定义：
某些量以超过真空光速的速度传输。

有特定条件下，光的相速度或群速度可能大于真空光速c。似乎信息也是可以超光速传输的，但这是与爱因斯坦的相对论理论相违背的，因为该理论认为超光速传输违反因果关系。但是。如果仔细研究已存在的物理理论（例如，电磁场的麦克斯韦理论，爱因斯坦的相对论和量子电动力学等），就会发现超光速传输并没有任何非因果效应。任何试图用传统物理理论来解释这一效应的都会陷入逻辑困境，因为中间存在的问题不容易被发现。 
两种相对简单的明显非因果传输的例子是： 

1. X射线或者近光学共振原子的相速度可能大于c。即使很简单的机械模型（例如，一系列耦合摆）都能得到相速度大于c。但是不能直接测量具体信号的相位波前，信号传输需要某些调制方法。 
2. 可以通过发送脉冲传输信息。这时信息的传播速度为群速度，即峰值传播的速度。群速度大于c得到超光速传输，这可以在隧道实验中得到。但是，在确定携带信息脉冲的峰值时容易出现错误。例如，高斯形状的脉冲在峰值被探测之前就已经被探测到了，因为在之前的一段时间检测到了光强。因此，需要考虑一种不同的信号，即在探测之前并没有任何信息传送的信号。采用这种信号时，最早能探测到它的时间即当他以真空光速c传播时。一个前驱的光以该速度传播，但是通常很弱而无法探测到，特殊情况下才能探测到。 

当然，现存的物理理论，例如，爱因斯坦的相对论，也可能需要重新考虑或被新的理论取代，甚至允许超光速传输。已经存在这种理论，但是目前为止，还没有可信的证据支持用新的理论取代相对论理论。

Definition: transport of some quantity with a velocity exceeding that of light in vacuum

Under certain circumstances, light can propagate with a phase velocity or group velocity greater than c, the vacuum velocity of light. At a first glance, this may suggest the possibility of superluminal transmission of information – in striking contradiction with Einstein's theory of relativity, which shows that superluminal transmission would be associated with a violation of causality, one of the most fundamental principles of physics. However, careful inspection of the accepted physical theories (in particular, of Maxwell's theory of electromagnetism, Einstein's theory of relativity, and quantum electrodynamics) shows that these definitely do not allow any non-causal effects. Any attempt to demonstrate such effects on the basis of these theories must fail for logical reasons and is thus based on fundamental misconceptions, although the flaw may not always be easy to identify.

Two relatively simple examples of apparently (but not really) non-causal transmission are given here:

• The phase velocity of light can be higher than c, e.g. for X-rays or near optical resonances of atoms. Even a simple mechanical model (e.g. a sequence of coupled pendulums) can exhibit a phase velocity above c. However, it is not possible to attach a real signal to such a phase front; signal transmission requires some kind of modulation.
• Information can be transmitted by sending (or not sending) pulses. The information then appears to propagate with the group velocity, i.e. the velocity of the peaks. Group velocities larger than c then appear to lead to superluminal transmission and can occur e.g. in tunneling experiments. However, the error lies in identifying the peak of the pulse with the temporal position of the carried information [1]. For example, a Gaussian-shaped pulse can be detected long before its peak due to the rise of intensity at earlier times. Therefore, a different kind of signal must be considered, where no information at all is sent out before a certain moment of time. For such signals, it can be proven that the earliest time at which that switching event can be observed is limited exactly by propagation with the vacuum velocity c [1]. A so-called precursor is traveling with that speed, but is normally too weak to be detected, except under very special circumstances [10].

Of course, it is conceivable that the currently accepted physical theories, e.g. Einstein's theory of relativity, will have to be revised or replaced with newer theories, possibly even allowing for superluminal propagation. Such theories exist already, but so far there is no convincing evidence that could support the withdrawal of relativity in the favor of an alternative theory.

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