Does the Universe Exist?

Here's a thought that boggles my brain from time to time - what if the universe didn't exist. I'm not talking about stuff, like chairs and people and planets. That's a little unusual, but it's not so hard to imagine a universe that's got nothing in it, an infinite void. But that's not what I'm talking about. What if the void didn't exist either? Not only was there nothing, there was nowhere for it to be.

This hurts my head for a while, but then it starts to seem plausible. Unperturbed nothingness seems pretty elegant. Why should there be physical laws at all, let alone arbitrary arrangements of matter and energy? Why does the universe exist at all?

Does it?

Let's assume that physicists eventually come up with a complete description of the universe, a theory of everything. (Not a theory of "everything", including waltzes and parakeets, but a complete description of the root level of reality, the 'building blocks' as it were.)

What's the difference between this set of rules and the actual universe? What gives life to the rules and makes the world they describe real, or is the mere possibility of a universe just as real?

To quote Stephen Hawking:

Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe?

What if it's nothing?

Okay, I admit this sounds totally bonkers, but consider the following thought experiment:

Let's assume for the moment that (1) you're a materialist, and you believe that cognition, self-awareness, intelligence, etc. are the output of computations (say) of the human brain.

Let's further assume that (2) we eventually develop artificial intelligence, maybe in a giant Game of Life, maybe some other way.

Unlike a robot brain, the AI we create isn't wired up to perceive the outside world, only events in its own simulated world - in essence, we've created a simulated universe (which may follow different laws than our own). So there's a society of simulated beings all chatting away with one another, or whatever they're doing. These aren't video game characters, mere sprites, these are rich, thinking beings that - when wired up to our world - behave just as richly as humans (though their thought processes and mental architecture might be very different), that just happen to be experiencing a completely virtual world.

Are these beings having a real experience, like we do? It would seem so, following from (1).

If you're still with me, here's where it gets weird. Is their experience somehow reliant on us running the simulation? Does it matter?

At first, it seems so. After all, if we never run the simulation, no experience for them, right?

Consider: if we pause the simulation, wait five minutes, and restart it, they'd never know. All their brains would freeze, along with everything in their little simulated universe, and be restarted, and they'd be none the wiser. In fact, if it's deterministic (like Conway's Game of Life) we could even rewind it a little and replay it. They're still motoring on like nothing had happened. These disturbances don't affect them at all.

In fact, run it backwards for a few years, turn it off, and when we get to the year 2076, start it over from the beginning. They still have no idea.

So it seems to me that the experience of these simulated beings is only loosely connected - if at all - to how and when we run the simulator.

Does it matter if we run it at all? If so, how?

If it doesn't matter if we ever run it, does it matter if we even invent it?

If not, it seems that a whole realm of existence and experience is 'out there', without us lending any energy to it whatsoever.

Back to us. If indeed we can find a simple theory of everything, maybe it's utterly unnecessary for anything to breathe fire into them. Perhaps all of experience is just the interaction between ephemeral possibilities of actuality. The universe doesn't really exist at all!