What keep the nucleous from colapsing into its self

[physicophile]

I was just wonder what force keep protons from "merging" into each other. As I understand, the reason I can't put my hand through a wall is due to the electromagnetic repulusion of electron. Protons on the other hand, repeal each other, but after a certian distance the strong nuclear force kicks in and they become attracted to each othere?


I guess my question can be rephrased why isn't baryonic matter allways an bose-enstien condestate.

 

[ShawnD]

I think it's due to the energy required to start that reaction. For example, hydrogen atoms can be joined to make helium atoms. This has been done many times before. The catch is that it requires extreme temperatures in the range of 800 million degrees Kelvin.

Nuclear reactions are just like any other reactions. Enthalpy still strongly favours the burning of gasoline at -100C, yet gasoline cannot burn at -100C. Why? Not enough energy to start the reaction.

edit: to sum up what I said, you need 800 million degrees Kelvin before the nuclei can get close enough for the strong nuclear force to overcome the coulomb force.

 

[interested_learner]

Can I guess something? Could it have something to do with the uncertainy principle? Could it be that if the protons condensed into one big lump, the combination of momentum and location would be greater than Planck's constant? I have heard that proposed as an explanation for why electrons don't collapse onto the nucleus. Maybe the bose-enstien condestate can only occur because the momentum of the protons are so low near absolute zero.

 

[ShawnD]

why electrons don't collapse onto the nucleus

I too would like to know the real answer for that. It just seems to make sense that eventually the nucleous would be slammed by an electron and the electron would never go away.

 

[marlon]

why electrons don't collapse onto the nucleus.

Here is the answer in the second post of our FAQ Thread

marlon

 

[interested_learner]

The FAQ is interesting and enlightening, but it really doesn't explain why the volume of space is located where it is. See:

http://www.chem1.com/acad/webtut/atomic/WhyTheElectron.html

for another interesting model.

 

[marlon]

The FAQ is interesting and enlightening, but it really doesn't explain why the volume of space is located where it is. See:

http://www.chem1.com/acad/webtut/atomic/WhyTheElectron.html

for another interesting model.

Err, what do you mean about "the volume of space and it's location" ? Also, the model presented in the FAQ is THE ONLY correct model coming from QM, and it says JUST THE SAME (in another way) as the content of your reference.

marlon

 

[RandallB]

I was just wonder what force keep protons from "merging" into each other...
Protons ... repeal each other, but after a certian distance the strong nuclear force kicks in and they become attracted to each othere?

I guess my question can be rephrased why isn't baryonic matter allways an bose-enstien condestate.

Haven't you answered your own question here. You already said Protons - "repel each other" so how could they merge into each other?
You might guess that the Strong force should do it; but that only increases (to some limit) as distance increases not decreases like charge force. Or as you already put it "after a certain distance the strong nuclear force kicks in".

I don't see where Bose-Einstein condensate would apply as that deals with electrons and nuclei working together. There the condensate is prevented until the kinetic energy contained in the movement we measure as temperature is removed.

 

[interested_learner]

Ha, ha, Marlan, I see what you mean. Well, it is a good try for an engineer. At least I am not completely wrong.

 

[marlon]

Ha, ha, Marlan, I see what you mean. Well, it is a good try for an engineer. At least I am not completely wrong.

Don't worry, the content of the link you provided is entirely correct. I just wanted to point out that it is just the same as what i have been saying.

regards
marlon

 

[heusdens]

I was just wonder what force keep protons from "merging" into each other.

Wrong question!

The question is not why don't collapse protons into each other, since protons are same electrically charged, and strongly repell each other.

So, the real question is: what keeps protons in the nucleus together?

The answer is: the strong nuclear force. The individual parts of the proton, the quarks, bind together because they interact with gluons, the carrier of the strong nuclear force, and interchange bosons (W+/W-, Z).

 

[Azael]

In the textbook(Introductory Nuclear Physics by Krane) I had in a nuclear physics class it says that the nucleon-nucleon interaction becomes repulsive at very small distances.

When I read that I assumed the strong force itself becomes repulsive at short distances.
But does it really mean the columb repulsion dominate over the strong force at extremely small distances?

 

[jdog]

But does it really mean the columb repulsion dominate over the strong force at extremely small distances?

Yes. The Strong force goes like exp(-ar)/r so it falls off very quickly when r is large, and the columb force (with it's 1/r^2 dependence) dominates when r goes to zero, So the strong force will only hold protons in place if they are close enough, and if they get too close, the columb repulsion takes over. i.e. it is in fact a stable equilibrium.

 

[Azael]

Yes. The Strong force goes like exp(-ar)/r so it falls off very quickly when r is large, and the columb force (with it's 1/r^2 dependence) dominates when r goes to zero, So the strong force will only hold protons in place if they are close enough, and if they get too close, the columb repulsion takes over. i.e. it is in fact a stable equilibrium.

But then there is the question of neutrons and what prevents them from "collapsing" into each other or into protons.

Do they come so close that the quark charges becomes significant?

 

[marlon]

But then there is the question of neutrons and what prevents them from "collapsing" into each other or into protons.

Do they come so close that the quark charges becomes significant?

In short : conservation laws ! At the quark level, which indeed is the level we need to look at when studying the weak interaction, a neutron is NOT the same as a proton !

But actually, you do not need to go THAT deep. I suggest you do some research. Try answering this question : what conservation laws need to be respected when a neutron decays into a proton and vice versa. Then you will understand why this decay does not happen "at random" in the nucleus.

marlon

 

[Azael]

In short : conservation laws ! At the quark level, which indeed is the level we need to look at when studying the weak interaction, a neutron is NOT the same as a proton !

But actually, you do not need to go THAT deep. I suggest you do some research. Try answering this question : what conservation laws need to be respected when a neutron decays into a proton and vice versa. Then you will understand why this decay does not happen "at random" in the nucleus.

marlon

My wording was a bit poor. I did not question why neutrons doesn't decay into protons in stable nuclei. I was questioning why a neutron won't get "sucked" into a proton. If the nucleon-nucleon interaction is purely attractive why doesn't the neutron and proton get sucked into each other.

About the neutron decay I can offhand guess two things that prevent neutron decay in stable nuclei.

One is the shell structure. If the proton shells are filled the proton after the decay would have to be put in a higher shell and this would probably make the decay impossible in most cases.

In even-even nuclei maby the needed breaking of a pair could make the decay energeticly impossible.

Are those guesses right??

I can't figure out offhand what prevents decay of neutrons in nuclei with odd-odd or even-odd proton-neutron number.

 

[Stratos]

My wording was a bit poor. I did not question why neutrons doesn't decay into protons in stable nuclei. I was questioning why a neutron won't get "sucked" into a proton. If the nucleon-nucleon interaction is purely attractive why doesn't the neutron and proton get sucked into each other.

About the neutron decay I can offhand guess two things that prevent neutron decay in stable nuclei.

One is the shell structure. If the proton shells are filled the proton after the decay would have to be put in a higher shell and this would probably make the decay impossible in most cases.

In even-even nuclei maby the needed breaking of a pair could make the decay energeticly impossible.

Are those guesses right??

I can't figure out offhand what prevents decay of neutrons in nuclei with odd-odd or even-odd proton-neutron number.

The fundamental interaction is not the strong interaction but the QCD (Quantum Chromodynamics ).
The nucleon (proton or neutron) is not a point like particle. In fact there are three quark inside and a sea of virtual gluons. The interaction is very complicated. The Physical Review D54 Particle and Fields, at page 194 present the cross section data for a proton-neutron up to 500 Gev (center of mass).

 

[Norman]

The fundamental interaction is not the strong interaction but the QCD (Quantum Chromodynamics ).

Care to elaborate why you think QCD and the strong interactions are not the same thing? How are you defining the "strong" interaction?

 

[Stratos]

Care to elaborate why you think QCD and the strong interactions are not the same thing? How are you defining the "strong" interaction?

You are right one must be careful.
I thought that you are talk about that (http://en.wikipedia.org/wiki/Nuclear_force) nuclear forces.
(Not a fundamental one)
I was talking about the QCD (http://en.wikipedia.org/wiki/Quantum_chromodynamics)
where the interaction where the color charge is the origin of the interaction (http://en.wikipedia.org/wiki/Color_charge)
The proton and the neutron have 0 total color charge. In fact all the mesons and all baryon have 0 color charge. The SU_color(3) group is responsible for that.
In high energy (a lot of Gevs) there is asymptotic freedom and one can use perturbation theory (Feynman diagrams). In the low energy (500 Mev is really low energy) one must use QCD latice using a super computer. This is very difficult and expensive.
In energy region of the nuclear physics we treat the nucleons (protons and neutrons ) as point like particles with a Classical potential. In such low energy this is not a bad approximation.

Now if one asked why one proton and one neutron they don't merge then he must use the full theory (no pertubative QCD).
In fact if we use ultra high energy one can create quark gluon plasma (http://en.wikipedia.org/wiki/Quark-gluon_plasma.)


PS.
1) I remember the decays of the Z0 during my Phd at CERN in the early 90s. When the Z0 decay to two quark we had two or three jet of particles.
2) Sorry for this lengthy message, but I just discover your forum and I am really excited!:tongue2:

 

[humanino]

Let me just add two comments.

First, the quark and gluon plasma has not been observed unambigously. As of today, it is still a theoretical speculative possibility, not an established experimental fact.

Second, one should not make such a difference between strong interaction and QCD. It is quite unusual to speak in those terms. In the field of strong interaction physics, everybody agrees that it is highly unlikely that QCD is not the correct and relevant model of strong interaction. Besides, were it at low (less than 100 MeV say) or high (more than 100 GeV say) energies one can actually make calculations using various perturbative technics (s.a., for instance, chiral perturbation theory at low energy, and direct application of Feynman's diagrams at high energy/momentum transfer). The real problem with non-perturbative QCD is in the intermediate energy range, not low energy. Even in the worse non-perturbative region for QCD, say between 100 Mev and a few GeVs, we do have at hand very convincing experimental facts establishing QCD as the correct model of strong interaction. And we are even able to perform some approximations and calculations, at 30% level. You would be extremely surprised by the efficiency of Regge-like (improved) calculations for instance in this regime, which rely merely on the most general properties of QFTs (causality, unitarity...).

 

[Stratos]

Dear humanino
Thank you for your comment.
a) Yes the quark and gluon plasma has not been observed unambigously.
b) I think that we both agree that “The fundamental strong interaction gives rise to the nuclear force“

In my post I was try to answer to [physicophile] first question.

“I was just wonder what force keep protons from "merging" into each other. As I understand, the reason I can't put my hand through a wall is due to the electromagnetic repulusion of electron. Protons on the other hand, repeal each other, but after a certian distance the strong nuclear force kicks in and they become attracted to each othere?
I guess my question can be rephrased why isn't baryonic matter allways an bose-enstien condestate. “

I understand that [physicophile] was talking about the nuclear force (Off course nuclear force Is not a fundamental force). [In molecular physics the van der Waals' forces are not fundamental... but arise from the polarization of molecules into dipoles (or multipoles)].

 

[Maxwells Demon]

why electrons don't collapse onto the nucleus.

Correct me if I'm wrong, I'm just a humble high school student..

The electrons would collapse into the nucleus if they weren't moving..? Its because they move, they don't hit.. They kinda falls but the nucleus curves away under them..

Just like the cannonball and the earth, you know..

All of you physics professors, be kind and correct me if I'm wrong, and please tell me the real reason..

 

[ZapperZ]

Correct me if I'm wrong, I'm just a humble high school student..

The electrons would collapse into the nucleus if they weren't moving..? Its because they move, they don't hit.. They kinda falls but the nucleus curves away under them..

Just like the cannonball and the earth, you know..

All of you physics professors, be kind and correct me if I'm wrong, and please tell me the real reason..

I could have sworn that someone has pointed out to you our FAQ. Please read that.

Zz.

 

[Maxwells Demon]

I can't find the FAQ, where is it??

 

[ZapperZ]

It's a sticky in the General Physics forum.

Zz.

 

[Maxwells Demon]

Thanks ZapperZ

 

[mormonator_rm]

I was just wonder what force keep protons from "merging" into each other. As I understand, the reason I can't put my hand through a wall is due to the electromagnetic repulusion of electron. Protons on the other hand, repeal each other, but after a certian distance the strong nuclear force kicks in and they become attracted to each othere?


I guess my question can be rephrased why isn't baryonic matter allways an bose-enstien condestate.

I think that the simplest, but still accurate, explanation of this is to tell you that there are TWO internucleonic forces at work here... First of all, we must consider the well known attractive potential, which is perpetuated by virtual pions. Since the pion carries no spin excitation (hence quantum numbers 0-+), it will not change the spin of either the emitting nucleon or the receiving nucleon. Since the nucleons are already arranged in spin pairs, sort of like Cooper pairs, they remain in a favorable arrangement when virtual pions are exchanged, and hence the force is attractive. Because pions are very light in the grand scheme of things, their Planck lifetime is relatively long, and hence the force becomes long-range. This is why the long-range nuclear potential is attractive.

On the other hand, there is also a short-range potential that is generally perpetuated by virtual rho mesons. Since the rho meson is about 6 times heavier than the pion, the range of the interaction is much shorter. Not only that, but the virtual rho meson, being a vector with spin excitation (and hence quantum numbers 1--), will change the spin sign for the nucleon that emits it. Since this spin reversal will make the nucleonic "Cooper pair" analogue become unfavorable upon emission, the potential is repulsive. This provides us with a short-range repulsive potential.

Put the two forces together, and what do you get? You get a long-range attractive force between nucleons, with a short-range repulsive core that prevents the nucleons from collapsing into each other. This is why your typical nucleus maintains a separation of about 1 fm between nucleons, but still does not come apart without great effort. Hence, we get a nucleus that does not collapse completely, yet shows enough attraction to keep itself from blowing apart due to electric repulsion.

I hope this answers the question.

 

[bjschaeffer]

I was just wonder what force keep protons from "merging" into each other. As I understand, the reason I can't put my hand through a wall is due to the electromagnetic repulusion of electron. Protons on the other hand, repeal each other, but after a certian distance the strong nuclear force kicks in and they become attracted to each othere?


I guess my question can be rephrased why isn't baryonic matter allways an bose-enstien condestate.

The main force that keeps protons from "merging" into each other is the so-called "tensor force" which is in fact the magnetic repulsion between the magnetic moments of the nucleons. The electrostatic repulsion between protons is not efficient enough.

 

[bcrowell]

The answer to the OP's question is that the density of nuclear matter is limited by the Pauli exclusion principle and the Heisenberg uncertainty principle.

For a fixed N and Z, if you make a nucleus more compact, you make it more bound in terms of the strong nuclear force, which is attractive. However, the Heisenberg uncertainty principle results in an increase in kinetic energy. The total energy is minimized at a certain density.

For varying N and Z, you do not get an increase in density by adding more nucleons. The reason for this is the Pauli exclusion principle.

In the textbook(Introductory Nuclear Physics by Krane) I had in a nuclear physics class it says that the nucleon-nucleon interaction becomes repulsive at very small distances.

When I read that I assumed the strong force itself becomes repulsive at short distances.
But does it really mean the columb repulsion dominate over the strong force at extremely small distances?

Yes. The Strong force goes like exp(-ar)/r so it falls off very quickly when r is large, and the columb force (with it's 1/r^2 dependence) dominates when r goes to zero, So the strong force will only hold protons in place if they are close enough, and if they get too close, the columb repulsion takes over. i.e. it is in fact a stable equilibrium.

This is incorrect. The nuclear force is often modeled by an attractive force that drops off exponentially, *plus* a repulsive core at very small distances. This is what Krane is talking about. It has nothing to do with the coulomb repulsion. The coulomb repulsion does not blow up at r=0, because the charge distribution of each nucleon is not pointlike.

The main force that keeps protons from "merging" into each other is the so-called "tensor force" which is in fact the magnetic repulsion between the magnetic moments of the nucleons. The electrostatic repulsion between protons is not efficient enough.

This is incorrect. Electromagnetic interactions are not really relevant to this entire discussion, nor is it particularly relevant to make a distinction between the charged protons and the uncharged neutrons. The Coulomb energy in the liquid-drop model http://en.wikipedia.org/wiki/Liquid_drop_model varies like Z^2, so it's negligible for light nuclei. Heavy nuclei don't have a different bulk density than light nuclei, so you can see that the electromagnetic interactions aren't strong enough to have a significant effect on this discussion.

But actually, you do not need to go THAT deep. I suggest you do some research. Try answering this question : what conservation laws need to be respected when a neutron decays into a proton and vice versa. Then you will understand why this decay does not happen "at random" in the nucleus.
marlon

This has nothing to do with the OP's question. The OP's question is a question about nuclear structure, not a question about beta decay.

As I understand, the reason I can't put my hand through a wall is due to the electromagnetic repulusion of electron.

This is not really correct. It's more closely related to the Pauli exclusion principle. The bulk electrical force between your hand and the wall is zero, since both objects have zero charge density on a macroscopic scale. If you look at the microscopic scale, the electromagnetic interaction between your hand and the wall may actually be attractive, i.e., your hand may tend to stick to the wall.

I guess my question can be rephrased why isn't baryonic matter allways an bose-enstien condestate.

Nuclear matter is cold and composed of fermions; you can't get a Bose-Einstein condensate with fermions. Condensed matter at normal temperatures can be composed of either fermionic or bosonic atoms, but you don't get a Bose-Einstein condensate because of the temperature.