Basis of science's belief that extraterrestrial life is likely?

Great Debates
1

Continuing form this locked GQ thread.

My contention is that there’s no scientific basis for claims that life exists elsewhere. We have absolutely no idea how life arose on Earth or what conditions were necessary for that to happen. This is borne out by the fact that one of the major groups of respected *scientific *theories about the origin of life on Earth is that it arrived here from outer space. A subgroup of that school argues seriously that proteins and even amino acids were seeded from interstellar nebulae. We really have that little idea how life arose or where.

As such the probability of life arising on Earth or any other planet could be anywhere from 1 (absolute certainty) to 1/10^87 (odds against being about the same as the number of particles in the universe). At the high end then life will arise pretty much everywhere it can. At the low end then it is extremely unlikely that life ever arose anywhere else but here. It doesn’t matter how many planets there are in the universe or how long they last, if the probability against life is higher than the number of particles in the universe then it’s pretty certain that we are alone.

The problem, to me, is that if life on Earth is the result of an improbable series of long shots, and they hadn’t occurred here, we wouldn’t’ be here to realise they hadn’t. So our single sample can not possibly tell us anything about the abundance of life elsewhere. It is the ultimate self-selected sample. Even if life required a planet identical in every way to Earth and had to evolve within a few million years at odds of 10^87 against, we would have to be the result of that improbability in order to know about it. Anthropic principle at work.

Also addressed in the last thread was the idea that because life originated fast on Earth, it must be easy and fast and thus highly likely. My rebuttal is that I have heard several cosmologists and biologists say that exactly the opposite is true. Their position is that life is so fragile while it is developing that in the vast majority of cases it gets destroyed well before it becomes robust enough to survive the trials the universe throws at it. The reason why our single sample developed so fast is because if had developed slowly, we wouldn’t be here to notice it. Since it is rare that life would develop rapidly enough to survive, then life must be perishingly rare elsewhere. In fact it may not exist anywhere else at all, because everywhere else it keeps getting destroyed before it can develop. The same single sample, Earth, can be used as evidence for the arguments both that life is common and that life only evolved in one place in the universe.

To forestall the nonsense in the other thread, even if the probability oflife arsing on any planet is the low one,nothing in this contention requires Earth to be special. Let’s assume hypothetically that the probability on any given planet is 1/10^87. That is the probability on all planets, 0n any planet. On any given planet. That includes Earth. Earth is not special in any way. It is exactly like all other planets in that regard. The probability of life arising on Earth is not special. In fact it is identical to every other planet in the entire universe, and that probability is 1/10^87. There are no conditions that exist in Earth that don’t exist elsewhere. Earth is totally unremarkable. It has exactly the same probability as life arising, and that probability is 1/10^87. Yet because it is so improbable it can’t have arisen elsewhere.

That’s my position. We can’t make any assumption about the probability of life elsewhere. It all hinges on how likely life is to arise on any given planet. We can never know what that is, or even make an educated guess, because the only sample we have must have had life evolve on it, no matter what the probability, because we are here discussing it.

On with the debate.

2

No facts here but if there is no life anywhere else, it kinda makes the universe pointless. I just think earth isn’t that special and there are probably billions of planets just like it somewhere.

3

From the earlier thread.

No dodge, it’s just that your entire position is a strawman. I’ve no doubt it’s unintentional. It’s actually a very common mistake amongst people who don’t understand probability.

The thing you need to realise is that just because some rendom event is highly unlikely, that doesn’t indicate that that conditions leading to that event must have been in any way special. Indeed, the very use of the word random demands that they were not special.

To help you understand, imagine that you have 10^87 X 100 sided dice. All the dice in are numbered sequentially in addition to their face numbers. Now you generate a series of a 10^87 numbers between one and 100. Then you throw your dice.

Now by chance you manage to generate exactly the sequence that you previously generated. Do you agree that the odds against that are staggeringly unlikely? I hope so. But the important point is, do you also understand that the fact this happened doesn’t indicate that there were any special conditions under which you threw them? That the result was just random?

Do you understand that if billions of people repeated the same test under the exact same conditions (or if you threw them billions of times) that there is no reason to believe same result would never come up again?
I know this can be hard to grasp, because our intuitive mind tells us that if something improbable happens, it must have occurred because of something special in the starting conditions. But that’s not true. Something can be improbable and happen randomly under perfectly standard conditions.

And of course the fact that it has happened once does not mean that it must happen every time the same conditions prevail. Life could evolve on Earth against all odds despite Earth being a perfectly ordinary rock, or even if Earth is downright inhospitable to the evolution of life.

Once you understand that fact, that an improbable event does not require an special starting conditions to occur, you will understand why the rest of your post is a strawman.

Which brings us back to your original question. At this stage we both agree that the chances of life evolving anywhere at all could be a uniform 1/10^87. In fact that probability is about as reasonable as any other.

So, since you have no reason to believe that the odds of life evolving twice aren’t orders of magnitude lower the chances of it never having done so, why would you bet that it has done so? You have already said that there is no reason to pick one probability of life evolving over any other. So if odds of 1/10^87 are as likely as odds of .9999, why would you bet in favour of the event?

This really is exactly like the cockfight in my scenario. You have absolutely no evidence or logic to favour betting either way. At odds of 1/10^87, which yuo admit are as likely as any other, then life won’t evolve twice even in a universe of universes, let alone in a few billion galaxies.

So why make the bet?

4

We’re either alone in the universe, or we’re not.

Either way, the implications are staggering.

5

Well, for starters, the simplest assumption for the size of the Universe is that it’s infinite. If it is, then we don’t even need to look at any of the other factors in the Drake equation.

And then there’s the fact, brought up in the other thread, that in the one case we know of where life started, it started extremely quickly. The fact that it started at all, we can’t conclude much from, because of the anthropic principle, but we could just as easily be having this discussion on a planet where life began billions of years after the planet cooled, so the anthropic principle doesn’t shoot down the “life started early” argument.

6
  1. As soon as the earth was suitable for life, life appeared. Though not proof, it does kinda hint that life isn’t that hard once the conditions are right.
  2. Large numbers of the different parts of life have been shown to spontaneously form from filling a flask with the chemicals that were on the earth when it formed, and zapping them with sparks ( lightning )
  3. The Universe is big. REALLY big. I’m not sure you’re getting how big it is. There are more stars in the universe than there are grains of sand on all the beaches on the entire earth. So even if life is very unlikely to start, which evidence doesn’t really show it is, you still have 5,000,000,000,000,000,000,000 rolls of the dice to get life. And that’s just now! The universe has been rolling those dice for around for 13,700,000,000 years, and will keep rolling them for many more billions of years to come.

All that strongly suggests life elsewhere is probable.

7

Why not? This seems like a non-sequitur.

If the probability against life ever existing is greater than the number of particles in the infinite universe, then why don’t we need to look at any other factors? Doesn’t such a low probability mean that it is improbable for life to arise twice even in an infinite universe?

This is the anthropic principle at work again. I addressed this in the OP, but I’ll repeat.

I have heard several cosmologists and biologists say that exactly the opposite is true. Their position is that life is so fragile while it is developing that in the vast majority of cases it gets destroyed well before it becomes robust enough to survive the trials the universe throws at it. The reason why our single sample developed so fast is because if had developed slowly, we wouldn’t be here to notice it. Since it is rare that life would develop rapidly enough to survive, then life must be perishingly rare elsewhere. In fact it may not exist anywhere else at all, because everywhere else it keeps getting destroyed before it can develop. The same single sample, Earth, can be used as evidence for the arguments both that life is common and that life only evolved in one place in the universe.

On planets where life never evolved fast, it was exterminated by asteroids, galactic collisions, passage through thermal vents etc. The only reason we can discuss this issue is because we are on a planet where life did manage to arise fast enough to become resilient. The norm might be that it takes half a billion years for the transition form organic soup to bacteria, but because complex organic molecules can’t survive catastrophes the whole planet is sterilised before life ever develops.

So only on planets where life can and did develop fast does it develop at all. That’s classic material for the anthropic principle.

8

Did anyone read the OP.:frowning: I know it was long, but still.

This is the anthropic principle at work again.

I have heard several cosmologists and biologists say that exactly the opposite is true. Their position is that life is so fragile while it is developing that in the vast majority of cases it gets destroyed well before it becomes robust enough to survive the trials the universe throws at it. The reason why our single sample developed so fast is because if had developed slowly, we wouldn’t be here to notice it. Since it is rare that life would develop rapidly enough to survive, then life must be perishingly rare elsewhere. In fact it may not exist anywhere else at all, because everywhere else it keeps getting destroyed before it can develop. The same single sample, Earth, can be used as evidence for the arguments both that life is common and that life only evolved in one place in the universe.

On planets where life never evolved fast, it was exterminated by asteroids, galactic collisions, passage through thermal vents etc. The only reason we can discuss this issue is because we are on a planet where life did manage to arise fast enough to become resilient. The norm might be that it takes half a billion years for the transition form organic soup to bacteria, but because complex organic molecules can’t survive catastrophes the whole planet is sterilised before life ever develops.

So only on planets where life can and did develop fast does it develop at all. That’s classic material for the anthropic principle.

No doubt. Not sure where you are going with this.

I know just how big it is. I mean, you think it’s a long way down the road to the chemist, but compared to the universe that’s just peanuts. And so forth.

See the probabilities against may be REALLY, REALLY big. I’m not sure you’re getting how big they could be.

The evidence doesn’t show anything about how likely it is. That’s the crux of the issue. Our self selected sample of one tells us nothing at all.

And what if the probability against is 5,000,000,000,000,000,000,000 ^5,000,000,000,000,000,000,000?

Really big numbers don’t help the case in anyway at all, because a really, really, really, really, really, really, tiny number multiplied by a really big number still produces a really, really, really, really, really, tiny number.

So I ask, can you show us how you calculated this probability?

9

Our single sample can tell us much. We know that life can arise from carbon, water, an energy source and some other chemicals and compounds to begin forming amino acids and presto! You have the building blocks of proteins from which all kinds of possibilities exist.

We know that these basic building blocks exist pretty much everywhere in the galaxy that astronomers point their telescopes and pardon me for saying this but it’s a big ass galaxy and it’s an even bigger ass universe.

Intelligent life? I have no idea but I find it harder to believe that life isn’t prevalent throughout the universe, even if it isn’t anymore complex than an alien slug.

10

[quote=“Blake, post:8, topic:560137”]

Did anyone read the OP.:frowning: I know it was long, but still.

So wait, where life never evolved, it got exterminated by asteroids? Where life doesn’t exist, it was killed even though it didn’t exist yet? That’s a very strange argument.

11

If we consider planets to be discrete data points we end up with the conclusions that Blake has proposed, that the determination of the probability of life in the Universe is absolutely indeterminate (we only know that it has a value greater than 1).

However, it seems there is an alternative procedure, one that can actually give us a rough lower bound on the probability of life arising somewhere else in the Universe. Given our ability to estimate the elemental composition of other planets, we may formulate a rough estimate of the number of earth like planets there are in the Universe. We thereby calculate the probability that the elements present on these earth like planets will spontaneously form into replicating molecules (I think the simplest we know of is RNA?). Couldn’t this be done merely by applying our understanding of molecular Physics? What is the chance these elements will collide in just the right way to form a working RNA, given our estimate of the number of opportunities available? Isn’t that simply a really complicated problem of many colliding particles? Of course there are hundreds of other parameters to be considered, but these too may be estimated from our limited sphere of observation. Sure, it would be an immensely complicated problem, but I think it provides a basis to work with.

I’m not proposing that the probability of life on other planets is necessarily high, merely that it can be theoretically calculated given our limited range of data.

12

You have it arse about face.

  1. Where life never evolved, intelligent life could never have existed to note the fact.
  2. Proto-life/organic soup molecules are so fragile that it cannot survive the regular catastrophes of the universe.
  3. Catastrophes occur at frequencies of less than 50 million years.
  4. Therefore proto-life must give rise to life within 50 million years of conditions becoming suitable, otherwise it will be sterilised and never evolve into life
  5. By combing points 1) and 5) we can conclude that intelligent life will only ever evolve on planets where life arises from non-life within 5 million years.

IOW a planet where life is able to arise within a very short period is one of the requirements for intelligent life to exist at all. The fact that we are intelligent and live on a planet where life arose fast can therefore tell us nothing. If Earth had been a planet where life took longer than 50 million years to arise, we wouldn’t be here to know about it.

Anthropic principle. Rapid origin of life is a necessary condition for the existence of intelligence. This doesn’t mean that on all planets things go from non-life to life that rapidly. We may be the only planet in the universe where that happened, and that is the *reason *why we are the only planet with life. Therefore our planet isn’t indicative of how easy or fast life normally arises, it’s just indicative of the fact that it *needs *to arise that fast for life to exist at all.

I the same way, the fact that Earth has lots of liquid water can’t be used as an indicator that all planets have water. Water was needed for intelligence to arise, so of course our planet has water. It’s possible (albeit unlikely) that no other planet in the universe has liquid water.

In short, we can’t extrapolate anything from conditions that were necessary for our ability to do extrapolation in the first place.

13

I’ll just point out that just because something is infinite in size/breadth/scope, doesn’t necessarily mean that everything that is even remotely/infinitesimally improbable will occur. There are an infinite number of real numbers between 1 and 2, but none of them include the number pi.

14

Unless we assume that the laws of physics are different depending on where you are in the universe, then Earth is no unique place. What happened on Earth is possible on any Earth-like planet, of which there must be billions. In fact, we don’t even know that life on Earth originated on Earth. Knowing what we know, life should exist elsewhere. To assume otherwise, we have to assume knowledge that we don’t have.

15

Why?

If we (Earthlings) are alone, we’ll never meet an extra-terrestrial life form.

If we aren’t alone, the odds of us meeting an extra-terrestrial life form in my lifetime make getting struck by lightning while holding a winning powerball ticket seem like sure odds.

And if we meet extra-terrestrial life after I’ve passed, what do I care?

The implications are ‘gee-whiz, wouldn’t that be cool’. if there is, or if there isn’t, it is so unlikely that anyone living today will learn the true answer the the implications are meaningless except as a brain and debate exercise.

16

But that probably tells us more about you than it does about the reality of extraterrestrial life. More than a few people believe it’s likely - but Blake makes the point that this belief is not based on evidence - it’s more like faith, or possibly religion.

An old tale has it that in ancient China no one was allowed to set eyes on the Emperor, but the question arose: how long is the Emperor’s beard? An answer was obtained by asking thousand of people for their opinion, the averaging the results. From this, a highly precise beard length can be calculated - but what good is that?

17

I think you mean greater than 0. Probability greater than one means it has already occurred. Actually, greater than 1 is impossible. 1 means it has already occurred.

No.

As outlined in the OP, we have no idea how or where life evolved. We have no idea if the process began in interstellar (or even intergalactic) space or on planets. If it began on planets we have no idea which planets. Life may have evolved on Jupiter and spread to Earth for all we know. Or maybe the necessary precursors formed in interplanetary dust clouds and only became life after they arrived on Earth… And whet were those precursors? Arguments have been put forward for amino acids, proteins, nucleic acids, lipids and even inorganic clays.

So, to start with tell us which of those scenarios you want to apply. For example, do you calculate the area of intergalactic nebulae, and then divide that by the number that pass around an Earth-like planet within, say, a billion years of cooling form a massive interplanetary collision?

Or do you just want to base it on Earth like planets? In that case the exogenesis proponents will tear your argument to shreds with their usual objections to a terrestrial origin of life.

It has been seriously suggested that maybe life really did only arise once and has spread throughout the universe from there. If that is the case then how would you even know what factors to include in the calculations?

I don’t see how, and nobody has proposed such a thing. We just have absolutely no idea what conditions are required for life so any equation would necessarily be able to produce any figure at all, depending on what inputs you want to use.

18

What part of “infinite” are you not understanding?

And your argument about “life had to evolve fast or not at all” still doesn’t make any sense, especially since the asteroid impacts and such that you claim would endanger proto-life are much rarer now than they were when life actually did evolve. If life had to get past those first hurdles before the next asteroid impact, then we would expect to see that life on our planet only arose in the past billion years or so, not that it had been present basically from the beginning.

19

Intelligence is neither here nor there. The question is only about life, not intelligent life. There is nothing special about the conditions on earth that led to replicating RNA molecules.

By the way, we do have that Mars rock which appear to have fossilized, nanobacterial lifeforms on it (a hypothesis which still has not been refuted nor has any better hypothesis been offered). That’s abiogenesis twice in one solar system.

20

That’s not really true. The Miller-Urey experiment showed that certain amino acids could be created if we assume certain conditions and apply an electrical spark. (Whether these conditions accurately reflect the primorial atmosphere is a subject of some dispute.) A few other experiments were able to produce a few other chemical compounds, such as some alcohols and organic acids. That’s still a far cry from saying that “large numbers” of the chemical components of life could be created in that manner.