M-theory

M-theory is a proposed 11-dimensional framework that unifies the five consistent superstring theories and includes membranes among its fundamental objects. Its full formulation remains unknown.

Superstring theory: A framework in which fundamental particles arise as vibrational modes of one-dimensional strings, with supersymmetry linking bosons and fermions. M-theory encompasses the five consistent versions of this framework.
Compactification: A construction in which extra spatial dimensions are curled into a compact geometry, leaving fewer dimensions apparent at large scales. Compactifying M-theory can produce lower-dimensional physics, including string-theory limits.
Second superstring revolution: A mid-1990s shift in string theory that emphasized dualities, branes, and the possibility of an underlying unified framework. This period established M-theory as a proposed unifying description.
M-theory definition: The unresolved problem of specifying M-theory as a complete, general quantum theory. No formulation is known that describes all backgrounds and scales in a single practical framework.
String duality: A set of equivalences relating string theories through transformations of their parameters or descriptions. Dualities reveal that apparently different string theories can describe the same physics.
Kaluza–Klein theory: A framework that interprets fields in extra dimensions as fields and forces in fewer dimensions. Its dimensional-reduction idea explains how an eleventh dimension can appear as a lower-dimensional parameter.
Edward Witten: An American theoretical physicist whose work reshaped mathematical physics and quantum field theory. His 1995 argument connected strong-coupling string theory to an eleven-dimensional framework.
Matrix theory: A conjectured nonperturbative formulation of M-theory in the infinite-momentum frame using matrix quantum mechanics. It gives a concrete candidate definition in a restricted setting, not a universal formulation.
D-brane: A dynamical object in string theory on which open strings can end. D-branes provide key evidence that string theory contains objects beyond strings.
Type IIA string theory: A ten-dimensional superstring theory with nonchiral spacetime supersymmetry and closed strings. At strong coupling, its extra dimension opens and its description approaches M-theory.

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