Anthropic Principles: What? Also: Why?


I’ve been listening to the Titanium Physicists podcasts for awhile now.

I first met the anthropic principles in maybe 7th grade before then.

My question is, “What do the anthropic principles mean?” and “why even bother?”.

As far as I’ve read, there’s two anthropic principals.

  • The weak anthropic principle states: “we exist, therefore the universe is in such a state, and has rules allowing us to exist” which is a simple tautology. Of course that’s true. Another way of putting it is “we exist, therefore we exist.” Stupid and unnecessary.
  • The strong anthropic principle states: “we exist, therefore we must exist.” which is wrong. Just because we exist doesn’t mean that we necessarily must come into being in this universe. Just because we exist doesn’t mean that it’s a requirement. There’s so many extinct species preceding us and so many extinct species to come that saying humans must necessarily exist is a stupid assumption. It’s groundless, because the only way for us to confirm it is to look into other universes without disturbing them, and Schrodinger and Heisenberg forbid such things.

I’d like to know other’s opinions on this matter. Why is the anthropic principle ever even mentioned by physicists?


We know that the speed of light, the charge of the electron, the mass ratio of the electron to the proton, the strength of Newton’s G, the relative weakness of the weak nuclear force (equivalently, the mass of the W and Z bosons relative to the mass of the proton and electron) take specific values. If we change these values even slightly at the time of the big bang, then the Universe would not have developed into one that could support our life: The sun would burn up too quickly, the chemistry of the periodic table would differ and would not support DNA. Nucleosynthesis would not have created the elements we need in the first place, neutrinos would orbit atoms as well as electrons, black holes would be everywhere, etc. etc.

The question is, suppose there’s a 4D graph of different values of G,h,c,e – which points in that graph correspond to universes that could support life, or intelligent life? The intuition seems to be that life (or even a recognisable universe) can only exist in a relatively narrow range of values of G,h,c,e compared to all possible values. So, why do we have the values that we do have?

The Weak Anthropic Principle stats that, for all we know, there are parallel universes out there, inaccessible to us, where radioactivity was so strong that every atom decayed in the first minute. There are universes out there where anything larger than a mouse collapses into it’s own black hole. We don’t find ourselves in those universes because they could not have supported us. We must find ourselves in a universe that is compatible with life, a kind of post-selection, so the argument about what percentage of the total space of allowed values of G,h,c,e support life is moot.

The Strong Anthropic Principle says that there is something fundamentally necessary about intelligent life, and that there’s some reason why the universe needs to create intelligent life, and so the other universes whose values of G,h,c,e exclude the development of intelligent life are excluded from being called into existence by some basic principle of physics that we haven’t discovered yet in detail, but which the authors would already label the ‘Strong Anthropic Principle’.

Of itself, it’s an interesting observation. Is there some new principle in Physics that corresponds to an anthropic principle? Fundamental physics has been basically cut and dried since the 1980’s … we know the basic laws, as far as we can tell, and all the work now taking place boils down to working out the consequences of those laws, and making educated guesses about where the next discrepancy between experiment and theory might be found, and so where the best places to look for discrepancies might be. The Anthropic Principle is a contribution to the question: what next? where to look?

If you wanted to be cynical, you could define a Weak Anthropic Tenure Principle, which states that, if you give philosophers of science permanent life-long jobs, some of them will create baseless, untestable theories to justify their existence. A corresponding the Strong Anthropic Tenure Principle that would state that there exists a deep underlying mechanism (called the ‘search committee’), that excludes the academics who can’t create baseless, untestable theories from ever becoming philosophers of science in the first place.

The closest that Physicists get to discussing anthropic principles is perhaps: Why does DNA coil clockwise versus anticlockwise? Why do the stereoisomers of the molecules in Oranges and Lemons taste different, although they contain the same atoms in the same numbers, arranged in mirror image of each other?

It turns out that the weak nuclear force favours a particular handedness of molecules at the level of about 1 part in 1,000,000, and that we are made from the molecules that are 1/1,000,000 lower in energy …


I take issue with the equivocation between “us” “life as we know it” and “life”. To assume that a universe that couldn’t support life as we know it also can’t support life is a form of Douglas Adams’s “puddle logic”. Life itself is fluid. To assume that since we exist in a certain configuration because a creator molded the environment to fit us is folly. There’s no evidence of that. But there’s mountains of evidence that life will adapt to any environment that will support it, far beyond our preconceptions and expectations.

Poppycock. What evidence is there for such a principle? How can it be tested? Just because we exist in this specific universe doesn’t mean that other universes can’t exist. Or that other universes must eventually develop our kind of life. Why?

A conspiracy theory to top it off? Bravo. I still don’t understand how either theory can be useful and testable simultaneously. It seems like dogma to me, quite frankly.


I’m not defending either position or posting conspiracy theories :slight_smile: I’m simply explaining what I think the terms mean. Now, the Strong Anthropic Principle is almost certainly poppycock. But, it actually is a valuable contribution nonetheless.

Here’s how it could be connected to something recognisable as science:

Hypothesis 1: Life can only exist in a narrow range of G,h,c,e
Hypothesis 2: The values of G,h,c,e have never changed in the course of the universe, because if they had done so, atomic physics would be markedly different and we would not exist

Methodology: We can test the values of h,c,e from the early universe by looking at atomic spectra (e.g. Rydberg series) of starlight from stars with large redshifts. The ratio h.c/e defines the fine structure constant. If that ratio had shifted by as much as one part in 10^-9 in the last 10 billion years, we would be able to tell with a careful study; we know where the stars are, let’s collect the starlight and see what we find. To make a comparison, we need good telescopes and accurate theories of how the values of h,c,e feed into the observable properties of atoms, and how the spectra of atoms would change with differing values of h,c,e (and we need the computational machinery to implement the theories) - we must be abel to predict atomic spectra ab-initio to the required accuracy for a comparison with observations of light from the early universe.

Then, doing that study is valid science, and I think such studies are ongoing. It’s also probably not the kind of question that would ever have been entertained or been readily explainable to a wide audience, were it not for the Anthropic Principle being part of the lexicon. The Anthropic Principle does have a value in prompting new, better, more specific questions, even if the gut reaction is to say ‘well it’s just obviously nonsense…’


Of course, we’re assuming that the observable universe actually exists outside of our observations of it.

Or are we?

We can assume that there is some kind of interference via observation of quantum states occurring at some point in the life time of the universe. Here we are.
Is it too much to ask that wave form collapse caused in some subset of condensed quantum information at some point in some potential dimension of some type can effect the values of some universal constants when those wave form collapses can only happen within the context of the subsets of potential vacuums with universal constants which would allow such wave form collapse?

And on and on…

And surely you don’t need a multiverse. Just one eensy-weensy, little infinity.


We don’t know where this universe ends and the next one begins.


welllll, assuming decoherence in some form, from the superposition of a universal wavefunction, your loaf might really just be converging beyond observable effects of structures impacting other structures within the observable horizon.

That might be, like, a really ridiculous understatement, the scale could be ten to the the grahams number (or something) larger than the minimum radius required for the formation of the observable universe but who the fuck knows!?

Point in any direction. Somewhere along the directon (oops, assuming spacelike infinity), there may be another @awjt pointing back (hmm, I should have said, pointing in the same direction but whatevs). And behind him, infinite more @awjt s.

At least one of them considers himself to be in a different universe within the multiverse and is right, I guess. Maybe. I like to think of it as a kind of data compression. If it’s already out there ‘in’ our universe, would the multiverse go to all of the trouble of reproducing it off in some other brane in some other part of the loaf? And if there is a correspondence, and those phenomenon are (locally) isomorphic then aren’t they the same thing anyway?

If something is exactly like something else, isn’t it actually that thing?



That sounds like something I would write. Wait am I you and you are me?
I’m pointing, but the miasmic principle is pointing back.
If you can find me, I will take us both out for pizza.


Tautologies aren’t necessarily stupid or unnecessary. “Survival of the fittest” is a famous tautology that has made a lot of difference, and the anthropic principle isn’t very different. People try to use the supposedly narrow range of universal constants in which life could happen as a proof of the existence of God (or at least something like meaning or purpose). That sounds pretty silly to me, and it flawed reasoning, but basically they point to the idea that it is very unlikely we are around and say we shouldn’t assume it just happened by chance.

The weak anthropic principle is really about intelligent life existing to ponder its own existence, not just about things that exist existing. The point is that given that there is intelligent life in the universe, we know the dials are tuned right that there can be intelligent life, and therefore the discussion of the dials being tuned right in this universe is irrelevant. If you want to say it was unlikely that this universe could support our lives, what you really have to argue is that it is unlikely that any universe could suppose life intelligence enough to reflect upon the properties of the universe. That is, you’d need to have some idea how many universes there are, the odds of them having various properties, and which combinations of properties could result in that - a lot of information we clearly don’t have. If any such universe ever existed, it would not be by chance we currently find ourselves in it. We are asking if there is a person who is seven feet tall, not measuring ourselves to see if we are seven feet tall.

I think the whole thing is silly anyway. We have absolutely no idea whether these constants could be different or what it would mean for them to be differeny. Could the speed of light in a vaccuum even be 2.9 x 108m/s? What if (if you’ll pardon the metaphor) it was instead 1.8 + 1.2i x 108m/s? What if universes with orthogonal values of h and c exist all around us but just can’t interact with us electromagnetically at all (i.e. dark matter)?

I don’t think people who speculate about universes without intelligent life realize how little universe you need to build life out of? Conway’s game of life produces a Turing machine.

So I propose the Medium Anthropic Principle: “we exist, and we have absolutely no basis on which to guess how likely that was or was not, or how likely it is there are or have been or will be (if temporal terms are even meaningful here) a universe in which intelligent life does/has/will exist again”


I’ve always understood it to be “I think therefore I am”, applied to the rest of the universe. In the weak sense, a form of establishing basic principles about the universe, basically, that we are here and that there are things to be known. A philosophical statement, not a scientific one.
The strong principle shows us a mode of thought were we set ourselves up as the center of the universe, the thing that must be explained for anything to make sense, its usefulness is in showing us that trying to explain our existence as a basic function of the universe is a mistake evolved primates make in the kind of universe that allows primates to evolve.

Carl Sagan in The pale blue dot:

“There is something stunningly narrow about how the Anthropic Principle is phrased. Yes, only certain laws and constants of nature are consistent with our kind of life. But essentially the same laws and constants are required to make a rock. So why not talk about a Universe designed so rocks could one day come to be, and strong and weak Lithic Principles? If stones could philosophize, I imagine Lithic Principles would be at the intellectual frontiers.”




Jebus, how does a banana do that?

It’s a transcendent answer.


It was written.


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