Fan, YaohuiMerrill, LucienneZhao, ChunnongJu, LiBlair, DavidSlagmolen, BramHosken, DavidBrooks, AidanVeitch, PeterMudge, DamienMunch, Jesper2015-09-252015-09-250003-6951http://hdl.handle.net/1885/15712We have observed negative optical torsional rigidity in an 80 m suspended high optical power cavity that would induce the Sidles–Sigg instability as a result of sufficient circulating power. The magnitude of the negative optical spring constant per unit power is a few μN m/W as the result of the optical torsional stiffness in the yaw mode of a suspended mirror Fabry–Pérot cavity. It has been observed to depend on the g-factor of the cavity which is in agreement with the Sidles–Sigg theory.This research was supported by the Australian Research Council and the Department of Education, Science and Training by the U.S. National Science Foundation NSF Grant Nos. PHY-0555453 and 0PHY-757968.http://www.sherpa.ac.uk/romeo/issn/0003-6951..."Publishers version/PDF may be used on author's personal website, institutional website or institutional repository" from SHERPA/RoMEO site (as at 25/09/15). Copyright 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in (Fan, Yaohui, et al. "Observation of optical torsional stiffness in a high optical power cavity." Applied Physics Letters 94.8 (2009): 081105.) and may be found at https://doi.org/10.1063/1.3088850Keywords: Circulating power; G factors; High optical power; Per units; Spring constants; Torsional rigidities; Torsional stiffness; Stiffness2009-02-2410.1063/1.30888502016-02-24