Virtual particles are real

The magnetic moment of a particle is proportional to its spin and to its g-factor, which is exactly 2 for a point particle with half-integer spin. However, the muon is constantly interacting with virtual particles, which wink in and out of existence with quantum-mechanical probabilities calculable to an incredible precision. This fluctuating cloud of particles modifies the $g$-factor. 

I knew that transient particles pop out from nothing and then really exist in our universe. Now they are being measured in the muon experiment. Intriguing.