Final answer:
The "second string" dimension refers to the additional dimensions in string theory beyond our familiar three spatial dimensions and one time dimension, influencing the vibration of strings that determine their manifestation as different types of physical phenomena.
Step-by-step explanation:
The term "second string" dimension in the context of your question refers to additional spatial dimensions in string theory, which is a theoretical framework in physics. In string theory, the basic constituents of reality are not particles, but strings that can vibrate in multiple dimensions. Although our conventional perception is limited to three dimensions of space and one of time, string theory posits the existence of up to 11 dimensions in total. The properties of these extra dimensions, including the "second string" dimension, influence the vibration patterns of strings. These vibrations determine whether the strings manifest as matter, gravity, or light in our observable universe. The complex mathematics of string theory and its extra-dimensional considerations remain largely theoretical, as experimental verification has not yet been possible due to the high energy levels required that are beyond current technological capabilities.