String field theory is a proposal to define string theory in such a way that the background independence is respected. String field theory can be understood as a quantum field theory with infinitely many fields which are unified into one master "string field". This string field, roughly speaking, assigns an operator to every one-dimensional curve (string) in spacetime, much like a quantum field assigns an operator to each zero-dimensional point in spacetime.
String field theory did not turn out to be helpful in the second superstring revolution because this revolution has revealed that other objects such as branes are as fundamental as the strings themselves. String field theory is based on the assumption that the strings are the fundamental objects, and it makes it more difficult (or impossible) to understand dualities within its framework.
There are several versions of string field theory - for example the boundary string field theory or the cubic (Chern-Simons-like) string field theory constructed by Edward Witten. In the late 1990s, both of them turned out to be very useful to understand tachyon condensation.
Stringtheory is a model of fundamental physics whose building blocks are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles) that are the basis of the Standard Model of particle physics.
Stringtheory as a whole has not yet made falsifiable predictions that would allow it to be experimentally tested, though various planned observations and experiments could confirm some essential aspects of the theory, such as supersymmetry and extra dimensions.
Stringtheory was originally invented and explored during the late 1960s and early 1970s, to explain some peculiarities of the behavior of hadrons (subatomic particles such as the proton and neutron which experience the strong nuclear force).
Originally, stringtheory was proposed as an explanation for the observed relationship between mass and spin for certain particles called hadrons, which include the proton and neutron.
But particles in stringtheory arise as excitations of the string, and included in the excitations of a string in stringtheory is a particle with zero mass and two units of spin.
This led early string theorists to propose that stringtheory be applied not as a theory of hadronic particles, but as a theory of quantum gravity, the unfulfilled fantasy of theoretical physics in the particle and gravity communities for decades.
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