Do Faster-than-Light Group Velocities imply Violation of Causality?

Hans-Otto Carmesin

In everyday life, in smartphones, we use electromagnetic waves for telecommunication. These waves propagate at the velocity of light. Is telecommunication at superluminal velocity (at ) possible? Einstein (1907) wrote that no action can travel faster than light, as such action would imply a causality violation. However, quantum physics includes actions that travel faster than light, see e. g. Einstein, Podolski and Rosen (1935). Accordingly, Einstein (1948) named such actions (at ) ‘spukhafte Fernwirkung’ or ‘spooky action at a distance’. But Aspect, Grangier and Roger (1982) provided such action at in experiments with pairs of entangled photons. While this experiment demonstrates the seeming transfer of information at , delayed-choice experiments do additionally demonstrate the seeming transport of energy and matter at , see Jaques (2008) and Manning et al. (2015). Do these actions at violate the principle of causality? Does quantum gravity escape causality violation, as Hobson, Efstathiou and Lasenby hope (2006, p. 346)? Can such action at be used in telecommunications and quantum computers? These questions are answered with help of the dynamics of volume in nature, the volume dynamics, VD, see Carmesin (2023a). The VD bridge general relativity and quantum physics. For these results, we provide a learning process, so that you can directly use the concept in your courses. The learning process has been tested in various learning groups, and experiences are reported.

Marijan Ribarič, Luka Šušteršič

Giovanni Valente

Herbert G. Winful

Experiments done with single photons in the early 1990's produced a surprising result: that single photons pass through a photonic tunnel barrier with a group velocity faster than the vacuum speed of light. This result has stimulated intense discussions related to causality, the speed of information transfer, the nature of barrier tunneling and the meaning of group velocity. The superluminality of tunneling photons is now textbook material, although the authors note that controversy still remains. Another paradoxical result, known as the Hartman effect, is that the tunneling time of the photons becomes independent of barrier length in the limit of opaque barriers. In this paper we examine the meaning of group velocity in the context of barrier tunneling. We ask whether a single tunneling photon can be described by a group velocity and whether the short group delays imply superluminal group velocity. We resolve the paradox of the Hartman effect and show that the predicted and measured group delays are not transit times but photon lifetimes.

G. Nimtz

Aephraim M. Steinberg, R. Y. Chiao

In the region between two lines of a doublet of an inverted medium, there exists a point of zero group-velocity dispersion, where highly superluminal effects may be observable without significant gain, loss, distortion, or broadening. The results of this group-velocity analysis hold for sufficiently narrow-band, analytic pulses, and do not constitute a violation of causality, although the group, ``signal,'' and energy velocities as defined by Sommerfeld and Brillouin may all exceed c or even become negative. No sharp disturbance in the pulse (a real signal) could propagate faster than light, but the scheme offers an unusual noiseless amplification scheme for the leading edge of a pulse, both at the classical and at the single-photon level.