7 ms·
If you jam a lot of protons and neutrons together they become stable thanks to gravity. I wonder where the crossover point is.
by adrianN 2y ago
If you jam a lot of protons and neutrons together they become stable thanks to gravity. I wonder where the crossover point is.
- stouset 2y agoAh yes, good ol’ element unnil^{54}ium, a proton star. That would make a great “Things I Won’t Work With” entry for Derek Lowe.
- LeifCarrotson 2y agoCorrect me if I'm wrong, but aren't nuclear-density collapsed matter stars exclusively zero-net-charge neutron stars? I suppose you could instantiate an "oops all protons" neutron star with a charge of 10^54 coulombs in Universe Sandbox, but I struggle to imagine how that would come to or continue to exist physically. More importantly, I think that the Coulomb's Law repulsion effect would more than cancel out the gravitational attraction effect - both laws work as Force = (constant x particle1 x particle2) / distance^2, and the electromagnetic force is much, much stronger than gravity at all distances.
- dmd 2y agoxkcd what if covered it: https://what-if.xkcd.com/140/ https://what-if.xkcd.com/140/
- acchow 2y agoMy understanding is that the repulsion between the positive charges (protons) far exceeds the gravitational force.
- vikingerik 2y agoFor normal nuclei it does, but when you get up into a solar mass sized blob of them, gravity exceeds it. That's because the repulsion is going in all directions and nets out against itself but all the gravity points inwards cumulatively. (Edit because we got a little confused in the replies: If it were all protons, they would repel and overcome gravity. But real matter isn't that, it's always protons and neutrons and electrons with close to no net repulsion so gravity wins.) Similarly, radioactive nuclei (alpha emitters and spontaneous fission) happen because the electromagnetic repulsion exceeds the strong force, which has a very short range and doesn't reach across those large nuclei.
- kaashif 2y ago> That's because the repulsion is going in all directions and nets out against itself but all the gravity points inwards cumulatively. Is that true? All of the protons repel all other protons with the electric force and attract with the gravitational force, and it seems like gravity is just much weaker at all distances... I haven't thought about this very hard though.
- vikingerik 2y agoThink of three protons in a line, A-B-C. Proton B is repelled in both directions so experiences no net force. Now think of 10^50 protons in a line. They're all experiencing net gravity towards the middle.
- kaashif 2y agoOkay, but A and B are both repelled outward and it explodes. So the conclusion is any proton star would just explode? That's what I already thought.
- mensetmanusman 2y agoDon’t we all. This is the reason general relativity and the standard model don’t fit under one framework (string theory has currently failed to combine them in a testable way), we don’t understand why or when gravity starts to become important.
- cvoss 2y agoI don't think that's accurate. We can pretty carefully calculate the forces involved and answer the question as posed. Related: We can calculate how strong a magnet has to be to hold your artwork on the fridge in opposition to the force of Earth's gravity. The GR vs Standard Model breakdown occurs at the extreme limits of GR, for example Planck scale regions of space and black hole singularities. A heavy nucleus is way too big to probe these limits and is well within the domain of physics where we don't see a conflict between the two theories.
- smaudet 2y ago> string theory has currently failed to combine them in a testable way String theory may well be wrong/currently untestable, however failure to invalidate the null hypothesis is not proof that somehow we don't have a meaningful understanding of other, proven theories/laws. Put a different way, if an apple falls from a tree on earth, I don't panic because I don't understand quarks or electron orbitals. I calculate the point/speed/force of impact to acceptable margins of error.
- bluepoint 2y agoI think the point is that string theory is a huge model with so many parameters that it can fit everything. This is overfitting. Because of that it does not have predicting power.
- FredPret 2y agoApparently the smallest a neutron star could theoretically be is 0.1 - 0.2 Solar masses. [0] And then 1.4 Solar masses is the upper limit. [1] [0] https://physics.stackexchange.com/a/143174/43351 https://physics.stackexchange.com/a/143174/43351 [1] https://en.wikipedia.org/wiki/Chandrasekhar_limit https://en.wikipedia.org/wiki/Chandrasekhar_limit
- kevinastone 2y agoThe Chandrasekhar limit is the maximum size of a white-dwarf, not a neutron star. It's usually defined as the minimum size of a neutron star (since it has to overcome electron-degeneracy pressure). The TOV limit[0] is the maximum size of a neutron star. [0] https://en.wikipedia.org/wiki/Tolman%E2%80%93Oppenheimer%E2%80%93Volkoff_limit https://en.wikipedia.org/wiki/Tolman%E2%80%93Oppenheimer%E2%...
- FredPret 2y agoAh yes - you’re right. I was thinking of neutron stars all along - which, correct me if I’m wrong, look like they have similar density to an atomic nucleus and for which the upper limit is apparently 3 Sols. [0] [0] https://en.m.wikipedia.org/wiki/Neutron_star#:~:text=Neutron%20stars%20have%20overall%20densities,1017%20kg%2Fm3 https://en.m.wikipedia.org/wiki/Neutron_star#:~:text=Neutron....
- nick238 2y agoHow much bigger can a neutron star be given the TOV limit vs. the Schwarzschild radius winking the whole thing into a black hole?
- deleted 2y ago[deleted]
- out_of_protocol 2y agoDon't forget electrons. Without them, you'll see something violent, up to instant black hole (relevant xkcd: https://what-if.xkcd.com/140/ https://what-if.xkcd.com/140/ ) P.S. Quote from the link: > Would this black hole cause the universe to collapse? Hard to say.
- downWidOutaFite 2y agoIt would not be an atom though.
- indoordin0saur 2y agoThe funnest way I've heard of conceptualizing a neutron star is that it's basically a giant atomic nucleus.
- alangibson 2y agoInteresting way of looking at it. Does it qualify as a nucleus if it doesn't have any protons tho?
- nick238 2y ago"Neutron star = nucleus" is very elided. The crust of neutron stars contains ordinary elemental matter with a density around that of a white dwarf (specific density of ~10⁶), but descending down to the core it explodes to ~10¹⁴, which is nucleus density and a little beyond. There's also the whole menagerie of nuclear pasta [1] where gnocchi and spaghetti you could maybe consider "nuclei" unto themselves. There's also waffles & lasagna, but when you get into the anti-spaghetti and anti-gnocchi realms, then you're into the "one nucleus" region. [1]: https://doi.org/10.1103/PhysRevC.96.025803 https://doi.org/10.1103/PhysRevC.96.025803
- indoordin0saur 2y agoA neutron star is about 20% protons.
- LeFantome 2y agoReally? Closer to the surface I imagine?
- mihaaly 2y agoJust let a single proton loose in the vicinity of a pure neutron star, wait to be attracted to it and tada, we made a super heavy and stable hydrogen isotope!
- Timwi 2y agoI do not believe that it would be stable. The proton would convert to a neutron and release an electron. If it were stable for the neutron star to have protons in or on it, then it would already have some.