These “strings” vibrate in multiple dimensions, and depending on how they vibrate, they might be seen in three-dimensional space as matter, light, or gravity. It is the vibration of the string which determines whether it appears to be matter or energy, and every form of matter or energy is the result of the vibration of strings.
String theory, as mentioned above, ran into a problem: another version of the equations was discovered, then another, and then another. Eventually, there were five major string theories. The main differences between each theory were principally the number of dimensions in which the strings developed, and their characteristics (some were open loops, some were closed loops, etc.). Furthermore, all these theories appeared to be correct. Scientists were not comfortable with five seemingly contradictory sets of equations to describe the same thing.
Some cosmologists are drawn to M-theory because of its mathematical elegance and relative simplicity. Physicist and author Michio Kaku has remarked that M-theory may present us with a “Theory of Everything” which is so concise that its underlying formula would fit on a T-shirt.Stephen Hawking originally believed that M-theory may be the ultimate theory but later suggested that the search for understanding of mathematics and physics will never be complete.
Stephen Hawking and Leonard Mlodinow, in the popular scientific book The Grand Design, take a philosophical position to support a view of the universe as a multiverse, and define it in the book as model-dependent realism which along with a sum over histories approach (see Path integral formulation of Quantum mechanics) to the universe as a whole, is used to claim that M-theory is the only candidate for a complete theory of the universe.
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