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I've heard this in many quantum mechanics talks and lectures, nevertheless I don't seem to grasp the idea behind it.

What I mean is, at which point is that our modern understanding of quantum mechanics led to a technological development so fundamental for today's computers that we could not have got it working other way?

Why is it not enough with Maxwell, Bohr, Lorentz, (Liénard)?

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Adding the word "modern" to the title of the question completely changes it. In modern computers you need semiconductors, and the whole theory of solid state physics (band structures, doping, etc) is based on a foundation of quantum mechanics - since electrons in semiconducting solids behave in a manner that is more wave-like than particle-like, with each electron occupying its own distinct state. Making a semiconductor work well requires in depth understanding of these things. –  Floris May 14 at 13:07
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@Floris Thank you, that's kind of the explanation I was looking for. –  harogaston May 14 at 13:38
    
I think it may be interesting to note that, however remotely we could have developed the technology first and understood it later! Atleast developed to experimental stages. –  Rijul Gupta May 14 at 13:48
    
@rijulgupta Yeah I understand, but that is not my point with this question. –  harogaston May 14 at 13:55
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@Floris your comment is really the Answer, maybe move it? –  Michael Martinez May 15 at 0:00

7 Answers 7

up vote 3 down vote accepted

Adding the word "modern" to the title of the question completely changes it. In modern computers you need semiconductors, and the whole theory of solid state physics (band structures, doping, etc) is based on a foundation of quantum mechanics - since electrons in semiconducting solids behave in a manner that is more wave-like than particle-like, with each electron occupying its own distinct state. Making a semiconductor work well requires in depth understanding of these things.

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The reason is very simple. Computers depend on electronics. Even the first diodes and triodes that the first bulky computers were made up of depend on the quantum mechanical nature of matter. The present ones with the chip technology are directly dependent on energy levels and bands of conduction etc in the electronics used. Semiconductivity is a quantum mechanical phenomenon.

Edit after the editing of the question

What I mean is, at which point is that our modern understanding of quantum mechanics led to a technological development so fundamental for today's computers that we could not have got it working other way?

The crucial point where quantum mechanical calculations became necessary was with the use of transistor technology, which has morphed to chip technology. It was with the invention of the transistor that control of quantum mechanical calculations was necessary for the leaps in progress we have made. For the vacuum tube computers it was not necessary except for explaining the tubes existence.The chip designs have reached the point of even needing to foresee the Casimir effect ( qm vacuum between charged plates).

Why is it not enough with Maxwell, Bohr, Lorentz, (Liénard)?

Maxwell is not enough because the classical theory cannot explain atoms molecules and solid state. Bohr is not enough because the primitive calculations could not be used in complicated lattices. Lorenz is irrelevant for solid state physics, the energies of the ions and electrons are low.

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It seems a very silly statement to say "computers wouldn't work without quantum mechanics". It is true that classical mechanics cannot explain the solid states of matter (and lots of other things), but vacuum tubes use glass, fillaments, metal... these things are all solid (condensed states of matter) too. So you can't explain vacuum tubes without quantum mechanics, or gears (made of solid substance), or yourself.... –  Dan S May 15 at 14:45
    
@DanS Sure , like the ankle bone is connected to the leg bone song, everything is connected. I answered the question on the level of whether before the understanding of quantum mechanics, just with good understanding of classical theories we could have built the computers we have now. –  anna v May 16 at 4:23
    
"chip designs have reached the point of even needing to foresee the Casimir effect" Do you happen to have a link that explains this need? –  Mehrdad May 16 at 5:58
    
@Mehrdad I would have to search the net for it again. I was looking for measurements of vacuum and fell on this. The reason the effect started being important was beacause of miniaturization, flat meta bit came too close to each other. this has a description syzygyastro.hubpages.com/hub/… . from first links for search "nanotechnoligy and casimir effect" –  anna v May 16 at 6:41

I find this quite an imprecise catch phrase. It's as correct as saying with out quantum mechanics there would be no atoms because electrons would have fall onto nuclei.

There would be computers but not like the modern ones. The first (electrical) ones didn't depend on quantum mechanical effects, they used vacuum tubes in place of transitors. Not to mention you can make mechanical computers running even on water (I mean for signal instead of electrical current). Not very efficient though.

What they probably meant is that quantum effects lie at the basis of semiconductivity and solid-state transistors which led to a true electronic revolution. They made computers available kind of like Ford made cars, both made production mass-scaled and cheap.

EDIT: When you add "modern". It's a very vague term to me. Modern as in non mechanical - VC electronical, using high-intergration chips (solid state transistors), or nowadays modern?

I am not sure whether inventors of transistor used QM models to explain their work, or the inventors of 1st micro-chip. Maybe they didn't have to, they just needed find good materials. Never-the-less, hot it cannot be explained with out using QM, but this knowledge is not needed for things to work or invent and develop them.

Also, I am sure that today QM theories are needed and used to develop better and smaller transistors. These theories are used to simulate and design most basic building blocks of the most advanced chips that are being produced nowadays.

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This is near-enough the answer I would have given if I had got here 20 minutes ago. Case in point - the Bletchley Park bombes. Not sure how the point you make in paragraph 3 is relevant; I think your answer would be better without... –  Floris May 14 at 9:45
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I am sorry but the cathode ray tube was one of the first indications of quantization of charge ( the electron) . electrons.wikidot.com/… . The tube technology also relies on QM –  anna v May 14 at 10:50
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@Floris What I was trying to say, is that scale of circuits got so small, that the magnitude of quantum effects in comparison got so big, it's not just something that makes transistors work. It's an effect like a gravitation to us. Before it was more like relativity. We know it's right but we don't care Newtonian transformations work just fine for us, because of scale, we operate on. Maybe I should reword it. –  luk32 May 14 at 11:42
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@annav - I think the triode can be explained without resorting to quantum mechanics - treating the electron as a classical particle without wave like properties works just fine. That doesn't mean that there are not quantum effects in play - but as your link exemplifies, people made CRT before they knew QM. I think we are interpreting the question differently? –  Floris May 14 at 11:54
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@annav - I realize that the behavior of the tube was explained a posteriori by QM. My point was that the tube was created without that knowledge - it worked although people didn't know why. The first triode was invented in 1906. The term quantenmechanik was first used in 1924. Yes, the work of Faraday, Boltzmann, Hertz, Planck and JJ Thompson - even the discovery of the photo-electric effect - happened before that date - but I don't know they were needed for the triode to exist. –  Floris May 14 at 13:03

Quantum mechanics led to a deeper understanding of field electron emission which was instrumental in developing the theory of electron energy bands and, in particular, an appreciation of the band gap. This let us work out the physics of semiconductors and develop models for selecting and refining semiconductor materials and treatments.

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The key word that makes the statement approximately true is "modern." There are many computing devices that can be (and have been) created using pre-transistor parts. Pascal and Leibniz constructed adding devices with gears. Babbage designed (but did not build) a programmable computer with gears, linkages and metal plates with holes in them. Completely mechanical calculators that could add were ubiquitous before World War II, and various other devices like tabulators and calculators that could multiply were available (for a relatively large price).

The classic paper that is considered to have started the modern era of computing is Claude Shannon's 1937 masters thesis, that demonstrated that boolean algebra can be used to design relay circuits (that were wiedely used in telephone switching networks). The first computers used some combination of electro-mechanical relays and vacuum tube diodes and triodes, the design of which depends on electron ballistics, which is really classical models (although it involves quantum particles.)

An area of active research now is biochemical computing, which programs protein feedback loops in e-coli bacteria to perform (extremely simple) boolean computations. And people fool around building logic parts out of tinker-toys, mechano, and hydraulic switches.

But... the transistor, and especially the MOSFET are the only devices we currently have that we can reliably manufacture by the billions or trillions. So almost all modern (say post 1965 or so) computers are constructed almost exclusively from transistors.

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I think the last digital computer using any vacuum tubes was around the mid '50s. Discrete transistors were used until the mid '60s, when various forms of what we today call integrated circuits started to be used. Everything beyond that, to today's microprocessors, was just refinement and shrinking it down. –  Phil Perry May 15 at 14:04

I've heard this in many quantum mechanics talks and lectures, nevertheless I don't seem to grasp the idea behind it.

There is no “idea” behind it. Just general fluff talk, like saying we wouldn’t have the lightbulb without Ohm’s Law (Fact: Lightbulbs were in existence before Ohm formalised his law, and the first practical Swan-Edison bulbs had one feature—high resistance—derived from Ohm’s law, among many other innovations. Ohmic resistance is not essential for a lightbulb at all).

What I mean is, at which point is that our modern understanding of quantum mechanics led to a technological development so fundamental for today's computers that we could not have got it working other way?

You’re mixing the existence of quantum mechanics with understanding of it. In Physics, theory usually follows observation (except for a few dramatic cases). Today’s computers depend on semiconductor-transistor action, which is explained by QM theory. QM theory enabled the refinement of transistors (eg: predicting what doping materials would produce what effects on which substrates). This, while good, is not fundamental to computing or computers at all. Whether we could have had it working any other way? Definitely yes. Whether it is possible to compete with today’s computers with non-semiconductor tech? That’s a hypothetical question! Could the Allies have lost WW II? Yes, but they didn’t. So it is with semiconductors—they were (and are) the best available mechanism for cheap, ubiquitous computing.

Why is it not enough with Maxwell, Bohr, Lorentz, (Liénard)?

It’s enough with Newton, Coulomb/Gauss, Faraday, and van der Waals. It only depends on what your definition of “modern” is. For example, imagine an advanded Babbage machine built with nanoparticles, featuring molecular gears and cogs. Now imagine said machine with electrical linkages (using dynamos and capacitors) to mimic a CPU pinout. Can this machine replace a Core i5 and run Facebook? Absolutely. Do we have such machines? No.

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Very nice answer but a bit to critical towards the OP. It's not only harogaston that confuses quantum mechanics (QM) the theory and QM as the set of all the observed stuff that seem incompatible with pre-QM notions . This is a semantic shortcut that almost everybody does and I agree it is not a good thing. It's like saying that we need QM to build houses because without QM we cannot deeply understand why it is that two macroscopic solid objects cannot pass through one another. Hence emphasising the refinement part was really needed in this discussion. –  gatsu May 15 at 8:16

The first programmable stored memory electronic computers were made with triodes. They do not need quantum mechanics to explain their operation. Lee de Forest invented the triode in 1906 http://en.wikipedia.org/wiki/Lee_De_Forest . The FET was invented by Lilienfeld in 1926 http://en.wikipedia.org/wiki/Julius_Edgar_Lilienfeld, and it being a majority carrier device its operation can be explained to an electrical engineer who will design it and with it very well without ever resorting to anything beyond phenomenological Maxwell's equations in the low-frequency limit. When it comes to bipolar transistors with their holes, minority carriers, etc., then one needs qm.

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