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Mitchell_Porter comments on Getting Over Dust Theory - Less Wrong
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That statement is just mystical dogma unless you can explain why they are the same thing. And just saying "all possible worlds exist" does not explain why they exist. Why not just some of them? Why any of them at all?
I won't say the truth of the mathematical universe hypothesis is self-evident or anything, but there's certainly nothing mystical about it. All it says is that there was never any reason in the first place to suppose there's a difference between possibility and existence. It's not like we have perfectly clear concepts of 'possibility' and 'existence' that clearly conflict with one another.
The best analogy I can think of is the statement that time is the same thing as a coherent set of universe-configurations. Why are they the same thing? Um, because that's what time is. What makes you think it's anything else? Likewise for existence: What makes you think it's anything other than possibility?
That's good to know. So the next time someone tells me that something might be a cure for cancer, I won't have to think about it or research it, because if it is possibly a cure for cancer, then it is actually a cure for cancer. And the next possible theory of 9/11 that I hear, I don't need to wonder if it's true, because, being a possibility, it's also an actuality. And I don't need to worry about what you actually mean by your writing, because whatever possible interpretation I come up with, that must be the actual intended meaning...
Wait - you're saying that's not what you meant... that all those possibilities which seem to simply not exist... you're saying that they do exist, but somewhere else, in some other world? Well, uh, that's an interesting idea... pretty radical... I don't see what evidence you could have for it... but I guess I have to admit it's possible, ha ha... But wait - isn't it also possible that these other worlds don't exist? So does that possibility - meta-possibility - "exist" somewhere too?
Now please note: I am not disputing your right to build byzantine multiverse theories and to engage in abstruse logic-chopping which will allow your new interpretation of the word "possibility" to become consistent. But it is absurd to say that "there was never any reason in the first place to suppose there's a difference between possibility and existence". You may as well say there was never any reason to suppose that the Flying Spaghetti Monster doesn't exist, or that there was never any reason to suppose that black isn't secretly white as well. There are clear differences between the everyday original meanings of possible and actual, and proposing to negate them by supposing that all possible worlds are actual is radical metaphysical innovation, and before I believe it, it is reasonable to request some evidence, or at least an argument in its favor.
This kind of thing "It is possible modal realism is false." is a nice example of a Godel-type statement that doesn't involve math.
I'm not particularly familiar with Tegmark's position (beyond looking at the website) but modal realism (which afaict is at least a pretty similar position) just declares "actual" to be a kind of indexical, like "here" or "now". Saying "we actually have a cure for cancer" is like saying "We presently have a cure for cancer." So modal realism (and I don't see why Tegmark IV couldn't do the same) doesn't negate the difference in meaning, rather it just interprets words that we're already confused about.
I'm similarly suspicious of the strategy as well (see the reply to Jordan I'm about to write). But I don't think the argument is dedicated to the negation of the difference between possible and actual.
Like what? The property of existence? How would a bit-by-bit description of an object with that property differ from a bit-by-bit description of an object without it?
By the way, here's the link to Tegmark's articles, if you haven't read them yet:
http://space.mit.edu/home/tegmark/crazy.html
It would have some extra bits saying "this object exists".
Sounds like the Ontological Argument. God exists because he's defined to have the existence bit set to 1.
Whether an object exists in some reality is a property of that reality, not of the object.
Okay, I guess it's my fault for not being precise enough. I meant a complete bit-by-bit description of an object, down to the subatomic level, or whatever level is necessary so that the description says everything that could ever be said about the object.
Such a description of, say, an apple, would differ from the description of a pear by many bits. But how would it differ from the description of a non-existent apple? You could add an 'existence' bit to the description, but it would be meaningless, because the apple already exists: A complete description of an apple is an apple. A description of a non-existent apple isn't the description of an existent apple plus or minus a few bits, it's not a description at all, it's zero bits.
You are really mixed up. What you are saying is nonsense and it should be obvious that it is nonsense. Suppose I have an incomplete description of an apple; in the form of words on paper, just to be specific. I have a few thousand words describing some hypothetical apple, its color, its taste, its size, and so on. Now suppose I add however many bazillion more words I need in order to make it a complete description. What are you saying - that at some point my stack of paper turned into an apple, even though it's still a stack of paper?
The thing is, you don't need to think like this in order to have a multiverse theory. It's only this peculiar neoplatonic desire to believe that reality is mathematics (or is computation, entirely abstracted from substance) which leads to the nonsense.
Of course not. A description by itself is nearly meaningless. It only becomes meaningful when it's being interpreted. Make the complete description of an apple the input of the right computer program, and the pattern resulting from the sequence of all states of the computation will be the apple.
Substance? What's that?
This is still nonsense. If I have a computer made only of naturally occurring atomic elements, and I use it to simulate a plutonium nucleus, are you saying I now actually have a plutonium nucleus there?
It's what things are made of.
"Actually have" is vague and should probably be tabooed. Yes, in that there is a causal structure there isomorphic to a plutonium nucleus; no, in that this causal structure has the wrong relationship to you.
Yes, that's exactly what I'm saying.
Of course, the 'simulated' nucleus exists within the system that is the simulation, just as 'real' nuclei exist within the system that is our universe. Thus it would be silly to ask why we can't create simulated uranium and take it 'out' of the computer to generate electricity, or something, because that's exactly like asking why, if our universe exists within another universe, the aliens living in the other universe can't just 'reach out' and take the Sun into their universe. The only way to (sort of) do both of these things would be to convert the pattern/thing we wish to take out of the inner system into the equivalent pattern in the outer system.
What would a mathematical description of substance look like? Not a description of a really simple object made of substance; a description of substance itself.
If you learned that we were in fact in a simulations, would you feel that you were not real? Would you have an overwhelming desire to escape, in order to become real?
Cogito ergo sum. If I am learning or feeling anything, I do actually exist (though my perceptions and beliefs may be false). It is impossible for me to not be real in the sense of not presently existing, however it is that the alleged simulation works.
Do you agree that it would be possible to feel real inside a simulation (whether the simulation is of one mind or a whole universe)?
IAWYC, but let me point out that you are describing your position rather than supporting it.
As a way of supporting it, imagine that there's some other universe with physics that encapsulate ours: the Dark Lords of the Matrix can cheaply run a faithful simulation of anything that happens in this universe on that universe's computers. It's clear to me that, given the setup you two are discussing, the Dark Lords would see that those extra bits aren't doing anything at all; they can be removed without altering anything that they could observe in the simulation.
Now, anything we can observe about our conscious experience is assumed to have an effect on our brains as we think of it, and thus the Dark Lords could observe it as well. (Namely, if there's a distinction between a "physically real" world and another one that's just "mathematically possible" with an identical copy of you, there's no difference from the Dark Lords' perspective between the description of your brain thinking "But I really exist!" and the description of your copy's brain thinking the same thing.) Note that this is an instance of the GAZP in action.
So by Occam's Razor, I don't think there's a justification for adding extra bits to the way the universe is described, when literally nobody within or outside the universe can be pointed to as having justification that those bits are one way or another.
Exactly. In fact, if possibility and existence are the same, the Flying Spaghetti Monster does exist.
The empirical inaccessibility of counterfactual worlds.
I'm working on a post about this. It actually turns out to be surprisingly believable.