I understand what is meant when we say the universe is "fine tuned", but I've seen it claimed it's is fine tuned "for life". Is it?

When I look at known or even speculated life in our universe, I see life clinging to a tiny crumbs separated by vast voids. Our life needs baryonic matter, only 5% of the universe. Its density is $10^{-30} g/cm^3$, but it clumps together leaving vast voids between the clumps. 99% is in stars and black holes and nebula utterly inhospitable to known life.

That leaves the parts "fine tuned for life" to be 0.05% of the universe. The planets, asteroids, and comets seem like just debris separated by voids billions of times larger than the life-sustaining clumps preventing life from spreading.

Even that seems to be used very inefficiently. Life requires the energy of a star, but it's spewed out in all directions. We only receive a tiny fraction of it. Similarly, life requires heavier elements created in supernova, but these are also spewed out in all directions. And it took billions of years to develop a being capable of asking "is the universe fine tuned for life?"

It seems like we live on the crumbs of creation warmed by a rounding error of the heat from our star. Taking this view, the universe appears extremely inhospitable to life. It could be a lot better tuned.

Is it that the universe is fine-tuned for life? Or is it that the universe is so large and life requires such an infinitesimal fraction of the universe that it's a near guarantee were we to roll the dice again that some crumb, maybe not even baryonic matter, could develop something complex enough to ask "why is the universe fine tuned for life"? Am I just restating the weak anthropic principle?

I understand this is straying into philosophy. If that's off topic, are my physical assumptions correct?


"There must be a god" said the puddle. "Otherwise, how did the dip I sit in come to fit me so perfectly?"

There are fine tuning issues that still absorb people's attention. But fine tuning specifically for life isn't really the point. After all, this is the only planet we are sure has life on it. So we don't really have any reason to think we know what the probability of life arising might be. Maybe life can arise in a wide range of conditions. Maybe we are "as good as it gets." Maybe we won the lottery.

If life hadn't happened we wouldn't be here to ask about it.

We do know that nearly all of the universe is drastically inhospitable to our specific kind of life. But we also know why our specific tiny spec of the universe fits us so well. It's because we have spent many millions of years adapting to it. I'm having difficulty remembering exactly when life is thought to have arisen. I seem to recall that single cell life is vastly much older than the Cambrian explosion. But even that is about a half billion years ago. The ones that were less well adapted were less likely to have progeny. For many millions of generations.

We don't know whether some other kind of life could have adapted to another dip in the ground. Maybe if we carefully inspected at least a few other planets in other solar systems, close up and carefully enough to be sure if there was life, maybe we would have a better notion.

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  • $\begingroup$ What ever is IS. We could say also that the universe is fine tuned for anything not just life, right? $\endgroup$ – Žarko Tomičić Jun 27 '18 at 21:19

The only unquestionable truth (dare I say, tautology) is that the universe MUST allow life, otherwise the universe in question cannot be observed or experienced. This is the philosophical basis for the Anthropic principle (AP). An observable universe might be otherwise arbitrary, unless constrained by other universal principles. But I am not aware of any other principle that is as universal as the AP.

There are many variations and refinements on the principle based on Bayesian analysis. We can further demand that an observable universe must not only allow for any kind of life, but intelligent life, otherwise we would not be here questioning the nature of natural laws

The principle implies that you can use it to argue for unnatural conditions of the universe. The limitation is that in order to keep the argument formal, you need also to provide convincing evidence that entropically favorable conditions are usually hostile to intelligent life, which is something that is impossible without a deeper understanding of the fundamental theories from which our known theories emerge

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  • $\begingroup$ So it implies there is no objective universe? $\endgroup$ – user45664 Jun 27 '18 at 22:57
  • $\begingroup$ what does an 'objective' universe means? how it is defined? $\endgroup$ – lurscher Jun 28 '18 at 0:57
  • $\begingroup$ Exists independently of human perception or thought--no humans required. $\endgroup$ – user45664 Jun 28 '18 at 1:15
  • $\begingroup$ an universe that cannot be observed goes beyond of the scope of the subject of physics. All universes presumably exist mathematically, even ours, in a Platonic sense, as a set of solutions of some unknown complex mathematical equations with unknown boundary conditions. This we are somewhat certain about. But physics concerns itself with what can be experienced, not with things that might exist beyond the scope of experience $\endgroup$ – lurscher Jun 28 '18 at 21:49

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