Six numbers: if any was altered in a very small degree, the universe would not have permitted life to develop.
For example, if gravity wasn't exactly this weak comparing to other forces in the atom, but not weaker, the universe either would have collapsed right after Big Bang, or would have expanded so fast that no stars, galaxies, planetary systems could've formed.
Thus, no potential for life.
Writing and readability
Rees makes his case of fine tuning with regard to life very convincing. The book is written for the lay reader, and it's decidedly a must-read for anyone interested in cosmology, astronomy, physics. It's rare that I find books for popularizing science that don't have the faults of being clumsy written or assuming a much higher level of knowledge that they're advertised for. To bring science down to earth is no easy enterprise, and Rees, reputable cosmologist, succeeds amazingly.
The book makes many comparisons of the numbers and ratios it speaks of, with every day examples, or it creates elocvently frame by frame images, to convey just how precise (small or big) the numbers are. This is done so well that it leaves you feeling you now really - really - know more, understand the universe better, and estimate the extreme unlikeliness of our universe to turn out just right for life.
The writing style does wonders to convey to the reader a powerful case for the fine tuning of each number.
However, the thesis doesn't stand up to basic philosophical/logical objections.
Rees compares what would happen if each number varies, assuming everything else is equal. One at a time. And concludes from it, that they're extremely fine-tuned. However, what would happen if you vary two numbers at a time? How about three? How about varying relations between one of these numbers and the other elements, which Rees combines with it (according to laws of physics of our universe) to yield his results of dead universes?
Varying one at a time is not a throughout investigation. I can't conclude from it *anything*.
Example. Let us say we have six integer numbers, and their sum is 1000. Let us say that a "life-permitting sum" is in the range 999 and 1001.
If I vary one of the numbers, with 1 (plus or minus 1), I still get about 1000. If I vary that number with 2 (plus or minus 2), I no longer get my goal sum. If I vary it more, the sum will never be in the goal range. I had only two permitted variations.
But, if I vary 2 numbers at a time, I can obviously "succeed" with significantly more variations. If I vary 3 numbers at a time, ever more.
If I also accept that the "law" can be changed (the function may be a sum, or a product, or an exponential function, etc), I can have way more winning combinations.
I may have more failures than successes, true, but I have a bigger pool of wins, meaning the numbers themselves are not "just about right".
Rees' fine-tuning thesis assumes varying one number at a time, which is only a slice of the research to get a whole range of (potentially) life-permitting universes. After it limits itself this way, it asks for an explanation for such an improbable event of each number being exactly right.
I'll say it's not a defensible position, from a logical-philosophical perspective.
Rees' possible answers to his question are the multiverse theory and the creationist hypothesis, with a nod in the direction of a possible unified theory that will eventually explain why these numbers had to be as they are. Since I don't think the question was entirely correct to begin with, I don't feel compelled to jump to his conclusions yet.
Critique of the critique
When I read the book, I was left with the question: is fine-tuning, in the current mainstream physics and cosmology, assuming variation of one number at a time, and drawing conclusions from only it?
I don't know, I'm no physicist, and while I feel I learned from this book (and it's Cosmology 101!), I will say that the reasoning itself at the basis of the argument is flawed. However, it seems I received my answer, from another direction.
This week, in the discussion on Manny's review of The Fallacy of Fine-Tuning, he quotes another cosmologist, Barnes, a supporter of fine-tuning theories. With this occasion (with this occasion I read Rees' book as well), I read more of Barnes' blog and articles, and I came upon this:
This gives me my answer, in no uncertain terms:
There is an objection to fine-tuning that goes like this: all the fine-tuning cases involve varying one variable only, keeping all other variables fixed at their value in our universe, and then calculating the life-permitting range on that one variable. But, if you let more than one variable vary at a time, there turns out to be a range of life-permitting universes. So the universe is not fine-tuned for life.
This is a myth. The claim quoted by our questioner is totally wrong. The vast majority of fine-tuning/anthropic papers, from the very earliest papers in the 70′s until today, vary many parameters.
Also, further down the page, Barnes refers to this book:
This myth may have started because, when fine-tuning is presented to lay audiences, it is often illustrated using one-parameter limits. Martin Rees, for example, does this in his excellent book “Just Six Numbers“. Rees knows that the limits involve more than one parameter – he derived many of those limits. But equation (1) above would be far too intimidating in a popular level book.
Indeed, I got my answer, spot on! The equation noted is above my (undergrad and forgotten) math level. However, the question I had while I was reading Rees' book had to do with internal logic of his thesis.
The road ahead
Which raises another question: what is then, the actual thesis/question of fine-tuning literature today?
According to Barnes, the only constraint in varying parameters to get possible universes, is for these universes to be logically possible. (non-contradictory)
That's a bold claim, if I ever saw one. And I mean bold. Particularly surprising when Barnes explicitly states that an universe is defined by (initial conditions, constants, laws of physics), and, according to him, *all three* are fair game, for the variance experiment, including laws of physics. I should add though, the claim strikes me as methodologically correct, because what else is there to assume about the possible universes? We can't necessarily assume they obey the physical laws that may have been set from the initial conditions (which we vary!) of our Big Bang for our universe. But the magnitude of the task, even if there are mathematical tools to make it more reasonable, leaves me in a combination of awe and disbelief.
Once she chose their universe(s) to examine, the fine-tuning-interested cosmologist then solves the equations for those possible universes. If the universe is not self-consistent, then it's trashed. Then estimate the probability for the universe under examination (actually class of universes) to be life-permitting.
The purpose: estimate the probability of life-permitting universes in the set of possible universes.
This is my current understanding of Barnes' paper and blog, and with them, a certain direction on fine-tuning today. I'd have more to say about those logically possible only universes (!), but I guess I'd better wrap up this review and read more.
Rees doesn't claim for his book to have another purpose than it does: to make general readership understand better a slice of one of the problems of cosmology today. He succeeds very well, perhaps too well. Apparently, we, lay readers, might get easily from it a too limited but powerful impression about the sides of the controversy of fine-tuning. :)
Luckily, the same is not the case on physics and cosmology. Controversies aside, I think this 101 in cosmology is one of the best written books popularizing science. Very recommended, and easy to read.
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