Google relies on at least four facts, all of them crucial, but especially the fourth one.
- Computer memory chips have become so cheap and so tiny that in an office-sized space you can pack enough random-access-memory units to store an utterly gigantic automatically maintained concordance to the whole Web, augmented with copies of huge portions of what is on those sites.
- Networks and processors have become so fast that your search command can be delivered to a server far away and checked against the gigantic index in just hundredths of a second.
- The number of sites containing all of the words on a list (rather than just some of them) goes down rapidly with the length of the list, and much more rapidly when the words have low probabilities of occurrence.
- Humans looking for a certain piece of information can on the whole be trusted to be smart enough to supply a list of words with the crucial property of having low probability in most texts but being guaranteed to occur in texts containing the desired information.
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