A complex vibrating object can vibrate in many ways. For instance, a guitar string can vibrate up and down or side to side; and (as shown in Figure 25 in Chapter 11), it can vibrate in many different ways.
The idea of string theory is that extremely tiny strings moving in a sufficiently complicated universe (typically including extra dimensions of space, see chapter 14) can vibrate in many different ways. To us, giant macroscopic creatures, these different modes of vibration and motion will make the single string look like many different types of particles.
As discussed in Chapter 17, particles arise from fields. One might naturally assume that strings similarly arise from string fields. Just as a single string with various modes of vibration can appear as many types of particles, a single string field can be viewed as a huge number of fields, of the sort discussed in detail in the Fields section of the book. If string theory truly describes our universe, then one might hope that a string field would contain the electric and magnetic fields, the Higgs field, the gravitational field, and all the others, along with huge numbers of fields as yet unknown. It would unify all the different types of fields into a single conceptual structure.
But at this point, this is just an interesting idea, one which some people find compelling for technical reasons, but which others find less so, for other technical reasons. Since a string field might not be relevant to the real world and is quite poorly understood even by professionals, I won’t go into more detail, and will instead leave it to string field theory experts to try to explain it.
