Reversible Simulations 

Physicist Sabine Hossenfelder is irate that non-physicists use the hypothesis that we live in a computer simulation to intrude on the territory of physicists:

The simulation hypothesis, as it’s called, enjoys a certain popularity among people who like to think of themselves as intellectual, believing it speaks for their mental flexibility. Unfortunately it primarily speaks for their lacking knowledge of physics.

Among physicists, the simulation hypothesis is not popular and that’s for a good reason – we know that it is difficult to find consistent explanations for our observations. After all, finding consistent explanations is what we get paid to do.

Proclaiming that “the programmer did it” doesn’t only not explain anything – it teleports us back to the age of mythology. The simulation hypothesis annoys me because it intrudes on the terrain of physicists. It’s a bold claim about the laws of nature that however doesn’t pay any attention to what we know about the laws of nature. If you try to build the universe from classical bits, you won’t get quantum effects, so forget about this – it doesn’t work. ..

For the purpose of this present post, the details don’t actually matter all that much. What’s more important is that these difficulties of getting the physics right are rarely even mentioned when it comes to the simulation hypothesis. Instead there’s some fog about how the programmer could prevent simulated brains from ever noticing contradictions, for example contradictions between discretization and special relativity.

But how does the programmer notice a simulated mind is about to notice contradictions and how does he or she manage to quickly fix the problem? If the programmer could predict in advance what the brain will investigate next, it would be pointless to run the simulation to begin with. So how does he or she know what are the consistent data to feed the artificial brain with when it decides to probe a specific hypothesis? Where does the data come from? The programmer could presumably get consistent data from their own environment, but then the brain wouldn’t live in a simulation. (more)

Video games today typically only compute visual and auditory details of scenes that players are currently viewing, and then only to a resolution players are capable of noticing. The physics, chemistry, etc. is also made only as consistent and exact as typical players will notice. And most players don’t notice enough to bother them.

What if it were physicists playing a video game? What if they recorded a long video game period from several points of view, and were then able go back and spend years scouring their data carefully? Mightn’t they then be able to prove deviations? Of course, if they tried long and hard enough. And all the more so if the game allowed players to construct many complex measuring devices.

But if the physicists were entirely within a simulation, then all the measuring, recording, and computing devices available to those physicists would be under full control of the simulators. If devices gave measurements showing deviations, the output of those devices could just be directly changed. Or recordings of previous measurements could be changed. Or simulators could change the high level output of computer calculations that study measurements. Or they might perhaps more directly change what the physicists see, remember, or think.

In addition, within a few decades computers in our world will typically use reversible computation (as I discuss in my book), wherein costs are low to reverse previous computations. When simulations are run on reversible computers, it becomes feasible and even cheap to wait until a simulation reveals some problem, and then reverse the simulation back to a earlier point, make some changes, and run the simulation forward again to see it the problem is avoided. And repeat until the problem is in fact avoided.

So those running a simulation containing physicists who could detect deviations from some purported physics of the simulated world could actually wait until some simulated physicist claimed to have detected a deviation. Or even wait until an article based on their claim was accepted for peer review. And then back up the simulation and add more physics detail to try to avoid the problem.

Yes, to implement a strategy like this those running the simulation might have to understand the physics issues as well as did the physicists in the simulation. And they’d have to adjust the cost of computing their simulation to the types of tests that physicists inside examined. In the worse case, if the simulated universe seemed to allow for very large incompressible computations, then if the simulators couldn’t find a way to fudge that by changing high level outputs, they might have to find an excuse to kill off the physicists, to directly change their thoughts, or to end the simulation.

But overall it seems to me that those running a simulation containing physicists have many good options short of ending the simulation. Sabine Hossenfelder goes on to say:

It’s not that I believe it’s impossible to simulate a conscious mind with human-built ‘artificial’ networks – I don’t see why this should not be possible. I think, however, it is much harder than many future-optimists would like us to believe. Whatever the artificial brains will be made of, they won’t be any easier to copy and reproduce than human brains. They’ll be one-of-a-kind. They’ll be individuals.

It therefore seems implausible to me that we will soon be outnumbered by artificial intelligences with cognitive skills exceeding ours. More likely, we will see a future in which rich nations can afford raising one or two artificial consciousnesses and then consult them on questions of importance.

Here I just don’t see what Sabine can be thinking. Today we can quickly make many copies of most any item that we can make in factories from concise designs. Yes, quantum states have a “no-cloning theorem”, but even so if we knew of a good quantum state to start a system in, we should be able to create many such systems that start in that same state. And I know of no serious claim that human minds make important use of unclonable quantum states, or that this would prevent creating many such systems fast.

Yes, biological systems today can be hard to copy fast, because they are so crammed with intricate detail. But as with other organs like bones, hearts, ears, eyes, and skin, most of the complexity in biological brain cells probably isn’t used directly for the function that those cells provide the rest of the body, in this case signal processing. So just as emulations of bones, hearts, ears, eyes, and skin can be much simpler than those organs, a brain emulation should be much simpler than a brain.

Maybe Sabine will explain her reasoning here.

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