The physical theory of objects isolated from their surroundings. Because it is very difficult to isolate large objects, quantum theory (also known as quantum mechanics or quantum physics) is essentially a theory of the microscopic world of atoms and their constituents. Among its main principles are the dual wave-like and particle-like behaviour of matter and radiation (wave-particle duality), and the prediction of probabilities in situations where classical physics predicts certainties. Classical physicsprovides a good approximation to quantum physics for everyday purposes, typically in circumstances with large numbers of particles.