Matrix theory can refer to two different things: the concept of living in a computer simulation, popularized by the movie "The Matrix," and a quantum mechanical model proposed in 1997 by Tom Banks, Willy Fischler, Stephen Shenker, and Leonard Susskind).

The Matrix theory popularized by the movie "The Matrix" is the idea that reality is an illusion and that we are all actually lying in pods of fluid, serving as nutrients for machines. While this idea sounds fantastical to most people, there are certain scientists and philosophers who believe that "The Matrix" raises some serious questions of whether we really are living in a computer simulation.

On the other hand, the Matrix theory proposed by Tom Banks, Willy Fischler, Stephen Shenker, and Leonard Susskind is a quantum mechanical model that describes the behavior of a set of nine large matrices. In their original paper, these authors showed, among other things, that the low energy limit of this matrix model is described by eleven-dimensional supergravity. These calculations led them to propose that the BFSS matrix model is exactly equivalent to M-theory).

In summary, Matrix theory can refer to the concept of living in a computer simulation popularized by the movie "The Matrix," or a quantum mechanical model proposed in 1997 by Tom Banks, Willy Fischler, Stephen Shenker, and Leonard Susskind.