A simple solution to the Fermi Paradox - why we haven't stumbled onto some alien broadcasts in space or have met ETs - may be that our Sun seems to be an "afterthought".

Any electromagnetic signal, which has been transmitted from even the most remote point in our galaxy more than 100,000 years ago will now have left the galaxy forever.

Most stars in our galaxy seem to have been created some 7E9 to 9E9 years ago. Our Sun is only about 4.6E9 years old.

According to the present star models, when our Sun reaches the age of about 6.5E9 years the market for long johns will be exhausted - the oceans will have evaporated and life as we know it will become extinct.

Assuming that an extraterrestrial civilization reached Earth a few E9 years back, even if they did build observation stations on Earth, plate tectonics may well have removed any trace of this incident.

Let's hope for the best, a 3-armed and 3-legged friend somewhere working on his emoh@ites project ::-))
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Split personality - who, us?

It would help if our aliens broadcast a contemporaneously overlapping (allowing for travel time) and continuous signal.

Any electromagnetic signal, which has been transmitted from even the most remote point in our galaxy more than 100,000 years ago will now have left the galaxy forever.

Bill is right.

If ET only sent a signal once, for 1 minute or for 100 years, and then stopped signaling us, whether it was 100,000 years ago or 100 years ago, then all SETI efforts are doomed to failure.

The chances that we would be looking in the right spot at EXACTLY the right time at EXACTLY the right instant when the radio signal hits the Earth are pretty much zero.


But we assume ET would know that and we don't think ET would do that. ...After all, we have to assume they're smart enough to realize that we wouldn't have a decent chance of detecting any signal that wasn't continuous over a very long period of time. We also have to assume that they would be smart enough to carry out such an endeavor only if it had a reasonable chance of success. We have to assume that ET would be smart enough to realize the chance of success would be zero if they signaled us only briefly and then stopped forever, and since SETI acts on the assumption that the ones making the pro-active effort are the aliens (they're doing the signaling), we also have to assume they would only make that effort is they had a reasonable idea of how to do it successfully.

SETI assumes that any civilization we might receive a signal from would be more advanced than us and would know they're more advanced than us, and would therefore know that for their signaling effort to succeed, the signal would have to be continuous for a very long time.

Lighthouses don't turn their light on for 1 minute and then stop forever, hoping that some ship just happened to be within range to see that light and happened to be looking in that exact direction during that 60 seconds when the light was on.

People tapping out SOS messages to be rescued don't send out an SOS just once or just for a few minutes hoping that a plane or ship happened to be in the area during that 2 minutes and happened to be listening to that frequency at the exact time the SOS was sent.


We have to assume that any signal from ET would be continuous over the course of a VERY long time, possibly millions of years or longer.
If the signal is not continuous over a very long time, then SETI will not work.

Any electromagnetic signal, which has been transmitted from even the most remote point in our galaxy more than 100,000 years ago will now have left the galaxy forever.

... We have to assume that any signal from ET would be continuous over the course of a VERY long time, possibly millions of years or longer.
If the signal is not continuous over a very long time, then SETI will not work.

I agree to most of your posting. There are, however, the problem of energy expenditure for a signal in all direction with a flux sufficient to be detected at a reasonable distance.

Also, it does not address my main point: That we may have come into existence too late - perhaps even billions of years too late.

But: Never forget the saying "Faint of heart never wins fair lady" (Thomas Hardy), and let's keep trying. I hope that we are not alone in our galaxy - like someone said (sorry, can't give credit where due) "such a waste of space".
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Split personality - who, us?

I agree to most of your posting. There are, however, the problem of energy expenditure for a signal in all direction with a flux sufficient to be detected at a reasonable distance.

But it wouldn't be a signal in all directions; it would be specifically sent to Earth.

I don't think anyone seriously believes that ET is wasting energy to blanket the cosmos in radio waves that will target every single star in the galaxy with the blind hope that one of those stars has an orbiting planet inhabited by extraterrestrials.

I think it's safe to assume that ET would possess the technology to detect exo-planets just as we do; arguably their ability to detect the planetary systems around stars would be significantly superior to ours considering that they would have to be older than us for SETI to work in the first place.

There's no reason not to believe that ET will have extensively analyzed and cataloged every single visible star in the galaxy; the technology and time required to do this isn't that great. In fact, the Kepler mission, launching in 2009, will extensively analyze the (potential) planets of over 100,000 stars within four years. Within a time-span of years, missions will be launched to not only detect Earth-size planets, but to analyze their atmospheres. The James Webb Telescope and ultimately the Terrestrial Planet Finder will be able to do this.

Considering that WE essentially possess the technical capability right now to not only detect the presence of an Earth, but to analyze its atmosphere, it's reasonable to conclude that an older civilization would also possess that capability.

Assuming such a civilization has observed and analyzed every visible star in the galaxy, it's reasonable to conclude that this civilization would be aware of the existence of Earth. It's also reasonable to conclude that this civilization would also be aware of the composition of Earth's atmosphere; since there is no natural mechanism other than biological processes to explain the level of oxygen in Earth's atmosphere, that civilization will also be aware that there is life on Earth.

Also, it does not address my main point: That we may have come into existence too late - perhaps even billions of years too late.

A good listen on that very topic:
"Are We Alone", the SETI radio show (episode on recent astrobiology conference)
http://radio.seti.org/past-shows.php#2008-05-05.php

Astrophysicist Charley Limeweaver discusses the fact that the occurrence of planets is a function of stellar metalicity, and since metals can only be created in post-Big Bang events like Supernove (only hydrogen and helium existed at the time of the Big Bang), the early universe did not have terrestrial, Earth-like planets since there were no metals from which they could form. According to Limeweaver's calculations regarding metalicity, terrestrial earth-like planets likely could not have formed in our region of the galaxy earlier than ~7.6 billion years ago.

Not only that, but *specifically addressing your issue*, Limeweaver further calculates that the "average 'Earth' in the universe, not just the galaxy, is about 1.8 billion years older than the Sun." (quoted verbatim)

Most stars in our galaxy (the vast majority) are M-Dwarfs/Red Dwarfs (not Sun-like stars); when you say that "most" stars in our galaxy are 7-9 billion years old, you're probably referring to these. They have extremely long, stable lifespans; estimated at 10s of billions, possibly trillions of years. Though Red Dwarfs have high metalicity, they were thought to be poor candidates to host life due to the fact that any planet in the theoretical habitable zone would be so close to the star that it would be tidally locked and its surface temperatures would be too extreme. These notions are now being thrown out as recent studies have proven the situation is far more complex than previously thought and stable atmospheres are plausible, perhaps even likely on such worlds.

As far as sun-like stars (G2-dwarfs), new ones are being born and old ones are dying all the time. I don't know of any reason to believe that we live in a time period beyond the capacity for most sun-like stars in our galaxy to develop and sustain life. Indeed, it seems the opposite is more likely to be true considering the time scales required for supernovae to bring about the build-up of metalicity beyond the galactic core.

As a further note:

It seems to me that anywhere an intelligent civilization evolves, they will inevitably possess the technical capability to avoid the effects of the death of their star.

The good news for us and any other intelligent civilization; these things happen slowly....VERY slowly.

Our Sun's temperatures will not be lethal to life on Earth for another 900 million years. That's a LONG time in which to plan.

Even if we had only 1 million years, that would also be a LONG time in which to plan.

Planets further away from the star can be colonized, migrations can be made to neighboring stars. These seem to me to be fairly straightforward, simple endeavors given the basic technical capability and powerful enough motivation (the death of your star should be a good enough motivator for any civilization).

Even if a civilization does not possess near-light-speed travel capability or worm hole technology or anything like that, traveling to the nearest neighboring stars would be fairly easy to accomplish. All you would need is stable, self-sustaining resources (like food) and enough room for however many individuals choose to travel. A journey of thousands of years might be a tough wait, but it would be guaranteed to escape the fate of the homeworld's star.

I actually think it's more likely than not that interstellar migration and colonization will occur long before the death of the homeworld's star anyway.

So the way I look at it, the average lifetime of stars is a useful variable with which to calculate the probability of the evolution of life or intelligence with regards to timescales, but I see it as inapplicable as a factor regarding civilizations that are already assumed to have arisen.

We have to assume that any signal from ET would be continuous over the course of a VERY long time, possibly millions of years or longer.
If the signal is not continuous over a very long time, then SETI will not work.

The first very weak radio broadcast on (from) Earth took place in 1906.

Thus an extraterrestrial civilization cannot have detected our signal from a distance of more than - at most - 102 light years.

For that reason it sems unlikely that an electromagnetic beam has been directed at us from a longer distance than this - and possibly outside the 3 to 30 cm frequency window, because our early broadcast were in the long wave spectrum.

More later.

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Split personality - who, us?

We have to assume that any signal from ET would be continuous over the course of a VERY long time, possibly millions of years or longer.
If the signal is not continuous over a very long time, then SETI will not work.

The first very weak radio broadcast on (from) Earth took place in 1906.

Thus an extraterrestrial civilization cannot have detected our signal from a distance of more than - at most - 102 light years.

For that reason it sems unlikely that an electromagnetic beam has been directed at us from a longer distance than this - and possibly outside the 3 to 30 cm frequency window, because our early broadcast were in the long wave spectrum.

More later.

Did you read this part of my post?

There's no reason not to believe that ET will have extensively analyzed and cataloged every single visible star in the galaxy; the technology and time required to do this isn't that great. In fact, the Kepler mission, launching in 2009, will extensively analyze the (potential) planets of over 100,000 stars within four years. Within a time-span of years, missions will be launched to not only detect Earth-size planets, but to analyze their atmospheres. The James Webb Telescope and ultimately the Terrestrial Planet Finder will be able to do this.

Considering that WE essentially possess the technical capability right now to not only detect the presence of an Earth, but to analyze its atmosphere, it's reasonable to conclude that an older civilization would also possess that capability.

Assuming such a civilization has observed and analyzed every visible star in the galaxy, it's reasonable to conclude that this civilization would be aware of the existence of Earth. It's also reasonable to conclude that this civilization would also be aware of the composition of Earth's atmosphere; since there is no natural mechanism other than biological processes to explain the level of oxygen in Earth's atmosphere, that civilization will also be aware that there is life on Earth.

All ET would have to be able to do is to analyze the chemical composition of our atmosphere via spectroscopy.

Let me make this clear:
*We* already possess this technology. Atmospheric analysis of exoplanets has already been done. Furthermore, the James Webb Space Telescope will be able to analyze the atmospheres of Earth-like planets.

If we were ever to detect a planet that contained the levels of Oxygen that are present in Earth's atmosphere, there would be no other feasible way to explain it; that planet would have life.


Earth's atmosphere has been sending out this biosignature to the universe for at least 1 billion years. That essentially means that any civilization in our galaxy (or indeed, the universe) that is able to see our planet (assuming we are not obscured by the galactic bulge), has had 1 billion years to detect the biosignature in our atmosphere.

If you don't believe me, here's an interesting article by Seth Shostak on this very subject:
http://www.seti.org/news/features/can-aliens-find-us.php

...and as far as our ability to detect the atmospheric composition of other worlds, listen to the Are We Alone radio show I linked to earlier.

Anyway, it seems certain that *if* older alien civilizations actually exist in the Milky Way Galaxy, then they are likely not only aware of the Earth but aware that it hosts life.

The question is, would an alien civilization predict that intelligence might one day evolve on Earth or exist already? Assuming they would, would they send a directed radio signal to Earth with the knowledge that an emerging civilization is likely to come to the conclusion that there are probably other civilizations in the galaxy (as they once did), and listen for potential communications from those civilizatons? If ET has any desire to communicate with potentially intelligent creatures on Earth, it would seem a simple matter to have a long-term project in which a radio signal is constantly directed toward Earth with the knowledge that some day, that signal will be detected. It may be possible that ET has some sort of "emerging civilization communicator" in orbit around their planet, constantly pinging a list of targeted planets on which life is known to exist.

The funny thing is, we may find ourselves in the role of ET some day.

travelling to the nearest neighbouring stars would be fairly easy to accomplish.

When one thinks of the resources that have been poured into various defence projects by the Earth's nations, this could have been well under way already.
Then again, we probably have the defence industry to thank for most of the advanced technology we enjoy today.

Someday, maybe, those that make the decisions may see the light.
But while it is not financially advantageous to embrace this idea, it will not happen in the immediate future.

Did you read this part of my post?

[quote]There's no reason not to believe that ET will have extensively analyzed and cataloged every single visible star in the galaxy; ...
...
Assuming such a civilization has observed and analyzed every visible star in the galaxy, it's reasonable to conclude that this civilization would be aware of the existence of Earth. It's also reasonable to conclude that this civilization would also be aware of the composition of Earth's atmosphere; since there is no natural mechanism other than biological processes to explain the level of oxygen in Earth's atmosphere, that civilization will also be aware that there is life on Earth.

I did read that part of your post.

In our galaxy there are about 100E9 stars - more stars enlarge the problem described below.

Minimum assumption: Let us assume that
at least stars from F5V to K5V (life spans 4E9 to 25E9 years) may have planets where life can exist.
# of stars: F5-9V: 1.5% + GxV: 9% + K0-5V: 7% = 17.5% af 100E9 = 17.5E9 stars

Minimum+minimum assumption: Let us assume that
at least 1/10 of these do have planets where life can exist.
# of planets: 1.75E9
at least 1/100 of these do have planets where life does exist, at least at a level where it was on Earth 4E6 years ago (oxygen etc.).
# of planets: 17.5E6

Minimum+maximum assumption: Let us assume that
at most 1/5 of these do have planets where life can exist.
# of planets: 3.5E9
at most 1/50 of these do have planets where life does exist, at least at a level where it was on Earth 4E6 years ago (oxygen etc.).
# of planets: 70E6

Maximum assumption: Let us assume that
at most stars from F3V to K9V (life spans 3E9 to 40E9 years) have may planets where life can exist.
# of stars: F3-9V: 2% + GxV: 9% + K0-9V: 14% = 25% af 100E9 = 25E9 stars

Maximum+minimum assumption: Let us assume that
at least 1/10 of these do have planets where life can exist.
# of planets: 2.5E9
at least 1/100 of these do have planets where life does exist, at least at a level where it was on Earth 5E6 years ago.
# of planets: 25E6

Maximum+maximum assumption: Let us assume that
at most 1/5 of these do have planets where life can exist.
# of planets: 5E9
at most 1/50 of these do have planets where life does exist, at least at a level where it was on Earth 5E6 years ago.
# of planets: 100E6

From our position in the galaxy we are able to see about 40% af all the stars, i.e. the planets, in the galaxy, 60% are hidden by the bulge and thick disk.

Minimum+minimum+minimum assumption:Let us assume that ET can see 40% of the planets
# of planets: 7E6

Minimum+minimum+maximum assumption:Let us assume that ET can see 50% of the planets
# of planets: 8.75E6

Minimum+maximum+maximum assumption:Let us assume that ET can see 60% of the planets
# of planets: 10.5E6

Maximum+minimum+minimum assumption:Let us assume that ET can see 40% of the planets
# of planets: 40E6

Maximum+maximum+minimum assumption:Let us assume that ET can see 50% of the planets
# of planets: 50E6

Maximum+maximum+maximum assumption:Let us assume that ET can see 60% of the planets
# of planets: 60E6

All ET has to do, is to direct a sufficiently powerful beam of electromagnetic radiation, in a frequency fit for detection at distances of, say, 40,000 lightyears, at anywhere from 7E6 to 60E60 positions in space and keep it running for 100E6 years or more.
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Split personality - who, us?

Oops! typo:
for "on Earth 4E6 years ago (oxygen etc.)"
read: "on Earth 400E6 years ago (oxygen etc.)"

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Split personality - who, us?

One more:
for "on Earth 5E6 years ago."
read: "on Earth 400E6 years ago."

Copy and paste *is* risky!

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Split personality - who, us?

In our galaxy there are about 100E9 stars - more stars enlarge the problem described below.

The latest estimates put the number of stars in the Milky Way at between 200-400 billion and possibly higher than that, though the bulk of these are low mass stars.

All ET has to do, is to direct a sufficiently powerful beam of electromagnetic radiation, in a frequency fit for detection at distances of, say, 40,000 lightyears, at anywhere from 7E6 to 60E60 positions in space and keep it running for 100E6 years or more.

Not sure I see how you came up with those numbers, but how about this?

If ET is within the 40-60% of stars which can a make direct, unobstructed visual observation of our star, and ET is older than our civilization, then they've already detected the Earth and are already aware that these is life here.

Earth's atmospheric biosignature has existed for at least 1 billion years. Any civilization able to directly observe our star has had at least 1 billion years to detect the presence of life here.

ET may have detected the Earth 1 million years ago, 10 million years ago, 100 million years ago, etc. They may be within 100, 200, 1,000 light years, etc.

If they decided to send a radio signal to Earth however long ago they detected us, that signal would only need to be strong enough to be detected within whatever distance the Earth lies from their planet or transmitter.

As far as Drake's Equation, I made my own guesses here:
http://setiathome.berkeley.edu/forum_thread.php?id=46028

In our galaxy there are about 100E9 stars - more stars enlarge the problem described below.

The latest estimates put the number of stars in the Milky Way at between 200-400 billion and possibly higher than that, though the bulk of these are low mass stars.

Some 73-78% (or more) of the app. 200-400E9 stars are M0-M9 dwarfs.

For a star to host plants with civilizations, it is my estimate, that the star must have a life span of 3E9 years or more. M dwarfs do have a longer life span, but they have an extremely small energy output.

It is my estimate, that only stars in the range F5V-K5V have an adequate energy output and life spans long enough to support a civilization.

If ET is within the 40-60% of stars which can a make direct, unobstructed visual observation of our star, and ET is older than our civilization, then they've already detected the Earth and are already aware that these is life here.

ET may have detected the Earth 1 million years ago, 10 million years ago, 100 million years ago, etc. They may be within 100, 200, 1,000 light years, etc.

There may be as few as 7E6 and as many as 60E60 possible planets supporting life, that has to beamed at, if my estimate holds.

As far as Drake's Equation, I made my own guesses here:
http://setiathome.berkeley.edu/forum_thread.php?id=46028

In my opinion, Drake's Equation is good entertainment, nothing more.

As I see it, the problem is that it is of no consequence for intelligent life in our galaxy today (in the universe, too), that a star is born in the year 2008 - we shall have to wait for 3-4-5E9 years before intelligent life has evolved on its planets - don't hold your breath.

What matters is how many stars were born 3-4-5E9 years ago.

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Split personality - who, us?

Some 73-78% (or more) of the app. 200-400E9 stars are M0-M9 dwarfs.

For a star to host plants with civilizations, it is my estimate, that the star must have a life span of 3E9 years or more. M dwarfs do have a longer life span, but they have an extremely small energy output.

It is my estimate, that only stars in the range F5V-K5V have an adequate energy output and life spans long enough to support a civilization.

Yes, I mentioned that the vast majority of stars in our galaxy are low mass M dwarfs. Indeed, if I'm not mistaken, only ~15% of stars in our galaxy are sun-like G dwarfs.

However, I've not heard the argument that M dwarfs lack the "energy output" to sustain a civilization... I'm aware that while our sun outputs most energy in the visible wavelength that is conducive for photosynthesis while M dwarfs release most energy in the infrared, I see no reason why, given a stable, habitable surface and atmosphere, life could not evolve given enough time. As far as civilization, I'm not sure how the star's energy output would factor in? Could you elaborate on that?

There may be as few as 7E6 and as many as 60E60 possible planets supporting life, that has to beamed at, if my estimate holds.

Specifically how do you arrive at those numbers and what basis do you have for the assumptions that lead you to those numbers?

In my opinion, Drake's Equation is good entertainment, nothing more.

Well, his equation is a perfectly valid formula and would result in a reasonable conclusion if all the variables were known. And the problem is that many of those variables are completely unknown. I agree, it's good entertainment since we can just plug our own personal guesses into the equation and have some fun speculating...

Yes, I mentioned that the vast majority of stars in our galaxy are low mass M dwarfs. Indeed, if I'm not mistaken, only ~15% of stars in our galaxy are sun-like G dwarfs.

However, I've not heard the argument that M dwarfs lack the "energy output" to sustain a civilization... I'm aware that while our sun outputs most energy in the visible wavelength that is conducive for photosynthesis while M dwarfs release most energy in the infrared, I see no reason why, given a stable, habitable surface and atmosphere, life could not evolve given enough time. As far as civilization, I'm not sure how the star's energy output would factor in? Could you elaborate on that?

Power consumption for food, heating, transportation, interplanet travel, etc. on the home planet(s), and for near light speed spaceships to distant (well, any) stars, for signalling to spaceships and to other possible civilizations, etc. etc.

Assuming a much lower insolation on Earth, we would not have fossil fuels, only stunted trees and bushes - go north, young man, or south. Which possibilities do you find for the development of an advanced civilization at 68º north or south?

As for establishing nuclear or similar power sources without the intermediate stadium of fossil fuels - stone age to nuclear age in one step? I seriously doubt that this is feasible.

One might assume that the power problem on a planet in orbit around a red dwarf could be set off by close proximity, which is correct - except that power is reduced by distance by 1/d**2 - which means that even if the equator is sufficiently warm to sustain a civilization, not far north or south it's very cold; and if the "top" of the northern and southern hemispheres are warm, then the equator and surroundings will be too hot.

There may be as few as 7E6 and as many as 60E60 possible planets supporting life, that has to beamed at, if my estimate holds.

Specifically how do you arrive at those numbers and what basis do you have for the assumptions that lead you to those numbers?

As always, when assessing the possibility of extraterrestrial civilizations, one has to make assumptions.

Mine are found in the posting from 21 May 2008 12:53:51 UTC. "Let us assume that ...".

The result is my assessed minimum number of planets to monitor and beam signals at.

If you allow for M dwarfs to have inhabitable planets with a possible bio fingerprint, you also increase the number of planets, which ET has to monitor and beam the signals to (your posting 18 May 2008 17:42:53 UTC: "We have to assume that any signal from ET would be continuous over the course of a VERY long time, possibly millions of years or longer.").

In that case ET will have to beam signals for millions of years at not just millions, but possibly billions of planets.

In my opinion, Drake's Equation is good entertainment, nothing more.

Well, his equation is a perfectly valid formula and would result in a reasonable conclusion if all the variables were known. And the problem is that many of those variables are completely unknown. I agree, it's good entertainment since we can just plug our own personal guesses into the equation and have some fun speculating...

What I see as the serious problem with Drake's equation is that a star born today does not entertain intelligent life/civilization tomorrow or next year - you'll have to wait for 3-4-5 billion years before its planets are of any interest.

For that reason the Drake "Rate of Star Formation in the Milky Way" is of no interest in assessing the number of possible civilizations.

My point is that the star formation rate 3-4-5 billion years ago is what matters.
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Split personality - who, us?

Power consumption for food, heating, transportation, interplanet travel, etc. on the home planet(s), and for near light speed spaceships to distant (well, any) stars, for signalling to spaceships and to other possible civilizations, etc. etc.

The energy of an M-Dwarf would provide more than enough power for whatever engineering such a civilization would need to sustain itself, and wouldn't have any influence on their capacity to build radio telescopes or radio transmitters. We have the capacity to directly detect extrasolar planets and transmit radio signals right now and we essentially don't use direct energy from the sun on any significant scale whatsoever.

Now whether or not such a star could provide enough energy for photosynthesis and ultimately complex animal organisms in this first place is an open question, but at a recent astrobiology conference, the answer seemed to be that it could.

Assuming a much lower insolation on Earth, we would not have fossil fuels, only stunted trees and bushes - go north, young man, or south. Which possibilities do you find for the development of an advanced civilization at 68º north or south?

Not sure what you mean, but I think we agree just in different ways.
Like I said, if such a star could not provide enough energy for widespread photosynthesis in the first place, then yes, this would be a moot point because intelligence wouldn't evolve in the first place.

As for establishing nuclear or similar power sources without the intermediate stadium of fossil fuels - stone age to nuclear age in one step? I seriously doubt that this is feasible.

Any planet on which intelligent, complex organisms evolved would presumably be just as rich in fossil fuels as the Earth is (or was) since intelligent organisms could only arise if they were preceded by a diverse wealth of complex creatures, and such creatures could only arise in a rich biosphere amenable to their existence in the first place.

One might assume that the power problem on a planet in orbit around a red dwarf could be set off by close proximity, which is correct - except that power is reduced by distance by 1/d**2 - which means that even if the equator is sufficiently warm to sustain a civilization, not far north or south it's very cold; and if the "top" of the northern and southern hemispheres are warm, then the equator and surroundings will be too hot.

The latest atmospheric models that I'm aware of suggest that it would be possible, if not likely, that Earth-like terrestrial planets within the habitable zone of M Dwarfs could possess thick atmospheres with higher pressures than on Earth acting as a global greenhouse to sustain habitable temperatures throughout the surface.

If you allow for M dwarfs to have inhabitable planets with a possible bio fingerprint, you also increase the number of planets, which ET has to monitor and beam the signals to (your posting 18 May 2008 17:42:53 UTC: "We have to assume that any signal from ET would be continuous over the course of a VERY long time, possibly millions of years or longer.").

In that case ET will have to beam signals for millions of years at not just millions, but possibly billions of planets.

I'm still not sure how you're arriving at those numbers, but why exactly would ET have to beam signals to "billions" of planets?

Are you suggesting that billions of planets within the unobstructed portion of the observable galaxy would possess detectable atmospheric signatures of biology, like Earth does?...

Earth is the *only* planet in our solar system that contains an atmospheric biosignature. There could be plenty of worlds with primitive microbes that do not possess atmospheric biosignatures (like Mars, Europa, or Titan). Indeed, this might be the norm.

...But planets with biospheres as direct and obvious as the Earth's are likely rare indeed, and I know of nothing to indicate otherwise. ET would know, just as we would, that the only way for such a potent biosignature to exist is the widespread existence of life throughout the terrestrial surface. I doubt very much that there are billions of such worlds throughout our galaxy. Indeed, I would guess the number of such worlds is likely in the thousands if not less.

Besides, you're making the mathematical mistake of including *all* locations in the Milky Way; ET will never find atmospheres with biosignatures in the galactic bulge because stars there are bombarded by too much interstellar radiation and are too strongly influenced by the gravities of other stars (since they are more tightly packed). Stars too far away from the bulge would lack the metalicity to support terrestrial planets on which life can evolve in the first place.

The number of star systems with enough metalicity, lack of gravitational interference, and lack of lethal interstellar radiation is likely a particular percentage of the galaxy; maybe 15-20% at the very most.

And ET wouldn't need to blindly signal every single one of those stars, it would only signal the specific planets on which it detected a biosphere.

Like I said, actually detecting every single one of those planets in the first place is a fairly quick task to accomplish, even if ET decided to scan *every single star* in the entire observable Milky Way rather than just the ones that are potentially habitable to begin with. Our primitive Kelper telescope will scan 100,000 stars for such planets within a period of four years. It launches next year, in 2009.

What I see as the serious problem with Drake's equation is that a star born today does not entertain intelligent life/civilization tomorrow or next year - you'll have to wait for 3-4-5 billion years before its planets are of any interest.

For that reason the Drake "Rate of Star Formation in the Milky Way" is of no interest in assessing the number of possible civilizations.

My point is that the star formation rate 3-4-5 billion years ago is what matters.

It seems to me like you're probably right about that.

The question to ask is, why they would broadcast at all? Even if they were sure we were here?

Don't get me wrong, I'm a believer.

But, the thing that worries me is not could they, but would they?

One question, why is our sun so much younger than the aveerage? There's got to be plenty of stars roughly the same age throughout the galaxy no?

<snip>One question, why is our sun so much younger than the aveerage? There's got to be plenty of stars roughly the same age throughout the galaxy no?

Main sequence stars: F5-9V: 1.5% + GxV: 9% + K0-5V: 7% = 17.5% af 100E9 = 17.5E9 stars or thereabout, star life span is importants in order to allow for life to emerge and civilizations to build.

Planets around M-dwarfs are a possibility (very long life spans), but there are a couple of problems like energy and quality of light vs. life requirements - not prohibitively so, but all we have to compare with is the one single pro tem. known life form.

Of these about 1/4 (give and take some) are of the same age as the Sun, but spread rather thinly over the spiral arms, which is where new stars are formed.

The star formation rate seems to have been stable for the last app. 1E9 years and the same, more or less, as it is today. See e.g. Persic og Rephaeli: "Galactic star-formation rates gauged by stellar end-products" = http://xxx.lanl.gov/abs/astro-ph/0610321 .

Most stars in our galaxy, however, seem to have been created some 7E9 to 9E9 years ago in one or more star bursts and the formation rate today seems to be about 1/4 of what it was then, see e.g.
Beacom: "The Cosmic Stellar Birth and Death Rates" = http://xxx.lanl.gov/abs/astro-ph/0602101
Hardin & al.: "Type Ia supernova rate at z ~ 0.1" = http://xxx.lanl.gov/abs/astro-ph/0006424
Blanc & al.: "Type Ia supernova rate at a redshift of ~ 0.1" = http://xxx.lanl.gov/abs/astro-ph/0405211

One has to give serious thougths to Fermi's paradox: "Where are they?"

If ETs have been around for a really long time - and still exist - why haven't we met them, why can't we hear them?

There may be simple answers to Fermi's paradox.

1. The very bad news: ETs know everyting about us, and don't like what they see.

2. The bad news: We are home alone.

3. The good news: ETs are around, they have noticed us, but presently prefer to just study us from afar.

4. The very good news: ETs have noticed us and they are doing their best to establish contact - only we are too darn primitive to understand their means of communication.

+ other explanations.

Like I wrote in a previous posting: Let's hope that somewhere a 3-armed and 3-legged friend is working on his emoh@ites project ::-))
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Split personality - who, us?

The question to ask is, why they would broadcast at all? Even if they were sure we were here?

Well, the way I've always answered that question is:

Would we broadcast?
Since we've already sent out several symbolic messages into interstellar space, not to mention the Voyager probes, without even knowing if anyone is even out there to receive them, it seems to me that they might share the same innate desire to communicate.

The other part of that is, they probably have an innate thirst for knowledge. I assume that any species which develops radio technology in the first place would have to be a species with an unquenchable thirst for knowledge, like we have. Such a desire to know would seem to be the driving force behind technological progress in the first place.

If they detect life on Earth, I'd guess that they'd want to *know* more about it, just like we would.....


But then, John brings up a good point; we have the Fermi Paradox staring us right in the face.
Earth's atmosphere has had a detectable biosignature for at least 1-2 billion years which would presumably be visible to any civilization in the galaxy which had an unobstructed view of our star.....

I like John's explanations. :)