Quantum physics, also known as quantum mechanics, is a branch of physics that describes the behavior of particles at the atomic and subatomic scale. It provides a more fundamental and comprehensive framework than classical physics for understanding the behavior of matter and energy at this scale. While there are some differences between classical and quantum physics, the laws of regular physics, which encompass classical physics, still apply in the macroscopic world as an approximation of quantum behavior.
Classical physics, which includes Newtonian mechanics, electromagnetism, and thermodynamics, provides accurate predictions for macroscopic objects under normal conditions. It is the realm of physics that we are most familiar with in our everyday lives. The laws of classical physics, such as Newton's laws of motion and the laws of conservation of energy, momentum, and angular momentum, are derived from and valid within a certain range of scales and energies.
On the other hand, quantum physics governs the behavior of particles at the atomic and subatomic levels. It introduces probabilistic descriptions, wave-particle duality, superposition, and entanglement, among other concepts that differ from classical physics. Quantum mechanics provides a more accurate and comprehensive understanding of nature at these small scales and has been extensively tested and verified through experiments.
Importantly, at the macroscopic scale, where classical physics is applicable, the behavior of objects is well-described by the average of large numbers of quantum particles. This is known as the correspondence principle, which states that the predictions of quantum mechanics converge to classical physics predictions when the number of particles involved becomes large.
Therefore, while quantum physics introduces new concepts and principles that are not present in classical physics, the laws of classical physics still hold as an approximation in the macroscopic world. Quantum physics builds upon and extends classical physics to provide a more complete description of the universe, encompassing phenomena that classical physics cannot adequately explain.