# Point particles

1. Sep 2, 2007

Is there such a thing as a truly point particle, ie one with dimensions? Alternatively, is possible for there to be any fundamental particle which is not a point particle?
It seems to me that an "electron" for example does not exist beyond its electric field, mass (resistance to motion) and other similar properties. In turn the electric field does not exist beyond its influence on other particles (if only one charged particle existed, there would be no reason to assume the existence of an electric field). So the only existence an electron has is in its relation to other things. In this sense, an "electron" as such does not exist at all as a "particle" and the concept of it having spacial extent is meaningless. By applying this logic to other fundamental particles its seems to me that you can deduce that all fundamental particles must be "pointlike".
Sorry if this is all a bit jumbled and badly explained, and i hope i put it in the right forum.

2. Sep 2, 2007

### Hurkyl

Staff Emeritus
I'm just going to comment on one specific point:

If "the concept of it having spacial extent is meaningless" is a correct statement, it thereby follows that it's meaningless to say a particle is pointlike, because that's the same as saying that particle has a spatial extent of a point.

3. Sep 3, 2007

The particle is pointlike in that it occupies zero volume. It is also correct to say (eg for an electron) that the particle has a centre of mass and centre of charge located at a specific point in space. The idea is there is no inherent "particle" as such that exists beyond the collective properties of electric field etc, which could be said to occupy a volume.

4. Sep 3, 2007

### Hurkyl

Staff Emeritus
But zero volume is still a volume. When you say something has zero volume, you are making the assertion that the concept of volume is applicable to that thing, and the measure of its volume is equal to zero. This is very different from the assertion that the concept of volume is inapplicable to that thing.

If you're going to take the position that there is only the quantum fields, then you're going to have to understand that those notions defined for classical particles simply do not apply. Once you understand that, then you'll have a better understanding of the new notions that we invent to talk about the quantum fields.

e.g. you know of one way to define the "position" of a field, by computing something like center of charge. (Not being well versed in QFT, I'm not sure if center of charge literally makes sense) This is a completely different notion than the classical idea of the position of a particle, although they share many similarities. Once you really understand that these are different ideas, you will have a better chance of understanding the "position" of a field.

5. Sep 17, 2007

### sd01g

It seems to me that it is impossible to measure something of zero volume. Hence, it does not empirically exist.

One might attempt to define or locate something as one dimension, but if it is actually there, it has three spacial dimensions.

6. Sep 17, 2007

### Hurkyl

Staff Emeritus
Why is that?

7. Sep 17, 2007

### sd01g

Consider the following 'points'.

1) Volumn is a mathematical concept for How Much not What Is.

2) Zero volumn is the exact, empirical equivalent of absolute nothing.

3) One may state that something has zero volumn, but it is just a statement of faith, because it was not determined by any empirical measurements. There is no measuring device that can detect or measure the zero point on the number line.

4) Zero volumn and 'zero-dimension points' exist only in the mind/brain. They are very usefull mathematical concepts but in the external, real, empirical world, they do not exist.

Thanks

8. Sep 17, 2007

### Hurkyl

Staff Emeritus
Sure. And so it would be rather peculiar that it would be possible to show something doesn't exist simply because its volume was equal to a certain number...

Please elaborate; I cannot think of a way to read this sentence that simultaneously makes sense, supports your argument, and appears true.

It takes no more faith than any other scientific statement.

The same could be said for every physical theory. In what relevant way does a volume of zero differ from a volume of 1 meter cubed, or any amount of mass or charge?

9. Sep 18, 2007

### sd01g

Maybe we are using different definitions of the word zero. My dictionary lists twenty different meanings/usages. I was wondering if you believe anything consisting of matter/energy can exist in only one or only two spacial dimentsions?

Also, do you consider the Big Bang Theory a proper theory even though it can never be verified by an actual observation?

Thanks

10. Sep 18, 2007

### ZapperZ

Staff Emeritus
There's a huge misunderstanding of how science works in this post.

When something hits you on the back, do you also automatically dismiss where it might come from simply because you or no one else made an 'actual observation' where it came from?

Science works in 2 ways in such a case:

1. It rules out large region of "phase space" where it can say with high certainty that the object didn't come from. Since it hits you on your back, you can rule out that it came from the front, unless there was some really strange circumstances.

2. But more importantly, if you can make a detection of how hard the object hits you, and figure out roughly in what direction it hits you, you can make a very reasonable guess at where it possibly came from.

The same can be said about the Big Bang. We may not have observed it directly, but what we observe now (and in the past based on distant observations) can be used as evidence that all point to such a scenario. It is an insult to those who have work in Cosmology to simply dismiss it as being something that can never be verified simply because there's no "actual observation", whatever that means.

This discussion of "point particles" is also rather strange. Point particles in physics means that the spatial extent of the object has no detectable consequence. So if you care that much about "actual observation", this would be it. Our "actual observation" based on measurements from area of study such as condensed matter physics, gives a direct result that many of these particles are point particles. To say that they have a spatial extent does not match our "actual observation". We have no ability to say that these have any. Not only that, the theory that describes them as point particles produces the SAME prediction as experiment.

Lacking anything to the contrary, that's all we are able to verify. Claiming anything else would be something made on purely speculative grounds devoid of any experimental observations.

Zz.

11. Sep 18, 2007

### sd01g

The intent was not to insult but to try to understand exactly what kind of "theory" is the Big Bang and what is it's probability of correctness when it cannot be verified or recreated empirically, in real time, as required by the scientific method.

The notion of the Big Bang is based not on observation, but on a rational, backward projection. It also requires a very suspect notion of moving 'space' faster than the speed of light to account for current observations. Its seems that 'space' can not be observed or moved--only matter/energy can be observed moving in space. Moving space is an impossibility for science and a very difficult challenge for philosophy.

Also, very importantly, the philosophy department gives us more latitude to speculate than the physics department.

Thanks

12. Sep 18, 2007

### ZapperZ

Staff Emeritus
Again, you have not understood what the "scientific method" is. It has isn't restricted to your requirement. And you seem to have ignored the example I gave. Many of the things you are using now are based on our 'extrapolation' of where things are. Would you believe that the band structure calculations being used to describe the semiconductors you are using in your modern electronics (including your computer) is based on calculations done at ZERO KELVIN? For many materials, only at T=0 is the only place where we can actually obtain any kind of solution to know and infer its behavior. Yet, using your criteria, this isn't a 'scientific method', since we have never made any "actual observation" of T=0. For some odd reason, a logical inference isn't part of your standard vocabulary, whereas it certainly is a BIG part of the vocabulary of scientists and scientific methodology.

This isn't about the Big Bang theory. Rather, it is about your false impression of how science works. Your objection to how we arrive at the BB can be used against all aspects of physics. Yet, you have made full use of the advancements made in physics, and the fact that they WORK, means that such methodology is valid. Can you say the same with your methodology and criteria?

That's OK. I won't hold that against you.

Zz.

13. Sep 18, 2007

### Hurkyl

Staff Emeritus
By 'zero', I mean the number that is one less than one.

14. Sep 18, 2007

### sd01g

A great example of the scientific method. The scientific method includes speculative hypotheses, projections, extrapolations, logical deductions/inferences and luck. But the scientific method is not completed--truth established--until it is verified & validated, in real time by an observer/experimenter or consumer. It requires that the 'chip', or the computer, work in the real world in real time and work every time it is challenged to do so.

The success/unsuccess of the empirical observation/experiment is what validates/invalidates all (or part) of that which went before.

The example of being hit in the back was not so good of an example if it is a one time event. The hypothesis and logical deduction/inference in steps one and two were only part of the sceintific method.

15. Sep 19, 2007

### ZapperZ

Staff Emeritus
But that is why we CONTINUE to make more observations to test the BB model!

By its nature, field of study such as cosmology would never have the same degree of certainty as condensed matter/material science. That is a given. However, that is no reason to dismiss it, because many of the models do not have JUST ONE observations to validate their existence. It is based on a series of different type of observations. In fact, experimental particle physics done in particle accelerators are also contributing to the validity of such models (check particle astrophysics).

All of these observations and the building of new observatories/telescopes are equivalent to doing experiments in cosmology. It IS part of the scientific method!

Zz.

16. Sep 19, 2007

### sd01g

That would be any number except zero. It is meaningless to state that something real has zero volume or zero area. Zero volume and zero area have never been observed or experienced. Why would anyone assume that anything real had zero volume?...maybe because the math is easier.

17. Sep 19, 2007

### ZapperZ

Staff Emeritus
You are forgetting one important thing here. Maybe when you get to such a scale, the concept of "length" or "space" no longer has any meaning. That can easily be argued that that is why assigning such things as "point" particles WORKED!

The problem here is that you continue to argue the physical nature of what we observe and understand of our universe based on what your "preference", rather than what has been shown to work, to be valid, or to have been consistent. It is extremely hard to dismiss that QED could agree with the experimental measurement of the electron gyromagnetic ratio to such a degree of accuracy. When a theory can match something that accurately, you simply cannot dismiss it with impunity. Yet, consider how QED treats such particles.

I think it is you who have to reconcile your insistence with the experimental and theoretical agreement here. It is you who need to come up with an explanation on why QED (and in fact, QCD as well) worked so well using such point particles.

Zz.

18. Sep 19, 2007

### Hurkyl

Staff Emeritus
How convenient.

The volume of an object is real number value, correct?
Zero is a real number, correct?
= is defined for any pair of real numbers, correct?
Therefore, the statement that the volume of an object is zero has meaning.

I suspect, however, that you intended to say something other than "lack of meaning".

Sure it has.

Because it's consistent with empirical data, and required by our theoretical knowledge?

19. Sep 21, 2007

### sd01g

Zero is a very special number. It is the only number that has no magnitude. It is the only number that is precluded from being a denominator in a fraction. Every number's value (except zero) is represented by a distance from the zero point on the number line to the point of that given number. Zero Volume has no magnitude. It is nothing real and contains nothing real.

However, zero volume and zero dimension points are useful mathematical concepts that can be applied to very small-- but not quite zero--volumes and very small-- but not quite zero--dimension points. These concepts can describe how real things move, predict future location, describe how much and how many...but for something to really be there, it must exist with three spacial dimensions.

For those who think that 'the math' is the really real, please disregard the above.

Thanks

20. Oct 5, 2007

We need a foundation

There is no such thing as a point particle. All elementary particles have spin which infers that they have some sort of internal structure. Point particles is a construct created by Coulombs electrostatic treatment of electric forces between charged particles.

Second, be carefull the way you throw 'fundamental particles' around conceptually. For instance, the proton, neutron and electron are all suppose to be elementary particles, which is interesting because we know that the neutron can decay, so what makes it elementary? The photon is considered to be elementary in the stricktest sense, even though it can form through the annihilation of an electron with a positron, and it can decay into each respectively.

Next, if you are having a hard time appreciating what a particle is, what it is composed of an where its boundary is, for example, individual electrons shot through a two slit barrier produce an interference pattern, so how does the electron interfere with itself-some suggest that it passes through both holes simultaneously...

The answer is clear, that there is no foundation for your intellect to group, organize, and arrange these concepts. The idea of a fundamental particle is particularly interesting because it suggests that below all of the particles is a fundamental particle, a building block for the diversity we see.

Consider, that if an electron is a particle with spin, that it has structure. If it has structure it has size, if it has charge, then it affects the space or fields around it, if it can do that it can still be a particle on a specific trajectory and at the same time affect distant objects without even touching them. In other words an electron may be a particle, but its physical attributes may extend beyond what we would call its particle dimensions.

Anyways, that's how I see it.

21. Oct 5, 2007

### Jimmy Snyder

I'm not the best one to form an opinion here, but in this case I believe you are exactly correct. In QFT, particles are assumed to have zero volume, that is to say are assumed to be point particles because the math is easier. Easier than if you didn't make the assumption, I mean. Lacking this assumption, you might for instance assume that particles were not points, but curved lines. The math is harder, but not impossible. But then it wouldn't be QFT would it? It would be string theory. Am I right about this?

As for measuring a volume and getting zero, I doubt that there is any practical way to do it. For instance, I don't think you can measure any distance and get an exact number. The best you could do is get a number and some error bars. Why should zero be any different?

22. Oct 6, 2007

### malawi_glenn

We dont't call it that the photon decays..
Why must spin has anything to do about size? Can you explain that from a QFT-point of view?

23. Oct 21, 2007

No, I'm using logic here. Logic dictates that for something to spin it must have a dimension. If it is a miniscule point in space- what spins? Also sorry Malawi if I used the term decay loosely...

24. Oct 21, 2007

### Hurkyl

Staff Emeritus
Why do you think something is spinning?

p.s. at the risk of going off onto a tangent, I would like to point out that objects do not have a dimension, so you'll have to be more specific about what you mean. Furthermore, I don't see the problem with the idea of a point particle actually spinning.

Last edited: Oct 21, 2007
25. Oct 21, 2007

### malawi_glenn

But there is a difference between the ordinary day word spin and physical quantity (quantized) spin... (as Hurkyl pointed out; who has said that they are spinning?) You can't use philosofical logic to describe physics, you must use the mathematical appraturs (formalism). And now QFT etc are the formalism that you use as a physicists.

I dont know if electrons has size or not, if the world are made up of strings or whatever; the thing I am concerned about is that you use the right way to proov things. I mean, you cant fly to the moon and say "Hey Guyds, you know what? I did not see God on my way to the moon, so therefore God does not exists!"

When QM was instroduced, it was contradiction with common sense. Human language and philosophical logic fails to describe nature. To describe nature we need the mathematical language.