I've long been enamored with the idea of learning from analog computers to build the next generation of digital ones. In some perspective all our computers are analog, of a sort - today's computer chips are effectively leveraging electron flow through a carefully arranged metal/silicon substrate, with self-interference via electromagnetic fields used to construct transistors and build up higher order logic units. We're now working on photonic computers, presumably with some new property leading to self interference, and allowing transistors/logic above that.
"Wires" are a useful convenience in the electron world, to build pathways that don't degrade with the passing of the elections themselves. But if we relax that constraint a bit, are there other ways we can build up arrangements of "organized flow" sufficient to have logic units arise? E.g. imagine pressure waves in a fluid -filled container, with mini barriers throughout defining the possible flow arrangement that allows for interesting self-reflections. Or way further out, could we use gravitational waves through some dense substance with carefully arranged holes, self-interfering via their effect on space-time, to do computations for us? And maybe before we get there, is there a way we could capitalize on the strong or weak nuclear force to "arrange" higher frequency logical computations to happen?
Physics permits all sorts of interactions, and we only really use the simple/easy-to-conceptualize ones as yet, which I hope and believe leaves lots more for us to grow into yet :).
Electricity is also a wave. The wires are essentially waveguides for particles/waves traveling at near luminal speeds. So in theory anything done with electricity could be replicated using other waves, but to make it faster you would need waves that travel faster than electrons through a wire. Photons through a vacuum might be marginally faster, but pressure waves though a fluid would not.
If bitflips are a problem in a modern chip, imagine the number of problems if your computer ran on gravity waves. The background hum of billions of star collisions cannot be blocked out with grounded tinfoil. There is no concept of a faraday cage for gravity waves.
Nitpick: gravity waves [1] pretty universally refer to waves in fluid media in which the restoring force is buoyancy. Ripples in spacetime are usually called _gravitational_ waves.
You're right that the speed of light remains a constant limitation on propagation delay, but the defining limitation on the speed of computation is rather the clock speed - how long it takes for each round of computation. Electrons are comparatively slow due to the time it takes to fill and stabilize a transistor. Our hypothetical new type of computer will have to be faster to converge, rather than faster to propagate.
You're right about the bit flips though. I don't know if a gravitational wave computer is actually ever going to be feasible, just an interesting dream for the far future. Hopefully there are more options to consider in the meantime :).
Gravity is a poor source of computation because it is incredibly weak - 10^-43 vs electron force. Even if you add several powers of 10 for all the metal wire harness and battery chemistry around the electrons, you still get far more usable force per gram from electricity and metal than you do from gravity.
That doesn't change the tradeoff; in a Big computer that's also a galaxy any of the stars used as an instrument for gravitational computation can't provide nearly as much compute as having a planet-sized electronic computer powered by that star.
Yeah but there are other factors. Resilience for example.
A simple black hole approaches the trajectory of that planet sized-computer and plop! All that computation gets condensed to 3 single numbers and all the information is lost (that last part is a very hot topic).
For a Galaxy computer on the other hand, blackholes could be the NOT gates.
If you have the ability to position stars and black holes as gates for computation, then the same ability enables you to ensure that they are positioned so that tinier computation around these stars can happen not disrupted - the resilience is enabled by the fictitious future technology even if you don't use that gravitational complexity.
> All that computation gets condensed to 3 single numbers
> For a Galaxy computer on the other hand, blackholes could be the NOT gates.
i.e. in the worst catastrophic case the former carries more information (3 numbers) than the best case of the latter (one bit).
Is it even theoretically possible to waveguide gravity? The electric field can be positive and negative, but gravity is unsigned -- there is no anti-gravity. This is probably related to what you're saying about faraday cages.
Gravitational waves can either stretch or contract spacetime relative to a baseline. Since the Einstein field equations are nonlinear, I think gravitational waves can be "refracted" when traveling through a region with a high baseline curvature, so maybe waveguides are possible. Gravitational lenses do lens gravitational waves in addition to light.
I realize this is a joke, but it isn't! Play a video of a ball flying up and then back down again and it'll be the same forward or backwards (up to air friction anyway).
If it wasn't a joke, then that was simply a misleading false statement.
Let's take the simple example of earth orbiting around the sun. Playing time backwards gets you a orbit in the opposite direction, while gravity becoming antigravity would mean that earth would get repelled by the sun and thus go off to infinity.
That's interesting. Playing time backwards long enough would see the earth disassembled into rocks, dust and gas, repelling each other and indeed flying off into <far away>. Same with the sun. But the short term orbit example challenges the intuition. Perhaps the answer is that the time-forward orbit is (conventional) downhill in spacetime, and the time-backward orbit is uphill in spacetime, but both trajectories are seen in conventional space as a curved path around the center of gravity.
> Playing time backwards long enough would see the earth disassembled into rocks, dust and gas, repelling each other and indeed flying off into <far away>.
No, playing time backwards long enough would see a hot earth exploding into rocks, dust and gas that are attracting each other - just the initial velocity is large enough and attraction is not strong enough to stop them from flying out into <far away>. They would be slowing down when flying off, not accelerating as if they were repelling each other.
They would then be joined by the dissolving sun and form a cloud of dust which some time later (i.e. earlier) would converge (because the dust is attracting itself) into some earlier massive star(s) out of whose remains our solar system was formed.
If an asteroid hits the earth, the gravitational potential energy (of an attractive gravity) gets turned into kinetic energy as it accelerates when approaching the earth and afterwards into heat as it impacts it; playing time backwards, the heat gets turned into kinetic energy, which then gets turned into gravitational potential as it distances itself from earth.
Electricity travels faster than the speed of electrons (which only travel at ~3 cm/s!), it travels proportional to the speed of light, it’s speed is instead described by the Poynting vector, an energy wave.
??? Indeed everything I have written is accurate, not sure your point since we are talking about electron directional velocity in a wire not the speed of energy propagation...
> In fact, electrons in conductive media do not travel at c, they travel at incredibly slow velocities, on the order of a fraction of a millimeter per second. The rate can vary, and the amount of current in the conductor is a function of the average speed of the electrons in it. [1]
Links [1] and [3] are wrong, and link [2] is correct but has nothing to do with this discussion. Link [1] is so full of errors it isn't even worth discussing. The pingpong ball analogy is wrong--it's all wrong. Link [3] commits the sin of ascribing single-electron behavior to parameters extracted from the Drude model. This is a semiclassical analogy and worked essentially thanks to units.
Here's the link you're looking for. http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/Fermi.html . Two electrons can't occupy the same state. In a metal of finite size, the momentum spectrum becomes quantized. Two electrons can occupy each k-state, one for spin up, one for spin down. Considering an empty metal, we can insert electrons one by one. They will find their lowest energy by packing into a sphere in k-space. Electrons inside the sphere have no states to scatter into, and there are no electrons occupying states outside the sphere. This means that only electrons on the surface of this sphere participate in conduction. The radius of this sphere is called the Fermi wavevector, and converting to units of velocity you get the Fermi velocity. All electrons participating in conduction travel at approximately the fermi velocity... at room temperature plus or minus a tiny fraction of a percent.
Drift velocity has everything to do with the discussion, which is why you brought it up.
I’m familiar with Pauli exclusion principle I’ve worked on real semiconductors.
Your last point is wrong, everything else you said is correct but it remains irrelevant since it does not contradict what was said. Both links are correct.
As you know the net fermi velocity of a fermion is 0. The directional velocity resulting from an electric field on an electron, the fermi velocity which becomes directional due to net flow, is the drift velocity. Which is what we care about.
You can do a simple experiment with NMR to measure the speed of electrons. Indeed they’ve done it and it corresponds to the “wrong calculations”.[1]
Edit: Good resource [2] to help you understand the difference between those two velocities:
> However, the drift velocity of electrons in metals - the speed at which electrons move in applied electric field - is quite slow, on the order of 0.0001 m/s, or .01 cm/s. You can easily outrun an electron drifting in a metal, even if you have been drinking all night and have been personally reduced to a very slow crawl.
> To summarize, electrons are traveling in metals at the Fermi velocity vF, which is very, very fast (106 m/s), but the flux of electrons is the same in all directions. That is, they are going nowhere fast. In an electric field, a very small but directional drift velocity is superimposed on this fast random motion of valence electrons.
> All electrons participating in conduction travel at approximately the fermi velocity... at room temperature plus or minus a tiny fraction of a percent.
> Your last point is wrong
> To summarize, electrons are traveling in metals at the Fermi velocity vF
your own quote. come on. I have a phd in this shit.
Many people on HN have a PhD in similar fields but that isn't relevant, though it's the smart people here that give us these thoughtful conversations on HN.
No one has disagreed with it, I explicitly agreed with you on the existence of Fermi velocity. I don't have the ability to downvote, but you were downvoted because you mentioned the fermi velocity in contradiction to electron flow even though it is the drift velocity that is pertinent in the original context of electricity (which requires a non-net zero velocity).
> So in theory anything done with electricity could be replicated using other waves
I sort of get this in a discrete digital logic scenario but out of curiosity as someone not big on Photonics, what would be the light 'equivalent' of an electrical AC signal? I'm kind of struggling to visual that.
> It employs two-dimensional quasiparticles called anyons, whose world lines pass around one another to form braids in a three-dimensional spacetime (i.e., one temporal plus two spatial dimensions). These braids form the logic gates that make up the computer. The advantage of a quantum computer based on quantum braids over using trapped quantum particles is that the former is much more stable.
"Wires" are a useful convenience in the electron world, to build pathways that don't degrade with the passing of the elections themselves. But if we relax that constraint a bit, are there other ways we can build up arrangements of "organized flow" sufficient to have logic units arise? E.g. imagine pressure waves in a fluid -filled container, with mini barriers throughout defining the possible flow arrangement that allows for interesting self-reflections. Or way further out, could we use gravitational waves through some dense substance with carefully arranged holes, self-interfering via their effect on space-time, to do computations for us? And maybe before we get there, is there a way we could capitalize on the strong or weak nuclear force to "arrange" higher frequency logical computations to happen?
Physics permits all sorts of interactions, and we only really use the simple/easy-to-conceptualize ones as yet, which I hope and believe leaves lots more for us to grow into yet :).