从超光速到时间机器？
From Superluminal Velocity To Time Machines?
摘要：
最近，各种实验已经显示出超光速的群速度和信号速度。实验主要是利用微波隧道效应、受抑全内反射和增益辅助异常色散进行的。根据教科书，超光速的信号速度违反了爱因斯坦的因果关系，这意味着因果关系可以改变，科幻小说中的时间机器可以建造。然而，这种天真的分析假设信号是时间维度中的一个点，忽略了它的有限持续时间。一个信号不是由一个点或它的正面来表示的，而是由它的总长度来表示的。另一方面，信号能量是有限的，因此其频带是有限的，后者是量子$h ν$场量子化的结果的基本物理性质。所有的超光速实验都是在相当窄的频带内进行的。窄带宽是避免分别由于隧道势垒或受激气体的色散关系而导致的信号的脉冲整形的必要条件。由于窄的频带宽度，信号的持续时间长，从而保持因果关系。然而，超光速的信号速度缩短了因果之间的时间跨度。
英文摘要：
Various experiments have shown superluminal group and signal velocities recently. Experiments were essentials carried out with microwave tunnelling, with frustrated total internal reflection, and with gain-assisted anomalous dispersion. According to text books a superluminal signal velocity violates Einstein causality implying that cause and effect can be changed and time machines known from science fiction could be constructed. This naive analysis, however, assumes a signal to be a point in the time dimension neglecting its finite duration. A signal is not presented by a point nor by its front, but by its total length. On the other hand a signal energy is finite thus its frequency band is limited, the latter is a fundamental physical property in consequence of field quantization with quantum $h ν$. All superluminal experiments have been carried out with rather narrow frequency bands. The narrow band width is a condition sine qua non to avoid pulse reshaping of the signal due to the dispersion relation of the tunnelling barrier or of the excited gas, respectively. In consequence of the narrow frequency band width the time duration of the signal is long so that causality is preserved. However, superluminal signal velocity shortens the otherwise luminal time span between cause and effect.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
