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I am currently trying to find out about what exactly Feynman diagrams are, and up until now I have mainly used the lecture notes 'Mathematical ideas and notions of quantum field theory' by Etingof. In this notes, Feynman diagrams are merely used to easier calculate asymptotic series expansions for correlation functions.

Now, I took a beginners course on particle physics some time ago, and there Feynman diagrams were introduced as graphical depictions of physical processes, where the edges of the diagram were interpreted as representing some real world trajectory.

Can someone explain to me in which way these interpretations are related? Where does the second way of looking at Feynman diagrams come from?

Edit: I think my question is less philosophical and more technical: As far as I know, Feynman diagrams mathematically arise in the following context: we get a bilinear form as second derivative of some 'action', and we get higher tensors from higher derivatives. Then for the correlation function of k fields, we look at all graphs with k 'external' vertices, and arbitrary internal structure, we assign the fields to the edges and the tensors from the action functional to the vertices, and then contract everything. So the question is: where does the interpretation as interaction process come in?

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    $\begingroup$ What you hear in a “beginner’s course” is simplified for beginners. $\endgroup$
    – G. Smith
    Commented Mar 6, 2021 at 18:11
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    $\begingroup$ @G.Smith in my experience it's only somewhat related to that. Beginner QFT courses can be quite clear about that, while sometimes even more advanced "particle physics" material is wrong about this. $\endgroup$
    – fqq
    Commented Mar 6, 2021 at 18:13
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    $\begingroup$ real world trajectory Quantum particles do not have classical trajectories. If they did, they would violate the Heisenberg Uncertainty Principle. So no line in a Feynman diagram is a classical trajectory. $\endgroup$
    – G. Smith
    Commented Mar 6, 2021 at 18:16
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    $\begingroup$ Your last comment makes your question much clearer. I suggest editing the question to incorporate that comment. $\endgroup$
    – G. Smith
    Commented Mar 6, 2021 at 19:42
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    $\begingroup$ I can't really offer much advice apart from keep reading different books on QFT until it somehow goes in. I have found the book by Srednicki to be useful. $\endgroup$
    – Tom
    Commented Mar 7, 2021 at 17:11

2 Answers 2

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Diagrams as abbreviations

Let's ignore physics for a moment. A Feynman diagram is a graphic abbreviation for a quantity like $$ \omega(x,y,z) = \int du\,dv\,dw\ A(x-u)B(y-v)C(z-w)D(u-v)E(v-w)F(w-u). \tag{1} $$ Lowercase letters represent points in spacetime. Each uppercase letter in the integrand is a function depending on the difference between two points. Some points ($x$, $y$, and $z$ in this example) occur in only one factor, and we don't integrate over them. Other points ($u$, $v$, and $w$ in this example) occur in more than one factor, and we do integrate over them. This particular example is represented by a Feynman diagram that looks like this:

enter image description here

Each factor in the integrand is represented by a line. Each variable is represented by a point, so if two factors share the same variable, their lines are connected to each other. More generally, as mentioned in the question, the factors $A,B,C,D,E,F$ in the integrand may also involve indices that may be contracted with each other in various ways, but this simple example illustrates the idea.

How Feynman diagrams arise in physics

In physics, quantities of the form (1) — Feynman diagrams — arise from using perturbation theory as a computational method in field theory. Perturbation theory is typically used when the equations of motion for the fields are nonlinear, which makes exact solutions unattainable, and when the nonlinearities (also called interactions) are weak. Then, by expanding everything in powers of the coefficient(s) of the nonlinear terms, we can sometimes get a good approximation to the exact solution. Perturbation theory is the art of doing that kind of expansion.

By the way, this is not specific to quantum field theory. Feynman diagrams are just as appropriate in classical field theory. One difference is that in classical field theory, Feynman diagrams don't have loops. Loops, like the triangular loop in the diagram shown above, are a special feature of quantum field theory. In this answer, I'll focus on quantum field theory, where the diagrams can be interpreted in terms of particles... well, sort of. More about this below.

Integrals like (1) arise when we start with a correlation function in the full nonlinear model and expand it in powers of the interaction strength — that's perturbation theory. Each diagram represents a term in this expansion. Each such term is a combination of quantities that we can calculate exactly in the linear (non-interacting) version of the model. In the example above, each of the factors $A,B,C,D,E,F$ represents a two-point correlation function in the linear version of the model. The things being correlated are field operators.

The question is, how is any of this related to particles?

Feynman diagrams and particles

In a quantum field theory with linear equations of motion, the field operators are typically directly realted to particles. Applying a field operator to the vacuum state typically gives a single-particle state. As a result, the vacuum correlation function of a product of two field operators can be interpreted as an inner product between two single-particle states. We can think of such an inner product as the amplitude (whose magnitude-squared is the probability) for a particle that is initially here to be detected there, where "here" and "there" may be separated in time and/or space. That's why we call the correlation function a propagator.

In a quantum field theory with nonlinear equations of motion, field operators typically do not correspond to particles in such a simple way. Applying a field operator to the vacuum state does not create a purely single-particle state, so an exact two-point correlation function (propagator) generally does not represent the propagation of a single particle. However, with some technical caveats, the state created by a field operator may include a single-particle term (as part of a quantum superposition with other terms), and we can extract the desired single-particle contribution to the propagator by applying an appropriate differential operator to it. This is the idea behind the LSZ reduction formula highlighted in the answer by user1379857. The LSZ reduction formula relates an exact $N$-point correlation function to a scattering amplitude for $N$ particles in the nonlinear theory.

Calculating correlation functions exactly is usually beyond our reach, though, so we resort to perturbation theory: we expand everything in powers of the interaction strength, the coefficient of the nonlinear terms in the equations of motion. Each term in this expansion has the form illustrated in equation (1), where $A,B,C,D,E,F$ are two-point correlation functions in the linear model, with the interaction strength set to zero. In the linear model, those lines would correspond to particles in the way described above. But the linear model is only being used here as an artificial device to calculate something in the nonlinear model. In the nonlinear model, interpreting those lines as particles is usually not valid. Experts often still refer to them as "particles" because it's easy, entertaining, or whatever. Sometimes they'll use the adjective "virtual" as a warning, but it's still just jargon.

However, there are situations in which the exact scattering amplitude in the nonlinear theory is dominated by one term in the expansion, and some of those situations really do involve a physically observable intermediate particle phenomenon that really does correspond fairly directly to an internal line in the dominant Feynman diagram! Furthermore, the distinction between those situations and more typical situations is fuzzy. That's why this answer is... fuzzy.

To really understand the relationship between Feynman diagrams and particles, I recommend working toward a nonperturbative understanding of particles in quantum field theory. Exact nonperturbative calculations are usually beyond reach, but we can still use nonperturbative principles as the foundation for how we think about quantum field theory. Perturbation theory is a tool in the toolbox. It's not the thing we're trying to build.

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    $\begingroup$ v. nice and helpful; thanks. One might add a comment along the lines that the terms one is integrating over may come in a small variety of types (e.g. propagator for different rest mass, charge, spin in particle physics) and the diagram can also track this by using a different line (straight, wiggly, etc.) for different type. One of the difficult parts of the maths is to know when one has correctly included all terms (at some order in pert. theory) without over-counting or missing any, and this perhaps is where the diagrams come into their own. $\endgroup$ Commented Mar 6, 2021 at 21:32
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The thing which ties Feynman diagrams to physical processes is called the "LSZ reduction formula." In a standard QFT class, the proof of the LSZ reduction formula (when done correctly) is really the first time you ask questions like, "what are asymptotic particle states" and "what does it mean for particles to interact." Basically, the formula tells you that in order to calculate the amplitude for some scattering process to occur, you just sum up the term from all of the Feynman diagrams with certain "external legs." I.e. if you want to calculate 2 electron --> 2 electron scattering, you sum the all the Feynman diagrams with 4 external electron legs. However, there is also a small subtlety, which is that the LSZ formula says that you actually must "amputate" the legs in order to get the amplitude. So even though you draw these diagrams, you don't actually include the propagators which come from the external legs. Only the internal legs contribute to the scattering amplitude.

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    $\begingroup$ It amazes me that some books with titles like ''Student-friendly beginner QFT'' contain no references to the LSZ formula. To my mind, this means that the author does not understand the subject. $\endgroup$
    – Tom
    Commented Mar 7, 2021 at 12:54
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    $\begingroup$ Frankly, it amazes me too. I personally found learning the basics of QFT a completely bewildering process, and often felt like yelling at the sky, "why didn't anyone explain this!?!" $\endgroup$ Commented Mar 7, 2021 at 15:38
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    $\begingroup$ Yes, this has happened to me hundreds of times learning QFT. I'm glad I'm not the only one. $\endgroup$
    – Tom
    Commented Mar 7, 2021 at 17:09
  • $\begingroup$ Is LSZ reduction formula the same idea as Wick's theorem? I've learned Wick's theorem but have skipped LSZ so far. $\endgroup$
    – Ryder Rude
    Commented Aug 27, 2022 at 10:06
  • $\begingroup$ They are different. Wick's theorem says that in order to compute an $n$ point correlation function of the form $\langle \phi(x_1) \ldots \phi(x_n) \rangle$, one must sum over all contractions of $\langle \phi(x_i) \phi(x_j) \rangle$. LSZ says that in order to compute scattering amplitudes, one must sum over Feynman diagrams where you amputate the external legs of the diagram. The proof involves defining what one means by asymptotic states, see physics.stackexchange.com/questions/210119/… . $\endgroup$ Commented Aug 27, 2022 at 19:12

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