Computing and the internet

Public-key encryption uses a linked pair of keys: a public key anyone can use to lock a message, and a private key that only its owner holds to unlock it.

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Data is split into packets, labeled with addresses, and passed between routers over cables, including undersea fiber, using shared protocols. No central control point exists.

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A QR code stores data as a square grid of dark and light modules that encode binary data, plus error-correction codes, so a camera can read it from any angle.

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The Apollo Guidance Computer was a real-time digital computer that solved navigation equations and controlled spacecraft systems, using rope-core fixed memory and a simple priority-based scheduler.

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The small-world effect is the finding that large networks, including social ones, have very short paths between nodes despite being highly clustered. A few long-range links produce it.

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Ordinary computers use bits that are 0 or 1; quantum computers use qubits that can be in superpositions and be entangled, which helps with some tasks but is hard to engineer.

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Moore's Law is the observation that chip transistor counts double about every two years. Transistor shrinking has slowed sharply, but whether that means the end depends on how you define the law.

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A Turing machine is an abstract mathematical model of computation: a head reading and writing symbols on an unbounded tape according to a finite rule table. It defines what is computable.

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Error-correcting codes store extra check bits alongside data. When data is read, the check is recalculated, so a flipped bit can be located and fixed automatically.

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Alan Turing's 1950 test asks whether a machine can converse so humanly that a judge cannot reliably tell it from a person. Whether passing shows real intelligence is disputed.

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The halting problem is proven unsolvable by any algorithm. P versus NP asks a different question, whether efficient finding is as easy as efficient checking, and it remains open.

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