Why do we need the anthropic principle?

Question

I've read debates about the anthropic principle (strong, weak, take your pick) but do not understand why it is even a subject of discussion. What observation is it meant to explain?

For example: does it explains why our universe has the fundamental constants it does? If so why is "life" required in the discussion? Furthermore how do we test an answer if we can't "see" into other hypothetical universes? Even if we could answer this, where does it lead--are we just asking why 1 = 1?

We are here in this universe. We see what its constants are. We cannot see into other universes. Isn't this the end of the train of thought?

Please help me understand why this is in any way important or where I've misunderstood the discussion.

I am not asking if this universe is fine-tuned for life, I am asking what value the so-called "anthropic principle" has in cosmological discussions, whether this universe is fine-tuned or not. In other words, what does it matter if it is or is not fine-tuned for life?

This forum is so restrictive. It seems to restrict questions without understanding them. Why? This is a valid question and is not addressed by other answers.

Answer 1

The anthropic principle can apply to questions about this universe, too.

For example, consider the question asked by early man: why is the world so hospitable to life, and to us? We have breathable air, food, water, a pleasant temperature, flat ground to stand on, gravity to stop things floating around, etc. It seems incredibly unlikely, unless it was by design. It seems to need an explanation - why do the laws of physics conspire to make life possible?

Then we find out about the rest of the universe. Almost all of it is extremely inhospitable: no air, no food, no liquid water, either burning hot or ultra cold, just dead rocks, ice, and dust floating in space. It turns out that the conditions for life are incredibly unlikely. The laws of physics do not make such conditions inevitable.

So how did we make that mistake? We observed that the environment we lived in was particularly conducive to life. It turned out that that was because we can only survive in places that are particularly conducive; our sample is biased by the fact that we could only sample from that tiny subset of the universe where we can survive. So of course any environment we could observe necessarily was one we could survive in, and we could deduce nothing about how likely or unlikely that was from observing it.

In other words, the Anthropic Principle is another name for sampling bias. Extraordinary coincidences can arise without any physical mechanism to explain them when the phenomenon is such that we can only only observe those cases where the coincidence occurs.

Its application to things like the fundamental constants is speculative and controversial. We at present have no explanation for why the constants are as they are. However, given that a universe with different constants would be hostile to life, one possible explanation would be that all the universes where they're different have no observers, and we only see the values we do because they mean that we can be here to observe them. There may be no explanation in physics for us to find for why they are as they are.

Or of course there may be one, and we just haven't discovered it yet. Since as you say we can't test the hypothesis, it can't ever be a scientific conclusion. It is, rather, possibly a question that science cannot answer. As with Turing's halting problem, some searches would be doomed never to give an answer, but give no way to ever tell whether it's doomed or whether we just haven't searched widely enough yet.

The speculation depends as well on Gell-Mann's 'Totalitarian Principle' of quantum physics - "Everything not forbidden is compulsory". Everything that can happen, must happen. The 'sum over histories' picture posits that every conceivable history happens, and those that are 'inconsistent' in some way interfere with one another and cancel out. But this leaves a lot of space for individually self-consistent alternative histories/universes that evolve independently of one another. But since it posits entities exist that can never be directly observed, we cannot experimentally confirm or refute it - we can only rely on aesthetics like simplicity or symmetry to decide if it serves as a good explanation.

Sampling bias is real. It's application to experimental physics is a real issue. Its speculative application to alternatives that are in principle forever unobservable, like other universes, is a logical possibility, but not one science can prove. But there may be other more metaphysical reasons for believing it.

Answer 2

I think physicists (and philosophers) remain somewhat divided on the role of anthropic reasoning. The least it does is draw our attention to the fact that the universe and its laws are closely knit in the sense that an apparently small change in many areas would have very large consequences on the evolution from the Big Bang till now.

I think it is fair to say that it is questionable whether the term "explanatory" can be attached to anthropic observations. By questionable I don't mean certainly wrong; I mean open to debate. We can observe, for example, that if a certain resonance in Carbon had a slightly different frequency then, if other aspects of physics were as little changed as possible, it looks as though the consequences for stellar evolution would be large, with the result that (arguably) life would not come about.

From this observation one line of logic is perfectly sound: we observe that life has come about so we deduce that the necessary conditions are in place, and in particular if other aspects of physics are unchanged then carbon must have the required resonance.

But does such reasoning amount to an explanation of why the resonance is there? That is where people disagree.

One can argue, "if there is to be life, the resonance is needed" but one cannot, without further justification, argue "we know there has to be life". One can argue that any state of affairs with ourselves in it is a state of affairs in which complex life has come about and persisted. That much is obvious. But it's not clear what to deduce from that.

I think anthropic reasoning has provided a useful service in drawing to our attention just how special the universe is. If one makes some rough attempt to construct a state-space of 'hypothetical universes' which appear to be feasible then the 'hypothetical universes' able to lead to life form a tiny tiny fraction of the total (ten to the power of minus some number which is itself huge). This does present us with a highly remarkable observation. But how to react is another matter. It is hard to claim that we really know what we are talking about when we do such calculations. The multi-universe discussions going on in some areas of contemporary physics do, it seems to me, lack the kind of caution which we look for in most scientific reasoning, especially in physics. I mean caution about being clear what our statements mean and how they relate to empirical matters.