What exactly is a particle - in particular in constrast to a wave

[birulami]

What exactly is a particle -- in particular in constrast to a wave

My first thread on this topic went soon downhill after two participants could not agree and someone locked the thread. But I have questions left.

The post went like this:

One of the biggest conundrums is the wave/particle duality of quantum systems as it is observed in two-slit experiments.

The interference pattern seen in a two-slit experiment suggests that the quantum system is a wave. But what exactly makes us say that the quantum system also behaves like a particle? My partial answer is:

1. On the screen behind the two slits we see individual, point-like marks very much like we would expect from a particle (imagine a little ball) hitting the screen, and certainly not from a wave front hitting the screen.
2. With sufficiently capable detectors it can be shown that the energy transmitted/released while creating the mark is quantized, i.e. it cannot continuously made arbitrarily small, like could be expected from a wave amplitude.

My question: Are there other points worth noting that suggest to say that the quantum system behaves like a particle? Is it correct to say that we don't know that it is a particle, we only make observations and interpret them? The question would then be: apart from the two observations above, are there other, different observations suggesting particle?

Among others, the following answer was quite helpful:

Greenstein and Zajac's book "The Quantum Challenge" argues that the key property of "particles" is that they cannot be detected in two locations at the same time.

My followup question would be: is this absence of the multi-location property what distinguishes the "particle" from a "wave", meaning that we can detect every wave in more than one location at the same time. (If it does not work for every wave, it is not clearly distinguishing).

To be honest: what I am after is a hard argument why we cannot dismiss the particle idea altogether. What is it that a particle can do that we cannot attribute to a wave plus its collapse?

 

[Ken G]

To be honest: what I am after is a hard argument why we cannot dismiss the particle idea altogether. What is it that a particle can do that we cannot attribute to a wave plus its collapse?

I think that if you define how this wave must "collapse", you will be imposing conditions on it that sound an awful lot like a particle. Or if you prefer, use the word "quantum", to avoid confusion with the classical term "particle" (like a baseball). In my nonexpert understanding, what defines a quantum is a set of "quantum numbers", like spin or excitation number in a discrete energy spectrum. Waves don't come with those "instructions"-- it sounds like your plan is to impose them at the time of "collapse". That would work as far as I know-- but it is that imposition that says you are doing a theory of particles, not strictly waves.

The way I think of it, which works perfectly well it seems, is that wave mechanics is what we use to make predictions involving quanta, i.e., particles. I don't see that as a "duality", but rather as a kind of separation of duties. You're right that the way we collapse the wavefunction (with our experimental design) determines the possible outcomes, but we are restricted in what types of collapse are possible-- for example, we can't collapse it to produce a single quantum showing up at two places at once. Those "collapse constraints" are defined by the quantum numbers, from whence we get quanta, from whence we get quantum mechanics. There are a lot of "particle physicists" who might be worried about being put out of work if we are to banish the concept of particle in place of "collapse constraints".

 

[Demystifier]

Different branches of physics have different DEFINITIONS of the notion of "particle".
For example, classical mechanics, nonrelativistic quantum mechanics, Bohmian interpretation of nonrelativistic quantum mechanics, quantum field theory in flat spacetime, quantum field theory in curved spacetime, all use different definitions of the concept of particle. Therefore, there is no unique answer.

For a review that, among other things, explains various notions of the concept of particle, see
http://xxx.lanl.gov/abs/quant-ph/0609163 [Found. Phys. 37 (2007) 1563]

 

[monish]

My first thread on this topic went soon downhill after two participants could not agree and someone locked the thread. But I have questions left.

Yes, that person is always hijacking threads and shutting them down when things don't go his way. I have reported him for this.

...To be honest: what I am after is a hard argument why we cannot dismiss the particle idea altogether. What is it that a particle can do that we cannot attribute to a wave plus its collapse?

Well that's the point: we don't know how to explain the collapse of the wave so we call it a particle. But the two examples you cited are not good cases for the particle argument, because in those instances the collapse of the wave is not so mysterious: it follows from a detailed analysis of the interaction of the electromagnetic wave with the Shroedinger function of the atom which acts as the detector.

Marty