> i hear this argument constantly, but i've never been able to understand it. you say the universe is big and has existed for a long time, but compared to what? if i say the universe is small and new, how would you counter that? i don't mean to come across as aggressive, but if there's any arrogance on this issue, i'd suspect it lies in thinking we have enough information to come to any meaningful position on the issue.

Basically, the universe is so ludicrously huge and so ludicrously old that it is as close to statistically impossible as one can imagine that human beings are the only things ever to evolve in such circumstances as to be sapient.

Note that the paper you have linked uses the term "detectable civilization", which is significant limitation due to our current level of technology, the age of the universe, and the propagation speed of light. It wasn't that long ago that we couldn't even detect extra-solar planets.

There is of course an argument to be made for us being "alone" in the universe in the sense that sapience is rare enough that pretty much all species, including ourselves, will die out before ever meeting another sapience, but the idea that in all the unimaginable vastness of the universe we are the only ones?

Without specific numbers and calculating chance of life like ours to occur I don't see how you can confidently make such claims. It could very well be the case that even for a universe this large, it was a statistical anomaly for it to have life. Maybe out of 1000 similar universes only 1 would happen to have life. How can you tell the odds so confidently?

Maybe even out of 1,000,000,000 of similar universes only 1 would actually happen to have life. It's similar with planets. If you only know of 1 planet within 1B to have life, how can you tell the odds are not low enough to have 1 universe out of 1B to have life?

This could be an awesome example of survivorship bias to assume because there's life here, there must be life elsewhere in the universe too. We only have experience with a planet where there's life, so we are innately biased to think that it might be more frequent occurrence than it really is.

Other than arguing that the infiniteness of the universe somehow indubitably lends credence to the emergence of some convergent sentience, could you or anyone point to something a bit more rigorous (such as the paper your parent comment linked to)?

We are aware of the evolution of one sapience in the universe. What argument do you provide that suggests this is such a mind bogglingly unique occurrence that it cannot have happened anywhere else in the universe's volume of over 400,000Gly^3 at any time during its age of 13.8 Gyr?

In other words, the fact that it has happened at least once very strongly suggests it has happened more than once.

"In other words, the fact that it has happened at least once very strongly suggests it has happened more than once."

No, it doesn't at all.

Maybe just the opposite.

The universe is mind-bogglingly large, but it's also really, really empty, and really, rally sparse. The numbers games starts to dwindle very quickly in terms of all the ingredients necessary.

And the Fermi paradox helps provide crudely indirect evidence.

If there was any validity at all to the 'common life theory' - then it would have started long ago and probably spread everywhere to the point wherein there would probably be ample evidence.

In the 'common life' scenario, it should be rare/impossible to find life that's disconnected from other life scenarios.

There's a decent chance we are alone.

I guess I disagree with the basis of the argument that it happened once, it has to have happened another time. I don't think a single instance of very unique set of circumstances is the basis of a pattern.

I don't think the argument is that it "has to have happened another time". The argument is that if it is possible to happen once, it's likely to have happened more than once, the "existence heuristic." It proves only that it might be worth looking, unless perhaps further research indicates that we on earth are not, in fact, intelligent life, and then the likelihood of intelligent life elsewhere in the universe falls off dramatically.

There is an argument that one instance of life evolving does imply there should be much more. I personally don't believe this though, though it is a valid argument. There is a counter argument too.

The common argument in favor of more life is that the probability of life evolving is some very small number, such that there must be a very large number of opportunities for life to evolve in order for life to actually evolve. At the same time, there are a large number of opportunites given the size of the universe. Given that there is presumably no relation between the "small" probability for life to evolve and the "large" number of opportunities in the universe, we don't expect these two numbers to conspire to be near the same size, which is what would have to happen for just one life to evolve. Rather, we expect there should be lots of life that evolved in different places or that the probability that any life evolved at all is extremely (vanishingly) small. So if this argument is correct and we know of one life form, we should expect many more in the universe. (Not to imply we would necessarily be able to find them.)

There is one potential problem with this argument. It assumes there is just one universe. If there are many universes then it is possible that life did not evolve in almost all of them. In universes where life did evolve, odds are it happened just once. And of course, that life (us, for example) would see that univese and the life that evolved there, however improbably that would be.

I read a saying "You know an argument is wrong if it invokes quantum mechanics." But I'll do it anyway. Some educated people believe quantum mechanics works this way - the univese effectively branches into parts that are essentially distinct (referred to as different universes too). It is not as outlandish as it may seem. The whole reason quantum computers are supposed to be powerful is that different things happen in parallel.

You don't have to buy the quantum mechanics part. If there are multiple universe for whatever reason then the probability argument above is not valid.

I do want to add one thing. The number of "opportunities" for life in our universe grows linearly with each dimension in the universe. (Lets just say to the fourth power of the size of the universe.) On the other hand, the size of probability grows/shrinks exponentially. So it is not hard to imagine between the two of these numbers, the exponential one wins.

The visible universe is so large it is effectively infinite. And that’s just the visible universe. With 125 billion known galaxies with a 100 billion stars in each it definitely feels arrogant to imagine that we are literally the only sentient beings.

It might still be that there is probably barely 1 advanced life form per galaxy. We know that organic molecules and related evolution requires air, water, temperature and stability that Earth like planets can afford. Unfortunately, to be Earth like planet, you don’t just need right distance from the star but abundance of water, minerals and so on. You must maintain magnetosphere otherwise water will be gone. You must have right moon otherwise axis will wobble with eradicate seasons. Even there life had been almost wiped out many times. There are half dozen human like species which were extinct and never panned out. All these goes on to show that even with 100B stars, you just need handful of probability to chope it down to one or so instances.

The non-existence proof of ETI also means laws of physics are cruel and cannot be violated. There is almost 100% chance that advanced life forms exist in other galaxies, may be even billion year old. But no one has figured out how to travel faster than light.

Old and big for something that would only have 1 planet with intelligent life. It may well be small and/or young from another PoV.

If anything I'd hazard our ignorance is more on the side of things we don't know we haven't observed than things we observed incorrectly.

> i hear this argument constantly, but i've never been able to understand it. you say the universe is big and has existed for a long time, but compared to what?

I think compared to how long it took for somewhat intelligent life to evolve on Earth, from the point of first life. Assuming* a comparable evolutionary cadence, the question then becomes - how often does life begin?

*This I assume because the cadence of living things on Earth is at the very least tied to the day/night cycles, winter/summer seasons, and lunar tides, none of which are especially unique.