“The anthropic principle – the idea that our universe has the properties it does because we are here to say so and that if it were any different, we wouldn’t be around commenting on it – infuriates many physicists, including [Marc Davis from UC Berkeley]. It smacks of defeatism, as if we were acknowledging that we could not explain the universe from first principles. It also appears unscientific. For how do you verify the multiverse? Moreover, the anthropic principle is a tautology. “I think this explanation is ridiculous. Anthropic principle… bah,” said Davis. “I’m hoping they are wrong [about the multiverse] and that there is a better explanation.””The anthropic principle has been employed in physics as a proposed explanation for the values of parameters in our theories. I’m no fan of the anthropic principle because I don’t think it will lead to big insights. But it’s neither useless nor a tautology nor does it acknowledge that the universe can’t be explained from first principles.
- The anthropic principle doesn’t necessarily have something to do with the multiverse.
The anthropic principle is true regardless of whether there is a multiverse or not and regardless of what fundamentally is the correct explanation for the values of parameters in our theories. The reason it is often mentioned in combination with the multiverse is that proponents of the multiverse argue it is the only explanation, and no further explanation is needed or necessary to look for.
- The anthropic principle most likely cannot explain the values of all parameters in our theories.
There are a lot of arguments floating around that go like this: If the value of parameter x was just a little larger or smaller we’d be fucked. The problem with these arguments is that small variations around one out of two dozen parameters leave out most possible combinations of parameters. You’d really have to consider modifications of all parameters together to be able to conclude there is only one supportive of life, which is however not a presently feasible calculation. And though this calculation is not feasible, the claim that there is really only one combination of parameters that will create a universe hospitable to life is on shaky ground already because this paper put forward a universe that seems capable of creating life and yet is entirely different from our own. And Don Page had something to say about this too.
The anthropic principle might however still work for some parameters if their effect is almost independent on what the other parameters do.
- The anthropic principle is trivial, but that doesn’t mean it’s useless.
Mathematical theorems, lemmas and corollaries are results of derivations following from assumptions and definitions. They essentially are the assumptions, just expressed differently, always true and sometimes trivial. But often, they are surprising and far from obvious, though that is inevitably a subjective statement. Complaining that something is trivial is like saying “It’s just sound waves” and referring to everything from engine noise to Mozart.
And so, while the anthropic principle might strike you as somewhat silly and trivially true, it can be useful for example to rule out values of certain parameters of our theories can have. The most prominent example is probably the cosmological constant which, if it was too large, wouldn’t allow the formation of structures large enough to support life. This is not an empty conclusion. It’s akin to me seeing you drive to work by car every morning and concluding you must be old enough to have a driver’s license. (You might just be stubbornly disobeying laws, but the universe can’t do that.) Though, this probably doesn’t work for all parameters, see 2.
- The anthropic principle does not imply a causal relation.
Though “because” suggests so there’s no causation in the anthropic principle. An everyday example for “because” not implying an actual cause: I know you’re sick because you’ve got a cough and a runny nose. This doesn’t mean the runny nose caused you to be sick. Instead, it was probably some virus. Alas, you can carry a virus without showing symptoms so it’s not like the virus is the actual “cause” of my knowing. Likewise, that there is somebody here to observe the universe did not cause a life-friendly universe into existence. (And the return, that a life-friendly universe caused our existence isn’t the case because life-friendly doesn’t mean interested in science, see 3. Besides this, it’s not like the life-friendly universe sat somewhere out there and then decided to come into existence to produce some humans.)
- The applications of the anthropic principle in physics have actually nothing to do with life.
As Lee Smolin likes to point out, the mentioning of “life” in the anthropic principle is entirely superfluous verbal baggage (my words, not his). Physicists don’t usually have a lot of business with the science of self-aware conscious beings. They talk about formation of large scale structures or atoms. Don’t even expect large molecules. However, talking about “life” is arguably catchier.
- The anthropic principle is not a tautology in the rhetorical sense.
It does not use different words to say the same thing: A universe might be hospitable to life and yet life might not feel like coming to the party, or none of that life might ever ask a why-question. In other words, getting the parameters right is a necessary but not a sufficient condition for the evolution of intelligent life. The rhetorically tautological version would be “Since you are here asking why the universe is hospitable to life, life must have evolved in that universe that now asks why the universe is hospitable to life.” Which you can easily identify as rhetorical tautology because now it sounds entirely stupid.
- It’s not a new or unique application.
Anthropic-type arguments, based on the observation that there exists somebody in this universe capable of making an observation, are not only used to explain free parameters in our theories. They sometimes appear as “physical” requirements. For example: we assume there are no negative energies because otherwise the vacuum would be unstable and we wouldn’t be here to worry about it. And requirements like locality, separation of scales, and well-defined initial value problems are essentially based on the observation that otherwise we wouldn’t be able to do any science, if there was anybody to do anything at all.