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Comment by grondilu | Hacker News Reader

> Mechanical computers are computers that operate using mechanical components rather than electronic ones.

For anyone who's excited about mechanical computers, perhaps it is worth reminding that an electron is about a thousand times lighter than a nucleon. Therefore, it's probably fair to say that mechanical computers will always be more energy consuming than electronic ones, because they fundamentally need to move atoms around to operate.

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Taking this to its logical extreme, photonic computing should be significantly more efficient than electronic computing. Eventually.

Is that the end-game? Is there anything that would theoretically get closer to the Landauer limit than photonic computing? It’s way out of my element but I suppose this is a good venue to ask the question.

https://en.m.wikipedia.org/wiki/Landauer%27s_principle

The big problem in photonic computing is actually making an optical transistor, i.e. a switch where the presence of photons coming from one source controls whether of photons coming from another source pass. This is harder than electrical transistors because photons are bosons and don't interact with each other, so even theoretically this is hard to imagine.

Papers that claim some progress pop up every once in a while but I haven't seen anything promising yet.

Yes, general photonic computing is mostly “theoretical” at the moment. Still, discussion of theory is important. I wish I could add more to your comment but I’m so far out of my depth that it would be simply misleading (blind leading the blind). I believe there’s theory saying it’s theoretically possible to create efficient photon<->matter interfaces which could achieve transistor-like behavior … but there’s too much I don’t understand to be able to evaluate whether there are inherent limitations which kill the practical application of the proposed theoretical mechanisms.

I think companies have come up with some practical applications of limited photonic “computing” at interface edges but I’ve heard that until we no longer need to convert photonics to electronics it won’t surpass electronics for general computing.

> so even theoretically this is hard to imagine.

Possibly a strech, but transistors are basically current amplifiers, so their optical equivalent should be... lasers. Indeed lasers are optical amplifiers. Whether or not they can be turned into logic gates as transistors can, I don't know.

Maybe not more efficient, but maybe more resilient to electromagnetic storms, not prone to overheating (maybe), etc... Maybe it's about fitting constrained scenarios.

It seems like they may be prone to overheating in some fashion. All that electricity and motion has to cause some kind of thermal load. Or am I way off base?

Yes. Friction is usually the limiting factor in mechanical systems. It causes a lot of heat, noise, stress, and wear on all interacting parts. It requires all sorts of messy approaches to mitigate, such as lubricants and bearings. Electricity is basically magic by comparison.

> but maybe more resilient to electromagnetic storms

If you mean solar flares, that's generally an issue with long transmission lines, as opposed to very small circuits.

Wouldn't that all depend on how much energy is used for computing, and how much for fetching and storing the bits involved? If the requirements involve slow computation with extremely long-term storage, perhaps mechanical computing can theoretically have an advantage.

Then again, Chuck Moore's GA144 shows there's still plenty of room when it comes to optimizing electron-based computing for those kind of extreme scenarios as well.

[0] https://www.youtube.com/watch?v=0PclgBd6_Zs

sure, but how many electrons do we typically move around as a single signal

Few, if any; instead, it's typically the propagation of an electromagnetic wave that transmits a signal: https://en.wikipedia.org/wiki/Speed_of_electricity

what is the electromagnetic wave made of, what's the substrate it is composed of and moving through?

The wave is electromagnetic energy passing through a waveguide (typically copper) mediated by electrons. See https://en.wikipedia.org/wiki/Waveguide

“Few”, yes. But definitely some. I don’t think you can have propagation of EM wave through a conduit without at least pushing one electron into the conduit and removing one electron from the other side of the conduit.

Yes, but it's subtle; see https://en.wikipedia.org/wiki/Drift_current and https://en.wikipedia.org/wiki/Drift_velocity and https://en.wikipedia.org/wiki/Electron_mobility for more details

I was pretty surprised about this since I had mistakenly believed that electrons had a velocity near the speed of light, which I think is only true in particle accelerators.

Indeed - I thought most college Physics 2 courses teach that electrons actually move quite slowly through conductors. It’s the “wave” which propagates near the speed of light, not the particles.

My mistake was being a biologist, and skipping or sleeping my way through the EE part of physics :) and then saying the wrong thing in front of some very smart people

Lord, I make that mistake on HN nearly every month. At least you didn’t have a “Putnam award” moment:

https://news.ycombinator.com/item?id=35079

I've had something similar- when I was deciding what grad school to go to, I was explaining how RNA enzymes work to some professor at UC Boulder, who ended up being Tom Cech (who won the Nobel for discovering RNA enzymes); he had to correct a lot of the details I messed up. I ended up going to UCSF and fortunately didn't try to explain prions to Stanley Prusiner.

In short, nearly everything I have learned is from saying dumb things in front of very smart people who instantly understood my misunderstanding and knew exactly how to explain it so I understood. That includes Sanjay Ghemawat and Jeff Dean telling me "your idea isn't so good, it's n-squared, here's a linear solution"

https://youtu.be/2Vrhk5OjBP8?si=gDXZKYeFkVoAs_LG

AlphaPhoenix did an amazing experiment to measure the speed of electricity FWIW. His other videos are incredible as well and explain EM physics in an absolutely outstanding way.

This veritasiun is relevant, complete with reddit discussion:

https://www.reddit.com/r/engineering/comments/qxrsrp/the_big...

What if you made a really big circuit consisting of a battery, switch, lightbulb, and a wire that goes out 300k km on either side making a circuit that should take 1s at the speed of light to travel through. How long after closing the switch will it take for the light to go on?

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