Summary: This five-part debate has covered a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth. Today in the final installment the participants examine why we haven’t found complex intelligent life, if indeed it does exist elsewhere in the universe. A thread of this debate also picks up on a comment made by Donald Brownlee in Part 4 -- that interstellar space travel may be impossible.
This five-part debate has covered a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth.
In Part 4, the participants discussed whether all life, here or anywhere, is ultimately doomed by the fact that stars swell and then die. Today in the final installment they examine why we haven’t found complex intelligent life, if indeed it does exist elsewhere in the universe. A thread of this debate also picks up on a comment made by Donald Brownlee in Part 4 -- that interstellar space travel may be impossible. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.
Michael Meyer: If there is intelligent life out there, why haven’t we found them yet?
Chris McKay: This is Fermi's paradox: Where are they? Or phrased differently: why aren't signs of galactic-scale intelligent life obvious in our telescopes? The simplest explanation for this is that we are the only, or at least the first, intelligent species in the galaxy.
Can anyone give a good argument for why our type of civilization would not be obvious over much of the galaxy after a million years?
David Grinspoon: If civilizations like ours were all over the galaxy, it would not be obvious. We are only listening, not broadcasting. We are not doing astroengineering.
True, we are leaking sitcoms and beer commercials, but these are not easily detectable over most of the galaxy and certainly would not be interpreted as signs of true intelligence.
So, in order to have an obvious presence, "our type of civilization" must become something quite different. Perhaps this is very rare or difficult. However, being a constitutional optimist, and considering the unimaginably vast reaches of time and space, I tend to think that sentient, long-lived civilizations should be out there somewhere. So, where are they?
The reasoning behind Chris's (and Fermi's) question implicitly assumes certain things about the behavior of advanced civilizations. It assumes they will keep expanding their populations and increasing the size of their civil engineering projects. Looking at the history of our civilization and extrapolating to our future, I understand why you could draw such a conclusion.
But it may be that truly sentient societies realize there is no future in unlimited expansion. We cannot keep expanding our population at our current rate. Even if we were somehow able to move out into space at the speed of light and colonize all available planets, we would still run out of space and resources and experience mass starvation within a thousand years. True minds will realize that such expansion is a dead end.
Of course, the problem with this kind of explanation for "the great silence" (Fermi's paradox) is that it must apply to every single civilization out there. It is hard to believe that every society that ever forms will transform themselves into sustainably living, granola munching, navel staring, contemplative Buddhists before creating some observable signs of their presence. So, we must search for another answer.
Frank Drake: A parallel question to this is: how long will the Earth’s technology be detectable? A few decades ago we thought the visibility would last a long time -- ever more powerful TV stations and radar installations were being built, and these are the strongest signs of our existence.
But there is only so much bandwidth in the useful electromagnetic spectrum. To transmit ever-increasing amounts of information, portions of the spectrum must be shared. This is only possible if signal strengths are reduced so that transmissions on the same frequency do not interfere with one another. The textbook example of this paradigm is the cellular phone system. This signal reduction means we are well on our way to becoming invisible.
So if the transmission of a rich cornucopia of information is what advanced civilizations do, they may become invisible. This is a rather counter-intuitive result, but a real one. This means that the detectable lifetimes of civilizations may be shorter than we have estimated, and hoped, alas.
David Grinspoon: Another possibility is that they may not want us to know they are there. It’s hard for us to fathom the possible motivations and behaviors of societies millions of years older than ours. It seems reasonable, however, to suppose that the differences between their capabilities and ours will be so great that it will be up to them, not us, how and when some kind of detection or contact is made.
It is possible that they have decided it should be against the law to let us know they are there (The "Zoo Hypothesis" or the "Prime Directive"). This might be because they are protecting us, studying us, protecting themselves from us or what we might someday become, or waiting until the time is right to initiate us into the Galactic Club.
The simplest explanation -- that we are the only, or the at least first, intelligent species in the galaxy -- requires an extreme violation of the Copernican Principle (which says the Earth is typical and common). This is especially true when you consider the generations of stars -- with possible habitable planets -- that lived for billions of years before our star and planet were even a twinkle in the eye in our parent molecular cloud. There has been so much time for someone to come along and achieve intelligence. Why should our present time be so special? It comes down to which unjustified pillar of scientific reasoning you prefer to violate: Occam's Razor (things are simple) or the Copernican Principle (our place is not special). Take your pick.
Frank Drake: Every discussion of alien intelligence assumes that they will come visit us. But the expense and danger of space travel are formidable. A strong reason why such enterprises are not carried out may be that radio communication works so much better, is far cheaper, and you get your answers at the speed of light.
Any reasonable transport of creatures across space calls for travel speeds that are a substantial fraction of the speed of light, otherwise it takes too long to go even to the nearest stars. But this exposes the spacecraft to serious hazards. Probably the most serious is the potential for collision with debris -- and we are learning that space is full of debris.
At relativistic speeds [approaching the speed of light], even a collision with a particle of a few grams results in something close in energy to a nuclear bomb blast. Not good news for the space travelers.
Also the energy requirements are ridiculous, at least to us. To send a spacecraft the size of a small airliner at one-tenth the speed of light requires as much energy as the U.S. now produces in more than a hundred years. And that just gets you someplace -- it doesn't provide for a landing or a return home.
To put it another way, it takes 10 million times as much energy to move a small space colony to another star as it takes to establish the same colony in the home system. And there is plenty of room at home. It is easily calculated that the energy of the Sun is enough to sustain more than ten thousand billion billion humans. That seems like enough. Why go to the great expense and danger of going to other stars? Truly intelligent life would laugh at the idea. The only ones who might try are the dumb ones, and they don't know how.
David Grinspoon: I agree that, given the time and energy constraints, any intelligent creatures would have to be nuts to attempt interstellar travel. But you would also have to be nuts to attempt to cross the ocean in a rowboat, and people have done that.
Why do we need to go one-tenth the speed of light? What’s the hurry? So what if travel times are thousands of years? From the perspective of an individual human life at this stage in our evolution, this seems like a long time. But will the galaxy never, ever, anywhere, produce a creature or cultural entity that doesn’t find this span of time daunting? Even at these slow speeds, if someone decided to start spreading across the galaxy they would be able to spread across the whole Milky Way in a few hundred million years, tops, which is still short compared to the life of the galaxy.
I also agree that radio communication makes much more sense than any form of interstellar travel for almost any purpose. Except it’s still more fun to go to the game than watch it on TV. I doubt we'll ever achieve warp drive or anything that makes interstellar travel so much faster, better, and cheaper that we can visit a new star system with shapely natives every week like Captain Kirk.
Still, isn't it extreme to declare that no one will ever travel the interstellar distances?
Donald Brownlee: I have always loved space travel in science fiction, but I take a very dim view of the likelihood that we will be able to send people more than just a short distance away. I know that a future without interstellar travel is a minority view, but it is not at all clear that technology could be developed to transport living humans to habitable places beyond our solar system.
I think that it is odd that so many people are sure that we will inevitably evolve to a Star Trek society, able to zip across the Galaxy like we drive to the next state. Beaming up and all that stuff seems so easy on TV. Our best bet with foreseeable technology is to use antimatter fuel, but even if we could build the hardware it would take all of our planet’s energy production for over a century just to make the fuel. Besides, there are additional problems in technology, funding, and human organization. New discoveries involving navigation and maneuvering are required to get to other earthly oases in space on a comfortable and timely basis.
Can all the UFOers really be wrong? Time will surely tell.
David Grinspoon: As many brilliant thinkers have pointed out, if a civilization survives to a certain point they could easily become immortal. That is, if they learn how to avoid asteroids and other natural disasters, tame any self-destructive instincts and learn to live sustainably, their lifetime effectively becomes the lifetime of the universe.
Yes, I know there are nasty things like gamma ray bursts and other hazards we haven't even discovered yet, but we are talking about technology and an understanding of nature, and of self-understanding, that are many millions of years beyond our own. Migrating between stars to stay alive will not be a hurdle for these "old ones." Comparing this idea to Star Trek or UFOs is a cheap shot that ignores the serious literature on this topic.
If you don't insist on making the trip within the current human life span, there are no huge technical hurdles.
Donald Brownlee: I am sorry that David considered my previous comments about Star Trek and UFOs to be a cheap shot, but I really do believe that the difficulty of practical interstellar travel is horrendously underestimated. In my opinion, the public is being bilked by wishful thinkers that like to write books and muse about futures that we would like to believe are our logical destinies.
Perhaps I take too much of a hard-nosed and practical view of this, but doing even simple things in space is difficult, unforgiving, and exceedingly expensive.
I am aware of the studies of anti-matter rockets, beamed energy, interstellar ram jets, etc., but all of these ideas have severe problems. As I see it, known physics will never deposit living people on Earth-like planets around other stars. Doing so would require "warp speed" and/or harnessing exotic phenomena such as wormholes or zero-point energy. Unless such radical developments occur, mundane ideas such as anti-matter rockets will not do the job.
We have gone to the Moon, we can go to Mars, but that is likely to be the limit that our resources and foreseeable technology will allow. At our current rate of progress, humans may not even make it beyond the International Space Station. Our bounds in space may be as limited as they are on Earth. We have covered the Earth but it seems highly unlikely that we will ever live more than a kilometer above or a few kilometers beneath its surface.
The suggestion that organisms could easily become immortal if they live long enough is intriguing. There are a number of issues here, including whether "immortal" means "relatively immortal" or "actually immortal." Forever is a very long time -- I suggest that nothing physical can ever be immortal. Infinite time is something that the universe cannot keep up with unless things like child universes pop up from time to time to refresh the landscape. If things aren’t miraculously refreshed, the universe just runs down over long time scales.
According to new information, the expansion of the universe has accelerated. Lawrence Krauss of Case Western University says that an accelerating universe "would be the worst possible universe, both for the quality and quantity of life… All our knowledge, civilization, and culture are destined to be forgotten. There's no long-term future." A most bleak forecast and at the totally opposite end of the spectrum from predictions of immortal beings.
David Grinspoon: I define "immortal" as lasting for the rest of the life of the universe, which may not be "really immortal" but may have to do.
If we accept the idea that some civilizations can solve the problems which threaten their survival, attain peace, stability, control their populations, learn to intelligently engineer their solar systems, etc., then "immortality" happens. By definition it is an irreversible transition, so the immortals must slowly be accumulating. None of us know, but my sense is that the universe is bio-friendly. I doubt there are any other planets with a peculiar history and biosphere closely resembling Earth’s, but I predict many, many inhabited worlds, and a large number with intelligence far in advance of anything we can even conceive of.
Don't you love predictions like this? It cannot be proven wrong!