The Believing Brain - Michael Shermer [177]
How can we test the multiverse hypothesis? The theory that new universes can emerge from collapsing black holes may be illuminated through additional knowledge about the properties of black holes. Other bubble universes might be detected in the subtle temperature variations of the cosmic microwave background radiation left over from the big bang of our own universe, and NASA recently launched a spacecraft constructed to study this radiation. Another way to test these theories might be through the Laser Interferometer Gravitational Wave Observatory (LIGO) that is designed to detect exceptionally faint gravitational waves. If there are other universes, perhaps ripples in gravitational waves will signal their presence. Maybe gravity is such a relatively weak force (compared to electromagnetism and nuclear forces) because some of it “leaks” out to other universes. Maybe.
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In late 2010, Stephen Hawking and Leonard Mlodinow presented their answer to the biggest of Big Questions (“Why is there something rather than nothing?,” “Why do we exist?,” and “Why this particular set of laws and not some other?”) in their book, The Grand Design. They approach the problem from what they call “model-dependent realism,” based on the assumption that our brains form models of the world from sensory input, that we use the model most successful at explaining events and assume that the models match reality (even if they do not), and that when more than one model makes accurate predictions, “we are free to use whichever model is most convenient.” Employing this method, the authors explain, “it is pointless to ask whether a model is real, only whether it agrees with observation.” The two models that describe light discussed above—the wave/particle models—serve as an example of model-dependent realism, where each model agrees with certain observations but neither one is sufficient to explain all observations. Hawking and Mlodinow explain the results of the double-slit experiment through the model developed by Richard Feynman called “sum over histories,” in which every particle in the double-slit experiment takes every possible path that it can, and thus it interacts with itself in its different histories (instead of interacting with particles in other universes in the alternate model presented above).
To model the entire universe, Hawking and Mlodinow employ “M-theory,” an extension of string theory that includes eleven dimensions (ten of space and one of time) and incorporates all five current string theory models. As in Feynman’s “sum-over-histories” model of light, Hawking and Mlodinow propose that the universe itself takes every possible path—experiences all possible histories—and this results in the most multiple multiverse imaginable. “In this view, the universe appeared spontaneously, starting off in every possible way,” Hawking and Mlodinow explain. “Most of these correspond to other universes. While some of those universes are similar to ours, most are very different. In fact, many universes exist with many different sets of physical laws.” Although, as we saw, some people call these different universes the multiverse, Hawking and Mlodinow claim that “these are just different expressions of the Feynman sum over histories.” Employing multiple models to explain multiple universes as nothing more than one system with multiple histories, Hawking and Mlodinow conclude, “For these reasons M-theory is the only candidate for a complete theory of