A particle–antiparticle pair which comes into being out of nothing and then rapidly annihilates without releasing energy. Virtual particles populate the whole of space in large numbers but cannot be observed directly. The principle of conservation of mass and energy is not broken provided the virtual particles appear and disappear so quickly that the change in mass or energy cannot be detected. However, if the members of a virtual particle pair move too far apart to rejoin they may become real particles, as happens in Hawking radiation from a black hole; the energy to make the particles real is extracted from the black hole. The lifetime of a virtual particle pair increases as the mass or energy involved decreases. Thus an electron and positron can exist for only up to about 4 × 10⁻²¹ s, but a pair of radio photons with wavelengths of 600 000 km could last for up to one second.