Dürr, Stephan2006-11-302006-11-302006-11-301863-7388http://hdl.handle.net/2003/2310910.17877/DE290R-1228If two Bose-Einstein condensates are prepared independently and then overlapped, a spatial interference pattern is observed. This prompts the question what determines the phase of the fringe pattern, and whether a condensate has a well-defined value of the phase. This problem has been studied in the literature in detail. The objective of this article is, to present an introduction to the subject and to summarize the discussion for a wider audience.enPhysics & Philosophy ; 4Bose-Einstein condensatephaseQuantum mechanics100530article (journal)