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."
"sometimes appears to leave before they enter"
Does this mean all those magicians are correct when they say the hand is tuly quicker than the eye ?
Seriously are they even close to certain that their detection methods are accurate or is this a side effect of the enviroment on the detection equiptment. Wouldnt be the first time.
Sig went tro...aahemmm.....fishing........
Just make sure it isn't heavy on math...
I disagree. Try to get a book that is really, really heavy on math. Because, in my experience, the only thing you might understand in quantum mechanics is the math. Doesn't matter if you're a fourth-grader or a physics professor.
Books that try to explain that stuff without giving the mathematical background tend to only give you that nasty feeling of believe.
Having read the paper (forgive me, I'm new here), I'd say the negative time effect is very similar to the FTL transmission results reported earlier. On page 6 of the paper, the caption to Figure 6 reads in part: Note the negative values of tau-sub e (the time spent inside the condensate region) in the region around ka ~/2: Wavepacket simulations show that here the peak of the transmitted wavepacket appears before the peak of the incident packet reaches the condensate. Now, IANA condensed matter physicist, but my best assessment of the effect is that it operates similarly to previously observed, similar phenomena of "FTL" transmission. The incident wave of light contains information for the entire wave embedded in the wavefront- so the peak of the wave is able to be reconstructed on the other side of the BEC, even though the peak itself has not reached the condensate yet. Essentially, since the entire wave of light is defined by a wavefunction, the entire wave can be constructed from the wavefront. Although the process can take zero or even negative time, it is not a violation of general relativity. The light itself propagates at the speed of light, as it must. Since the peak of the wavepacket is recreated before it is actually destroyed, it would seem to be moving faster than the speed of light. However, the information that completely describes this peak is embedded into the wavefront, which travels at the rather pedestrian speed of light (and for a BEC, it is almost pedestrian- the amazing dispersive effects have been shown to reduce c to around 38 miles per hour, an effective refractive index of over 17 million!). While something is technically traveling over a nonzero distance in zero or even subzero time, no unique information can be transmitted in this manner- since the peak is constructed from information in the wavefront, the peak must be composed of information contained in the wavefront. The wavefront is moving at the speed of light, and taking all of the information it has with it at exactly that speed. If we had an ultrafast and ultrasmall computer conceivably, we could dispense with the Bose-Einstein condensate and do this thing ourselves. The wavefront enters a detector, the information is broken down, and the hypothetical supercomputer we have at our disposal uses the information in the wavefront to calculate the wavefunction. It then spits out a wavepacket with characteristics identical to those of the incident packet, and does so before the peak of the incident packet even reaches the detector. The effect is somewhat analogous to the movement of lights on a scrolling theatre marquee. The scrolling itself can actually occur faster than the speed of light, but since the "information" is just a discrete on/off light, no useful FTL message can be encoded. In the same way, the wavefront carries information faster than the speed of light, but the information merely codes for the rest of the wavepacket! Thus, it is not a violation of relativity. If anything, it is an affirmation- the weirdness of quantum mechanics, what with the wave/particle nature of light, is weird in such a way that useful messages cannot be sent faster than the speed of light. On a completely different note, I was amused to see someone referenced in this paper that /.ers might recognize, if they had actually read the paper. On page 6, the authors propose explanations for this effect, and they suggest a many-body interference mechanism devised by Ray Chiao et al. Raymond Chiao, some of you may remember, is the physicist who had a /. story not long ago about the possibility of a gravitational Meisser effect for superconductors (Can Superconductors Block Gravitational Fields?).
"FDA staff reviewers expressed concern about the number of patients who were left out of the study because they died."
This seems deep and mysterious, but it is just a trick. To understand the trick you just need to understand two terms and one concept.
Here is the first term. If you change something, your change will cause changes to propagate outwards. That rate is called the group velocity. This is the rate at which changes propagate, and cannot exceed the speed of light (thanks to Einstein).
Here is the second term. If you sit and watch the waves go by, the peaks of the waves have an apparent motion. That rate of motion is called the phase velocity. The phase velocity is the most easily measured apparent motion.
Here is the concept. After you have been sending a constant stream of waves for a while, the phase and group velocities have nothing to do with each other! In particular this paper just says that the phase velocity can be made negative, that is the waves look like they are moving backwards. Mildly amusing, but commonplace.
If you want to visualize this, draw a 2 vertical lines on a piece of paper. Those lines are light-weight plastic barriers. On either side you have water, and inside you have something else - oil say. Visualize a stream of waves coming from right to left. They hit the first barrier, part bounces, part goes in. They hit the second barrier, most bounces, part goes out. The part that bounces from the second to the first, well most bounces, part goes through. And back and forth we go.
The incoming wave train sets up a resonance in the middle third. Depending on the details of that resonance, the waves in the middle section may move forward, stand still (if you do it just right) or even go backwards. When they go backwards, ohmigosh, the wave is leaving before it goes in, we have waves moving backwards in time!
Amazing, isn't it? And isn't it astounding that when you stop the waves coming in, between the two barriers your waves keep on bouncing back and forth for a while, and most emphatically the stoppage does not arrive on the other side before you stopped?
Cheers,
Ben
I was quite surprised to see our paper on /. Please note, that the mentioning of negative transmission times is aimed at an audience who will not see this as a very spectacular thing. Hopefully fascinating, but not sensational! :) ). Whether this is a good thing for physics is not clear to me: Will it help recruitment of new students? Or will they be turned off and go and do "useful things"?
Concepts like faster-than-light tend to draw a lot of attention from the general public (well, at least part of it