# What's the probability of a universe capable of life?

1. Jun 11, 2010

### brushman

What's the probability of a universe capable of life? In other words, what's the probability that the laws of the universe allow for life?

And if the probability is low, does that imply there are infinite universes? Or there is a "god"?

2. Jun 12, 2010

### Chalnoth

Nobody knows. To know this, we would need to know the correct theory which describes how regions of the universe like our own are generated.

3. Jun 12, 2010

### mathman

It is possible that there is only one universe. In that case the probability would be one.

4. Jun 12, 2010

### Visigoth

Your question would be based on an assumption of a multiverse - and should a multiverse exist, there is no reason to believe it would be finite. In which case, the probability would either be a binary composite approaching 0 or approaching 1. As the above poster pointed out, based on our current evidence we can only assume it is 1.

5. Jun 12, 2010

### bapowell

Why do all answers to this question overlook the terribly complicated and unknown processes by which life gets started once (and if) a habitable place in the universe exists? You might succeed in finding another Earth in our very universe (surely many such planets exist) but until you understand how nucleic acids synthesize out of the primordial soup, questions like this are moot.

6. Jun 12, 2010

### Visigoth

Questions like this are rarely very formal in nature, and are mostly just speculation based on absolutes - it's brain food! I doubt the OP was looking for a scientific explanation, insofar as it can be given. Baseless speculation tends to only be interesting if we ignore the idiosyncrasies which complicate the problem. :)

Physicists generally don't concern over the structural dynamics of the paint on a spaceship with the ability to travel through time. :P

7. Jun 12, 2010

### bapowell

Right, except I don't think my objection is a marginal detail like paint on a time machine. It's completely central to the discussion. The question of whether or not intelligent life exists elsewhere in the universe is deeply important, and fun to think about. But people tend to get lost in debates about cosmological constants, bubbles in the string landscape, the Drake equation, etc. This are 0th order requirements for the existence of life. However, it's the sort of '1st order' problem -- the actual development of life -- that is not too well understood at present and seems to be fully ignored by most physicists. I'm a physicist myself, so it's not like I'm some pissed off biologist that's disgruntled that my science is being ignored ;) I just think we should at least acknowledge that we are ignoring abiogenesis completely when we are supposedly discussing whether a certain universe or planet is 'capable of life'.

8. Jun 12, 2010

### Visigoth

Ah, certainly then, I agree with you. I suppose I'm just not too fond of acknowledging the trifling nuances! :P

In all seriousness though, isn't it fair to suggest that in two Universes which appear to mirror the fundamental cosmological tenets of one another, that the probability of abiogenesis occurring in one Universe is equivocal to that in the other? In which case, wouldn't the probability of the process occurring be wholly irrelevant?

Just my thoughts. :)

9. Jun 12, 2010

### Chalnoth

I'd just like to add a small point: though our current evidence is wholly and utterly insufficient to come close to pinning down what portion of the universe contains life, the evidence most definitely seems to point in the direction of vast volumes of space that are barren and lifeless, with some small regions here and there (like our own) that contain the occasional life-bearing planet.

10. Jun 13, 2010

### mathman

I wonder where you got this information. I have been under the impression that our sun was a rather ordinary star. I would assume that similar stars would have similar planetary systems, including some earthlike planets. I doubt if they are particularly rare.

11. Jun 13, 2010

### Visigoth

I think what he's suggesting is that the amount of empty space far exceeds the amount of occupied space in the Universe. Certainly, there's trillions and trillions of stars in our galaxy alone, but the amount of empty space between those stars, and between our galaxy and others, is exponentially larger.

12. Jun 13, 2010

### Chalnoth

You're both a little bit off from what I meant. What I meant is that I expect that the vast majority of Hubble volumes are entirely devoid of life.

13. Jun 14, 2010

### mathman

Is this your guess or is there some study which leads to this conclusion?

14. Jun 14, 2010

### Chalnoth

Well, as I said, without knowing how hubble volumes like our own get started, we can't really know this for sure.

But, even in the standard model of particle physics there are spontaneous symmetry breaking events that would be different in different locations, and we expect there are likely more such events encoded in still higher-energy physics. Perhaps many more. This would be an indication that the low-energy physical laws which we experience would be a product of a peculiar series of random events in the early universe which would be different from place to place.

The second bit of suggestive evidence comes from dumb examinations of the physical constants: slight changes in most of them result in a universe which would be devoid of life. Now, these are rather dumb examinations, because without the correct theory explaining how new Hubble volumes get started, we don't have the probability distribution for these constants. But taking either a dumb flat prior or a logarithmic one seems to indicate that the region of parameter space available to life is likely to be a tiny fraction of the whole.

The final bit is a philosophical point. The second point above is basically a brute fact that must be dealt with (I'm sure I could come up with references if you really need them). This fact forces us into one of two possible situations:

1. In reality, only a minuscule fraction of the entire apparent parameter space is actually possible. It just so happens that this tiny bit of the apparent parameter space is also be the tiny bit that is conducive to life.

2. A significant fraction of the parameter space is possible, and the parameter space is explored.

The first possibility sounds to me quite magical, and from a philosophical standpoint is almost certain to require a theory with more parameters to specify the smaller parameter space (and thus made less likely due to Occam's Razor). In general, specifying a class of objects requires fewer postulates than specifying a single member of the class.

From this I conclude that a very large, mostly-uninhabitable universe, somewhat like a larger version of what we see in our own universe with its vast expanses of uninhabitable space, is by far the most likely possibility. The apparent discovery of spontaneous symmetry breaking in high-energy physics appears to be pointing us more strongly in this direction as well.

15. Jun 15, 2010

### mathman

Aren't the physical constants supposed to be the same throughout the observable universe?

Last edited: Jun 15, 2010
16. Jun 15, 2010

### Chalnoth

Yes, but we also know that the observable universe is but a small fraction of the whole.