  Models for cosmic ray (CR) dynamics fundamentally depend on the rate of CR
scattering from magnetic fluctuations. In the ISM, for CRs with energies
~MeV-TeV, these fluctuations are usually attributed either to 'extrinsic
turbulence' (ET) - a cascade from larger scales - or 'self-confinement' (SC) -
self-generated fluctuations from CR streaming. Using simple analytic arguments
and detailed live numerical CR transport calculations in galaxy simulations, we
show that both of these, in standard form, cannot explain even basic
qualitative features of observed CR spectra. For ET, any spectrum that obeys
critical balance or features realistic anisotropy, or any spectrum that
accounts for finite damping below the dissipation scale, predicts qualitatively
incorrect spectral shapes and scalings of B/C and other species. Even if
somehow one ignored both anisotropy and damping, observationally-required
scattering rates disagree with ET predictions by orders-of-magnitude. For SC,
the dependence of driving on CR energy density means that it is nearly
impossible to recover observed CR spectral shapes and scalings, and again there
is an orders-of-magnitude normalization problem. But more severely, SC
solutions with super-Alfvenic streaming are unstable. In live simulations, they
revert to either arbitrarily-rapid CR escape with zero secondary production, or
to bottleneck solutions with far-too-strong CR confinement and secondary
production. Resolving these fundamental issues without discarding basic plasma
processes requires invoking different drivers for scattering fluctuations.
These must act on a broad range of scales with a power spectrum obeying several
specific (but plausible) constraints.
