[sgentle]

If anyone's looking for a really solid "accessible to a layperson but you're still going to have to put in some work" series on this stuff, I wholeheartedly recommend PBS Space Time on YouTube. It's paced like your standard 10-15 minute light educational fare, but the core is serious physics.

Here's their 7-part series on Quantum Field Theory: https://www.youtube.com/playlist?list=PLsPUh22kYmNBpDZPejCHG...

An excerpt from part 5, "The Secrets of Feynman Diagrams":

The overall interaction described by a set of Feynman diagrams is defined by the particles going in and the particles going out. These are the particles that we actually measure. We know their properties - for example, their energy and momentum. And they obey Einstein's mass-energy equation. We say that these particles are on the mass shell, or just on shell. They sit on the shell structure you get when you plot Einstein's equation of energy, momentum, and mass. On the other hand, everything that happens between the ingoing and outgoing tracks has questionable reality.

Each possible diagram that results in the same ingoing and outgoing particles is a valid part of the possibility space for that interaction. The particles that have their entire existence between vertices within the diagram but don't enter or leave are called virtual particles. Their correspondence to anything resembling real particles is debatable. They are also, by definition, unmeasurable. Otherwise, they'd be one of our ingoing or going particles. These particles do not obey mass-energy equivalence. So they are off shell. These particles aren't even limited by the speed of light or the direction of time, which leads to all sorts of fun.

[graycat]

Nice.

And thanks for the link.