Chain Reaction
A self-propagating sequence of reactions where each event triggers the next — most famously in nuclear fission, more generally in any cascade.
Origin
The chemical concept goes back to early 20th-century work on combustion. The version that fixed it in the public imagination is nuclear: in 1933 Leo Szilard, walking in London, conceived that if a neutron-emitting reaction could trigger more such reactions, the chain would go critical. Fermi achieved the first sustained nuclear chain reaction in a squash court under the University of Chicago bleachers in December 1942 — the project that produced the atom bomb. The phrase has had a heavy second life ever since.
Modern usage
Used for any cascading event — financial contagion, social-media pile-ons, a single decision that triggers ten others. 'It set off a chain reaction.' Often invoked in the same paragraph as 'critical mass.' The Daft Punk song with the title also helps the term feel fresh.
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