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  3. Classical physics defines the vacuum as a state of absence: a vacuum is said to exist in a region of space if there is nothing i

Classical physics defines the vacuum as a state of absence: a vacuum is said to exist in a region of space if there is nothing i

admin2010-06-30  17
问题      Classical physics defines the vacuum as a state of absence: a vacuum is said to exist in a region of space if there is nothing in it. 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 antielectron, can be created out of the void(空间). Particles created in this way have only a fleeting existence; they are annihilated (消亡) almost as soon as they appear, and their presence can never be detected directly. They are called virtual particles in order to distinguish them from real parades, whose lifetimes are not constrained in the same way, and which can be detected. So it is still possible to define the vacuum as a space that has no real particles in it.
      One might expect that the vacuum would always be the state of the lowest possible energy for a given region of space. If an area is initially empty and a real panicle is put into it, the total energy, it seems, should be raised by at least the energy equivalent of the mass of the added panicle. A surprising result of some recent theoretical investigations is that this assumption is not invariably true. 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, If the reduction in energy is great enough, an electron and a positron will be instantly 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(衰减) into a state of lower energy; that is one in which real particles are created.
     The necessary condition for the decay of the vacuum is the presence of an intense electric field. As a result of the decay of the vacuum, the space permeated by such a field can be said to obtain an electric charge, and it can be called a charged vacuum. The particles that materialize in space make the charged vacuum likely to be found in only one place: in the immediate vicinity of a super heavy atomic nucleus(原子核), one with about twice as many pro tons as the heaviest natural nuclei known. 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. Experiments attempting to achieve this are now under way.
It can be inferred from the passage that scientists are currently making efforts to observe which of the following events?
选项 A、The decay of a vacuum in the presence of virtual particles.
B、The decay of a vacuum next to a super heavy atomic nucleus.
C、The creation of a super heavy atomic nucleus next to an intense electric field.
D、The creation of a virtual electron and a virtual positron as a result of fluctuations of a vacuum.
答案B
解析 由文章最后两句话可知，真空衰减的必要条件是出现强大的电子场。当真空附近出现一个原子核时，这种情况才会发生。相关实验正在进行中。
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