Does Spacetime Exist? If spacetime does not exist, it certainly does so in a curiously conspicuous way, not at all like other geometric abstractions. Consider this mindbender: spacetime does not exist at all, but it doesn't exist in the void even more than it doesn't exist in the universe. Is that the description of something that does not exist? Relativity theory treats spacetime as a mathematical abstraction-a four-dimensional coordinate system analogous to the three dimensions of Cartesian geometry. It does not exist, per se. It is not a "thing" in any tangible sense. It merely serves as the background needed to describe the behaviors of the "real" things that exist within it. It is no more than the stage on which the cosmic drama plays out. That's the theory. What if spacetime exists after all? Are we not obliged to take this stuff seriously and start asking questions? What are its properties? Where did it come from? Is spacetime a simple or a composite thing? Can it be broken into simpler parts? If not, are we compelled to wonder if spacetime is actually the fundamental substance of the universe? And how does this remarkable stuff come together to form the physical world around us? The Substance of Spacetime sets out to investigate these very mysteries and the results are truly exciting. Treating spacetime, not merely as a coordinate system, but as a real physical substance, opens a window onto reality that would otherwise be impossible to even contemplate. If spacetime exists, everything changes.