# Seeking Point Particle Explanation

1. Aug 5, 2009

### AndyDufresne

Seeking "Point Particle" Explanation

Ahoy,

For some time I've been reading through various topics in theses Forums as a Guest, but decided to move beyond that. I'm unfortunately not adept with Science or Mathematics (I'm more adept in Language and in Words), but I've always found them fascinating to me nonetheless and try my best understand them as well as I can. So I thought I would preface my question with the hope respondents keep my general inexperience and lack of knowledge in mind.

I'd like some explanation on the concept of Point Particles--- dimensions, size, the lack-thereof, etc (analogies would be splendid---as I mention, language and words are more my forte).

Thanks ahead of time to those who respond and take the time to explain!

--Andy

2. Aug 5, 2009

### jnorman

Re: Seeking "Point Particle" Explanation

andy - wikipedia is often a good place to start with this kind of question. a point particle has no spatial dimensions, ie, it occupies no volume. electrons and quarks are considered to be point particles, thus everything which we perceive as matter is composed of particles which have no volume - very fun, huh?

3. Aug 5, 2009

### denisv

Re: Seeking "Point Particle" Explanation

I'm not sure if thinking of particles as points is very useful in quantum mechanics.

4. Aug 5, 2009

### WaveJumper

Re: Seeking "Point Particle" Explanation

The apparent volume of matter comes from the interaction of the electromagnetic, strong nuclear and weak nuclear force through mediating force carrier bosons. Not from purported zero dimensional point particles.

5. Aug 5, 2009

### ZapperZ

Staff Emeritus
Re: Seeking "Point Particle" Explanation

So what is the volume of, say, the free particle when you start solving the free particle Schrodinger equation at the beginning of a QM course?

Zz.

6. Aug 5, 2009

7. Aug 5, 2009

### DrChinese

Re: Seeking "Point Particle" Explanation

Please remember that the Heisenberg Uncertainty Principle (HUP) is at play with quantum particles. A fundamental particle (say an electron) has no constituent structure and behaves "as if" it is a point particle. (Theory gives us that as well.)

You could say it has no volume (because you can confine it to any arbitrarily small volume of space). Or you could say it has a large volume (because it has a precise momentum). But quantum particles most definitely exist, by the usual meaning of the word.

8. Aug 5, 2009

### ZapperZ

Staff Emeritus
Re: Seeking "Point Particle" Explanation

Maybe you should turn things around and ask yourself "what do I mean when something has a volume?"

Do you think in classical ways in which an object has definite boundary of occupation in space? Think about it! It certainly looks that way with your eyes, but what if you look at it using an electron microscope? Or what about using an atomic force microscope? Do those "definite boundaries" look definite anymore? So what "volume" are we talking about then?

You are trying to force a square object through the round hole. Rather than looking at the square object, you are complaining about the round hole. Same thing here. Maybe your concept of a "volume" really has no meaning at that scale. So trying to force it down to where it doesn't belong can easily be the problem!

Zz.

9. Aug 5, 2009

### wittgenstein

Re: Seeking "Point Particle" Explanation

"Do you think in classical ways in which an object has definite boundary of occupation in space? Think about it! It certainly looks that way with your eyes, but what if you look at it using an electron microscope? Or what about using an atomic force microscope? Do those "definite boundaries" look definite anymore? So what "volume" are we talking about then?"
ZapperZ
But isn't that indefiniteness a result of our technical inability rather than an objective trait of the object? If an object has an actual indefinite boundary would that not mean... well.. suppose I draw a line and have a particle that we call A. Then A would be on one side of the line and also the other. I realize that this has similarities to QM but that still leaves QM as something that cannot be understood. By understood I mean this; if I create a geometry based on the principle of a square circle and am able to make predictive computations with that geometry can I really be said to know what a square circle is?

10. Aug 5, 2009

### ZapperZ

Staff Emeritus
Re: Seeking "Point Particle" Explanation

So you think an atom has a definite boundary, and that the "fuzziness" is simply due to our technical inability?

Zz.

11. Aug 5, 2009

### wittgenstein

Re: Seeking "Point Particle" Explanation

Or is QM not about "whats out there" but how we can get what we want from reality? We want to be able to make predictions and if we do, it does not matter if the model violates logic . I am asking a question about QM that has bothered me for quite some time. Does QM tell us anything about reality or is it only about making predictions. I realize that "making predictions" says something about reality (future reality) . But to me its like knowing how to do math without knowing why the math works.

12. Aug 5, 2009

### ZapperZ

Staff Emeritus
Re: Seeking "Point Particle" Explanation

Aren't you in the wrong thread to ask these questions? Did you forget where you were? Thread hijacking isn't exactly allowed.

Zz.

13. Aug 5, 2009

### wittgenstein

Re: Seeking "Point Particle" Explanation

On a personal level I'd love to understand something that violates normal logic. I have even heard about paraconsistent logic. But I have also heard what to me are contradictory claims. 1. There is nothing fantastic* about QM and 2. That A does not necessarily = A.
* By fantastic I mean anything beyond human understanding.

14. Aug 5, 2009

### wittgenstein

Re: Seeking "Point Particle" Explanation

You don't have to get angry. So you are saying that asking if QM describes reality or is only a system for predicting results belongs in the philosophy section? This is directly related to the point particle concept.

15. Aug 5, 2009

### ZapperZ

Staff Emeritus
Re: Seeking "Point Particle" Explanation

I said "wrong thread", not wrong sub-forum.

This thread has a specific topic. You are trying to derail it with a different topic.

Zz.

16. Aug 5, 2009

### wittgenstein

Re: Seeking "Point Particle" Explanation

I think that "trying to derail it" is a bit strong. I was trying to respond to your question," So you think an atom has a definite boundary, and that the "fuzziness" is simply due to our technical inability?" Basically, I was asking is the 'fuzziness' actually there or only a useful model that aids calculations.

17. Aug 5, 2009

### ZapperZ

Staff Emeritus
Re: Seeking "Point Particle" Explanation

You did not answer my question. Do you think an atom has definite boundary or not the way you see the volume of a block of wood?

Zz.

18. Aug 5, 2009

### wittgenstein

Re: Seeking "Point Particle" Explanation

I would say that if an atom does not have any clear boundaries then in that "fuzziness" area one could say that yes this area is part of the atom and not part of the atom. A is A and not A. Of course I am not saying that that is not the way it is. I am only saying that if a lack of boundaries is an objective fact ( and not the result of our technical inadequacies) then it is something that we humans cannot conceive of.

19. Aug 5, 2009

### ZapperZ

Staff Emeritus
Re: Seeking "Point Particle" Explanation

Er... You HAVE seen the wavefunction for, say, the hydrogen atom before you wrote this, haven't you?

For example, look at the s-orbital. Where do you think it "ends"? And where do you think the "boundary" of that atom is? And where is the "technical inadequacies" here?

And in case you missed it the first time around, here is our https://www.physicsforums.com/showthread.php?t=5374". Pay particular attention to our policy on speculative post.

Zz.

Last edited by a moderator: Apr 24, 2017
20. Aug 5, 2009

### wittgenstein

Re: Seeking "Point Particle" Explanation

So, if an object does not have a clear boundary then there are parts of it that are part of the object and not part of the object? So you are saying in the example you gave "there are parts of the wavefunction that are part of the wave function and at the same time not part of the wave function? "If you say "NO" to that then you are saying," at every point in the area under viewing it is clear that that point is part of the object or is not a part of the object. That is a clear boundary. Yes, even when looking at a photo I have difficulties telling if the photo is blurred ( technical difficulties) or that the object does not have clear boundaries. But that is a red herring or at least I do not know why you mentioned it. As for speculation, so no new ideas or new questions allowed?

21. Aug 5, 2009

### DrChinese

Re: Seeking "Point Particle" Explanation

As I mentioned to one of your posts in another thread, you should be careful when mixing "normal" words and the language of QM. As ZapperZ is pointing out, the formulae are the true formalism. And they really don't have problems! It is only when you try to fit them in to our everyday meaning that the issues arise.

So you can debate certain semantics without making much progress because the HUP is still the HUP regardless of how you choose to color it. Etc. for the other equations. Is it a point? Or does it encompass the entire universe? These questions are debated endlessly and Zz correctly wants to keep this thread on track. Obviously this has been a problem in the past, where a thread about A morphs into a thread about B. Don't hesitate to start your own thread, hey it's free!

22. Aug 6, 2009

### ZapperZ

Staff Emeritus
Re: Seeking "Point Particle" Explanation

I have no idea what you just said here. All I asked is if you've looked at the wavefunction of a hydrogen atom that has been solved in any undergraduate QM class. You didn't answer, which leads me to presume that you haven't. Yet, you've made all of these guesses based on ... on ... what?

It appears that practically all of your "arguments" have been based not on established physics, but based on a matter of TASTES! Look back at all of your posts here. Not one of them are actually based on physics. That is why I kept asking you questions based on not only physics, but on empirical observations, trying to get you to think not only analytically, but also to try to ground this on at least something that we know to be VALID. Unlike philosophy, physics discussion, at least at this level, can't be based on that, especially when there are already well-established, documented evidence that can be used in this case.

My argument that this is really you trying to force a square object through a round hole has plenty of support. My examples of trying to look objects using more precise and finer instruments (SEM, AFM, etc) are the empirical evidence. I can even point to you arguments that provide a conclusion that it is the CLASSICAL world that you observe with your eyes that is not quite right. If you study a quantum phenomenon with "coarse-grained" observation, you can https://www.physicsforums.com/showpost.php?p=1520644&postcount=58"! So if anything, it is the classical world that has the "technical inadequacies", not the quantum level. The appearance of definite boundary to allow you to talk about volume is an illusion as the result of poor, coarse observation!

In other words, I just didn't spew all of this out based on simply a matter of my tastes. It is based on things that I've personally have looked at (SEM, AFM), and also based on published papers. We simply can't make things up as we go along in this forum, and certainly not in the physics subforums.

As for making speculation of "new ideas", which part of the guideline here which you had agreed to that you didn't understand?

Zz.

Last edited by a moderator: Apr 24, 2017
23. Aug 6, 2009

Staff Emeritus
Re: Seeking "Point Particle" Explanation

Merely that we hope you have the integrity to follow the rules you agreed to on overly speculative posts.

24. Aug 6, 2009

### Demystifier

Re: Seeking "Point Particle" Explanation

It depends on what one means by "useful".

If you only want to efficiently calculate the probabilities of different measurement outcomes, then particles as points are usually not very useful. (See however
http://prola.aps.org/abstract/PRL/v82/i26/p5190_1
for an exception.)

If, on the other hand, you want to have an intuitive visual picture of the stuff you are calculating, then a picture of particles as points may be very useful. However, one must be very careful with such a picture because it may lead some to a wrong conclusion. A consistent way of thinking about particles as points is provided by the Bohmian formulation of quantum mechanics. If used according to the well defined prescriptions, it cannot yield a result that is not consistent with statistical predictions of standard QM. Even if Bohmian interpretation is wrong in the sense that particles are not really points having trajectories, the Bohmian formulation is still useful in the sense of providing a useful intuitive picture.

25. Aug 7, 2009

### yossell

Re: Seeking "Point Particle" Explanation

I'm not sure I know what's meant when one's asked if one has seen the wavefunction of a hydrogen atom. Are we talking about the abstract mathematical wavefunction that assigns complex numbers to various base states? Or are we talking about the plots of probability distributions derived from the wavefunction such as those found here:
http://sevencolors.org/tag/atom orbitals

In either case, I'm not sure what conclusions I'm supposed to draw as a matter of pure physics about the boundary of an atom. If the probabilities are interpreted as corresponding to a genuine fuzziness in the positions of the quantum objects themselves then I think I see the argument, but this is partly an interpretational matter - not all interpretations of QM hold the eigenvalue-eigenvector link, that an object *only* has a sharp value when it is in an eigenstate of the relevant operator.

yossell.