[HN Gopher] Reversible computing with mechanical links and pivots
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Reversible computing with mechanical links and pivots
Author : tennysont
Score : 92 points
Date : 2025-04-30 17:35 UTC (5 hours ago)
(HTM) web link (tennysontbardwell.com)
(TXT) w3m dump (tennysontbardwell.com)
| jstanley wrote:
| > Specifically, the Landauer's principle states that all non-
| physically-reversible computation operations consume at least
| 10^21 J of energy at room temperature (and less as the
| temperature drops).
|
| Wow! What an absurd claim!
|
| I checked the Wikipedia page and I think you actually meant
| 10^-21 J :)
| godelski wrote:
| FYI, total global energy production is a lot less than 10^21 J.
| It's south of 10^19 from what I can google...
| kragen wrote:
| Depends on which aspects of energy production you're
| concerned with and over what time period. Global marketed
| energy production is about 18 terawatts, which is about 1021
| J every year and 9 months. The energy globally produced by
| sunlight hitting the Earth, mostly as low-grade heat, is on
| the order of 100 petawatts, which is 1021 J every hour and a
| half or so. Global agriculture is in between these numbers.
| tennysont wrote:
| Fix! Ty!
|
| P.S. I once calculated the mass of the sun as 0.7kg and got
| 9/10 points on the questions.
| qoez wrote:
| For things like machine learning I wonder how much extra
| performance could be squeezed out by simply working with
| continuous floating values on the analog level instead of
| encoding them as bits through a big indirect network of nands.
| tails4e wrote:
| This is something that has been tried, basically constructing
| an analog matrix multiply/dot product and it gives reasonable
| power efficiency at into levels of precision. More precision
| and the analog accuracy leads to dramatic power efficiey losses
| (each bit is about 2x the power), so int8 is probably the sweet
| spot. The main issues are it is pretty inflexible and costly to
| design vs a digital int8 mac array, and hard to port to newer
| nodes, etc
| hermitShell wrote:
| I have wondered this and occasionally seen some related news.
|
| Transistors can do more than on and off, there is also the
| linear region of operation where the gate voltage allows a
| proportional current to flow.
|
| So you would be constructing an analog computer. Perhaps in
| operation it would resemble a meat computer (brain) a little
| more, as the activation potential of a neuron is some analog
| signal from another neuron. (I think? Because a weak activation
| might trigger half the outputs of a neuron, and a strong
| activation might trigger all outputs)
|
| I don't think we know how to construct such a computer, or how
| it would perform set computations. Like the weights in the
| neural net become something like capacitance at the gates of
| transistors. Computation is I suppose just inference, or
| thinking?
|
| Maybe with the help of LLM tools we will be able to design such
| things. So far as I know there is nothing like an analog FPGA
| where you program the weights instead of whatever you do to an
| FPGA... making or breaking connections and telling LUTs their
| identity
| thrance wrote:
| You lose a lot of stability. Each operation's result is
| slightly off, and the error accumulates and compounds. For deep
| learning in particular, many operations are carried in sequence
| and the error rates can become inacceptable.
| Legend2440 wrote:
| Deep learning is actually very tolerant to imprecision, which
| is why it is typically given as an application for analog
| computing.
|
| It is already common practice to deliberately inject noise
| into the network (dropout) at rates up to 50% in order to
| prevent overfitting.
| red75prime wrote:
| Isn't it just for inference? Also, differentiating thru an
| analog circuit looks... interesting. Keep inputs constant,
| wiggle one weight a bit, store how the output changed, go
| to the next weight, repeat. Is there something more
| efficient, I wonder.
| Calwestjobs wrote:
| TLC,QLC,MLC in ssd is it. so it is used already. and it gives
| you limits of current technology.
| ziddoap wrote:
| >*TLC,QLC,MLC"
|
| For those unaware of these acronyms (me):
|
| TLC = Triple-Layer Cell
|
| QLC = Quad-Level Cell
|
| MLC = Multi-Level Cell
| rcxdude wrote:
| It's possible, but analog multiplication is hard and small
| analog circuits tend to be very noisy. I think there is a
| startup working on making an accelerator chip that is based on
| this principle, though.
| grumbelbart wrote:
| There are optical accelerators on the market that - I believe -
| do that already, such as https://qant.com/photonic-computing/
| PendulumSegment wrote:
| This is very interesting because according to one of the authors
| of the mechanical computing paper(personal communication) they
| never dynamically simulated the mechanisms. It was purely
| kinematic. So this web browser simulation is new work.
| Reversibility might disappear once dynamics are modelled.
| mitthrowaway2 wrote:
| Indeed. The web simulation clearly applies damping, which is an
| irreversible element. A truly reversible process should
| probably be built around minimally-damped oscillating elements,
| so that the stored energy never needs to dissipate.
| PendulumSegment wrote:
| Even if damping is removed they might not be reversible.
| Logic gates that were found to be individually reversible,
| were found to have difficulties operating when connected in a
| circuit: https://core.ac.uk/download/pdf/603242297.pdf
| rkp8000 wrote:
| A great pedagogical article on thermodynamic vs logical
| reversibility, for those interested:
| https://arxiv.org/abs/1311.1886 (Sagawa 2014).
| Animats wrote:
| 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...
| kragen wrote:
| 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
| zozbot234 wrote:
| 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.
| kragen wrote:
| 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.
| gene-h wrote:
| 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
| kragen wrote:
| 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.
| gradschool wrote:
| 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?
| kragen wrote:
| 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.
| gsf_emergency wrote:
| 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?
| kragen wrote:
| 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.
| gsf_emergency wrote:
| 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 :)
| kragen wrote:
| 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.
| gsf_emergency wrote:
| > _. 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)?
| gsf_emergency wrote:
| Not entirely.. terminal Br were also required to keep the
| molecule on the Silver tracks..
| kragen wrote:
| 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.
| 7373737373 wrote:
| I'd love to see a watch manufacturer try to build a watch-sized
| purely mechanical computer
| coumbaya wrote:
| This is the concept behind the computers in The Diamond Age right
| ? Or am I mistaken ?
| fintler wrote:
| It's very similiar. The rod logic in diamond age (Eric Drexler
| was the one who originally came up with it) moves linearly --
| not rotationally like this does. It's also reversible.
| fintler wrote:
| Classical reversible computing feels like it would be a good way
| to interface with a quantum computer (since it's also reversible
| in theory).
| danbmil99 wrote:
| Quantum computation came directly out of reversible computing.
| Look for example at the Fredkin and Toffoli gates.
| mathgradthrow wrote:
| A big problem with the idea of physical reversible computing is
| the assumption that you get to start with a blank tape. Blank
| tapes are trivial to acquire if I can erase bits, but if I start
| with a tape in some state, creating working space in memory
| reverisbly is equivalent (identical) to lossless compression,
| which is not generally achievable.
|
| If you start with blank tape then it isn't really reversible
| computing, you're just doing erasure up front.
| kragen wrote:
| I don't think your criticism is applicable to any reversible-
| computing schemes that I've seen proposed, including this one.
| They don't _assume_ that you get to start with a blank memory
| (tapelike or otherwise); rather, they propose approaches to
| constructing a memory device in a known state, out of atoms.
| mathgradthrow wrote:
| What do you think you're saying here? Building a memory
| device in a known configuration is erasing bits.
| kragen wrote:
| Yes, building a memory device in a known configuration is
| erasing bits. Once you've built it, you can use it until it
| breaks. As long as you decompute the bits you've
| temporarily stored in it, restoring it to its original
| configuration, you don't inherently have to dissipate any
| energy to use it. You can reuse it an arbitrarily large
| number of times after building it once. If you want to
| compute some kind of final result that you store, rather
| than decomputing it, that does cost you energy in the long
| run, but that energy can be arbitrarily small compared to
| the computation that was required to reach it.
|
| Consider the case, for example, of cracking an encryption
| key; each time you try an incorrect key, you reverse the
| whole computation. It's only when you hit on the right key
| that you store a 1 bit indicating success and a copy of the
| cracked key; then you reverse the last encryption attempt,
| leaving only the key. Maybe you've done 2128 trial
| encryptions, each requiring 213 bit operations, for a total
| of 2141 bit operations of reversible computation, but you
| only need to store 27 bits to get the benefit, a savings of
| 2135x.
|
| Most practical computations don't enjoy quite such a
| staggering reduction in thermodynamic entropy from
| reversible computation, but a few orders of magnitude is
| commonplace.
|
| It sounds like you could benefit from reading an
| introduction to the field. Though I may be biased, I can
| recommend Michael Frank's introduction from 20 years ago:
| https://web1.eng.famu.fsu.edu/~mpf/ip1-Frank.pdf
| rcxdude wrote:
| Yes, but erasing the tape once is much better than erasing the
| tape many times over.
| danbmil99 wrote:
| Now that I think of it, if using damped springs, the system would
| not be reversible. Energy is dissipated through the damping, and
| the system will increase in entropy and converge on a local
| energy minimum point.
|
| Another way of looking at it: there are 4 states going in (0 or 1
| on 2 pushers) but there are only 2 states of the 'memory'
| contraption, so you lose a bit on every iteration (like classical
| Boolean circuits)
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