... if we think of empty spacetime as some immaterial substance, consisting of a very large number of minute, structureless pieces, and if we then let these microscopic building blocks interact with one another according to simple rules dictated by gravity and quantum theory, they will spontaneously arrange themselves into a whole that in many ways looks like the observed universe. It is similar to the way that molecules assemble themselves into crystalline or amorphous solids.It seems to me that they are leaving something out. The flock emerges because the birds already exist. If we had to start by producing the birds out of nothing, the whole business might take a bit longer. Well, we shall see.
Spacetime, then, might be more like a simple stir fry than an elaborate wedding cake. Moreover, unlike other approaches to quantum gravity our recipe is very robust. When we vary the details in our simulations, the result hardly changes. This robustness gives reason to believe we are on the right track. If the outcome were sensitive to where we put down each piece of this enormous ensemble, we could generate an enormous number of baroque shapes, each a priori equally likely to occur-so we would lose all explanatory power for why the universe turned out as it did.
Similar mechanisms of self-assembly and self-organization occur across physics, biology and other fields of science. A beautiful example is the behavior of large flocks of birds, such as European starlings. Individual birds interact only with a small number of nearby birds; no leader tells them what to do. Yet the flock still forms and moves as a whole. The flock possesses collective, or emergent, properties that are not obvious in each bird's behavior.
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