Excitations of the vacuum

"The vacuum, which we perceive as empty at everyday scales and even at atomic scales, is not really emptiness, but a richly structured entity known as a ‘ quantum field’ . Elementary particles are higher-energy configurations of this entity : ‘ excitations of the vacuum’ . So, for instance, the photons in a laser are configurations of the vacuum inside its ‘ cavity ’ . When two or more such excitations with identical attributes (such as energy and spin ) are present in the cavity , there is no such thing as which one was there first , nor which one will be the next to leave. There is only such a thing as the attributes of any one of them, and how many of them there are."