String theory is a theoretical framework in physics that aims to unify the fundamental forces of nature, including gravity, with quantum mechanics. It suggests that the basic building blocks of the universe are not particles but tiny, vibrating strings. These strings can oscillate at different frequencies, giving rise to different particles with varying properties, such as mass and charge. In string theory, particles are not considered fundamental but rather different vibrational states of the underlying strings.
String theory also requires extra dimensions beyond the familiar three spatial dimensions (length, width, and height) and one time dimension. The exact number of extra dimensions required depends on the specific version of string theory being considered. In some formulations, there are a total of ten dimensions, while in others, there may be eleven dimensions.
Regarding the name "M theory," it is an extension of string theory that was proposed in the mid-1990s. The "M" in M theory originally stood for "membrane" or "mother," as it was believed to incorporate higher-dimensional objects called membranes or branes. These branes are akin to higher-dimensional analogs of strings. However, as our understanding of the theory evolved, it was realized that "M" theory is not solely confined to membranes but encompasses other objects as well, such as strings and particles. Therefore, the exact meaning of the "M" in M theory remains somewhat ambiguous, and it is simply referred to as "M."
It is worth noting that M theory is not a complete and fully developed theory yet, and its precise formulation is still an active area of research. It is considered a promising candidate for a theory of everything that unifies the fundamental forces and provides a consistent framework for understanding the universe at its most fundamental level.