Probability of life.

[Tank]

There is no such thing as a natural miracle. A miracle is by definition supernatural. It's use in a natural context is misuse of the word.

Re-reading the opening chapter of The Incredible Unlikeliness of Being: Evolution and the Making of Us by Prof. Alice Roberts, I found where I had read the term natural miracle.
Apart from being a famous biologist and anthropologist she is also the former president of the British Humanists.

Yes I know her. Still doesn't mean she's right. And appeals to authority hold no weight here.

[zorkan]

But have you read the book?
The first chapter takes my breath away.

I don't like authority either.
If someone says I am right and you are wrong, I turn my back.

[Asmodean]

It's not a matter of antiauthoritarianism - just that an argument of "Well, this here smart person *point* thinks so" is unpersuasive.

Look at it this way; Let's say I approve of Brexit while Tank can't stand it. (By the way, this is an example. Actual opinions may vary and are not that relevant to this discussion) Is the validity (as opposed to quality or eloquence) of our arguments for and against it dependent on our respective IQs? What happens when two for all intents and purposes equally "smart" people come to mutuially-exclusive conclusions about the same issue, based on the same data?

"Yes, but Einstein/Jesus/my mother said so!"

Well, so what? They may think that their conclusions are accurate, as may you - but what does that mean to me?

What I would like, is something akin to this, for instance, "Einstein thought that Newtonian mechanics were inaccurate/incomplete because they would not organically conform to explaining and sufficiently predicting the motion of certain celestial objects. I agree with that line of thinking because Mercury."

Now we can discuss whether or not you have a point, using Einstein's body of work as reference. Otherwise, you need not have invoked his name at all, except perhaps to credit him for what you "copy/paste" into your argument.

EDIT: revised a little for clarity between citing sources and arguments from authority.

[zorkan]

I like the football crowd analogy.
Ask each individual to write down what they saw after the game and you are going to get any number of different accounts.
Might be able to explain this by reference to Einstein's famous theory.
So who do you trust? No one.
There is no authority.
Can you even describe what that life form is? You can't.
You can only label it as a human, a cat, or whatever your language allows.
You can conclude it's an arrangement of atoms done in a certain way which is successful in surviving its environment.
But why?
 

[Recusant]

Perhaps I've got the wrong end of the stick but it seems that "why?" is seeking meaning. If so it's a philosophical question rather than a scientific one. It's a worthy philosophical question I think.


I got here via a search for "anthropic principle" so might as well post this science item here. 

"Many physicists argue the universe is fine-tuned for life – our findings question this idea" | The Conversation

Physicists have long grappled with the question of why the universe was able to support the evolution of intelligent life. The values of the many forces and particles, represented by some 30 so-called fundamental constants, all seem to line up perfectly to enable it.

Take gravity. If it were much weaker, matter would struggle to clump together to form stars, planets and living beings. And if it were stronger, that would also create problems. Why are we so lucky?

Research that I recently published with my colleagues John Peacock and Lucas Lombriser now suggests that our universe may not be optimally tailored for life. In fact, we may not be inhabiting the most likely of possible universes.

We particularly studied how the emergence of intelligent life is affected by the density of "dark energy" in the universe. This manifests as a mysterious force that speeds up the expansion of the universe, but we do not know what it is.

The good news is that we can still measure it. The bad news is that the observed value is way smaller than what we would expect from theory. This puzzle is one of the biggest open questions in cosmology, and was a primary motivation for our research.

We tested whether "anthropic reasoning" may offer a suitable answer. Anthropic reasoning is the idea that we can infer properties of our universe from the fact that we, humans, exist. In the late 80s, physics Nobel laureate Steven Weinberg discussed a possible anthropic solution for the observed value of the dark energy density.

Weinberg reasoned that a larger dark energy density would speed up the universe's expansion. This would counteract gravity's effort to clump matter together and form galaxies. Fewer galaxies means fewer stars in the universe. Stars are essential for the emergence of life as we know it, so too much dark energy would suppress the odds of intelligent life such as humans appearing.

Weinberg then considered a "multiverse" of different possible universes, each with a different dark energy content. Such a scenario follows from some theories of cosmic inflation, a period of accelerated expansion occurring early in the universe's history.

Weinberg proposed that only a tiny fraction of the universes within the multiverse, whether real or hypothetical, would have a sufficiently small dark energy density to enable galaxies, stars and, ultimately, intelligent life, to appear. This would explain why we observe a small dark energy density – despite our theories suggesting it should be much larger – we simply could not exist otherwise.

A potential pitfall in Weinberg's reasoning is the assumption that the fraction of matter in the universe that ends up in galaxies is proportional to the number of stars formed. Some 35 years later, we know that it is not that simple. Our research then aimed at testing Weinberg's anthropic argument with a more realistic star formation model.

[Continues . . .]

A graphic giving a rough synopsis of their findings:

Image credit: Readapted from D. Sorini, J. A. Peacock, L. Lombriser

The paper is open access:

"The impact of the cosmological constant on past and future star formation" | Monthly Notices of the Royal Astronomical Society

Abstract:

We present an extended analytical model for cosmic star formation, with the aim of investigating the impact of cosmological parameters on the star formation history within the ΛCDM paradigm. Constructing an ensemble of flat ΛCDM models where the cosmological constant varies between Λ = 0 and 10⁵ times the observed value, Λ_obs⁠, we find that the fraction of cosmic baryons that are converted into stars over the entire history of the universe peaks at ≈27 per cent for 0.01 ≲ Λ/Λ_obs⁠ ≲1.

We explain, from first principles, that the decline of this asymptotic star formation efficiency for lower and higher values of Λ is driven, respectively, by the astrophysics of star formation, and by the suppression of cosmic structure formation. However, the asymptotic efficiency declines slowly as Λ increases, falling below 5 per cent only for Λ > 100Λ_obs⁠.

Making the minimal assumption that the probability of generating observers is proportional to this efficiency, and following Weinberg in adopting a flat prior on Λ⁠, the median posterior value of Λ is 539 Λ_obs. Furthermore, the probability of observing Λ ≤ Λ_obs is only 0.5 per cent. Although this work has not considered recollapsing models with Λ < 0⁠, the indication is thus that Λ_obs appears to be unreasonably small compared to the predictions of the simplest multiverse ensemble. This poses a challenge for anthropic reasoning as a viable explanation for cosmic coincidences and the apparent fine-tuning of the Universe: either the approach is invalid or more parameters than Λ alone must vary within the ensemble.