The above characteristics are limited in real scientific communities. Publications are sometimes lost or difficult to retrieve. Concurrency is limited by resources including personnel and funding. Sometimes it is easier to rederive a result than to look it up. Scientists only have so much time and energy to read and try to understand the literature. Scientific fads sometimes sweep up almost everyone in a field. The order in which information is received can influence how it is processed. Sponsors can try to control scientific activities. In Ether the semantics of the kinds of activity described in this paragraph are governed by the actor model.