Final answer:
The "first string" dimension refers to dimensions in string theory where one-dimensional strings vibrate to create different particles. These strings operate in an 11-dimensional space, and their vibrations determine their manifestation as various forms of matter and forces. The theory is complex and remains untested due to current technological limitations.
Step-by-step explanation:
The term "first string" dimension in the context of string theory refers to the dimension in which one-dimensional fundamental objects, known as strings, exist and vibrate. In string theory, there are multiple dimensions beyond the four dimensions of spacetime (three spatial dimensions plus time) that we commonly experience. These strings are posited to be the ultimate constituents of all matter and forces, including gravity and light. String theory proposes a universe made up of tiny strings which, depending on their modes of vibration, manifest as different particles in our familiar four-dimensional perspective. The complex nature of string theory has led to challenges in testing it empirically; not even the most powerful particle accelerators have yet reached the energy levels required to confirm the presence of strings or to explore these higher dimensions directly.