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Jay V van Zyl
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The anthropomorphic personification of the em dash walked into a bar...
The anthropomorphic personification of the em dash walked into a bar...

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Faraday cages are one of the most basic tools in electrical anything. They're based on the principle that if you place a hollow, conductive container inside an electromagnetic field, then no matter what that field is on the outside, the container shields it: inside the container, the field is zero. These also work in reverse: if you put a field source inside a conductive container, the container will prevent that field from getting out.

This is pretty useful if you want to do something that could produce dangerous fields, like use microwaves to heat food. By wrapping it in a Faraday cage, you make sure that the resulting fields don't also heat everything in their vicinity.

Now, most Faraday cages aren't solid, conductive containers; it's been known for a long time that a wire mesh works just as well. Except it turns out that it doesn't.

Faraday invented the cage in 1836. From then until roughly the 1940's, the correct functioning of mesh cages has been a combination of lore and practical engineering: if you really care that your cage works (like in a microwave oven), you build it and measure what happens. The theory of them was worked out by Feynman in the 1940's – except it turns out that Feynman simply did it wrong. (In particular, he looked at wire meshes with constant charge on them, not constant voltage; the math was right, it simply solved the wrong problem)

According to Feynman's solution, what matters for a working Faraday cage is the proximity of the wires. Roughly, the depth into the cage at which it provides the needed field suppression drops exponentially as the wires move closer together. It turns out this isn't right: fields decay only linearly with wire spacing. What really matters is the thickness of the wires: the suppression does scale exponentially with that.

In practice, this explains a lot of open mysteries, like why your cell phone works inside an elevator, but not inside an underground parking garage. Elevators, under the old theory, should have been pretty good Faraday cages; how do the radio signals, which are just EM fields, get out? It turns out they aren't very good Faraday cages at all. Likewise, garages don't tend to have deliberate EM shielding on them, but they do have lots of rebar, and windows which are often grated. Put those together and the new theory tells you that you have a great Faraday cage.

Also in practice, this team now has a good method for calculating how Faraday cages will actually work ahead of time. It's not rocket science; it's simply solving the differential equations of electrodynamics for a cage. You can see some of the results in pictures below, where the density of lines indicates the field strength. (In all of those pictures, an EM source is to the right of the cage)

The moral of this story: if everyone assumes that there's a good theory for something, but nobody can actually find it worked out in detail, there's a good chance that there actually isn't one.

Via +rone 

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I fell in love with a couple of people in April/May. ;-) (Including total strangers I saw only once.)

(Via J on FB, do you prefer attribution or privacy? ;)

#fb  

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Dearest friends

Here is a comprehensive list of email sign-offs and what they mean.

Hugsies
Jay

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Added to bucket list.
A Phoenix Aurora over Iceland
Image Credit & Copyright: Hallgrimur P. Helgason; Rollover Annotation: Judy Schmidt
http://apod.nasa.gov/apod/ap160316.html

All of the other aurora watchers had gone home. By 3:30 am in Iceland, on a quiet night last September, much of that night's auroras had died down. Suddenly though, a new burst of particles streamed down from space, lighting up the Earth's atmosphere once again. This time, unexpectedly, pareidoliacally, they created an amazing shape reminiscent of a giant phoenix. With camera equipment at the ready, two quick sky images were taken, followed immediately by a third of the land. The mountain in the background is Helgafell, while the small foreground river is called Kaldá, both located about 30 kilometers north of Iceland's capital Reykjavik. Seasoned skywatchers will note that just above the mountain, toward the left, is the constellation of Orion, while the Pleiades star cluster is also visible just above the frame center. The new aurora lasted only a minute and would be gone forever -- possibly dismissed as an embellished aberration -- were it not captured in the featured, digitally-composed, image mosaic.
Photo

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Turms out that DeepMind is only human, after all.

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"Machines have conquered the last of the games. Now comes the real world."

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"The predicted warming rate in 1990 ranged from 1.9°C per century to 4.2°C per century. The observed warming rate, however, peaked at 1.75°C per century in 1975. Since 1985, it has been on a steep decline. Note that this does not mean the world isn’t warming. We’re talking about how fast it is happening. Over the last few decades, global warming has been real, but it has been slowing down dramatically."

After millennia of rangebound temperatures on earth, despite some serious volcanic activity, numerous meteorite strikes and badarse solar storms, some humans still have enough ego to think that the current, slowing blip of an upcycle is "our fault" and even the "most pressing problem facing humanity". Even if warming was serious and human-caused, I still rank alleviating poverty and eradicating disease as WAY more important than taking on the impossible task of controlling global climate.

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This is HUGE, y'all. Up to now, reasonably good amateur players could kick any computer's arse at Go. Beating a 9d is every AI's wet dream, and DeepMind has done it twice in a row now. You can bet that the foxy machine's grandkids are going to be hearing about this for years.

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"The Fukushima meltdown is not evidence of how dangerous nuclear reactors are. It was a 40-year-old reactor, badly operated by a monopoly that cut regulatory corners, and was struck by two concurrent natural disasters, an earthquake and a tsunami, both of which exceeded its design tolerance. That it caused no deaths and very limited environmental radiation exposure is a testament to the safety even of old, second-generation reactor designs. Modern third-generation reactors would be even safer."


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