String theory and M-theory
String theory and M-theory
Since the 1990s, many physicists believe that
11-dimensional M-theory, which is described in
some limits by one of the five perturbative superstring theories, and in another by the
maximally-supersymmetric 11-dimensional supergravity, is the
theory of everything. However, there is no widespread consensus on this issue.
A surprising property of string/M-theory is that extra dimensions
are required for the theory's consistency. In this regard, string theory can be
seen as building on the insights of the Kaluza-Klein
theory, in which it was realized that applying general relativity to
a five dimensional universe (with one of them small and curled up) looks from
the four-dimensional perspective like the usual general relativity together
with Maxwell's electrodynamics. This lent credence to the
idea of unifying gauge and gravity interactions, and to extra
dimensions, but didn't address the detailed experimental requirements. Another
important property of string theory is its supersymmetry, which
together with extra dimensions are the two main proposals for resolving the hierarchy
problem of the standard
model,
which is (roughly) the question of why gravity is so much weaker than any other
force. The extra-dimensional solution involves allowing gravity to propagate
into the other dimensions while keeping other forces confined to a
four-dimensional spacetime, an idea that has been realized with explicit
stringy mechanisms.
Research into string theory has been
encouraged by a variety of theoretical and experimental factors. On the
experimental side, the particle content of the standard model supplemented with neutrino
masses fits into a spinor representation of SO(10), a subgroup of E8 that routinely emerges in
string theory, such as in heterotic string theoryor (sometimes equivalently)
in F-theory. String theory has
mechanisms that may explain why fermions come in three hierarchical
generations, and explain the mixing
rates between quark generations.On the theoretical side, it
has begun to address some of the key questions in quantum
gravity,
such as resolving the black hole information paradox, counting the correct entropy of black holes and allowing for topology-changing processes.It has also lead to many
insights in pure mathematics and in ordinary,
strongly-coupled gauge
theory due to the Gauge/String
duality.
In the late 1990s, it was noted that one
major hurdle in this endeavor is that the number of possible four-dimensional
universes is incredibly large. The small, "curled up" extra dimensions
can be compactified in an enormous number of
different ways (one estimate is 10500 ) each of which leads to
different properties for the low-energy particles and forces. This array of
models is known as the string theory landscape.
One proposed solution is that many or all of
these possibilities are realised in one or another of a huge number of
universes, but that only a small number of them are habitable, and hence the
fundamental constants of the universe are ultimately the result of the anthropic principle rather than dictated by
theory. This has led to criticism of string theory,arguing that it cannot make
useful (i.e., original, falsifiable, and verifiable) predictions and regarding
it as a pseudoscience. Others disagree, and string theory remains
an extremely active topic of investigation in theoretical physics.
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