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I am trying to find proofs of the stability of an atom, says, for simplicity, the hydrogen atom. There are positive answers and negative answers in various atom models.

The naive "solar system" model of a negatively charged electron orbiting the positively charged nucleus is not stable, it radiates electro-magnetic energy and will collapse.

The Bohr-Sommerfeld atom model seems to make stability a postulate.

The Schroedinger equation seems to give a "proof" of the stability of the hydrogen atom, because we have stable solutions corresponding to bound states.

Does anybody know if the Dirac equation or Quantum Electro-Dynamics can be used to prove the stability of a hydrogen atom?

Many thanks in advance for any references where I can learn more about this.

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This might interest you books.google.com/… –  Yakov Shlapentokh-Rothman Jan 21 '13 at 18:02
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The book by Lieb and Seiringer is indeed the ultimate reference here. –  Abdelmalek Abdesselam Jan 21 '13 at 18:20
    
Quick clarification: the accepted answer I believe is for a collection of atoms. But for a single Hydrogen atom, the stability pretty much arises from the Schrodinger solution... you can calculate that the probability the electron will reside inside the nucleus is a nonzero but very small percentage. –  Chris Gerig Jan 21 '13 at 20:14
    
@Chris Gerig: The probability that the electron is inside the nucleus isn't relevant to the stability of hydrogen. –  Ben Crowell Jan 22 '13 at 0:35

3 Answers 3

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I think you can find more in Lieb and Seiringer's book "The Stability of Matter in Quantum Mechanics", or see also Freeman Dyson http://www.webofstories.com/play/4415 and the book review http://arxiv.org/abs/1111.0170.

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In particular, the reason for the stability of matter (i.e. why matter doesn't collapse in on itself) is due to the quantum degeneracy pressure (i.e. the Pauli exclusion principle). On the flip-side, if you want to talk about why we can stand on the ground without falling through it, then the dominate cause is electrostatic repulsion (i.e. the electromagnetic force). –  Chris Gerig Jan 21 '13 at 20:11
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@Chris Gerig: "the reason for the stability of matter (i.e. why matter doesn't collapse in on itself) is due to the quantum degeneracy pressure (i.e. the Pauli exclusion principle)." I don't think this is accurate. In section I, Lieb proves the stability of an isolated hydrogen atom, where the exclusion principle is irrelevant. If that calculation had come out a different way (say, because we changed the behavior of the electric force), then matter would be unstable for reasons having nothing to do with the exclusion principle. –  Ben Crowell Jan 22 '13 at 0:44
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"if you want to talk about why we can stand on the ground without falling through it, then the dominate cause is electrostatic repulsion (i.e. the electromagnetic force)." I don't think this is right either. Neither electromagnetic interactions nor the exclusion principle suffice to explain the normal force between your foot and the ground. You need both, and Lieb is forced to invoke both in section II: "The extra factor $N^{2/3}$ is essential for the stability of matter; if electrons were bosons, matter would not be stable." –  Ben Crowell Jan 22 '13 at 0:51
    
I didn't say it was the sole cause, you definitely need both, but electric repulsion is the dominant cause. And stability of matter is different from the separation of two pieces of matter. –  Chris Gerig Jan 22 '13 at 1:40
    
When you talk about the reason we don't fall into the ground, you're referring to the normal force between your foot and the dirt. When you talk about "why matter doesn't collapse in on itself," you're talking about internal normal forces within the matter (plus the fact that the individual atoms don't collapse). In both cases, we're discussing the microscopic explanation for a normal force. The explanation is fundamentally the same in both cases, and in both cases it requires both electrical interactions and the exclusion principle. –  Ben Crowell Jan 22 '13 at 1:45

The first thing to say is that ordinary matter is actually not stable. Suppose a baseball-sized rock finds itself in the vacuum of outer space in the very distant future, isolated by the universe's accelerating expansion within its own cosmological horizon. Even within the standard model of particle physics, the rock will eventually decay by quantum-mechanical tunneling into more stable forms of matter. Over extremely long time scales, the result is believed to be that it will become a microscopic black hole, which then evaporates into other particles (mostly photons). (You will hear people say that this is the ultimate fate of all matter in the universe, which isn't actually right.) This kind of thing is discussed in Adams and Laughlin.

You asked about the stability of the hydrogen atom in various theories. There are some reasons to believe that the proton is unstable (google "proton decay"), in which case the hydrogen atom isn't actually stable. However, it is stable within specific models. Others have pointed out the Lieb paper, which in section I makes a specific technical argument about one type of stability for individual atoms according to one model. The model is the Schrodinger equation with a pointlike proton.

First off, there are really two things that are required in order to show that hydrogen is stable in this model, and Lieb only focuses on one of them, which is stability against a collapse of the electron's wavefunction so that it becomes bounded within an arbitrarily small distance from the proton.

The other type of stability that has to be demonstrated is stability against the electron's escape. Stability against escape is nontrivial. For example, the interaction between two neutrons is essentially purely attractive, and yet the two-neutron system is believed to be unbound. This is because the range of the force is so short (about $10^{-15}$ m). If the neutrons were to be confined within that distance of one another, they would have to have high kinetic energy, so they would fly apart. The reason hydrogen is bound is that the electrical force is long-range.

For hydrogen's stability against collapse, Lieb's argument is more complicated than it needs to be, because he unrealistically assumes a pointlike proton. Since protons are not really pointlike, compressing the electron to an arbitrarily small space $\epsilon$ near the center of the proton gives an electric field whose energy diverges to infinity like $1/\epsilon$. (If the proton were pointlike, then the external field would go to zero in this limit, so this argument would fail.)

Your question about quantum field theory is an interesting one. I think the nicest way to approach this is to look at the dimensionless and dimensionful quantities that you can form out of the relevant parameters. Most of the interesting physics can be understood in terms of two of these. There is the fine structure constant, $\alpha=ke^2/\hbar c\approx 1/137$, and the Bohr radius, $a_o=\hbar/mc\alpha$, where $m$ is the mass of the electron. In hydrogen, the typical velocity of the electron is $\alpha c$, and since this is small compared to c, you don't really need quantum field theory for hydrogen. The Schrodinger equation, which is nonrelativistic, is an excellent approximation. However, if you make a hydrogenlike atom consisting of a nucleus with atomic number $Z$ plus a single electron, the velocity in units of $c$ is on the order of $Z\alpha$. For large $Z$, this shows that you need relativity, and quantum field theory.

The Bohr radius is the only quantity you can form here with units of length. That suggests, without the need for explicit solution of the Schrodinger equation, that not only does hydrogen not collapse to an arbitrarily small size (as shown by Lieb's argument), but we expect it to reach a certain size which is basically the Bohr radius times some factor of order unity.

Adams and Laughlin, http://arxiv.org/abs/astro-ph/9701131

Lieb, Rev Mod Phys 48 (1976) 553, http://www.pas.rochester.edu/~rajeev/phy246/lieb.pdf

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