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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."
I think Bose-Einstein condensates are used in the making and / or broadcasting of sitcoms and movies. You're watching a movie or sitcom, and you already know what's going to happen. My guess is that you've already seen it just slightly before.
... would a particle have been emitted anyway?
Ie: Is there enough information in the 'onset' part of the wave to cause the reconstruction of a particle at the other end, similar to the other 'faster than light' story? I'm betting there is.
While I'm ranting: Why does the dot keep posting stories about obviously-misinterpreted science news while ignoring *serious* news like the cure for 1/3 of cancers in mice from a week ago?
.f00Dave
One minute you're striking a BEC, the next minute 10 seconds have gone by.
I am a Karma Library.
Stephen Hawking explains these concepts marvellously in his book, A Brief History of Time. It's an easy read, but also very informative.
I have discovered a truly wonderful explanation of this phenomenon, which unfortunately the lameness filter will not let me post!
The first time I leanerd quantum mechanics, I didn't understant it.
The second time I learned quantum mechanics, I thought I understood it.
The third time I learned quantum mechanics, I knew I didn't understand it.
Lord, bless my users that they may stop being such fucking idiots!!
I remember that something similarly shocking was published back then in 1948.
;-)
The article in question was:
"The Endochronic Properties of Resublimated Thiotimoline"
Let's give some credit to pioneers!
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 :-)
I haven't got the first clue about "the scattering properties of a Bose-Einstein condensate held in a finite depth well", and I doubt many people here have.
/.
To prove my point, most of the mod'ed up comments here have been mod'ed as 'Funny', rather than 'Interesting' or 'Informative'.
Seems symptomatic of most hard science posts on
That's it. My post-pub waffle is over.
They will never know the simple pleasure of a monkey knife fight
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