How Do Extraterrestrials Know We Exist?

[DaveC426913]

Not only that. The oceans are too big to allow the potentially combining molecules to come close to each other, so it is very likely that life began in small ponds. But small ponds would never accumulate enough active molecules, without being fed by the sea. This means that we need tides to feed the ponds. It is possible that a large and nearby moon is necessary to abiogenesis.

The oceans are too big? Hardly. That just means more area for reactions to occur.

It's not like the interesting reactions have to occur out in the middle of the ocean at midwater. No, they'll occur in the shallows, where there's lots of sunlight and where there's lots of nutrients and lots of mixing. There's no dearth of active molecules there.

While I grant that tides are a good location for reactions, I do not grant that they are critical. I think your logic is unnecessarily Earth-centric - which is what we're trying very hard to avoid here.

 

[CEL]

The oceans are too big? Hardly. That just means more area for reactions to occur.

It's not like the interesting reactions have to occur out in the middle of the ocean at midwater. No, they'll occur in the shallows, where there's lots of sunlight and where there's lots of nutrients and lots of mixing. There's no dearth of active molecules there.

While I grant that tides are a good location for reactions, I do not grant that they are critical. I think your logic is unnecessarily Earth-centric - which is what we're trying very hard to avoid here.

What I said is that the vastness of the oceans don't allow high concentration of the molecules. If the reacting molecules are far apart from each other, the probability of them meeting and combining is small.
As for geocentrism, we know that life has arisen on Earth. We don't know how, so we formulate hypoteses. We suppose that life can exist elsewhere, but we have not found it yet. So, we might expect that the conditions for the appearance of life should not be very different from those on Earth.

 

[DaveC426913]

What I said is that the vastness of the oceans don't allow high concentration of the molecules. If the reacting molecules are far apart from each other, the probability of them meeting and combining is small.

I got that.

As for geocentrism, we know that life has arisen on Earth. We don't know how, so we formulate hypoteses. We suppose that life can exist elsewhere, but we have not found it yet. So, we might expect that the conditions for the appearance of life should not be very different from those on Earth.

Yes, but that narrows the search, eliminating possibilities; it doesn't keep it as broad as possible.

This subthread started because CRG postulated an alternate method for life that didn't use water (https://www.physicsforums.com/showpost.php?p=2211424&postcount=50"). I argued against it, based on what I consider "showstopper" factors - factors that I think are almost universal, and are not merely based on the path Earth life followed.

So, now I'm playing Devil's Advocate, and trying not to narrow the possibilities more than they absolutely need to be narrowed. The more fundamental the dependencies, the more compelling my argument for water-based life. Claiming "no life because no tides" is not a universal showstopper IMO, and therefore a relatively weaker argument.

 

[CEL]

I got that.
Yes, but that narrows the search, eliminating possibilities; it doesn't keep it as broad as possible.

This subthread started because CRG postulated an alternate method for life that didn't use water (https://www.physicsforums.com/showpost.php?p=2211424&postcount=50"). I argued against it, based on what I consider "showstopper" factors - factors that I think are almost universal, and are not merely based on the path Earth life followed.

So, now I'm playing Devil's Advocate, and trying not to narrow the possibilities more than they absolutely need to be narrowed. The more fundamental the dependencies, the more compelling my argument for water-based life. Claiming "no life because no tides" is not a universal showstopper IMO, and therefore a relatively weaker argument.

Now it is my turn to play Devil's Advocate. If you want to broaden the possibilities, why insist on water?
We agree that a liquid medium is necessary to allow the building blocks of life to come close to each other. This liquid must be abundant. Hydrogen and Helium are the most abundant elements in the Universe, but since He does not combine with anything, we must search for hydrogen compounds. The elements that form stable compounds with hydrogen are oxygen, sulfur, nitrogen and fluorine. The compounds: water, hydrogen sulfide, methane and hydrogen fluoride are all gaseous, unless submitted to pressure and in certain temperature limits, so we need an athmosphere to provide this pressure.
Additionally, the athmosphere will provide protection against the UV radiation from the star, which would tend to destroy the complex molecules needed for life.
It is very unlikely, but a silicon based life in a hydrogen fluoride ocean is a possibility.

 

[DaveC426913]

Now it is my turn to play Devil's Advocate. If you want to broaden the possibilities, why insist on water?
We agree that a liquid medium is necessary to allow the building blocks of life to come close to each other. This liquid must be abundant. Hydrogen and Helium are the most abundant elements in the Universe, but since He does not combine with anything, we must search for hydrogen compounds. The elements that form stable compounds with hydrogen are oxygen, sulfur, nitrogen and fluorine. The compounds: water, hydrogen sulfide, methane and hydrogen fluoride are all gaseous, unless submitted to pressure and in certain temperature limits, so we need an athmosphere to provide this pressure.
Additionally, the athmosphere will provide protection against the UV radiation from the star, which would tend to destroy the complex molecules needed for life.
It is very unlikely, but a silicon based life in a hydrogen fluoride ocean is a possibility.

You showed up late to the meeting. I refer you back to https://www.physicsforums.com/showpost.php?p=2211199&postcount=40". Water is the universe's "almost too good to be true".

 

[jreelawg]

Maybe there is a frozen ocean somewhere with a volcanic heat source at the bottom, and the heat source is enough to melt the ice directly above the ground. There would be maybe a sort of puddling like effect over hot spots, and those places may also be places where chemicals are released from the volcanic activity. You would have puddles of hot pressurized water saturated with earthy elements, and volcanic chemicals.

Maybe at a place like this, organisms have the ability to lay dormant in ice. Then one day, something big collides with the body, and sends huge chunks of life filled comets hurdling into space. One of those chunks crashes into a place like earth, and life as we know it begins.

 

[CEL]

You showed up late to the meeting. I refer you back to https://www.physicsforums.com/showpost.php?p=2211199&postcount=40". Water is the universe's "almost too good to be true".

I don't believe that life non carbon based could happen and that the three other solvents I mentioned could replace water, but it is possible. Methane, at least, is nearly as common as water in the universe.
Silicon, like carbon, can form long chains, but those chains are less stable. Even if a silicon based life can exist, I don't think it could achieve the complexity needed for intelligence.
As I said in my first post, the only place in the Universe where we know absolutely that life exists is Earth. So, we must look for earthlike worlds in our search for extraterrestrial life. I only mentioned silicon and the other solvents because you said the search should be broaden.

 

[ibcnunabit]

But let's say they have some kick a** planet-finding telescopes, they could find our planet from hundreds or even thousands of light-years distance. For them to see the Great Wall of China, the lights from our cities, or even the cities themselves, would be extremely difficult.

Not to mention that the light carrying the immage would be hundreds or even thousands of years old. And that meens no Great Wall of China and no light from our citys, right.

Even a single light-year is too far to resolve an image of a man-made object (even the Great Wall) with an optical telescope. From that distance, they would only be able to use spectroscopy (or some other technique we haven't discovered yet) to tell anything significant (besides maybe mass, distance from the sun, and density). Then, perhaps they'd send out a probe, and if the signal from a probe flyby or landing were promising, an eventual visit.

 

[DaveC426913]

I don't believe that life non carbon based could happen and that the three other solvents I mentioned could replace water, but it is possible. Methane, at least, is nearly as common as water in the universe.
Silicon, like carbon, can form long chains, but those chains are less stable.

If they have the potential to be building blocks for complex molecules then there's no reason why we shouldn't see precursors to these molecules here on Earth. Just because organic life is here doesn't mean it prevents other reactions from taking place. I don't mean life, I just mean we ought to see methane and silicon forming complex molecules spontaneously.

 

[CEL]

If they have the potential to be building blocks for complex molecules then there's no reason why we shouldn't see precursors to these molecules here on Earth. Just because organic life is here doesn't mean it prevents other reactions from taking place. I don't mean life, I just mean we ought to see methane and silicon forming complex molecules spontaneously.

Does water form complex molecules spontaneously on Earth?

 

[DaveC426913]

Does water form complex molecules spontaneously on Earth?

Water and carbon do, yes.