# Distance of Atoms **URL:** \ **Category:** Factual Questions\ **Created:** [June 17, 2015, 10:58pm UTC](https://boards.straightdope.com/t/distance-of-atoms/722797 "2015-06-17T22:58:15Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![Washoe](https://avatars.discourse-cdn.com/v4/letter/w/ecae2f/32.png) [@Washoe](https://boards.straightdope.com/u/Washoe)\ **Post date:** [June 19, 2015, 1:31am UTC](https://boards.straightdope.com/t/distance-of-atoms/722797/21 "2015-06-19T01:31:35Z")
> [@Chronos](#): > > As in, the electron clouds of the atoms are overlapping. So our neutrino gets a free pass not because it doesn’t bump into anything, but because it doesn’t _interact_ with anything? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 19, 2015, 3:55am UTC](https://boards.straightdope.com/t/distance-of-atoms/722797/22 "2015-06-19T03:55:30Z")
Right, I’m not saying that the PEP is incorrect. It is, 100%. It’s just that it’s not a matter of PEP _or_ electromagnetic interactions: The electromagnetic interactions are also 100% correct. As an aside, the Pauli Exclusion Principle actually says a lot more than just “no two particles can occupy the same state”. That’s just one implication of the version of the Principle that applies to fermions. There are other implications, and there’s also a version of the Principle that applies to bosons. It’s just that the “no two fermions can occupy the same state” bit is the only part that’s easy to express in common language. **Washoe** , that is correct. Electrons, quarks, photons, and neutrinos are all, so far as we can tell, point particles (if they have a size, it’s so small that we can’t tell the difference). Certainly, their physical size (if any) is significantly smaller than their interaction cross-sections. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [June 19, 2015, 5:17pm UTC](https://boards.straightdope.com/t/distance-of-atoms/722797/23 "2015-06-19T17:17:33Z")
> [@Washoe](#): > > So our neutrino gets a free pass not because it doesn’t bump into anything, but because it doesn’t _interact_ with anything? Well they do interact, just not electro-magnetically. They were originally detected via proton-neutrino interaction via the weak force. Some things to note: 1. The range of the weak force is tiny: 10[sup]−17[/sup] meters or so. So the neutrino has to get really up close to a proton. 2. The wavelength of a neutrino is also small. This limits it’s own range of interaction. 3. Neutrinos are moving really fast. So fast it was thought they were actually moving at the speed of light, but perhaps just really, really close to it. 4. So neutrinos fly by protons in no time at all. It’s a fluke of chance that one would interact with a proton every once in a long while. (Which gets into the design rules of neutrino detectors: lots of protons packed together. But you also want to be able to “see” the resulting gamma rays. And a gamma ray moving thru water is good for that. Plus, the more material, the better, so it’s gotta be cheap. Isolate from other radiation sources. Etc.) ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [June 20, 2015, 1:17am UTC](https://boards.straightdope.com/t/distance-of-atoms/722797/24 "2015-06-20T01:17:23Z")
According to [Wikipedia](https://en.wikipedia.org/wiki/Electron_degeneracy_pressure), Freeman Dyson first showed that the force that stops two objects from penetrating each other is not electromagnetic, as had been believed, but is due to the Pauli Exclusion Principle. The PEP gives rise to so-called electron degeneracy pressure. This is the same pressure that keeps a white dwarf from collapsing to a neutron star, unless its mass exceeds the Chandrasekhar limit. Note that the pressure depends strongly on the electron number density and the mass of the electron, but does not depend on the electron charge, so it is not an electromagnetic effect. Neutrons, which have no electric charge, but also obey the exclusion principle (fermions) also exhibit this degeneracy pressure. It is important to say, however, that the electron degeneracy pressure is felt only by other electrons, so when you stand on the floor, it is the electrons in your feet that feel the pressure of the electrons in the floor. In that sense, you could say it is an electronic force, but not an electromagnetic force. ---
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [June 20, 2015, 2:03am UTC](https://boards.straightdope.com/t/distance-of-atoms/722797/25 "2015-06-20T02:03:13Z")
Electron degeneracy pressure is a property of degenerate matter, not the normal matter specified in the OP. There is no such thing as separate electric and magnetic forces, just electromagnetism. ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [June 20, 2015, 2:33am UTC](https://boards.straightdope.com/t/distance-of-atoms/722797/26 "2015-06-20T02:33:24Z")
> [@rsa](#): > > Electron degeneracy pressure is a property of degenerate matter Indeed, and the electrons inside a normal metal, for example, are highly degenerate. Most of the bulk modulus of metals is due to degenerate electron pressure. (For example, [see this](http://farside.ph.utexas.edu/teaching/qmech/Quantum/node66.html).) > [@rsa](#): > > There is no such thing as separate electric and magnetic forces, just electromagnetism. Quite true. I didn’t say “electric,” I said “electronic” as in, it is due to electrons. I grant that that is not what people usually mean when they say “electronic.” ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 20, 2015, 1:47pm UTC](https://boards.straightdope.com/t/distance-of-atoms/722797/27 "2015-06-20T13:47:53Z")
Yes, neutrons are also subject to exclusion, but again, what counts as the “same state”? The answer to that question will always lead you to one of the fundamental forces (in the neutron’s case, mostly the strong force). ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [June 20, 2015, 5:09pm UTC](https://boards.straightdope.com/t/distance-of-atoms/722797/28 "2015-06-20T17:09:27Z")
Chronos, I am not an expert on quantum field theory, but if the exchange interaction comes from the fundamental force, why is there no charge in the expression for the degeneracy pressure? The formula for the neutron pressure is identical to that for electrons, the only difference is the mass of the particle. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [June 20, 2015, 9:04pm UTC](https://boards.straightdope.com/t/distance-of-atoms/722797/29 "2015-06-20T21:04:38Z")
> [@ftg](#): > > 1. The range of the weak force is tiny: 10[sup]−17[/sup] meters or so. So the neutrino has to get really up close to a proton. > > 2. The wavelength of a neutrino is also small. This limits it’s own range of interaction. > > 3. Neutrinos are moving really fast. So fast it was thought they were actually moving at the speed of light, but perhaps just really, really close to it. > > 4. So neutrinos fly by protons in no time at all. It’s a fluke of chance that one would interact with a proton every once in a long while. These are oft-quoted statements about how to think about fundamental forces, but they have serious problems. Item (1) can be sort of true if you squint just right (e.g., ignore the strong nuclear force [which also has a short range yet nucleons have large interaction probabilities], and ignore neutrino interactions above 50 GeV or so). Item (2) has no bearing on the strength of the force. The wavelength of a particle depends only on its momentum. So, a 30 MeV/_c_ neutrino and a 30 MeV/_c_ electron have the same wavelength, but the electron is much more likely to interact. In Items (3) and (4): the speed sort of has a bearing on the interaction probability, but it isn’t to do with how long the particles spend near each other. In fact, that story leads to a backwards answer for neutrinos. The faster a neutrino is going, the more likely it is to interact, contrary to what Item (4) would suggest. [Previous page](https://boards.straightdope.com/t/distance-of-atoms/722797.md?page=1)