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See Drexler's mechanical nanotechnology from 1989.[1]

There's a minimum size at which such mechanisms will work, and it's bigger than transistors. This won't scale down to single atoms, according to chemists.

[1] http://www.nanoindustries.com/nanojbl/NanoConProc/nanocon2.h...

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It seems like you've misremembered the situation somewhat.

Merkle developed several of his families of mechanical logic, including this one, in order to answer some criticisms of Drexler's earliest mechanical nanotechnology proposals. Specifically:

1. Chemists were concerned that rod logic knobs touching each other would form chemical bonds and remain stuck together, rather than disengaging for the next clock cycle. (Macroscopic metal parts usually don't work this way, though "cold welding" is a thing, especially in space.) So this proposal‚ like some earlier ones like Merkle's buckling-spring logic, avoids any contact between unconnected parts of the mechanism, whether sliding or coming into and out of contact.

2. Someone calculated the power density of one of Drexler's early proposals and found that it exceeded the power density of high explosives during detonation, which obviously poses significant challenges for mechanism durability. You could just run them many orders of magnitude slower, but Merkle tackled the issue instead by designing reversible logic families which can dissipate arbitrarily little power per logic operation, only dissipating energy to erase stored bits.

So, there's nothing preventing this kind of mechanism from scaling down to single atoms, and we already have working mechanisms like the atomic force microscope which demonstrate that even intermittent single-atom contact can work mechanically in just the way you'd expect it to from your macroscopic intuition. Moreover, the de Broglie wavelength of a baryon is enormously shorter than the de Broglie wavelength of an electron, so in fact mechanical logic (which works by moving around baryons) can scale down further than electronic logic, which is already running into Heisenberg problems with current semiconductor fabrication technology.

Also, by the way, thanks to the work for which Boyer and Walker got part of the 01997 Nobel Prize in Chemistry, we probably know how ATP synthase works now, and it seems to work in a fairly similar way: https://www.youtube.com/watch?v=kXpzp4RDGJI

The interesting question is how much energy is lost to mechanical friction for a single logic operation, and how this compares to static leakage losses in electronic circuits. It should also be noted that mechanical logic may turn out to be quite useful for specialized purposes as part of ordinary electronic devices, such as using nano-relay switches for power gating or as a kind of non-volatile memory.

That's one of many interesting questions, but avoiding it is why Merkle designed his reversible logic families in such a way that no mechanical friction is involved, because there is no sliding contact. There are still potentially other kinds of losses, though.

>mechanical logic (which works by moving around baryons) can scale down further than electronic logic, which is already running into Heisenberg problems with current semiconductor fabrication technology.

I think I must be missing something here, I thought this was working with atoms. Are you saying that someday mechanical logic could be made to work inside the nucleus? Seems like you might be limited to ~200 nucleons per atom, and then you'd have to transmit whatever data you computed outside the nucleus to the nucleus in the next atom over? Or are we talking about converting neutron stars into computing devices? Do you have a good source for further reading?

No, no, not at all! That kind of thing is very speculative, and I don't think anybody knows very much about it. What I'm saying is that the position of a nucleus is very, very much more precisely measurable than the position of an electron, so it has a much weaker tendency to tunnel to places you don't want it to be, causing computation errors. That allows you to store more bits in a given volume, and possibly do more computation in a given volume, if the entropy production mechanisms can be tamed.

We routinely force electrons to tunnel through about ten nanometers of silicon dioxide to write to Flash memory (Fowler–Nordheim tunneling) using only on the order of 10–20 volts. That's about 60 atoms' worth of glass, and the position of each of those atoms is nailed down to only a tiny fraction of its bond length. So you can see that the positional uncertainty of the electrons is three or four orders of magnitude larger than the positional uncertainty of the atomic nuclei.

> Chemists were concerned that rod logic knobs touching each other would form chemical bonds and remain stuck together, rather than disengaging for the next clock cycle.

The rod & shaft designs are passivated. This kind of reaction wouldn't happen unless you drove the system to way higher energies than were ever considered.

> Someone calculated the power density of one of Drexler's early proposals and found that it exceeded the power density of high explosives during detonation

I think this is a (persistent) misunderstanding. His original work involved rods moving at mere millimeters per second. There are a number of reasons for this, of which heat dissipation is one. However all the molecular mechanics simulations done operate at close to the speed of sound in the material, simply because they would otherwise be incalculable. There is sadly a maximum step size for MD simulations that is orders of magnitude lower than what you would need to run at realistic speeds.

> You could just run them many orders of magnitude slower, but Merkle tackled the issue instead by designing reversible logic families which can dissipate arbitrarily little power per logic operation, only dissipating energy to erase stored bits.

The rod logic stuff is supposed to be reversible too. Turns out it isn't though. But it could be close enough if operated at very low speeds or very low temperatures.

The rod logic stuff is WAY smaller than the rotational logic gates in TFA. For some applications that matters, a lot.

If you are going to go for the scale of these rotational systems, you might as well go electronic.

I appreciate the corrections, but what scale do you think the link-and-pivot systems are intended to operate at?

(Also, passivation doesn't eliminate van der Waals bonds.)

And why wouldn't it work? Linear slide like mechanisms consisting of a silver surface and single molecule have been demonstrated[0]. The molecule only moved along rows of the silver surface. It was demonstrated to stay in one of these grooves up to 150 nm. A huge distance at this scale.

[0]https://www.osti.gov/servlets/purl/1767839

It can work (see my sibling comment) but it's tricky. The experiment you link was done under ultra-high vacuum and at low temperatures (below 7 K), using a quite exotic molecule which is, as I understand it, covered in halogens to combat the "sticky fingers" problem.

You seem to be knowledgeable about this topic. The reversible component designs in the article appear to presuppose a clock signal without much else said about it. I get that someone might be able to prototype an individual gate, but is the implementation of a practical clock distribution network at molecular scales reasonable to take for granted?

I'm only acquainted with the basics of the topic, not really knowledgeable. It's an interesting question. I don't think the scale poses any problem—the smaller the scale is, the easier it is to distribute the clock—but there might be some interesting problems related to distributing the clock losslessly.

Not an expert, but would this count as molecular scale :)?

https://en.wikipedia.org/wiki/Chemical_clock

(This version can be done at home with halides imho: https://en.wikipedia.org/wiki/Iodine_clock_reaction)

To your question: I suppose all you need is for the halide moieties (Br) in your gates to also couple to the halide ions (Br clock?). The experiment you link was conducted at 7K for the benefit of being able to observe it with STM?

That's a different kind of clock, and its clock mechanism is a gradual and somewhat random decrease in the concentration of one reagent until it crosses a threshold which changes the equilibrium constant of iodine. It isn't really related to the kind of clock you use for digital logic design, which is a periodic oscillation whose purpose is generally to make your design insensitive to glitches. Usually you care about glitches because they could cause incorrect state transitions, but in this case the primary concern is that they would cause irreversible power dissipation.

The experiment was conducted at 7K so the molecule would stick to the metal instead of shaking around randomly like a punk in a mosh pit and then flying off into space.

Yeah you're probably right about the clocks but I hope that wouldn't stop people from trying :)

>The experiment was conducted at 7K so the molecule

Br is good at sticking to Ag so I suspect the 7K is mainly (besides issues connected to their AFM^W STM setup) because the Euro dudes love ORNL's cryo engineering :)

Br's orbitals are filled here because it's covalently bonded to a carbon, so it's basically krypton. Experiments with moving atoms around on surfaces with STMs are always done at cryogenic temperatures because that's the only way to do them.

>. Hence, the Br atoms kept the molecules on track, likely because their interaction with the surface substantially contributed to the barrier for molecular rotation

Yeah that's a reason people prefer AFM (but then they won't be able to do manipulation)?

[Br- is a "good leaving group", not so much at 7K maybe. You are also right in that, above all, they don't want their molecule sticking (irreversibly) to the (tungsten) tip ]

Not entirely.. terminal Br were also required to keep the molecule on the Silver tracks..

Those are some of the halogens I'm talking about. It's a little more polarizable than the covalently-bonded fluorine, so you get more of a van der Waals attraction, but still only a very weak one.

I'd love to see a watch manufacturer try to build a watch-sized purely mechanical computer

That's clearly feasible; the mechanical complexity for a mechanical computer is on the order of a Curta calculator, and I outlined some promising approaches to macroscopic mechanical digital logic 15 years ago in https://dercuano.github.io/notes/mechanical-computers.html. Since then MEMS has advanced significantly and gone mainstream, and photolithographic and reactive-ion-etching-based silicon fabrication has been used for other purposes, including watchmaking, with macroscopic silicon flexure components going into first TAG Heuer's Guy Sémon's Zenith Oscillator in the Zenith Defy Lab https://www.hodinkee.com/articles/zenith-defy-lab-oscillator... and then mainstream watches:

https://wornandwound.com/no-escapement-an-overview-of-obtain...

https://monochrome-watches.com/in-depth-the-future-of-silico...

https://www.chrono24.com/magazine/innovative-escapements-fro... (warning, GDPR mugging)

https://www.azom.com/article.aspx?ArticleID=21921

https://www.europastar.com/the-watch-files/those-who-innovat...

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