考研考试

(46) {{U}}Classical physics defines the vacuum as a state of absence, which is said to exist in a region of space if there is nothing in it.{{/U}} In the quantum field theories that describe the physics of elementary particles, the vacuum becomes somewhat more complicateD、Even in empty space, particles can appear spontaneously as a result of fluctuations of the vacuum. For example, an electron and a positron, or anti-electron, can be created out of the voiD、Particles created in this way have only a fleeting existence; they are annihilated almost as soon as they appear, and their presence can never be detected directly. (47){{U}} They are called virtual particles in order to distinguish them from real particles, whose lifetimes are not constrained in the same way, and which can be detecteD、{{/U}} Thus it is still possible to define that vacuum as a space that has no real particles in it.
One might expect that the vacuum would always be the state of lowest possible energy for a given region of space. If an area is initially empty and a real particle is put into it, the total energy, it seems, should be raised by at least the energy equivalent of the mass of the added particle. (48) {{U}}A、surprising result of some recent theoretical investigations is that this assumption is not invariably true, and there are conditions under which the introduction of a real particle of finite mass into an empty region of space can reduce the total energy.{{/U}} If the reduction in energy is great enough, an electron and a positron will be spontaneously createD、Under these conditions the electron and positron are not a result of vacuum fluctuations but are real particles, which exist indefinitely and can be detecteD、In other words, under these conditions the vacuum is an unstable state and can decay into a state of lower energy; i.e., one in which real particles are createD、
The essential condition for the decay of the vacuum is the presence of an intense electric fielD、(49) {{U}}As a result of the decay of the vacuum,{{/U}} the space permeated by such a field can be said to acquire an electric charge, and it can be called a charged vacuum. The particles that materialize in the space make the charge manifest.An electric field of sufficient intensity to create a charged vacuum is likely to be found in only one place: in the immediate vicinity of a super heavy atomic nucleus, one with about twice as many protons as the heaviest natural nuclei known. (50) {{U}}A、nucleus that large cannot be stable, but it might be possible to assemble one next to a vacuum for long enough to observe the decay of the vacuum.{{/U}}Experiments attempting to achieve this are now under way.