# What do we mean when we say “position of particle”?

a particle is in a superposition until it is observed. if observed the system collapses and chooses one position or something like that. I heard somewhere that if the particle is again observed it will be in the same position as the first time it was measured. but it seemed weird to me. because in case of atoms, electrons are supposed to have momentum. then how it can be in the same place in the atom?

so I guess the idea that I am getting by the phrase "position of particle" is not what it actually is. so will someone please explain? please in simple words minding that I am a newbie in this area

• In quantum mechanics, the "position of a particle" is the reading that your position detector outputs when you apply it to the system that contains the particle. Any more putatively "fundamental" definition is philosophy, not physics. And a "position detector" is defined as - my word, look at the time, I'm off to bed. – tparker Oct 18 '17 at 5:57
• Do you know what a quantum particle is? Do you know how quantum particles are described? – Andrei Geanta Oct 18 '17 at 9:57
• It is unclear what most physicists really mean on the fundamental level by position of a particle. Maybe you want to read up on the de-Broglie-Bohm theory. – Luke Oct 18 '17 at 11:51
• @Arthur yes I think.I don't have academic knowledge about them.but I learnt a bit from google and you tube – user136782 Oct 18 '17 at 13:52
• @user136782, Ok, no problem. You will measure the same position as long as you measure it fast enough so that the state of the system does not evolve in time. – Andrei Geanta Oct 19 '17 at 13:47

What we mean depends on the context we are discussing. There isn't a single "meaning" for the term. But it is certainly NOT true that an electron, once observed, will remain in its observed position. I am extremely dubious that you "heard" that somewhere. In general, you can understand a bit of the reasoning with regard to the way subatomic physics works. All particles carry momentum. In order to observe an electron, you have to interact with it (by definition). So, what are you going to interact with it with?? Right, a particle. And that particle can transfer (or take) some momentum from the electron during the interaction. Which means that the electron will likely move because of the interaction. QM has a lot of non-intuitive characteristics. One of them is that the (macroscopic) concept of "position" is of marginal use for subatomic physics. Distance between particles is far more important, but because of superposition, location is not well defined (on a subatomic scale). This isn't only true with subatomic particles. Think about (as one example) a lake. What is its "position"? Well, we can (more or less) pin-point the center of the lake, but what about the shore? Can we draw a razor sharp line (or curve) which has lake/water on one side of it and ground/sand/soil/rock on the other? No. It will depend on wind, waves, rain, evaporation, tides, etc. The best you can do it locate a line beyond which the lake usually doesn't exist (high water mark). This is arbitrary and often not even accurate. Let's think about that a bit more: if we can't "really" define the (exact) border, then can we really define the lake's center? So, do we really know what we mean when we talk about the lake's position? Well, for practical purposes we can. What we mean by "position" of the lake depends on the discussion's context. It's more important that we understand what we mean by a lake's position than it is to understand a subatomic particle's. Why? because one is far more useful than the other. Teachers like to contrast the differences between QM and Classical physics by high-lighting the differences. In fact, many of the "strange" things of QM have analogs in our macroscopic world. What is the sound of one hand clapping? Is that a meaningful question? Does a (single) hand have a property called clapping?

• Well said sir, well said. – Brad S Oct 19 '17 at 17:43

I think you're bit confisung the concepts... I can try to explain what I learnt (I'm a mathematician and I'm a neophite too so if someone find in my answer something unclear or, even worse, uncorrect please feel free to edit the answer or let me notice).

Position of a particle

A particle is always in a superposed state until you don't measure it and/or the particle is not interacting with the surrounding space. Superposed means that it can occupy actually every place in the space with a certain probability. If you measure its position then the particle istantaneously collapses in a real state and you see it. (EDIT: For real state I mean that it's wavefunction collapses and the particle is found in a well defined position in space).

Electrons are supposed to have momentum, then they move

If the two measurements are made immediately one after the other or you don't allow time evolution in your system (i.e. the system is fixed and then the momentum $p=0$ -for the sake of simplicity we can consider it as its classical counterpart) then the electron will be found ever and ever in the same position. But if you consider also time evolution, the secret now is that the system is collapsed in a real state and now it obeys to the Schrodinger Equation law.

I suggest this very clear article explaining your doubt http://quantumartandpoetry.blogspot.fr/2009/06/

• What do you mean by real state? – Andrei Geanta Oct 19 '17 at 13:55
• After measurement, if the state is let alone, it will evolve acording to the Schrodinger's equation. – Andrei Geanta Oct 19 '17 at 13:58
• You're absolutely right... For real state I took a "philosophical licence" somehow, but actually the term is missleading... I'll edit the answer. – Erik Pillon Oct 19 '17 at 16:16
• Position will change after observation because it's not a "good quantum number", i.e. it doesn't commute with the Hamiltonian. In classical terms, it changes over time because it's not conserved. – J.G. Oct 2 '18 at 21:25

Until the particle is "observed" there is no information in the universe about it. The word observed is not a good term, I would suggest interacting. This interacting is how causality is propagated. I also view momentum as a relationship not a fundamental quantity. You can calculate it but not measure it.

• Ok, but What do we mean when we say “position of particle”? – AccidentalFourierTransform Oct 2 '18 at 21:52