I was looking at a feynman diagram to show the decay of a neutron into a proton (by a down quark decaying into an up quark by emitting a W^- boson which then decays into an electron and an electron antineutrino). In a nutshell, Beta^- decay. Here is the said diagram

 

 

this article is not questioning beta^- decay, but asking about the directions of the arrows on the diagram. As you can see, the electron antineutrino is pointing towards the vertex, whereas the electron is pointing away from it. now this would seem to mean that that the antineutrino is going back in time. this is not the only diagram i've seen that does this.

 

http://images-mediawiki-sites.thefullwiki.org/10/1/1/4/4483182193902691.png

 

same affair here, the positron goes back in time, and so does the antiquark. please could someone explain what this is suggesting, whether it is just a standard procedure in feynman diagrams, or if i've just found dud diagrams. thank you

Standard protocol, owing to the charge + parity + time reversal symmetry involved. An antiparticle is indistinguishable from a particle traveling backward in time.

so basically, an antiparticle travelling forward in time will be drawn as though it were travelling back in time?

so basically, an antiparticle travelling forward in time will be drawn as though it were travelling back in time?

 

Drawn as a particle, yes.

Everything you are saying it is very interesting. The feynman diagrams can help us to predict some interactions. How easy do you think, it is? How can i do this?

Feynman diagrams actually is (to some extend) the way I usually determine whether some particle interaction is possible, and also to some extent how likely it is. What you do is trying to construct graphs with the correct incoming and outgoing particles; from the complexity of the graph and the type of internal connections and lines one can often roughly guess the reaction probability. It's not exact of course (Feynman diagrams are exact in the sense of being addends in a complicated mathematical expression, but evaluating this requires plugging in numbers and a bit of math), but often sufficient as a first orientation or for more general statements.