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BigJim
2003-Jul-18, 09:40 PM
What are your values for it? If you don't know of or remember the Drake Equation, here it is. The Drake Equation:

N = R * Fp * Ne * Fl * Fi * Fc * L

N is the number of intelligent civilizations in our galaxy that we can communicate with, R is the number of sun-like stars formed per year, Fp is the fraction of sun-like stars with planets, Ne is the average number of Earth-like planets in a typical planetary system, Fl is the fraction of such planets thtat develop life, Fi is the fraction that develops intelligent life, Fc is the fraction of intelligent civilizations that develop communications, and L is the average lifetime of an intelligent civilization.

I will post my values for the Drake Equation:

R = 4 stars per year. I think this number is reasonably accurate, and I have seen similar numbers in many books. This is the only number we can really estimate at present.

Fp = .5. I think this value is conservative - only 50% of sun-like stars developing planets. In actuality the number is probably higher, but I think .5 is a reasonable vaule.

Ne = .1 I think that even if sun-like stars develop planets, many will not be habitable, for a variety of reasons, including giant planets that may eject them from the systems, or being slightly too large or small. So I think .1 is a reasonably conservative value.

Fl = 1. Scientists think that life probably arises when it can, so 1 is probably a good value for Fl. Of course, there are those who argue that life is the result of an incredibly fortuitous series of events, and the answer is still unclear. But I think that 1 is good.

Fi = .3. In keeping with conservative values, .3 seems like a decent value for the fraction of planets with life that develop intelligence. On the one hand, many worlds like Mars and Europa may only develop microbial or aquatic life. On the other hand, some say that dolphins could perhaps be considered a form of intelligence. So I think .3 is a good compromise.

Fc = .3. Again, a conservative value. Dolphin-like intelligent creatures probably would not develop communication capabilities, and some civlizations simply might not develop radio communications or any communicational capability. But I think many would, so .3 would be a good average.

L = 50,000 years. This is obviously the most speculative. While I think that truly successful civilizations should last much longer than this, there are the issues of how long they communicate using radio wavelengths of light, and there probably are shorter-lived civilizations and longer ones, too, so all things considered, 50,000 should be a good value for this parameter.

So, inputting my values for the Drake Equation, you get:

N = 4 * .5 * .1 * 1 * .3 * .3 * 50,000

N = 900 intelligent civilizations in the galaxy with which we can communicate.

So, what are your values, and why?

skyglow1
2003-Jul-18, 09:56 PM
what are the chances that their radio waves will actually reach us?

skyglow1

mike alexander
2003-Jul-18, 11:40 PM
I have no idea.

I stopped this exercise a long time ago when I found myself saying things along the lines of: "I think that... it seems reasonable.... probably about..." and realized these are totally without foundation.

Much more interesting is that we are on the edge of getting a statistical sampling of actual planets in the local neighborhood. Then it gets interesting.

BigJim
2003-Jul-19, 12:19 AM
True- I think that within 15 to 20 years we should have a reasonably accurate number for Fp and maybe even a good estimate for Ne. And if we send a manned mission to Mars which finds current or past life, they we can also get a good idea of Fl.....

pmcolt
2003-Jul-19, 12:29 AM
Just for laughs:

N = R * Fp * Ne * Fl * Fi * Fc * L

R = 5 stars per year. I like 5; it's a nice round number.

Fp = 0.5. I'll say that half of all stars have planets.

Ne = 3. Taking our solar system to be average, I'm willing to call Venus, Earth, and Mars "earthlike" in the sense that they could've developed similarly to Earth.

Fl = 0.75. I think that jiggly organic molecules eventually turn into wiggly little critters more often than not.

Fi = 0.3. Of the three "earthlike" planets in our solar system, one developed intelligent life. I think.

Fc = 0.2. We have to develop an intelligent species which is capable of tool use, and which has some sense of progress and advancement, and which goes on to use familiar radio technology for communication purposes. One-fifth sounds like a good number, but I don't want to say what I pulled it out of.

L = 2000 years. Human civilization has lasted many times longer than that, but we've only had radio for a bit over 100 years, and we've only just recently had the capability of destroying ourselves.

So N = 5*0.5*3*0.75*0.3*0.2*2000 = 675 civilizations in the galaxy capable of communicating. Given the size of the galaxy, that means we have a whole lot of empty space out there.

snowcelt
2003-Jul-19, 08:34 AM
I forget how I figured out the various numbers; but, 1 seemed to be the number. Therefore, there are 150,000,000,000 tech civs in the universe.

eburacum45
2003-Jul-19, 11:39 AM
Working from the other direction, (the number of sunlike stars in the galaxy) I came to the same approximate estimate as pmcolt; but I tend to think intelligence develops much less frequently, but lasts longer as a civilisation; we are at the start of our civilisation, not the end (Brandon Carter's mathematical argument not withstanding).

In fact long-lived civilisations may sometimes assist other civilisations to last longer than they would otherwise;
alternatively these long-lived, widespread civilisations might cause widespread loss of intelligent life when and if they collapse.

I am also of the opinion that most intelligent life will be very different to our own, and in a majority of instances will not attempt to contact or travel to other solar systems... but this is all entirely speculation, of of course.
More unsubstantiated speculation here (http://www.orionsarm.com/xenos/SearchForNonTerragenIntelligence.html)

ToSeek
2003-Jul-19, 01:34 PM
I go along with the Rare Earth folks and have Fi as a very, very small number, so that we're probably the only intelligent civilization in the galaxy.

kilopi
2003-Jul-19, 04:30 PM
N is the number of intelligent civilizations in our galaxy that we can communicate with, R is the number of sun-like stars formed per year, Fp is the fraction of sun-like stars with planets, Ne is the average number of Earth-like planets in a typical planetary system, Fl is the fraction of such planets thtat develop life, Fi is the fraction that develops intelligent life, Fc is the fraction of intelligent civilizations that develop communications, and L is the average lifetime of an intelligent civilization.

I'm not sure if that is the standard definition of the Drake equation, but I'd like to point out that in order for the "units" to work out right, L has to be the average length of time that an intelligent civilization transmits communications. You used a value of 50,000 years for that, whereas we've only done it for about a hundred years. If our civilization ends as soon as Thomas Gold says, L will be 500 years or less. Anyway, it's quite probable that our communication method will change--which also has to be a limiting factor on L. So, 500 years seems like a more reasonable value for L.

Your assumption that, given an "Earth-like" environment (whatever that means), that life is absolutely sure to develop (probability of 1) is very optimistic. Reducing it to say 10% might be wise, if not still too optimistic.


So, inputting my values for the Drake Equation, you get:

N = 4 * .5 * .1 * 1 * .3 * .3 * 50,000

N = 900 intelligent civilizations in the galaxy with which we can communicate.

So, what are your values, and why?
Factoring my changes in, that would divide your value by 1000.

So, using those values, we can expect to find about 0.9 intelligent civilizations in our galaxy. That may be about right, doncha think? Maybe a tad high. :)

dgruss23
2003-Jul-19, 06:50 PM
I go along with the Rare Earth folks and have Fi as a very, very small number, so that we're probably the only intelligent civilization in the galaxy.

I with you on this ToSeek! JS Princeton and I battled it out on the Venus and Creationism thread (if I remember right). We couldn't reach any common ground because he felt that we could not evaluate the possibilities from what we've learned about our own planet since a sample size of "1" is a poor sample. So we couldn't agree on any particular factors as "restrictions".

We still have a lot to learn on the matter, but in my opinion the three biggest restrictions to large numbers of civilizations are (1) climatic stability. By that I mean on Earth for example that solid, liquid and gaseous water (could be other substances on other worlds) have been present on the surface throughout most or all of the Earth's history since the heavy bombardment. In that sense Ice ages are not considered a sign of an unstable climate. A lot of factors such as our large Moon may play important roles in climate stability.

(2) The contingent nature of the evolutionary process. There doesn't seem any guarantee that we would appear built into the evolutionary process and it seems our ancestors got "lucky" at times.

(3) The Age and size of the galaxy. Even if a given number civilizations have appeared, unless that number is much higher than Rare Earth arguments suggest is likely, then the odds that any other civilizations are within communication range at this instant is exceedingly small. We could be "alone" without being the only civilization in the galaxy.

BigJim
2003-Jul-19, 08:17 PM
What exactly is this book's argument for why Fi should be so low?


(1) climatic stability. By that I mean on Earth for example that solid, liquid and gaseous water (could be other substances on other worlds) have been present on the surface throughout most or all of the Earth's history since the heavy bombardment. In that sense Ice ages are not considered a sign of an unstable climate. A lot of factors such as our large Moon may play important roles in climate stability.

And I think that .3 is a reasonable estimate for this reason. Take the places in the solar system where life has or may have arisen - Earth, Mars, Europa, and Titan are the most likely places. Now, say maybe only Mars and Europa developed life. That makes one-third of the environments in which life develops result in intelligent life. If there are more places where microbial life may have arisen, yes, that would bring Fi down, but it would also bring Fl and Ne up. So all things considered, I don't see why an estimate of Fi should be low.


Anyway, it's quite probable that our communication method will change--which also has to be a limiting factor on L. So, 500 years seems like a more reasonable value for L.

But we could still attempt to communicate using radio. And if we start doing optical SETI or any other wavelengths, then L would have a resultant increase. I think that most intelligent civilizations would last far longer than 500 years or even 50,000 years, and would probably be able to make their presence known to us in ways other than radio, such as us seeing exhaust from anti-matter spacecraft engines (it's possible), Dyson spheres, or detecting any signals at any wavelengths.


L has to be the average length of time that an intelligent civilization transmits communications. You used a value of 50,000 years for that, whereas we've only done it for about a hundred years

I understand that. See above.


(2) The contingent nature of the evolutionary process. There doesn't seem any guarantee that we would appear built into the evolutionary process and it seems our ancestors got "lucky" at times.

But on Earth-like planets, wouldn't evolution favor the development of intelligence?

What would we consider hive-mind civilizations? Are they really civilizations, or more like one giant organism?

dgruss23
2003-Jul-20, 01:44 AM
BigJim wrote: And I think that .3 is a reasonable estimate for this reason. Take the places in the solar system where life has or may have arisen - Earth, Mars, Europa, and Titan are the most likely places. Now, say maybe only Mars and Europa developed life. That makes one-third of the environments in which life develops result in intelligent life. If there are more places where microbial life may have arisen, yes, that would bring Fi down, but it would also bring Fl and Ne up. So all things considered, I don't see why an estimate of Fi should be low.

Stephen Jay Gould made a nice point in his book Wonderful Life. After the Cambrian Explosion, all the phylum that exist today were present but there were numerous additional phylum - or body plans. The only chordate was a creature called Pikaia which happened to survive an extinction event that eliminated about 1/2 of the previously existing phylum. No new phylum have appeared since that extinction. So if the chordate phylum had been wiped out in that extinction event there might very well be no vertebrates on this planet. That doesn't mean that intelligence couldn't have ever evolved, but it does illustrate one of the many instances in which our lineage could have been completely wiped from existence.

Perhaps I've been too influenced from reading Gould's work, but if you look at the geological history of the planet, it is mighty remarkable that we are here debating this. Despite the last 540 million year history of abundant multicellular life, only recently have large brained (relative to body mass) intelligent species appeared - Whales/Dolphins providing the only other real examples besides the homonids.

In my view the climatic stability is the real limitation to abundant complex ecosystems - but if you stretch your requirements beyond complex ecosystems such as what we have on Earth to intelligence, then IMO you reduce the numbers out there by at least two orders of magnitude. But even if fi was 0.3, in my assessment that still would leave very few civilizations.


But on Earth-like planets, wouldn't evolution favor the development of intelligence?

Its hard to know - we pay a price for our large brains. They consume a tremendous amount of our bodies resources. In an evolutionary sense, there probably has to be a real benefit to be gained by such an adaptation before natural selection will push a species in that direction. When you consider the long time span within which intelligence could've developed against the fact that it actually did in us very recently, its not clear that we were inevitable.


What would we consider hive-mind civilizations? Are they really civilizations, or more like one giant organism?

If they did exist and we're intelligent, does that change the odds dramatically?

I think a lot of questions will be answered when we've been able to determine whether or not life exists or once existed elsewhere in our solar system.

eburacum45
2003-Jul-20, 05:47 AM
The Drake Equation cannot be considered without the Fermi Paradox- there is very little that can stop a civilisation that decides to replicate itself across a galaxy, so the first one that decides to do so should be here on Earth already.
So this tends to suggest that one or more of the following is correct:
the Rare Earth Hypothesis,
it is very difficult to travel from one star to another,
it is very difficult to replicate in a new solar system once you get there,
most civilisations that develop are not interested in colonisation or interstellar communications.

There are probably others, but these alone will explain the empty skies.

It is perfectly possible that humanity will become introverted as it develops further; we may decide to exploit the energy of the sun, and the planets and asteroids for living space; but electronic virtual media might become so entertaining that we never bother with the long, uncomfortable trip to the stars.
This is such a sensible strategy (at least while the sun is still Main Sequence) that it seems likely that many civilisations could become introverted in the same way.

ocasey3
2003-Jul-20, 06:16 AM
I think that humans will never tire of adventure and exploration.

kilopi
2003-Jul-20, 01:23 PM
I think that humans will never tire of adventure and exploration.
Some already have. In fact, I'm pretty sure that the tendency for adventure and exploration is in the minority. The questions are, will the tendency die out completely, will it be frustrated by the expense, will the technology to support it ever be produced?

Emspak
2003-Jul-23, 02:57 PM
to kilopi et al. :

On your point about exploration as a tendency being in the minority, in an evolutionary sense it is better if that is the case-- yuo want some percentage of a species that goes out and looks for new food sources, et cetera, and a larger percentage that stays at home. This is a hedge -- becuase you don't want to risk all your genetic marbles on a bet that may not pan out. This is one reason why biologists noted that human colonization has followed a familiar pattern -- a few people tire of living around their relatives, run out of resources, or whatever. They go off and colonize while the majority stay at home. Even in the modern period, the majority of people from whatever country did not migrate, usually. That is why countries depopulated by emigration (like Ireland) are so unusual.

dgruss23
2003-Jul-23, 03:28 PM
That's a great point Emspak. I wonder where we would be if there was no longer any exploration. Robert Zubrin makes a nice point in The Case for Mars that humans do have a drive to explore and expand their horizons. If we stop exploring in some form do we stagnate?

When people landed on the Moon the world was riveted. So while most of the population will not do the exploring, a large percentage of the population will be interested in and inspired by those that are doing the exploring.

But "exploring" can mean a lot of different things. The poet may consider what he or she writes "exploring" the use of language. The scientist considers research the exploration of what is unknown.

It seems natural that at some point people will want to in person explore the solar system and beyond.

A.DIM
2003-Jul-23, 06:18 PM
Two very relevant articles:

Regarding the Fermi paradox (http://www.newscientist.com/hottopics/astrobiology/alienshiding.jsp).

And this on Intelligent life (http://www.newscientist.com/hottopics/astrobiology/weweremeant.jsp).

pmcolt
2003-Jul-23, 07:42 PM
I'm not quite sure what to make of the first article. At first, it would suggest that T is very small, but the signals aren't really undetectable, just untraceable.

T could very well be small. We only think to monitor radio and optical wavelengths in SETI programs, but it's entirely possible that many civilizations use tight-beam or cable systems with no leakage for us to detect. Or as the article suggests, they could use a type of communication which we currently don't understand.

If T (argh, it's called 'L' in this thread) is only on the order of a century, then using my numbers for an example, N would be in the low double digits.

On the other hand, the second article suggests that fi might be large... the moral of the story: what do we know, really?

snowcelt
2003-Jul-24, 11:48 AM
eburacum45. Why would you think that the Leslie/Carter argument is weightless? In your calculation you place weight upon the duration of a civilization. This is a phenomenon which is, charitably, 5000 years old on this planet: with EM ability during only the last couple of centuries. If your estimate is like pmcolt's, but with a lesser variable placed upon sun-like stars, high civilization would have to be even longer lasting. Did not the Leslie/Carter Argument not, in your mind, do nothing but shorten the life span of a intelligent species? I find the L/C argument compelling. I have played with the argument for years and have never seen a way that it can be cavalierily dismissed. I would love to hear a rebuttal. :D

eburacum45
2003-Jul-24, 01:54 PM
Brandon Carter and John Leslie's argument can only apply to a continous population of humans; it is apparent to me that the population of the Earth will soon cease to be simply human, so will be not affected by the numbers game that these mathematicians are attempting to play.

Additionally if we consider a set of alien civilisations with a wide range of ages, each one of those civilisations will have passed through a stage when the Carter/Leslie argument seems to apply, just as this argument appears to apply to us now;
whenever a civilisation becomes mathematically competent enough to formulate an argument like this, they are also likely to be in approximately the same relationship to their own population growth curve as we are.
The Carter /Leslie argument is an artifact of our present developmental stage; it was not valid three hundred years ago, it will not be valid three hundred years from now. It appears only true now because we do not know if it is true; if we knew it was not true (by surviving the next three hundred years) we would not need to ask.

We are a sample of one, as so many people like to point out; it will take only one five million year old alien civilisation to disprove Carter, and if we find millions of of them among the 10^22 stars of the observable universe we will be able to better determine the true value of T.

waynek
2003-Jul-25, 12:30 AM
I haven't had a chance to read the entire thread, but to follow the format of the original post I'd like to give my values for the Drake equation. I am copying this from an exam I took 12/5/1995 where the same question was asked:

4a. N = R* Fg Fp Ne Fl Fi Fc L

R* - 10 This is fairly well understood as an average from astrophysics.

Fg - 0.1 The star must be stable enough for life, and have a long enough lifetime for evolution to go.

Fp - 0.5 From observation of accretion disk and infrared excess of stars (remember, this was 1995).

Ne - 2 Assuming 10 planets in a system, 1/5 would be suitable. Since we use our system as a model, and we don't have any evidence that life ever existed on Mars or Venus, I would say 1, but giving the others some credit, I'll count them each as 1/2.

Fl - 0.01 Just because life happened quikly on the Earth, that doesn't mean it's easy, just that everything was just perfect, ie (Jovian planets to clean out system, good rotation, etc.)

Fi - 0.01 Need just the right ballance of catastrophy and stability (false starts should be common, so intelligence relitively rare).

Fc - 0.1 Assume 1/2 are land based, 1/2 of those are interested, and 2/5 of those have a compatible communication system.

L - 10^5 - If they last more than 10^6 years, we would be seeing them cruising around, and they would likely only try talking to us 1/10 of the time.

N = 10*0.1*0.5*2*0.01*0.01*0.1*10^5 = 1 (believe it or not, I didn't plan this)


I guess we're it, so don't bother running "SETI @ Home" anymore. And the most amazing part, I was able to put my hands on this test I took nearly 8 years ago. :o I pretty much stand by it, along with the observation that Fl is probably the least well understood, although many are just pure guesswork.

Pinemarten
2003-Jul-25, 02:23 AM
what are the chances that their radio waves will actually reach us?

skyglow1

This has me thinking.

Our SETI receivers may be set to the wrong freq. Wouldn't an advanced race attempt to communicate to lesser developed peoples on a freq that we would be able to interpret?

This would have many purposes. Accelerate our technology on the chance that they may learn from us. Prevent us from developing in the 'wrong' direction; and possibly stopping a threat to them. Influencing our beliefs to be 'one of the federation'. The list can go on.

When advanced civilizations first started expanding their influence on Earth, we were benevolent and maelevolent; but either way we did not believe in a 'prime directive'. We exploited for 'good' or for what we 'thought was good'. I can't see this being different on a larger scale.

I don't believe we, or any race, will ever achieve FTL. Intersteller communication may be the only 'contact' and that's it. Therefore, no species can ever be a threat to another one. I assume more advanced ones than us have proven this, and are expending efforts on communication only.
They would probably communicate in a way that any reasonably advanced planet could understand, and we may not be quite 'tuned in' yet.

beskeptical
2003-Jul-25, 07:57 AM
Your assumption that, given an "Earth-like" environment (whatever that means), that life is absolutely sure to develop (probability of 1) is very optimistic. Reducing it to say 10% might be wise, if not still too optimistic.

..........So, using those values, we can expect to find about 0.9 intelligent civilizations in our galaxy. That may be about right, doncha think? Maybe a tad high. :)

Less than one? Intelligence may not be abundant on the Earth, but I don't think you can say there is none here.

If you have 4 billion years or more and an Earthlike planet, our data though limited, does put the odds of life developing at 100%.

We know of one Earthlike planet, Earth. And, on all the Earthlike planets that we know of, life developed. Granted our sample size imposes limitations on the conclusions we can draw from the data. But the data for now indicates 1 was the correct number to put in the equation.

kilopi
2003-Jul-25, 08:56 AM
Less than one? Intelligence may not be abundant on the Earth, but I don't think you can say there is none here.

If you have 4 billion years or more and an Earthlike planet, our data though limited, does put the odds of life developing at 100%.
I'm reserving judgement, as usual. Intelligence is a combination of a lot of factors, it includes such non-obvious things as compassion and a sense of humor. It's not just a Turing test, in my opinion.

Iain Lambert
2003-Jul-25, 10:29 AM
Of course, this is the Anthropic principle. In a sample size of 1, you're either going to see that all of your planets have developed life, or you're not in any position to comment either way. :D

dgruss23
2003-Jul-25, 01:12 PM
One question that is interesting is what the size of our sample is. Certainly Earth is the only life bearing planet that we know has life. But does that mean that our sample size for understanding the requirements for life is 1?

I don't think so because scientists have been identifying the characteristics of the Earth that make it habitable - not just for "our type of life" but really for any complex ecosystem. We can compare those characteristics against other planets. For example, some have proposed that part of maintaining an atmosphere thick enough to keep a stable climate is the process of plate tectonics and volcanism. But a smaller planet will cool more quickly, its volcanism will slow down, and the planet will no longer be able to maintain a thick (relatively speaking) atmosphere (Mars).

So I do not agree that we only have one data point. We can look at the other objects in the solar system and determine whether or not their situation is consistent with our emerging picture of the characteristics needed to maintain the Earth's habitability.

Chuck
2003-Jul-25, 03:24 PM
The Drake Equation leaves out stuff we haven't thought of nor discovered yet. We should throw in another ten terms of 1% each to account for them.

kilopi
2003-Jul-25, 04:22 PM
But we might have to throw in another dozen factors of x10, just to correct the other errors. :)

ToSeek
2003-Jul-25, 04:32 PM
The Drake Equation leaves out stuff we haven't thought of nor discovered yet. We should throw in another ten terms of 1% each to account for them.

I disagree: you could break down the given factors further, but as they stand I would defend them as exhaustive.

kilopi
2003-Jul-25, 05:10 PM
I disagree: you could break down the given factors further, but as they stand I would defend them as exhaustive.
I would too. Dimensional analysis--that's how I figured out that Big Jim's original definitions were off (http://www.badastronomy.com/phpBB/viewtopic.php?p=117322#117322). You multiply them all together, the various subcategories have to cancel.

Grey
2003-Jul-25, 05:13 PM
Intelligence is a combination of a lot of factors, it includes such non-obvious things as compassion and a sense of humor. It's not just a Turing test, in my opinion.
This is probably off topic, but I'd say that passing a Turing test is indeed a sign of intelligence. I just think that lacking traits like compassion and a sense of humor would be a dead giveaway in such a test. :D

kilopi
2003-Jul-25, 08:54 PM
That's easy for you to say--I flunked mine.

beskeptical
2003-Jul-26, 09:38 AM
One question that is interesting is what the size of our sample is. Certainly Earth is the only life bearing planet that we know has life. But does that mean that our sample size for understanding the requirements for life is 1?

I don't think so because scientists have been identifying the characteristics of the Earth that make it habitable - not just for "our type of life" but really for any complex ecosystem. We can compare those characteristics against other planets. For example, some have proposed that part of maintaining an atmosphere thick enough to keep a stable climate is the process of plate tectonics and volcanism. But a smaller planet will cool more quickly, its volcanism will slow down, and the planet will no longer be able to maintain a thick (relatively speaking) atmosphere (Mars).

So I do not agree that we only have one data point. We can look at the other objects in the solar system and determine whether or not their situation is consistent with our emerging picture of the characteristics needed to maintain the Earth's habitability.

But I thought the equation called for the number of 'Earthlike' planets in the galaxy, not the number of planets. The question then becomes would life develop on all Earthlike planets? In that case we have one Earthlike planet in our sample and it has life. We have no Earthlike planets without life. 100% of our sample of Earthlike planets have life.

As to your other thoughts, I don't think we can rule out life on all the other planets and moons in our solar system until we send more probes out. I do think we can rule out life with advanced technologies on other bodies in the solar system.

dgruss23
2003-Jul-26, 04:08 PM
beskeptical wrote: But I thought the equation called for the number of 'Earthlike' planets in the galaxy, not the number of planets.

The greater the number of planets, the greater the chances that there are other Earthlike planets out there. It also depends upon how far you want to carry the calculation. Are fi and ft important? It depends upon whether you want to know how frequently complex ecosystems have developed or whether you want to know how frequently civilizations we can communicate developed.

It also depends upon what a person means by "Earthlike". I'm most interested in the number of planets that do support complex ecosystems (and I'm usually thinking terrestrial because that would be our best chances for communication) and here I've always been defining complex ecosystems as those with multicellular producers, consumers, decomposers and so on.

You could argue Venus is "Earthlike" in its mass and radius. However, other conditions have prevented Venus from supporting a complex ecosystem. You could point to the terrestrial nature, axial tilt ,and rotation period of Mars and say that is "Earthlike", but its smaller radius and loss of vigorous volcanic activity seems to have prevented it from maintaining a thick enough atmosphere to keep its surface warm enough for complex ecosystems.

JS Princeton insisted that we only have the Earth as a data point. I disagree. We've learned what features are necessary components of the Earth's terrestrial ecosystems. Why don't all the other terrestrial planets and moons have terrestrial ecosystems too? Why doesn't Mars have "Earthlike" conditions? If terrestrial ecosystems were easy to produce and could manage to exist in a wide range of planetary conditions, then surely with the Earth having such ecosystems, other bodies in our solar system ought to as well. But they don't and the fact that they don't is directly tied to the fact that they lack certain geophysical and solar system conditions that the Earth possesses. That makes the other planets data points. And it also means that there must be some restrictions on the development of complex ecosystems.

Lets say for example that someone said (no one has to my knowledge, but for making a point) that a planet must have a radius >5000 km to have complex terrestrial ecosystems. If Mars had complex terrestrial ecosystems, it would disprove that immediately. However, since Mars does not have complex terrestrial ecosystems or the surface conditions needed to support them it acts as a data point. It confirms that in the absence of tidal forces to keep the planet's interior heated, that a small enough planet that starts with liquid water will cool off too quickly, lose vigorous volcanic activity and freeze. It does not "prove" that a planet must be >5000 km to have complex ecosystems, but what it does confirm is that the geophysical conditions needed to maintain a thick enough atmosphere are unlikely to exist on a planet as small as Mars. The Moon and Mercury illustrate the same point.

Notice that we don't have to go into the adaptibility and creativity of life and evolution to make this point. Its strictly geophysical. So arguments such as "Well maybe under different conditions life could adapt ..." really don't come into play. The point is that certain geophysical conditions are needed to allow for a planet to be habitable so that will be able to exist and play its adaptation game. And the other planets act as data points which support the argument.


The question then becomes would life develop on all Earthlike planets? In that case we have one Earthlike planet in our sample and it has life. We have no Earthlike planets without life. 100% of our sample of Earthlike planets have life.

You're right ... so in the context of what I've said above my point would be that the other planets act as data points on the geophysical end of it not on the life end of it.


As to your other thoughts, I don't think we can rule out life on all the other planets and moons in our solar system until we send more probes out. I do think we can rule out life with advanced technologies on other bodies in the solar system.

I agree again! Earth is clearly the only planet with complex terrestrial ecosystems. But certainly Europa and some other places might be possible sites for other types of ecosystems of varying complexity.

Betenoire
2003-Aug-19, 07:18 PM
I know precisely nothing about the frequency of formation of sunlike stars, so I'm sticking with R = 4.

As the sun is not just any kind of star but a particular type of star formed from stellar nebulae, I'd expect they form a majority of the time with planets due to the disks of debris to be found around them. Fp for me is .7

I'm in the camp that life may well exist on other planets and moons in the system. Life is persistent. Therefore Ne is a pointless number. Life might develop on non-Earth like planets. It is a consideration, instead, under Fl.

Europa seems a good bet, as it does have massively deep oceans with evident signs of flow within them (The ice surface is cracked and mounded, like tectonic plates, though I'm merely suggesting that this is the result of the subsurface water immitating magma, not of any actual magma action). Undisturbed water likely won't spontaneously develop life, as there is a necessity for a large number of chemicals to interact with each other.
Mars is a slimmer possibility in my view, depending on the nature and duration of its past water flow and atmosphere.
Venus even slimmer, and certainly not anymore (Its surface temp is many times boiling). Other planets more or less useless.
We have little knowledge of the makeup of other solar systems beyond "Some Jupiter-esque planets are out there." Smaller Earth-like planets and other Fl contributors are not known to exist or not to exist in these systems, so this is all SWAGs. But there ARE gas giants, and they could have Europa's of their own. Out of four gas giants in our system with moons, we have at least one life-potential moon. And as I say, we have no information on smaller planets or moon counts in those systems, so I'd say one in four of those also has that. My Fl guess is therefore .25.

For the life developing into intelligent life factor, I think that, if you give any sort of life a fighting chance (half a billion years without annihilating it), it will make the conditions for it to develop more and survive any other disruptions. Eventually (maybe in much longer than the nearly four billion years it took us to show up) something will come about. So I say .25 for Fi, also, as Europa and Mars and others demonstrate that life probably won't get that fighting chance.

Fc, in my view, is much more like .9. Firstly, our definition of "intelligence" is very arbitrary, more or less "Something like us." That can possibly lower Fi in this consideration, or it means that any intelligent life is going to develop the technology for communication given a good fifty thousand years, really not asking too much from a planet given the scale of geologic time. Dolphins, were we not here, would climb onto the shores and start slashing and burning the forests for us. Chimps and gorillas are also potential intelligence-generating genuses (genii?).

Doomsdayists may argue that the L factor is narrow, we're going to kill ourselves or do something else very quickly. I don't buy it, humans are worse than cockroaches in that you just can't kill them. L could be indefinite to an extreme optimist, and at least a few thousand years. There are considerations for a change in communication method (quantum entanglements are my favorite) but I expect there will still be some radio receiver that will pick up transmissions from another civilization sent out centuries before, if only because we'll still not have funded our astronomers enough to have a radio map of the stars. So my suggestion for L is that the first number, 50,000, is decent, and maybe even too lean.

My equation works out to N = 4 * .7 * .25 * .25 * .9 * 50,000
N = 7,875
That's what I get for growing up watching Star Trek.

I think the Drake equation is rather narrow in its scope, as already dealt with in the Ne factor. There is an implicit assumption that all civilizations take the same amount of time to develop and that stellar neighborhoods play no role (The galactic center I would expect is sterile, out here at the fringes though we get all the right conditions more often than not) and probably a dozen other factors I'm not considering. I'm a molecular biologist, not a physicist, unfortunately, so I can't really hold up much of a debate on this. Thanks for reading this far, though.

Emspak
2003-Aug-20, 02:41 AM
Well, your point about the definition of intelligence being narrow is well taken, but you need not even assume terrestrial type chemistry-- that is, you need not assume that the chemistry of life has to be water-sugar-oxygen. You could go with (I dunno, you're the molecular biologist) something like ammonia-nitrogen-ethanes or some such.

The key issue is stability and detectability over interstellar distances. To be seen over any distance, you have to have radio or do some pretty big engineering. Thus far, we have seen no large-scale alteration of the galaxy or stars, at least not out to where we can detect anything.

No sign on the radio front either, so we can say pretty confidently that there are no civilizations like ours -- that is, that use radio on anything like the scale we do -- within a certain radius. Even without making any assumptions at all about the frequency your aliens transmit on, given that radio maps at a pretty wide range of frequencies exist for the whole sky, I'd say there's nobody within 1,000 light years or so at least. Or at least, nobody transmitting on frequencies that we typically see and use.

Now, obviously there are all sorts of frequencies we haven't checked. But, assuming there is someone out there with a planet that is even remotely like ours, if they use radio sooner or later they will hit on something that punches through the atmosphere, no matter what it is made of. Every gas has a window, after all. (If they are water-breathers it is tougher).

So, why no signs?

Figure this: you need heavy elements, so that is a constraint on the generations of stars that are at all likely to have planets. You need stability, so that means planets that persist with environmental conditions that don't go too far out of whack too often. That means you aren't going to find anyone further back than, say big bang + 5 billion years or so.

Knowing that, you can say that you won't get stable orbits in the center of the galaxy because stars are too darn close together. .Won't get heavy elements way out becauser there aren't enough of them to create heavy enough bodies to form planets of any stripe besides the odd gas giant.

That narrows the radius in the galaxy (from the center) to something between about 20,000 light years out or thereabouts to maybe 40,000. We're in the middle of that.

So, we can pretty confidently say that in terms of what we know about stability-- and remember-- it doesn't depend on what kind of life it is -- that if there is anyone out there transmitting, they haven't started in the last 70,000 years, give or take. That is, if they started a radio-capable civilization transmitting at any frequency likely to be detected over that distance, they must have done so less than 70,000 years ago 'cause the signal isn't here yet.

If it was much more than that, you see, and they transmitted, we'd have heard them.

Did we miss them? Well, if they transmitted for 100 years but started 100,000 years ago and blew themselves up, got whacked by an asteroid, or all decided to take up crochet and communicate with smoke signals, we'd have missed out if they stopped before we got a chance to hear.

But every civilization within that radius all discovering and abandoning radio within a realtively short time, just so we missed it, seems a little farfetched. So is the converse-- we all started so close together in time that our signals haven't reached each other yet because we are too far spread out.

Welcome to the Fermi paradox, folks.

Any number of contemporaneus civilizations that hits the 1,000 mark runs into this problem. Given an average separation -- a back-of-the-envelope shows it to be on the order of one every 300-400 light years -- that means something is up -- we'd have noticed them by now. Remember, the Drake equation assumes the number of civilizations at any given moment.

Since SETI searches haven't thoroughly gone over that sphere, but when a really complete SET survey is done that does cover that radius, I'll say Drake must be <1000, at least for technical civilizations.

beskeptical
2003-Aug-20, 06:01 AM
No sign on the radio front either, so we can say pretty confidently that there are no civilizations like ours -- that is, that use radio on anything like the scale we do -- within a certain radius. ...., I'd say there's nobody within 1,000 light years or so at least. Or at least, nobody transmitting on frequencies that we typically see and use.

.....That means you aren't going to find anyone further back than, say big bang + 5 billion years or so.

Knowing that, you can say that you won't get stable orbits in the center of the galaxy because stars are too darn close together. .Won't get heavy elements way out becauser there aren't enough of them to create heavy enough bodies to form planets of any stripe besides the odd gas giant.

That narrows the radius in the galaxy (from the center) to something between about 20,000 light years out or thereabouts to maybe 40,000. We're in the middle of that.

Did we miss them? Well, if they transmitted for 100 years but started 100,000 years ago and blew themselves up, got whacked by an asteroid, or all decided to take up crochet and communicate with smoke signals, we'd have missed out if they stopped before we got a chance to hear.

But every civilization within that radius all discovering and abandoning radio within a realtively short time, just so we missed it, seems a little farfetched. So is the converse-- we all started so close together in time that our signals haven't reached each other yet because we are too far spread out.

Since SETI searches haven't thoroughly gone over that sphere, but when a really complete SET survey is done that does cover that radius, I'll say Drake must be <1000, at least for technical civilizations.

Some good points but <1000? There are an awful lot of galaxies out there that are more than a few thousand light years away. How weak are our generated signals? How weak are they over distance given the inverse square law? So at what distance would they be too weak for our instruments to detect? We can pick up the BB remnants but that was BIG. Aren't our signals relatively tiny?

Emspak
2003-Aug-20, 11:46 AM
Most signals we transmit are weak, once you get past a certain distance. But in certain frequencies they are quite strong. We transmit at an awfully wide range-- everything from 60Hz power transmission to cell phones and satellite communications. So one or more of those frequencies will punch through the atmosphere and someone noted in another thread that while you wouldn't be watching "I Love Lucy" you'd know something was up, because the signal modulation would be intact. You could even tell it wasn't a neutron star because the frequency and amplitude curves would be wrong.

So basically, aliens know we're here if they are anything like 50 or 60 light years out.

We could probably pick up the equivalent from about the same distance, maybe further. I'd have to sit down and do the math. But my guess is that we could see if something was going on a pretty fair distance out, say 100 light years or more.

THat's the point I was making -- given the average expected separation and given what we're likely to detect, it doesn't look good.

Now, someone coud be transmitting from Andromeda. That would be a much tougher call, I am not sure we would notice in the noise. But if that's the case then transmitting back and forth is probably not possible for a long while, since doing so requires an awful lot of juice. But more importantly, we can tell that nobody has done any large-scale galacto-engineering in the last 2 million years or thereabouts. We'd have seen it.

Same goes for other galaxies. Now, maybe nobody builds dyson spheres. Maybe no civilization ever engineers galaxies. But we can imagine doing so, and could you really say that all civilizations, everywhere, decide against it?

Even at speeds like Voyager's, you'd colonize a radius out to say, Tau Ceti, in 100,000 years. 10 million years gets you to the nearest stars, and 100 million starts to get noticeable from outside the galaxy. 1 billion years covers a very sizeable chunk-- and this assumes our technological development stops tomorrow.

So looking out to the limit of the Hubble, it sure as heck does not look like anybody who arose int he last 5 billion years or so decided to a)get off the home planet and b) erect a gian't McDonald's sign or anything like it on galactic scales. And they have had the time. A galaxy 5 billion light years off is showing us its first generation of stars that have the metallicity of the Sun. Further in we'd have seen any projects anybody started since then.

Like I said, it don't look good.

Manchurian Taikonaut
2004-Mar-04, 06:12 PM
http://www.setileague.org/software/unknown2.wav

SETI have had near results before, but nothing real...was 28th of july 2002 an Aliens contact signal?