Our Mathematical Universe: My Quest for the Ultimate Nature of Reality

by Max Tegmark

As we discussed in detail in the previous chapter, the MUH says that a mathematical structure is our external physical reality, rather than being merely a description thereof. This equivalence between physical and mathematical existence means that if a mathematical structure contains a self-aware substructure, it will perceive itself as existing in a physically real universe, just as you and I do (albeit generically a universe with different properties from ours). Stephen Hawking famously asked, “What is it that breathes fire into the equations and makes a universe for them to describe?” In the context of the MUH, there’s thus no fire-breathing required, since the point isn’t that a mathematical structure describes a universe, but that it is a universe. Moreover, there’s no making required either. You can’t make a mathematical structure—it simply exists. It doesn’t exist in space and time—space and time may exist in it. In other words, all structures that exist mathematically have the same ontological status, and the most interesting question isn’t which ones exist physically (they all do), but which ones contain life—and perhaps us.