Everything you say is correct except pions (pi-mesons) are comprised of a quark anti-quark pair so they aren’t really elementary particles. But otherwise I don’t know exactly what you are looking for.
Hideki Yukawa realized that the short range nature of the strong residual nuclear force required a particle with a mass about 200 times that of an electron, and when the pi meson was discovered this was confirmed.
The proton and neutron are surrounded by a cloud of virtual pions that are constantly being emitted and reabsorbed, and since the virtual pions are massive they can only exist for a very short period of time which accounts for the short range of nuclear force.
Thanks, Ring for clearing that up. I really had been confused for a long time concerning the strong force, whether it was the force binding quarks or nucleons. I never read about pions being mediating particles or being the strong force in the nucleus. A long-existing question has been answered.
I’m no expert on Quantum ChromoDynamics but the following helps me to keep the pion/gluon thingy straight.
As I’m sure you know both the electromagnetic force and gravity get weaker the farther apart the objects are, however the strong color force acts just the opposite. It gets stronger as the particles get farther apart. Therefore, if gluons mediated the force between nucleons we’d never be able to observe a lone proton or neutron. (they’d be color confined to white bound states just as quarks are)
However, this is not to say that gluons aren’t involved in the residual strong force.They’re involved in the process of creating the virtual pions.
Ring, I actually knew that the quark’s attractive force gets stronger as the distance increases. That’s why they have never been able to isolate a single quark. (At least, the last I read, which, admittedly, has been quite a while. Have they isolated one recently?) However, I did not attribute that to the gluons, but to the quarks themselves. I just was confused about what was meant by the strong force. Apparently, there are two strong forces, one between (or among, if you are an English purist) the quarks and one between the nucleons, and I did not know that the pion can be a vector.
Strictly speaking, there are only quarks in the nucleus, which are continually interacting through the color force, which is mediated by gluons. However, you can often get away with the approximation that the particles are protons and neutrons, interacting via the strong force (which is actually much weaker than the color force), which is mediated by pions. The strong force is actually an effect of the color force, but it’s the primary effect involved in a stable nucleus.
You can also think of the mess of virtual particles as fields, by the way: Virtual photons make up the electric field, etc. Whether a region full of electric fields can be considered “empty” is mostly just a matter of semantics, though.
Now there’s a new way to think about it. Sounds right too. After all a baryon’s nothing more than 3 quarks, and there’s no shell or anything around the quarks that I know of. It also, somehow or other, doesn’t seem quite right.
Chronos how about a free proton? would you consider this nothing more than an assembly of 3 quarks bouncing around? Is an atom nothing more than a bunch of quarks and leptons? Somehow this doesn’t seem right , but I’ll be damned if I can come up with why not.
I took this post to the language department and they assure me that this is, indeed, written in English. Now, I’ve been speaking English for neigh on 2.3 decades now and yet I can’t understand a single damn thing said in this post.
I’m going to go beat my head against the wall and see if I can wake up those extra hundred IQ points I have stored in my cranium for just such emergencies.
I beg to differ – virtual paryticles are indistinguishable from real particles – they are real particles. They can certanly interac with real particles, at which time they are just as real as anything else. Any excess energy they seem to have over a background of zero is limited in time by the uncertainty principle, but if they interact with a “non-virtual” prticle from elsewhere, they’ll just “steal” from its energy, and the Cosmic Energy Budget will still balance.
If virtual particles aren’t real and can’t interact with the real world, wht he hell do you think they are? Why do we postulate their existence? We migh as well say there are virtual angels in their, dancing in sace smaller than the head of a pin.
In an old Justice League of America, the Atom as perched in an Oxygen atom, thinking “I can breathe here in this oxyge atom”. This started my re-adolescent brain thinking: “Wait! If he’s inside the oxygen atom, then how can he be breathing it?” And that started me thinking bout those photons the Atom saw one time. I mean, what light was he seeing the photons with? And so I eventually ended up as physicist. Now I don’t even belie ve in The Atom anymore. So was it worth it?
I wish I could actually access the board more often since it is so packed with hits I rarely can get on any more. This would have been a fun discussion to get into the thick of.
I think the regulars have beautifully answered the question. I have just one small addition: In every neutral, unexcited atom there is technically no place where there isn’t some probability of finding the electron. This is a rather disturbing feature of quantum mechanics, but as the first allowed wavefunction is simply spherically symmetric with no nodes, no matter where you look “in-” and “out-” side of an atom you have a small but non-zero probability of finding an electron.
As for what an atom is, a bunch of quarks and leptons is actually a pretty good model as far as I’m concerned. Of course, there are also gluons and photons and pions and W and Z weak force carriers, etc., all up in the mix, but if you know the relevant particle physics cross sections, the energy scale, and rules of quantum chromodynamics and quantum electrodynamics, quarks and leptons are all you need to make an atom.
As to whether virtual particles are “real” or not, they are real when they have to be and not real when they don’t have to be. Take for example an isolated photon: this particle will not spontaneously decompose into an electron-positron pair if it is in isolation, but given the right amount of energy and the right situation: BLAMMO, you’ve got pair production. The question is a bit “tree falling in the forest” to me. Only the answer this time seems to be (to me) that there actually was no sound made! If nothing could “see” the pair production, it didn’t occur (or, in other words, we have no way of knowing whether it actually occurred).
No one ever said nature had to be understandable, only consistent.
A proton is constructed from two up quarks and one down quark. There are 6 flavors of quarks, but only two up quarks and one down quark constitute a proton. I thing we’re just moving up a scale of reality.
A human being is constructed from cells. Is a human being just a collection of cells?
I think Chronos is saying something else I’m just not sure what.
Ring, I had some real problems with that website you linked to as it seemed to me to be somewhat sloppy and inaccurate. e.g.
**
That’s just not correct. The Casimir Effect is caused by virtual particles, however that’s not what “the small force they exert on real particles” is called.
Anyway, this “small force” isn’t that small and it’s not called anything in particular because it’s the same force that a “real” particle would exert. It just happens to be of very short duration.
Virtual particles can be “detected” in all sorts of way, e.g., the Lamb shift. They are also fundamental to QED which is the most accurate scientific theory ever developed.
This distinction regarding “detecting” virtual particles directly is a bit of a red herring anyway. How do you detect “real” particles “directly?” You don’t. You interact with them in some way which causes a change in the some other particle or system. You then say this change was caused by the particle you “detected.” But you observe virtual particles in exactly the same way. The only difference is that these interactions are constrained by the uncertainty principle.
Most (maybe all) physicists that post at sci.physics do not consider virtual particles to be real.
Virtual particles are not ”on their mass shell.” I.e. they do not obey the special relativistic equation
m[sup]2[/sup] = E[sup]2[/sup] -p[sup]2[/sup] (c = 1)
Since they are not on their mass shell there is no experiment that can be devised that can detect them.
They are not constrained to travel at or below light speed.
Virtual particles are called virtual particles. Real particles are called real particles. There’s a reason for this.
I haven’t done an extensive search but Gordon D. Pusch is a respected physicist and you can read what he says at:
I really don’t know anything about this but I wonder if the question is actually very meaningful. Is this a scientific equivalent of “what is the sound of one hand clapping”. Ideas of space and vacuum are macroscopic notions that just don’t apply at the sub-atomic world view. Some of our models require us to visulise the atoms as ‘little solar sytems’ or clouds of probability, but I suspect that this has more to do with getting ideas inside human heads than what is actually real. The reality is unimaginable because there is no good macroscopic analogue of what the mathematical model tells us is going on - and of couse the models are subject to revision.
umm, That IS disturbing. I’m not correcting you, it’s just that that sounds really strange, even for quantum physics. Seems like an electron would need to keep a certain distance from the nucleus at all times.
I am not a physicist (but I play one on tv!), but I vividly remember my 2nd semester adv. engineering physics teacher making the analogy that if you scaled a nucleus of (some element?) up to the size of a pea, and put it on the 50-yard line of a football field, the first electron shell (yes, I realize that’s the Bohr model and not quantum mechanics) would be well outside the end zones of the field.
His point was, we often draw or otherwise depict atoms as somewhat compact, probably because it’s more convenient on paper, and it’s also somewhat psychologically satisfying. Addressing the OP, is it not correct to say that atoms are considerably more ROOMY than we generally give them credit for? Can anyone put numbers to the pea-football field example? Although I realize the probability stated above is non-zero, the electrons do generally keep their distance from the nucleus, don’t they?
If by “certain distance” you mean infinitesimally close to the center of the nucleus, then yes, that’s right. Of course, the center of the nucleus is simply a point and not exactly measurable. Certainly if one COULD measure this point, one would find that the electron was NEVER there no matter how many measurements were made. In practice, though, one measures not at the center and so there is a small but non-zero chance that you will find the electron wherever you measure.
This is why the Bohr model is wrong. It is only right to say that the most probable location of the electron is at that place. However, there is a chance can be anywhere on the field (except that center nodal point).
It all depends on what you mean by compact. Most of the mass of an atom is found at the nucleus which is very compact. Certainly the most probable way for the atom to arrange itself it with the electron a considerable normalized distance away from the nucleus. However, that’s only the most probable. Sometimes we talk about the “electron cloud” model to try to give a better conceptual understanding. The electron is not at any given position in the atom like a planet is at a given position when it revovles around the sun. Rather, it is in a state that has probability of being in many different positions.
Yes, they do. A typical size of an atom is about an angstrom (10^-10 m). The nucleus is on the order of a fermi (10^-15 m).
