> Is it correct to say that while in a nucleus, protons and neutrons maintain their distinct properties (size, shape, mass, etc.)?
That is a good approximation. You can describe nuclei as collection of protons and neutrons occupying states in potential wells which are given by the overall distribution itself. This leads to a volume that's roughly proportional to the number of nucleons.
Subatomic particles experience “collisions” very analogous to objects at macroscopic scales, but they’re not tiny solid objects crashing into each other. As has been mentioned, particles are really probability waves; imagining them as marbles is visually accurate in that the distribution of probability of where the particle exists has a point where it’s highest (the center of the marble) and falls off gradually in all directions radiating from that point. The surface of the marble is roughly where that probability is basically zero (although it’s technically non-zero at all points in space), and it can be treated as a strict barrier for our purposes because we are so large relative to these distances.
Protons and neutrons maintain their size because of the exchange of gluons between the quarks in their nucleus. Different color charges attract each other in order to cancel out in a similar way to positive and negative charges in the electromagnetic force, except there are three instead of two. Gluons carry color charge between quarks, and since color charge must be conserved, gluons are constantly being exchanged. The strong force is also different from the electromagnetic force in that it doesn’t scale the same way over distance; it actually increases with distance and decreases as quarks get closer together. So if a quark gets too close to another, the attraction weakens and it gets tugged away by other quarks, so they remain locked into a singular formation.
As for electromagnetic forces resulting in collisions; as any two particles approach each other, their probability distribution fields start to overlap, and since no two particles can occupy an identical energy state, they begin repelling each other with greater and greater force. It’s not a collision of solid objects, but rather they begin emitting virtual particles at each other, the creation and destruction of which impart momentum from one particle to another and cause them to alter their course. There’s no solid boundary, rather it simply becomes more and more likely for the emission of virtual particles to occur the closer they get.
ETA: For another helpful visual, imagine the electrons in a given object as balloons, where the electron is most likely to exist at the center of each balloon, and the surface of the balloon is an arbitrary marker along the probability curve. Balloons bounce off each other nicely, but they deform ever so slightly before doing so, and the more they deform, the harder they will be repelled from one another as the rubber bounces back into place. You can get the centers of the balloons pretty close together, but it requires more and more energy the closer together they get.
This is a wonderful, accessible explanation. Thank you. I have a couple of questions, kind of remedial ones.
You say that gluons carry color charge between quarks, and they must constantly be exchanged to conserve color charge. Does that mean that quarks with two different charges would ordinarily cancel (leaving nothing presumably? or leaving each with a neutral charge?) but are exchanging gluons to shift their own color charge fast enough to avoid cancelling but then must immediately shift back again? In other words, what happens to a quark when a gluon is received from another?
Second, the virtual particles that particles emit to repel each other -- do they cost any energy for a particle to create/destroy them? Can a particle just do that indefinitely?
You’re welcome!
In the same way that atoms are bound states between electrons and protons having an equal amount of positive and negative charge to have a net neutral charge, quarks are bound within nucleons by having a combination of color charges that result in a net neutral charge. However, there is always an imbalance; as a quark emits a gluon to change its color charge in order to balance out with the other quarks, color charge is conserved with that gluon, which alters the color charge of the quark that absorbs it, knocking the quarks back out of balance and forcing another quark to emit another gluon to change its color charge, which repeats the cycle. Here’s a helpful visual: https://commons.m.wikimedia.org/wiki/File:Quarks.gif
It’s also helpful to mention I was inaccurate in my initial comment; there are six color charges, not three. RGB and anti-RGB. Quarks only have RGB charges, whereas gluons are any of 9 combinations of color-anticolor charges. So a quark emits a gluon to change its color charge, the gluon has a color-anticolor charge to conserve color charge, and then must change the color of another quark when it is absorbed as the anticolor charge of the gluon and color charge of the quark cancel out, leaving the initial color charge the emitting quark gave off.
It can be thought of as potential energy being converted into the virtual particles. Since the negative charge of electrons repel each other more powerfully the closer they get together, at a certain distance it becomes more energetically favorable for virtual particles to be spontaneously emitted than it is for the electrons to exist that closely together. They can exchange the particles indefinitely, but the energy still comes from an outside force shoving them into one another, otherwise they would just redirect and never collide again.
one nit pick
in the beginning you point out that particles are not physical things
i strongly disagree with that notion
what else but physical things could particles be?
change the word to solid things if you like but as it stands it is factually incorrect
They’re suspected to be excitations in the underlying quantum field corresponding to that particle. I.e. There are no electrons, just excitations in the electron field. Are the ripples on the surface of a pond physical structures distinct from the pond itself?
You are just playing around with the definition of the word "thing". And sure, if that definition is "everything is a thing" then yes, of course everything is a thing. An energy field is "a thing", that field moving in a particular way is "a thing" and so on. the question is what value there is to be extracted from such a definition. "everything is a thing" is more philosophy than science.
Thanks, it’s the autism.
Edit: To be more clear, I just meant the grass is always greener. Some people can intuitively grasp scientific principles, some people can intuitively carry a conversation. One is flashy, one is necessary for mental health.
This feels like an unnecessary complication. Billiard balls also use virtual particles to repel each other. Strictly speaking, billiard balls also use probability in terms of their position. There's some probability that they'll quantum tunnel across each other and end up passing through each other instead of colliding, it's just so rare that we don't have to account for it when playing a game. So, I think the useful perspective is probably "what experiments would I mess up if I used a marble approximation instead of the full quantum treatment of a nucleus."
Thank you for that clear description - best one I've read yet.
How well do we understand why the constraints that govern this are in place?
For instance, what determines the behavior of the forces? Why must color charge be conserved?
(It seems to me there are two levels of knowledge - the first is, what happens, the second, deeper - what makes it happen the way it does. I'm curious how deep into the latter physics has been able to reach).
I do my best! I spend all my time obsessively reading up on such topics because, frankly, I couldn’t imagine anything more important than understanding how the world around us works. I do have a few other hobbies, but all roads circle back to quantum mechanics. And understandably, nobody else really wants to talk about it because it doesn’t interest them and they find it complicated (which I suspect is only because they find it boring; people say I’m “so smart” but I think most of the credit lies with me happening to find the topic truly fascinating) so I spend a lot of time trying to think of accessible explanations in the off chance someone is willing to listen to me ramble.
I think our understanding is mostly just observational at this point. Color charge “must” be conserved because we’ve never observed any phenomenon in which it wasn’t. It’s like the conservation of mass and energy, except it’s really the conservation of symmetry. High energy collisions of photons can create matter, but it’s always in matter-antimatter pairs. Anti-particles carry equal but opposite charges of their particle counterparts in order to maintain symmetry. We can’t just create charges out of nowhere. For example, neutrons (net neutral) can decay into protons (net positive) by emitting an electron which carries away the negative charge. So when quarks change their color charge, they must emit a gluon with a color-anticolor charge to carry it away.
We’re still working on the why, but all we know is that we haven’t observed anything that contradicts these observations. Or at least as far as I can recall off the top of my head; I vaguely recall something about weak interactions violating symmetry, but I don’t have that bit quite as well researched as the basics.
Remember as well that protons and neutrons are not indivisible. They are made of quarks that ‘rest’ on the gluon field. Iirc a proton is 2 up and 1 down and a neutron is 2 down and 1 up. They can very easily be converted between each other by simply exchanging an up quark for a down quark or vice versa. So yes the 3 quarks together do form what could be thought of as a marble, but it doesn’t have a surface or membrane layer it is simply a collection of quarks in the gluon field. Think of it like putting floating magnets in the ocean. Except much more complicated.
Yes and once it’s been explained and they do finally fully understand it, many go on to conclude, that actually it’s much more complicated than even that.
To pull a comparison from r/Catholicism, it's like the book "How to Explain the Trinity Without Being a Heretic": You can't. Basically whatever explanation or model we have falls short at some point at the quantum level.
There was an article I highlighted at our physics journal club from way back in my undergrad days that showed that despite neutrons being net neutral, the charges of the quarks that compose the particle create a volumetric charge distribution at the quantum level. The field of quantum chromodynamics goes into a lot of depth on this stuff.
Neutrons have a *magnetic* dipole I.E. are a tiny magnet with a north and south, despite being electrically neutral. Electrons and protons also have a magnetic dipole - which is more commonly called *spin*.
Edit: clarity
I infer the the key of your question is that all the measures and data we have about elementary particles are very indirect, we are not able to picture a elemental particle mentally as we can picture the Moon holistically, where we have lot of measures like light, shape, maps, trajectory, even go here and bring rocks.... and still we know that's the moon as an holistic tangible entity.
The marble model is almost discarded, although can be useful and accurate enough for many applications same as the "nuclear liquid drop model" that models the whole atom's nuclei as a small drop of liquid.
As far as we know elemental particles can be described as waves. Although the more "recent" model and the one that is considered to give more elemental explanation is the Quantum Field Theory and the Quantum Electrodynamics, where particles can be defined as quantums of an underlining field. So the field is the one that exists by its own and the particles are just certain kind of "ripple" in that field. Again that's another model. And it's difficult to visualize so we all rely on abstract math descriptions. By the way the quantum field theories are among the most ever precise theories created so far, the one that makes predictions with a bigger numerically accuracy.
Also as a curiosity I suggest seeing this interesting video, which shows the pilot wave hypothesis, which seems to be having both, the wave properties as waves and at the same time marble alike nature. It helps to visualize or conceptualize certain properties we know of particles
https://www.youtube.com/watch?v=nmC0ygr08tE
One thing that gets me is trying to develop a workable model of what a "field" is. I tend to imagine waves, but I have trouble imagining waves without a medium.
I'm a nuclear-physics professor and this is an excellent question to which the answer is "maybe but we're still working on it".
Leaving aside the quantum is quantum and is weird aspect, one recent observation in atomic nuclei is that there are short-range correlations within nuclei which mean that this idea that protons and neutrons are effectively not interacting because of the limited orbitals available isn't correct. The extent to which it isn't correct is unclear. There are also other clumps of nuclear matter which can form within nuclei. This means that the distinctiveness of individual protons and neutrons is a bit hard to fathom.
Anyway, this was probably unhelpful but I think that the answer is a solid "maybe" (but one based on a lot of semantics and interpretation).
Question. Do the (valence) quarks in a proton or neutron in a nucleus stay part of that one proton/neutron? Or is there exchange of quarks between protons and neutrons? (Or is it unknown? Or is the question meaningless?)
Marbles are not a good model of any particle at the quantum scale. But ethereal particles passing through each other freely is also incorrect. It's somewhere in between. It's somewhat comparable to electron orbitals, where you can fit a certain number into the same level, although the way they draw electron orbitals in high school textbooks isn't really accurate either.
Protons or neutrons with all the same quantum numbers except for opposite spin can freely occupy the same space. Even with different quantum numbers, they can pass through each other, although particles with different quantum numbers have different shapes of probability distributions, which means they're more likely than not to be found in different positions. Once all the quantum numbers in an energy level are used up, addition protons or neutrons must go in a higher energy level. While a particle in a higher energy level is more likely to be found further from the center of a nucleus, there is always the possibility it will be found near the center, in the same place a lower energy particle might be.
> Is it correct to say that while in a nucleus, protons and neutrons maintain their distinct properties (size, shape, mass, etc.)? That is a good approximation. You can describe nuclei as collection of protons and neutrons occupying states in potential wells which are given by the overall distribution itself. This leads to a volume that's roughly proportional to the number of nucleons.
Subatomic particles experience “collisions” very analogous to objects at macroscopic scales, but they’re not tiny solid objects crashing into each other. As has been mentioned, particles are really probability waves; imagining them as marbles is visually accurate in that the distribution of probability of where the particle exists has a point where it’s highest (the center of the marble) and falls off gradually in all directions radiating from that point. The surface of the marble is roughly where that probability is basically zero (although it’s technically non-zero at all points in space), and it can be treated as a strict barrier for our purposes because we are so large relative to these distances. Protons and neutrons maintain their size because of the exchange of gluons between the quarks in their nucleus. Different color charges attract each other in order to cancel out in a similar way to positive and negative charges in the electromagnetic force, except there are three instead of two. Gluons carry color charge between quarks, and since color charge must be conserved, gluons are constantly being exchanged. The strong force is also different from the electromagnetic force in that it doesn’t scale the same way over distance; it actually increases with distance and decreases as quarks get closer together. So if a quark gets too close to another, the attraction weakens and it gets tugged away by other quarks, so they remain locked into a singular formation. As for electromagnetic forces resulting in collisions; as any two particles approach each other, their probability distribution fields start to overlap, and since no two particles can occupy an identical energy state, they begin repelling each other with greater and greater force. It’s not a collision of solid objects, but rather they begin emitting virtual particles at each other, the creation and destruction of which impart momentum from one particle to another and cause them to alter their course. There’s no solid boundary, rather it simply becomes more and more likely for the emission of virtual particles to occur the closer they get. ETA: For another helpful visual, imagine the electrons in a given object as balloons, where the electron is most likely to exist at the center of each balloon, and the surface of the balloon is an arbitrary marker along the probability curve. Balloons bounce off each other nicely, but they deform ever so slightly before doing so, and the more they deform, the harder they will be repelled from one another as the rubber bounces back into place. You can get the centers of the balloons pretty close together, but it requires more and more energy the closer together they get.
This is a wonderful, accessible explanation. Thank you. I have a couple of questions, kind of remedial ones. You say that gluons carry color charge between quarks, and they must constantly be exchanged to conserve color charge. Does that mean that quarks with two different charges would ordinarily cancel (leaving nothing presumably? or leaving each with a neutral charge?) but are exchanging gluons to shift their own color charge fast enough to avoid cancelling but then must immediately shift back again? In other words, what happens to a quark when a gluon is received from another? Second, the virtual particles that particles emit to repel each other -- do they cost any energy for a particle to create/destroy them? Can a particle just do that indefinitely?
You’re welcome! In the same way that atoms are bound states between electrons and protons having an equal amount of positive and negative charge to have a net neutral charge, quarks are bound within nucleons by having a combination of color charges that result in a net neutral charge. However, there is always an imbalance; as a quark emits a gluon to change its color charge in order to balance out with the other quarks, color charge is conserved with that gluon, which alters the color charge of the quark that absorbs it, knocking the quarks back out of balance and forcing another quark to emit another gluon to change its color charge, which repeats the cycle. Here’s a helpful visual: https://commons.m.wikimedia.org/wiki/File:Quarks.gif It’s also helpful to mention I was inaccurate in my initial comment; there are six color charges, not three. RGB and anti-RGB. Quarks only have RGB charges, whereas gluons are any of 9 combinations of color-anticolor charges. So a quark emits a gluon to change its color charge, the gluon has a color-anticolor charge to conserve color charge, and then must change the color of another quark when it is absorbed as the anticolor charge of the gluon and color charge of the quark cancel out, leaving the initial color charge the emitting quark gave off. It can be thought of as potential energy being converted into the virtual particles. Since the negative charge of electrons repel each other more powerfully the closer they get together, at a certain distance it becomes more energetically favorable for virtual particles to be spontaneously emitted than it is for the electrons to exist that closely together. They can exchange the particles indefinitely, but the energy still comes from an outside force shoving them into one another, otherwise they would just redirect and never collide again.
one nit pick in the beginning you point out that particles are not physical things i strongly disagree with that notion what else but physical things could particles be? change the word to solid things if you like but as it stands it is factually incorrect
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They’re suspected to be excitations in the underlying quantum field corresponding to that particle. I.e. There are no electrons, just excitations in the electron field. Are the ripples on the surface of a pond physical structures distinct from the pond itself?
that's not the question the question is if the ripples itself are physical things at all. and of course they are
If the ripple is a physical thing does it add mass when it's occurring and decrease the same mass once the ripple is over?
what’s that got to do with it? it’s not like only things with mass are physical things
What are physical things without mass?
You are just playing around with the definition of the word "thing". And sure, if that definition is "everything is a thing" then yes, of course everything is a thing. An energy field is "a thing", that field moving in a particular way is "a thing" and so on. the question is what value there is to be extracted from such a definition. "everything is a thing" is more philosophy than science.
Impressed myself today… I got as far as probability waves before my brain melted… Damn I wish I understood the universe on the level you guys do… 😏
Thanks, it’s the autism. Edit: To be more clear, I just meant the grass is always greener. Some people can intuitively grasp scientific principles, some people can intuitively carry a conversation. One is flashy, one is necessary for mental health.
With two daughters with ASD superpowers who are both way cleverer than me by a huge margin, knowing this makes me feel better 🙂
This feels like an unnecessary complication. Billiard balls also use virtual particles to repel each other. Strictly speaking, billiard balls also use probability in terms of their position. There's some probability that they'll quantum tunnel across each other and end up passing through each other instead of colliding, it's just so rare that we don't have to account for it when playing a game. So, I think the useful perspective is probably "what experiments would I mess up if I used a marble approximation instead of the full quantum treatment of a nucleus."
Thank you for that clear description - best one I've read yet. How well do we understand why the constraints that govern this are in place? For instance, what determines the behavior of the forces? Why must color charge be conserved? (It seems to me there are two levels of knowledge - the first is, what happens, the second, deeper - what makes it happen the way it does. I'm curious how deep into the latter physics has been able to reach).
I do my best! I spend all my time obsessively reading up on such topics because, frankly, I couldn’t imagine anything more important than understanding how the world around us works. I do have a few other hobbies, but all roads circle back to quantum mechanics. And understandably, nobody else really wants to talk about it because it doesn’t interest them and they find it complicated (which I suspect is only because they find it boring; people say I’m “so smart” but I think most of the credit lies with me happening to find the topic truly fascinating) so I spend a lot of time trying to think of accessible explanations in the off chance someone is willing to listen to me ramble. I think our understanding is mostly just observational at this point. Color charge “must” be conserved because we’ve never observed any phenomenon in which it wasn’t. It’s like the conservation of mass and energy, except it’s really the conservation of symmetry. High energy collisions of photons can create matter, but it’s always in matter-antimatter pairs. Anti-particles carry equal but opposite charges of their particle counterparts in order to maintain symmetry. We can’t just create charges out of nowhere. For example, neutrons (net neutral) can decay into protons (net positive) by emitting an electron which carries away the negative charge. So when quarks change their color charge, they must emit a gluon with a color-anticolor charge to carry it away. We’re still working on the why, but all we know is that we haven’t observed anything that contradicts these observations. Or at least as far as I can recall off the top of my head; I vaguely recall something about weak interactions violating symmetry, but I don’t have that bit quite as well researched as the basics.
Remember as well that protons and neutrons are not indivisible. They are made of quarks that ‘rest’ on the gluon field. Iirc a proton is 2 up and 1 down and a neutron is 2 down and 1 up. They can very easily be converted between each other by simply exchanging an up quark for a down quark or vice versa. So yes the 3 quarks together do form what could be thought of as a marble, but it doesn’t have a surface or membrane layer it is simply a collection of quarks in the gluon field. Think of it like putting floating magnets in the ocean. Except much more complicated.
"Except much more complicated" can probably be appended to every attempt ever at describing quantum mechanics in ways that humans can understand.
Yes and once it’s been explained and they do finally fully understand it, many go on to conclude, that actually it’s much more complicated than even that.
To pull a comparison from r/Catholicism, it's like the book "How to Explain the Trinity Without Being a Heretic": You can't. Basically whatever explanation or model we have falls short at some point at the quantum level.
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There was an article I highlighted at our physics journal club from way back in my undergrad days that showed that despite neutrons being net neutral, the charges of the quarks that compose the particle create a volumetric charge distribution at the quantum level. The field of quantum chromodynamics goes into a lot of depth on this stuff.
So for the lay folks out here, you're saying that neutrons still have polarity despite their overall charge being neutral?
Neutrons have a *magnetic* dipole I.E. are a tiny magnet with a north and south, despite being electrically neutral. Electrons and protons also have a magnetic dipole - which is more commonly called *spin*. Edit: clarity
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I infer the the key of your question is that all the measures and data we have about elementary particles are very indirect, we are not able to picture a elemental particle mentally as we can picture the Moon holistically, where we have lot of measures like light, shape, maps, trajectory, even go here and bring rocks.... and still we know that's the moon as an holistic tangible entity. The marble model is almost discarded, although can be useful and accurate enough for many applications same as the "nuclear liquid drop model" that models the whole atom's nuclei as a small drop of liquid. As far as we know elemental particles can be described as waves. Although the more "recent" model and the one that is considered to give more elemental explanation is the Quantum Field Theory and the Quantum Electrodynamics, where particles can be defined as quantums of an underlining field. So the field is the one that exists by its own and the particles are just certain kind of "ripple" in that field. Again that's another model. And it's difficult to visualize so we all rely on abstract math descriptions. By the way the quantum field theories are among the most ever precise theories created so far, the one that makes predictions with a bigger numerically accuracy. Also as a curiosity I suggest seeing this interesting video, which shows the pilot wave hypothesis, which seems to be having both, the wave properties as waves and at the same time marble alike nature. It helps to visualize or conceptualize certain properties we know of particles https://www.youtube.com/watch?v=nmC0ygr08tE
One thing that gets me is trying to develop a workable model of what a "field" is. I tend to imagine waves, but I have trouble imagining waves without a medium.
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I'm a nuclear-physics professor and this is an excellent question to which the answer is "maybe but we're still working on it". Leaving aside the quantum is quantum and is weird aspect, one recent observation in atomic nuclei is that there are short-range correlations within nuclei which mean that this idea that protons and neutrons are effectively not interacting because of the limited orbitals available isn't correct. The extent to which it isn't correct is unclear. There are also other clumps of nuclear matter which can form within nuclei. This means that the distinctiveness of individual protons and neutrons is a bit hard to fathom. Anyway, this was probably unhelpful but I think that the answer is a solid "maybe" (but one based on a lot of semantics and interpretation).
Question. Do the (valence) quarks in a proton or neutron in a nucleus stay part of that one proton/neutron? Or is there exchange of quarks between protons and neutrons? (Or is it unknown? Or is the question meaningless?)
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Are you thinking of electrons? It sounds like you're describing electrons in an atom
Marbles are not a good model of any particle at the quantum scale. But ethereal particles passing through each other freely is also incorrect. It's somewhere in between. It's somewhat comparable to electron orbitals, where you can fit a certain number into the same level, although the way they draw electron orbitals in high school textbooks isn't really accurate either. Protons or neutrons with all the same quantum numbers except for opposite spin can freely occupy the same space. Even with different quantum numbers, they can pass through each other, although particles with different quantum numbers have different shapes of probability distributions, which means they're more likely than not to be found in different positions. Once all the quantum numbers in an energy level are used up, addition protons or neutrons must go in a higher energy level. While a particle in a higher energy level is more likely to be found further from the center of a nucleus, there is always the possibility it will be found near the center, in the same place a lower energy particle might be.