Daily Archives: February 1, 2009

Going Into Overtime

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Entangled Particles Face Sudden Death

[I]n a paper published today in the journal Science, two physicists show that entangled particles can suddenly and irrevocably lose their connection, a phenomenon called Entanglement Sudden Death, or ESD.

“The degree of information entangled can disappear faster than the information itself,” said Joseph Eberly, a physicist at the University of Rochester, who, along with Ting Yu, co-authored the paper. “It’s completely non-classical physics.”

I don’t do experiments with entangled particles and I haven’t read the paper yet, but I was a little surprised to read that the model up to this point had been that entanglement was lost slowly. I had always gotten the impression that entanglement was much more a binary condition, so you wouldn’t describe particles as being a little bit entangled, any more than you would say someone was a little pregnant. I suspect this has to be tied to the question of how fast a wave function collapses.

Science 30 January 2009:
Vol. 323. no. 5914, pp. 598 – 601
DOI: 10.1126/science.1167343

I'm Leaving, On a Jetpack

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Real Water Rocket Guy – Analysis to come

[S]eems like something that would be on Swans on Tea

Well, now it is

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Rhett now has a detailed analysis posted in which he explains how the thrust is generated, and estimates the power the pump must have to do this.

One thing to note about this is that it works because much of the propellant is not being carried onboard. The rocket is lifting the water in the feeder hose, but that’s all — after a few seconds, that water has been expelled. But the jetpack continues to fly, because additional water is being supplied. This is one of the big problems for rockets. They need to carry enough propellant to lift the payload, and all of the propellant and fuel (for some systems, e.g. ion drives, the propellant and the energy source aren’t the same thing). This is why rockets are really inefficient, and have such a small ratio of payload/rocket mass.