What is a virtual particle?
Quick Answer
A virtual particle is a temporary quantum entity used to describe interactions between particles. It is not directly observable as a free particle like an electron or photon.
The Full Story
Virtual particles appear in the calculations of quantum field theory as intermediate steps in particle interactions. They can help explain forces such as electromagnetic attraction and repulsion, but they do not obey the same on-shell energy-momentum relation as real particles. Richard Feynman popularized the use of diagrams in the 1940s to represent these processes. Virtual particles are a useful way of modeling quantum behavior, though they are not little particles flying around in the classical sense.
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Suggested video title for this topic:
"The invisible ‘messengers’ of quantum force"