When two electrons – negatively charged particles – draw near, they repel each other. How can we envision the propagation of the repelling force from one electron to the other? Physicist Harald Fritzsch: "A fast moving electron can be pictured as a charged, massive objet surrounded by a cloud of photons, which move together with the electron through space. When two electrons move past each other, the two clouds blend." In the process, an exchange of photons happens. "Since the photons carry energy and momentum, the result is a change in the momenta of both electrons and thus in their direction of travel, which can be interpreted as the effect of a force." So it is the photons that arise as force particles and communicate the repulsive force from one electron to the other. These photons are called 'virtual photons'. They are – according to the laws of quantum mechanics – particles that can be created and annihilated in the vacuum, without any external influence, just by themselves. The vacuum is full of such quantum fluctuations, as physicists like to call them.