Tuesday, July 13, 2021

An argument against a giant multiverse

Tariq Nazeem emailed me a really cool and simple argument against certain kinds of gigantic multiverses. I’ve tweaked the argument a little, and here it is.

Start with this, as the target of the reductio ad absurdum:

  1. Every metaphysically possible kind of substance exists.


  1. It is metaphysically possible to have a substance that has the causal propensity to turn every colorable object red every second.

(Colorable objects are things are like trees and dogs, but not numbers, photons or electromagnetic fields.)

Well, it follows from (1) and (2) that:

  1. So, there is a substance that has the causal propensity to turn every colorable object red every second.

  2. So, every colorable object turns red every second.

  3. But I am a colorable object that does not turn red every second. (Empirical observation)

  4. Contradiction!

My initial objection to Nazeem’s argument was that in a typical philosophical giant multiverse theory, the multiverses exist in separate spacetimes. (I think Nazeem’s own target was a view where they were in a single spacetime, but that is a less common view.) But then I realized that there is nothing absurd about a substance affecting things that are not spatiotemporally connected to it—classical theists think God is like that. Substances have causal propensities that specify the types of things they affect and the circumstances in which they affect them, and there is nothing absurd about a specification of these things and circumstances that makes no reference to a spatiotemporal connection. Therefore, (2) is pretty plausible, notwithstanding the fact that the colorable objects might exist in other spacetimes than the substance making them red does.

A different objection to this argument is what to say about conflict. What if reality included a substance that constantly made everything colorable red and another substance that constantly made everything colorable blue? Would I then be red all over and blue all over at the same time? But that’s impossible.

I am not completely clear on what to say to this objection.

One thought is: So much the worse for our giant multiverse—there are metaphysically possible pairs of kinds of substances, like the constant-reddenner and the constant-bluer, that simply cannot both be exemplified.

But on the other hand, maybe the possibility of conflict suggests that when we fully specify the causal propensities of a substance, we need to specify how they would interact with other causal propensities. Thus, we might have a constant-reddener that in the absence of other color-setters turns everything red, but in the presence of a constant-bluer, turns everything purple. However, it seems metaphysically possible to also have an overriding-reddener which makes everything red notwithstanding whatever other things exist. Then there could also be an overriding-bluer which makes everything blue notwithstanding whatever other things exist. And again this refutes (1).


Unknown said...

I think Professor Rasmussen had the same "conflict" objection in mind, in his book how reason can lead to God he dreams up the possibility of every conceivable universe existing simultaneously as an explanation to why we exist, but he too realizes that certain possibilities preclude others (the possibility that a blackhole that spans all of reality and the possibility that there isn't a universe inside a blackhole).

I think this thought experiment at the very least provides a compelling case that reality is specific, consistent and homogenous.

-Tariq Nazeem

Tom said...

I sometimes struggle to make sense of "metaphysical possibility" as it's used here. In this world, the Reddener would need to change the structure of everything else to make it red (assuming red isn't just a product of our minds?), and even then it could only do so within the bounds of the speed of light. So it's clearly not physically possible.

But then you have metaphysical possibility, which seems to describe magic as much as anything else. The Reddener turns everything red without reference to any laws of nature or other causal structures/physically possibilities, it just does it, somehow, even across lengths greater than the speed of light and different universes (and is it even a coherent suggestion on metaphysical pictures like the "vast mosaic of matters of particular fact," where everything is separate and a matter of the chance distribution of fundamental physics? but those ideas are weird too, since they make everything in the world an astounding coincidence)

So metaphysical possibility seems to work out to just be some sort of conceivability, which strikes me as a weak basis to allow something so extraordinary.

But maybe this is a problem with all modal notions, since we by definition can't have empirical evidence for them (we only know what happens in this world from empirical examination, any ideas about other possibilities are at least abstracted). And causation is a bit of a weird subject too, since most attempts to reduce it to something else just allow the same weird features to pop back up elsewhere.

Anyway, I'm not sure this made a lot of sense, but suffice it to say I find a lot of suppositions made in philosophy about these topics thoroughly confusing.