What’s a Proton, Anyway?

The Large Hadron Collider is mainly a machine for smashing protons into each other.  But what’s a proton?

First and foremost, it’s a mess.  A total mess.   As ugly and chaotic as a hydrogen atom is elegant and simple.

Ok, then, what’s a hydrogen atom?

The elegant simplicity of Hydrogen: the heavy proton almost stationary at the center, and the lightweight electron moving at ~1% the speed of light within the grey-shaded region. (M. Strassler 2011)

It is the simplest example of what physicists call a “bound state” — the word “state” basically just meaning a thing that hangs around for a while, and the word “bound” meaning that it has components that are bound to each other, as spouses are bound in marriage.  In fact, the image of a married couple, especially one with one spouse weighing a lot more than the other, is probably the one you want.  A proton sits in the center, barely moving, while floating around on the edges, moving a lot faster than you and me but much slower than the speed of light, the ultimate speed limit, is a lightweight electron.   It’s a peaceful vision of marital bliss.

Or so it appears, until we look into the proton itself.  The inside of the proton itself is more like a commune packed full of single adults and children: pure chaos.  It too is a bound state, but what it binds is not something as simple as a proton and an electron, as in hydrogen, or even a few dozen electrons to an atomic nucleus, as in more complicated atoms such as gold, but zillions (meaning “too many and too changeable to count usefully”) of lightweight particles called quarks, antiquarks and gluons.  (More on them elsewhere.)  It is impossible to describe the proton’s structure simply, or draw simple pictures, because it’s highly disorganized.  All the quarks and antiquarks and gluons inside are rushing around as fast as possible, at nearly the speed of light.

Snapshot of a proton -- and imagine all of the quarks (up,down,and strange -- u,d,s), antiquarks (u,d,s with a bar on top), and gluons (g) zipping around near the speed of light, banging into each other, and appearing and disappearing. (M.Strassler 2010)

You may have heard that a proton is made from three quarks.  Indeed here are several pages that say so.  This is a lie — a white lie, but a big one.  In fact there are zillions of gluons, antiquarks, and quarks in a proton.   The standard shorthand, “the proton is made from two up quarks and one down quark”, is really a statement that the proton has two more up quarks than up antiquarks, and one more down quark than down antiquarks.   To make the glib shorthand correct you need to add the phrase “plus zillions of gluons and zillions of quark-antiquark pairs.”  Without this phrase, one’s view of the proton is so simplistic that it is not possible to understand the LHC at all.

The Tiny White Lie: Wikipedia's sterotypical image of a proton as two up quarks and a down quark bound together.

In short, atoms are to protons as a pas de deux in a delicate ballet is to a dance floor crowded with drunk twenty-somethings bouncing and flailing to a DJ.

This is why, if you are a theorist trying to understand what the Large Hadron Collider will observe in its collisions of protons, you’ve got a challenge.  It isn’t so easy to make predictions for collisions of objects that you can’t characterize in a simple way.  But fortunately, starting back in the 1970s, following ideas of Bjorken from the late 1960s, theoretical physicists found a relatively simple and workable technique. Still, the technique only works to a certain extent, typically only accurate to ten percent or so (though occasionally better.)  For this and several other reasons, the reliability of our calculations at the LHC is always somewhat limited.

Now, what about all those quarks, antiquarks and gluons — what are these particles?  that’s another article…

One more thing about the proton.  It’s tiny.   Really tiny.  If you blew up a hydrogen atom as big as your bedroom, the proton would be a tiny grain of dust almost too hard for you to see.  In fact, it’s because the proton is so small that we can ignore the chaos within when describing the hydrogen atom as simple.  More precisely, the distance across a proton is 100,000 times smaller than the distance across a hydrogen atom.

For comparison, the distance across the sun is only 3,000 times smaller than the distance across the solar system (measured from the planet Neptune’s orbit.)  That’s right — the atom is emptier than the solar system!  Think about that when you look up in the sky at night!

But you then might well ask, “Wait a second! You’re telling me that the Large Hadron Collider somehow smashes protons together that are 100,000 times smaller than atoms?  How can anyone possibly do that?!”

Excellent question.

97 responses to “What’s a Proton, Anyway?

  1. Dear Professor Strassler,

    I am not a student nor have I ever studied physics. I just a 49 year old guy who is fascinated by the info in your website. So please forgive me if the following question sounds a bit stupid.
    If the many crowded particles within a proton are moving very close to the speed light, then is time dialated within the proton? And if so, would the proton constitute a point in space-time where time moves very slowly as compared to what exists outside of the proton?
    Thank you in advance, for your time (as well as creating such a great website, especially for folks like me)

    John Chartier

    • Such questions are welcome.

      The challenging part of this question is that one has to remember that time is not, in some absolute sense, dilated. It depends on your point of view.

      First, a general comment. Time — or more precisely, the length of time between two events — is not the same for two observers who are moving relative to one another. [Neither is space — or more precisely, the distance between two events. But there is a combination of the time separation and the space separation of two events which is the same for the two observers… so not everything is equally slippery.]

      If a proton is sitting still relative to us, time for the proton as a whole is not dilated. That said, from our point of view, it is dilated for each quark and antiquark and gluon, which are moving relative to us. Now how does this manifest itself?

      If you were traveling along with one of the quarks, you would say that it takes a certain amount of time to cross from one side of the proton to the other. But I, sitting outside the proton, and not moving with respect to it, would say that it takes longer. The reason that this is consistent is that you would see the proton flattened by length-contraction, so you would think it takes a short time to cross it. I would say the proton is perfectly round, so I would not be surprised that it takes longer.

      Each quark and gluon (or more precisely, each observer who is stationary with respect to one of the quarks or gluons) sees the proton somewhat flattened in the direction that the quark is moving, and thinks it takes relatively little time to cross the proton. Standing outside the proton, we think the proton is round, and that it takes any quark or gluon moving near the speed of light about the same amount of time to cross from one side to the other.

      If the proton is moving very fast relative to us (as at the LHC) then we see the proton as highly flattened in its direction of motion. It looks like a flying pancake. And we also see the relative motion of the quarks and gluons, as they try to cross the proton, as slower — time dilation makes us think that the motion of the quarks and gluons across the proton is slowed down, and that it takes them longer to cross than if the proton were at rest in front of us. [This slowed-motion is actually very useful in the theory of high-energy proton-proton collisions… but that’s a technical point, just added for your amusement.]

      Nothing about relativity is easy to think about. But I hope that was helpful to some degree.

      • so u said that time dilation is relative… then what actually is happening? does the actually get dialated or not?
        time dialation always seems to confuse me!

        • Each observer sees the other’s clock as running slower. This sounds impossible. Meanwhile, each observer sees the other as compressed along the direction of motion (length contraction). This also sounds impossible. The magic in Einstein’s formulas (or more precisely, his interpretation of Lorentz’s formulas) is that the two seemingly impossible things, combined together, become possible, consistent, and correct.

      • i am still a student and i am unable to understand your diliated talks could you please elaborate it in simpler language with real life examples.

  2. Prof. Strassler,
    Is the proton made up of only up, down and strange quarks and gluons, or are there other kinds of quarks in their quark-antiquark pairs thrown in? If there are no top, bottom, or strange quarks in the proton, what particles do you find them in?

  3. I’m just like John, also 49 year-old guy that wishes he would have taken chemistry – or something more academic in high school than just shop classes. For the last several years I’ve been recording and re-watching as many science programs as possible on my PVR (Naked Science, Through the Wormhole, Wonders of the Universe), and lately have spent “zillions” of hours reading Wikipedia – and other websites (like yours) – on atomic theory. I understand the ‘basics’ of the standard model, and am just now able to ‘grasp’ what those funky 3D electron orbits are all about.

    Although I don’t truly understand what antimatter is – and therefore, can’t believe the universe started out with a slightly lower ratio to that of ‘conventional’ matter, I do believe antimatter exists, and annihilate with its opposite particle (as I’ve seen with continuous electron–positron annihilations from a beta-decaying object). However, this is the first time I’ve ever heard of protons being comprised of a “dance” of quarks, antiquarks and gluons… with 2 up quarks and one down quark ‘not able to find a dance partner.’

    This ‘postulation’ leaves me with a number of questions, the most troubling being – why isn’t the relatively “enormous” energy of these observable in such hadron calculations?

    Also, I know that in accelerators, adding energy to a proton increases the mass of such – and “heavy particles” have been created and detected by this method for decades, but in your “snap shot” of a proton, you have charm and strange quarks. Surely the mass of these exotic particle (however briefly they exist) would wreak havoc with gravity, and make the measurement of every day objects unfeasible.

    Some other questions I have are – Is a neutron also composed of a matter-antimatter ‘mosh pit?’ What about electrons – are the also in a ‘stand-off’ with positrons in their sub-shells?

    Thanks (RE: “What’s a Proton, Anyway?”)

    PS: Please don’t conclude I’m being facetious.

    • You ask “why isn’t the relatively ‘enormous’ energy of these observable in such hadron calculations?”

      I’m not an expert, but I believe that the answer is “it is”. If I’m not confused, the vast majority of the observed mass of the proton comes from the energy in (or represented by, the distinction seems to get a bit fuzzy in quantum mechanics) the “zillions” of quark/anti-quark pairs and gluons. Only a tiny fraction of the total proton mass comes from the rest mass of the up up down trio.

      Neutrons are similar, just with a different set of unpaired quarks. Electrons, however, are not. I think this has something to do with the fact that electrons carry no color charge so they don’t get tied into knots by the strong force.

    • After reading a bit more of this site, I realize something must be wrong with my understanding here. I don’t know if the usually quoted mass of the proton (approx 1 GeV/c^2) is supposed to be a “rest” mass, or if that even makes any sense for a composite particle. Is the motion energy of all those quarks and gluons zipping around at nearly the speed of light a part of this mass or not? What about the potential energy associated with the color charge? Are the paired quark/antiquark pairs “virtual” particles resulting from the mediation of all that energy, whose rest mass somehow “doesn’t count”, or something else entirely?

      • 1 GeV (actually .938) is indeed the rest mass of the proton. As you point out, there is positive motion energy from all those particles running around in there, as well as some amount of positive mass energy, and then there is also a very negative potential energy from the fact that all those particles are tightly bound in there. We do not have a simple description of a proton analogous to a hydrogen atom, where you can work out where all the energy comes from. It’s a big complicated mess, but in the end the sum of the energies for a proton at rest is 0.938 GeV. Yes, highly relativistic bound states are a lot more complicated than nice simple non-relativistic atoms.

        Does that help at all? Maybe not very satisfying… but hopefully clarifying…

      • (I tried to post this a couple of times over the weekend, but it seems to have gone to /dev/null — at least I hope it went there rather than Matt’s inbox. My goal was to assist in addressing some of these questions, not to create extra work. Let’s see if this goes any better when I strip out the links that were in previous versions…)

        Andrew, I think you’ve pretty much got it. There is always a level of impressionistic imprecision in non-mathematical language. That said, sometimes terminology can be important to physical understanding, as in the case of mass: as Matt explains in a comment to his Higgs FAQ (7 September, in response to Alfa), the mass of a particle is constant and does not change with speed. “Rest” mass is a potentially misleading — and unnecessary — concept. For more information, I recommend the 1989 Physics Today article “The Concept of Mass” by Lev Okun, which you can find through a Google search.

        Mass makes as much sense for composites like protons as it does for apparently-elementary (non-composite) particles like electrons and quarks. However, while the known elementary particles gain mass through their interactions with the Higgs field(s), the masses of composites come from the energy (including the mass-energy) of their constituents. In the case of the proton, its mass is roughly 100 times larger than the sum of the masses of the three unpaired quarks that make it a proton (by determining its electric charge and other properties). Describing the other ~99% is where things can get impressionistic.

        Personally, I find it more useful to think in terms of fields. One way of phrasing things is to say that most of the proton’s mass comes from the energy of the strong interaction that ties together the up-quark and down-quark fields to form the proton. The strong interaction is strong enough that it produces many (specifically, “zillions”) of disturbances in the fields.

        I think it’s fine to consider these disturbances “virtual” particles, which are not particles at all. But there is a more subtle issue that I should at least mention. At first glance, there seems to be more than enough energy in these fields to produce “real” (as opposed to “virtual”) gluons, up quarks, down quarks and even strange quarks. However, these would-be particles are themselves confined to the proton by the strong interaction, and therefore are indistinguishable from the more general disturbances in the fields. This phenomenon of confinement is crucial to the physics of protons and similar composites, but it is difficult to explain in any detail.

        Finally, to touch on an issue raised by Sean and James, the strong interaction binding the proton together can indeed produce disturbances in the strange-quark, charm-quark and other fields. However, the larger the mass of the particle associated with a field, the more energy is needed to produce disturbances that have significant effects. Much effort over many years has gone into theoretically calculating and experimentally measuring the effects of strange-quark fields on the proton and neutron. While this issue isn’t entirely settled, I would say there is emerging consensus that these strange-quark contributions to the properties of the proton are fairly insignificant; those of charm-, bottom- and top-quark fields are negligible.

  4. I view the proton as a nearly perfect marriage, where divorce doesn’t happen because none of the parents want to be stuck with the kids.
    The forces at play in a proton seem to form natures most perfect vacuum bottle. It contains and saves nearly primal energy for slow release in proton-proton interaction in the sun.
    Do we know why the proton “surface” is the asymptotic limit for the quarks that would tend to escape its domain?
    The rest mass of protons have been determined to a high degree of accuracy. Using your dance floor analogy (with zillions of quarks and gluons) do some protons have more or fewer dancers than others or is there some force or property that requires precisely some very large number to be present for a proton to occur?

  5. Okay, I started reading thiswebsite last night,and haven’t looked at the comments section yet, so this question/theory may have already been asked/postulated… If space is infinite (in both directions, e.g. large and small), wolud it be possible for our universe to exist as either a proton (stars and “black holes” corresponding to up and down quarks, respectively), or for other mirror universes to exist as such (hand drawing a hand drawing a hand ad naseum)?

    • I think what you are asking is this: what do we know about the universe beyond the part that we can see? Might we ourselves — or rather, the part of the universe that we can observe — be part of an even larger structure?

      Yes, that is possible. But until you or I or someone else can think of a way to study the question experimentally, it remains a pure speculation that cannot be turned into science.

  6. You say the mass of proton comes from the energy of the strong force, but I thought to have bound states like hydrogen or helium you give out energy to bind the particles so they are in more energetically favourable positions/lower potential and thats why they have less mass then their seperate nucleons/electrons. Binding energy is the energy you need to put in to separate them. Why does the strong force do the opposite with the quarks inside the proton?

  7. A simple question :- If all mass is made of atoms and 99.999999 % of an atom is just space :- The entire mass within the universe must be 99.999999 % space. Why are we looking for dark matter to make up the missing mass.

    • No, there’s a fallacy in your original assumption.

      (a) All mass is not made from atoms. Only ordinary matter under familiar conditions is made from atoms. The atoms in stars have been broken up into electrons and atomic nuclei — and stars makes up a bit of the mass in the universe too. Neutrinos have mass too, there are huge numbers of them throughout the universe, and they have nothing to do with atoms.

      (b) Just because atoms are mostly empty space has nothing to do with whether something is missing or not. You can’t assume because your refrigerator is mostly empty that something is missing from it. Emptiness and missing are logically different things.

      The reason we know something is “missing” is because we can indiretly estimate the total mass of all the objects in the universe, and (separately) we can estimate the total mass made from known types of particles, and the two aren’t close to equal. [To understand how we learn this is a complicated story, and it would take a few articles to explain it, so I won’t try it now.] That tells us that there is some other class of massive objects (probably particles of some type) out there in the universe, whose nature we don’t know.

      But this has nothing to do with the fact that atoms are empty. That’s just a feature of atoms. The point is that there aren’t enough atoms (or neutrinos or other known particles) to equal, in mass, the total mass in the universe.

  8. Hi Matt, one question about the bustling proton: when you say that the proton is filled with “zillions of quark-antiquark pairs” does that mean that it contains more pairs than a similarly sized space of vacuum? i.e. are these zillions of pairs actually part of the proton or are they just part of the jumble of background energy?

    • The quark-antiquark pairs are part of the proton. If you just look at the normal udd quark formation for a proton, you would find that there would be a large energy deficit.

  9. If the rest mass of a proton is finite (0.938 GeV) and is the sum of the positive motion, positive mass, and large negative potential (binding) energies of the constituent elementary particles, is the number of elementary particles making up the proton finite, or is the number of elementary particles bounded by some limit? And does that limit vary between a proton at rest and a proton in motion?

  10. I’ve not heard your description of a proton before, (many pairs of quarks and anti-quarks plus some unmatched). Just wondered if the space just outside the proton/or atom had a similar number/density of these pairs and if so (or even, if not) then what makes up the boundry of what you are describing as the porton.

  11. Hi, Dr. Strassler. I’ve been reading your articles lately (thank you for taking the time to explain things to lay people (which includes me)). The following question popped into my head this morning: Preface: I’ve read something like the proton’s mass is accounted for by the extreme speeds of the quarks, as the (rest) masses of the (three) quarks is not enough to add up to a proton’s mass. Question: If there are zillions of quarks in a proton, would the quarks’ masses then add up to the mass of a proton, even IF all the zillions of quarks were NOT moving at extreme speeds? Thanks for your help! -Kevin

  12. Hi Matt, When you say “zillions of quarks”, what is an order of magnitude of the actual number, if this is known? Are we talking thousands or millions, or more like 10^20? The latter may sound preposterous to some readers; but I seem to recall Leonard Susskind mentioning in his book “The Black Hole War” that a staggering number of photons, something like 10^30, are emitted and absorbed per second by an electron. So it wouldn’t be that surprising if the same sort of numbers applied to quarks and anti-quarks in a proton. Best Wishes, and thanks for this site, John Ramsden.

    • It is not possible to count; it isn’t even well-defined, because the number is constantly changing, and many of these “particles” aren’t really particles at all; they are virtual “particles”, which means they are really more generalized disturbances in the quark fields, not the nicely behaved ripples that are real particles. http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/ Worse, inside a proton (a very small place indeed) it is hard to distinguish clearly the real particles from the virtual ones, so there is no point in trying to count the real ones either.

      When Susskind gives you the 10^30 number, he is referring to virtual photons; these are not really particles. In particular, a virtual photon can have any mass, whereas a real photon is always massless; a virtual electron can have any mass, whereas a real electron always has a mass of 511,000 electron volts / c-squared.

      Moreover, Susskind is apparently ignoring the fact that photons spend some of their time as virtual electron/positron pairs. By the logic that he uses here, you would also conclude that the electron is surrounded by a staggering number of electrons and positrons. Now that really does sound crazy, because shouldn’t that screw up the mass of the electron? No, because these are not real particles; they are disturbances in the electron field, and they don’t have the mass of the electron.

      I’m honestly a little shocked that Susskind would make such a confusing and potentially misleading statement in a book for the public. It sounds very cool at first glance, but unless I’m completely misunderstanding what he’s trying to say, it’s physically very misleading. When I was a graduate student and took a quantum field theory course from Susskind, he certainly did not explain things in this way; nor would I explain it this way to graduate students in my own classes. I don’t see the point in compressing the facts into a form that confuses rather than clarifies.

  13. I was wondering, with “zillions of quark-antiquarks” inside a proton, how was it determined that there are two more up quarks than top anti-quarks and 1 more down quark than down anti-quarks? How is it possible to determine that without counting all of them? I guess it has probably something to do with statistics…

  14. Hello Matt, as lots here I am a lay person with a passion to know stuff. First thank you for your site. I am so grateful that you can interpret things so clearly and honestly for us. I have a very simple elementry question: how does a field propagate through space-time? Does it do so at the speed of light? When for example the Higgs Field is mentioned I have the impression that it is implied that it is everywhere all at once. If that is the case is it something like a space(and time maybe) dimension that exists everywhere and can be “disturbed” by ripples. Also can the fields of the fundamental forces be considered as a type of “dimension”?

    I am stunned that all electrons and quarks etc all have exactly the same mass, charge, etc. for me it seems that there is something truly fundamental being “rattled”

    This is the first place I have ever read in lay terms that the structure of a proton is a mess! Makes lots of sense to me now. But still am mystified why eletrons have a charge of -1 and quarks sit at +2/3 and -1/3. It’s even more crazy to consider that the charges of a proton and electron cancel out each other exactly. What mechanism is in place that makes this so? Where is the link between quarks an leptons? -John Nichas

    • Field’s don’t propagate; they just exist everywhere. The Higgs field that we find throughout the universe is a constant; it doesn’t need to go anywhere.

      Waves in fields (and their quantum versions, which we call particles) do go somewhere. They will travel at the speed of light only if the particle is massless. Otherwise they can travel more slowly, or even sit at rest.

      No, fields are not a sort of dimension. See http://profmattstrassler.com/articles-and-posts/some-speculative-theoretical-ideas-for-the-lhc/extra-dimensions/extra-dimensions-how-to-think-about-them/dimensions-of-physical-space/

      It is a prediction of quantum field theory that electrons are ripples in the electric field, and all such ripples are identical.

      The link between quark and lepton charges isn’t precisely known. But many theories predict that they should balance precisely in this way: for instance, grand unification of the three non-gravitational forces requires it. And consistency of the quantum field theory of the three non-gravitational forces with gravity also requires it, via a subtle mathematical consistency condition.

  15. After reading this article i have come across a question. “If the quarks, antiquarks and gluons move with such high speed then why they do not come out of the proton?”

    • Because when they try to escape, the strong nuclear force pulls them back toward the other quarks, antiquarks and gluons. Really, it’s the same kind of reason that the earth doesn’t escape from the sun, and electrons don’t escape from atoms — but the speeds are much higher because the strong nuclear force is so powerful.

  16. Ok, but what is the source of this nuclear force?

    • Your question is a bit ambiguous, so I’m not sure I’m answering it.

      The strong nuclear force is, like electric forces, a fundamental force in nature.
      These types of fields are fundamental ingredients in the universe, as far as we know; there is no explanation for them in terms of other things.
      Inside the proton, the strong nuclear forces are generated by the strong nuclear fields emitted by quarks, antiquarks and gluons, just as electrical forces that hold an atom together are due to electric fields emitted by the protons in the atomic nucleus and the electrons in the atom’s outskirts.

  17. Well, you invited stupid questions from the general public. I’m just an old lady wanting to understand some of this before I go. Here’s what I don’t understand about your description of the innards of a proton: how can there be quark/anti-quark pairs? Wouldn’t an anti-quark be like anti-matter? And when they are close enough to be paris, why don’t they just obliterate each other????

  18. I have a theory which states that all atomic particles are the result of a single energy source that manifests itself as electrons, protons and neutrons via some very special mechanics…so…my question is…

    Can we be sure that all of these quarks and things aren’t atomic-shrapnal created by the battering that is inflicted upon protons inside the Hadron Collider, and, which, wouldn’t otherwise, exist??

  19. And for this one..Great article Sir !!!

    • Professor Matt..since I am Quite new in Your site..I haven’t read most of the article to be honest..so I was thinking whether You have written an article on Proton Decay already. If You have written one..Please link me to that while replying..

  20. Thanks for the explanation professor, wikipedia didn’t emphasize that their picture of the proton is too simplified, and now I understand what they mean by “A modern perspective has the proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of sea quarks.” Btw professor, I’m a layperson and don’t understand many things about quantum physics, my algebra skill is rusty and I still suck in calculus, but interested in quantum physics nonetheless. I’m confused with Gev, is it true that the density of the proton is 10^14 gram per cubic centimeter? Two 1Gev proton were accelerated and smashed on a head-on collision and among the debris higgs boson was found having 125 Gev mass… did the event obeyed E^2 – p^2c^2 = m^2c^4?

  21. Moreover professor, I’m curious about the ideas of Bjorken.. I’m not sure if it’s the same thing some of your colleagues called ‘trickery’ because no computer on earth could possibly store in its memory the 10000 trillion numbers of the matrix calculations? And I’m intrigued, they said the standard model predicts we don’t exist, and yet we are here. So 1 is not equal to .99999… after all, even though by algebra it’s equal? They said the apparently substantial stuff is actually no more than fluctuations in the quantum vacuum.. the true proton is the sum of all the possibilities going on at once. Can I conclude that .0000…1 probability won the lottery and this applies too to the Baryon asymmetry of the big bang theory?

  22. Hi professor! A faculty at Stanford University clarified my question into “if a proton has a mass of 1 GeV/c^2 and the Higgs boson has a mass of 125 GeV/c^2, how does that work?” and he wrote:
    So incoming protons have energies
    E_1 = 4000 GeV
    p_1 = 4000 GeV/c
    E_2 = 4000 GeV
    p_2 = – 4000 GeV/c
    That cleared my confused mind, and he used the formula “E^2 – p^2c^2 = m^2c^4″ and came up with “S= ( 8000 GeV )^2″ as the maximum mass of the event… I’m confused again that he cancelled the momenta because of opposite direction. I feel it would be impertinent if I will argue with a Stanford professor on our first encounter. Is my skepticism about cancellation of opposite momenta justified? Regards. :-)

    • Your skepticism is not justified, but you even know it is true. Suppose two cars of equal size traveling in opposite directions with equal speed have a head-on collision. You know intuitively that the wreckage will not be moving to the right or left… the collision is symmetric. That’s the intuition behind the statement that the momenta of the two cars, which are of equal size but opposite direction, cancel each other.

  23. Yes my skepticism is not justified, he already answered my question satisfactorily about how a couple of smashed 1-Gev protons had produced a 125 Gev higgs boson.

    I noticed that my skepticism arises from my inference that I didn’t tell. I inferred that the cancelled momenta is converted to heat and I didn’t see it in the equation. Perhaps I’m not accustomed yet to the subtle equation of relativistic physics, in chemistry the produced heat is indicated by the letter delta on the right side of the equation.

    I inferred that the cancelled momenta is converted to heat energy because of my thought experiment. Firing two lead bullets on a head-on collision in a vacuum chamber I thought will melt the bullets, the heat came from the cancelled momenta… I could be wrong about it.

    • What you’re missing is an important question: “what is heat?”.

      Heat-energy is the motion-energy of all the component particles of a system, less the motion-energy of the system. So if you shoot two bullets at each other, all the individual electrons and nuclei start moving around, breaking bonds with each other, and you end up with a pool of molten lead.

      But when you do that with elementary particles there aren’t any component parts to move around. So when there’s “spare” energy knocking around in a macroscopic system, we get heat; but when there’s “spare” energy in a particle system, we get the generation of particle-antiparticle pairs. There aren’t any particles to dump that excess energy into, so entirely new particle (pairs) get created.

      Sometimes, you will see a composite particle raised to a higher-energy state; at lower energies in atomic interactions, those are pretty common; but in particle physics, it’s mostly particle-pair genesis.

      • Romulo Binuya

        Thanks, that makes two of us who believe that two lead bullets fired in a vacuum chamber on head-on collision will melt. I know heat in terms of classical physics, and the principle that makes the microwave oven works. Heat is energy as I understood it in terms of quantum physics, but we have no knowledge what energy really is, We do not have a picture that energy comes in little blobs of a definite amount, we just designate the photon to carry it and mathematically explain it.

        I believe that the particles and energy from smashed protons were pre-existing thus were released and not created.

  24. Thanks for a great article.
    Protons and neutrons seem to be very stable particles. Can you explain how such stability emerges from the internal maelstrom you describe. Hows does order arise from this chaos?

  25. Im looking for the protons mass and have been reading quite a few of these articles but remain a bit baffled.
    Its been said that it is not from the Higgs field which only accounts for a tiny portion of the overall mass, but is that strictly true? Isnt the mess of particles around those core quarks interacting with the Higgs field and would this fully account for the mass of the proton?

    If not, (and I suspect it wouldnt else the Higgs faq would seem a bit disingenuous) where is the mass coming from, the only answer I have been given is ‘energy’ which is unsatisfying. It doesnt seem sensical to say that the quarks require the Higgs field to have mass, but the additional energy just plain has mass. (Which is the only response I can find.) So how does energy have mass? Do we know?

  26. Isn’t it the higgs boson field had already done its job in the big bang? I wonder what the higgs field is doing now, perhaps maintaining the job it had done?

    • The Higgs field’s role in the Big Bang is unknown, and may have been completely minimal.

      The Higgs field does its main “job” now, because without it, ordinary matter would not form. There would be no atoms. Now.

  27. These quark/anti-quark pairs in the proton, why do they not annihilate each other? I am thinking maybe they do, but that you then get another matter/anti-matter pair that in turn gets you back to a quark/anti-quark pair. Is that right?

  28. Oh and one last thought. The idea of a proton full of anti-quarks, well doesn’t that quite neatly solve the mystery of why there is more matter than anti-matter. It would seem that there isn’t that much more, rather that just like a first past the post voting system – when performing in a way that people consider most unfair, the matter keeps nipping victory by very small orders of magnitude!

  29. Thank you for your answer. It is so neat, the way that the quark + anti-quark process matches in reverse the gluon + gluon process. And it is interesting to think that so much is happening in a proton. When I was a boy, I would sometimes look at something in a microscope and think it strange that I could see something beautiful; but that this beauty was hidden from sight. I used to think that it was almost as if the universe was making a lot of effort that was somehow redundant – quite ego-centric of me! But it seems no matter how big your microscope, you will always end up seeing something complex and dynamic.
    My point is that if you compared the constituent parts of a proton and an anti-proton, you would match up zillions of quark/anti-quark pairs, they would seem almost the same thing.

  30. Despite the craziness that Matt points out going on inside the proton, it is quite stable. Outside the nucleus, a neutron lasts about 15min until it decays into a proton and an electron. I don’t believe anyone has observed a proton decaying. I think that is very interesting.

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  32. Condensed Matter Physicist

    I find your description of the structure of the proton somewhat confusing. It seems you justify these “zillions of quark-antiquarks” through perturbation theory. Is this viewpoint even valid, since the proton is a bound state? Secondly, perturbation theory works in QCD only at large energies, which I suppose is the case with the quarks in a proton, but the perturbative approach cannot be considered fundamental. When you say “zillions”, I assume you basically mean an infinite number, since you can go to infinite order in perturbation theory.

    I think you also forget other constituents of the proton. If you start to count all the virtual particles inside, why not include the virtual photons, electrons, positrons, W & Z bosons, Higgs, everything? And if this is the case, then is it not possible to construct any virtual combination of the particles? Therefore, the proton is not made of just “zillions of quark-antiquarks and gluons”, but inside every proton you can “find” every person on Earth and actually everything else in the Universe, as well. ;)

    I think it is great that you try to explain these complicated things to the laypeople and virtual particles do offer a picture, which is more intuitive than many other ways to say it. I just wonder if people take this description too literally. Instead of talking about “zillions of particles” inside the proton, I would just talk about the quark fields and the gauge fields.

    I am not an expert in the field of elementary particle physics, so if I have misunderstood something fundamental about QCD, please correct me.

    • Well, your questions are ones that touch on how we explain difficult problems to ourselves, as scientists, and to non-scientists. In short, there are both scientific and pedagogical issues here.

      The problem with talking about the quark and gauge fields is that this is generally even more confusing to non-experts than the notion that the proton is full of particles.

      “zillions” is not intended to mean infinite; it is intended to represent only that there’s a lot going on. The number of particles inside the proton is large but not definite, so the number cannot in fact be counted.

      The need to understand that there are many particles inside the proton stems from the physics of the LHC. http://profmattstrassler.com/articles-and-posts/largehadroncolliderfaq/whats-a-proton-anyway/checking-whats-inside-a-proton/ If you want to explain processes like quark + antiquark –> Z or gluon + gluon –> Higgs, you don’t want to explain every time where the antiquarks and gluons come from. [You could try to argue that a proton has only valence quarks and that the antiquarks and gluons are emitted from valence quarks as virtual particles. But not only is this more confusing than stating that the antiquarks and gluons were there as real particles inside the proton, it is inaccurate.]

      Similarly, if you want to talk about quark masses and say that the up quark mass is a few GeV/c^2, you will have trouble explaining why a proton is so heavy. Of course you can say that all the extra energy is stored in quark and gluon fields, but most people have even more trouble visualizing that then they do visualizing a sea of particles running around.

      Of course, *none* of these pictures and ways of speaking is accurate. The proton is indeed a small non-perturbative object and the particles inside it are certainly not “on-shell” (i.e. defining which ones are real and which ones are virtual isn’t possible.) I’ve begun the process of addressing this here: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-structure-of-matter/protons-and-neutrons/ But in the end, even particle physicists and quantum field theorists do not have a precise way of talking about a non-perturbative bound state. We measure its properties; we calculate its properties using numerical simulations; but we cannot express it in simple mathematical or verbal terms.

      By the way, the probability of finding a gluon or an up anti-quark inside the proton is very very high; the probability of finding a W or top quark is exponentially low (i.e. will *never* happen no matter how many experiments you perform). More subtle are photons, and indeed, people are beginning to include them in their calculations, because precision has reached the point where we need to account for them too. And yes, there will be (extremely rare) electron-positron pairs in there too…

  33. Are all protons identical? Is the proton in hydrogen identical to a proton you would fins in phosphorous or plutonium?

  34. hi professor, plzzzz explain sir, that how excess neutrons in a nucleus can cause unstabalised nucleus ((as we know that the neutrons are needed to provide stability in the sense by providing nuclear forces)) then how sir, it can do so?????

    • The strong nuclear force in some sense involves, as one of its processes, exchanging one proton with one neutron [i.e. proton neutron].

      If it weren’t for electric repulsion of the protons — i.e., if you turned off the proton’s electric charge — the most stable nuclei (i.e. those with largest [negative] binding energy) would have equal numbers of protons and neutrons. Any sufficient excess of protons or of neutrons would make a nucleus that could become more stable (i.e. end up with much more binding energy) by undergoing a transition where a neutron becomes a proton (plus an electron and an anti-neutrino) or the reverse.

      With electric repulsion, things shift. It costs energy to have protons close together, so the most stable nuclei have more neutrons than protons, rather than equal numbers. But what I said above is still true; if you have way too many neutrons, the nucleus can reduce its energy by converting one to a proton, an electron and an anti-neutrino.

  35. Professor Strassler, I am watching DVD’s from the Great Courses about ‘Dark Matter, Dark Energy …” I was surprised that Professor Carroll explicitly states (so far – disk 2) that a proton is composed of two up and two down quarks, and a neutron is composed of the reverse. I went back on line to find that some blog sites are stating that while there are an infinite number of notional ‘sea’ quarks, there are only three with valences. That doesn’t make any sense to me – what are these super-quarks? I much prefer your description of a sea of morphing quarks which always has an additional three up and down quarks (if I got that at all right). So, what about those special quarks with valences? And what about Professor Carroll’s description – is he just keeping it simple for the lay folk?

  36. Whoops! I meant to say he said a proton is composed of two up and ONE down.

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  42. michael anderson

    Dear Dr. Strassler,

    Firstly, thanks so much for your blog. Keep up the great work!

    A question about protons: I’ve heard that part of the reason people think there are three quarks in a proton is that when protons are made to collide, the data suggests that there are three little nuggets within the proton. Is this true? How is this possible? And if it is true, how is this reconciled with your description, where there are many more than three quarks inside?


  43. You stated “That’s right — the atom is emptier than the solar system! ” but isn’t dark matter suppose to compromise bulk of the universe?

    1. Can/does an atom contain subatomic “dark matter” (not anti-matter)?

    2. If colliders can produce antimatter, why did the original matter that it was part of existed and not annihilated by the antimatter in the first place? What keeps antimatter attached to matter if it can be separated in a collider?

    3. When the gluons disappear, what do they become? When they reappear, is it the same gluon? How could we tell?

    4. This is weering off a bit – but if nothingness can “exist”, which is really the only way to explain the the big bang inflation theory (faster than the speed of light, etc), then how can the pre-Big-Bang universe catalyze in to any other state without a catalyst, since there was nothing else but a singularity? Wouldn’t that mean
    a) the pre-Big-Bang state steadily became unstable, which is a dead end
    b) the pre-Big-Bang state was a transitional point in time smaller than the inflation time point, so something was before it
    c) there was a catalyst, as in external information, thus, singularities can’t truly exist and this is not a universe
    d) there is a perpetual big bang/big crunch cycle, which would explain b), and some other things
    e) god-did-it
    f) i’m nuts

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  47. if quarks are always found in pairs why do we have an odd number of quarks in a proton/neutron?

  48. Professor Strassler– May I use your image of the proton (“MyProton”) to try to improve Wikipedia?

  49. Robert Watters

    Professor Strassler, my limited understanding, is that less than 2% of a protons mass is derived from the three valence quarks ( via the Higgs mechanism ), most of the remaining protons mass , I am lead to believe, is derived from gluon particle field itself ( Quantum Chromodynamic Binding Energy).
    Is the QCBE mass also derived via the Higgs mechanism?

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  51. The more I here about physics like this, the more physics reminds me of A Game of Life. Would it be an accurate comparison to say that with energy states being the cells, the particles we know are just the stable cellular automaton? I think using the game as an example would make it much easier to teach the concept of wave-particle duality if my interpretation is correct.

  52. proton decay has been estimated 10^33 years but hoe come we can measure such a long time what is the basic idea behind such experiments?

  53. Dear Professor,
    Proton is much simpler than what you have in your picture…
    Think double layers…
    I have some ideas about particle decay, why and how fast.
    It’s up to you if you want to discuss,

  54. Answer to “What is a Proton?”:

    MpRp=4LM <– Solution to the muonic proton charge radius problem and the source of mass of matter.

    Mp=Mass of proton
    Rp=Radius of proton
    L=Planck Length
    M=Planck Mass

    Mp=1.67262177774e-27 kg
    L=1.61619926e-33 cm
    M=2.1765113e-8 kg

    Calculated Rp=


    Calculated Rp=0.84123643e-13 cm

    The muonic hydrogen proton charge radius measurements of 2010 and 2013 approach this simply calculated


    This is what Richard Feynman was looking for – a source of the mass of matter and it should be in

    fundamental units, Planck units, and that is what MpRp=4LM is.

  55. Does anyone knows why this equation matches so well the 2010 and 2013 muonic hydrogen measurements of the cproton charge radius, Rp?

  56. Protons of what are sent through the Hadron Collider?
    How are they obtained and fired through the Collider?

    • Through a small cylinder of hydrogen. The hydrogen is then stripped of the electrons, and further accelerated.

      Then, the same math that can’t analyze a single proton (see proton radius problem/puzzle) is used to analyze two colliding protons. Only the insanely crazy would attempt.

  57. really an awesome article … i have read in books and on Wikipedia also that protons are made of only three quarks but now i know the truth … thanks a lot for sharing it with us… but i have a question that … how proton’s (+1) charge is defined if its made of lots of quarks and gluons ???

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