{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"456a8ed6dcb37bb4c892a8a05db45a4a842699de60d38a7dd425fe49de6087d9","created":"2026-10-03T07:17:54Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"c60829d2fb33d5894e17752ed99700a66be404e416a2be330427ddce1117fb00","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"classification","assisted_by":[],"external_id":"astro.disk-dynamics.spiral-longevity","kind":"well-posed","literature_status":"contested","n":"1","parents":[],"plain":"Spiral arms could be a fixed pattern that stars pass through, like a traffic jam, or short-lived patterns that keep reforming. Simulations mostly give short-lived arms, while classic theory assumes lasting ones.","posed_since":"1964","precise":"Decide whether spiral structure in isolated, unbarred disk galaxies is a quasi-steady density wave with a single pattern speed lasting many rotations (Lin and Shu 1964) or a sequence of transient, swing-amplified modes each lasting a few rotations. Observational discriminants include pattern speed versus radius, azimuthal age gradients across arms, and Gaia kinematic structure.","problem_ref":null,"references":"","settled_by":"Measured pattern speeds as a function of radius in a sample of grand-design and flocculent spirals plus Milky Way arm kinematics from Gaia.","status_note":"","title":"Are spiral arms long-lived density waves or recurrent transients?","topic_ref":"f19e1364f833efe76f28e89a114cdd946c8b004cdb07a402cf8e6ddc3975459a"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"1106e6b89cc32456af042c43547b238992ed5e85b1977532d59a11c7c3ddd25b","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"36de31ce7396c7becd2a6150bbe97ace3cbcd0ec5acbc3e405802b055f0fc755","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"buQxVvihXBaCtsBtbzYigXO5HIhuPV5MSj4EcExt61VLQMPbKsJ2IzoTlLwjUG_5X6-Ytb8s-_YlMdkVeaHGCA"},"schema":"pubphys.envelope/1"},"record_hash":"1106e6b89cc32456af042c43547b238992ed5e85b1977532d59a11c7c3ddd25b","leaf_index":522}