The universal doomsday argument still relies on SSA, because under SIA, I’m equally surprised to exist as an early person whether most civilizations are short or long. If most civilizations are long, I’m surprised to be early. If most civilizations are short, I’m surprised to exist at all. I could have been any of the late people who failed to exist because most civilizations are short. In other words, the surprise of existing as an early person is equivalent in both cases under SIA, so there’s no update. Only under SSA am I certain I will exist but unsure where in the universe I will be.
I think that SIA is generally* valid but it uses all its power to prove that I live in the infinite universe where all possible observers exist. After that we have to use SSA to find in which region of the multiverse I am more likely to be located.
*I think that the logically sound version of SIA is “if I am in a unique position, generated by some random process, then there were many attempts to create me” – like many earth-like-but-lifeless planets are in the galaxy.
Another point is that the larger number of short civilizations can compensate for their “shortness.” We can live in the region of multiverse where there are many short civilizations and almost all of them die off.
The universal doomsday argument still relies on SSA, because under SIA, I’m equally surprised to exist as an early person whether most civilizations are short or long. If most civilizations are long, I’m surprised to be early. If most civilizations are short, I’m surprised to exist at all. I could have been any of the late people who failed to exist because most civilizations are short. In other words, the surprise of existing as an early person is equivalent in both cases under SIA, so there’s no update. Only under SSA am I certain I will exist but unsure where in the universe I will be.
I think that SIA is generally* valid but it uses all its power to prove that I live in the infinite universe where all possible observers exist. After that we have to use SSA to find in which region of the multiverse I am more likely to be located.
*I think that the logically sound version of SIA is “if I am in a unique position, generated by some random process, then there were many attempts to create me” – like many earth-like-but-lifeless planets are in the galaxy.
Another point is that the larger number of short civilizations can compensate for their “shortness.” We can live in the region of multiverse where there are many short civilizations and almost all of them die off.