Exploring Feynman Diagrams: The Concept of Time Reversibility

[Trip2]

what does Feynman's diagram prove? i know it deals with something "going back in time"

 

[muppet]

It doesn't "prove" anything exactly. It's a depiction. Asking what it proves is like asking what Raphael's "Transfiguration of Christ" proves. But the idea is that they are a representation of subatomic processes. In some of these processes, a particular kind of particle known as "antiparticles" are created. These antiparticles have a few strange properties- if they ever meet their "normal" matter counterpart, they annihilate each other- both particles cease to exist, and energy is given off (as light/heat/gamma-ray radiation etc.). In the mathematical description of these particles, you take the description of a "normal" particle and replace the variable describing how they behave with time with its own negative (amongst a couple of other changes). So wheras a normal particle experiences t seconds, an antiparticle experiences -t -as if it's gone backwards in time.

 

[Trip2]

o ok i kind of see where your getting at.so in reality there is no particle or anti-particle going back in time?

 

[f95toli]

But the idea is that they are a representation of subatomic processes.

Actually, they are much more general than that. Diagrams are also used in e.g. many-body problems in solid state physics. They are basically a very general tool used in field theory that can be used to perform certain calculations (if I remember correctly each diagram represents a term in an expansion of the S-matrix), i.e. they are not only used to illustrate various processes; the extra "loops" etc seen in more complicated diagrams do actually have a specfic meaning.

 

[Trip2]

Actually, they are much more general than that. Diagrams are also used in e.g. many-body problems in solid state physics. They are basically a very general tool used in field theory that can be used to perform certain calculations (if I remember correctly each diagram represents a term in an expansion of the S-matrix), i.e. they are not only used to illustrate various processes; the extra "loops" etc seen in more complicated diagrams do actually have a specfic meaning.

so what in reality "goes back in time" or does it just mathematicly go back in time? becuase on wiki i looked up feynman diagram and saw the word "go back in time" and assumed something is going back in time.

 

[muppet]

Actually, they are much more general than that. Diagrams are also used in e.g. many-body problems in solid state physics. They are basically a very general tool used in field theory that can be used to perform certain calculations (if I remember correctly each diagram represents a term in an expansion of the S-matrix), i.e. they are not only used to illustrate various processes; the extra "loops" etc seen in more complicated diagrams do actually have a specfic meaning.

As I understand it, the extra loops are allowing for possibilities such as events relating to virtual particles? I seem to remember reading that to accurately determine the probability of a particular process you'd have to draw infinitely many Feynman diagrams, but that the correction associated with each diagram grew smaller and smaller as the number of vertices increased ...

I think that most particle theorists would say that anti-particles actually go back in time- they do, after all, work on the assumption that their maths describes reality! Obviously, as we can't go back in time, you could never "see" an anti-particle going backwards in time; its creation and annhiliation would always be stored in our memories the same way every other kind of event we see is.
There's a separate thread for particle physics: perhaps if an admin moved this you might find people more knowledgeable about the subject there?

 

[Trip2]

As I understand it, the extra loops are allowing for possibilities such as events relating to virtual particles? I seem to remember reading that to accurately determine the probability of a particular process you'd have to draw infinitely many Feynman diagrams, but that the correction associated with each diagram grew smaller and smaller as the number of vertices increased ...

I think that most particle theorists would say that anti-particles actually go back in time- they do, after all, work on the assumption that their maths describes reality! Obviously, as we can't go back in time, you could never "see" an anti-particle going backwards in time; its creation and annhiliation would always be stored in our memories the same way every other kind of event we see is.
There's a separate thread for particle physics: perhaps if an admin moved this you might find people more knowledgeable about the subject there?

i don't think it right becuase causality violation's would occur.

 

[atyy]

When you do the expansion (1+x)ⁿ, you get the binomial coefficients, which have a combinatorial meaning - so you could actually draw corresponding "binomial diagrams". In this case, doing the algebra is usually easier than drawing the diagrams.

Feynman diagrams are just another series expansion, whose coefficients likewise have a combinatorial meaning (combinations of spacetime interactions). In this case, it happens that it's easier to draw the diagrams before doing the algebra.

 

[Haelfix]

Understand that these diagrams are essentially a mneumonic for physicists. Each line represents a mathematical term, each vertex or wiggly line another term and so forth. They're a pictorial way of putting together a mathematical expression in a comprehensive way.

For any physical process there are infinitely many feynman diagrams that contribute to the actual physical result, but for calculational purposes its useful to only keep say the first few diagrams (b/c typically, but not always, the subsequent diagrams tend to be suppressed or small relative ot the first few).