Slashdot Mirror
More Strange Bose-Einstein Condensate Behavior
Allen Varney writes "According to a story on EurekAlert, an arXiv preprint server paper titled 'Scattering of atoms on a Bose-Einstein Condensate' reports that atoms striking a BEC sometimes appears to leave before they enter. 'This doesn't imply a breaking of the light-speed barrier, time travel or anything overly exotic but is a property of waves being broken down into component parts and being reassembled slightly differently. [...] As an atom hits the BEC, it is absorbed into the collective state but still exists as a vibration. The vibration travels through the BEC but can escape as an atom once more. The study reinforces the similarity between atoms as waves and light as waves.' Slashdot has talked about supposed faster-than-light travel once or twice (or more) before."
Stephen Hawking explains these concepts marvellously in his book, A Brief History of Time. It's an easy read, but also very informative.
On reading the article it seems the clue lies in the group velocity. Is it then possible that speed of tunneling can exceed that of the group velocity?
Looking closely it is not given that is a limit, though I would expect the speed of the tunnelig "transport" would be lower than that of light in vacuum, c\sub{0}.
Is it really so simple as just making sure the group velocity is slow? Of course wave velocity cannot end up faster than the group, or the wave exceeeds the signal...
I didn't understand 90% of the maths, and I suspect it would be impossible to do so without a maths or physics degree...but I found this section in plain english explains quite clearly what exactly is meant by particles leaving before they arrive:
At low k values, we observe that t becomes negative over a rather wide range. For wavepackets with momentum components mainly in this range, a peak in the transmitted wavepacket can appear before the peak of the incident wavepacket has reached the condensate. This is confirmrmed by wavepacket simulations.
So it's about the peaks of waves rather than actual particles, which makes more sense.
Second, there's no "magic" in it. As they say in the article, the peak of the transmitted wavepacket appears before the peak of the incident wavepacket has reached the condensate.
At the beginning of the century (1914), Brillouin and Sommerfeld already showed that, when a plane EM wave with a sharp forward front propagating in vacuum is incident upon a transparent medium, its shape is changed and precursor waves form, with a velocity approaching c in vacuum, corresponding to the high-frequency components for which the (relative) permittivity goes to 1.
In excitable media (and I assume the same happens with atoms in a BEC) the effect is even more spectacular, because these fast components (or, as in this case, the leading edge of the pulse) can get amplified and then leave the medium before the "bulk" of the incoming pulse even enters it.
Moreover, before leaving the medium this "fast" pulse is split in two, and the reflected component can interfere destructively with the "lazy" pulse, wiping it out. Hence the "illusion". Needless to say, Einstein is still right :-)