Was that the transparent to x-rays story? Only worked for miniscule amounts and it's extremely unstable, if I recall correctly. Sorry, Scotty. :)

That's an interesting brute force method, but not exactly what I was thinking of.

Oh, you were being literal. I'm not sure how your idea would work, since the impact of photons is extremely small compared to that of sound waves.

Yes, but you'd be using a much smaller wavelength, x-ray or higher, need to work out the angle of incidence for internal reflection from the front surface, (Might need very smooth or highly regular structure of Al to even hope to get anything) and you'd need a highly coherent x-ray source and beam splitter... simplistically when you applied the source illumination back in antiphase to what bounced back from the front surface, you'd get some kind of "picture" of what was happening.... as long as the source illumination is bright enough, the thermal noise is low enough, the aluminum structure is uniform enough and the alignment of the planets is favorable.... This sort of thing mayyyy become more practical with advances in making large metal mono-crystals, and engineering a metal specifically to be easy to see through.

But anyway, that wouldn't work to see through the aluminum panels under your car, because any dirt on it would mess things up.

It's the noise low enough part that concerns me.