The Bedlam Within Protons and Neutrons

My Structure of Matter series has been on hold for a bit, as I have been debating how to describe protons and neutrons.  These constituents of atomic nuclei, which, when combined with electrons, form atoms, are drawn in most cartoons of atoms as simple spheres.  But not only are they much, much smaller than they are drawn in those cartoons, they hide within them a surprising commotion, one that cannot be anticipated from the relatively simple structures of atoms and of nuclei.

As I’ve described in my new article, along the lines of this short article and this more detailed one that I wrote some time ago in the context of the Large Hadron Collider, the story that scientists tell the public most often, that “a proton is made from two up quarks and a down quark”, is not in fact the full story — and in some ways it is deeply misleading.  The structure of protons and neutrons is so entirely unfamiliar, and so complicated, that scientists neither have a simple way of calculating it, nor an entirely agreed-upon way to describe it to the public, or even to physics students.  But I believe my way of describing it will be satisfactory to most particle physicists.

The new article is not entirely complete; it is perhaps only half its final length.  I’ll be adding some further sections that cover some subtle issues.  But since I suspect many people won’t feel the need to read those later sections, the completed part is written to stand on its own.  If you like, take a look and let me know if you have questions, suggestions or corrections.

26 responses to “The Bedlam Within Protons and Neutrons

  1. We can’t blame Lucy.

  2. Your Structure of Matter series has now started explaining things I didn’t already know. This is an interesting article so far. Here’s one “but” that I hope to see explained in the next layer:

    Although the masses of the quarks are only a small fraction of the masses of the nucleons, based on the explanation so far I’d still expect the difference in mass between a proton and a neutron to be a result of the difference between an up quark and a down quark, but the stated difference between up and down quark masses, 0.004 GeV/c², is about three times the difference between the cited proton and neutron masses, 0.0013 GeV/c². Does the proton contain more kinetic or binding energy for some reason, compensating for two thirds of the mass difference?

    • The proton has to carry around an electromagnetic field while the neutron does not. Because of this, physicists didn’t understand early on why the neutron was the more massive particle. The down quark mass being larger than the up quark mass saves the day.

  3. Thank you very much for your post, Professor Strassler!

    There are a few points I’d like to understand better; maybe you can clarify them in subsequent posts:

    1) You mention that the classical “quark composition” is significant and there is a significant distinction between valence quarks and sea quarks. My naive understanding is that the “valence quarks” are those quarks which, if hypothetically free and then combined (say, 2 up + 1 down), would produce the baryon of that “quark composition”, uud in this case = a proton. In other words, that while a proton and a neutron may have innumerable quarks, anti-quarks, and gluons, it is the “quark composition” that makes a proton a proton and a neutron a neutronl

    Is this correct?

    2) I understand that there are other “ingredients” that differentiate baryons from each other – a delta-+, for example, has the same quark composition as a proton, but, if I recall correctly, all its quarks have the same spin, unlike the proton. If no individual quark can be distinguished as a “valence quark”, how are the delta-+ and the proton distinguished?

    3) What meaning do “quarkonium” quark compositions have, if there is no excess of quarks of any flavor? – how is a neutral pion distinguished from an eta, J/psi or upsilon? (For that matter, how is a spin-zero quarkonium meson distinguishable from a vacuum or a photon [here my ignorance shows, I guess...])

  4. Tienzen (Jeh-Tween) Gong

    Indeed, this is one of the best article on describing both proton and neutron in terms of the established consensus. But, I think that there are two genuine questions should be asked.

    a. Among many baryons, proton and neutron are rock bottom building blocks for bio-lives. “Should” one of them or both contain the “seed” of life? If not, what is rationale for not to? If yes, then where is the hint of this seed in your description?

    b. Both quantum and determinism are realities, that is, there must be a bridge between them. While both proton and neutron are, indeed, quantum particles, the quantum algebra shows that they are the bridge of these two realities.
    Proton (quantum) + electron (quantum) = hydrogen atom (quantum),
    then, H + H + O (oxygen) = H2 O (water, determinism).
    “Should” the seed of unification of quantum and determinism be part of their (proton and electron) attributes? If not, why not?

    These two are genuine “questions,” not speculations. Do you agree? If not, why not? If you do agree with them, why not mention about them? Have you ever heard about these questions? Do you know that there are articles addressing them?

    • I’m not sure I understand what you’re asking – maybe Professor Strassler can – but I think your questions are metaphysical and philosophic, rather than scientific, and are thus unlikely to be answered here.

    • Hmm,

      “Among many baryons, proton and neutron are rock bottom building blocks for bio-lives.”

      Well, no. Protons and neutrons are composite particles. And they are the building blocks for nuclei which in turn are one of the two building blocks of atoms. Yet the chemical properties of atoms are largely determined by their (outer) electron shell arrangements. So, even if there is a ‘seed of life’, there isn’t much rationale for it being attributed to protons or neutrons.

      “H2 O (water, determinism).”

      Why do you think that a water molecule does not exhibit quantum behaviour?

      • Tienzen (Jeh-Tween) Gong

        @Surreptitious Evil
        It is no point of arguing about which one carries more weight [(proton + neutron) vs electron]. If the “seed” of life is in electron, it will be very fine. The point of the issue is whether this seed is in physics or in the breath of God.

        Today, we do know what this seed should look like. Life is an information processing machine, that is, it needs a bio-computer. There are, in fact, two kinds of bio-computers in life, the DNA language and the protein language. It is reasonable to assume that they both share the same kind of CPU.

        We also know that the best CPU is a Turing computer. The Life Game of John Conway showed that a “glider” can be the base for constructing a Turing computer. Thus, if one bio-building block (proton, neutron, electron or the whatnot) carries a sub-structure which is a glider, then the seed of life is in physics.

        Furthermore, a glider is a cellular automaton, 100% deterministic. Thus, if a quantum particle carries a glider as its sub-structure, its destiny will also be deterministic. In this scenario, the both questions are resolved with one stone.

        @ Hasirpad
        It is your rights to view these two questions as metaphysics or philosophic issues. When these two questions are addressed with a “particle theory”, they are the issues of physics. Indeed, there is such a particle theory addressing these two questions. I cannot provide you the link to that theory as I must not promote any theory here. You can try to Google it.

        • “The point of the issue is whether this seed is in physics or in the breath of God.”

          Particle gnosticism. Wow …

          “Today, we do know what this seed should look like. Life is an information processing machine.”

          Is it really? You may be confusing ‘life’ and ‘intelligence’.

          “We also know that the best CPU is a Turing computer.”

          This is also rubbish. We know that a ‘Turing machine’ can be used to simulate any linear program. That makes it versatile not ‘best’.

          You are not trying to have a particle physics discussion. You are trying to have a metaphysics discussion using belief-derived ‘evidence’. That you are looking for evidence in particles doesn’t make it physics any more than me drawing a quick map for somebody who is lost is ‘art’, merely because I’ve used the same mechanisms that an artist would.

  5. I’m sorry Matt, but I don’t like this picture. Gluons are virtual particles. Just as there are no actual photons flitting back and forth in a hydrogen atom, just as magnets don’t shine, there are no actual gluons flying around in a neutron. It isn’t some seething bedlam plastic bag ready to burst open to reveal a host of free quarks. Magnetic moment and beta decay tells us it has structure, and that this structure is not the same as the proton. We’re dealing with quantum field theory here, deep inelastic scattering tells the proton has a definitie tripartite form, but its field is part of what it is, so it isn’t small, and field is all it is, so things like gluons are field quanta, just “chunks” of field. For better picture, look to TQFT and start with a trefoil: http://commons.wikimedia.org/wiki/File:Trefoil_knot_arb.png . Cut it with your gedanken scissors, give it a half twist, then join it back up for spin ½. Then give it a circulating flow at c. Follow this round it with your finger clockwise from bottom left calling out the crossing-over directions: Up, up, down. Make it elastic like the bag model. Make it so fat it starts looking spherical. Then draw lines on it to divide it up into little elastic-cube field quanta and call the stress a quark and the tension a gluon. Only don’t stop at the spherical surface, because the proton’s electromagnetic field is part of what it is too, and look to gravitomagnetism for frame dragging. Then do low-energy proton-antiproton annihilation to gamma photons, and think of them as mere distortions in a lattice that’s already made up of little cubes. In your picture all those fundamental quarks and gluons have gone. In this picture, they haven’t.

    • “Virtual” means “unobservable” not “nonexistent”. When two charged particles interact electromagnetically, the photon they exchange by definition starts at one particle and ends at the other. Intercepting it so you could see it is the same as saying the particles didn’t interact in the first place.

      Gluons are the quanta of the field, yes. You don’t think disturbances in the strong field exist inside the nucleon as the quarks move and interact?

  6. I don’t understand. If protons and neutrons are really chuck full of quark/antiquark pairs wouldn’t they undergo annihilation, resulting in photon emission and reduction of the nucleon mass down to just a few MeV of the u and d quark mass?

    “Oh, no Dino there is so much going on in the nucleon it’s more like the Sun, where photons are constantly emitted and re-absorbed, it takes forever for a photon to escape… In fact we think it will take 10 to the 34 yrs for the proton to evaporate to a positron and energy.”

    But I thought that there was a law of conservation of baryon number that keeps the proton (and antiproton) stable. Experimentally, has this law ever been violated? You haven’t actually seen proton decay have you?

    “How about the creation of Universe itself? How do you think the universe came to consist of matter over antimatter? Otherwise the universe could never get started in the first place. That law of conservation of baryon number is so 20th century!”

    If you say so…

  7. Here is an interesting video on the standard model that may give a visual of what’s going on. I don’t represent that it is a perfect picture but for those of you that like to “picture” things, it could be helpful: http://www.youtube.com/watch?v=K6i-qE8AigE&feature=player_embedded

  8. Dino: check out positronium at http://www.cs.cdu.edu.au/homepages/jmitroy/workdesk/psatom.htm .
    Note that “To a first approximation it can be regarded as a sort of light hydrogen atom”. Then look hard at electron, positron, antiproton, and proton properties, and arrange these 4 particles into a 2 x 2 table. Do it based on properties alone. THEN add the column headings Matter and Antimatter. Now when you ask the question “Where did all the antimatter go?” you get a slightly different answer.

    A proton isn’t stable because “it’s the law”. That’s a non-answer/ The proton is stable because of its structure or its nature or whatever you want to call it, be it balanced / harmonic / symmetric / etc. The neutron isn’t. Wait 15 minutes and you’ve got a proton, and electron, and an antineutrino. This ought to tell you that the neutron structure is very different to the proton structure. And that they’re aren’t just bedlam bean-bags.

  9. Thanks for taking the trouble to spell this out. You know the two sayings: 1 the devil is in the details, 2 god is in the details. Without wishing to get all theological, the foundational conceptual issues you address are really interesting and important and too often glossed over. Your efforts to explain these as simply as possible, but not more simply, will hopefully influence others going forward. Keep up the good work!

  10. Dr. Strassler :
    In your personal opinion , do you accept the idea that the DARK SECTOR could in principle have its kind of SM , consequently , its full fledged universe …..galaxies , stars , planets……AND life forms ?

    • What is “the DARK SECTOR”? Do you mean ‘cold dark matter’? If so, although an extensive set of CDM particles is possibly, the observed density and distribution of CDM makes it very unlikely that it forms interacting structures (other than gravitationally bound clouds) of any sort. Hence no stars or smaller objects.

      Also, to form anything other than very large gravitationally bound clouds, CDM particles would need to inter-react with each other more than they do with SM particles and forces. Which would be likely to make some of them thermal – which would significantly change their interreaction with the SM universe. Making them more like heavy neutrinos. So, although this is _possible_ it is currently unexpected.

  11. Unless the dark sector includes its own kind of interaction unobserved forces ??

  12. S.E. ; Excuse me , but on what capacity you answer in the name of Dr,Strassler

  13. No Sir as i am asking a professor in physics in person.
    Are you as such ?

    • Tienzen (Jeh-Tween) Gong

      aa. sh.:

      I don’t think that a (any) professor in physics can answer your question, as he/she does not even know what the role of the Standard Model particles plays for the rising of bio-life. When the five issues below are understood, your question will be answered.

      1. Both Cabibbo angle (θc about 13 degrees) and the Weinberg angle (θW from 28 to 30 degrees) are the two key parameters in the Standard Model (SM). These two should be the first criterion for a correct particle theory, that is, they both must be “derived” by a correct particle theory. Of course, the Standard Model fails on this task (criterion), as they are only the “Free” parameters in SM.

      2. The Alpha (α, electron fine structure constant) should be the second criterion. Alpha is the “lock” for the universe, as it locks three measuring rulers (ħ, the spin charge; e, electric charge; c, light speed) of the universe into a constant relationship. When these three rulers are locked, the universe is allowed to roam free with its evolution. Thus, this Alpha, as “the” most important lock/key for the universe, must be “derived (directly calculated)” in a correct particle theory. Of course, Standard Model again fails on this task (criterion).
      Note: C (light speed) is also a lock which locks the (space/time) into a fixed relation. So, space and time can be set free. Yet, the Alpha is the final lock.

      3. That the expansion of the universe is accelerating is now a “Fact”. This fact should be the direct consequence of a correct particle theory. Again, SM fails on this criterion.

      4. The Planck data showed three key features of this universe.
      a. The dark/visible ratio (69/26/5). Again, the “Standard Model proper” cannot make any linkage to this issue. Its baby (SUSY) is making some wild guess without the ability to match with this ratio. Yet, this ratio can be numerologically matched with an iceberg model. Thus, the correct particle theory must encompass a substructure which is iceberg-like.

      b. Neff is 3. What the number for Neff should be? This must be directly “derived (calculated)” in the correct theory. The Standard Model fails on this again as the Neff = 3 is the gadget data (fact) for SM but is not calculated (theoretical) result.

      c. Dr. Guth’s inflationary scenario is consistent with the data. Again, the inflation is not the direct “consequence” of the Standard Model, and it must be an add-on. The physics meaning for inflation is topological, the universe is a topological plane instead of a topological sphere. The Standard Model cannot even address this topological issue.

      5. Neutrino oscillation is again a physics “Fact” now. The Standard Model has no slightest clue to explain this. Yet, all physicists happily brush this cruel fact away, delinked it in order to preserve the greatness of the SM. With “energy” rule, the particle “decays” to a lower energy state. On the contrary, the neutrino can “decay upward” to become a heavier neutrino. Obviously, “decay upward” is not truly a decay in linguistic sense. So, it is called “oscillation”. Thus, neutrino demands a new physics, in addition to the “energy” rule. This oscillation can only be done with the “music-chair” rule, such as,
      1 = (2, 3) = (2, (1, 2)) = (2, 1, 2)
      That is, 1 is the “complement” of (2, 3), and it can play music-chair. A correct particle theory must encompass a substructure of this music-chair.

      The five facts above are physics facts, not metaphysics or philosophic issues. But, the Standard Model fails on each and every one of them. Should these five be the criteria for the correct particle theory? This is a great question if no theory can meet these criteria. But, this is not the case. When this issue gets clear, your question will be answered automatically.

  14. Dear Matt
    I wonder if you have an email address? I do particle physics and astronomy in New Zealand at the University of Auckland and would like to enquire about using one of your figures in a paper, with acknowledgment.
    Philip Yock

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