Astro2020 Science White Paper: Magnetic Fields and Polarization in the
  Diffuse Interstellar Medium
  Magnetism is one of the most important forces on the interstellar medium
(ISM), anisotropically regulating the structure and star formation that drive
galactic evolution. Recent high dynamic range observations of diffuse gas and
molecular clouds have revealed new links between interstellar structures and
the ambient magnetic field. ISM morphology encodes rich physical information,
but deciphering it requires high-resolution measurements of the magnetic field:
linear polarization of starlight and dust emission, and Zeeman splitting. These
measure different components of the magnetic field, and crucially, Zeeman
splitting is the only way to directly measure the field strength in the ISM. We
advocate a statistically meaningful survey of magnetic field strengths using
the 21-cm line in absorption, as well as an observational test of the link
between structure formation and field strength using the 21-cm line in
emission. Finally, we report on the serendipitous discovery of linear
polarization of the 21-cm line, which demands both theoretical and
observational follow-up.
