Forgotten barriers

In the production of temperatures below 1 mK, heat sources which are usually ignored at higher temperatures become important and even dominant. These heat sources include a wide range of physical phenomena and are often interesting physical problems in themselves. They can come from either outside the refrigerator (cosmic rays and vibrations, for example) or inside (released by various relaxation mechanisms like ortho-para conversion in hydrogen or the relaxation of mechanical stress). Some of these mechanisms are discussed and the amount of heat they can produce will be estimated where that is reasonable. Although they are potential barriers in the way of producing temperatures well under 1 mK, most can be reduced greatly by straight-forward methods. (Auth.)