Anthropic Principle says

physical Universe must be compatible with the conscious life that observes it.

I thought it is just philosophy mumbo-jumbo and it's not physics.

But I could be wrong; I wonder whether is it possible to formulate it into a specific scientific theory that is falsifiable, at least in principle?

The argument that "the theory is true because we are here; our existence proves the theory and if we are not here, the theory is disproved" is extremely unsatisfying, mainly because it can be formulated to support any kind of philosophical mumbo-jumbo.


6 Answers 6


The statement you quote is a truism, so not much science in it. There are however two ways you can extract science from the anthropic principle.

One is that it can give you non-trivial constraints on parameters in current theories. In most cases the existence of intelligent life itself is too much of a detail that enters the argument. It's more that you'd be saying we have observed that the universe is such that e.g. carbon exists in such and such an abundance, and this is only possible if parameter x is in some specific range, so you've constrained your parameter. That constraint might be more or less interesting, but nothing unscientific about this.

Second one is that you might try to come up with a theory in which the "evolution of life" is well-defined in scientific terms, such that you can argue there's some optimization principle going on which results in some specific theory, that being the one we observe. However, there's some arguments that our standard model might not be the only one supporting the evolution of life (Google for: Weakless Universe). In any case, taking this road is going to involve a lot more than only physics because first you have to find a way to define "life" in mathematical terms.

  • 5
    $\begingroup$ But cannot we just use the same reasoning without invoking the Antropic principle? For example, say "carbon is experimentally observed, so the theory should have such and such constraints, uranium is also observed so the constraints should be such and such" etc. Why do we need the Antropic principle for this kind of reasoning? $\endgroup$
    – Anixx
    Commented Mar 29, 2011 at 0:51

The anthropic principle (at least in it's weak form) is neither philosophy nor science. It is simply a statement about the admissibility of evidence.

Any precondition to an observation is not evidence about the whatever you are observing.



I don't think the argument can be stated as "The theory is true because we are here; our existence proves the theory and if we are not here, the theory is disproved" because, as far as I know, the anthropic principle is not a statement about the truth of something. Instead, is a statement regarding the necessary conditions to produce intelligent life aware of certain features of its environment. The specifics are given depending on which anthropic principle you are talking about.

Obviously the anthropic principle is not a theorem or a law or anything of that sort, but it adds what the common sense would add: In case you're dealing in physics with something that is restricted by a result you're expecting to obtain (for example, a process that permits the creation of life), then by avoiding solutions not allowing this final result, you're implicitly using the anthropic principle. However, I would find hard to argue that we actually need a fancy name for this kind of situation.


Step 1 is to understand the exact laws of our universe, back to the big bang. We hope and expect that we will eventually end up with some mathematical equations or structures that are very constraining: Basically, that there will be only one simple self-consistent theory that can possibly be the laws of physics of the universe (or multiverse). Of course, people expect that this theory is string theory: They expect that any plausible self-consistent theory that includes general relativity and QFT is either string theory or something mathematically equivalent to string theory.

Step 2, you extrapolate these laws to learn about unobservable things, including whether there are other universes besides our own. In what we understand of string theory so far, it seems that, yes there have to be many universes, each with different fundamental constants. (Our universe had better be one of the possibilities of course!)

Step 3A, IF it is possible to find a sensible measure of the frequency in which any given type of universe appears: In principle, it is possible to estimate the expected number of intelligent life-forms in each universe. Then by doing a weighted average you can get a probability distribution of what the value of each parameter would be for a randomly-selected intelligent life-form. Then we can see where we fit in this distribution. If we are within a couple standard deviations of the center, good. If we're many standard deviations away, out in the tail, it counts against the likelihood of these laws of physics. I mean, if we have a Bayesian analysis to decide "what are the laws of physics", the finding that our universe is a real outlier among universes with intelligent life would lower our judged likelihood that string theory is the correct one. (Maybe it is still correct, because the other alternatives to string theory, if any, require even more unlikely coincidences.)

Step 3B, IF it is NOT possible to find a sensible measure of the frequency in which any given type of universe appears ("there are infinitely many of each so they don't have a well-defined ratio"): Then the best we can do is show that our universe is one of the possibilities. We will never be able to explain why we are in one mathematically-allowed life-supporting universe rather than another, because there is no explanation.

The overwhelming majority of the "scientific theory" in the anthropic principle comes in steps 1 and 2. Step 3 is a research program to (possibly) get some "predictions" which we can compare to reality, which adds some extra information and falsifiability to the theory. That's all good, but still, there is vastly more information and falsifiability in step 1 than in step 3. That's why, since Step 1 is still very far from complete, people's time is by and large better spent working on Step 1 rather than Step 3.


Anthropic reasoning is as good a scientific method as any others, in that it can produce experimentally testable predictions.

For instance it has predictions (For historical reasons these are mostly post-dictions I think) of the ranges of certain cosmological constants. Anthropic reasoning would say that these number must be within certain ranges because otherwise the universe would not be capable of supporting life, but we observe life so the constants must lie within ranges capable of supporting life.

So the basic argument goes like this "We know A is impossible without B, but we know A is true, therefore B is true", which is perfectly valid reasoning.

(A=>B and A) => B

Edit: Could downvoters please state what they dont understand?

This is the definition of a scientific method I use:
-Take observation, in this case that life exist.
-Use a formal system to derive testable predictions based on observation:
Done as follows: Life is observed to exist, therefore physical constants must be within certain ranges.
-Perform experimental test of prediction:
Constants observed to lie within predicted values.

Observation fits prediction, the definition of a succesfull scientific theory.

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    $\begingroup$ This is not valid at all. Say, there are a billion multiverses, all with different constants etc. Now let only one of the multiverses have the capability of supporting complex molecules and hence life. If the living beings in that universe follow such a thought pattern, they would be wrong, as there were many universes and the fact that they are there is a result, not a reason. Or in other words we are alive because the constants are adequate, not constants are adequate because we are alive. $\endgroup$
    – Cem
    Commented Jan 18, 2011 at 23:05
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    $\begingroup$ @Cem I dont understand how you think that the reasoning would be false in your example. It is logically equally valid to say the constants are adequate because you are alive, thats exactly the point the anthropic principle shows. $\endgroup$ Commented Jan 18, 2011 at 23:11
  • $\begingroup$ @kakemonsteret: You see, this is not science. What you just proposed is not a scientific argument, it is exactly what Graviton meant by "philosophical mumbo-jumbo". And please do not say "shows" rather say "suggests" as anthropic principle makes no sense physically. $\endgroup$
    – Cem
    Commented Jan 19, 2011 at 16:47
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    $\begingroup$ Ok, let's consider the A and B in your system. A: Life, B: Suitable constants. I am not defying this. A definitely implies B. What you are saying, though, A implies that B is there FOR A, which is not in the initial suggestion. Logically linking two propositions with an if does not necessarily link them with a for or because of, and there is a reason why there are no such linkers in logic in maths. Also, this argument is pointless beyond belief as anthropic reasoning is not a testable scientific theory by any means because there is no measurement for "intention" in the physical universe. $\endgroup$
    – Cem
    Commented Jan 19, 2011 at 17:25
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    $\begingroup$ Most of the time, when two people argue, it is because of the difference between their definitions, rather than the actual topic of argument itself. In this case, it was my mistake as I was reminded of another principle a colleague of mine had mentioned about and after reading anthropic principle's definition at Wikipedia I realized I was talking about a whole other issue, that is why I said your definition of A and B was inappropriate. I sincerely apologize for the misunderstanding. $\endgroup$
    – Cem
    Commented Jan 20, 2011 at 15:56

From what I gathered from Wikipedia, the Anthropic principle has been used to explain the age of the universe and values of physical constants. The argument is that if the values of physical constants changed by a tiny amount, this would have huge consequence of life as we know it. Hence the perfect values for the constants. But in a recent paper Don Page has argued that the value of cosmological constant is a bit larger. If it had been smaller and negative the universe would have had better conditions to support life (it would increase galaxy and star formation rates, thereby increasing the chance of life supporting environments). I think this flies directly in the face of the Anthropic Principle.

Further, we have studied conscious life only on earth. There is no conclusive evidence to support that life elsewhere in the universe (if it existed) would have the same characteristics as that seen on earth.

Also see the article on arXiv blog: http://www.technologyreview.in/blog/arxiv/26276/


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