Do physicists distinguish things that can be explained through the movement of matter as opposed to some unseen or non-material forces?

Question

Do physicists distinguish things that can be explained through the movement of matter as opposed to some unseen or non-material forces? For example, in quantum mechanics, the act of observing something may have an effect on the result of an experiment on a wave. This arguably cannot be explained through the movement of particles, whereas the movement of a ball that was thrown can be explained through the movement of particles. Is there an adjective or a noun that allows us to distinguish between the two. Is there such a word in physics?

Answer 1

Yes. The word you are looking for is probably kinematics. It refers to studying the motion of particles, without really caring much for the causes of that motion. For example, describing the ballistic trajectory that a bullet or a baseball takes would be kinematics.

However, we don't really explain much through kinematics. Usually, all we can do is describe the path that something would take as a function of initial parameters like position and velocity.

Dynamics on the other hand refers to the study of motion of things due to particular forces which are known. Pushing a block across a rough surface or an Atwood machine come to mind.

Answer 2

The movement of a quantum ball colliding with a macroscopic (i.e. classical) wall can be treated in quantum mechanics just as well as in classical mechanics - this is a matter of doing the math correctly. (Obviously, one has to use the quantum laws in this case.)

The philosophical difference however comes from the different status that observation has in the classical and the quantum physics. Observation might be the word that you are looking for. Classical physics assumes that observations can be, in principle, done with infinite precision. On the other hand, quantum mechanics, relativity (and perhaps also statistical physics) take into account that the observations are done using the devices and methods from the real material world, which limits their power. There is no reason why we could extrapolate the familiar behavior of the objects beyond the limits of our measurement devices, and the experimental evidence is that we can't.

If we can explain the movement of a ball in terms of molecules, it is because we made an assumption about how these molecules move, and that we can observe their movement. This assumption us however cannot be implemented in practice (because the molecules are too small to be directly observed abd because their great number makes calculation impractical - hence one needs to use quantum and statistical physics.)

Answer 3

You may be suggesting the distinction between locality and nonlocality (historically called action at a distance)? Classical gravitational and electromagnetic forces are good examples of action at a distance. You might bring up a number of objections to these examples, such as GR or QFT reducing some historically nonlocal effects to local effects, but I'm assuming you're just looking for terms, not technicalities?

I should also point out that the standard pedagogical preference is the Copenhagen Interpretation of Quantum mechanics. This makes the claim that it is the act of measurement that causes the collapse of a wavefunction. You can imagine trying to probe a hydrogen atom to determine the position of its electron. Well, how would you do that? You might blast another electron at the hydrogen atom and analyse the coulombic interaction. Ah, but that requires a transfer of energy that's on the order of the energy of the initial system, so you are forced to interfere with the initial state of the system; i.e., a collapse of the wavefunction. On the other hand, you can send a torrent of photons at a billiard ball, and not noticeably affect the system, but that's simply because the energy of a photon is negligibly small compared to that of a billiard ball in a classical setting. This is not a complete example, as there are many technicalities, but it should highlight the possible local interpretations of the observer effect. There are also many other interpretations that have local/nonlocal flavors.

Answer 4

I think the word you may be looking for is classical.

Roughly speaking, we use classical to refer to objects in which quantum behavior has been statistically "averaged out," simply because there are so many particles involved. So in the case of your thrown ball, for example, the quantum behavior of the atoms that make up the ball is too small to be observed at the everyday scale, so it looks to us like the ball has a definite position and velocity. We call an object classical when this happens, i.e., when the quantum behavior is so small compared to the whole object that it can be safely ignored.

However, it is important to remember that classical behavior is not fundamental: instead, it is a phenomenon that emerges as a special case of the theory of quantum mechanics when many, many particles are involved. So the distinction you draw between "the movement of particles" and "unseen or non-material forces" is not a fundamental distinction, but instead only makes sense for objects of certain sizes. Even your thrown ball is, at the most basic level, made of atoms that exhibit quantum behavior, and interact mainly via the electromagnetic force (which you might consider to be "unseen or non-material").

I would also add that any act of observation in quantum mechanics can be described in terms of "movement of [quantum] particles," since forces are mediated by the exchange of bosons. Even when you look at your ball, what that really means is that photons bounce off of the ball and later hit your optical nerve. It's just that the particles involved with quantum observation exhibit quantum behavior.