Particles near $10^{20}$ eV are the most energetic particles known to
us in the universe, also called ultra high energy cosmic rays. Their
observations have led us to build the largest detector systems in the
world, in the South the Auger air-shower array, and in the North the
Telescope Array.  With these and earlier arrays events have been
detected of an energy up to $3 \, 10^{20}$ eV, which is a macroscopic
energy.  There have been two predictions: one that due to interaction
with the microwave background the spectrum should show a turnoff near
$5 \, 10^{19}$ eV (1966); this has been confirmed by two experiments,
HiRes and Auger (2007, 2008).  Second, that active galactic nuclei,
probably radio galaxies, should be the accelerators, based on the
non-thermal optical spectra of knots and hot spots in radio galaxies
(1987); this is now tentatively confirmed by Auger (2007, 2008), but
contradicted by HiRes (2008).  I will go through some fundamental
problems with the predictions, which teach us about active galactic
nuclei.  Apart from differentiating various remaining options, such as
gamma ray bursts, how to generate these particles, and their source
population, there is one major difficulty: the lack of understanding
of the cosmological web of magnetic fields, which may influence the
propagation of high energy particles; here it is especially important
to understand the role of our local cosmic neighborhood and a possible
galactic magnetic wind.  I propose, that magnetic scattering leads to
a steep distribution function of scattering angles of the deviation
from a straight line path for the arriving particles.   I will discuss
the observational and theoretical limits for an exemplary set of
models, the predictions like chemical abundances, that result from
these models, and how present and future observations will test our
conclusions, especially with the Telescope Array (TA), the Auger
Array, the neutrino observatory IceCube, the TeV Cherenkov
$\gamma$-ray telescopes, and the future space observatory EUSO.  We
face a number of exciting challenges for particle physics, cosmology,
astronomy, and our deep understanding of matter.