An interesting question to ask is: what is the time that tunneling particles spend inside the barrier? The experimental setup to demonstrate photon tunneling can be modified, as shown in Figure 117, to measure the total tunneling time of the photons, that is tv + th. This strange, two-part timing is due to the nature of frustrated total internal reflection. The photon is not a point particle when not being observed, but has a wave equation that extends out into the gap, so the reflection appears to take place behind the surface of the first prism, resulting in a shift down the surface before reflection, D in Figure 117, called the Goos–Hänchen Shift.
Figure 117 Setup used by Nimtz and Stahlhofen to measure the tunneling time of λ = 33 mm photons. This tunneling time has been shown experimentally to exceed the speed of light.
In an experiment carried out by Günter Nimtz and Alfons A. Stahlhofen at the University of Cologne in 2007, a very small value was found for the tunneling time, resulting in velocities faster than light.37 That is, photons crossing the barrier seem to take less time than they should at light speed. In the actual experiment, using λ = 33-mm microwaves, it was found that both reflected and transmitted beams left their respective prisms at exactly the same time. With the distance d set at 60 mm, the microwaves should have taken 20 ps to cross the gap. However, this time was not detected. The experiment was accurate to ± 5 ps, which shows that tunneling inside the barrier happens in zero time! This would mean that a tunneling particle travels faster than light, something that is forbidden by Einstein’s Theory of Relativity.
The most common interpretation given by physicists is that only the beginning of the information signal (the leading edge of the “group velocity”) moves at speeds faster than the speed of light. However, since the signal itself is smeared through time due to Heisenberg’s Uncertainty Principle, a signal that could convey information actually moves no faster than the speed of light, which means that superluminal (faster than the speed of light) communication does not happen, since the actual signal velocity remains less than the speed of light. In spite of this, the experiment by Nimtz and Stahlhofen is claimed by many to be the one and only observed violation of Einstein’s Special Relativity. As we will see later, communication at speeds faster than light would enable transmitting information to the past, which opens up a whole set of paradoxes that give physicists a headache.
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