[Physics] Lifetime of a virtual particle

Question

It is common to read that the lifetime of a virtual particle is given by the uncertainty relation:
$$\tau \sim \frac{\hbar}{E}$$
on the premise that the virtual particle 'borrows energy'. This statement is infact wrong (at least I think it is) since energy is conserved in Feynmann diagrams and thus no energy needs to be borrowed. Given this, how do we actually determine the lifetime of a virtual particle, and why is it not just the same as the real particle?

Answer 1

That relation you quote has noting to do with borrowing energy. It is just Heisenberg's uncertainty principle. However, in my humble opinion, it is best not to ascribe any "reality" to virtual particles. They are just pictorial representations of terms called propagators which appear when performing perturbation theory on a Quantum Field Theory. There is very little understanding to get along the route you took in your question.