They are responsible for the electromagnetic force that holds matter together, making them a key component in our understanding of the physical world.
[1][2] Virtual photons are thought of as fluctuations in the electromagnetic field, characterized by their energy, momentum, and polarization.
The polarization of a virtual photon is determined by the direction of its momentum and its interaction with the charges that emit or absorb it.
Virtual photons are said to be "off-shell", which means that they do not obey the usual relationship between energy and momentum that applies to real particles.
They are also responsible for the Casimir effect, which is the phenomenon of two uncharged metallic plates being attracted to each other due to the presence of virtual photons in the vacuum between them.