What forces are at work when neutrons are ejected?

[Dennis P Brown]

I answered the question on the energy of the neutron 'ejected' from the nucleus very clearly far up in this thread. Whether one wants to believe scientific fact (and that does not mean something else isn't causing the process, only that is how science works) or just go off into speculation is fine. But the answer does not require a force - no more than a photon needs a force to start at the speed of light - that is just as 'counter intuitive’ as the neutron issue yet no one here is asking how that is possible.

So, once more I will answer a question using accepted science which will run against the grain ... but a neutron star does not decay in fifteen minutes for the same reason a neutron does not decay in a nucleus - it is less stable if it decays than if it remains a neutron. A neutron lowers its energy when it is bound. The only way to confirm this is to calculate the energy in each case - trying to use a easy to understand example in our macro world just will not cut it for quantum effects. Can't get around that.

[Jeff Robertson]

Another thing to keep in mind when discussing nuclear-related topics such as this is that "force" is a very classical, borderline obsolete term. While it may be appropriate to talk of forces on a macro scale, the concept of forces (in the sense that particles are "pushing" or "pulling" on each other) holds no meaning on the nuclear level (or in the quantum world at all, for that matter). Modern quantum field theory (and most modern quantum theories) states that these "forces" are really the exchange of particles carrying momentum/energy. For the electromagnetic force this is the well known photon, for the strong interaction this is the gluon, and the weak interaction is governed by the exchange of W and Z bosons (some versions of quantum field theory predict an analogous particle for gravity called the "graviton," although gravity is such a small force in comparison that it would be nearly impossible to detect said particle).

That said, if you're trying to explain neutron emission in terms of some force pushing or pulling the neutron out of the atom, you may be left feeling unsatisfied. In fact, if you're trying to reconcile a neutron's behavior with any sort of classical concept of motion or dynamics, chances are good your answer is either wrong or incomplete. Steve's ideas are interesting to think about, but with all the success quantum mechanics has had in explaining subatomic particles, I doubt that neutron behavior can be fully explained with such archaic concepts (although they could very well play a part, this is all just speculation after all).

While I'm more inclined to believe the solutions Dennis or myself proposed (since quantum has been the most successful theory on the nuclear scale to date), I agree with Richard's notion of playing devil's advocate. We can't assume we "know" anything about the neutron. We know nothing for certain about its shape, size, constituents, or behavior. We can't even observe a neutron directly (have you seen a neutron before? because I haven't!), we can only observe its effects on the environment and infer that a neutron must be in play.

All of that said, I'm still a fan of my energy balance proposal . Within every particle there is an internal tug-o-war between the particle's kinetic/thermal energy and the potential energy due to surrounding particles. If the kinetic energy is greater then the particle is free to roam; if the potential energy is greater, then the particle is bound to the system (this is one of the reasons why planets orbit around the sun, and electrons are bound to nuclei). This is how plasma is formed - we give enough energy to the atoms until the nuclei and electrons break free from each other, resulting in a "soup" of ions and electrons freely roaming. Energy conservation is one of the few physics concepts that has survived from classical mechanics all the way through modern quantum theory.

When a neutron is bound to the nucleus, it means that its potential energy due to the strong force is stronger than its internal energy. When some event, such as the collision of an additional neutron with the nucleus, shifts the atom, the neutron is pushed out of equilibrium for a moment. If this shift causes the neutron to temporarily lose enough potential energy so that it drops below its internal energy, the neutron will be able to break free from the nucleus. If it is not enough to upset the neutron's balance, then it'll simply settle back down into equilibrium. These are just my thoughts at least.

Apologies for the long post, this always seems to happen when I start talking about quantum!

Jeff

[Frank Sanns]

Jeff Robertson wrote:


> While I'm more inclined to believe the solutions xxxxx or myself proposed (since quantum has been the most successful theory on the nuclear scale to date), I agree with Richard's notion of playing devil's advocate. We can't assume we "know" anything about the neutron. We know nothing for certain about its shape, size, constituents, or behavior. We can't even observe a neutron directly (have you seen a neutron before? because I haven't!), we can only observe its effects on the environment and infer that a neutron must be in play.

Neutrons have no external electric charge so they should and do pass right though solid objects. It is the Coulomb force that keeps normal matter looking and acting the way it macroscopically does. Similarly a visible photon should not interact with a neutron both because of the wavelength of a visible photon would be too long and the lack of charge would make a neutron invisible. A neutron star then should be invisible or should I say transparent to visible light. Gravitational lensing would be expected though as the gravitational forces would still do their work.


> When a neutron is bound to the nucleus, it means that its potential energy due to the strong force is stronger than its internal energy. When some event, such as the collision of an additional neutron with the nucleus, shifts the atom, the neutron is pushed out of equilibrium for a moment. If this shift causes the neutron to temporarily lose enough potential energy so that it drops below its internal energy, the neutron will be able to break free from the nucleus. If it is not enough to upset the neutron's balance, then it'll simply settle back down into equilibrium. These are just my thoughts at least.
>

And what of anti color charge with neutrons? Anti electrical charge is easy to appreciate but when it involves the color charge, all bets are off. Anit matter has its own consequences and ramifications. Is this anti potential energy or positive potential energy or double potential energy? At high packing densities does degeneracy become equivalent to the environment in an atom or vice versa?

Frank Sanns

[Jeff Robertson]

Well I was being facetious with my comment about not being able to "see" neutrons, perhaps I should have made it more obvious. Low energy photons do not interact with neutrons because, as you said, they're electrically neutral. The quarks that make up a neutron (for those who don't know, three "quarks" are what come together to form a neutron) do have electric charges, though, so I believe high energy photons do interact with the quarks inside a neutron. Not completely positive though (no pun intended), if anyone can verify this?

As for anti color charge, I'm not exactly sure what you're getting at. My understanding of anti particles is that they're identical except for having opposite spin and electric charge. If this is true, this would imply that the electromagnetic potential energy of an anti particle would have the opposite sign as the potential energy of its particle counterpart, which is what I think you were saying. The potential we've been talking about in this thread, though, is due to the strong interaction, and as far as I can tell the strong nuclear force interacts with matter and antimatter in the same way.

I'm rather rusty on quarks and all that fun stuff, so take everything I say with a grain of salt.

Jeff

[Frank Sanns]

Jeff,

Quantum Chromo Dynamics (QCD) is just one of the many theories related to hadrons which of course includes quarks and obviously then neutrons and protons. I used it as an example because anti matter is not anti coulomb charge matter. Negative and positive electric charges neutralize but they do not annihilate like matter and anti matter (anti color charge). The color charge is an attribute just like a Coulomb charge but with quite different properties. Also spin or mass alone does not cause annihilation .

Quarks, which are the accepted building blocks of hadrons (protons and neutrons in the case of the original thread), have properties that include electric charge of -1/3 or +2/3, mass, spin, and color charge. It is this latter that is quite interesting in the stability or instability of quark composites like protons and neutrons. Call it whatever your will but what is referenced as color charge is what has a profound effect on stabilities, and reactivities. The attributes mentioned above are definitely the smoking gun of ejections from nuclei.

Frank Sanns

[Dennis P Brown]

Understanding how neutrons are ejected by a 'force' or other cause from a nucleus is interesting speculation and I see this thread has provide the opportunity to 'fill in the theory you desire' here. Of course, appeal to alternate explanations is only speculation unless such an alternate theory provides a better - that is more accurate - calculation for the energy that is experimentally measured.

Hence, the basic idea of momentum and the uncertainty principle does provides both the reason and an accurate calculation of the kinetic energy of the neutron under these situations and this approach is the scientifically accepted method and provides the accepted explanation - until someone offers a more accurate calculation by offering a model that provides similar or better answers than they are just using speculation, not science. Readers need to understand the difference.

Aside: I do not wish to imply that speculation is not useful in science - it can be extremely useful besides being fun.

[Richard Hull]

Jeff got my drift as devil's advocate in my post.

I might be classically educated, but I am not classically bound.

I understand that if one is gainfully employed in the world of physics one must spout the party line or face dire consequences. I have seen a number of retired physicists go rogue in their thinking, free at last of the peer pressure to muse openly.

For me, Quarks and all there colors and flavors, charms, etc are a joke we play on ourselves for balance sake in explaining physics to a level 4 layers below direct normal world laboratory observation. They are that which was and are no more.

If one is willing to apply big bang energies in the tiny space of a proton then you will see what was before protons and not the constituents of a proton. In the energy ball around this event what we see is stable for nanoseconds or less as the energy ball expands reverting the near big bang space back to 3 kelvins space. What we have recorded was a ghostly apparition, a resurrection, only to be returned to the grave from which it arose.

Richard Hull

[Frank Sanns]

Richard, I am with you on this. That is why, while I listed spin and color, I only refer to them as attributes.

Spin, a misnomer that many students think is classical spin when in fact has nothing to do with classical rotation. Then there are fields. Yes those invisible yet powerful nothingnesses. Things leak to and from them but from where and by what mechanism. Does not work for me.

The world of the very small is where the real physics are, and not this macroscopic world that is simply a construct of the electrostatic forces. Things are not as they appear and by the time the mind is developed enough to go beyond, it has already been heavily jaded by what has been seen and learned, and experienced. Science has been stuck for a century, no doubt because of the jade.

Frank Sanns

[Steven Sesselmann]

My 5 cents worth...

I go along with Richard's view, that these particle events create hot conditions similar to those that might have existed in the past, but where I beg to differ is that what we see has anything to do with the past.

To observe how these things looked in the past we would have to observe them from the past, which of course we can not. As observers we are observing the particle smash from ground potential, which is nothing at all like what the particles see.

A particle pair creation in the lab which for us lasts only nano seconds, may for the particles created, last an eternity.

Until we fully understand the observer dependency, we are never going to make sense of it.

In these CERN experiments we should not attempt to understand what we see, but instead we should model the whole event from the perspective of the particles, try to "be" the particle!

Then we will see that it looks a lot like our world, if not exactly like our world.

Remember how long it took before Copernicus unraveled the mystery of the planetary motions.

Steven

[Jeff Robertson]

Side note, I was discussing this with an old professor of mine and he believes conservation of momentum is the only law needed to explain neutron ejection. Neutron comes in from the left, strikes the nucleus, knocks a neutron out from the right side. No need for any quantum voodoo or anything beyond classical mechanics.

It makes sense if you think about it. I mean, if a neutron strikes a nucleus then that momentum has to go somewhere right?

Jeff

[Frank Sanns]

The original question on this thread is a bit ambiguous as a neutron is not ejected from a stable nucleus. The nucleus has to be unstable because of an unfavorable neutron to proton ratio or energy input from outside of the nucleus. Conservation of momentum alone can not account for the interactions within the nucleus from either situation. While the Fermi energy(Pauli Exclusion for fermions) and momentum transport may explain the rules, it says little about the inner mechanisms.

Frank Sanns