Qual é o cérebro Positronic cunhado por Asimov?

Asimov foi muito vago sobre a construção desses cérebros. Presumivelmente eles usam positrons , que são antielétrons. O que sugere que eles devem ter campos magnéticos poderosos. Eles também são descritos como sendo construídos de uma liga de platina e irídio.

Infelizmente, essa é toda a informação que parece haver.

Por que "positrônico"?

When I first began writing science fiction stories, the positron had been discovered only six years before as a particle with all the properties of an electron except for an opposite charge. It was the first (and, at that time, still the only) bit of antimatter that had been discovered, and it carried a kind of science-fictional flavor about it.
    That meant that if I spoke of “positronic robots” rather than “electronic robots,” I would have something exotic and futuristic instead of something conventional.
_{—The Word I Invented, 1980}

Como funciona o cérebro positrônico?

[P]ositrons are very evanescent particles, at least in our world. They don’t survive more than a millionth of a second or so before they bump into one of the electrons with which our world is crowded, and then the two annihilate each other.
    I had a vision, therefore, of “positronic pathways” along which positrons briefly flashed and disappeared. These pathways were analogous to the neurons of the animal nervous system, and the positrons themselves were analogous to the nerve impulse. The exact nature of the pathways were controlled by positronic potentials, and where certain potentials were set prohibitively high, then certain thoughts or deeds became virtually impossible. It was the balance of such potentials which resulted in the Three Laws.
    Of course, it takes a great deal of energy, on the subatomic scale, to produce a positron; and that positron, when it encounters an electron and is annihilated, produces a great deal of energy on the subatomic scale. Where does that positron-producing energy come from and where does the positron-annihilation energy go to?
    The answer to that is that I didn’t know and didn’t care. I never referred to the matter. The assumption (which I didn’t bother to state) was that future technology would handle it and that the process would be so familiar that nobody would wonder about it or comment upon it—any more than a contemporary person would worry about what happens in a generating plant when a switch is flicked and a bathroom light goes on.
_{—The Word I Invented, 1980}

De que é feito o cérebro positrônico?

When I wrote my first few robot stories in 1939 and 1940, I imagined a “positronic brain” of a spongy type of platinum-iridium alloy. It was platinum-iridium because that is a particularly inert metal and is least likely to undergo chemical changes. It was spongy so that it would offer an enormous surface on which electrical patterns could be formed and un-formed.
_{—Cybernetic Organism, 1987}

Fontes adicionais

The mere fact that I talk about positronic robots and say they are guided by the Three Laws of Robotics has no actual predictive value from the engineering standpoint. Imagine, for instance, a discussion between an interviewer (Q) and myself (A).
    Q. What is a positronic robot, sir?
    A. One with a positronic brain.
    Q. And what is a positronic brain?
    A. One in which positronic shifts take the place of the electronic shifts in the living human brain.
    Q. But why should positronics be superior to electronics for the purpose?
    A. I don’t know.
    Q. How do you keep your positrons from combining with electrons and forming a flood of energy that will melt down the robot into a puddle of metal?
    A. I haven’t the vaguest notion.
    Q. For that matter, how do you translate positronic flows into the “Three Laws of Robotics”?
    A. Beats me.
    I’m not ashamed of this. In writing my robot stories it is not my intention to describe robot-engineering in detail. It was merely my intention to describe a society in which advanced robots were common and to try to work out possible resulting consequences.
_{—Future? Tense!, 1965}

It was usually called the “positronic robot” series, because the electric currents in the brains were flows of positrons rather than electrons. I did that just to make the brains sound part of a futuristic technology, but some of the less sophisticated readers thought that this was based on sound science and would ask me to give them additional information on how it worked.
_{—In Memory Yet Green, 1979}

[W]hen in 1939 I began to write robot series, I gave my robots “positronic brains” as a glamorous science fictional variation of the flat and uninspiring “electronic brains.”
_{—Opposite!, 1987}

Since I began writing my robot stories in 1939, I did not mention computerization in their connection. The electronic computer had not yet been invented and I did not foresee it. I did foresee, however, that the brain had to be electronic in some fashion. However, “electronic” didn’t seem futuristic enough. The positron—a subatomic particle exactly like the electron but of opposite electric charge—had been discovered only four years before I wrote my first robot story. It sounded very science fictional indeed, so I gave my robots “positronic brains” and imagined their thoughts to consist of flashing streams of positrons, coming into existence, then going out of existence almost immediately. These stories that I wrote were therefore called “the positronic robot series,” but there was no greater significance than what I have just described to the use of positrons rather than electrons.
_{—My Robots, 1987}

[I]n 1939, at the age of nineteen, I determined to write a robot story [...] Since I needed a power source I introduced the “positronic brain.” This was just gobbledygook but it represented some unknown power source that was useful, versatile, speedy, and compact—like the as-yet uninvented computer.
_{—Introduction: The Robot Chronicles, 1990}

Do blog de Frederik Pohl:

Why were they positronic? I asked him that once and he said, “Because the positron had just been added to the list of particles and no one knew what it could and couldn’t do.”
_{—Isaac, Part 3 of quite a few, 2010}

Essencialmente, é um dispositivo de enredo usado para significar "tecnologia futura desconhecida".

Asimov deliberadamente não falou sobre os aspectos técnicos de seus robôs, para que ele pudesse se concentrar nos personagens envolvidos. Esta é uma das principais razões pelas quais suas histórias escritas nos anos 1940 e 1950 não são muito antigas; porque ele não falou sobre transistores e tubos de vácuo, ele falou sobre tecnologias fictícias como o cérebro positrônico.

Eu me lembro de ler em algum lugar Asimov dizendo que quando ele estava escrevendo suas primeiras histórias de robôs, o Positron tinha acabado de ser descoberto (1932) e assim foi uma nova ideia de ponta.

"Positrons viajando sobre uma esponja como substância" é, creio eu, a maneira como foi expressa.

Asimov nunca explicou detalhadamente nenhuma das tecnologias que usou em seus trabalhos.

O cérebro positrônico é o melhor exemplo disso. Há uma entrevista nos anos 70 (quando ele trabalhou em artigos científicos reais) quando ele definiu uma aproximação de um cérebro positrônico:

"É um cérebro feito pelos homens, então qualquer cérebro, sempre o nosso é positrônico"

De acordo com The Caves of Steel , esse cérebro tem detalhes muito interessantes. Não pode ser construído sem as Três leis, porque não existia nenhuma teoria matemática de tais cérebros. A única teoria matemática que existe, de acordo com o romance, somente permite a construção do cérebro positrônico que segue as Três leis. É impossível no nível de hardware fazer o contrário (tais construções são desconhecidas).

Tenho certeza de que, sim, Asimov queria usar um nome não trivial para o computador e a palavra "cérebro eletrônico" era simples e trivial. O "cérebro positrônico" adicionou um encanto aos seus romances e tornou possível tecer muitos romances para essa "positrônica", que se combinam a uma história ficcional para a humanidade daqui a 20.000 anos (veja seu "Império Galáctico"). O termo "positrônico" era a chave necessária para tornar "reais" suas histórias sobre a salvação da humanidade com base no fato de que ele era ateu.

Isso é correto, ele é usado positrônico porque era generalista e moderno na época. É preciso evitar o uso de máquinas ou métricas do mundo real em ficção científica, senão elas se tornam rapidamente desatualizadas, mesmo que você pense que não. Por exemplo, o andróide Data disse que ele poderia realizar 60 trilhões de cálculos por segundo, o que era bionicle em 1987, mas hoje não é impressionante como os supercomputadores hoje ultrapassam esse número em 100x facilmente.