Preface: Virtual Entities in Science

It is not only since the sudden increase of online communication due to the COVID-19 situation that the concept of the “virtual” has made its way into everyday language. In this context, it mostly denotes a digital substitute for a real object or process. Virtual reality is perhaps the best-known term in this respect. With these digital connotations, “virtuality” has also been used in science and research: Chemists use virtual laboratories, biologists do virtual scanning of molecular structures, and geologists engage in virtual field trips. But the concept of the virtual has a much longer tradition, dating back to long before the dawn of the digital age. Virtual images and virtual displacements were introduced in classical physics already in the late seventeenth and eighteenth centuries, respectively. They represent auxiliary objects or processes without instantiation, with the purpose of efficiently describing specific physical systems. In today’s physics, the term “virtual” is...