Wednesday, 8 August 2007

Light faster than light

Browsing through arXiv repository of preprints I have found a short paper Macroscopic violation of special relativity by
Authors: G. Nimtz, A. A. Stahlhofen.

At first I was surprised by the bold statements of the authors:
We demonstrate the quantum mechanical behavior of evanescent modes with digital microwave signals at a macroscopic scale of the order of a meter and show that evanescent modes are well described by virtual photons as predicted by former QED calculations.

Several QED and QM calculations predicted that both evanescent modes and tunneling particles appear to propagate in zero time.

All three properties - the violation of the Einstein energy relation, the zero time spreading, and the non observability of evanescent modes - can be explained by identifying evanescent modes with virtual photons as predicted by several authors, see for instance references. Tunneling and evanescent modes are properly described by quantum mechanics

The (unpublished) paper's form is `unacademic' (i.e. not TeX-ed...) and at first I had doubts as to the credentials of the experiment. Was it another out-of-nowhere Einstein basher? But further search has showed that this is a paper from a long series of publications involving light and microwave signal processing in unusual setups. For references it suffices to search arXiv for au:Nimtz_G

Signaling faster than light speed? Not only possible but observed?
In the light of a recent discussion with a friend (also on this blog...) how would one describe it in a quantum way? or photons as billiard balls way?


Xawer said...

Sceptical remarks by ex-experimentalist:
1. As I understand the paper, it just shows, proves, or just give some good arguments towards recognizing classical evanescent waves as virtual photons in QED view. Interesting. But the title of the paper seems to be added by redactor used to work for tabloids.
2. It proves that evanescent waves can be observed on macroscopic distances if you use microwaves. Nothing surprising. Identical experiment is described in The Feynman Lectures on Physics (vol.II chapter 33). 20 years ago it was demonstrated for students during introductory course of physics at Warsaw University. Actually it was my hard work (as prof. A.K.Wróblewski's assistant) to set it up…
3. The paper claims that packet delay had been measured. I can’t see any picture showing how the packed delay depends on the gap width, there is no discussion of possible experimental errors, no clue what was the maximum gap width they used, no information about absolute end-to-end delay time, nothing more than The measured time delay in both reflection and transmission of the digital pulse is about 100 ps.
4. Authors say nothing about the shape of wave packet they use. It would be quite interesting, but probably it would also make it obvious, that they couldn’t measure delays with accuracy authorizing them to use words like about 100 ps. From uncertainity principle, you can see that in order to measure time with precision of 100 ps the energy of photons must be uncertain by more than 10GHz*h. As microwaves of 9.15GHz were used, band spectrum had to be wider than 5-15GHz.
5. And finally – philosophical remark. Virtual photons, like in evanescent modes, travel with infinite speed. I haven’t been aware of this implication of QED. Interesting point. But it hasn’t spoiled (yet) my good feeling about consistency of the Nature and Physics. It doesn’t mean that the information can be transmitted faster than c. Any information must be coded as wave packet of some minimal size – several λ. So we need several λ/c of time to send the information, and similarly – several λ/c to receive it. Then signal must go through the thick prisms – another 10λ or so to be travelled at the speed c/n (n=1.6) – so signal is delayed by yet another several λ/c. Prisms must be thick comparing to λ to see the FTIR effect. But the maximum distance we can see FTIR is no more than several λ. So the signal is inevitably delayed more than we could kick it into future. Well – this is something I believe - as I haven’t made the calculations and I am not even quite sure if I am able to make them…

Wanderer in the country of blindfolded said...

I agree that the title is tabloid type. And indeed this is what has caught my attention.

One of the reasons of the post is not its physics (Which is interesting but to properly judge it one needs indeed to go into details mentioned by xawer) but to show how sometimes the way physics is presented. It is quite interesting that the boundary between real research and cranks may be difficult to discern.

For those who - as xawer - would like to get the answers to look through the references to earlier and more detailed work of the authors, these works are much more detailed.