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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."
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.
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