[diminoten]

Yes, the brain is made up of atoms, but our modern physics does not, at least not with the degree of certainty you're waving about, tell us that it is therefore, necessarily, simulatable.

I believe this to be the case, and there is some evidence that this is the case, but it is not anywhere near as certain as you're claiming.

As for your word choice re: impractical, the word shouldn't be used in place of 'impossible', which is the correct word you're looking for.