  Large scale, strong magnetic fields are often evoked in black hole accretion
flows, for jet launching in the low/hard state and to circumvent the thermal
instability in the high/soft state. Here we show how these ideas are strongly
challenged by X-ray polarization measurements from IXPE. Quite general
arguments show that equipartition fields in the accretion flow should be of
order $10^{6-8}$ G. These produce substantial Faraday rotation and/or
depolarization for photons escaping the flow in the 2-8 keV IXPE bandpass,
which is not consistent with the observed data. While we stress that Faraday
rotation should be calculated for each individual simulation (density, field
geometry and emissivity), it seems most likely that there are no equipartition
strength large scale ordered fields inside the photosphere of the X-ray
emitting gas. Strong poloidal fields can still be present if they thread the
black hole horizon rather than the X-ray emitting flow, so Blandford-Znajek
jets are still possible, but an alternative solution is that the low/hard state
jet is dominated by pairs so can be accelerated by lower fields. Fundamentally,
polarization data from IXPE means that magnetic fields in black hole accretion
flows are no longer invisible and unconstrained.
