Non Chronological Simulation

It is quite possible - at least in theory - to write a simulation which calculates the simulates not in chronological order. Simulation here means that in the end possible histories of the simulated world are derived. The program not necessarily must track events in the simulation in the order of their occurrence in the simulation world.

Aye, you can simulate histories almost as easily as futures so long as you have a clear mechanism to select a legal past state from a present condition. E.g. if you were playing chess backwards, you could move almost everything but pawns in the same way you normally can, pawns could only move backwards, and pieces starting in the graveyard could be introduced after moving an enemy piece away (though limits like having only one white bishop and one black bishop would remain). The simulation becomes considerably more complicated if you need to approach a particular past (e.g. if you need to play chess backwards until all the pieces are in their original configuration), but that's the same sort of issue you encounter if you're attempting to run a regular simulation to a particular future.

The problem below presents a different challenge: simulations with causal loops, where causes in the future introduce effects in the past and vice versa. This would be the same as simulating time travel, where you need to deal with kill-your-own-grandfather problems, or even just sending a letter into the past that would affect the future in such a way as the letter wouldn't have ever been sent.

From SimulationRelationshipToParent:

Consider 'time'. Time as we understand it in our universe is literally a partial-ordering of observations (which is why time is relative), and we keep 'accurate' time by focusing on 'regular' observations (daily cycle of sunrise/sunset, moon going round sun, hands going round clock, vibrations of a cesium atom under pressure of a laser beam, etc.) So, what would it take to make time move backwards in a software simulation? We could take a simulation that has moved forward and attempt to 'rewind' it, but that would be our experience, not that of the agent within the simulation (who has, at the end, no experience of time going backwards). Time going backwards would, for the agent, would require the agent be able to observe and react to events and observations that haven't happened yet, resulting in effects that happen prior to their cause. To make this happen in the simulation, we'd need to deliver the effects before cause to the agent... which would leak into our universe as the ability to determine and observe the effects before the cause (i.e. the ability to receive a message before it has been sent). So, again, this is no-go.

The interesting thing is that normal story-telling effectively means mentally performing NonChronologicalSimulation - albeit on a very coarse scale. If you could program a constraint solver for the above example to first 'tell the rough story', the fitting in the details may be much easier than brute force trying all pasts/futures.

Simulations follow rules to get from a present state to a future state... story tellers aren't so limited. "Telling a rough story" might be a way to write the constraints, though, but simulating towards a particular end isn't nearly as trivial an exercise as simulating towards an open end.

Now the question is: could some sentient life come to reality into such a simulation? If so, and if all data within my computer is encrypted, that means that any arbitrary noise is a universe containing sentient life, if only you had the decryption key... In other words, our reality is only one of the infinite interpretations that could be made out of the "universe noise", other could exist as well (including realities where the universe is infinite in time and space). This is the plot of PermutationCity (GregEgan) -- PhilippeDetournay

Formally defining sentience has proven non-trivial - a lot of people aim such definitions at attempting to ensure humans are the only sentient animal. Definitions for it have included a range of questions such as: can it recognize itself in a mirror? is it capable of subjective thought? can it suffer? The only thing universally recognized is that most humans are sentient... because we defined ourselves to be so. For any given formal definition of sentience, there is little fundamental reason that you couldn't create a software entity capable of meeting it. Some people might consider it impossible for an electronic being to really 'suffer', but even our own neurobiology implies that sufferance is simply stimulation of a particular center of the brain that discomforts us and thus we aim to avoid.

Further complicating things, "sentience" may prove to be continuous. The debate may in some ways resemble DefinitionOfLife.

See also SimulationArgument

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