Empty space is not so Empty: The Casimir Effect

Empty space is supposedly filled with energy! This is not based on an untested theory. Quantum Field 
Theory, the successful theory behind the Standard Model, tells us that there is energy due to quantum 
fluctuations in the vaccuum. This energy is in general everywhere and infinite. It is therefore part 
of a background. Energy measurements,  say of energy levels of atoms,  etc. are energies measured with respect to this background. The obvious question is, "are there cases where we expect this vaccuum  energy to be detectable?". The answer is yes. One is supposed to observe it in gravitation. The other case is when fields are constrained to boundaries, say between two plates. Then there is a difference between the infinite energy of empty space with boundaries and the infinite energy of empty space without boundaries. This difference is called the Casimir energy and it leads to a force between the two plates. One can make theoretical calculations of the Casimir energy and force. I will illustrate some simple examples. The experiments are difficult because the force is so tiny. I will briefly discuss some of the latest experiments.