Knowledge

Is it possible that the Universe and how we perceive it are just fractions of what is really out there? How would we know that the universe is not some completely different place that we could not even begin to undestand or perceive? For example ants live their lives without ever knowing of our existence so how would we know that there is not a lot going on in this world that we can not sense?

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The idea that our familiar universe is only a fraction of reality isn't just an abstract possibility. Many serious thinkers argue that this hypothesis is in fact strongly supported by physical theory.

Two distinct sets of considerations are relevant here. One is to do with the interpretation of quantum mechanics. Quantum mechanics says that the world is basically chancy: nothing determines when a radioactive atom will decay, for example; this has a certain probability, and then either does or doesn't happen. However, it has proved surprisingly difficult to give a physically cogent account of quantum chance. Advocates of the Everettian interpretation of quantum theory (a.k.a. the 'many-worlds theory') maintain that the only viable theory is that the universe is constantly splitting into branches, one for each possible outcome of every chance event. At one level this is mind-spinningly extravagant: for starters, it means that each of us is constantly splitting into trillions of irreversibly divergent future selves, one for every result of every chancy event we interact with. But Everettians argue that we'd better believe it, given that all the alternative interpretations of quantum mechanics conflict with basic principles of physical theory.

The other reason for believing in extra universes is the 'fine-tuning' argument. Many basic constants of nature (such as the constant of universal gravitation) have values which seem suprisingly well-suited for the emergence of life. If they'd been just a bit bigger or smaller, evolution wouldn't have gotten off the ground. One possible explanation for this fine-tuning is that God arranged things so as to make life possible. But another explanation is that reality contains many universes, including ones with 'bad' constants of nature. If this were so, then it would scarcely be a surprise that we living beings find ourselves in one of the universes with 'good' constants of nature. The philosopher Derek Parfit has argued that, given these explanatory options, we should prefer multiple universes to God--for surely an all-powerful architect would have made a better job of a purpose-built universe than this one.

These are quite different arguments, which posit quite different kinds of extra universes. But they both give us reason to suppose that there's a lot more to reality than there seems.

David has given you two arguments for extra universes based on scientific considerations. These sorts of case are particularly neat because, as I understand it, we are physically cut off from other universes: there is no causation between worlds. But your ant analogy suggests that you also wonder whether there might not be more than we can sense or know in our own universe.

There is indeed much more in our world than we can sense, and our scientific theories describe a lot of it. But is there much more in our world than we can know, once we take account of how far beyond what we can sense science takes us? To be unknowable, it seems that features of the world would have not just to be unobservable but causally isolated from what we can detect. (That is why David's examples were neat.) But there are many events in our world with which we cannot interact causally, given how limited is our own existence in spacetime. And even where there is causal interaction, I suppose that, like the ants, we not clever enough to figure it all out.

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