{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"a4bf19367f28bb46cc4579889f07d8f2314eeb8ec948a4db15dced2097433966","created":"2026-10-03T07:18:11Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"d638707d22625f7e90ec020035c1be23a0a8230a0cf1c9f873860a03c2f650ba","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"yes-no","assisted_by":[],"external_id":"stat.mpemba-relaxation.granular-experiment","kind":"well-posed","literature_status":"open","n":"1","parents":[],"plain":"A granular gas (many grains bouncing and losing energy in each collision) is predicted to show a hotter sample cooling below a target temperature before a cooler one, if the hot sample's speed distribution has the right shape. No experiment has yet confirmed this.","posed_since":"2017","precise":"Homogeneous gas of inelastic hard spheres or disks with restitution coefficient $r < 1$, freely cooling or coupled to a stochastic thermostat. Kinetic theory predicts that the granular temperature curves $T(t)$ of two samples cross when their initial temperatures and initial fourth Sonine coefficients $c_{2}$ ($c_{2} = (d/(d+2)) \\langle v^{4}\\rangle/\\langle v^{2}\\rangle^{2} - 1$ in d dimensions, which vanishes for a Maxwellian) are suitably chosen. The question is whether this crossing occurs in a laboratory granular gas (vibrated monolayer, levitated or microgravity system) with measured $T(t)$ and $c_{2}(t)$ in quantitative agreement with kinetic theory.","problem_ref":null,"references":"","settled_by":"Particle-tracking measurement of $T(t)$ and $c_{2}(t)$ in a homogeneous granular gas showing the predicted crossing with prepared initial $c_{2}$.","status_note":"Predicted by Lasanta et al. (2017) and studied in many kinetic-theory and simulation papers through 2026 (for example sheared gases, arXiv:2605.17881); no experimental observation was found.","title":"Experimental observation of the Mpemba effect in a granular 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