In string theory, strings are not made of energy in the same way that particles are not made of energy. Instead, strings are fundamental objects that can vibrate and propagate through spacetime. They are described as one-dimensional entities with tension, length, and various modes of vibration.
In string theory, particles and their properties, such as mass and charge, are manifestations of different vibrational modes of the fundamental strings. The specific vibrational pattern of a string determines the type of particle it represents. For example, a string vibrating in one way might correspond to an electron, while a different vibrational pattern could correspond to a photon.
Energy plays a crucial role in string theory. The energy of a string is related to its vibrational modes, and different vibrational patterns correspond to different energy levels. The energy of a string is quantized, meaning it can only take on certain discrete values. The lowest energy state of a string corresponds to its ground state, which represents the particle with the lowest mass and no excitations.
So, while strings in string theory have energy associated with their vibrational modes, it is more accurate to say that strings are fundamental entities with tension and length, and their energy is determined by their vibrational patterns and modes. The interactions and dynamics of these strings give rise to the diverse phenomena and particles that we observe in our universe.