Physics nuts

OmegaSupreme
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Physics nuts

I hope this is not too inappropriate a topic but I know some of you boys and girls are very learned in physics.  I often wondered if the Pauli exclusion principle, PEP, is violated in neutron stars as electrons and protons merge to become neutrons.  Are there any cases such as black holes that we know of where the PEP is violated?  What are your insights on this matter?  I have only a sophomore level education in physics unfortunately and cannot answer these questions myself.  Please deliver me from my unbounded ignorance.


deludedgod
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Quite the opposite. The

Quite the opposite. The Pauli exclusion principle is precisely the reason that neutron stars form. Recall that in ordinary matter, a set of n spheres form the first of the quantum numbers, and the spin eigenvalues form the 4th. In general, most of these n-spheres will not be filled with electrons, but as the density increases in a forming neutron star (where electrons will be stripped from parent atoms) the electrons are forced to occupy higher n-spheres (or principle quantum numbers, the two terms are interchangeable). This gas cannot release thermal energy because the electrons have become "locked" and therefore do not relax to lower principle quantum numbers. That is why white dwarfs are luminous. This fermionic gas is termed degenerate matter and, once a star runs out of fuel, will result in a white dwarf, but if the Chandrasekhar limit is exceeded then the star will collapse into a neutron star but the neutron star cannot collapse further because two neutrons cannot occupy the same quantum state, which is a consequence of PEP. (if the Oppenheimer limit is exceeded then a black hole would be formed).

EDIT: See Post #9

"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.

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Cpt_pineapple
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The Pauli Exclusion

The Pauli Exclusion principle states that two particles cannot occupy the same quantum state.


That means they cannot have the same quantum properties like spin or energy levels.

 

It's probably the same thing with neutron stars. The electron and proton have different masses, so they occupy different energy levels and probably spins.

 

 

 

 

 

 


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I think he was asking how

I think he was asking how the proton and electron can come together under the PEP.

 

 


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deludedgod wrote:Quite the

deludedgod wrote:

Quite the opposite. The Pauli exclusion principle is precisely the reason that neutron stars form. Recall that in ordinary matter, a set of n spheres form the first of the quantum numbers, and the spin eigenvalues form the 4th. In general, most of these n-spheres will not be filled with electrons, but as the density increases in a forming neutron star (where electrons will be stripped from parent atoms) the electrons are forced to occupy higher n-spheres (or principle quantum numbers, the two terms are interchangeable). This gas cannot release thermal energy because the electrons have become "locked" and therefore do not relax to lower principle quantum numbers. That is why white dwarfs are luminous. This fermionic gas is termed degenerate matter and, once a star runs out of fuel, will result in a white dwarf, but if the Chandrasekhar limit is exceeded then the star will collapse into a neutron star but the neutron star cannot collapse further because two neutrons cannot occupy the same quantum state, which is a consequence of PEP. (if the Oppenheimer limit is exceeded then the degenerate matter wouldn't form a neutron star anyway)

Thank you Cpt Pinapple and Deludedgod.  You've answered my question nicely.  I didn't realise until later on that the PEP applies to IDENTICAL fermions so an electron and a proton may combine to form a neutron.  So the neutron star cannot collapse any further due to the PEP huh?  I guess the next logical question would be "How close does the PEP allow identical fermions with the same set of quantum numbers to get?"  I think the quantum physics book from years ago explained that electrons with the same set of quantum numbers have zero probability of being in the same space at the same time because their wave functions are antisymetric.  This is a good way of putting it mathematically but I fail to see the physical reason for it.  I mean, is there an underlying force keeping them apart (you didn't read that from me) or is there something else going on?  Or is this one of those things where we know it is true but do not know exactly what causes it?

You had me at "WTF?"


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OmegaSupreme wrote:  I

OmegaSupreme wrote:

  I guess the next logical question would be "How close does the PEP allow identical fermions with the same set of quantum numbers to get?"  I think the quantum physics book from years ago explained that electrons with the same set of quantum numbers have zero probability of being in the same space at the same time because their wave functions are antisymetric.  This is a good way of putting it mathematically but I fail to see the physical reason for it.  I mean, is there an underlying force keeping them apart (you didn't read that from me) or is there something else going on?  Or is this one of those things where we know it is true but do not know exactly what causes it?

 

 

Because they would be essentially the same partical.

 

Two electrons with the same spin, energy etc... would be indistiguishable

 

So two similar electrons occupying the same space would essentially be one electron.

 

 

 

 


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As to how close two fermions

As to how close two fermions can get would be the next energy level

 

For example:

If

E=[n+1/2]h-bar*w

So n=1 then the next similar fermion will be n=2

 

 

See Band Theory.

 

 


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Cpt_pineapple

Cpt_pineapple wrote:

OmegaSupreme wrote:

  I guess the next logical question would be "How close does the PEP allow identical fermions with the same set of quantum numbers to get?"  I think the quantum physics book from years ago explained that electrons with the same set of quantum numbers have zero probability of being in the same space at the same time because their wave functions are antisymetric.  This is a good way of putting it mathematically but I fail to see the physical reason for it.  I mean, is there an underlying force keeping them apart (you didn't read that from me) or is there something else going on?  Or is this one of those things where we know it is true but do not know exactly what causes it?

 

 

Because they would be essentially the same partical.

 

Two electrons with the same spin, energy etc... would be indistiguishable

 

So two similar electrons occupying the same space would essentially be one electron.

 

 

 

 

Yeah, that could be a problem huh?  That's an interesting point you make about two electrons with the same set of quantum numbers being indistinguishible since the only qualities they have are their spin, angular momentum...And actually for them to be close to each other in the first place is a little weird since they have a repulsive electrical force between them.  I haven't studied the strong and weak nuclear forces but understand they come into play when subatomic particles get close to each other at which point electrical repulsion becomes weaker than nuclear attraction.  Thanks for your input Cpt Pineapple and btw, I really like your Velma picture.

You had me at "WTF?"


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OmegaSupreme wrote:Yeah,

OmegaSupreme wrote:

Yeah, that could be a problem huh?  That's an interesting point you make about two electrons with the same set of quantum numbers being indistinguishible since the only qualities they have are their spin, angular momentum...And actually for them to be close to each other in the first place is a little weird since they have a repulsive electrical force between them.  I haven't studied the strong and weak nuclear forces but understand they come into play when subatomic particles get close to each other at which point electrical repulsion becomes weaker than nuclear attraction.  Thanks for your input Cpt Pineapple and btw, I really like your Velma picture.

 

 

 

By "same place", I mean same Energy level

 

For the hydrogen atom for example, two electrons in the 1s orbital would have to have different spins, or else, there is only one electron there.

 

 

 

 

 

 


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I want to return to this

I want to return to this thread because I'm bored and because my first explanation was far too unclear, upon rereading. The stability of any star depends on the outward pressure of fusion equilibriating the internal collapse due to gravity. Once fusion stops, the star must contract under gravity.

Begin with a star of less than 8 solar masses in the red giant or supergiant phase ( all stars leave the main sequence eventually to become red giant. If the star is >4 solar masses, it will become a red supergiant after the red giant phase) of its life. Depending on its mass, the star core when fusion stops will either be composed of helium (if the star stops fusing after the proton-proton cycle), carbon and oxygen (if it stops after the triple-alpha process), or neon and magnesium. In any case, the fusing shells around the core create tremendous pressure and temperature in the core, which is confined within the fusing shell that surrounds it, but it makes the outer layers extremely tenuous until they just leave altogether. At this point, the star is a white dwarf, but the core is still contracting under gravity once fusion stops (which, like everything else) depends on how massive the star).

The white dwarf is in the center. The tenuous gas surrounding it is a planetary nebula.

Once this occurs, the electron degeneracy pressure is the only thing which stops the continued collapse of the star, since there is no energy from outward radiation pressure. The more massive the star, the greater the inward force of contraction under gravity. For these low mass stars (the overall star <8 solar masses, and the dwarf that remains after the outer layers leave <1.44 solar masses),  the electron degeneracy can stop the continued contraction of the star. This is a consequence of Pauli exclusion. This (core <1.44 solar masses) is called the Chandrasekhar limit. The reason I mention this is because it is necessary to understand neutron star formation.

The principle quantum numbers are shown above. In the core of the white dwarf, the density is such that the electrons occupy all of the possible quantum numbers. In normal matter, this does not occur. This is called degenerate fermi gas. The outward pressure of the electrons because all quantum numbers are occupied will halt contraction only if the Chandrasekhar limit is not exceeded.

Now, begin with an arbitrary high mass (>8 Solar masses) star in the red supergiant phase of its life. All of these stars will be to go past the formation of neon and magnesium. They will be able to fuse silicon to make iron ash in their core. The nucleosynthesis process must stop here because iron is the most bound nuclei.  All of these stars above 8 solar masses undergo a supernova as part of the last stage of their life. As with any fusion reaction, the production of photons in the core during nucleosynthesis occurs. In high mass stars with an iron ash core surrounded by magnesium, neon and oxygen (surrounded by carbon, surrounded by helium, surrounded by hydrogen) the gamma radiation is sufficiently energetic to break the nuclei in the core until the star core is composed almost entirely of protons, neutrons and electrons. Once this occurs, the inward pressure on the core cannot sustain the occupation of principle quantum numbers by electrons. They must be forced into the protons (they have nowhere else to go). As a result, the core is composed wholly of neutrons This process results in a virtually instantaneous tremendous increase in density of the core. This again is a consequence of Pauli exclusion. If the core contraction is too high for the pressure from electron degeneracy due to PEP to stop the contraction, then the only place the electrons can go is into the protons. Because the Chandrasekhar limit is exceeded, the contraction is too great for electron degeneracy to halt. But the neutrons are also bound under the PEP. Two neutrons cannot occupy the same quantum state. So as the star contracts, the neutron degeneracy pressure is what halts the contraction this time, instead of electron degeneracy pressure. But this time the process does not just stabilize. The PEP repulsion of neutrons is tremendous. The outward degeneracy pressure overshoots the inward contraction, so the star expands to an equilibrium state between neutron degeneracy and gravity. The overshooting of the inward contraction by Pauli exclusion is catastrophic. A massive shockwave is sent through the star which blasts off every outer layer and leaves a tiny core of neutrons. Now we have a stable neutron star. Just for emphasis, when I say "tiny core" I mean really fucking small and dense. Imagine the sun compressed to the size of Manhattan (this is just a scale comparison. Obviously this cannot happen to the sun because  the sun is too small to exceed the Chandrasekhar limit). This whole process is a consequence of Pauli exclusion. This process is a stunning demonstration of the accuracy of predictions of quantum mechanics.

But one process that does violate Pauli exclusion, and that no physics, not even quantum mechanics, can describe, is what happens when the Oppenheimer-Volkoff limit is exceeded. This is a logical extension of what we saw above. If above a certain mass (the Chandrasekhar limit) the electron degeneracy due to PEP cannot stop inward contraction, there should be another mass limit above which the neutron degeneracy due to PEP cannot stop the inward contraction. Once gravity overpowers neutron degeneracy, there is no outward pressure which can halt its continued contraction into a black hwole. The mass limit (the core >3.2 solar masses, or the original star before everything was blasted off>40 solar masses) is called the Oppenheimer-Volkoff limit. Once the Oppenheimer-Volkoff limit is exceeded, the neutrons have nowhere to go, but their outward degeneracy pressure cannot stop the inward contraction. The star is too massive. Eventually, the mass will be forced into a volume such that the span of the radius is less than the Schwarzschild radius. Once this happens, the event horizon is formed, and a black hole is born. There is no physics which can describe the physical matter that undergoes this process. As an illustration of the mind-bending density that results when the contraction overcomes neutron degeneracy, the Schwarzschild radius of Earth is just under one tenth of one millimeter. This is the amount of space we would have to compress the mass of Earth into in order to make a black hole out of it.

Once the mass is compressed to less than the Schwarzschild radius, the bounding surface of the SR will be the event horizon of the black hole. No information can leave the event horizon, and as per gravitational time dilation, anyone far from the event horizon observing someone travelling into a black hole would simply see them stop at the event horizon (actually, they wouldn't see them at all since gravitational time dilation is equivalent to gravitational redshift, and the light would simply be redshifted to infinite wavelength).

 

"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.

-Me

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deludedgod
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Also, before I forget, the

Also, before I forget, the reason that quantum mechanics cannot describe what happens to matter after the neutron degeneracy is overcome in stars which exceed the OV limit is because once it collapses (and nothing stops it from doing so now) it would not exhibit any degeneracy. Degenerate matter is just a general term for matter in which exclusion is the principle contributor to outward pressure. Quantum mechanics characterizes matter in terms of particles occupying quantum states. No two particles can occupy the same quantum state or they would be the same particle. The Heisenberg Uncertainty principle in this case tells us that (delta-p)(delta-x)=h-bar/2. This leads to some very odd conclusions about degenerate matter. Even at temperatures near absolute zero, its particles would be moving with very high average speed. In order for exclusion to have a noticeable effect, there must be extremely little uncertainty in x and thus a very high uncertainty in p. Once the neutron degeneracy is overpowered, then whatever is formed as a result would have no quantum states at all, as any quantum states would invariably give rise to an exclusion which would resist the compression. This is extremely strange since it means that once the Schwarszchild radius is reached (that is, the span of the volume occupied by the matter is less than the Schwarszschild radius) there is no quantum mechanical description of the matter. Obviously, nobody has ever seen an event horizon (and by definition, it would be impossible to see what was beyond the event horizon) but black holes cause extreme curvature of space-time, and by the equivalence principle of general relativity, they cause light to bend. So an event horizon would probably look something like this:

"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.

-Me

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