The discussion centers on the conservation of momentum and energy in quantum mechanics (QM) versus classical mechanics. It establishes that while conservation laws hold in both realms, in QM, these laws manifest as statistical rather than deterministic due to measurement uncertainties. The expectation values of momentum, energy, and angular momentum are conserved, but the precise values are not always well-defined. This distinction highlights a fundamental difference in how conservation is interpreted in classical and quantum frameworks.
PREREQUISITESPhysicists, students of quantum mechanics, and anyone interested in the foundational principles of conservation laws in both classical and quantum frameworks.
But you have to admit, don't you, that the meaning of conservation law is different in classical mechanics and quantum mechanics? After all, classically we say that the system has the same value of momentum at all times, whereas in QM we say that the system may not even have a well-defined value of momentum at all times.The_Duck said:Momentum, energy, and angular momentum are all exactly conserved in QM, just as in classical mechanics. The fact that systems can be in superpositions of different momenta, energy, or angular momentum doesn't change this.
Ken G said:Yes, it seems to me that saying an energy expectation value is exactly conserved is not quite the same thing as saying the energy itself is exactly conserved. Any completely closed system that has a definite energy, such that we have something exact to conserve in the first place, should be in a "stationary state"-- meaning that it doesn't do anything at all until it interacts with something external. Thus in formal quantum mechanics, the concept of a closed system that has anything happening in it is having a little fight with the concept of a conserved exact energy for that closed system, though it's not clear if that is really what is meant by energy being "exactly conserved." If it is only the expectation value that is exactly conserved, then that's an awful lot like being statistically conserved, because no finite number of trials used to ascertain the exact conservation will obtain exact conservation. Perhaps the issue is semantic, since the same could be said for classical mechanics experiments-- they will only verify exact energy conservation in a kind of statistical limit. Perhaps we can say that the theory of quantum mechanics is a theory of exact energy conservation, but in practice, quantum systems, like classical systems, can only be demonstrated to conserve energy in a statistical sense.
Because we have measurement uncertainty, which certainly never approaches anywhere near as small as quantum mechanical uncertainties.dipole said:Why can classical mechanics only verify energy conservation in a statistical limit?
It's an issue of what counts in the theory-- what we are pretending to be true, or what is actually a demonstrable success of a theory. It is an unproven assumption of standard interpretations of the theory that it is completely deterministic (and not at all a necessary assumption to use the theory effectively, as we no longer make the assumption that classical physics is an exact theory). My point is that this is a rather ironic interpretation of classical physics, because in any real effort to verify that the theory is working, you will never be able to treat it as a theory that acts on exact values of observables-- you would always falsify the theory if you really interpreted it that way! Instead, the theory is interpreted as mapping from an uncertain domain into an uncertain domain, and although there is no specified smallest uncertainty as there is in QM, the uncertainty encountered is always way larger than in QM.Let's pretend we can measure the particle's energies without disturbing them, which you can in the classical way of thinking.
It has never been known if classical physics has that property either, it is merely the simplest way to cast the theory. I can easily create a version that makes all the same testable predictions as the usual interpretation of those laws, without invoking that assumption. It was always a kind of lazy and untested assumption that never played any important role in classical physics, and now that we know it isn't true, we still use classical physics in exactly the same way we did before.That doesn't work in QM though, because you have no way of knowing exactly what the particles are doing at t_0 + t even if you knew everything about them at t_0.
But why do we say that it is possible to know a position exactly in classical physics? Is that ever a necessary component of any aspect of classical physics? It certainly isn't true that we can do it in practice, so why on Earth would we have a need to say it is true "in principle"? Just what kind of principle is that anyway?dipole said:What I'm talking about are measurements made within the framework of that theory and their idealizations: In classical mechanics, there's no fundamental reason why you can't know the position of a particle exactly, whereas in quantum mechanics position isn't even well defined, which is the source of the uncertainty, not some flaw in your instrument or technique.
Good point, but note that the total uncertainty in any possible measurement of the location of the center of mass of a baseball is way larger than the uncertainties encountered in the HUP. I'm told the LISA satellite would be the first effort to ever get macroscropic locations with quantum mechanical precision, but that's still something of a pipe dream at this point.marty1 said:The measurement of momentum is more and more exactly known the more particles you are measuring at one time because the more particles you measure the less you know about the exact position of each one. There are bizzillions of atoms in a baseball. You can be certain of its total momentum because you know absolutely nothing about the exact position of ANY of those particles.