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Displays in the shop window could in this way be activated by touching the outside of the window (Figures 2a and 2b explain the role of the evanescent field in this case in somewhat more detail). In one application, the optical touch key was used on shop windows, on the inside of which the keyboard was semi-permanently glued in optical contact, as shown in Figure 1. In this type of non-mechanical key, the finger interfered with the evanescent field at total internal reflection in a rectangular prism as illustrated in Figure 1, Figure 2a and Figure 2b. One practical application of the evanescent field was the optical touch key – a forerunner to the touch keys in present-day iPads and the like – which was patented in 1978 and marketed by OptiSensor AB. Superluminal Speed of the Evanescent Field? Wikipedia has extensive articles on “Faster-than-light” and “Quantum_entanglement”, which are recommended as sources for further references in this vibrant field. A macroscopic equivalent to quantum tunnelling is the evanescent field at total internal reflection, which will here now first be discussed. Before that, tunnelling in quantum mechanics, which is another possible example involving such superluminal velocities, will be discussed in Sec.
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One such example is quantum entanglement – what Einstein called ‘spukhafte Fernwirkung’ (‘spooky action at a distance’), and which will here be discussed at some length in Sec. However, it now also seems clear – even if maybe not fully understood – that some physical phenomena would seem to transcend the limitation thus imposed by the speed of light. According to Einstein’s theory of special relativity, the speed of light would also seem to be the maximum speed with which information can travel, otherwise we would seem to be at risk to get involved into problems about cause and effect with regard to reference systems with different relativistic velocities. From Maxwell’s equations follow that electromagnetic waves in vacuum propagate with the speed of light.