearing*

------------------------------
Bottom line

*A repulsive Casimir bearing using a fluid medium:*

   - *Can genuinely levitate a nanoscale rotor *
   - * Provides passive, stable confinement *
   - * Allows low-friction rotation *

*But:*


   - * It trades vacuum operation for fluid damping *
   - * And requires very careful material selection*


*=========*
*John K Clark*
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