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My understanding is that the repulsion between the positive charges (protons) far exceeds the gravitational force.
by acchow 2y ago
My 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.
- wakamoleguy 2y agoA is repelled outward, B is not repelled, C is repelled outward. With gravity, A is attracted inward, B is not attracted, C is attracted inward.
- mrb 2y agoYour idea doesn't generalize. With A-B-C-D-E, only C experiences no net forces along the horizontal axis, while all the others (A, B, D, E) are repelled outward. Therefore the electromagnetic force is not cancelled as you seem to imply. Of course, in practice, we don't know what the inside of a black hole is like, as quantum gravity is very much unsolved.
- smaudet 2y agoWe like to say gravity is "weak", however it seems to operate on many orders of magnitude larger distances. Even if you make the argument that the energy complicit in the strong/weak forces is cumutavily much greater than the energy due to gravity, I'm still not sure "weak" is the right word for the job... Gravity onstensibly runs at infinite speed over infinite area (the effect of gravity waves non-withstanding).
- __MatrixMan__ 2y agoI think that gravity starts winning once it becomes strong enough to encourage the positive charges to evacuate, leaving neutrons where there once were protons, like the juice leaving a squeezed orange.
- KMag 2y agoDon't forget electron capture and positron emission converting some of those protons into neutrons.
- dredmorbius 2y agoGravity is strictly accumulative. All other forces are paired with positive and negative carriers (electromagnetic, weak nuclear, strong nuclear). Protons can generate neutrons through beta decay. I suspect something like this happens in neutron stars, which don't spontaneously fly apart into a proton cloud, so far as we've observed. This SE comment explains why neutron stars don't contain (many) protons: <https://physics.stackexchange.com/a/149656 https://physics.stackexchange.com/a/149656> (The ratio seems to be ~100:1 neutron:proton.)
- arcadi7 2y agoprotons can generate neutrons and neutrons can generate protons . why are there no nuclea with only neutrons ? — these could be as big as you want because there is no electric repulsion . the answer is that only neutron nucleus would have — by pauli exclusion principle — very large kinetic energy . so nucleus turns couple of its neutrons into protons to lower the kinetic energy of neutrons . same reason for small fraction of protons in neutron stars
- dredmorbius 2y agoThere are nuclei with only neutrons. We call those "neutron stars". They're ... both big (by atomic scales) and massive (atomic and astronomical).
- arcadi7 2y agoand they _always_ have few percent of protons and electrons — every single one of them .
- arcadi7 2y agoand they _always_ have few percent of protons and electrons — every single neutron star .
- adrianN 2y agoHow do we know this? That seems difficult to observe from a couple of light years away.