Z. Naturforsch. 2017; 72(9)a: 881–884 Letter Günter Nimtz* and Horst Aichmann Zero-Time Tunneling – Revisited DOI 10.1515/zna-2017-0172 Received May 23, 2017; accepted June 8, 2017; previously published online July 14, 2017 Abstract: Since 1931, the nonclassical process of tunneling was conjectured to have a zero-time delay in the barrier. These theories have been rejected and denied. However, photonic and recent electronic tunneling experiments have proven the zero-time prediction. Tunneling is due to virtual wave packets in electromagnetic, elastic, and Schrödinger wave fields up to the macroscopic level. In this article we cite theoretical and experimental studies on zero-time tunneling, which have proven this striking behavior. Keywords: Field Independent Tunneling; Special Theory of Relativity; Virtual Particles; Zero-Time Tunneling. Up to this very day, zero-time tunneling has been vehe- mently biased against theoretical predictions since 1931 and against experimental results since 1992. Many review articles on tunneling time have been published – see, e.g. Refs. [1–5]. However, they present no quantitative results that might help interpret the experimental data. The second of the four cited reviews above deals with the arrival time in quantum mechanics [2]. This review represents a painstaking theoretical analysis of the time measurements in quantum mechanics. However, it does not explain the observed tunneling time of signals in tel- ecommunication systems for instance, or in a photonic set up of frustrated internal reflection. The latter represents the quantum mechanical tunneling analog. One of the recent review articles introduced a novel tunneling time approach [3]. Its author concluded that a tunneling barrier is a resonator. This strange model was fre- quently published by the same author in leading physics journals such as Physical Review Letters [4, 5]. We were surprised by the referees’ recommendations, as the author has shown and pointed out that, according to his resonator model, a signal is not causally transmitted: the back and front of the signals are interchanged by transmission. Obvi- ously, author and referees were not aware that the tun- neling process with a causal signal transmission has been applied in telecommunication systems for many decades; for instance, as a coupling device see Figure 1. In addition, a resonator stores real positive energy. The tunneling evanes- cent mode has a negative energy that cannot be measured. The fifth cited review article deals essentially with the electron tunneling time of the ionized atoms in strong laser fields. The latter fields reduce the Coulomb potential so that electrons can tunnel. Many of the known theories have been analyzed with respect to laser-induced tun- neling [7] but no final recipe has been found. However, these sophisticated experiments do permit the estimation of tunneling times. It was concluded that the time delay in the barrier is zero, though none of the theoretical work on zero-time tunneling has been mentioned [7, 8]. The first calculations resulting in no appreciable delay in tunneling were performed by Condon [9] and MacColl [10]; a quantitative one was carried out by Hartman in 1962, and another much later zero-time tunneling calculation was presented by Low and Mende in 1991 [9–12]. The quantita- tive calculations of Hartman are based on the Schrödinger equation and the Wigner phase time. The measured short tunneling time originates at the barrier entrance boundary. This scattering time represents the transmission time as well as the reflection time of an incident signal. The experiments have shown that barriers are traversed in zero-time in agreement with the Hartman’s quantum mechanical calculations (Hartman was inspired by metal- insulator-metal tunneling experiments. Esaki invented the tunneling diode at the same time). Hartman sent one-dimensional Gaussian wave packets through a rectangular tunnel barrier and calcu- lated the tunneling time with respect to Wigner’s phase time. The results are presented in Figure 2. The data show no additional time loss after entering the length regime of opaque barriers, i.e. if the exponent of the transmission probability (aε) ≥ 1 holds. This result is called the Hartman effect today. The Hartman approach was judged to repre- sent the best one by the critical analysis of Refs. [15, 16]. Based on the analogy of the Helmholtz and Schrödinger equations, tunneling experiments were first *Corresponding author: Günter Nimtz, II. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany, E-mail: g.nimtz@uni-koeln.de Horst Aichmann: KWF, R & D, Zur Bitz 1; 61231 Bad Nauheim, Germany, E-mail: h-aichmann@t-online.de