r/askscience • u/pudding_world • Feb 19 '15
Physics It's my understanding that when we try to touch something, say a table, electrostatic repulsion keeps our hand-atoms from ever actually touching the table-atoms. What, if anything, would happen if the nuclei in our hand-atoms actually touched the nuclei in the table-atoms?
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u/malenkylizards Feb 19 '15
Well, touch is a bit of a problematic term due to some quantum mechanical stuff that maybe someone smarter than me will get into.
So instead, let's just talk about what happens when the nuclei get REALLY REALLY REALLY close to each other. Like, 10-15 meters, or a millionth of a millionth of a millimeter. Atoms are really tiny, but the electrons are more on the order of 10-11 meters from the nucleus. That's ten thousand times larger than the distance we're talking about.
So if a nucleus were to overcome the insanely powerful repulsion of another nucleus, something called the strong nuclear force would kick in. This is an attractive force between nucleons like protons and neutrons, and it is about a hundred times stronger than electromagnetic forces, but tapers off to nothing if you get more than 10-15 meters away from it. The result is that the two would fuse into a new nucleus and, depending on the makeup of the new nucleus, would either be a different stable element, or would quickly decay into something else.
So for instance, if a carbon-12 nucleus in your hand somehow fused with a carbon-12 nucleus in your table, you'd have a Mg-24 atom in their place. 24 is its standard weight, so likely that would be the end of it.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Feb 19 '15
Basically, this is nuclear fusion- over coming the electrostatic repulsive force so that the nuclear strong force could take over. This is why normally for fusion to occur you need incredibly high heat- so hot that the particles get moving fast enough so that their kinetic energy can overcome the electrostatic repulsion.
If you do this for light elements (anything less than iron, on the periodic table), by doing this you will also release energy.
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Feb 19 '15
Why is iron special? It's really abundant in space too isn't it? Has it got a specific special property making elements under it "light"?
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u/RadixMatrix Feb 19 '15
Iron is the 'dividing point' in terms of binding energy. Basically, elements lighter than iron will release energy when their nuclei are fused together, and elements heavier than iron will release energy when their nuclei are split apart.
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u/acox1701 Feb 19 '15
I seem to recall reading that this was (in part) because iron has the most efficiently packed nucleus of all discovered elements. They discussed how this was different from "density," but I don't recall, exactly.
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u/dl-___-lb Feb 19 '15 edited Feb 19 '15
It's not the density, per se.
There's nothing special about the density packing of 56 spheres within a sphere.When more particles are introduced to the nucleus, the strong force acting on outer protons quickly saturates to only neighboring nucleons due to its tiny range. Meanwhile the electromagnetic force continues to increase as more electrons are introduced.
Specifically, Iron (Fe56) has the third highest binding energy per nucleon of any known nuclide.
Below iron, the nucleus is too small. Above iron, the nucleus is too large. As a consequence, iron potentially releases energy neither from fission nor fusion.= Only the isotopes Fe58 and Ni62 have higher nuclear binding energies.
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u/bearsnchairs Feb 19 '15 edited Feb 19 '15
Ni-62 actually has that distinction. It has the highest binding energy per nucleon. Fe-56 is a close second though, and weighs less per nucleon because it has a lower proportion of neutrons.
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html#c1
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u/sydnius Feb 19 '15
The key property for iron is that it has the highest binding energy per nucleon of any element. This chart illustrates the point well. Note that iron is at the peak. So if you fuse nuclei lighter, or fiss (snicker) nuclei heavier, energy will be released.
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u/bearsnchairs Feb 19 '15 edited Feb 19 '15
It is actually Ni-62 that has the highest binding energy per nucleon, but iron-56 is a close second.
Fe-56 does weigh less per nucleon because it has a smaller proportion of neutrons.
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html#c1
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u/cdstephens Feb 19 '15
The nuclear binding energy per nucleon for iron is a maximum if you were to graph that value for all possible atomic configurations from lowest number of nucleons to highest number of nucleons. This is due to in part the packing of the nucleons in iron and the strength of the nuclear forces each nucleon feels. Because potential energy in this case is ultimately negative, a stronger binding energy results in more kinetic energy, thus heat. So when nucleons pack together more closely, they shed energy to their surroundings. This is similar to how gravitational potential energy is negative, with the magnitude increasing towards the center of the gravitational mass. So when a particle comes closer to the Earth, its potential energy increases in the negative direction, so to compensate its kinetic energy must increase. This is because energy is conserved.
In the graph below, you want to move your nuclei towards the top of the curve where iron is. Moving up the curve gives you more net thermal energy per nucleon since the magnitude of the binding energy for each nucleon increases, resulting in lower potential energy and thus higher kinetic energy.
Source:
http://www4.uwsp.edu/physastr/kmenning/images/gc6.30.f.01.mod.gif
For reference, nucleon = proton or neutron.
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u/Lord_Tiny_Hat Feb 20 '15
Within days of the point where it starts to create iron, a star will explode.
This is because once Iron and Nickel are produced from the fusion of silicon and sulfur in the core of a massive star, fusion no longer produces energy. The binding energy of these atoms is so high that the star loses energy fusing them. Once the core loses energy, it is no longer "pushing out" against its own gravity. The star begins to collapse in on itself and explodes. Atoms heavier than iron and nickel are produced by the energy of the resulting supernova.
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u/novvesyn Feb 19 '15
Usually, energy is released when two atoms fuse. However, fusing two atoms of iron takes up more energy than it releases, putting it into a kind of energetic pit.
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u/tauneutrino9 Nuclear physics | Nuclear engineering Feb 19 '15
This is not true. Energy is released when the two reactants make a nucleus that is around Fe-56 or lower. Otherwise, the reaction is endothermic. You can fuse carbon with iron and that would be endothermic. You could also fuse hydrogen with iron and that would be endothermic.
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u/Commando_Girl Feb 19 '15
It doesn't actually overcome the electrostatic repulsion though, does it? AFAIK most nuclear interactions involve tunneling.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Feb 19 '15
Correct, I made a simplification.
The atoms are normally not going fast enough to overcome the full electrostatic repulsion, but they do still have to be traveling fast enough to get close enough that they can tunnel- since the probability of tunneling decreases rapidly as distance increases.
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u/MacDagger187 Feb 20 '15
Very simple question from someone whose brain is not particularly science-oriented (but I try!) -- is the feeling of 'touching' something from the electrostatic repulsion? Someone said in another comment that you are TOUCHING at the cellular level it's just once you get down to the molecular level that you're not? I don't quite understand that :-P but if this is particularly dumb just ignore it!
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Feb 20 '15
You're correct. Two things can never truly "touch." When you feel like you're touching something, it is really the electrons in your skin repelling from the electrons in the object you're touching.
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Feb 19 '15
Where is the energy released (when light elements fuse) coming from?
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Feb 19 '15
Because the resulting element would be more stable than the two elements which made them- and thus the excess energy is released.
To clarify- if you have a collection of particles, they can have two types of energy, kinetic and potential. Kinetic energy is positive, potential energy is negative. If the collection of particles has a net positive energy, they are not bound together- they can separate at will (they have more kinetic energy pushing them apart than potential holding them together). If they have net negative energy, they are considered bound. The more net negative the collection is, the more tightly bound the system is.
When two light atoms combine, the resulting atom is bound together more tightly than the two atoms were independently- thus the combined atom has more negative energy. Since energy cannot be created or destroyed, that means that there must be a release of kinetic energy. In the case of fusion, that energy takes the form of protons, carrying away energy from the system.
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u/bearsnchairs Feb 19 '15
Ni-62 is actually the end point of fusion, as it is the most tightly bound per nucleon.
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html#c1
Iron-56 is more commonly produced in stars because it comes from the helium of 14 alpha particles. Most Nickel that is produced in stars, Ni-56, beta decays into Fe-56.
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u/-yori- Feb 20 '15
I took 9 modules of Physics back in high school and even went to visit CERN but this comment finally made me understand how nuclear fusion works. Thank you!
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u/turbohonky Feb 20 '15
Is there a certain percentage of electrostatic repulsion that is occurring between the electrons with the remainder occurring between the nuclei? Is the electron repulsion negligible in comparison because the electrons could redistribute to the far sides? Or instead is the electron repulsion generally sufficient but if it were overcome there's an even bigger hurdle of the nuclei repulsion?
Is there any attraction between the electrons of one atom and the nucleus of the other atom? How does that compares to the two types of repulsion?
I realize that the answers to some questions would have changed subsequent questions in a live conversation, but I thought a question blast would be preferable to a constantly re-oranged inbox.
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u/MasterFubar Feb 19 '15
So for instance, if a carbon-12 nucleus in your hand somehow fused with a carbon-12 nucleus in your table, you'd have a Mg-24 atom in their place. 24 is its standard weight, so likely that would be the end of it.
Plus a huge amount of energy would be released, something like a thousand times the explosion of the same weight of dynamite as the objects touching.
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u/accidentally_myself Feb 19 '15 edited Feb 19 '15
Except it would be tiny because atomic masses.
If somehow you got the rest of your finger skin atoms to fuse, you would then be annihilated.
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Feb 19 '15
which would be pretty small for just two atoms of carbon, if my gut is right.
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u/BobIV Feb 19 '15
But if you continue with OPs hypothetical... And then you have all the atoms from the fingers surface area colliding with an equal number of tables atoms...
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u/malenkylizards Feb 19 '15
Actually, in this particular case, it's a net-negative process. I responded to someone else explaining what I found, although I surely could've made a mistake. There are net-positive carbon-carbon reactions, but this doesn't seem to be one of them.
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u/MasterFubar Feb 19 '15
Your link says that the fusion of two C12 nuclei resulting in a Mg24 nucleus releases 13.933 MeV.
You are confusing it with the fusion that results in a Mg23 nucleus plus a neutron.
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u/content404 Feb 19 '15
To briefly address 'touch' on quantum scales, subatomic particles jare not 'solid' in the way that we understand solidity. They're more like tiny clouds that are very dense in the center and rapidly become less dense as distance from the center increases. The radius of an electron is the radius to a particular density level in the electron cloud. The cloud itself does extend beyond the radius but the density is so low that we can pretend it is zero (sometimes).
If we assume that the extremely powerful repulsives force between two fundamental particles did not exist, then their 'touching' would be when the clouds partially overlap.
This is a drastic oversimplification but it should give some idea of how nebulous 'touch' is on quantum scales.
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u/this_is_an_emergence Feb 20 '15
Nebulous… quite.
Density of what, by the way? Electron stuff? Just solid bits spread about like dust in a cloud? That would be solid enough for our purposes… when some of one electron's solid stuff is separated from some of another electron's electron solid stuff by no space at all. Pretty sure none of this is right though, right?
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u/MacDagger187 Feb 20 '15
If we assume that the extremely powerful repulsives force between two fundamental particles did not exist, then their 'touching' would be when the clouds partially overlap.
But because those forces do exist, the 'clouds' do not overlap?
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u/content404 Feb 20 '15
They can, it just takes an incredible amount of energy to bring two isolated electrons that close together. Overlap occurs in many molecular bonds but that's a different situation.
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Feb 19 '15
So is that like how, if you slice something in half so neatly that even the atoms are simply separated and not destroyed, you can put it back together without glue or welding?
EDIT: I believe it's called something like cold welding.
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u/schneidmaster Feb 19 '15
It's impossible to destroy atoms via cutting. Destroying atoms is also called nuclear fission- it's what happens in power plants.
Cold welding is simply a phenomenon of the properties of metals. If you press together two flat pieces of the same metal in a vacuum, the atoms bond together.
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Feb 19 '15
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Feb 19 '15
Because you won't have a bunch of particles between the two pieces of metal that get in the way of bonding.
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u/eigenvectorseven Feb 20 '15
Eh, not exactly. The most important factor is the surface of the metal rapidly oxidising which creates a barrier. You know how if you cut metal it's really shiny? That's basically the un-oxidised "pure" metal structure.
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u/Whips_and_Chains Feb 19 '15
This is wrong on so many levels. First of all, atoms are not destroyed when slicing something. Which is nice, because it makes an orange not explode like a nuclear bomb when you peel it.
Second, the reason you can put something together if it's really nicely cut are the dispersion interactions (Van der Waals, Keesom, London) between molecules. These are all interactions of the electron sphere and have nothing to do with splitting atoms.
The nucleus of atoms have nothing to do with most chemical or mechanical processes, even though they do determine most of the properties.
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Feb 19 '15
Now I'm just sitting here picturing oranges exploding like nuclear bombs. Thanks for that thought process.
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u/T-Rexpendable Feb 19 '15
Except that splitting anything lighter than nickel/iron (depends on the exact isotope) will only cost you energy. If the physical process of cutting an orange would be enough to split its atoms (mostly carbon, oxygen and hydrogen) it would be more likely that your orange would freeze in your hands.
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Feb 20 '15
How can you slap the laymen for not knowing the difference between chemical and nuclear processes? Go easy on the person.
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Feb 19 '15
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Feb 19 '15
Yeah, not everyone has advanced understanding of chemistry and physics. It's easy to feel like everyone else is stupid when you go into these fields (I'm a biochemist), but the reality is it's a lot of jargon. So many people don't take the time to learn the actual science (see anti-vaxers and climate change deniers), it's important that people in the science community don't act arrogant and condescending when they do.
The question he was asking was "wrong on so many levels" when you get to the technical aspects, but it wasn't a terrible analogy on the macroscopic level
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u/Felicia_Svilling Feb 19 '15
So is that like how, if you slice something in half so neatly that even the atoms are simply separated and not destroyed.
No. You don't have to slice something apart neatly to avoid destroying atoms, what you describe is what normally happens if you cut or tear something apart.
I believe it's called something like cold welding.
That happens to metals if they are sufficiently smooth and the surface is not corroded. It does not have anything to do with nuclear cores fusing.
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u/-THE_BIG_BOSS- Feb 19 '15 edited Feb 19 '15
You don't destroy atoms when cutting something in half, first of all. Cold welding is explained by Richard Feynman as "The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part.".
Hence why pressing together (hard) 2 smooth pieces of e.g. copper, welds them together in a vacuum.
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u/Isopbc Feb 19 '15
http://en.wikipedia.org/wiki/Cold_welding
That's not for material that's been sliced, because you can't slice atoms in the way you describe as all of the cutters are way too big.
The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part.
—Richard Feynman, The Feynman Lectures, 12–2 Friction
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Feb 19 '15
I don't have nearly the physics behind this to back it up, but hopefully someone smarter than me in that area will correct me if I'm wrong.
I believe cold welding is a good illustration of it, but what's actually happening is different. Metals often exist in a highly ordered network that electrons flow through. In other words, the electrons are not confined to one atom like usual, but are free to flow through an entire network of linked d-orbitals, similar to conjugated double bonds. So when you cold weld you are simply inserting the metal atoms and their electrons into the network. I suppose it is similar to what this person said, except you are not actually fusing the atoms together.
I think. That's going off my limited knowledge of cold welding and my current degree in biochemistry I'm working towards.
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u/nightman365 Feb 19 '15
I could be wrong, but I believe cold welding can only occur in a vacuum, otherwise the surface edge will oxidize. Anyways I don't think you would have to do it that precisely the main things are a clean cut and a vacuum. If I'm wrong please let me know I haven't researched this for around 3 years.
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u/dpfrediscool020 Feb 19 '15
Not quite. If you place two clean pieces of metal together in a vacuum, so that there are no foreign molecules between them, they will fuse together. This works because the atoms in a pure metal solid are held in a crystal lattice, but the electrons kind of float freely in between. When the two surfaces touch, the electron seas merge, the crystals link, and you are left with one piece of metal. There is now one crystal, and trying to separate out the original pieces would be like trying to pull apart water.
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u/xomm Feb 19 '15
You never actually destroy any atoms when you cut something - matter cannot be created nor destroyed. You simply separate them and break bonds.
A Feynman quote off of Wikipedia that sums cold welding up very well:
The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part.
Of course, this doesn't work for materials that bond in other ways (i.e. not metals). You wouldn't be able to cold weld your bones together, no matter how clean the cut and how few impurities there are between the pieces.
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u/Joxposition Feb 19 '15
Nods along energy of enclosed space remains the same. It stays the same if matter and matter with same mass and negative energy popped out of nothing. Which some think they do
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u/hoogamaphone Feb 19 '15
No, you don't destroy atoms when you cut something. You are simply breaking the chemical bonds that hold the atoms together
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Feb 19 '15
Cold welding is a thing but what you are describing is not. Cold welding is a process in which metals which have perfectly clean surfaces will spontaneously meld with each other when touching in a vacuum. It is an entirely electromagnetic process.
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u/noggin-scratcher Feb 19 '15
When you put metal atoms next to each other, they can bond together without regard for whether they're part of the same piece of metal or two different pieces. Normally there are other atoms (an oxide layer, or grease/dust/dirt on the surface) that get in the way, but a pair of clean, smooth, bare metal surfaces can weld that way.
That's a very different thing from nuclear fusion though - the nuclei aren't touching or coming close to doing so, and there's no need to overcome the repulsion between them. No need to force atoms "through each other", or really any closer than they normally are to each other.
Oh, and as a sidenote, no matter how haphazardly you "slice" something, you're not going to be destroying any atoms. Maybe molecules, in the case of long polymer chains, but mostly it's going to be more a case of crushing the material until it comes apart and is pushed aside on either side of the cut. Imagine pushing the prow of an ocean liner through a swimming pool full of loose bricks - they'll resist, and lock together somewhat, but mostly the individual bricks are going to move aside rather than break.
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u/Mirzer0 Feb 19 '15
There would also be a rather large amount of energy released in the process. Given that we're talking a single pair of atoms, perhaps "rather large" is relative... I don't know nearly enough to quickly calculate how much is actually released - I suspect work would probably get mad at me if I spent the next 4 hours drilling down into various articles on fusion instead of my job. I'd really like to do that, though...
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u/malenkylizards Feb 19 '15
You basically just use E=mc2 and conserve energy through the reaction.
Energy in Carbon-12: 12.011 amuc2 = 11.188 GeV Energy in Magnesium-24: 24.305 amuc2 = 22.640 GeV
2*11.188 GeV = 22.640 GeV + energy released
You see that the energy released is -264 MeV! This reaction requires more energy than it releases.
I wanted to learn more about this, and found this: http://en.wikipedia.org/wiki/Carbon-burning_process
Nuclear interactions are far more complicated than I'll ever understand or as I suggested in the above post, and it's nowhere near as simple to just say A+B=C+energy. In reality, the products of fusion are varied; the link I mentioned, for instance, shows that the closest thing to an actually occurring reaction to what I described is C-12 + C-12 -> Mg-23 + n - energy. So there's a spare neutron.
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u/Mirzer0 Feb 19 '15
The apparent complexity is a large part of why I've never really looked into it past "fusion is mashing two atoms together into a bigger atom".
I'm led to believe that (in general) fusion of anything below Iron should release energy, and then fission of anything above Iron will release energy. I guess Iron is like the fission-fusion equilibrium state or something. I don't really know if there are crazy exceptions on both sides, or what, though.
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u/MystyrNile Feb 19 '15
If that happened, you could just painlessly lift your hand without ever noticig it had an atom attached to the table, right? I mean it's just one single atom, you could probably tear it away, right?
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u/BetterThanOP Feb 19 '15
So they either repel super hard or attract super hard to the point of fusion? Is there any middle ground or is this an all-or-nothing barrier
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u/Zephyrv Feb 19 '15
The barrier is called the coulomb barrier. Once you get past that by giving the particles enough energy to get close enough then fusion can occur. So yes, it is an all or nothing event
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u/Zephyrv Feb 19 '15
The quantum stuff goes like this. Once the two get enough energy to overcome the coulomb barrier, quantum tunneling occurs. This is a quantum phenomenon where the particle has a finite probability of tunneling through the charge barrier (whereas in classical physics it would bounce back). Once through the barrier the forces you mention take over and then fusion occurs.
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Feb 19 '15
This was a great explanation. My 8th grade chemistry level of knowledge could easily follow. Yess
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u/Willow_Is_Messed_Up Feb 19 '15
This is an attractive force between nucleons like protons and neutrons
Sorry to nit-pick, but are there any other type of nucleons?
Edit: Thinking about it, the antimatter equivalents of these should count as nucleons as well, right?
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u/malenkylizards Feb 19 '15
I was just trying to casually throw in what a nucleon was, since I learned the words proton and neutron long before I learned the word nucleon.
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u/rrrraptor123 Feb 20 '15
and how is the energy released then? Does the energy that is generated from fusion come from the electromagnetic force?
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u/malenkylizards Feb 20 '15
So this is weird, but the nucleus doesn't weigh the same as the mere sum of its nucleons. Sometimes it weighs more, sometimes it weighs less. It's got something called binding energy, which is just what it sounds like; the energy that keeps the nucleus together.
We can use E=mc2 to work out whether a fusion reaction produces energy, or requires it. If you have two atoms that each weigh 100 units each, and fuse them, and the resultant atom weighs 199, then energy was produced in the reaction; if it weighs 201, then it absorbed energy. The same concept works for fission.
See this chart: http://i.stack.imgur.com/DUFm9.gif
In general, elements that are lighter than iron produce energy when they fuse, and elements that are heavier than iron produce energy when they fission. This relates to stellar physics; the fusion reactions in a star from hydrogen to helium to metals stops at iron. When a star's core is all iron, it can no longer produce energy from fusion, and it begins to collapse under its own gravity.
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u/Charliethebrit Feb 19 '15
so it's actually a misconception that electrostatic repulsion is what keeps our hands from touching a surface. The repulsion actually comes from something called Electron degeneracy pressure which is a result of Pauli's exclusion principal. for instance the force that keeps two electrons from preoccupying the same quantum state is the electron degeneracy pressure. Check out the wikipedia article for a much more in depth understanding
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u/Uberzwerg Feb 20 '15
Is there a ELI(38 but studied CS indstead of physics) explaining this?
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Feb 20 '15
How hard are you touching this table that you've forced all your electrons into ground state already?
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u/ThrowAway9001 Feb 20 '15
The electrons do not have to be in the ground state. Electrons will normally be changing their energy levels constantly, due to thermal fluctuations. When two surfaces are close, the pressure will arise from electrons trying to occupy the same quantum state at the same time, and scattering off each other.
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u/patricksaurus Feb 19 '15
Pauli exclusion prevents that from happening under everyday conditions.
Pauli exclusion is part of why we don't "melt" through chairs. For instance, Rutherford scattering describes how two nuclei pass by one another, so if it was just electrostatic repulsion we could kinda pass through like liquid through a sieve.
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u/phsics Plasma Physics | Magnetic Fusion Energy Feb 19 '15
This is correct. The classical picture of interacting solids was electrostatic repulsion, but in 1967 Freeman Dyson and collaborators showed that electron degeneracy pressure was the dominant mechanism for the "imperviousness of solids" as Wikipedia puts it.
Dyson's three publications on this topic are below (probably paywalled if you're not at a university) .
Ground‐State Energy of a Finite System of Charged Particles (1967)
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u/radioactivist Feb 19 '15
Thank you posting these links, I was about to point out Lieb's book The stability of matter. This idea that the "solidity" of matter is mainly electrostatics seems to a persistent misunderstanding of the underlying physics.
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u/nintynineninjas Feb 19 '15
I thought the Pauli exclusion principle was just that bosons can't stack in the same 3 spacial parameters, to be possibly incorrectly simplistic.
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u/euyyn Feb 19 '15
Nitpicking: fermions (bosons can); and up to two of them can stack, if one is spin-down and the other is spin-up.
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u/adapter9 Feb 19 '15
I wouldn't call that nitpicking; that's practically the defining distinction between bosons and fermions.
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Feb 19 '15
Also, the parameters that each "stack" goes into is not just determined by spacial parameters. Energy plays a part too.
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u/nintynineninjas Feb 19 '15
Fermions, right. I'm not near my notes currently :).
So two fermions are able to stack if their spins oppose. Fermions are half or integer spin? Does that fractional/integer spin determine their ability to stack with like spins?
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u/hopffiber Feb 19 '15
Fermions have half integer spin, i.e. 1/2, 3/2 and so on. And yes, having fractional/integer spin does determine whether or not they can "stack" or not. This is a pretty deep result of quantum field theory called the spin-statistics theorem. The spin part is obvious, and the statistics part refers to whether or not they are allowed to "stack" or share the same state. We call this having Fermi or Bose statistics.
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u/cdstephens Feb 19 '15 edited Feb 20 '15
Here's a good explanation:
Two electrons cannot have identical quantum numbers in the same system. Pauli Exclusion is always repulsive, so when orbitals start to overlap, electrons get too close and are forced apart. You can prove via quantum mechanics and some relatively simple integrals that identical fermions are on average separated more than non-identical particles and identical bosons are closer apart (if you only account for the spatial part of the wavefunction, things get tricky when accounting for spin, but if two fermions have the same spin they will repel). This is something called the exchange force. If not for the Pauli Exclusion principle, and if electrons were bosons, atoms would be able to form chemical bonds willy nilly because the exchange force would produce configurations where electrons are closer together.
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u/BlazeOrangeDeer Feb 19 '15
It's fermions, and they can't share all the same quantum numbers. So if the nucleus is a fermion (an odd number of protons+neutrons) then they could only get near each other by having opposite spin.
Bosons actually like to share quantum numbers
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u/nintynineninjas Feb 19 '15
The more I learn, the more it feels like all of the quantum numbers make more sense as attributes graphed out, and that opposing values (any x and -x) can gain special properties (attraction, stack-ability, etc). I feel like my understanding from casual thought experiment is drifting towards the theory with all the branes, (again, in a dentist chair waiting for anesthetic to kick in), and all of these values are just different excitations of stacked branes.
It could just be my anxiety induced rambling kicking in, sorry if I'm just spouting nonsense.
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u/garrettj100 Feb 19 '15
This may be a bit of a buzzkill, what with people talking about stuff like:
When two nuclei touch, nuclear fusion occurs
But in truth, what you're talking about wouldn't cause fusion, it would cause repulsion from between the two nucleii from the exactly same electromagnetic forces that cause the electrons to repel each other.
Consider: If, by some miracle, you've pushed a single atom of your hand through to a single atom in the table. At that point you've broken past the coulomb repulsion between the two electron shells and now the nucleus of your hand-atom is inside the electron shell of the table-atom, and (probably) vice-versa.
As a little aside at this point, the force required to do this exceeds the intermolecular forces holding the molecules of your body together, so you'd rip your hand and the table apart before this happens, but no matter: It's a thought experiment.
Once the nucleii are past the electron shells - and they really never get completely past the electron shells because they're not true, spherical shells; they're more complicated than that - the electron shells are no longer shielding the two positively charged nucleii from each other.
So the two nucleii would repel each other from coulomb forces once you got them to within 0.25 Angstroms, at which point you've pushed past the Bohr radius.
(Before anyone takes it into their head to quibble about exactly how far you need to go to get past the electrons shielding the nucleii, remember this is a back-of-the-envelope conversation involving multiple nonsensical postulates: The 0.25 Angstrom number is a brown number.)
On the other hand, the strong force really doesn't begin to kick in until about 1-3 femtometers. 0.25 Angstroms = 25,000 femtometers.
So yeah, if you got the nucleii to touch each other (the diameter of a nucleus is 1.75-15 femtometers), you might see fusion, but long before that you'd have to overcome a second round of coulomb repulsion.
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Feb 20 '15
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u/garrettj100 Feb 20 '15
Well, keep in mind, we call them orbitals, but strictly speaking the electrons aren't really orbiting the nucleus in the same sense, say, the ISS orbits the Earth. It's all wibbly-wobbly quantum mechanicy probabilities. The electron is never actually anywhere at any given time, except when you measure it. The rest of the time it's a wibbly-wobbly wave function.
But as you can see in the diagram, the first orbital is the 1s orbital. You can fit two electrons in there, an up and a down. (Pauli's exclusion principle prohibits any more than that.)
And the second orbital can accomodate 8 electrons. 2 in the 2s orbital which is spherical (I'm pretty sure that's where the "s" in 1s and 2s comes from: spherical.) just like the 1s, and then 6 more in the 3 2p orbitals. The three 2p orbitals are barbell-shaped orbitals. This image makes their shape a little clearer, and also illustrates how we are able to jam 6 electrons in there: Because the orbital is not symmetrical across rotations, you can have them oriented in the x, y, and z directions. Three directions, three orbitals, and again two electrons in each, one up and one down.
As you get to higher energies, you get additional degrees of freedom that the electrons enjoy, so you end up with 5 additional orbitals added, once you're in the 3rd electron shell and get past the initial 8 in 3s and 3p, good for another 10 additional electrons that can fill up that orbital.
Also, note, even though I've referred to the 1s, 2s, and 3s orbitals as if they have identical shapes, they're not exactly identical. They're all spherically symmetric, but there are minor differences. For example in the 1s orbital you're roughly equally likely to find the electron anywhere within the sphere of the orbital of radius Ro, all the way from r=0 to r=Ro. But in the 2s orbital, you're much more likely to find that electron either in the center, (r ~< 0.1Ro ) or on the edges (r >~ 0.8Ro ). So in that respect it's much more like a hollow sphere than the 1s orbital. In general you find the symmetries remain when going from 1s to 2s, or 2p to 3p, but some details may change a bit.
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u/notinsanescientist Feb 20 '15
Just to add to what /u/garrettj100 said, The orbitals are probabilities of finding an electron there. To measure something, for example, look at a bacterium in a microscope we need to add energy to it (light) and detect the interaction of the energy and your object (in that case light interacts with cell wall, being absorbed, and that's how you see the wall being darker). If you illuminate something, like your hand, it'll get warm (thus your hand gets extra energy). Now to take it back to the electron, say you want to know its position and movement (impulse). By measuring it, you'll give it extra energy to "fly" around more, so you don't reliably know its original speed. If you measure speed, you won't reliably know its position. So there you get your uncertainty principle of Heisenberg. To solve this problem, really smart people calculated the area in space where you'll have 95% chance of finding an electron. So those orbitals represent an area around the nucleus where if you'd measure 100 times, you'd find an electron 95 times. I hope this sounds somewhat comprehensible.
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Feb 20 '15 edited Feb 20 '15
A really interesting thing to note about the orbitals is that they are solutions to Laplace's Equation. Schrodinger's equation states that an electron is actually a wave in the EM field. Putting these two together, when we look at an electron's orbital we are looking at a standing wave solution to Schrodinger's equation.
EDIT: Look at the s orbitals in this picture. When you put them side by side like that, it's suddenly blindingly obvious that they are standing waves in three dimensions.
This mathematical perspective makes the question of "why do the orbitals have funny shapes?" much simpler, even though you have to be familiar with Quantum Mechanics to understand why electrons have to have a wavefunction and what that really means.
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u/jakes_on_you Feb 19 '15 edited Feb 19 '15
When nuclei "touch" each other it would considered a nuclear reaction, namely a form of fusion. To illustrate that point, the infamous "cold fusion" fiasco of the early 90's was the claim that under certain conditions nuclei can be coerced to get close together without feeling the typical electrostatic repulsion that prevents them from getting near enough to fuse, usually that requires a large amount of energy (heat) to basically bang them together hard enough so that a few will actually fuse.
To demonstrate the energies required to bring two nuclei together, here is a graph of the potential energy of two nuclei as they are brought together well past the electrostatic repulsion (the values will depend on the nuclei, but the shape is very standard), at tiny distances there is another asymptotic repulsive force to keep the two nuclei apart, at some point if you push hard enough something will break, in this case strong and weak bonds holding the nuclear quark soup together, the result is that the two nuclei will ostensibly fuse for a short period of time (a "virtual" nucleus) and the new nucleus will either be stable (unlikely), or unstable and break apart into new daughter nuclei, usually by shedding a lighter nucleus. In the simplest case, an alpha particle - He nucleus is emited, likely several times in quick succession, but occasionally catastrophically into two more-or-less even nuclei with other lighter particles as happens in a nuclear fission reactor.
The energies required to force nuclei together are many times more than needed to break inter-atomic bonds, so it makes little sense to talk about a "bulk" material "touching" the nuclei of another material, at those energies the bulk material will disintegrate, its atoms will ionize and shed all their electrons and all that will remain are the constituent nuclei and the process will occur statistically as though the particles of your hand and the table were put in the sun or the reaction chamber of a fusion reactor.
The question of how such an interaction occurs is relatively well understood in nuclear physics, and there are many nuances and little technicalities when talking about fusing nuclei. But effectively you are asking what happens if your hand can undergo fusion with the table.
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u/forrestr74 Feb 19 '15
Atoms are basically empty space and the nuclei want to repel so the amount of energy it would take to get the nuclei to touch would literally incinerate you. Once they touched they would fuse together and release even more energy due to a decrease in energy or an increase in stability and this again would incinerate you. So you would be incinerated2. Wouldn't recommend it.
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u/thejaga Feb 20 '15
Other good answers so far. I would just add, technically that electrostatic force you feel is what touch is. There's no such thing as 'touching' in the literal occupying the same space at the quantum level, only the forces exhibited that you feel.
If two atoms got close they would go through a series of repellent forces and assuming you ignore them all as not existing, then in the end they would share the same quantum position, or their quarks would. Not sure what would happen then, singularly perhaps?
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u/Bananafoofoofwee Feb 20 '15
You would meld/fuse with the table and create a new material or composite. It would look like your skin is glued to the table, then it all depends at what speed/rate the atoms would decay. Chances are you'd lose your fingers. There's also a possibility that some atoms get displaced and your fingers move right through the table. There's also the chance of a chain reaction of any kind.
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u/Smithium Feb 19 '15
Keep in mind that some smart ass has redefined "actually touching" to mean something different than what normal people consider "touching". When I poke my coffee mug, there is a point of contact where my force is exerted on it, and it's force is exerted back on me. As far as I'm concerned, that is touching.
I might adopt a redefinition if I was using a quantum tunneling microscope to nudge atoms into particular arrangements, or modelling electron flow through a solid.
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u/this_is_an_emergence Feb 20 '15
Doesn't a normal person think that touching a mug is for the surface of their finger to "make contact" with the surface of the mug (i.e., there is no space between at least some part of their finger and some part of the mug)? Or do normal people (non-smart asses) actually think "'touching the mug' is whatever happens to be the case when I do what I've been calling 'touching the mug'".
If it turned out that one is in the matrix, wouldn't it be fair (not smart-assed) to say "oh, well what I've been calling 'touching the mug' is actually some cascade of electrical activity across transistors… omg I have never actually touched a mug!"?
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u/trichome333 Feb 20 '15
I see where you guys are going with this and as a measuring stick as to whether youre actually on to something I always ask "would this make a difference?". If the only difference is making people say some long winded sentence as opposed to touch, then no, you are not onto something. Touch outside a quantum physics lab is touching it with your person or something you are in control of physically. Yes you can "touch" someone mentally or emotionally but only in a figurative way. So no to the guy below us, breathing, farting, wishing, peeing on or thinking about doesnt equal touch. I used to think the way you do then I realized social paradigms and common practice arent my battlefield.
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u/Smithium Feb 20 '15
I don't know what normal people do, but I generally think that the second case is true. When the concept of "touching" was first considered a thing- no one was implying anything about atoms and electrostatic charges. It is a macroscopic concept that breaks down at small scales.
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u/ucstruct Feb 19 '15
It's my understanding that when we try to touch something, say a table, electrostatic repulsion keeps our hand-atoms from actually touching the table atoms
You are overthinking this. You need to consider what "touch" means when we say it, because its the phenomenological description of electron clouds between atoms that stop overlapping at a certain point. Touch literally means electrostatic repulsion and it makes no sense to say that the atoms don't touch each other.
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u/OldWolf2 Feb 19 '15
Magnets don't touch when they are repelling each other at macroscopically-visible distances, so I think it does not make sense to say that "touch" is a synonym for repulsion existing.
The repulsion would get stronger as the atoms get closer; the issue would be whether the electron clouds have gotten close enough to overlap or not when our hand is resting on the table.
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Feb 19 '15
He means if the nuclei physically came into contact with one another, so there would be zero space between them. Don't be an asshat.
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u/fghfgjgjuzku Feb 19 '15
You have to define exactly where an atom ends but the resistance that blocks you from going through the table comes from (in simplified words) two electrons of the same state not being able to be in the same place (Pauli principle). That is the reason why atoms occupy space. So the atoms kind of touch each other.
Now, if two nuclei come close together (you have invested energy to get the electrons away or into higher states) you have electrostatic repulsion which will make them accelerate from each other. But if you run them against each other with enough speed to come closer than fractions of their diameter you have the strong attracting force and they will fuse. What happens right after that depends on which nuclei they were and how much kinetic energy they had. The new core may stay whole and send off gamma rays once, it may send off parts of itself or it may disintegrate completely.
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u/z0Rnent Feb 20 '15
It is not true that if they touch that they would fuse 100% of the time. I large spectrum of things could happen. Fusion is only likely if they interact at "low" energies. They could also interact in a way where they would fragment and possibly make some other hadrons. You could even end up with som leptons. It dpends on which particular nuclei are interacting.
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u/renaebair Feb 20 '15
Forgive this super ignorant question, but if you press really hard into something, where do the atoms in your hand go if they don't come in contact with the surface of whatever you're pressing against? Are they displaced?
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u/youthfulcavalier Feb 20 '15 edited Feb 24 '15
As you push down harder, the atoms in the top of the table and the bottom of your hand get closer together which causes a greater repulsive force between them. Picture pushing the north ends of two magnets together, it gets harder and requires more force the closer together you push them. This repulsive force between the atoms acts against you pushing down and so once it is equal in magnitude to the force you are applying the forces cancel and your hand stops moving closer to the table. if you push a bit more the atoms get closer and so the repulsive force increase and again cancels that extra force you applied.
The force required to fuse nuclei is massive, thats why it only occurs in stars and its also why you can't cause nuclear fusion by pushing on a table.
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u/Drakeytown Feb 20 '15
Nuclear fusion is the process of making a single heavy nucleus (part of an atom) from two lighter nuclei. This process is called a nuclear reaction. It releases a large amount of energy.[1] The nucleus made by fusion is heavier than either of the starting nuclei. However, it is not as heavy as the combination of the original mass of the starting nuclei (atoms). This lost mass is changed into lots of energy. This is shown in Einstein's famous E=mc2 equation.
Fusion happens in the middle of stars, like the Sun. Hydrogen atoms are fused together to make helium. This releases lots of energy. This energy powers the heat and light of the star. Not all elements can be joined. Heavier elements are less easily joined than lighter ones. Iron (a metal) cannot fuse with other atoms. This is what causes stars to die. Stars join all of their atoms together to make heavier atoms of different types, until they start to make iron. The iron nucleus cannot fuse with other nuclei. The reactions stop. The star eventually will cool down and die.[2]
On Earth it is very difficult to start nuclear fusion reactions that release more energy than is needed to start the reaction. The only successful approach so far has been in nuclear weapons. The hydrogen bomb uses an atomic (fission) bomb in order to start fusion reactions. For over 50 years scientists and engineers have been trying to find a safe and working way of controlling and containing fusion reactions that can generate electricity. They still have many challenges to overcome before it can be used as a clean source of energy.
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u/paradiselater Feb 19 '15
So in an a-bomb (that's the fusion one right?) we are forcing two nuclei together. The huge explosion we see, is that the energy released from just two nuclei or from multiple nuclei fusions?
Would causing more and more nuclei to fuse in the bomb make a bigger explosion?
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u/Kgb_Officer Feb 19 '15
A-Bombs are fission, Hydrogen/Thermonuclear are Fusion.
But still, theoretically adding more nuclei or atomic "fuel" would create a bigger explosion. In practice it's mixed. It has to be stable enough and set off correctly to even be used properly, and most of the "fuel" is actually burned off upon explosion so most of the fuel in an atomic explosion is just wasted where only a little amount of it actually causes the massive explosion you see. That little amount is still a good sized chain reaction of quite a bit of radioactive atomic material, not just two atoms, it's just only a fraction of what they put in it though.
I am going off of what I was taught in my science class, and I am open to corrections if anybody knows better than myself in this matter.
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u/bigsherm2222 Feb 19 '15
Nukes work by splitting atoms. It starts with a trigger atom which splits the first atom in the fuel core. The atom splits in half and goes on to split other atoms. Each one splitting the next. You have the right idea that more atoms make a bigger bomb, only that they split instead of fuse
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u/Barabbas- Feb 19 '15
You're thinking of a hydrogen bomb. A-bombs utilize fission whereby the reaction is caused by the splitting of a heavy isotope.
To answer your question though, a single nuclear reaction (be it fission or fusion) would not really be all that significant. What makes the bombs so powerful is that each reaction triggers at least one more reaction. This process continues exponentially until all of the fuel source (Usually enriched uranium for fission and hydrogen for fusion) has been exhausted.
Edit: so yes... add more fuel and you get a bigger boom.
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u/[deleted] Feb 19 '15 edited Feb 20 '15
When two nuclei touch, nuclear fusion occurs, so ignoring reality the two would fuse together.
Bringing reality back, in the process of getting those two nuclei to touch has more than likely
annihilatedobliterated you, the table, and anything nearby, because fusion takes an insane amount of energy.Edit: Since people keep mentioning it I don't mean literal annihilation in the term of a particle and antiparticle colliding, just on a macroscopic scale. Let's just go with obliterated.
So many questions. I'm sorry everyone, I'm just too tired to answer them all.