 For large gaps between the prisms the tunnelling time approaches a constant and thus the photons appear to have crossed with a superluminal speed.However, the Hartman effect cannot actually be used to violate relativity by transmitting signals faster than "c", because the tunnelling time "should not be linked to a velocity since evanescent waves do not propagate".The evanescent waves in the Hartman effect are due to virtual particles and a non-propagating static field, as mentioned in the sections above for gravity and electromagnetism.