Observations of accreting black holes often provoke suggestions that theirjets precess. The precession is usually supposed to result from a combinationof the Lense-Thirring effect and accretion disc viscosity. We show that this isunlikely for any type of black hole system, as the disc generally has toolittle angular momentum compared with a spinning hole to cause any significantmovement of the jet direction across the sky on short timescales. Uncorrelatedaccretion events, as in the chaotic accretion picture of active galacticnuclei, change AGN jet directions only on timescales \gtrsim 10^7 yr. In thispicture AGN jet directions are stable on shorter timescales, but uncorrelatedwith any structure of the host galaxy, as observed. We argue that observationsof black-hole jets precessing on timescales short compared to the accretiontime would be a strong indication that the accretion disc, and not the standardBlandford-Znajek mechanism, is responsible for driving the jet. This would beparticularly convincing in a tidal disruption event. We suggest that additionaldisc physics is needed to explain any jet precession on timescales shortcompared with the accretion time. Possibilities include the radiation warpinginstability, or disc tearing.
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