Imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn't it? In fact, it fits me staggeringly well, [it] must have been made to have me in it!'

This is a quote from Douglas Adams' book The salmon of doubt and it's of course ridiculous. The hole wasn't made to have that particular puddle in it, rather the puddle exists because the hole was there in the first place. But the idea illustrates a problem when it comes to our attempts to understand the Universe: we can't see beyond our particular "puddle", which makes it easy to draw false conclusions about the world.

"The sheer scale of the Universe affects how we reason about science," explains David Wallace, a philosopher of physics at the University of Oxford. "It affects the way we reason from observations to predictions as to which theories are true." Modern theories suggest that the Universe really is unimaginably large — perhaps it's infinite, but even if it's not, it's so large it may as well be. Some theories even suggest that we live in a multiverse, with individual universes constantly popping in and out of existence like bubbles in a bubble bath (see here to find out more). "If that's the case, if the universe really is much, much larger than we can see, and most of it we'll never see because light from it will never reach us, then asking the standard scientific question of 'what happens?' has to be replaced by the question 'what happens in the part of reality we can observe?'," says Wallace. "And since what we really want to know is what happens, what's the true theory, we need to think really hard about how to relate these two issues."

This means that, ironically given our insignificance within the vast cosmos, we need to take ourselves into account when trying to understand the Universe. We need to try and understand our place in the world and the limits to what we can see before drawing conclusions. An example comes from the values taken by the constants of nature that determine the nature of our patch of the Universe. The gravitational constant, which measures the effect of gravity, the fine-structure constant, which measures the strength of electromagnetism, or the cosmological constant, which measures the effect of the mysterious dark energy that drives the expansion of our Universe. If you take into account what we know about how universes form, it turns out that the values these constants take are quite unlikely. If you constructed a Universe at random following the laws of physics we know, then the chance of its constants of nature taking the values they do in ours would be minuscule.

What does this tell us? Some people have been tempted to make the same mistake as Douglas Adams' puddle. It appears that the constants need to take the values they do for life to exist, so it seems that the world has been made to have us in it. There must be some sort of god, then, and we are at the centre of god's creation. But this is nonsense. "When you remember that the observations we make have to be made by us in a Universe that can support our existence, it's not so surprising to see the cosmological constant [for example] having the value it does," says Wallace.

So what accounts for those unlikely values of the constants of nature? The multiverse theory provides an answer: if there really are a huge number, or infinitely many, universes, each with its own constants of nature, then it's not so surprising anymore that one of them takes the values we see in ours. It's a bit like finding out that your friend has won the lottery. That would be a surprise, but not if you knew that she'd bought millions of tickets covering many different six number combinations.

But while the idea of a very large cosmos with lots of variation can provide some answers, it also poses new questions. As the lottery example shows, large size can make the improbable possible. Some physicists are contemplating the possibility that we are merely disembodied and deluded brains resulting from rare but possible quantum fluctuations (see here to find out more) or that each one of us could exist multiple times in the cosmos. Whether we really are who we think we are obviously makes a big difference to what we can hope to discover about the world we live in.

All this is tricky territory though. Talking about things we will never be able to see, such as the other universes in a bubble multiverse, takes us to the boundary of science and across into philosophy. The answer, presumably, is to keep thinking — for example, astrophysicist Fred Adams is busy calculating what proportion of other universes in a multiverse could be similar to our own, shedding some more light on just how likely, or unlikely, ours is (find out more in this podcast). Others are looking for ways of finding indirect evidence of other universes, for example, they might have collided with ours early on in its life and left traces in the cosmic microwave background (see this article to find out more). And yet others are working out the probability that we really are the kind of observers we think we are (see this article to find out more). In this project we will keep you informed on people's research regarding our roles as observers of the cosmos.

And what about the puddle? Well, it paid for its mistake. As Douglas Adams wrote,

This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it's still frantically hanging on to the notion that everything's going to be all right, because this World was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for.

About this article

David Wallace is a philosopher of physics at the University of Southern California, having previously received PhDs in physics and in philosophy at the University of Oxford. His 2012 book on the many-worlds interpretation of quantum mechanics, The Emergent Multiverse, was joint winner of the 2013 Lakatos Prize for philosophy of science.

Marianne Freiberger is Editor of Plus. She interviewed Wallace at the 5th FQXi International Conference in Canada in August 2016.

This article is part of our Who's watching? The physics of observers project, run in collaboration with FQXi. Click here to see more articles and videos about questions to do with observers in physics.