Hal Puthoff reveals they studied a physical sample
( This appears to be the same sample sent into Art Bell and analysised by Linda Moulton Howe.)
The story behind that sample with LMH is on you tube

It was a multilayered bismuth and magnesium sample. Bismuth layers less than a human hair. Magnesium samples about ten-times the size of a human hair. Supposedly picked up in the crash retrieval of an Advanced Aerospace Vehicle. It looks like itís been in a crash. The white lines are the bismuth; the darker areas are the magnesium separations. So the question was what about this material, so naturally we looked in all the national labs, we talked to metallurgists, we combed the entire structure of published papers. Nowhere could we find any evidence that anybody ever made one of these.

Secondly, some attempts were made to try to reproduce this material, but they couldnít get the bismuth and magnesium layers to bond.

Thirdly, when we talked to people in the materials field who should know, they said we donít know why anybody would want to make anything like this. Itís not obvious that it has any function.

Well, years later, decades later actually, finally our own science moves along. We move into an area called metamaterials, and it turns out exactly this combination of materials at exactly those dimensions turn out to be an excellent microscopic waveguide for very high frequency electromagnetic radiation terahertz frequencies. So, the wavelength is 60 microns, which is a pretty small size. But it turns out because of the metamaterial aspect of this material, those bismuth layers that act as waveguides can be one twentieth the size of the wavelength, and usually when you make a waveguide itís gotta be about the size of the wavelength. So, in fact this turned out to be a material that would propagate sub-wavelength waveguide effects. Why somebody wants to do that we still donít know the answer to that.

But anyway, itís amazing weíve gone through this and this is the kind of structure we go through a lot. You get a material sample with unusual characteristics to be evaluated, the method of manufacture is difficult to assess or reproduce, the purpose of the function is not readily apparent Ė as with our sample here, and then as our own technical knowledge moves forward we finally see a possible purpose or function comes to light. That sequence is repeated over and over in this particular area.

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