Antropic principle = We're alone ?

The antropic principles , as I understand it, states that the universe we live in, is most probably, a very generic universe with just the enough amount of “quirkiness” to allow for intelligent life. The kind of intelligent life that questions its nature that is.

Does this suggest that we are probably the only intelligent life in this universe?

The catastrophy that wiped out the dinosaurs might have been the cherry on the cake of events that lead us to be the rare, maybe unique species that the Fermi paradox suggests we should be.

What do you people think?

There are “hard” and “soft” anthropic principles, neither of which are quite equivalent to your summary.

The “soft” anthropic principle is that any explanation or theory of the universe is constrained by the presence of life, ie. “We’re here” must be the starting point of any sound hypothesis.

The “hard” anthropic principle takes this further, stating that there couldn’t have been a different universe since “we’re here”. If those conditions hadn’t existed, there would be nobody to wonder whether they might have been different.

(I must admit, I never really understood the implied causality in the “hard” priciple - it always struck me as a bit “Gaia”.)

However, neither version places any constraints on the nature of the organic life in the universe. Whether it exists in one or many trillion locations is, to the universe, a triviality compared to whether or not it can exist at all. Several cosmic coincidences attest to that (eg. balance between gravitational attraction and electromagnetic repulsion, resonances of carbon nucleii etc - read John Gribbin’s “Cosmic Coincidences”).

If your question is “Has an incredible series of coincidences led to life evolving on Earth first in this galaxy?”, this may be the case. The sun was one of the first third generation stars around give or take a billion years, the capture of the moon in Earth’s infancy might well be essential due to the effect tides have in “pushing” evolution, and the size, composition and distance from the sun might all be “just right” for those primordial Goldilockses.

If we are alone in the universe, that would suggest to me that we are existing in a cosmos that is only just hospitable to life-

if the anthropic priciple works because we select the universe we live in simply by existing, it would imply to me that many universes also exist where the conditions for life are even more favourable.

Most universes with life should have more than one instance of life- some will be positively teeming.

I can feel a Brandon Carter-type argument in there somewhere but can’t quite get to it.

Sci-fi worldbuilding at

Damn, eburacum, why didn’t I select the universe where I can live forever and shoot laser beams out of my eyes?

The criterion is that you exist, not that you can enjoy it.

Looks like I’ll just have to be satisfied with the two laser pointers taped to my cycling helmet.


Let me restate,

So, having this universe the choice to configure itself in any of infinite ways, it seems natural that most of those configurations would be sort of boring and lifeless.
I think the problem we face is to try to visualize what is the nature of the most common family of universes able to harbour at least one intelligent self questioning lifeform.

Having figured that out, the question is: Can any conclusion be derived as to how many of these lifeforms are probable given the hypothesis that this type of universe will try to “cut corners” wherever possible?

SentientMeat -> You affirm that the universe has already achieved the hardest parts, like balancing critical forces and such. So that the amount of different life forms ditributed in this universe cannot be predicted since it’s an easy “extra” for the universe to create trillions or just one.

I guess the difficult part is to figure out what variables are available to tinker with at the moment of randomly producing a generic(ish) universe.
(did that make sense?)

Any other thoughts?

To follow on with my rather dubious line of reasoning-
if it is true that there are many universes to choose from, we would expect to find ourselves in one of the universes with many individual inhabited planets to choose from-
or is that just bad maths?

My argument would probably fall down if there are an infinite number of worlds to choose from, because every option would then have infinite instances.

Otherwise it seems to me that far from suggesting that there is a small number of worlds with intelligent life in our own universe, the anthropic principle suggests the opposite.

Sci-fi worldbuilding at

If it’s probabilities we’re talking about, one can surely separate the two since they are independent?

Assuming that, of all possible universes, those suitable for organic life are somehow “favourably weighted”, this surely has no bearing on the specific physical conditions required within that universe. There is no causal link that I can think of between the “balance of g” and the fact that eg. you need three generations of stars to form enough interesting atoms or the probability of capturing a moon of just the right size at just the right time in a planet’s evolution?

It might be “easy”, it might be incredibly unlikely. The point is that it is not related to the “coincidences” on a cosmological scale, and so I do not see how either loneliness or teeming skies follow from the anthropic principle.

Let’s simplify the finite/infinite planets issue by talking about density of intelligent lifeforms instead of a set amount of them.
Let us therefore establish we are trying to figure out if the anthropic principle makes any prediction as to the density of lifeforms in a generic life-bearing universe.

For the purpose of the following argument I’m going to make a fantastic simplification. Let us say there is only one single variable that defines the nature of any single universe. Lets call it C.
C is an unbounded positive real number. It is reasonable to assume that most of the values of C yield lifeless universes. Let us say that a life bearing universe can only be achieved if the constant C is set at a value between 4 and 5. So, the anthropic principle states that any intelligent being must not be marveled at the fact that the universal constant C of the universe he lives in is at a such a remarkably low figure, say 4.32.
Lets say this intelligent being wants to figure out what are the chances of there being a great or small density of other lifeforms like him sharing his universe.
It all seems to depend on the shape of the function relating average lifeform density and the C constant in the range 4<C<5.
If it is a low flat line with a spike, then chances are the lifeform density is low. If it is a fat tall bell shape curve, chances are high.

The anthropic principle seems not to make any predictions as to the average density of lifeforms in the restricted range of the C constant.

Or does it?

I hear you Sentient

How about this (get ready):

Following from my last post, and assuming an absolute lack of any other knowledge about the function relating C to lifeform density: is it reasonable to determine the average integral of a an average function in any interval?
Is there such a concept?
Can we say that some mathematical functions are more “common” than others?

It occurs to me that this particular function is restricted. It has to start at a zero value and end at a zero value. Does that help?

Does that the above sound like I’m high on marijuana :slight_smile:

I guess the problem is that, right now, we only have one data point to work with.

The universe is so big, however, that even if there was a very slight chance of intelligent life forming (say one in a billion for any given star over the course of it’s existance) there should still be several planets with intelligent life in our galaxy.

I find it difficult to believe that there is anything special about our solar system that would make it the only one or even one of a few where life formed. It’s kind of like renting a hotel room for the first time and being amazed that there happens to be a TV and a bed inside. As far as we know, the processes that created life here are as common as TVs and beds in hotel rooms (and if there wasn’t a bed inside, we probably wouldn’t have been there in the first place to contemplate it).

My argument from chance goes like this-
It is nearly the same as Brandon Carter’s doomsday argument- (have you discussed that here at all?)
If there were only four universes with life to consider
Each with a different chance of life forming, each with a different value of **ChaosGod’**s C constant
We’ll name them after colours-
Red has 1 world with self-aware life
Yellow has ten
Green has 100
Blue has 1000

You are born by chance into one of those worlds- before your birth you select the world by picking a coloured ball out of a cloth bag corresponding to your future universe.
Most of the time you are born into the Blue universe.
It’s as simple as that.

Sci-fi worldbuilding at


I’m not familiar w/ hard and soft anthropic princple, but there are weak and strong verisons:

read about it here.


“Weak Anthropic Principle (WAP): the observed values of all physical and cosmological quantities are not equally probable but they take on the values restricted by the requirement that there exist sites where carbon-based life can evolve and by the requirement that the Universe be old enough for it to have already done so.” (The Anthropic Cosmological Principle by John Barrow and Frank Tipler, p. 16)

“Strong Anthropic Principle (SAP): the Universe must have those properties which allow life to develop within it at some stage in it’s history.” (The Anthropic Cosmological Principle, p. 21)

In addition to the WAP and SAP, there are the Participatory and Final Anthropic Principles. The Participatory Anthropic Principle states not only that the Universe had to develop humanity (or some other intelligent, information-gathering life form) but that we are necessary to it’s existence, as it takes an intelligent observer to collapse the Universe’s waves and probabilities from superposition into relatively concrete reality. The Final Anthropic Principle states that once the Universe has brought intelligence into being, it will never die out. These two are also very speculative.

Sounds like the Participatory and the Final versions would assume that we are alone.

I’ll just add that it takes a “smarter” person than me to understand the Anthropic Principle. I tried to wade thru the definitive book on the subject (The Anthropic Cosmological Principle by Barrow and Tipler) several years ago, and just couldn’t do it.

I see what you mean. Your argument suggests that if there are many universes which contain civilizations, we would have a higher chance of ending up in one with a great number of them.
UNLESS,… the vast, VAST, majority of those universes were scarcely populated. In that case, even though there was a tiny fraction of odd-ball, densely populated ones, chances are we wouldn’t be in them.

PD. I’ve been introduced to the doomsday argument. I don’t know if it’s been discussed in the boards before but, using the “Interesting Subject Principle” , I’d guess that it already has.
John Mace
I’ll read up about the Participatory and Final Anthropic Principles as soon as I can.

PD Doesn’t the weak principle sound stronger than the Strong Principle in your definition?

Not my definition, but Barrow and Tipler’s.

Focus on the word must in the SAP. That’s my take.

John, thanks - yes I was working from memory and “weak/strong” is indeed the standard usage (I think I was confusing “hard/soft” consciousness problems or something).

Again, I never really understood anything but the weakest version of the anthropic principle. Why should a universe need consciousness? It functioned perfectly well for 12 billion years without someone going around collapsing all the wavefunctions. The stronger versions all seemed to me to be merely a desperate attempt to limit the sheer improbability of a life-bearing universe, since I cannot conceive of a mechanism whereby the later existence or not of offal-which-can-think affects the intitial conditions of that universe.


Yes, but how long is a piece of string? Until one knows the exact form of the function F(x) you will merely be integrating over F(x)dx. There is no such thing as an “average function” - are you suggesting counting all the different mathematical functions the human mind can conjure and seeing which was most common?

What you seem to be proposing is a Gaussian - the function which decribes essentially any random process, and wondering “is the Gaussian spiky or bloated?” (ie. is the variance [symbol]s[/symbol] small or large?). Again, like msmith points out, we cannot tell until we find more data points.

eburacum’s illustration is a useful one, although one should remember that since life evolves (ie. the probability of selecting the blue ball increases from 0 over time) we still have no clue at what stage the selection is taking place at (ie we might be in a blue-ball universe but we’re still the first one). As I said, there is evidence that the sun is one of the first few stars which could shine on living matter.

As long as we are talking probabilities, wouldn’t it be highly improbable that we are the first ?
Unless,… the doomsday principle is right, and we are the first, … AND last

the “interesting subject” principle works:

As long as we are talking probabilities, wouldn’t it be highly improbable that we are the first ?

As improbable as being the second, or the nineteenth, or the 1024th.