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?

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what are the chances that their radio waves will actually reach us?

skyglow1

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.

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.....

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.

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.

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

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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.

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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.
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.

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.

What exactly is this book's argument for why Fi should be so low?

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.

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.

I understand that. See above.

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?

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.

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.

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.

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.

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I think that humans will never tire of adventure and exploration.

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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?

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.

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.

Two very relevant articles:

Regarding the Fermi paradox.

And this on Intelligent life.

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?

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.

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.

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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. I pretty much stand by it, along with the observation that Fl is probably the least well understood, although many are just pure guesswork.

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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.

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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.

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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.

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.

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.

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.

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But we might have to throw in another dozen factors of x10, just to correct the other errors.

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

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