# Tag Info

59

The alpha particle is a quantum mechanical system, and it is not clear what we might mean by drawing pictures of billiard balls arranged according to classical polyhedra.In particular, the alpha has quantum numbers $J^\pi=0^+$, so it has complete spherical symmetry. In a shell model picture, which provides a simple guide to the exact 4-body wave function, ...

44

Protons and neutrons are in orbitals within the nucleus which have angular momentum , so the statement of "stationary charges" is true only to first order. The magnetic fields in laboratory experiments are not strong enough to induce a proton or a neutron to exit the nucleus. In astronomical observations, neutron stars and magnetars are studied and ...

27

Most of the forces induced by a point particle follows the 1/r^2 rule No, it's the forces mediated by point particles with no mass and charge that follow the the 1/r^2 rule. then why does strong force don't obey it? The inverse square law is a consequence of the particles having no mass and/or charge. Such particles have long/infinite lifetimes and can ...

26

This is covered by a few existing answers (see for example About free quarks and confinement) though surprisingly it doesn't appear that anyone has asked this exact question before. Anyhow, the answer is that the colour force is mediated by particles called gluons just as the electromagnetic force is mediated by photons. The difference is that while photons ...

26

Kaons and sigmas contain strange quarks, so the "ground state" particles must decay by changing quark flavour. The strong and electromagnetic interactions cannot change flavour, but the weak interaction can, hence they can only decay weakly. The strong interaction can easily produce $s\overline{s}$ pairs, which can subsequently pair up with lighter quarks ...

23

Great question! My answer would be that in order to get a bound state, we need to have a potential that is deeper than the kinetic energy the two particles have. We have a better chance of getting a potential of the right depth to bind two nucleons if: (1) They don't repel charge-wise. Compared to nucleon interaction the Coulomb force isn't that strong, ...

21

$c$ is not first and foremost the speed of light. It is first and foremost the universal speed limit of a cause-effect relationship - if $A$ influences $B$ (in the same inertial frame) causally, and if $B$ is a distance $d$ from $A$, then the minimum time that must elapse before the influence can reach $B$ is $d/c$. Since the interactions you name are ...

21

There is another fundamental force of nature apart from the electromagnetic and the gravitational force. This is the strong nuclear force. Its presence is in between the interactions of protons and neutrons themselves or between protons and neutrons. Unfortunately, the strong force has no macroscopic effect as to feel the interaction themselves because the ...

20

First, the strong force acts on scales where our classical idea of forces as something that obeys Newton's laws breaks down anyway. The proper description of the strong force is as a quantum field theory. On the level of quarks, this is a theory of gluons, but on scales of the nucleus, only a "residual strong force", the nuclear force remains, which can be ...

16

The closest you will ever come in nature to pure neutron matter is the nuclear matter in neutron stars, but it is not pure either, being suffused with protons and electrons. Neutrons are 1.4 MeV heavier than protons and tend to undergo beta decay to $pe\bar{\nu }$, unless stabilized by the exclusion principle and an abundance of protons, electrons, or ...

15

Both the EM force and gravity are long range forces. The mediator is massless. Now the gluon is massless too. So far so good. But gluons and quarks live in confinement. That is why the strong force is considered short range. Confinement is a tricky beast, you cannot use gluons to propagate through vacuum vast (or any) distance to transfer information. ...

14

Suppose that $\text{U}(3)$ was the gauge group. We can decompose this as $$\text{U}(3)=\text{U}(1)\times\text{SU}(3),$$ which implies that in addition to the $\text{SU}(3)$ that has eight generators corresponding to eight gluons, there would be an additional generator for $\text{U}(1)$. The latter in principle corresponds to an additional gauge boson, but ...

14

The two neutron state (the dineutron) is known to be unbound. As far as I know no definitive calculations have been done for the trineutron, quadneutron,etc states but they are expected to be unbound as well. A couple of the comments have mentioned neutronium, but I'm not sure that counts as a bound state. We expect it to exist only where gravitational ...

12

You probably know that the electrons in atoms occupy a series of energy levels, the $1s$, $2s$, $2p$, etc orbitals. Although the structure of nuclei is complicated, basically the same idea applies to nuclei as well as atoms. This happens because you can't put more than one fermion into the same quantum state. The electrons in atoms are fermions, and so are ...

11

When you say that the binding energy of, say, a hydrogen atom is negative you are comparing two states A hydrogen atom, where the two particle are in close proximity to one another A free proton and a free electron where the two particles are arbitrarily far away from one another (where they are free). The same thing applies when one says that a bound ...

11

The electrostatic repulsion force is long-distance, and the nuclear attraction is short-distance. So, protons do repel, and this is precisely what makes really large nuclei unstable. Secondly, electrons in the S-wave orbital have nonzero wavefunction at the nucleus, so effectively they are able to penetrate into the nucleus, and that is what makes reverse-...

11

QCD and nuclear force So, first off: The interaction holding a nucleon together (quark to quark) is indeed quantum chromodynamics (QCD). It is mediated by gluons. The interaction holding a nucleus together ("between nucleons" as you say) is not strictly speaking QCD. It is a residual interaction of QCD, similar to how the Van der Waals force between ...

10

Your existing answer talks about quark confinement, but stable nuclei can't really be described using quark and gluon degrees of freedom. Also your existing answer doesn't answer your title question: why don't nuclei collapse to a point? To first approximation, nuclei do collapse into a point. The diameter of a nucleus is typically about $10^{-5}$ the ...

10

Your question is founded on misunderstandings. The theory of what goes on inside the nucleus is neither simple nor intuitive. "Meson" is simply a name for any particle that is a bound state of a quark and an anti-quark, just like "baryon" is a name for a bound state (like a proton or a neutron) of three (anti-)quarks. It does not denote a particular ...

9

A very tempting mental model of an atom, reinforced by many illustrations in books, has protons and neutrons as "large" spheres in the nucleus and electrons as "small" spheres somewhere near the nucleus. If you assume that all of these particles are made of some "stuff" that has roughly the same density (which is the case for everyday solid and liquid ...

9

All couplings in QFT are measured in Lorentz-Heaviside rationalized natural units. That is, for instance, for the electric charge, $$\alpha = \frac{e^2}{4\pi\varepsilon_0\hbar c} \approx 1/137 .$$ In these units $\epsilon_0=1$, so the elementary electric charge is simply  e = \sqrt{4\pi\alpha} ~\sqrt{ \hbar c} \approx 0.30282212 \sqrt{ \hbar c}...

9

The answers by Alex and userTLK are correct but incomplete. It is true that whilst the strong force essentially only acts between nearest neighbours, whilst coulomb repulsion acts between all protons, it is actually the weak force that prevents the building of extremely large nuclei. For example, one must explain why you can't build nuclei with more and ...

9

The answer as you can see from the comments differs whether you are asking about the residual strong force (nuclear force) or the strong force between quarks inside neutrons and protons. residual strong force You can read a lot on this site about whether the nuclear force is attractive at large distances and repulsive at short distances. Though, this is ...

8

Let us clear up a few basic concepts. Physics is about studying the way nature is . To start with, this could be descriptive, as it was for centuries before Newton. Descriptive does not answer "why" but "how" questions, in the same way that a map does not really answer "why" the landscape is like that, but describes as ...

8

I don't know of a geometrical explanation, but there is an explanation within quantum mechanics. In gauge theories forces are "carried" by vector bosons. For example, the electromagnetic force is carried by the photon, and the nuclear force (related to the strong force) is carried by the $\pi$ meson. The lifetime of the photon is infinite, so the outgoing ...

8

It is instructive to look at chart of isotopes , number of neutrons on the x axis and protons on the y. The stable (black) isotopes diverge from the diagonal, more neutrons are needed to neutralize the coulomb repulsion of the protons, for stability. The main forces are the coulomb force (repulsive) and the strong force (attractive) , but the specific ...

8

The electromagnetic repulsion between two protons is a long-range force, depending on $1/r^2$, where $r$ is the separation of the two protons. The electromagnetic repulsion between two protons is not the reason that they do not stick together; if they are forced together (or can tunnel through the Coulomb barrier) then short-range strong nuclear forces are ...

8

It is not clear how much you know about elementary particles and interactions. This is the table of elementary particles in the standard model of physics And these are the forces with which the elementary particles interact and finally create matter as we see it everyday. The quarks within the proton and neutron interact mainly with the strong force, ...

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