With the Pierre Auger Observatory, the subject of ultra-high energy cosmic rays is about to do the step from hypothetical physics to observational astronomy. While the phenomena explaining these particles, which reach energies eight orders of magnitude above the LHC, still seem to be outside the scope of accepted theory, their high energy solves by itself the major observer's problem in cosmic ray research: the veiling action of cosmic magnetic fields. Due to their presence, cosmic rays of lower energy are devoid of any clear directional information on their origin. Only at the very highest energies, where the flux of cosmic rays goes down to a few particles per square kilometer per century, magnetic deflection becomes low enough for the universe of cosmic ray sources to gain structure, comparable to a landscape seen through a pane of frosted glass. The Pierre Auger Observatory, with its effective area of about 3000 square kilometers, and other current and future experiments will collect enough cosmic ray events at these energies that a statistical reconstruction of this landscape appears possible for the first time. At this point, however, the relation of this quest to cosmic magnetic fields undergoes a qualitative change: be used only as an excuse for our ignorance so far, for which it was enough to know that cosmic magnetic fields exist, it now becomes necessary to understand how they are structured and distributed. Unfortunately, ignorance bumps into ignorance at this point: While there are a lot of results meanwhile on magnetic fields in our and other galaxies, and our models are taking shape on these scales, the strength and structure of magnetic fields on larger scales is still a matter of speculation. The difficulty here is that the information on cosmic magnetic fields is hidden in a puzzle of various observations, and limited mainly by our ability to interpret our data at very low energies. The idea that extremely high energies may carry an important piece of this puzzle has not entered this field yet.

Can we hope that these two areas - the research on ultra-high energy cosmic rays, and on magnetic fields in the Universe - can mutually fertilize themselves? Can double ignorance result in knowledge? Or does the cat bite its tail, and we are lost in ignorance in the end? At this workshop, we brought together some of the leading experts in both fields, to exchange both their knowledge and open questions. Hereby, we do not attempt to take full account of the actual research in both fields. Instead, the main goal of this workshop is to start the dialog between these two communities. For this reason, this workshop is not packed with talks, but a high fraction of time is left for discussions. In the end, we do not expect that we have more answers; rather we can regard it a success if we have started to ask the right questions.

jprachen@mpa-garching.mpg.de