The Fabric of the Cosmos the book, such as discussions revolving around classical physics, quantum mechanics, and cosmological physics. Chapter 2, "The Universe and the Bucket", features space as its key point. The question posed by Greene is this: "Is space a human abstraction, or is it a physical entity?" The key thought experiment is a spinning bucket of water, designed to make one think about what creates the force felt inside the bucket when it is spinning. The ideas of Isaac Newton, Ernst Mach, and Gottfried Leibniz on this thought experiment are discussed in detail. Chapter 3, "Relativity and the Absolute", focuses on spacetime.
Interaction point Interaction point In particle physics, an interaction point (IP) is the place where particles collide in an accelerator experiment. The "nominal" interaction point is the design position, which may differ from the "real" or "physics" interaction point, where the particles actually collide. A related, but distinct, concept is the "primary vertex": the reconstructed location of an individual particle collision. For fixed target experiments, the interaction point is the point where beam and target interact. For colliders, it is the place where the beams interact. Experiments (detectors) at particle accelerators are built around the nominal interaction points of the accelerators. The