on the right-hand side is the quantum covariance of and . Thus, for large positive times, the uncertainty in grows linearly, with the coefficient of equal to . If the momentum of the initial wave function is highly localized, the wave packet will spread slowly and the group-velocity approximation will remain good for a long time. Intuitively, this result says that if the initial wave function has a very sharply defined momentum, then the particle has a sharply defined velocity and will (to good approximation) propagate at this velocity for a long time. Relativistic quantum free particle There are a number of equations describing relativistic particles: see relativistic wave equations. See also Wave packet Group velocity Particle in a box Finite square well Delta potential References Quantum Mechanics, E. Abers, Pearson Ed., Addison Wesley, Prentice Hall Inc, 2004, Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles (2nd Edition), R. Eisberg, R. Resnick, John Wiley & Sons, 1985, Stationary States, A. Holden, College Physics Monographs (USA), Oxford University Press, 1971, Quantum Mechanics Demystified, D. McMahon, Mc Graw Hill (USA), 2006, Elementary Quantum Mechanics, N.F. Mott, Wykeham Science, Wykeham Press (Taylor & Francis Group), 1972, Quantum mechanics, E. Zaarur, Y. Peleg, R. Pnini, Schaum's Outlines, Mc Graw Hill (USA), 1998, Specific Further reading The New Quantum Universe, T.Hey, P.Walters, Cambridge University Press, 2009, . Quantum Field Theory, D. McMahon, Mc Graw Hill (USA), 2008, Quantum mechanics, E. Zaarur, Y. Peleg, R. Pnini, Schaum's Easy Outlines Crash Course, Mc Graw Hill (USA), 2006, Concepts in physics Classical mechanics Quantum models