Observations of Gravitational Lensing

Abstract

The first detection of a gravitational lens phenomenon happened in 1979 [Wals-CW], when an impressive number of theoretical studies of the lens effect had already been published (cf. Chapter 1). By now the majority of scientists working in gravitational lensing and most papers appearing in journals deal with observations. Basically, there are two types of observational projects on lensing: the individual object approach, where the (often serendipitous) discovery/confirmation of a gravitational lens system or a refined new obser­vation is reported. A complementary approach deals with gravitational lens surveys: Scien­tists systematically search for lenses among candidates that fulfill certain criteria (defined by source redshift, brightness, position on the sky, detectability in the radio/X-ray/infrared regime, morphology, or others). The first approach is more frequent, although the second one is more liked by theoreticians, who prefer well-defined samples in their attempts to make (statistical) sense of the observations.

For better or for worse, there is little evidence that we have any idea of what reality looks like from an absolute point of view. We only know what the world looks like from our point of view…

In Heisenberg’s words, ‘What we observe is not nature itself but nature exposed to our method of questioning. Our scientific work in physics consists in asking questions about nature in the language we possess and trying to get an answer from experiment by the means that are at our disposal.’ And Bohr’,it is wrong to think that the task of physics is to find out how nature is. Physics concerns only what we can say about nature.’

… Physics has a clear criterion for the choice of a language—physicists choose the language that allows their predictions most closely to fit their observations. Bruce Gregory