[AnonNo15]

Regarding you second point - you don't need to simulate the Universe at 1:1 scale to reliably replicate the past states of a certain region. For example, Earth itself can be simulated at full resolution only within a thin shell centered on it's surface, say +/-10 km from geoid. There are no observers at the core, so a simplified model would do, as long as plate tectonics is reproduced accurately enough.

[andyjohnson0]

A short story by Stephen Baxter, Touching Centauri, touches on this issue of model resulution and observers. [Spoilers ahead]

The protagonists use a powerful laser to bounce a pulse of energy off an exoplanet. This forces the entities who (it emerges) are simulating the universe (or maybe just our local portion of it) to massively increase the accuracy and extent of the simulation, so that the returned pulse behaves as physically expected. Previously they had been simulating the remote star system in a low resolution way due to the lack of observers there. The sudden need to increase the simulation's extent and resolution overwhelms their computational resources and the simulation crashes. A resource exhaustion attack on "reality", in other words.

[simonh]

That doesn't work though. Even a game of billiards is so complex and so dependent on external influences that a change in state in an electron at the edge of the observable universe can have an observable effect on the outcome.

http://www.anecdote.com/2007/10/the-billiard-ball-example/

[AnonNo15]

It doesn't prevent simulating A universe. You can then compute a difference between the simulated past state and real past state based on (partial) information about the past state.

Then you go back and iterate again, eventually improving the accuracy of simulation until there is no distinguishable difference between simulated state and real past state.

The simulation might end up being not a perfect copy, but it would reach the point where it will be impossible to find any discrepancies.

[simonh]

If you have a way to find discrepancies between a simulation and reality, why do you need the simulation anymore?

I don't buy it. Even simulating just the earth and its environs would probably require a computer as big as a galaxy worth of matter. There's just no conceivable payback that could ever be worth the costs.

[AnonNo15]

You have to actually compute the simulated state and compare it to known bits of the past state to get the results.

Yes, simulating Earth to sufficient precision might require resources of the Type III civilization. However, if something can be done, why shouldn't it be done?

The payback is twofold, actually:

First, you can get provide the meaningful afterlife to your ancestors.

Secondly, once you have hardware and software to simulate a virtual universe, you can create a multitude of them, with different properties and qualities. Think of the same logic behind splitting the physical computer into virtual machines.

[simonh]

I think a Type III civilization will have better things to do that precisely simulating me taking a crap every day. To me this argument smells the same as simplistic suggested solutions to real world problems. 'Why don't we just do X', such as why not just take all the excess food in the world and feed it to the starving people in Africa? Wow, how simple. Why did nobody think of that. Why wouldn't people in the future just simulate all of reality thousands of times? But easy to say or imagine != easy to do. I have severe doubts if even a Type II civilization can ever be viable.