# How do neutrons bounce off one another?

1. Jul 3, 2014

### Wittyname6

My understanding of what allows me to touch, say, a desk is that the desk electrons become very near the electrons on my hand and the electric force repels them. However, if I have 2 electrically neutral particles, such as neutrons, why do they not just pass through each other or something?

My best guess would be that the neutrons are made up of quarks which do carry charge. It is actually that the quarks are getting near each other and repelling. The neutron itself is electrically neutral only in total.

If this is the case, what if I took a neutrino, and fired it at another neutrino. I'm not exactly sure what a neutrino is, but would they not occupy the same physical space at an instant?

2. Jul 3, 2014

3. Jul 3, 2014

### Wittyname6

Hey, thanks for your quick responses.

I looked at the Wikipedia page for a bit, but I was quickly getting bogged down in things I didn't understand.

Perhaps I could give you an idea of how I'm thinking about it and you could point me in the right direction?

There are 4 forces: weak, strong, gravitational, electromagnetic.

Between the 2 neutrons.
Weak: Don't have a great understanding of this. I think it's responsible for radioactive decay. Given that, I don't think they would stop neutrons from passing through one another. Am I mistaken?

Strong: Holds quarks together (not quite sure how, why, or which ones). I don't think it repels, so it could not be responsible for neutrons bumping together.

Electromagnetic: The particles are neutral, so this would not come into account. Unless we treat the neutrons as packets of quarks in which case, I could see the individual quarks repelling each other.

Gravity: Again an attractive force, and it would be took weak to do anything anyway.

4. Jul 3, 2014

### Chronos

Subatomic particles have a property termed cross section. This relates to the probability that particles will collide. Further complicating matters is this property can be energy dependent. A particle, like a neutron, can have a smaller cross section when traveling fast than when slow. Some particles, like the neutrino, have an incredibly tiny cross section. It is capable of penetrating a light year or more of lead without suffering a collision. The cross section of particles is studied by shooting vast quantities of them at each other and measuring scatter. This is basically how a particle collider works. Beams of particles are collimated and accelerated to high speeds in opposing directions using powerful electromagnets. The beams are then directed to collide and they look for remnants of collisions. The frequency of these collisions are used to calculate cross section. Since a neutron has no electrical charge you have to use some ingenuity to collide them. This is normally achieved using a charged ion, like deuterium, which includes a neutron.

5. Jul 3, 2014

### Wittyname6

I'm confused about how a particle like a neutrino or a neutron has a cross section in the first place.

When I think of two particles colliding, I think of the two particles approaching each other, and then some force repels them after they get too close. For two atoms, it is that the electrons are growing too close, and the electric force repels them. But I can't think of any force which would cause the neutrons to change direction.

6. Jul 3, 2014

Staff Emeritus
If there were not a repulsive component to the strong nuclear force, why do nuclei have size?

7. Jul 3, 2014

8. Jul 3, 2014

### Wittyname6

Hello Vanadium 50, that is very closely related to my question. An admittedly brief google search seemed to indicate that the strong force is only attractive. Perhaps I was mistaken. I'll take a look at those links, 256bits, thank you.

9. Jul 3, 2014

Staff Emeritus
Whatever you read must be wrong - we know that nuclei have finite size, therefore we know that there must be a repulsive part when nuclei interact.

10. Jul 3, 2014

### Wittyname6

That stuff wasn't wrong, it's just that when I googled "Does the strong force repel" I only got things saying that it attracts.

I know that they have finite size and repulse, but I didn't understand why. My guess would be that a neutron is a bundle of quarks, which do have electric charge, and when the neutrons are close together, those electric forces, which cancel out over long distances, can start to be felt. The same way that I don't feel electric forces from my desk until the electrons get close enough.

It seems you are saying that it is not the electric force, but the strong force that is responsible. So I have two questions: Why is my theory about quarks not right, and could you explain the finer workings of the strong force? I thought that it attracted quarks and that's it. What am I missing?

11. Jul 3, 2014

### Wittyname6

Hello 256bits,

I took a look at those pages, but as usual with Wikipedia's science pages, I quickly ran into concepts and terminology I wasn't familiar with, and clicking on them only lead to more terms and concepts I wasn't familiar with.
That's why I come to a forum, I'm looking for an explanation from a human who might be able to see what I'm not understanding a little better. I'll definitely look at them when I have a better understanding of things though.

12. Jul 4, 2014

### blintaro

Hey OP,
You raise a pretty interesting question, one that I hadn't really thought about before. On a large, everyday scale it seems intuitive that stuff made of matter just hits, transfers momentum and bounces off, but on a small scale if electric forces (as you pointed out there are only four forces in existence) are ultimately responsible for making things "hit" each other and bounce off, then how can a neutron do so? I think it must have something to do with the nuclear forces as well, although I must confess I'm not that far into my education. Definitely be sticking around to see if anyone can offer an explanation!

13. Jul 4, 2014

### 256bits

14. Jul 4, 2014

### Wittyname6

Yeah, I took a look at that thread before posting, but after a couple of posts the conversation spirals off into something I don't understand too well. Looking at that previous thread, it seems that the reasons are the strong force, the electromagnetic force, and the Pauli exclusion principle.

Some of the previous posters mentioned the strong force. I still don't quite understand how that works in repulsion though, is there an equation for the strong force that might shed some light? I've done google searches on the equation, but I've gotten several different things and they are all extremely confusing. Perhaps someone who understands how the equation works a little better could explain?

The electromagnetic force was also mentioned. I still kind of like my theory about the quark charges only becoming noticeable at close distances. Am I on the right track, or is it something else?

Finally I hear about the Pauli exclusion principle. I really only have a very vague notion of what the exclusion principle means. Internet searches seem like they will explain it, but I would need to do a lot of digging to understand everything. Can someone offer a simplified picture to get me on the right track?

15. Jul 5, 2014

### 256bits

Particles are separated into fermions and bosons.
Fermions, such as the electron, proton, neutrons, neutrino, have a quantum number with half spin.
Bosons such as the photon, and some atoms, have an integer spin.

Fermions cannot collect together and have the same quantum state. This gives rise to the chemical nature of atoms and the quantum shells, which I am sure you have heard about. Two electrons, each having an opposite spin from the other can occupy a shell. The next electron that an atom possesses must reside in another shell, and so on.

This is the basis behind the Pauli exclusion principle, as proposed in the first part of the 20th century.

you said,
Certainly that is true. But what if you press down more and more? Can you overcome the electromagnetic repulsion of the electrons, and push your hand through the desk?

Well no. The electrons like a certain size of shell and resist extra pressure being put upon them with what is called degenerency pressure, which is a result of the Pauli Exclusion principle. Your extra pressure is trying to push electrons together so that more than 2 will fill up a shell. The more the pressure the more the degenerant pressure. Ordinary matter that one deals with has some degenerant pressure but, for the most part electromagmagnetic forces dominate.

Planets, with a much great interior pressure, and no great internal heat source to provide thermal pressure, are held up, as you can now imagine by a good deal of degenerative pressure from the electrons.

From this graph, from the site listed, gives an indication of the repulsive quality at small distances, for Argon, which which even if attracted to one another at first, the repulsion due to Pauli exclusion dramatically increases as they become very close to one another.
http://www.quora.com/Physics/Is-it-...t-explain-why-I-do-not-fall-through-the-floor

For some celestial objects, the pressure is so great, that the only repulsion that is holding the thing up is for all the electrons to become degenerative and you have a white dwarf.

If the pressure is even greater, the electrons are forced to merge with the protons in the nucleus to form neutrons, and the neutrons will provide the degenerative pressure to hold the object up in what is called a neutron star.

So here, even if the strong force is only attractive, and increases with distance, that would be the case for only the quarks exchanging gluons within each individual neutron. Two neutrons trying to merge under great pressure, or energy, will behave similar to the electrons under pressure, and Pauli Exclusion will again come into play, but this time for the neutrons.

That probably is too simple of an explanation of what goes on, but even if it does not satisfy a good answer, you will read as much as you can to acquire a better understanding. Perhaps my low percentage of knowledge of nuclear interactions has helped you out some.

Cherios.

16. Jul 5, 2014

### Wittyname6

Yes, thank you! Even if it's a simple answer, it definitely points me in the right direction, which is exactly what I was looking for. The article you posted looks great too.