2 ms·

In essence, you need to enumerate and sum up all possible ways everything can happen in a very little piece of the universe that corresponds to the "reaction ce

In essence, you need to enumerate and sum up all possible ways everything can happen in a very little piece of the universe that corresponds to the "reaction center" of the experiment. All possible ways you can pop out particles from the vacuum that interacts with your experimental particle and pop back into the vacuum etc. It's a totally impossible calculation to do in full, so a large chunk of theoretical high energy physics is about finding shortcuts to do that. All these ways contribute to the reaction probabilities (which are the ones you get a number on at the end of the day). The muon g-2 experiment is so interesting because there is a vast number of pretty exotic particles and particle pathways that have to be taken into account, and if our understanding of any of those is wrong, or there are (even more interestingly) particles that can pop out and contribute that we don't even know about, it will show up in the muon g-2 number as compared to the theoretical calc.

frutiger 3y ago
> All possible ways you can pop out particles from the vacuum that interacts with your experimental particle and pop back into the vacuum etc. This is a neat statement, but really those diagrams are notation for elements in a perturbation expansion series. Nothing in the theory says that these particles actually pop out and actually interact with the incoming/outgoing momenta. In fact they travel off-shell so we know that they cannot be physical.
- l33tman 3y ago
  If you're referring to Feynman diagrams, sure in the modern treatment they don't mean what people usually think they mean, but it's also not accurate to state that particles don't pop in/out. It's a simplification for sure, but you really do need to enumerate all the field configurations and there is really no difference from a first principles point of view between your "physical" particles and the "off-shell" excitations. You can argue about the reality of the contributing terms but it sure looks like they're needed or you won't get the right results.

frutiger 3y ago

> All possible ways you can pop out particles from the vacuum that interacts with your experimental particle and pop back into the vacuum etc. This is a neat statement, but really those diagrams are notation for elements in a perturbation expansion series. Nothing in the theory says that these particles actually pop out and actually interact with the incoming/outgoing momenta. In fact they travel off-shell so we know that they cannot be physical.

l33tman 3y ago
If you're referring to Feynman diagrams, sure in the modern treatment they don't mean what people usually think they mean, but it's also not accurate to state that particles don't pop in/out. It's a simplification for sure, but you really do need to enumerate all the field configurations and there is really no difference from a first principles point of view between your "physical" particles and the "off-shell" excitations. You can argue about the reality of the contributing terms but it sure looks like they're needed or you won't get the right results.