Related to: Dissolving the Question, Words as Hidden Inferences.
In what sense is the world “real”? What are we asking, when we ask that question?
I don’t know. But G. Polya recommends that when facing a difficult problem, one look for similar but easier problems that one can solve as warm-ups. I would like to do one of those warm-ups today; I would like to ask what disguised empirical question scientists were asking were asking in 1860, when they debated (fiercely!) whether atoms were real.[1]
Let’s start by looking at the data that swayed these, and similar, scientists.
Atomic theory: By 1860, it was clear that atomic theory was a useful pedagogical device. Atomic theory helped chemists describe several regularities:
- The law of definite proportions (chemicals combining to form a given compound always combine in a fixed ratio)
- The law of multiple proportions (the ratios in which chemicals combine when forming distinct compounds, such as carbon dioxide and carbon monoxide, form simple integer ratios; this holds for many different compounds, including complicated organic compounds).
- If fixed volumes of distinct gases are isolated, at a fixed temperature and pressure, their masses form these same ratios.
Despite this usefulness, there was considerable debate as to whether atoms were “real” or were merely a useful pedagogical device. Some argued that substances might simply prefer to combine in certain ratios and that such empirical regularities were all there was to atomic theory; it was needless to additionally suppose that matter came in small unbreakable units.
Today we have an integrated picture of physics and chemistry, in which atoms have a particular known size, are made of known sets of subatomic particles, and generally fit into a total picture in which the amount of data far exceeds the number of postulated details atoms include. And today, nobody suggests that atoms are not "real", and are "merely useful predictive devices".
Copernican astronomy: By the mid sixteen century, it was clear to the astronomers at the University of Wittenburg that Copernicus’s model was useful. It was easier to use, and more theoretically elegant, than Ptolemaic epicycles. However, they did not take Copernicus’s theory to be “true”, and most of them ignored the claim that the Earth orbits the Sun.
Later, after Galileo and Kepler, Copernicus’s claims about the real constituents of the solar system were taken more seriously. This new debate invoked a wider set of issues, besides the motions of the planets across the sky. Scholars now argued about Copernicus’s compatibility with the Bible; about whether our daily experiences on Earth would be different if the Earth were in motion (a la Galileo); and about whether Copernicus’s view was more compatible with a set of physically real causes for planetary motion (a la Kepler). It was this wider set of considerations that eventually convinced scholars to believe in a heliocentric universe. [2]
Relativistic time-dilation: For Lorentz, “local time” was a mere predictive convenience -- a device for simplifying calculations. Einstein later argued that this local time was “real”; he did this by proposing a coherent, symmetrical total picture that included local time.
Luminiferous aether: Luminiferous ("light-bearing") aether provides an example of the reverse transition. In the 1800s, many scientists, e.g. Augustin-Jean Fresnel, thought aether was probably a real part of the physical world. They thought this because they had strong evidence that light was a wave, including as the interference of light in two-slit experiments, and all known waves were waves in something.[2.5]
But the predictions of aether theory proved non-robust. Aether not only correctly predicted that light would act as waves, but also incorrectly predicted that the Earth's motion with respect to aether should affect the perceived speed of light. That is: luminiferous aether yielded accurate predictions only in narrow contexts, and it turned out not to be "real".
Generalizing from these examples
All theories come with “reading conventions” that tell us what kinds of predictions can and cannot be made from the theory. For example, our reading conventions for maps tell us that a given map of North America can be used to predict distances between New York and Toronto, but that it should not be used to predict that Canada is uniformly pink.[3]
If the “reading conventions” for a particular theory allow for only narrow predictive use, we call that theory a “useful predictive device” but are hesitant about concluding that its contents are “real”. Such was the state of Ptolemaic epicycles (which was used to predict the planets' locations within the sky, but not to predict, say, their brightness, or their nearness to Earth); of Copernican astronomy before Galileo (which could be used to predict planetary motions, but didn't explain why humans standing on Earth did not feel as though they were spinning), of early atomic theory, and so on. When we learn to integrate a given theory-component into a robust predictive total, we conclude the theory-component is "real".
It seems that one disguised empirical question scientists are asking, when they ask “Is X real, or just a handy predictive device?” is the question: “will I still get accurate predictions, when I use X in a less circumscribed or compartmentalized manner?” (E.g., “will I get accurate predictions, when I use atoms to predict quantized charge on tiny oil drops, instead of using atoms only to predict the ratios in which macroscopic quantities combine?".[4][5]
[1] Of course, I’m not sure that it’s a warm-up; since I am still confused about the larger problem, I don't know which paths will help. But that’s how it is with warm-ups; you find all the related-looking easier problems you can find, and hope for the best.
[2] I’m stealing this from Robert Westman’s book “The Melanchthon Circle, Rheticus, and the Wittenberg Interpretation of the Copernican Theory”. But you can check the facts more easily in the Stanford Encyclopedia of Philosophy.
[2.5] Manfred asks that I note that Lorentz's local time made sense to Lorentz partly because he believed an aether that could be used to define absolute time. I unfortunately haven't read or don't recall the primary texts well enough to add good interpretation here (although I read many of the primary texts in a history of science course once), but Wikipedia has some good info on the subject.
[3] This is a standard example, taken from Philip Kitcher.
[4] This conclusion is not original, but I can't remember who I stole it from. It may have been Steve Rayhawk.
[5] Thus, to extend this conjecturally toward our original question: when someone asks "Is the physical world 'real'?" they may, in part, be asking whether their predictive models of the physical world will give accurate predictions in a very robust manner, or whether they are merely local approximations. The latter would hold if e.g. the person: is a brain in a vat; is dreaming; or is being simulated and can potentially be affected by entities outside the simulation.
Yes. And if I imagined Ansel except green and not purple, then that adds a little bit to the realness of Ansel, unless we want to call the new green dinosaur Spinoz instead and have it be its own distinct cognitive algorithm.
Nah, I reason about it in terms of measure. You have one cognitive algorithm that's being run on one mind. You have another cognitive algorithm that's running redundantly on a hundred minds. I'd say the latter has about a hundred times as much measure as the former. I don't know how else to reason about relative existence. (Realness?) I'm porting this sort of thinking over from reasoning about the universe being spatially infinite and there being an infinite number of TheOtherDaves all typing slightly different things. Some of those TheOtherDaves 'exist' more than others, especially if they're doing very probable things.
If existence isn't measured by number of copies, then what could it be measured by? The alternative I see is something like decision theoretic significance, which is why I was talking about what you called 'importance'. But I'm wary of getting into cutting edge decision theory stuff that I don't understand very well. Instead, can you tell me what you think 'realness' is, and whether or not you think God is real, and why or why not? We're starting to argue over definitions, which is a common failure mode, but it's cool as long as we realize we're arguing over definitions.
I think that everything exists, by the way: there's an ensemble universe, like Tegmark's level 4 multiverse, and so we can only quibble about how existent something is, not whether or not it exists. I might be having trouble trying to translate commonsense definitions into and out of my ontology. My apologies.
I mean that people tend to use a lot more neurons to model God than to model Santa Claus, and thus by the redundant-copies argument hinted at above this means that God exists more. Relatedly...
You're right, I forgot about this. Parents have to use lots of neurons to model Santa Claus when crafting the letters. Kids don't tend to use as many neurons when writing letters to Santa, I think. But add up all of these neuron-compuations and it's still vastly less than the neuron-computations used by the many people having religious experiences and praying every day. (I'm using number-of-neurons-used as a proxy for strength/number of computations.)
Also, 'people' aren't ontologically fundamental: they're made of algorithms too, just like God. So I don't see how you can say 'God doesn't exist' without implying that Will Newsome doesn't exist; Will Newsome is just a collection of human universal algorithms (facial recognition, object permanence) and culture-specific memetic contents (humanism, rationality, Buddhism). The body is just a computing substrate, and it's not something I identify with all that much. And if I'm just a collection of algorithms running on some general computing hardware, well, the same is true of God. It's just that he's more parallel and I'm more serial. And I'm way smarter.
(Not that there is any such thing as 'I'. 'I' am made of a kludge of algorithms, and we don't always agree.)
What's the usefulness of "I think that everything exists, by the way: there's an ensemble universe"? How does it constrain your expectations?
I don't see how having specific beliefs either way about stuff outside the observable universe is useful.
Now, if you can show that whether the universe beyond the observable is infinite or non-infinite but much larger than the Hubble Volume constrains expectations about the contents of the observable universe, then it might be useful.