How Do Extraterrestrials Know We Exist?

[junglebeast]

The sample size is one.

One civilization has survived at least 12,000 years.

The question was, "What's the expected evolutionary timeframe before a civilization destroys itself?" Since our civilization hasn't destroyed itself yet, we have zero observed samples of the amount of time it takes for a civilization to destroy itself. All we know is that the first sample is >= 12,000 years. It could be 14,000 or 100,000 or 100,000,000...it doesn't allow even a crude estimate of the mean of the distribution.

 

[DaveC426913]

There is only one way that they can tell from interstellar distance that intelligent creatures inhabit this planet...RADIO.
...
The number of star systems within 15 light-years is about three dozen. There would have to be 10 billion technically sophisticated societies in the Galaxy to have a reasonable chance of finding one camped out among the nearest three dozen stars. Thats optimism of a high level indeed.

There are a number of assumptions in your assertion that I'm not willing to take at face-value.

1] Just because they didn't have proof-positive of intelligent life does not mean they would be completely uninterested in our planet. We humans are currently raving about the first planets with oxygen because of the potential for life - and any kind of life at that. It is not unreasonable that alien species might be doing the same thing. That pushes back their theoretical departure time by billions of years, which is a large fraction of the age and breadth of the universe.

2] You assume a randomly-distributed incidence of alien life across the galaxy. If one puts any truck at all in the panspermia theory, it is far, far more likely that incidences of life cluster.

 

[Vanadium 50]

It's from an (in)famous CNN poll that said

Not clear how only 54% believe in ET but 64% believe in a coverup!

I read this as 54% believe in ET and 64% believe ET has visited: meaning that 10% believe ET doesn't exist: but has visited!

 

[Belzy]

Well we do know of at least one tech society.

Nice little pocket in the outer rim.

Well on another note we have a lot of room in our own solar system to conquer first. So if ET is out there maybe it will find us before our sun grows and consumes us.

 

[Kalibr]

If "they" are at the sort of technological level we are at, it's be easier to just send out a signal instead of attempting to master intersolar travel. I'm guessing some sort of pulse, similar to the modern day EM Pulse would be used, but on a massive scale.

This is all assuming they don't follow the pessimistic prediction that all civilisations will destroy themselves before they attain the sort of technological level required for communications across the stars. I have a few even more outlandish theories, but they say simple is better, and making a pulse signal will certainly be the easiest to produce. It also works as a handy filter so that only civilisations that can actually intercept the signal and understand it for what it is will respond.

Of course, they'll have to repeat the pulse at some sort of rate in case a civ happened to miss the first. Of course, if they'd had made the pulse cycle once every 3.14 years that'd really hammer the point across, in case the observers think it's a natural event.

 

[veattaivatsan]

First of all,we,human beings were a chance and result of a non-linear events which has happened from the birth of universe! It doesn't mean that other planets capable of holding life can march in the same direction.I mean the events might have been different. The base for life may not be amino acids,DNA or genes.There may not be an evolution in the same way as earth. ET,if they are,may have lived upon non-oxygen(gases apart from oxygen) and may not drink water or depend upon light.They might depend on some other yet-to-be-discovered gas and may have some other basic of life.
ok,So there might not be telescope,or space travel or even those ExtraTerrestrials may not even look up at the sky! if so,then why we waste time in sending radio waves and messages to the rest of universe??
If by chance,they have evolved in the same manner as we were,then it is not compulsory that they have same theories and same means of approaching the outer space! and if they have carried out evolution in science as same as we had,then it is possible that they will respond to our radio waves and come and frighten us!

but the probability (of human like evolution-similar to what happened as of now in earth)as of now is 1 in N,where N is the increasing age of universe!and 1 is us!

I don't know why we look for oxygen and water when ETs might have thrived upon some other gases! even before oxygen catastrophe in earth,organosms were anaerobic!

This is my own ideas,may not be correct!

 

[CRGreathouse]

Of course, they'll have to repeat the pulse at some sort of rate in case a civ happened to miss the first. Of course, if they'd had made the pulse cycle once every 3.14 years that'd really hammer the point across, in case the observers think it's a natural event.

Yeah, just as long as their year is as long as ours. 99,139,006 seconds might not be as special as you'd like to think.

Edit: Maybe send signals spaced by (half-life of tritium) / pi and (half-life of tritium) / e, alternatingly?

 

[CRGreathouse]

The base for life may not be amino acids,DNA or genes.

Seems very likely to me. I'd be shocked if a molecule as complicated as DNA was independently derived on another planet. (Amino acids, not so surprising.)

There may not be an evolution in the same way as earth.

Why not?

ET,if they are,may have lived upon non-oxygen(gases apart from oxygen) and may not drink water or depend upon light.

Sure. But we'd expect any developed organisms to be based on small atoms, since larger atoms generally take more generations of stars to form. So methane-breathers, sure; xeon-breathers, probably not.

But life needs low entropy, and by thermodynamics this means they'll need energy. Light seems to be the easiest way to get it, directly or indirectly. While others are possible I see it as less likely as a basis for intelligent life ( = high energy requirements).

They might depend on some other yet-to-be-discovered gas and may have some other basic of life.

Probably not, since I expect we've discovered all reasonably stable low atomic weight atoms and their easily-formed compounds.

I don't know why we look for oxygen and water when ETs might have thrived upon some other gases! even before oxygen catastrophe in earth,organosms were anaerobic!

I'm not quite sure either. Life on Earth is pretty tied to water since we're close to its triple point and it's abundant, but surely there are other compounds that could fulfill that role.

 

[junglebeast]

Yeah, just as long as their year is as long as ours. 99,139,006 seconds might not be as special as you'd like to think.

Edit: Maybe send signals spaced by (half-life of tritium) / pi and (half-life of tritium) / e, alternatingly?

You have the right idea, wrong approach -- half life of tritium must be measured in some arbitrary units.

The only numbers that would remain constant for interpretation would be ratios. ie, send out a binary pulse signal where the ratio of the duration of the pulse to the duration of the silence was equal to a number.

 

[DaveC426913]

I'm not quite sure either. Life on Earth is pretty tied to water since we're close to its triple point and it's abundant, but surely there are other compounds that could fulfill that role.

Water is unique in many important ways that are very useful to any type of life. I can't even begin to list more than a few.
- it's quite abundant in the universe
- it's one of the simplest molecules (and so very abundant) made out of two abundant elements.
- It is nearly a universal solvent (making it an excellent "mixing bowl" of useful chemicals).
- It is easily ionizable into acid and alkali.
- It is small enough to behave atomically as well as molecularly (combine this with above elements, and you get a substance that get inisde a cell membrane and do a lot of interesting things to molecules it finds in there).
- It's solid phase floats, which means it freezes top-down instead of bottom-up (which means life and survive seasonal changes, which greatly expands life's range).

The list just goes on-and-on.

Same thing can be done for carbon-based molecules. There just aren't any substitutes.

Carbon has the unique ability to spontaneously form long, complex chains in a huge array of configurations. It can form single-bonds, double bonds, positive bonds or negative bonds. Again, it's light and simple (only element #6).

Hydrogen, the most abundant element, likes to attach to carbon. The second most abundant element, helium, is useless, since its non-reactive. You can go down the periodoc table one by one, eliminating elements that do very little, and the results are virtually inescapable.

Think about the arrangment of the table and what that means to the formation of molecules. Usually, it's got to be a combination of something from the left (positive) and something from the right (negative). You can't have Lithium Hydride for example. NH3 (ammonia) is pretty much the very first valid combination.

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.

 

[apeiron]

Have you come across any good papers that do go through the chemistry tool kit this way, perhaps even a bayesian analysis?

 

[Integral]

Our solar system is completely unremarkable, being a tiny grain of sand in a huge beach. The chances of a alien civilization discovering us is quite small.

However, you can eliminate the need for discovery if the "visitors" are time travelers. A future civilization may be peeking back filling in holes in the history books. As far as I know the energy requirements for time travel would be similar to that for near light speed interstellar trips. Only now the "visitors only have to hold on to the earth, they do not have to discover it.

 

[DaveC426913]

However, you can eliminate the need for discovery if the "visitors" are time travelers. A future civilization may be peeking back filling in holes in the history books. As far as I know the energy requirements for time travel would be similar to that for near light speed interstellar trips. Only now the "visitors only have to hold on to the earth, they do not have to discover it.

OK, now that's a bit speculative. Even for this thread.

 

[Integral]

OK, now that's a bit speculative. Even for this thread.

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

 

[junglebeast]

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

Because science suggests that the existence of alien life is highly probable, whereas time travel is a completely made up thing.

 

[Integral]

Because science suggests that the existence of alien life is highly probable, whereas time travel is a completely made up thing.

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.

Time travel is no less physical then interstellar travel.

 

[CRGreathouse]

You have the right idea, wrong approach -- half life of tritium must be measured in some arbitrary units.

Huh? The half-life for tritium is a set amount of time -- about 3.88e8 seconds. This time could be recognized by other civilizations, even if they measure it as 2.62e6 flarbs.

Admittedly, they'd have to share our 'Pythagorean' tendency to measure things in whole-number ratios, but that seems to be a reasonable choice.

 

[junglebeast]

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.

Well, this thread is not really about "alien life forms that have visited Earth." It is simply a rational discussion about the means by which an alien life form might visit Earth, and the associated probability of such an event someday occurring (or having occurred).

Time travel is no less physical then interstellar travel.

Yes, it is. Time travel is a made up idea concocted by science fiction writers with no evidence of possibility, whereas acceleration of a mass through space (which is all that is required for interstellar travel) is provably possible and well understood as a direct result of Newtonian physics.

 

[DaveC426913]

Yes, it is. Time travel is a made up idea concocted by science fiction writers with no evidence of possibility, whereas acceleration of a mass through space (which is all that is required for interstellar travel) is provably possible and well understood as a direct result of Newtonian physics.

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.

Interstellar travel OTOH, as you point out, is completely possible (whether it's feasible or not is a different matter, but that is entirely up to the proposed aliens' philosophy).

 

[CRGreathouse]

Water is unique in many important ways that are very useful to any type of life.

Agreed.

Same thing can be done for carbon-based molecules. There just aren't any substitutes.

Agreed.

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?

 

[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