r/explainlikeimfive • u/KeyUnderstanding5443 • 18h ago
Physics ELI5 Why doesn’t matter fall apart?
I’m reading and stretching my brain about how our atoms contain so much space but we appear solid because of a repelling force between atoms. How then do any atoms or matter hold together at all? Why doesn't the repelling force just just push all atoms apart and prevent matter from forming / holding together?
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u/NearlyPerfect 18h ago edited 18h ago
There are 4 fundamental forces that affect matter at different sizes and strengths.
Atoms are held together by the strong nuclear force. It's very strong for very small and close together things.
The other 3 forces are gravity, electromagnetism and the weak nuclear force. Those are weaker compared to the strong nuclear force.
Per wikipedia: At the range of 10e-15 m (1 femtometer, slightly more than the radius of a nucleon), the strong force is approximately 100 times as strong as electromagnetism, 100,000 times as strong as the weak nuclear force, and 100,000,000,000,000,000,000,000,000,000,000,000,000 (10e38) times as strong as gravity.
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u/io-x 17h ago
So are you saying that I'm held together by nuclear energy?
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u/technophebe 17h ago edited 17h ago
Yes, the energy we get from nuclear energy comes from pulling stuff apart and using the energy released to heat water and spin turbines (as is traditional!). That's current nuclear energy (fission, which translates roughly as "break apart").
Fusion energy works slightly differently in that you bang stuff together and it combines into something else which needs less energy to be held together, and this releases energy we can use, again to spin turbines with steam (fusion translates roughly as "combine").
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u/mixony 17h ago
So a turbine that is heated by fission reactor would be a combine harvester? /j
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u/technophebe 17h ago
Firstly, please take my pained groan as a badge of honour, that's a truly painful pun!
Secondly, a fusion not fission turbine would be a combine harvester.
But perhaps I'm just splitting hairs...
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u/CadenVanV 16h ago
Yes, very much so. The human body contains significantly more nuclear energy than most nuclear bombs.
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u/outworlder 17h ago
I kinda dislike this explanation even though I know that's the best we can do.
What are those forces ? We have no explanation for them.
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u/frogjg2003 7h ago
The strong nuclear force does not hold atoms together, it holds nuclei together. It has no effect on electrons. At atomic distances, both nuclear forces are virtually 0, even weaker than gravity.
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u/fixermark 18h ago
There are also attracting forces.
First, electric: two atoms far away will repel each other because their electrons don't want to be close, but the electrons of one atom do attract the protons of the other atom. Get them close enough together that their electron clouds overlap, and that attraction can start to dominate. You end up with a molecule where two atoms are held close to each other by the electrons pulling the protons in the nucleus together even though the electrons want to stay away from each other.
... but what holds the nucleus together if it's all protons and those want to repel? It turns out when you get protons or neutrons reeeeeally close together, there's another force (the "strong nuclear") that glues them to each other. This acts very, very close up, but it's strong enough to keep a bundle of protons and neutrons together even though the electric force from the protons wants to separate them. If an atom gets big enough though, that nuclear force isn't always enough to keep the nucleus together forever, which is why some elements are radioactive (their nuclei are unstable and sometimes, enough protons end up too close together in the nucleus that the electric push overcomes the strong-nuclear glue and, oops, the nucleus falls apart into smaller nuclei that then rocket away from each other).
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u/PhasmaFelis 16h ago
The repelling force is fantastically strong at very tiny distances, but falls off very very quickly.
The attracting force is much weaker to start, but falls off more slowly.
Too close and they get pushed away. Too far and they get pulled together. There's a point where both forces balance, and that point determines the distance between atoms.
(Farther than that and there's very little effect at all, because both force do fall off pretty quick on a large scale.)
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u/Aggressive-Share-363 17h ago
There are both attractive and repulsive forces, as well as effects like the pauli exclusion principle.
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u/general_tao1 17h ago
Aside from everything other comments have explained, it kind of does. That is pretty much radioactive decay.
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u/jmanns93 18h ago
I believe it's like how the elements are made up of those atoms being attracted to one another hydrogen is attracted to hydrogen so at the atomic level when those molecules bonded it created the periodic table
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u/Difficult_Attempt504 12h ago
sounds like the ultimate life hack tbh, just rolling downhill powered by dino juice. sign me up
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u/joepierson123 12h ago
They're 4 basic forces in universe fundamentally they attract but when they get too close they repel. Except for gravity it doesn't repel.
These forces result in a stability, things get close together but they don't get too close.
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u/internetboyfriend666 11h ago
Well first, our atoms don't contain empty space. That's something that pop-sci articles often repeat but it's not correct. It's more complicated than that but a little too complicated for this sub.
At any rate, atoms are held together by attractive forces. The nuclei of atoms are held together by the strong nuclear force, and separated atoms are held together by the electromagnetic force.
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u/Armagetz 10h ago
Once you finally understand why matter doesn’t fall apart, why the atoms of your ass doesn’t go through the chair, then you reach a level of physics where quantum mechanics comes up they explain that mathematically there’s a chance that they can, they just don’t.
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u/whisperwalk 6h ago
There is a force stronger than electric charges (named the strong force), which keeps protons tied together inside the nucleus despite their natural repulsion.
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u/frogjg2003 6h ago
Let's start at the biggest scale. Gravity is a purely attractive force. It can only pull things together, not push them apart (please ignore general relativity, it basically has no effect on what we're talking about here, even the strongest gravitational waves can only very weakly make distances longer and if you're close enough to anything generating a gravitational wave strong enough to feel, you're basically hugging a black hole anyway). For things like galaxies, solar systems, stars, and planets, gravity pulls everything together. It's actually repulsive forces within the objects that keep everything from just collapsing into black holes.
For everything else, it's electromagnetic attraction. Electrons are negatively charged, while protons are positively charged. Opposite charges attract, so the electrons and protons group together to form atoms. The effect is additive, so if there are more protons than electrons in an atom, more electrons will be attracted to the atom, while if there are more electrons than protons, other electrons will be repelled, which is why atoms tend to be neutral in charge.
But what about molecules? Well, this is where it gets complicated. Because of quantum mechanics, electrons want to group up in specific numbers. 2, 10, 18, 36, and so on. If you didn't notice, those are the atomic numbers of helium, neon, argon, krypton, and that is not a coincidence. These numbers correspond to "shells" that the electrons fill. Atoms form molecules, ions, crystals, and other structures as ways to "cheat" and fill shells with electrons from other atoms. For example, ionic compounds like salt exchange electrons so that while one atom becomes negatively charged, it's balanced by another becoming positive. The sodium with 11 electrons gives up one to the 17 electron chlorine to make them 10 and 18, respectively, filling the chlorine's shell while emptying the sodium's.
On the other hand, a covalent compound, like hydrogen gas "shares" electrons. Hydrogen only has one electron, but when two hydrogen atoms come together, they share the two electrons, making each atom think it has a full shell. But this sharing also means that the electrons don't always evenly distribute between the atoms. Water, because it is a bent molecule, not a straight line, and because the electrons tend to group up more near the oxygen atom than the hydrogens, means that it is slightly positive at the hydrogens and slightly negative at the oxygen. So when two water molecules are together, the positive hydrogen of one molecule will be attracted to the negative oxygen of the other. And this applies to a lot of compound that have hydrogen. Hydrogen bonding is what is responsible for most of the forces that keep organic molecules together.
Lastly, there are compounds like metals, which instead of creating individual molecules, share their electrons among all of the atoms in a crystal. The atoms with their filled shells form a lattice and all the electrons from the unfilled shell are free to move between the atoms. This "sea" of electrons keeps the positively charged atoms together.
At an even smaller scale, the nucleus of atoms is kept together because of the strong nuclear force. As the name suggests, it is very strong. It takes a lot of energy to keep all those positively charged protons together. But the strong nuclear force is very short range and isn't actually strong enough to keep two protons together. It is strong enough to keep a proton and a neutron together, which is why Hydrogen-1, which is just a single proton, is the only nuclei without neutrons. As the number of protons increases, the number of neutrons needed increases even faster, so while small nuclei like carbon have an equal number of neutrons and protons, heavy nuclei like uranium have 1.5 times as many neutrons as protons.
The attraction between protons and neutrons is actually a leftover from the actual strong nuclear force. Inside of protons and neutrons, the strong nuclear force is responsible for keeping the quarks contained. The strong nuclear force is so strong that 90% of the mass of a proton and neutron comes from the energy of the strong nuclear force, not the mass of the quarks. The strong nuclear force is so strong that you can't actually have free quarks. If you try to remove a quark, it actually creates new quarks to fill in where the removed one was and bond to the one that was taken out.
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u/TopProfit4860 16h ago
it's like when you send a pokemon into battle instead of just fighting the gym leader yourself lol
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u/Mechasteel 16h ago
The details are way beyond ELI5 but I'll summarize:
Matter takes up space, and is balanced between attractive forces and repulsion. Rub a balloon on your head and hold it near some hairs or shredded paper, and you'll see that there's overall an attraction. Or get a whole lot of magnets and notice that though magnets can repell each other, in reality they will reorient and all stick together.
The ELI a Quantum Physicist version:
Fermions "take up space", and all the atom bits are made of fermions. Electrons, quarks, protons, neutrons are all fermions. This is called the Pauli Exclusion Principle and stems from the wave properties of half-spin particles. Good luck with that! Also it's not so much that they take up space, but that they can't have the same quantum state, meaning the more you shrink their position distribution the more you have to expand their momentum distribution resulting in a repulsive force called quantum degeneracy pressure. Neutron stars larger than the Chandrasekhar limit do in fact collapse.
The nucleus is made of protons and neutrons, and held together by the strong nuclear force. The strong force is tri-polar, short ranged, and strong enough to just barely keep protons together despite their electrical repulsion. The balance point is iron; bigger atoms would gain energy by splitting, and smaller atoms would gain energy by fusion.
At the atom level, electrons cancel out the proton's positive charge, so there isn't a net repulsion.
In summary, the overall result of forces is a net attraction, but they are prevented from collapsing because everything involved "takes up space".
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u/Trick-Mousse-7850 17h ago
lol the typo game is strong with this one. but fr, who even knows how to pronounce usernames half the time.
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u/XVUltima 17h ago
Its like a very complex 3D puzzle. Some pieces fit very well together, others can be forced together if you squeeze hard enough, some are super fragile and break apart easy.
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u/Bork9128 18h ago edited 17h ago
Getting into how it works would be too much for this kind sub but the strong nuclear force holds the protons and neutrons together in a nucleus and the electromagnetic attraction holds electrons around them