Equally dramatic changes of perspective have come from revelations on the inward front, with the advent of atomic theory in the eighteenth century, the discovery of subatomic particles at the start of the twentieth century and the advent of quantum theory shortly thereafter. The crucial achievement of the inward journey is that it has revealed that everything in the Universe is made up of a few fundamental particles and that these interact through four forces: gravity, electromagnetism, the weak force and the strong force. These interactions have different strengths and characteristics, and it used to be thought that they operated independently. However, it is now thought that some (and possibly all) of them can be unified as part of a single interaction.
Figure 1.1 illustrates that the history of physics might be regarded as the history of this unification. Electricity and magnetism were combined by Maxwell's theory of electromagnetism in the nineteenth century. The electromagnetic force was then combined with the weak force in the (now experimentally confirmed) electroweak theory in the 1970s. Theorists have subsequently merged the electroweak force with the strong force as part of the Grand Unified Theory (GUT), although this has still not been verified experimentally. As discussed in Section 1.1, the final (and as yet incomplete) step is the unification with gravity, as attempted by string theory or M-theory.
A remarkable feature of these theories is that the Universe may have more than the three dimensions of space that we actually observe, with the extra dimensions being compactified on the Planck scale (the distance of 10_33 cm at which quantum gravity effects become important), so that we do not notice them. In M-theory itself, the total number of dimensions (including time) is eleven, with 4-dimensional physics emerging from the way in which the extra dimensions are compactified (described by what is called a Calabi-Yau manifold). The discovery of dark dimensions through particle physics shakes our view of the nature of reality just as profoundly as the discovery of dark energy through cosmology. Indeed, we saw in Section 1.1 that there may be an intimate link between these ideas.
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