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Comment by Razengan | Hacker News Reader

And some people still aren’t sure if there is other life beyond Earth. :)

That's entire galaxies. Not just stars, and that's just the observable universe.

We are still being surprised by the lifeforms we observe here on Earth, and the environments and conditions they can thrive in (including the outer hulls of our spacecraft), and we still have yet to discover and catalog all the life on our planet. Life may even come in the form of "artificial" constructs that build, learn, teach and reproduce on their own, created by "natural" life somewhere then set free into the cosmos.

How, when given the evidence of practically infinite planets out there (considering that almost every star may have at least one planet, and there are even rogue planets [0] that don't orbit any star, and that asteroids and comets or even dense nebulas could also harbor life), can we even consider the possibility that this is the only planet with life?

This game is Big, folks: Even if 90% of all planets had life there would still be billions if not trillions of them without any life, and the incomprehensibly vast distances (including voids, like the one our own galaxy is near or inside of [1]) between them may mean millions of years before life from one planet encounters life from another (which is good I suppose; every species would have some room to expand into at their own pace.)

And of course, not all intelligent civilizations will be "successful."

We will either die out on our home planet or expand.

We may not encounter other life for hundreds or even thousands of year after we develop interstellar travel, or the life that we encounter may turn out to be mundane and eventually unexciting (I mean, imagine being an spacefaring species discovering us; planetlocked and still collectively figuring many basic things out.)

[0] https://en.wikipedia.org/wiki/Rogue_planet

[1] https://en.wikipedia.org/wiki/Milky_Way#Environment

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> How, when given the evidence of practically infinite planets out there [...] can we even consider the possibility that this is the only planet with life?

Human minds aren't good with very large numbers. What seems "practically infinite" to us - isn't, really.

The number of all planets in the universe is said to be in the ballpark of 10^24.

Meaning (obviously) if you keep on dividing it by 10, in just 24 steps you get down to 1.

Meaning if there are 24 factors crucial for life to evolve, and each has a 10% chance of occurring - it's actually quite likely only one planet would win on this lottery. (At a given point in time, that is - that's another aspect of course).

Or 12 factors, but each with a 1% chance. And suddenly it doesn't seem like a number big enough to warrant certainty claims...

Basically it is the "wheat and chessboard" problem in reverse.

You start with a huge pile of planets, but once you start splitting the pile, it shrinks very very fast.

Personally I'm not sure either way, and I think being sure (either way) is just naive.

I believe there is microbial life out there, and I accept a slim chance we may actually discover it - say, over the course of the next few centuries - but I'm sceptical about the existence of intelligent life, and even if it did exist, I see the chances of any contact to ever occur as zero.

I agree with you in that we will probably never have contact with another sentient species of extra terrestrial origin. The distances are to vast, the lifetimes of civilisations and even species too short and so on.

However I do think that you vastly underestimate the probability of life. Take a look at https://en.wikipedia.org/wiki/Earliest_known_life_forms . See how quickly life appeared once the earth had cooled down? It's almost immediately!

I'm pretty convinced that almost all star systems have some complex energy conversion cycles that could be classified as life. Let's not make the mistake of assuming all life has to be carbon based.

See how quickly life appeared once the earth had cooled down?

https://en.wikipedia.org/wiki/Panspermia

I find this a compelling possibility.

Highly unlikely.

DNA's 'half-life' is about 500 years. Meaning that each nucleotide-bond has a 50% chance of breaking every 500 years.[0] At the time scales of interstellar travel, DNA based life is very unlikely to survive.

EDIT: This then means that DNA based life was likely a local phenomenon. This also means that other planets may have done the same and that a 'interstellar seed' is not required.

[0] https://www.the-scientist.com/?articles.view/articleNo/32799...

But it's fairly likely to be carbon-based, as carbon is the simplest existing element with the maximum amount of valence electrons. And generally, simpler (= lower ordinal number) means more stable, as well as more likely to arise (as less energy is required, e.g. in stars).

There are other theoretical biochemistry possibilities[1]. Silicon, for example, works well in low-pressure environments. It's an immense stretch of the imagination to think about life-forms that use semiconductors to do useful work, but is certainly a fun thought experiment.

[1]: https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi...

Yes, it's possible, just less likely. How much less likely is hard to tell, but I if you hear hoofsteps, you first think of horses, not zebras.

It may not be based on elemental matter, or matter at all.

Hmm, true. But then, interaction with elemental matter life forms (like us) would become terribly difficult; it might be hard to even notice there's a life form existing in anti-matter.

Do you think the periodic table contains all the elements in the universe?

Yes, since from what I understand, any heavier elements would be so unstable they wouldn't last a billionth of a second.

That's the general assumption, even though there might be another stable "island" of fairly stable isotopes for higher-order elements than observed so far. But as I said, they'd need to be created first through nuclear fusion of lower elements, so their existence is obviously less likely, let alone life forms evolving on top of them.

> See how quickly life appeared once the earth had cooled down?

What if life can only appear as a planet is finishing to cool down?

> I see the chances of any contact to ever occur as zero.

Yes, especially if you take into account the speed that entire galaxies travel in space. It's mind boggling high. So the effort for any civilization to build technology that allows them to travel through intergalactic space until they get "hooked" by Milkyway's black hole magnetic field is infinitely complex. That's where the whole idea around worm holes makes sense.

We don't know all the factors under which life can exist. Even the lifeforms that we do know of, occupy a very broad spectrum in all metrics, existing at all sorts of extremes.

We even have evidence of life we have never seen, will never encounter, and may never even have conceived as being possible to exist without that evidence: the enormous dinosaurs.

There's a huge difference between what evolved organisms can adapt to, and the conditions necessary to start life.

Look, humans already adapted to being in space for prolonged periods of time. We adapted to every climate. But drop a naked human in a random spot on our planet, and he will die with high degree of probability. And that's after billions of years of evolution.

Out of which some 3 billion years had to pass before unicellular forms of life evolved into multicellulars.

Who knows if this jump alone should be taken for granted - plus, not many planets could provide 3 billion years of "good weather" (or staying habitable) to start with.

> Out of which some 3 billion years had to pass before unicellular forms of life evolved into multicellulars.

We might be a slow developer and poor representative sample in that respect, but without more data it's hard to tell.

> plus, not many planets could provide 3 billion years of "good weather" (or staying habitable) to start with.

I suspect the opposite will be true, red dwarfs are the most common type of star and are stable for trillions of years. Even if the average time it took multi cellular life was 30 or 300 billions years then it might not matter for life evolving there. They do come with some other caveats though: https://en.wikipedia.org/wiki/Red_dwarf#Habitability

That is the odd thing about the history of life on Earth isn't it? Prokaryote popped up almost as soon as was conceivably possible. And then. Nothing. For billions of years. The plausible explanation is that you need Oxygen for Eukaryotes and that's how long it to change the chemistry of a planet. It just seems weird to me.

"Let me make my position clear. The miracle, and I do not mean it in the religion sense, I mean it in the evolutionary sense, the miracle of the evolution, is the cell. While there are theories involving an RNA world and selforganising, it remains a mystery. Once you had the eukaryotic cell from the point of view of evolution and development it was downhill all the way, very very easy. "

Lewis Wolpert : https://link.springer.com/chapter/10.1007/978-3-0348-8026-8_...

Just a reminder - we were naked for most of our history and were able to survive.

But not in nearly as many emvironments as we can today. We can assume, over time, life learns to live under absurd conditions; but its hardly safe to think it can start in those same conditions.

I think humans are a bad example - we use our own relatively highly developed intelligence to solve problems, and that means we had to feel safe enough from the environment long enough to develop that. But if we look at older organisms, like the tardigrade, there are some incredibly robust creatures out there.

However, all the life we know of is part of the same Earth system. Just because some extremophile exists in a freezing desert or boiling pool does not necessarily mean that the same environments on other planets would have those organisms.

Life on Earth evolved and spread under specific conditions - our gravity, sun brightness, geology, magnetic field, large moon, etc etc. It then spread to all possible environments on the planet and evolved to live there.

Unfortunately, it could be that the 'growth conditions' for life are quite specific even if - once it gets going - life can evolve to more general environments.

> Even the lifeforms that we do know of, occupy a very broad spectrum in all metrics, existing at all sorts of extremes.

But all lifeforms we know evolved from one common ancestor, it's one tree of life. As far as we know life evolved here only once.

So we know that once life happens it can become completely ubiquitous, but it doesn't say anything about that crucial first step.

> But all lifeforms

That's not nessisarily true. Single celled organisms have a rather fluid exchange of genetic information even our 'single cells' are really several different organisms. It's possible life evolved many times on earth and current single celled organisms and thus all life has a mismatch of ancestors.

Further, life may regularly evolve even today, but simply get eaten due to the stiff competition.

PS: We know life evolved at least once, but the high end could be multiple times a second on earth.

> It's possible life evolved many times on earth

It's also possible it evolved elsewhere and rode here on a meteor - or was brought here by some other lifeform several orders of magnitude more advanced than us, using earth as their own cosmic terrarium.

Why not both. We could also be descendants of an invasive alien species that killed off all of Earth's native lifeforms.

It's good practice to assume the simplest possible solution. However, the full range of the full range of what might have happened is ridiculously large.

Great reality check. I'll remember this point every time I read an article spouting the "there MUST be life out there because there are SO many planets" line.

> Personally I'm not sure either way, and I think being sure (either way) is just naive.

I think it's worth pointing out that there's lot of space between "absolutely sure there is" and "absolutely sure there isn't", and I doubt any fairly intelligent person actually holds either of those absolute positions. Which is to say that really, the arguments are over the degrees to which we think it's likely there is life, or intelligent life, elsewhere.

    Meaning if there are 24 factors crucial for life to
    evolve, and each has a 10% chance of occurring - it's
    actually quite likely only one planet would win on
    this lottery.

Is it? What is the probability?

I believe the post above was working out math that would make it so the probability would be 1 / #(of planets in the universe), which, following the law of large numbers, would mean that given an infinite number of universes, those universes would tend to have exactly one planet (Earth in our universe's case) that would have life.

In other words, the odds of there being life elsewhere in the universe is:

a very large number of planets * a very small chance of life forming = ???

We don't really know how small the chances of life are. It may be that the chances of life forming are so small it dwarfs the number of planets in magnitude. It may be an incredible miracle that we live.

It seems that whenever an article says "there are more planets than we expected", someone concludes "this makes it more likely that there is life somewhere out there", but it's also fair to conclude that "this means the odds of life forming are lower than we previously thought" since we haven't actually found any other life. As Fermi asked, where is everybody?

Phrased differently: the expected number of such planets is 1.

In most variations of such a calculation the expected number of planets would be 0. The fact that the actual answer is >= 1 might suggest that the calculation is not an accurate representation of reality.

I'm not sure I'm understanding. It is a well-defined probability question to ask about the "expected number" of planets with life if there are 10^24 of them each with probability 10^-24, and the answer is 1.

Yes it will be approaching 1, the same as for 10^24 coin tosses the number of heads would be approaching 50% (or 5*(10^23)).

Given the definition of "expected value," it is identically 1 (as opposed to merely "approaching it"). Sorry, I'm not trying to be pedantic.

https://en.wikipedia.org/wiki/Expected_value

You're absolutely correct - not being a native English speaker, I read "expected value" as a common phrase, not a probability theory term. Thanks

I wonder how long much longer it will take for the astronomers to get the technology that would allow them to hunt for life in other stellar systems so that we can check all this out.

Depends on the size of the dish, really. You need to resolve the 'signs of life' at stellar distances.

Street-Math time!:

Lets assume that means you need to resolve a meter sized elephant on another planet. Lets choose Vega, as that is ~25ly away, really the furthest star we can go to (and return from) and still have someone alive here to have a beer with afterwards (50 year round trip). 25ly is about 2.3E19 meters away.

A 1 meter sized elephant at 2.3E19 meters is about 4.3E-20 radians.

Lets use wavelengths in the visible spectrum. Hydrogen's Balmer peak at 656nm is a nice red color to look at (though stellar issues abound here).

The resolution of a telescope is approximately : R = L/D. R is resolution, L is the wavelength, D is the diameter. Rearranging we get the diameter to be : D = L/R. Plugging in a L of 656nm (~6.6E-7m) and a R of 4.3E-20radians and you get D.

D is about 1.5E13 meters.

For reference the earth is about 1.2E7 meters, about 6 orders of magnitude smaller. 1.5E13 meters is about 14 light-hours, much larger than the Earth-Sun distance.

Soooooo, I think it's going to be a while ...

;)

That's an interesting argument. But do you have some of specific conditions that might qualify?

@ASmallFiction had a beautiful, succinct take on this recently:

"Are we alone in the universe?" she asked.

"Yes," said the Oracle.

"So there's no other life out there?"

"There is. They're alone too."

https://twitter.com/ASmallFiction/status/946608733982822401

I like that it makes sense.

I'd say there certainly is life out in the Universse and it's quite possible there is intelligent life.

But the distances are so large that we will never meet any other intelligent life before our own civilization vanishes.

A nice way to think about it.

While, of course, it is technically possible, it seems very reasonable to infer that we may be in a small window in which we haven't connected to other intelligent life. In just 100 years we have gone from being barely able to fly at all, to being close to becoming a multiplanetary lifeform.

The evidence becomes more and more every day that life is not a one in a trillion miracle but a natural product of certain environments. To assume we will never have contact with other lifeforms seems like a huge jump.

> And some people still aren’t sure if there is other life beyond Earth. :)

Until we actually observe it, no, we can't be sure; the probability of it existing is high simply due to the sheer amount of dice rolls there are in the universe, but the likelihood of us observing it is low. We already know there is and was no intelligent life in our solar system besides what's on Earth; there's still hopes of finding traces of microbial life on Mars and maybe that ice moon. But unless we get a signal from far away (what the SETI project has been scanning for for decades now), we'll not find evidence of alien life in this generation.

I also doubt humanity will find a way to travel fast enough to get to another solar system within a human lifespan. If we do, it'll probably involve bending the fabric of space-time and the Great Old Ones will return from Beyond the Observable Universe and will start eating us.

> We already know there is and was no intelligent life in our solar system besides what's on Earth

We do not know all that was on Earth. We wouldn't have known about dinosaurs if we didn't chance upon their remains, and we only recently learned that they had feathers.

There is probably no way to know everything that has happened on this planet.

–

And that brings another facet of the point in the GP:

It's not just the distances between different planets that hinders life from meeting other life; even life on the very same planet is prevented by vast periods of time from ever encountering each other.

>I also doubt humanity will find a way to travel fast enough to get to another solar system within a human lifespan.

The next solar system is only four lightyears away. Given the will to break nuclear test ban treaties we think we could develop nuclear pulse propulsion drives that achieve about 10% the speed of light. Even after accounting for acceleration and deceleration that gets us there in a human lifespan.

Granted, even at that speed generational ships make a lot of sense, and the selection of targets is still somewhat limited. But we can get to our neighbouring solar systems in "reasonable" time if we want to, with very doable development effort

A better strategy is to start mining asteroids. Sell mined stuff on earth to make enough money to build ships and lift necessary bootstrapping material out of the gravity well.

Also plenty of water out there and sunshine, burning hydrogen as fuel should be possible.

Now you will have enough machinery in space to mine building stuff and enough fuel. Gradually build permanent colonies in space.

It won't get all humans out of earth soon or ever. But it will put some humans permanently out of earth. Then keep building colonies as population in space grows.

Once you have done this you have done two things:

1. Eliminate a existential threat to humanity more or less permanently.

2. A slow but steadily growing population of humans in space.

Several decades/centuries after that you could have humans permanently living and working in space. From there its a matter of population growth, resources are a plenty and you have a universe for the taking.

Once you have a massive presence in space interstellar travel by definition is a question of acquiring real estate to keep up with population growth.

Even if there were bars of pure gold just sitting there on asteroids (or even floating in orbit) for us to take, it would cost more to get them than just mining them on Earth. People underestimate the stupendous economic cost of doing anything in space.

Oh well not gold.

But if you told them they could get unlimited clean fuel. There could be plenty of funding to extract water/ice from asteroids and send it back to earth.

>Even after accounting for acceleration and deceleration that gets us there in a human lifespan...

It gets _something_ there in a human lifespan. The only really well though out models for that are Daedalus and Icarus, and they would involve flybys only of probes the size of a hockey puck, or a washing machine at most. The cost of which would be civilisation impoverishing.

I think it's a very reasonable chance that humanity will never, as a functioning human populated civilization, ever spread beyond this solar system. Now 'never' is a pretty long term statement, but no matter how advanced our technology becomes, it's still likely to be constrained by the physics we know. It looks like spreading beyond this system would cost a very large fraction of all the resources we have in the system to start with, if it ever becomes practical at all.

If we drop the requirement that someone who is alive on the ship when it leaves Earth has to be alive when it arrives at the destination, it becomes much more feasible.

The basic idea is to combine two kinds of ship, neither of which we know how to make work individually, to form one that we can make work.

The first is the generation ship. The problems with those are that (1) we don't know for sure how to make a completely self-sustaining ecosystem that can keep a large enough population for viable colonization alive for the hundreds of generations it would take to reach their destination, and (2) such a population would require a freaking large ship, raising all kinds of challenges.

The second is the cryogenic suspension ship. The problem there is that we don't know how to unfreeze them without killing them.

Can we address the generation ship problems? Both problems stem from the need to have a large population. If we didn't need a large population, we could use some of the space that would have otherwise been consumed by colonist families to bring food to supplement the food produced by the ship's ecosystem, and eliminate the rest of the space they would have occupied.

Assume we can develop a food that can be stored indefinitely and that has the caloric density of rice (200 calories/cup). There are ~4 200 cups in a cubic meter, which gives 840 000 calories in a cubic meter. At 2000 calories per day, that's 420 days of food. A storage unit 10 meters on a side gives 1150 years worth of food for one person. And this is assuming that the person is getting all their calories out of storage, not depending on the ship's ecosystem.

If the storage food only needs to be a supplement designed to make up for things that we can't figure out how to get from the ship's ecosystem, then the numbers get even better.

This is where the cryogenic suspension comes in. We can't suspend and revive people--but we can do eggs and sperm indefinitely and revive them. Our cryonic generation ship will carry the colonists as frozen eggs and sperm. The only warm people on board would be the crew to operate the ship, and to raise the next generation crew.

Note that the next generation crew could come from the frozen eggs and sperm rather than from mating among the current generation crew, so we don't have to worry about a small crew becoming severely inbred during the trip.

If we can ever develop robots to the point that a robot can raise and educate a human baby without any human supervision, then we can leave out the crew and the food and the ecosystem, and just send robots plus frozen eggs and sperm.

> ... or a washing machine at most.

The Orion project was designed for sending a city-sized space ship to Alpha Centaury in ~100 years.

The hard part seems to be how to build a city-sized ship, not how to move it.

My apologies, I meant Daedalus. I can’t find any references to the Orion project you describe, though there have been several real spacecraft and related systems by that name.

You can start at Wikipedia:

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

Basically nothing of it was actually built (nuclear propulsion wasn't an well accepted idea - for good reasons that won't matter as soon as we start building in space). There was just some very small scale prototypes.

In comparison, project Daedalus moved much faster.

Those were launch systems or in-system vehicles though, and much cruder and less efficient than Daedalus which was optimised for interstellar travel.

We already have that technology, more or less... A orion type drive is can be built using 1960's technology, the rest is just engineering, not new physics. A orion type drive can take you as close to c as you can afford. At 0.9999999c you can travel to A.Centauri in 1/2 day. Granted getting to 0.9999999 and back again is gonna take a fair pile of h-bombs.

Just to be clear, that would be 1/2 day for the people on the spacecraft. For the people you leave behind on earth you'd still be gone 4 years each way.

And you still have to slow down from 0.9999999c

That was my reaction as well. If you're being scientific about it, how can you be sure at all? We have observed exactly zero instances of life beyond Earth. We're all free to believe whatever we like. I have beliefs of my own, but I don't expect anyone to share them unless they happen to believe it too. I just hope people who are so sure of extraterrestrial life can comes to terms with the fact that they are making a leap of faith.

To nitpick: there is life on the International Space Station (beyond Earth). I don't mean 'in it'; I mean on it. Bacteria colonize some external surfaces.

So life can apparently adapt to extra-Earth locations. The leap of faith is then, that it hasn't left Earth, or that life on Earth didn't come from somewhere else to begin with? Because either scenario becomes plausible. It'd be strange to believe it didn't happen.

So long as people acknowledge that what they think is a belief or hypothesis, I love to hear it. I guess everyone is forced to believe something un-provable about life. Looking at all of those galaxies forces you to come to terms with how little any of us really know for sure.

> I guess everyone is forced to believe something un-provable about life

> Looking at all of those galaxies forces you to come to terms with how little any of us really know for sure.

Only if you are forced to look at all that - in detail - and since that's not essential to a mundane life, there is no obligation to believe anything much.

I mean, I'm trying to make a point about believe in distinction to knowledge. There is yet a weaker modus operandi, which I'd call awareness for now, but that's a loaded term, too. If you hold two thoughts about contradicting possibilities in mind at the same time, that's not believe. Believe happens when externalities force a decision, also called leap of faith. Then you choose to believe in one possibility. Knowledge on the other hand means the possibility of round about 1.

If we ever create life from nothing, identify conditions where that would happen,estimate number of locations Galaxy that would have planets with said condition...surely that would provide scientifically informed estimation of extraterrestrial life.

Any estimations about life outside Earth only become viable once we understand the factors that gave rise to it first, and their probabilities of occurrence. If the total probability is less than 10^24 (say) we might expect life elsewhere, but if it's 10^100 probably not. But either way, gut feel and intuition about this are pointless - by the anthropic principle, we're speculating about the possibilities of life only because we're already winners of the lottery - almost like someone lucky enough to win a lottery twice or thrice in a row wondering why it doesn't happen to everybody.

Either way, I actually think that the Dark Forest principle applies heavily in the universe (https://en.wikipedia.org/wiki/The_Dark_Forest) - no halfway intelligent civilization should assume the intentions of any other civilization are benign until proven to be so, and will always take steps to either hide themselves or shoot first and investigate later.

There's always so much guesstimation and wishful thinking going on. It makes sense though because we see this magnificence and want explanations. It's frustrating because we learn so much about what we don't know, and worse, how much we'll probably never be able to know.

> can we even consider the possibility that this is the only planet with life?

Yes, we can. I don't personally think life is all that rare in the universe- we agree there. But I think it's perfectly fair that it's possible we're alone.

It's really a question of how many unlikely events lead to us existing and how improbable those events are. People always start with "Let's imagine only 0.1% of planets have conditions for life! How rare that must be!". But is 0.1% a fair estimate? What if the true number is 1E-20? Or 1E-40? 0.1% is easy for humans to understand and think about so we always start there. Then apply the same reasoning to the jump from single-cell life to multi-cell life, to the odds of become larger animals, the odds of becoming intelligent.

We don't know what the odds are, so we always use odds that sound low but are actually quite large.

Just like humans are biased towards not seeing how big the universe is, you (like everyone else) are biased towards using probabilities in the realm of human understanding. That is just as wrong.

The question isn't how big the universe is, it's the comparison of how-big to how-unlikely.

Infinity is fun :)

It reminds me of that (totally wrong) quote in The Hitchhikers Guide To The Galaxy that the population of the universe is mathematically zero.

The Universe is a very big thing that contains a great number of planets and a great number of beings. It is Everything. What we live in. All around us. The lot. Not nothing. It is quite difficult to actually define what the Universe means, but fortunately the Guide doesn't worry about that and just gives us some useful information to live in it.

Area: The area of the Universe is infinite.

Imports: None. This is a by product of infinity; it is impossible to import things into something that has infinite volume because by definition there is no outside to import things from.

Exports: None, for similar reasons as imports.

Population: None. Although you might see people from time to time, they are most likely products of your imagination. Simple mathematics tells us that the population of the Universe must be zero. Why? Well given that the volume of the universe is infinite there must be an infinite number of worlds. But not all of them are populated; therefore only a finite number are. Any finite number divided by infinity is zero, therefore the average population of the Universe is zero, and so the total population must be zero.

Art: None. Because the function of art is to hold a mirror up to nature there can be no art because the Universe is infinite which means there simply isn't a mirror big enough.

Sex: None. Although in fact there is quite a lot, given the zero population of the Universe there can in fact be no beings to have sex, and therefore no sex happens in the Universe.”

I know it's wrong on purpose, but still...

> it is impossible to import things into something that has infinite volume because by definition there is no outside to import things from.

If you cut a 2d plane with a line, then both halves are infinite, yet both have "outside". You can repeat this and get infinite (aleph 0) number of divisions, each still as infinite as the initial plane.

Same way you can cut infinite 4d universe into countably many infinite 4d universes.

> not all of them are populated; therefore only a finite number are

By same logic: not all integer numbers are even, therefore only a finite number of numbers are even.

Wouldn't you get aleph_1 divisions, if, by methods left as exercise to the reader, making a cut for each real number? For example, how many angles are there in a circle? But the real kicker is, that taking the 2D surface of a ball, certainly the area is finite even if there are no bounds at all.

Addendum: The set of cuts would then include all points of the 2D-surface. So in principle, as the number of cuts approaches real infinity (pun intended; if you might call it that) the area left between the cuts approaches zero. That's certainly not infinite. But this cannot be done with only circle and straight edge - thus in no system of only two dimensions, pretty much by analogy - yikes - I mean the apparent isomorphism between Cartesian and polarized coordinate-systems is the only one I know in 2D.

Certainly, cutting is the arch example of proportional rationing, so the word alone implies rational numbers. Then, cutting is the act of removing a set of points from one set. So, the same construction over the real line but circled only a quarter around a midpoint is effectively removing two quandrants, half of the circle. So then you could say you have two infinite sets, but exactly because they don't touch (interact with) each other.

I know this is not meant to be taken seriously, but some claims don't even make sense from a purely mathematical point of view.

> Area: The area of the Universe is infinite.

Right now there is more evidence for the universe to be finite rather than infinite.

But apart from that: Just because it has no borders doesn't mean it is infinite. This is a classic failure in understanding infinity. For example, the area of the surface of a torus is finite, yet when living on a torus surface you won't find any borders. Same for higher dimensions.

> Imports: None. This is a by product of infinity; it is impossible to import things into something that has infinite volume because by definition there is no outside to import things from.

This has nothing to do with infinity. This is solely due to the definition of a universe.

A finite universe doesn't import anything as well, and this is no contradiction whatsoever.

Also, an inifinitely large "part" can still import and export, despite being infinite. For example, take an infinite universe and split it into two halfs. Those "parts" import and export, despite being infinite.

> Right now there is more evidence for the universe to be finite rather than infinite.

There is the observable universe and there are good reasons to believe that there is "some universe" beyond the observable universe. The observable universe is finite, the reason for that is the geometry of spacetime and the consequences of it (cosmological horizon/particle horizon). But I would not make absolute statements about what lies beyond the horizon(s), since there is (and can be) no data that would justify a statement about the infinity of space or the falsification of it.

But if there can be no data about it. Does it even make sense to talk about it as existing?

IIUC everything in the observable universe is sufficiently far away from anything else that any causal relationship with anything is either from before inflation or within the universe we can observe.

And given how the existence of matter seems to be rather connected to time, it self a by product of interaction, it seems to follow that there is, relatively speaking, nothing beyond the horizon.

That is an interesting metaphysical objection. Does a particle exist, if you cannot measure it? There is certainly no reason to think it does. But is there reason to categorically deny is existence? Very hypothetically, you might build a "Laplacian supercomputer" that could answer this question. But until this can be done (wich I doubt very much), the agnostic approach, to leave the question as undecidable, is prudent here.

>> IIUC everything in the observable universe is sufficiently far away from anything else that any causal relationship with anything

I don't think that is correct. Suppose you have a particle that is very very near the horizon. This particle has its own observable universe and we are at the edge of its horizon. Now, this particle can have casual contact with the other parts of the universe (the parts beyond). But by the time the interaction takes place, it has already slipped out of our horizon. So from the perspective of the particle casual contact is possible. But from our perspective: Not even a chance to detect this interaction.

But what does it mean for a particle to be near the horizon? The horizon is ours, thus the particle must be near our horizon, which means either the horizon we might observe today, sans opaqueness of the initial universe this means a point in spacetime before particles even existed. Or the furthest point we might observe in the future, which means sending the observer at C in any direction. Now, would the observer ever reach a point where she’d be able to observe any trace of interaction with anything that wasn’t observable from earth at the start of the journey? Let’s take a photon, or graviton, traveling towards the observer such that the observer would interact with at a distance from here expanding at C, that would be our particle “very very near the horizon”. It would never reach earth so strictly speaking it is outside our horizon so in a sense it is proof of existence outside the horizon. But let’s expand the definition of horizon to include all particles that could be measured by the observer, could any one of them have a history including an interaction with a particle originating outside this expanded horizon?

> But if there can be no data about it. Does it even make sense to talk about it as existing?

To be honest, that question never made any sense to me, because this whole discussion is merely about refining a semantic detail of the common word "existing". It says nothing about the particle itself, but just about our language.

For me, a far more interesting question is: Is it relevant? (for science?)

And the answer is: As long as it doesn't interact in any way with us, it is certainly not relevant whatsoever.

> This has nothing to do with infinity. This is solely due to the definition of a universe.

> A finite universe doesn't import anything as well, and this is no contradiction whatsoever.

It's not far-fetched that a satirical sci-fi series could have multiple universes with travel between them.

The Hitchiker's Guide actually both does and doesn't if you look at the "Whole Sort of General Mish Mash" joke. Though the WSOGMM joke is pretty explicit that it's one infinite mish-mash with infinite perspectives and universes are just an illusion.

> Also, an inifinitely large "part" can still import and export, despite being infinite. For example, take an infinite universe and split it into two halfs. Those "parts" import and export, despite being infinite.

He actually covers this in the quote on WSOGMM.

> The reason they are not universes is that any given universe is not actually a thing as such, but is just a way of looking at what is generally known as the WSOGMM, or Whole Sort of General Mish Mash. The Whole Sort of General Mish Mash doesn't actually exist either, but it is just the sum total of all the different ways there would be of looking at it if it did.

> The reason they are not parallel is the same reason that the sea is not parallel. It doesn't mean anything. You can slice the Whole Sort of General Mish Mash any way you like and you will generally come up with something that someone will call home. Please feel free to blither now.

You can split the universe anyway you like, it doesn't make 2 universes, just a divide in the universe. Your 2 divides are exporting/importing but the universe as a sum is not.

Of course, we're just arguing over a joke in a fictional universe now. But if you're going to do that you should get the fictional physics right :P

If you take a pragmatic stance and consider only what you can measure, then the observable universe is certainly limited. And the rest is, as you pointed out, a matter of semantics.

This part of the Hitchhikers' Guide might not have aged well, but because it contributed to that effect.

I realise I'm arguing against a work of comedy, but:

> But not all of them are populated; therefore only a finite number are.

There are infinities of different sizes

My understanding is that the infinity of "all integers greater than zero" and of "all integers greater than zero and divisible by 2" are in fact infinities of the _same_ size.

The same would apply for "divisible by 100" or "divisible by 1 million". This would apply to the fraction of worlds that are populated, whatever fraction of them are.

As best I understand it, all those reduce to aleph-zero because they're countably infinite and can be mapped to the natural numbers. But aleph-one exists and is provably bigger than aleph-zero despite both being infinite.

https://en.wikipedia.org/wiki/Aleph_number http://www.alephzero.co.uk/what.htm

Of course. Of course. You are totally right.

https://www.youtube.com/watch?v=23I5GS4JiDg

We don't know the probability of life arising from matter. We haven't replicated that emergence yet. If that p is something like 1/10^100 due to some compounding factors, we are alone in the observable universe.

You'll never know that value with any certainty as there may be multiple ways life could emerge from non-life on Earth (not just one) and ways it could emerge on other worlds that we could not know.

This is an equally excellent argument that aliens speak English.

If the universe is large enough, with enough inhabited planets, it's very likely some alien, somewhere, speaks perfect English.

At the same time, it's extremely likely we never encounter them.

For very large values of large enough that is always true. Even on the condition that they use the Latin alphabet and have exactly the same set of words and exactly the same set of meanings of words, there are still n factorial ways of assigning a meaning to each word. For n=1000 words that is already gigantic. Even if there is a civilisation that speaks English words on every planet in the observable universe, the probability that any one of them assigns the same meaning to words practically zero.

I'm tired of hearing these types of extremely confident lay-assertions about life beyond earth. It's true that the number of planets is incomprehensibly large, but it could also be true that the probability of a particular planet developing life is incomprehensibly small. In physics and chemistry, there are some odds that remain so extremely small even when you roll the dice many many times. Stop using "incomprehensibility" as an argument since it can go both ways.

Let’s talk numbers.

There’s 250 billion stars in our Milky Way galaxy. Our local supercluster has 100,000 galaxies in it. Previous estimates were that there are 100-200 Billion galaxies in the observable universe and this article suggests the figure is 10x as much as that.

The Kepler telescope has been discovering planets around nearby stars for a while now, and the impression we get are that at least half of the stars seem to have planets around them.

With 250 Billion stars in our galaxy and 200 Billion galaxies or more just in the observable universe (also cosmologists believe the actual universe is larger than the observable universe by an unknown amount), that leaves us with somewhere in the neighborhood of 25 Billion trillion (that is, 25 Billion * 1 trillion) planets in the observable universe. 100 Billion + worlds just in our galaxy.

We also believe that the laws of physics are the same across the universe and that water and other basic elements are abundant throughout the universe.

So to me it seems like even with extremely remote probabilities of life existing, to me it seems most likely that there would be lots of planets where life started.

Nice intuition. Too bad intuition has nothing to do with quantifiable numbers. "Extremely remote probability" is a human idea that you have in your head. It has nothing remotely scientific in it.

There's an extremely remote probability that the atoms in your body might teleport to the Great Wall of China. How much are you willing to bet that it happens?

When we talk about something so big that it might as well be infinite, we call it "astronomical". We've all heard the awesome facts: we know we're an invisible dot in our town; we know the town is an invisible dot on the planet; we know the planet is an invisible dot in the solar system; we know the solar system is an invisible dot in the galaxy; we know the galaxy is an invisible dot in the universe.

But, as impressive as that all sounds, that's describing scale with exponential growth, but exponential growth pales in comparison to combinatoric growth.

The number of ways you can arrange 100 people is far greater than the number of estimated atoms in the universe. And not _just_ greater, greater in a way that is difficult to explain. A drop in a bucket? No. A drop in the ocean? No. All our metaphors are physical, and so they're inadequate, but let's just it's beyond all that.

Of course, that doesn't mean the probability of other intelligent life is best assumed to be of a combinatoric nature, but let's get ahead of ourselves and let our awe of the universe's size cloud our judgement. It's far from infinite, and silly to assume intelligent life is certain.

> That's entire galaxies. Not just stars

Ehm... sorry to be a bit pedantic, but the distinction between the two has no effect on your argument here. Either way there are ten times as many environments in which stuff happens.

Also, as b3lvedere's comment points out elsewhere, you really shouldn't trust gut instincts when it comes to reasoning about infinities. Unless you're a mathematician specialized in the subject, you're pretty much guaranteed to be wrong about them.

(having said that, I'm also more inclined to believe that there's plenty of life out there. I'm not as hopeful about complex life-forms escaping planets though)

>How, when given the evidence of practically infinite planets out there (considering that every star has at least one planet, and there are even rogue planets [0] that don't orbit any star), can we even consider the possibility that this is the only planet with life?

For the "rogue" planets, it's easy -- we can be pretty sure they don't have any life.

For the others, we'll believe it when we see it.

> For the "rogue" planets, it's easy -- we can be pretty sure they don't have any life.

Why?

Even if you assume all life requires heat and/or light, a rogue planet could produce heat/light from geothermal or other chemical activity.

A rogue planet drifting through certain nebulas might have more surrounding material for the processes of life to work with.

The life on a rogue planet may even be "artificial", say a colony of self-sufficient, self-replicating AI machines that originated elsewhere, but otherwise in-differentiable from "naturally-evolving" life.

> A rogue planet drifting through certain nebulas might have more surrounding material for the processes of life to work with.

Keep in mind that nebulae are in reality less dense than industrial-grade vacuum. From the point of view of a rouge planet passing through one, it's pretty much still empty space.

What about the regions of nebulas that are about to coalesce into stars? Wouldn’t they have some kind of protostellar “stuff” at point that makes the region more active than the rest of the average nebula?

https://en.wikipedia.org/wiki/Molecular_cloud

https://en.m.wikipedia.org/wiki/Great_Filter

https://www.haystack.mit.edu/hay/staff/jball/etiy.pdf

Dark Forest

The tyranny of ‘c’

It's extremely odd that we don't see autonomous probes from other civilizations that are used to monitor and disable technology that exposes existential risk to them. (For example, runaway AI would jeopardize all life if it were to expand at light speed.)

Can you imagine the likelihood of detecting such probes?

Can we even detect any of the probes that we ourselves have sent out, without knowing exactly where they are and how to communicate with them?

Maybe if they are advanced enough to send those probes specially if it's for _their_ defense then it's pretty well cloaked and small?

It makes me excited and happy to think of other life out there, waiting to meet us. But sad that I will probably not be around to witness it.

mankind has been extremely lucky with our environments, among others:

- Jupiter & Saturn pick out most of the comets, so we encounter massive reset only very rarely

- thanx to hitting other planet long time ago, we have a spinning molten metal core inside planet generating magnetic shield for all nasty radiation

- moon gives us tides & stability

- our sun is just the right size and age to be stable

- we're located at quiet part of the galaxy, which itself is pretty stable

I think some primitive life (ie stromatolytes equivalent) can exist on many places, simply because it happened on our planet very early when it was very violent place to be - so random arrangement of matter into self-reproduction is feasible.

But to evolve into specialized multi-cellular organisms took more than billion of years, a step that might not happen at all.

And to create an intelligent life is another massive step up (or accident). There is Drake equation to manifest it, but basically all variables in it can be anything including 0, so it's kind of useless.

So from my perspective there are couple of options:

- universe is too young (if you consider average star age compared to estimated universe age), so we're one of the first sentient arrivals, others are behind in evolution

- we're just not interesting enough to be contacted by superior races - just look at the news, I wouldn't blame them. We're 'meh' category at best, although most like to think of themselves in very different perspective

- we're fucking alone, weird random anomaly (by far the most scary option of them all)

There is a wonderful TV series from BBC called Earth: Power of the Planet, which explores this topic to great details, highly recommended!

A good book that explores some of these factors for our environment is Rare Earth by Peter D. Ward and Donald Brownlee.

While it's true that life "out there" may look vastly different, I feel like sometimes ET-optimists use that as a hand-wavy defense that overlooks two curious details: First, we haven't found any different-looking-life-out-there on, say, Venus, or Mars... so clearly even in a solar system with some "lucky" aspects for life there are plenty of environments where life just doesn't "find a way." Second, the relative quickness with which our kind of life appeared in Earth's history suggests an oddity that we haven't even found other examples of our-general-kind-of-life out there.

A good book that explores some of the proposed solutions to the Fermi Paradox is Where is Everybody? by Stephen Webb. (It does a good job using math to explain why it's a little surprising that we haven't found ET life even within our own galaxy, and it does a good job knocking down most of the sociological explanations by noting that it would require every other civilization in the galaxy to behave that way with no exceptions.)

Earth was lucky but Mars and Venus (now) have been unlucky.

Mars lost its magnetic field long ago, but it had one once.

Venus, well, it's an acid rain oven but it could have been different.

Both could have had conditions for life (as we know it) once.

> we're one of the first sentient arrivals

Given we didn't arrive until 14.5bn years into the universe's existence, I'd find that extremely hard to fathom.

Our solar system is +-4bn years old, so there have been only cca 3 previous generations of similar stars (and it's kind of nice to have heavier elements from the first generations of stars in abundance - I would expect forming life just from He and H wouldn't work well).

Let's ignore the 'bn' part - this place is 14-old. Our system is 4-old. Life is 3-old. Multi-cellular is 2-old. Sentient is 0.00001 old. With usual star life has +- 10 to escape, or it will be destroyed. It doesn't sound so far-fetched to me.

But maybe it's all different - plenty of life all around the place, even super smart & evolved, but maybe speed of light simply can't be broken no matter what. That would dampen any rise of galactic civilization. Or some other unbreakable constraint.

> there have been only cca 3 previous generations of similar stars

That assumes they're consecutive, not overlapping. Assume it takes 4-old, space them out by 0.1-old, and you could have 100+ generations between the first and us.

You're assuming that all life will be like ours. This is a safe bet, but there could be very many ways for life to exist.

Maybe, but unlikely. Life as we know it depends on some rare chemistry.

Carbon is the only atom [I know of] that can form long chains of double bonds (even triple though they tend to be unstable enough to not matter). Nothing else allows the chemical complexity needed for self reproduction.

Water is a weird molecule. Many chemical reactions happen in water that would be uncontrolled or not happen at all without water. The polar properties, the fact that is expands when it freezes (that is frozen water floats on liquid), and acid/bases are all interesting properties that are very useful for life. I suppose that something else could fill this role, but since water is common in the universe I doubt it.

Yes life elsewhere is unknown and so we cannot say for sure it will not look like our, but chemistry is a known.

Can you suggest another one?

• Robots that perform all the functions humans do, including making copies of themselves and teaching each other.

• Hive-mind species where there are no individuals.

• Species without any gender or sexual reproduction, whose individuals are spontaneously "born" in specific locations and conditions on their planet.

• "Species" consisting of a single, massive/long-lived individual.

• Parasitic intelligences that "infect" various other, unrelated species and direct them towards the goals of the parasites.

• Lifeforms floating around in gas giants or even in nebulas.

• Life-bearing planets themselves being a lifeform; that "reproduces" when the civilizations it supported upon itself go on to terraform other planets to become like their home planet.

The question is, how would it form by itself?

The fundamental thing about life as we know it is not just that it's made of the few particular elements it uses. It's about those elements being a sweet spot for molecular nanomachinery. It's not clear that any other combination of elements might be able to form a stable replicator and bootstrap itself into large-scale complex system. In fact, it might be that life can only arise by itself using materials our life is made of, directly because of laws of physics in our universe.

Or put in another way - if abiogenesis of our life is improbable, a spontaneous creation of robots - large scale machines with digital brains - is orders of magnitude less likely still.

When dom96 said "there could be very many ways for life to exist" I understood that to mean there could be very many different "biological substrates" (for want of a better term), nothing to do with how the species organises itself socially. The challenge is to come up with a hypothesis for "life" that's not based on long carbon chains.

I agree that only our imagination is the limit when discussing potential life. If matter that is not completely frozen (ie atoms stop moving) will eventually organize into some self-replication, any energy source could be enough.

It could be on the surface on active star, neutron star (someone already tackled that in sci-fi), all kinds of dwarfs, nebulas, maybe even something living off dark energy? Or black hole.

Energy means movement, and if matter/energy will eventually arrange into something 'alive', it might be enough.

Until a flying saucer lands on my yard I have to assume there is no life in Universe.

Me, I think there is no life beyond Earth.

> How, when given the evidence of practically infinite planets out there (considering that almost every star may have at least one planet, and there are even rogue planets [0] that don't orbit any star, and that asteroids and comets or even dense nebulas could also harbor life), can we even consider the possibility that this is the only planet with life?

Who gives a shit?

If our spaceships landed on a distant planet populated by a vast hyper-intelligent civilization of decidedly not-alive robots, would we shrug, say "no life here," and head home bored?

Maybe life isn't what we're looking for?

Maybe chemistry is not so poor that it has to repeat itself by re-creating something that we would consider to be alive. Maybe the non-living things are vastly more interesting.

Maybe finding distant life is like visiting a distant country and finding a McDonalds.

Maybe "let's find life" is just a failure of imagination. It's SETI, not SETL.

One could flip your argument and say defining life as purely organic is a failure of imagination.

The thing about searching for "life" over "intelligence" is that life doesn't necessarily need to be intelligent for it to be interesting.

I also think the SETI example you've made is a tad unfair because their process dictates that their search be for intelligence (crudely put, radio transmissions). Whereas when you're visiting a planet, you can observe a wider scope of life as it need not be intelligent (as we can perceive anyway) for markers to be identified.

> One could flip your argument and say defining life as purely organic is a failure of imagination.

That's right! So how would you define life? Or the meta-question, why bother? We gain nothing from figuring out what's "alive" and what has "merely" a rich complexity.

"Search for life" is a category error.

> The thing about searching for "life" over "intelligence" is that life doesn't necessarily need to be intelligent for it to be interesting.

If something is interesting, why do we care if it's "life?"

Anything we find is likely to operate on some scale vastly different than what we're familiar with. We won't even agree on whether it's alive.

> Whereas when you're visiting a planet, you can observe a wider scope of life as it need not be intelligent (as we can perceive anyway) for markers to be identified.

My hope is we'll find markers that are clearly weird and that aren't evidence of life but maybe something more and weirder that we don't understand.

Isn't that even more interesting? Why hope to find life, when we know life is already possible?

> That's right! So how would you define life? Or the meta-question, why bother? We gain nothing from figuring out what's "alive" and what has "merely" a rich complexity.

> "Search for life" is a category error.

To be honest I think you're over thinking the whole "life" thing. It's just a label. You could replace that term and still have the same arguments. eg "What would someone define as 'interesting'? What does that category even mean?". Or "complex", or "intelligent" or any other label.

> Why hope to find life, when we know life is already possible?

Possible, yeah. But it's unique in that we are constantly finding a multitude of other phenomena in the universe yet we've only ever observed life on our own planet. That alone makes this topic interesting in my opinion.

I don't see why it has to be an either / or though. Nobody is suggesting we search for life at the sacrifice of learning more about black holes...or whatever.

Geology is interesting I will grant that. However if you subtract life earth is much less interesting. All the rocks you can find on earth can be found on other plants in our solar system. You can go most places on earth examine all the rocks and say for each "very similar to some other rock from elsewhere". But the life in each place will be very different.

This is not to subtract from geology which is an interesting and useful study, just to say it is unlikely we would discover anything very new if we could somehow examine every planet in the universe. However if we add in life we would expect a lot of interesting variety to study.

> Isn't that even more interesting? Why hope to find life, when we know life is already possible?

We know it's possible, but we've also seen it happen only once. The observational data we have suggests the universe is mostly dead - that is, not complex. Or, in other words, boring. Discovery of any complex process on a comparable level to even simplest of what we call life would be a civilization-changing breakthrough. It would prove the universe is not boring.

(It would also ultimately kill off most of our existing religions.)

Most existing religions are silent on the subject of life elsewhere. Since they do not say "it doesn't exist" finding life doesn't mean anything (it might even grow as they have a new group of life to convert).

Religion tends to be very careful to stay in the area of not providing a testable hypothesis which means that new alien knowledge won't prove it wrong.

Christianity, at least as far as I have been taught it, seems to implicitly conflict with intelligent extraterrestrial life, as at the very least it would lead to questions whether the aliens also are in sin, whether Christ's sacrifice applies to them or did they have their own equivalent of Jesus, and generally how special humans are in God's grand cosmic plan.

> Religion tends to be very careful to stay in the area of not providing a testable hypothesis which means that new alien knowledge won't prove it wrong.

That's true, but until very recently, the idea of life elsewhere wasn't really considered seriously.

It depends on the preacher. Some are more traditional eg arguing that Earth is unique for hosting life because God created the Earth and that the universe is just there to demonstrate the epic scale of God's power.

Source: I'm not religious but I've been dragged to plenty of sermons by the extended family.

> It would also ultimately kill off most of our existing religions

I think you may be underestimating the tenacity of religions

Maybe. But I would love to see how mainstream religions of today try to handle a definitive proof of a separate tree of life elsewhere in space.

I'm going to guess they'll handle it the same way as they handled definitive proof that the Earth is more than a few thousand years old, that the Earth revolves around the Sun, evolution by natural selection etc etc.

Unconditional belief in the unknowable gives you an awful lot of wiggle room when it comes to evidence.

> If our spaceships landed on a distant planet populated by a vast hyper-intelligent civilization of decidedly not-alive robots, would we shrug, say "no life here," and head home bored?

As a different take on that: Suppose we develop "robots" that can sustain themselves, mine resources, produce [ever improving] copies of themselves, and learn on their own and teach each other. Then we send them to colonize a distant planet.

Fast forward a hundred years. Those robots now have a "civilization" of their own.

By what criteria would that civilization fail to be counted as "life" by an species looking for such?

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