I think the condition from the beginning is that you’re picking a unique girlfriend who knows all microphysical facts about your universe, including the content of any letters you have or will ever write.
Quantum mechanics forbids that much knowledge. If you take the present state and apply the advanced Green function to it, you don’t end up with one distinct letter written. Similarly, if you take a future state where you wrote one particular set of letters and apply the retarded Green function to it, you don’t get the present state. So she isn’t your girlfriend yet… but she will be once you write or receive your last letter from her.
I think the condition from the beginning is that you’re picking a unique girlfriend who knows all microphysical facts about your universe, including the content of any letters you have or will ever write.
Quantum mechanics forbids that much knowledge. If you take the present state and apply the advanced Green function to it, you don’t end up with one distinct letter written. Similarly, if you take a future state where you wrote one particular set of letters and apply the retarded Green function to it, you don’t get the present state. So she isn’t your girlfriend yet… but she will be once you write or receive your last letter from her.
Doesn’t that just mean you’re picking one who’s simulating this universe?