How Do Extraterrestrials Know We Exist?

[DaveC426913]

But suppose that a planet was formed with a different elemental makeup from Earth in its crust. (I will ignore, for the moment, the major issue of the interior composition.) Oxygen is too rare to allow oceans on the scale of Earth; carbon, hydrogen, calcium, silicon, and lithium are more prevalent. Life is based on carbon compounds, like on Earth, but with much smaller amounts of oxygen. Methane, calcium carbide, carbon tetrafluoride, calcium hydride, boron carbide, cyanogen, lithium fluoride, diborane, hydrogen cyanide, etc.

Plausible?

Well, http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements" , Oxygen is the third most abundant element in the universe. Once you eliminate Helium, you have H and O as the top 2.

But let's grant your supposition. How - without a liquid primordial soup in which to mix - would these chemicals you list combine in numbers required to form a process of life?

 

[3dfan]

All our knowledge of galactic technological socities is based on a human perception and interpretation of them, there are no facts, only thoughts and opinions and if "they" exist, "they" will find us be sure

 

[veattaivatsan]

So, while we have access to only one example of life, we have access to the same chemistry rules that the rest of the universe uses. The palette of 92-odd elements is the same everywhere in the universe. And of those 92, only the first dozen or so are useful and reactive enough to build any kind of complex molecules. And of those dozen or so, there are only a set number of ways they can combine.

Well sir, the 92 elements are the ones discovered as of now(and their abundance too)!We had not focussed or read the whole universe!there may be new element(nor an isotope of element in earth) somewhere in asteroid or planet. And these 92 elements were classified for our purpose.It doesn't mean that there are only 92 elements in this universe.moreover,these reactive elements are reactive in earth-like conditions. but there may be different condition(not yet achievable by human means) in other parts of universe where un-predictable(non-reactive) elements may react and constitute as building block of different type of life there. So ,we cannot say that this is the end of periodic table.After a century , more elements may be added that we cannot even predict! correct my statements If iam wrong!

 

[junglebeast]

There is no evidence for alien life, just as there is no evidence for time travel. Both are completely speculative. I think that's what Integral was trying to say.

But there is evidence for alien life -- we know that life can evolve on a planet (eg, Earth) -- and we know that there are billions of other planets. This is very strong evidence that life also evolved on other planets. Traveling back in time, however, creates logical paradoxes and has no evidence at all.

Strong evidence and fairly well understood scientific process >> no evidence and logical contradiction.

 

[ImAnEngineer]

But there is evidence for alien life -- we know that life can evolve on a planet (eg, Earth) -- and we know that there are billions of other planets. This is very strong evidence that life also evolved on other planets. Traveling back in time, however, creates logical paradoxes and has no evidence at all.

Strong evidence and fairly well understood scientific process >> no evidence and logical contradiction.

We know that there are billions of other planets, but we don't know any planet that's exactly like earth. Even though you might think it's evident that there is alien life since the universe is big, other people may find it not evident. We haven't found any alien life as of now. So I wouldn't call the argument of it being likely evidence.

 

[Andre]

I'm not aware of any evidence as in the scientific method, I don't think the hypothesis (of incredibly many planets times incredibly tiny chance of live = some live) can be ticked off, mathematically, as evidence. It's just a hypothesis and I doubt it's even a scientific one, since it does not seem to be falsifiable.

Moreover, the probability of Earthlike planets may be orders of magnitudes smaller than estimated (Drake equation) if it requires a disproprotional large moon to stabilize it's spin axis and to avoid the chaotic zone.

 

[Ivan Seeking]

We have no scientific evidence for alien life. I think junglebeast is referring to logical confidence through inductive reasoning.

There is still controversy over the Mars rock, so in principle that could eventually serve as scientific evidence for life on Mars.

 

[Integral]

Well sir, the 92 elements are the ones discovered as of now(and their abundance too)!We had not focussed or read the whole universe!there may be new element(nor an isotope of element in earth) somewhere in asteroid or planet. And these 92 elements were classified for our purpose.It doesn't mean that there are only 92 elements in this universe.moreover,these reactive elements are reactive in earth-like conditions. but there may be different condition(not yet achievable by human means) in other parts of universe where un-predictable(non-reactive) elements may react and constitute as building block of different type of life there. So ,we cannot say that this is the end of periodic table.After a century , more elements may be added that we cannot even predict! correct my statements If iam wrong!

You are wrong. We understand the structure of the elements and that structure is displayed in the periodic table. Give it a look, there are no holes.

 

[DaveC426913]

Well sir, the 92 elements are the ones discovered as of now(and their abundance too)!We had not focussed or read the whole universe!there may be new element(nor an isotope of element in earth) somewhere in asteroid or planet. And these 92 elements were classified for our purpose.It doesn't mean that there are only 92 elements in this universe.moreover,these reactive elements are reactive in earth-like conditions. but there may be different condition(not yet achievable by human means) in other parts of universe where un-predictable(non-reactive) elements may react and constitute as building block of different type of life there. So ,we cannot say that this is the end of periodic table.After a century , more elements may be added that we cannot even predict! correct my statements If iam wrong!

Integral beat me to it.

The periodic table is universal. The behaviours of the elements are predictable.

While the emergent properties of chemistry can get very hard to predict (any organic chemist and anyone who studies proteins can attest to that), the building blocks are quite limited in scope.

While it is true that exotic lifeforms are not ruled out, it does weigh greatly against them in our search.

Look at it this way:

I hold out a bag to you. It contains 4997 red marbles (which represent the number of ways organic chemistry might produce life) as well as one blue, one yellow and one green marble (representing some more exotic combinations of elements that might produce life).

I tell you you can only reach into the bag a dozen times and that you can only have equipment to study one colour: red, blue, green or yellow (representing our limited resources in the search for life). Each colour marble requires its own set of equipment.

Which equipment would you buy? Red, blue, green or yellow?

 

[junglebeast]

There is still controversy over the Mars rock, so in principle that could eventually serve as scientific evidence for life on Mars.

Indeed, breaking news indicates there may be a fundamental problem with the Mars rover's search for life! Or, as Dave might say, they chose the Yellow detector instead of the Red one,

http://www.newscientist.com/article...bots-may-have-destroyed-evidence-of-life.html

We have no scientific evidence for alien life. I think junglebeast is referring to logical confidence through inductive reasoning.

And how is that not "scientific evidence" ? Evidence of a theory can be based on either deductive or inductive logic..

In deductive logic, one first thinks of a theory, then finds testable hypothesis, tests the hypothesis, observes the results, and finally concludes that the theory is true by deduction.

In inductive logic, one first makes observations, notices a pattern, forms hypothetical explanations, tests them, observes the results and proposes a consistent theory.

From our observations, we can tell that

a) there have observed many rocky exoplanets that are within reasonable distances of their stars to possibly support life
b) the number of observable star systems is so incomprehensibly preposterously overhwelmingly large that the number of planets having similar conditions to Earth is also incomprehensibly preposterously large...assuming that the observable fraction is representable (that follows directly from the "cosmological principal")
c) the same basic elements are available on all planets
d) through random processes, these chemicals may organize into self-replicating sets allowing natural selection to do the rest
e) therefore, the probability of this self organization occurring on some other planet is overwhelming

What is there to disagree with? It seems we have as much evidence for this as we do of anything

 

[Ivan Seeking]

And how is that not "scientific evidence" ? Evidence of a theory can be based on either deductive or inductive logic..

A theory or argument is not scientific evidence. The testing part is where you get into trouble. We don't know what process lead directly to life from non-life, so we have no way to show that this would be common to other planets. And even if we could it would only be evidence that life could exist elsewhere - that we expect it to exist. It wouldn't be direct evidence that other life does exist.

 

[veattaivatsan]

Well,I will choose one that detects red!because everytime,I use the instrument in the bag,I have the highest possibility to get red ball! but think of this- if we dare to use the blue or green one and if when we get those balls,it will be great leap forward.Eventhough the probability is low,the result is of high value.It will change the way we search for ETs.I mean , there is no discovery without risk.Why can't we search for lives built upon non-reactive elements,besides searching for ETs built upon earthly elements as we do now??
Im just college student and asking questions from layman view!please don't get irritated by my arguments!

 

[Ivan Seeking]

There is NO evidence of alien life forms visiting earth, only speculation on the cause of some unexplained and unrepeatable observations. Any effort to explain without meaningful evidence is speculation.

Not true. We have plenty of evidence for visiting aliens, but it is all anecdotal.

 

[Ivan Seeking]

Well,I will choose one that detects red!because everytime,I use the instrument in the bag,I have the highest possibility to get red ball! but think of this- if we dare to use the blue or green one and if when we get those balls,it will be great leap forward.Eventhough the probability is low,the result is of high value.It will change the way we search for ETs.I mean , there is no discovery without risk.Why can't we search for lives built upon non-reactive elements,besides searching for ETs built upon earthly elements as we do now??
Im just college student and asking questions from layman view!please don't get irritated by my arguments!

We put our money on the most likely to succeed. If we had an infinite number of research dollars, it might be a different story. Beyond that, the scientists involved want to be successful. They want to pursue the research most likely to yield results.

 

[CRGreathouse]

Well, http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements" , Oxygen is the third most abundant element in the universe. Once you eliminate Helium, you have H and O as the top 2.

Yep. But since the abundance of elements in a particular star system, and thus on a given planet, depends on the generation of the stars from which it is formed, it wouldn't be unusual for local variations to occur. Further, though it would be (presumably) unusual, there could be any number of stellar catastrophes that might alter the balance of elements in one or more planets.

But let's grant your supposition. How - without a liquid primordial soup in which to mix - would these chemicals you list combine in numbers required to form a process of life?

Dunno. But you'd have a lot of time and a lot of space for finding out. The surface area of a planet times the number of such planets times billions of years.

 

[Ivan Seeking]

Hey any talk of visitation by aliens is speculation, how is mine any more so. I have simply resolved the issue of discovery.

To ask how they would find us is not the same as speculating as to how they would get here, but if you asked which is more likely, time travel, or travel at speeds beyond the speed of light, I think we get into trouble very quickly. From where we sit right now, I think it is fair to say that time travel may at least be theoretically possible, where travel beyond speed C is not. But I'm not up on the latest on wormholes and other exotic ideas. There is also the idea of alcubierre [warp] drive... I saw that recently one paper came out claiming that there are fundamental problems with that idea. Again, not sure of the latest on that one.

 

[Ivan Seeking]

I also think it is fair to say that the anecdotal evidence for very strange and unexplained "things", such as in the most interesting UFO reports, is far stronger than any anecdotal evidence that ET is the pilot. So even if we were take some of the most exotic and impressive reports literally and absolutely, it would not constitute direct evidence for visiting ETs. It would only constitute evidence for an advanced technology of unknown origins. For example, the evidence that two F4s tangled with a UFO over Tehran, in 1976, is much better than any evidence for alien abductions. In the former case we have official military (DOD) reports describing the encounters, many witnesses, multiple RADAR tracks, high quality sources, physiological effects on the pilots, etc. In the latter case, with rare exception, we have only isolated cases of one or two persons telling a story.

 

[DaveC426913]

... but if you asked which is more likely, time travel, or travel at speeds beyond the speed of light, I think we get into trouble very quickly...

True, except that "travel beyond the speed of light" is not required in order for them to visit us.

 

[Ivan Seeking]

True, except that "travel beyond the speed of light" is not required in order for them to visit us.

True, given the the modification that it "may not be required for some potential 'them'".

 

[CEL]

Well,
But let's grant your supposition. How - without a liquid primordial soup in which to mix - would these chemicals you list combine in numbers required to form a process of life?

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.

 

[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.