QCD and QED Theory in Layman's Terms

This is my first thread on the boards. I am currently a high school senior and have a Physics question for the bright people who post here.

I am currently involved in doing a report on the 2004 Nobel Prize for Physics. The Prize was given asymptotic freedom in QCD theory. To understand QCD, it would be awfully helpful to understand QED theory. Both of these theories are extremely technical and for a student in AP Physics C, difficult to grasp.

I have found a good amount of information on Quark Theory and the Standard Model, so I luckily do not need help there.

So my question is as follows: Can anyone here help to explain QCD and QED theory in layman’s terms or direct me to a website that contains information not in the form of equations?

Thank you all very much.

I highly recommend Alice in Quantumland : An Allegory of Quantum Physics, by Robert Gilmore. It’s a good, easy read with clear explanations of the basics of QED (no relation) and other aspects of quantum physics.

I second that. His The Wizard of Quarks: A Fantasy of Particle Physics is also a good read on particle physics.

I’d like to offer up Feynman as well, but I found QED to be uncharacteristically tedious, especially (I would imagine) to someone who didn’t already have a relatively solid grasp on QM.


“Listen buddy, if I could explain it in fifty words or less, it wouldn’t be worth a Nobel Prize.” - R. P. Feynman


That being said though, Feynman routinely made easy chewing of many complicated topics in physics. The Feynman Lectures are quite pleasent reading, and at least a good part of Vol. 1 doesn’t really require any math to comprehend. (It should be noted that it was actually jointly “authored” and edited by Feynman, Den Leighton, and Julian[?] Sands.)

There’s nothing about QED that’s straightforward enough to explain to an average nine year old, but I just found Feynman’s description of it, well, accurate (of course) but tedious. I don’t claim to be able to do better, but it just didn’t grab me. I guess I want to see the cartoon version. :smiley:


Really, I’d read up on general QFT (quantum field theory) first, since QED’s a relatively simple QF-style-T and QCD’s a harder one. Before that, even, I’d learn about classical field theory. Unfortunately, I don’t know many different “layman’s” refs than what have already been given, but if you aren’t allergic to mathematics I recommend Rubakov’s Classical Theory of Gauge Fields for the classical picture. Gauge fields aren’t absolutely essential for QED, but they’re a very useful way of thinking about the subject, and they’re essential for how to generalize to QCD.

Phew! He’s going to need some advanced calculus and vector analysis to get into gauge fields. A high school student, even one in AP Physics isn’t going to get to far into that. In the physics undergraduate curriculum, I don’t think they even do much more than touch on QCD, and then only in the most vague technical sense.

If I read the OP correctly, the poster is looking for more of a non-technical (or at least, non-upper division calculus) explaination of QCD, if such a thing even exists. Part of the problem with trying to explain QED and QCD without math is that the concepts are so non-intuitive that trying to talk about them in English is bound to result in confusion, and it takes someone who not only understands the theory but is able to communicate with creative and clarifying analogies.


I would actually beg to differ, on that one: Feynamn diagrams are a big part of QED, and you could probably train most nine year olds to construct valid Feynamn diagrams (though I can’t promise that the kid would know what to do with them, of course).

To further explain: The goal in any theory of particle physics is to be able to calculate two sorts of numbers: Cross sections and lifespans. A cross section is a measure of the likelyhood of two particles interacting to produce some particular collection of output particles (which might or might not be the same as your original particles). It’s measured in area units, and can rougly be thought of as the size of your target for your particle accelerator. A lifetime is the average amount of time a particular particle will last, before decaying into other particles (this can be infinite, for some particles).

It turns out that both of these quantites are related to a quantity called the amplitude, which describes the process. Feynman found that one could calculate amplitudes with the aid of simple diagrams, constructed according to a simple set of rules. For QED, the diagrams consist of squiggly lines, representing photons, lines with arrows on them, representing electrons/positrons, and vertices where the lines meet, consisting of one arrow-line coming in at the vertex, one arrow-line going out, and one squiggly line.

For instance, suppose we’re interested in the cross-section for a photon scattering off of an electron. That is, we have a process where one photon and one electron go in, and one photon and one electron come out. At the bottom of the page (representing the time before the interaction), we have a squiggly line heading towards the middle of the page for the photon coming in, and an arrow-line pointing up representing the electron (if it were a positron, the arrow-line would point down). At the top of the page, we also have a squiggly line coming out, and an arrow-line pointing up coming out, since at the end of this particular process we still have a photon and an electron. Now, in the middle of the page, we want to make some combination of lines connected together with those vertices, such that we connect all of the lines at the top and bottom of the page. For instance, we might connect our two incoming lines at a vertex, with the third leg of the vertex being another arrow-line all by itself pointing up, and then attach another vertex to that arrow-line that turns into the arrow-line and the squiggly line at the top. Once we’ve connected everything, we have a Feynman diagram for that process.

Now, for that Feynman diagram, there’s a set of rules we can use, based on the number of vertices we have, the number of lines, and how the vertices are connected, to determine an amplitude for that particular Feynman diagram. If we draw all of the Feynman diagrams for a given process, and add up all of the amplitudes for them, we get the total amplitude for the process, and from that we can calculate the cross section or the lifespan.

The problem with this is that, for any given process, there are an infinite number of valid Feynman diagrams you can draw. Fortunately, though, in QED and electroweak theory, the amplitude of a diagram decreases as the diagram gets more complicated (more vertices), so if you only draw the simple diagrams, you can get a good estimate of the true amplitude (the more complicated diagrams you draw, the better the estimate). In QCD, the theory of the strong force, however, this doesn’t usually work: For most QCD processes, the amplitude increases as the diagrams get more complicated, and there’s no limit to how complicated a diagram can get, so you can’t do the sum.

Asymptotic freedom refers to the fact that, for very high particle energies, QCD diagrams do get weaker as they get more complicated. So if and only if your particles are high enough energy, you can use Feynman diagrams to determine cross sections in QCD.

Nice explaination, Chronos. That’s a heck of a lot clearer than my H-bomb text explained it. 'Course, it went into all that icky math stuff. :slight_smile:

But I still don’t think you are going to get a 9 year old (well, not an average one, anyway) to really comprehend the implications of QED. Crank through Feynman diagrams, yeah, but explain in their own words how to apply the principle of renormalization lets you collapse the infinite number of calculations to a finite prediction. Heck, most physics undergrads have a tough time really grasping the theory in it’s completeness. :wink:

But you make me want to return and finish off that physics degree.


Well, no. I don’t think you’re going to get a Nobel laureate to really comprehend the implications. As Feynman himself said, “Anyone who understands quantum mechanics doesn’t understand quantum mechanics.”. But it’s not quite so dire as to say that there’s nothing at all in the theory accessible to a schoolchild, and in fact, the most useful tool in the theory bears a remarkable resemblance to children’s doodles.


An apt metaphor. QM and its children are a prime example of why, “the more you know, the more you know you don’t know.”

Again, an excellent explaination. If only my modern physics profs had been so succinct, I would have spent far fewer evenings scratching my head and tossing pencils at the library ceiling to see if they’d stick. :smiley:


Thank you all very much for your help so far. Chronos gave a pretty helpful explanation (although, I am still going to have to look at it again to get the full grasp).

As for my mathematics level, I am currently taking Differential Equations and have already taken a course in Multivariable Calculus. Hopefully, I should be able to understand some of the mathematics behind all of this.

I will absolutely take a look at the books you all have mentioned. My physics teacher (who was actually taught by Feynman at CalTech) recommended his book as well, so I will certainly give it a thorough look through.

Once again, thank you all very much, and I look forward to reading further responses.


Perhaps a picture would help? I should have included one earlier. Unfortunately, I can’t seem to find any images on the Web of the particular diagram I described, so here’s an ASCII rendition:

                    future ^

                   \       /
  Outgoing photon   \     7   Outgoing electron
                     \   /
                      \ /
                       ^   Virtual electron
                      / \
                     7   \
Incoming electron   /     \    Incoming photon
                   /       \

                     past  v

(I couldn’t figure out how to draw wavy lines, so pretend the red ones are wavy)

Some other popular-level books to check out:
Fearful Symmetry, A. Zee
Facts and Mysteries in Particle Physics, M. Veltman

For QED, try Feynman. Not a quick and easy read, but worth the effort if you stick with it.

Unfortunatly, my book won’t be published until later this year. :frowning: Look for The Theory of Almost Everything, coming soon to a bookstore near you…

There’s a terrific Scientific American article on QCD, written probably 10-20 years ago. That’s actually where I’d recommend you start, if you can find it.