[lisper]

This story has been all over the news in the last 24 hours, but it's sending my bogometer well into the yellow because Faraday cages are a simple and effective defense against any electromagnetic interference. (That's why you can safely peer into your microwave oven while it's pouting 1500 watts into your leftover lasagna.) So this feels like a Saddam-has-WMDs-style disinformation campaign to me. It also has a whiff of desperation to it since we really so seem to be pretty helpless to stop NK from developing the ability to deliver nukes to the continental U.S. without more collateral damage to Seoul than the American public is likely willing to accept (to say nothing of the residents of Seoul!)

I fear that the ethos of tell-a-lie-often-enought-and-it-becomes-the-truth has permeated very deeply into the U.S. government. That approach often works in politics. In physics, not so much.

[leggomylibro]

I love the explanation I got from my high school physics teacher for why microwaves aren't dangerous to be near; anyone comment on whether it's actually accurate? Or whether I may be misremembering it?

'You can look into the microwave safely because the holes in the grate are smaller than the amplitude and thicker than the frequency of the microwave.'

[whatshisface]

That can't be right because the amplitude of EM waves has nothing to do with spatial displacements (unlike guitar strings). The way it works is that every point in space has an E (electric) and B (magnetic) field vector which can oscillate around a 3D space. The vectors point towards other places in space, but their meaning is a field strength and direction at a single point. So this diagram[1] is actually describing the EM fields along a single line, not in the volume of space that the (purely illustrative) arrows are drawn in.

The correct understanding of Faraday cages is that their holes must be small relative to the wavelength: this is easy to remember because on a very large wavelength scale you would expect the tiny holes to not be very noticeable, and on a very short wavelength scale you're essentially talking about the light that you can see your food with.

https://en.wikipedia.org/wiki/Light#/media/File:Light-wave.s...

[leggomylibro]

Huh, interesting; thanks. The more you know...

...the more likely you are to misremember things and mislead people, apparently. Leadership, hooo!

[lisper]

You've got it backwards: the holes are smaller than the wavelength of the microwaves, and the material is thick enough that the eddy currents induced by the microwaves don't generate enough heat to melt the grating.

[schemathings]

Was curious .. google turned up this http://rabi.phys.virginia.edu/1060/1999/final_answers.html

A satellite dish is actually a mirror telescope for microwaves, with its receiving antenna located at the real image of the distant satellite. But unlike the smooth, shiny mirror of a light telescope, a typical satellite dish is made of dull metallic mesh. Microwaves reflect almost perfectly from this mesh because

Answer: (A) its holes are much smaller than the wavelength of the microwaves.

[wrs]

Not sure what "thicker than the frequency" means, but it's because they're much smaller than the wavelength, which is a few inches, as you can demonstrate with some marshmallows. (Google "measure speed of light with microwave oven".)