From David Deutsch's The Beginning of Infinity:
Take a powerful computer and set each bit randomly to 0 or 1 using a quantum randomizer. (That means that 0 and 1 occur in histories of equal measure.) At that point all possible contents of the computer’s memory exist in the multiverse. So there are necessarily histories present in which the computer contains an AI program – indeed, all possible AI programs in all possible states, up to the size that the computer’s memory can hold. Some of them are fairly accurate representations of you, living in a virtual-reality environment crudely resembling your actual environment. (Present-day computers do not have enough memory to simulate a realistic environment accurately, but, as I said in Chapter 7, I am sure that they have more than enough to simulate a person.) There are also people in every possible state of suffering. So my question is: is it wrong to switch the computer on, setting it executing all those programs simultaneously in different histories? Is it, in fact, the worst crime ever committed? Or is it merely inadvisable, because the combined measure of all the histories containing suffering is very tiny? Or is it innocent and trivial?
I'm not so sure we have the computing power to "simulate a person," but suppose we did. (Perhaps we will soon.) How would you respond to this worry?
That depends whether you think that you are increasing the probability of a Boltzmann brain coming into existence by a greater factor when you stir your coffee, in comparison to when the memory of the computer is randomized.
I'm sure the coffee contains enough atoms for a Boltzmann brain to form. However, the entropy of the coffee is already high from your point of view before you stir the coffee, i.e. the probability of the coffee containing a Boltzmann brain is changed little by your stirring it.
It also depends to some extent on the size of the computer memory in question. We can infer that it is vast, since it is capable of simulating a human. However, is it just sufficient to do that or much bigger still?