I just saw a TV document about counterfeited paintings and discovering if they are original or not. They used all kinds of funny stuff like some proton cannon to measure the x-rays sent back and electron microscope to see the paint particles...

Now my question is this: if the fake painting looks just like the original, no one could tell the difference if not *very* conversant with counterfeits, why isn't it as valuable as the original? Sure the original author author would lose money but what if s/he had been dead for 500 years already. Then that problem doesn't exist.

We shall assume that this painting is not like Mona Lisa which is safely where it is and if another one was brought up it could be judged as a counterfeit without even looking at it.
A long lost or a totally new ([claimed to be] found from someone's attic) painting from some known author would be a case we are talking about.

Is it just the name that costs, the subject, the paints, the way it's made with (brush draws etc.), what? Why?