Hello, ladies and gentlemen, fellow lifeforms. My name is Alasdair Richmond. I teach philosophy at the University of Edinburgh and amongst my interests is the anthropic principle and that's what I want to introduce you to today. I'm going to begin by placing a bet with you. I'm going to make the following bet. I bet that, like me, you grew up on the Earth's surface, you have a body chemistry based on carbon, and you live in a world that has, as far as you can perceive, only three spatial dimensions. Now, as you may have noticed, these are all examples of anthropic effects. And anthropic effects can define a very wide range of different physical and observational circumstances. They can take in phenomena from chemistry, from physics, from astronomy, geology, and can even reach down into the very structure of space and time itself. Now the term 'anthropic principle' was coined by Australian physicist Brandon Carter and first appeared in print in 1974. It since had a lot of usage and has acquired a lot of different associations. Some of them slightly misleading. But the key idea that Carter was trying to express is this, the kind of observer we are will set restrictions on the kind of physical conditions that we're likely to observe. We are physically based observers, we require very complicated internal structures, internal organs, internal chemistry. And creatures like ourselves are only going to be found in places where the right sorts of conditions will be found to obtain. We are context-sensitive observers and so we won't be arranged randomly in space and time. Now, ancient science put the Earth and human beings in a very privileged position. The ancient earth-centered or geocentric astronomy, codified in the work of Ptolemy in about the third century AD, placed the Earth at the exact geometrical center of the universe with all the other planets and stars arranged in concentric orbits around it. Gradually, however, problems with the geocentric astronomy accumulated and, by the middle of the 16th century, astronomers led by the work of Copernicus had moved over to a heliocentric or sun-centered astronomy, according to which the Earth was now simply one planet orbiting around a much larger central body. But this process of gradual dethronement of humanity from a privileged or unique center continued. Discoveries from geology pushed the age of the Earth and the universe back and back into so-called deep time. Astronomers discovered an ever larger, ever more complicated, ever more widely ramified universe. And then, in the middle of the 19th century, in 1859, with Darwin's publication of the Origin of Species, came a wide-ranging suggestion that we could explain the origins of life, the origins of biological complexity without reference to external oversight or design; but with reference, instead, to non-intentional processes, processes that operated purely through natural law. So gradually, from ancient times up to the last 150 years, humanity's seemingly come to have a less and less central role in nature. And one might be tempted to go the opposite extreme from placing humanity at the center and conclude that we are absolutely unexceptional creatures and that the region of this, the universe we observe is in no way remarkable. Now, anthropic reasoning tries to strike a balance between excessive anthropocentrism, excessive insistence on importance and excessive insistence on our typicality, on the other hand. The anthropic principle says the kind of observer you are is likely to condition the kind of conditions that you observe. Anthropic reasoning reflects the very important philosophical principle that when you're trying to explain something, you want the thing that you're trying to explain to appear probable or typical or to be expected. It's an explanatory virtue to make the thing you're trying to explain appear probable and not an explanatory virtue to make the thing you're trying to explain seem improbable or unusual. So when assessing explanations, favour those explanations that put you and creatures like you in the most probable or typical or expectable location. Now, a wide range of physical phenomena that appear otherwise unrelated nonetheless have something very important in common. They seem to be fine-tuned and specifically fine-tuned into that narrow band of values that permits life. Be it facts about, carbon chemistry, be it facts about the freezing of water, facts about the structure of space, facts about the age of stars, many phenomena that we observe in the physical universe couldn't be even slightly different from the way they are and still have observers like us around to be able to take an interest in them. Now from this apparent cosmic fine-tuning, some thinkers have wanted to conclude that the best explanation for the structure of the physical universe is that the physical universe was designed. The physical universe was an artifact created intentionally according to a plan, and that plan was to generate observers like us. Now, such arguments from anthropic effects to apparent design may actually go some way beyond what anthropic reasoning itself actually suggests. Here's an example. Get a hold of a telescope, have a look at the night sky, and you will rapidly come to realize that you can observe a wide range of stars of different ages. You can see nebulae, young, bright, blue-white stars, older, yellow, medium-term stars like our sun, all the way through to swollen red giants, neutron stars, and even, if you have a very good telescope, the occasional black hole. Well, why such diversity? Well, partly, it's because we are physically context-sensitive observers. Observers like us require a very narrow range of physical conditions, but a very wide range of physical elements. Our bodies contain elements ranging from light, abundant elements like hydrogen all the way through to heavier, rarer elements like iron and sodium. The heavier elements are only formed in the heart of stars, the process of stellar nucleosynthesis. The fusion process by which light nuclei combine and fuse to form heavier nuclei. We are, quite literally, star dust. The heavier elements of which we are composed can only form inside stars and can only be scattered across the universe ready to coalesce into planets and, ultimately, into us after several stellar generations have been and gone. So it's not a coincidence that you live in a period of the universe's history after several generations of stars have been and gone. Okay, here's a bit of, audience activity. Raise your hands if you have a body chemistry that's not based on carbon. Specifically put your hand in the air if you have a body chemistry which is based on sulphur, silicon, or selenium. No takers? No. Rather unsurprising. It's not a coincidence that your body chemistry is based on carbon. Carbon has unique properties that make it exceedingly apt for generating long chain molecules. Specifically, carbon is a tetravalent element. Its outer electron shell contains four valency opportunities and it is therefore extremely good for forming very long chain molecules, not just with other examples of itself, but forming long chain molecules with other elements. If you want to build life, if you want to build the kind of cellular replicators that make life possible, having carbon around is a very good place to start. Now, as you can see, I'm holding in my hand a model, of the solar system. And the reason for that is I want to illustrate what a small range of the solar system we're able to inhabit. Now, as I've just said, if you're going to build life, carbon chemistry is a very good place to start. But the physical conditions that allow carbon to form and to form long chain molecules are only found in comparatively narrow sets of circumstances. Counting outward from the sun, we've got eight planets in the solar system. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. And Pluto, recently downgraded to dwarf planet status. But if you consider the very wide range of physical bodies in the solar system, not just the planets, but their associated moons, you will notice that many of them have serious disadvantages as possible abodes of life. Many of them, like Mercury, are too hot or awash with very strong doses of solar radiation. Many of them, on the other hand, like Uranus and Neptune, are too cold. Consider the moons of Jupiter, like Io, which is awash with continual sulphurous volcanic activity. Well, I'm going to bet that you didn't grow up on Mercury or Venus, on a moon of Jupiter, in the boiling thermosphere of Neptune, nor did you grow up inside the surface of a star. You grew up in a very narrow range of temperature and pressure conditions. And those temperature conditions in the solar system are found on one world and one world only. The world that orbits the sun in what's referred to as a Goldilocks Zone. Not too hot, not too cold, but just right. And that planet, of course, is the Earth. Now, another anthropic bet. If you look around you, in your immediate spatial environment, I bet that you will observe that it has three spatial dimensions, neither more nor less. You can arrange objects and processes around you along three independent spatial axes. One running front to back, one running left to right, and a third running up and down, and that's not a coincidence either. There are very good anthropic reasons for thinking that life in any physically plausible form cannot possibly exist in spaces that have other than three distinct independent axes. Imagine you lived in a plane, a flat surface that has a forward back and left to right axis, but no depth. In a world like that, it's almost impossible to construct a nervous system. Nerve impulses can only travel in and out of cells by effectively crossing at a series of interconnections, rather like logic gates. The fidelity, the reactiveness of such a nervous system is hopelessly compromised. So it's not a coincidence that you don't live in a flat plane. On the other hand, spaces with more than three dimensions, it's extremely difficult to enclose volumes, so forming cells, forming complex internal organs, is virtually impossible. Now, the range of anthropic effects that I've just surveyed may suggest that the universe we inhabit is actually just a very small fraction of a much larger ensemble. Many philosophical and physical theories postulate that the universe that we inhabit is one aspect of an enormously larger ensemble of worlds sometimes known as a multiverse. Now, there are different ways of conceiving it, the multiverse. Some from philosophy, some from quantum physics. But what they all have in common is the idea that this physical universe is a tiny subset of an enormously larger ensemble that can contain all the physically possible ways that the universe can be, all the physically possible combinations of conditions and forces. If that's true, if there is a multiverse, and the full range of existence comprises all the possible combinations of forces, all the possible arrangements of chemistry, all the possible arrangements of space and time, then it's not surprising that we happen to inhabit a universe that contains just the right combination of conditions necessary for our survival. So it may be that the correct inference to draw from anthropic effects is the our universe is part of a much larger ensemble of discrete, equally concrete, but all-encompassing worlds known as a multiverse, which can contain every physical possible arrangement in matter and forces in space and time. Now, I said at the beginning that the term 'anthropic principle' has acquired several different meanings, some of them misleading, some of them even conflicting with each other. And I want now to talk about three important issues where a little clarification of what anthropic reasoning means can be very helpful. Firstly, the name itself. Anthropic comes from a Greek root, antropos, meaning man as in anthropomorphic, human-shaped. And Brandon Carter, for all that he coined the term 'anthropic principle', later came to think that perhaps the word 'anthropic' had acquired unfortunate associations or connotations. For one thing, the anthropic principle is not about human beings specifically. And it's certainly isn't about men. It isn't human-centered, it isn't male-centered. Rather, it's a principle that can be used by any physically based context-sensitive observer. The anthropic principle reflects the link between the kind of observers that, that we are and the kind of physical conditions that we observe. Any physically based context-sensitive observer can run its own version of anthropic reasoning. So really it's an observer principle and not a human-centered principle, that's the first problem. Okay, second problem, the anthropic principle is sometimes dismissed as if it expressed a mere tautology. A mere logical truth, empty of empirical content. A statement like, for example, 'all swans are swans'. Or 'all hot things are hot things'. Well, if the anthropic principle expressed a mere tautology, it wouldn't be scientifically very interesting. However, it's not entirely fair to dismiss the anthropic principle as a tautology. Notice the anthropic principle is not saying, 'you have evolved where you have evolved', which would be a tautology. Rather, the anthropic principle is saying, 'it is overwhelmingly more likely that you have evolved in conditions suitable for your evolution', and that is not a tautology. In a sufficiently big universe or a sufficiently big ensemble of universes, there could be some very, very unlikely, some very, very ill-adapted observers eking out a living somewhere. But remember the philosophical principle that I sketched earlier. We favour explanations that make our location probable or typical or to be expected. You may be a very unlikely observer in very atypical observational conditions, but all else being equal, you shouldn't favour explanations that make you atypical or unusual. Now, the third problem that I want to try and clarify concerns the distinction between two forms of anthropic principle, sometimes referred to as the weak anthropic principle and the strong anthropic principle. Now, the strong anthropic principle is sometimes presented as if it's an inevitable consequence, or necessary implication, of the weak anthropic principle. And that is maybe not the case. And confusion arises from not attending to the distinction between the two. The weak anthropic principle is the one that's closest to Brandon Carter's original formulation. And as I've kept saying, Carter's original formulation says the kind of observers we are will set restraints on the kind of conditions that we are likely to observe. We are overwhelming more likely to observe the sorts of conditions conducive to our evolution. Now, read correctly, the strong anthropic principle generalizes the weak anthropic principle to the universe as a whole. And says the evolution of observers like us suggests it's overwhelmingly likely that the universe is such as to permit the evolution of creatures like us. But note, it's expressed in terms of probability. The strong anthropic principle is sometimes glossed as saying that the universe must be such as to create life. As though the range of possible universes that could have existed was restricted, or selected from, on the basis that it had to generate life. And that may be a misleading inference to draw. But care must be taken here. It's sometimes assumed that the strong anthropic principle should be interpreted in a categorical sense, as though saying that the universe must necessarily have to exist in such a way as to generate life. But this categorical inference is maybe not the correct way to read anthropic reasoning. The difference is essentially between a categorical and a conditional form of inference. And here's an example to try to make the distinction between the two kinds of inference a bit clearer. Suppose you're a frog who has grown up in a pond. And you think to yourself, 'well, as a typical frog evolving in a pond, I'm more likely to have grown up in conditions that allow the possibility of frog spawn'. So that's a safe anthropic conditional inference. If I'm a normal frog, I'll have grown up in conditions where frog spawn exists. A less plausible version would be to say, 'my presence in this pond demonstrates that the universe was categorically designed with a view to generating frogs'. My existence as a frog in a narrowly pond-centered section of the universe establishes that ponds and frogs are part of the universe's plan. Now, that frog may have reasoned correctly, but such may also be rather a presumptuous frog. The point about the sustainable version of the strong anthropic principle is that it says merely in a conditional form, if we have evolved in this universe it is overwhelmingly likely that this universe contained conditions suitable for our evolution. It merely generalizes and broadens out the weak anthropic principle inference that Carter originally offered. So one way to avoid the presumptuous frog inference is to reflect on the phenomena that I introduced earlier and think that the apparent fine-tuning of the physical universe may support the hypothesis that this universe is just one segment of a multiverse or universe ensemble. If all the physically possible ways that a universe could be exist as concrete places in their own right, it's not surprising that we inhabit a very narrowly circumscribed section of the multiverse. So perhaps the real lesson of anthropic inferences is that from the Earth to the solar system to the very structure of space and time itself, what exists may be far larger than we used to think.
