quantum mechanics and string theory Quantum mechanics and string theory are two of the most significant branches of physics that attempt to describe the behavior of the universe at a fundamental level. While both theories have made incredible contributions to our understanding of the world around us, they remain separate and distinct approaches to understanding the cosmos. No Deposit Bonus Quantum mechanics is a theory that describes the behavior of matter and energy at very small scales, such as the behavior of subatomic particles. It was developed in the early 20th century and has since become one of the most successful theories in all of science. According to quantum mechanics, particles do not have defined properties until they are observed or measured. This means that the act of observation can change the outcome of an experiment, and particles can exist in multiple states simultaneously. This concept is known as superposition.