> X-ray microtomagraphy

Neat! But, can it work in principle to pick out the structure and arrangement and composition of molecules? (I'd guess yes, but am not sure).

> We're talking about interstellar commerce here

Interstellar distances are ~ pc distances; the GWs we are looking for with LIGO are >> Mpc or << pc with GRAIL/GOCE/GRACE and their follow-ons. These observatories are sensitive to sources at radically different distances, but since such sources are not what's under study (and in particular it is hard to correlate their GW signals with other observables), their signals are excluded.

One could build a GW observatory to study an exoplanet in pretty fine detail (down to whatever the appropriate word for its seismology would be); there are a lot of unknowns about exoplanets, but their mass quadrupoles are determinable independently, and GW observatories essentially just look for changes in mass quadrupoles. What's responsible for any given change is the hard part, after one has built the (distributed) observatory.

GRAIL II would have done exactly that for the moon and (it was hoped) for a couple of other objects in the solar system. GRAIL itself was an approximation of this broad technique for the moon.

> Seismic events are many, many orders of magnitude more energetic than a painting being moved around

Sure, amplitude-wise. They have very distinct frequencies though. The argument you want to advance is that there are lots of similar objects (8kg rectangles, roughly) that generate a lot of noise, and worse, some of them are in the same art gallery (and with similar orientation) as the subject of observation. Worst of all, picking it out if it never leaves the Louvre wall is implausibly difficult; but if it's moved around the gallery, and in particular, it's back does not always face the gallery wall it normally hangs on, that opens a lot of "technology will improve" arguments, such as you use with the "perfect copier".

> all it would take to foil it would be for the copy to move close to the original at any point in time

The emissions from a bump on a rotating sphere is strongly determined by the bump's position. I happily admit that you can contrive ever-greater screening, though, but the cat would be out of the bag that you were deliberately trying to frustrate distinguishing between the original and the copy.

Which rather raises the question, why? Why not just hang the copy and ship the original to the aliens as agreed?

> no sensible being could possibly care ... if they have to resort to such extreme measures

I think you have the ordering wrong; they might resort to such extreme measures because they care. Their valuation function might not be identical to yours (why the Mona Lisa rather than any other Earth painting? or any painting they could make locally, in the style of an arbitrary Earthling?).

Or, more pointedly, maybe when 'all you have to do is to try to define what "the same thing" actually means', you, other people, and aliens' efforts might lead to different definitions.

> when 'all you have to do is to try to define what "the same thing" actually means', you, other people, and aliens' efforts might lead to different definitions.

Well, yeah, they might. But I'm assuming the aliens understand physics, in which case only one definition makes sense. You can snooker the irrational and the ignorant into paying for anything regardless of what species they are.

On which note, I have this nifty bridge you might be interested in? It's available at a great price. And it is the original!

Incidentally, we seem to be discussing the plot of _City of Death_, Douglas Adams's Doctor Who story. :D