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SETI: What has it ruled out?

  1. Feb 2, 2010 #1
    I just watched this podcast of a lecture by Paul Davies ( http://royalsociety.org/The-Eerie-Silence/ ), which made me wonder: What's been learnt so far, from searches for radio signals, about the likelihood of an alien civilisation within, say, 10 ly, 100 ly, 1000 ly... How does the data that's been studied constrain estimates of how common radio-using civilisations are in our part of the universe? For example, it it reasonable to say that if there was a civilisation similar to our own in the Alpha Centauri system, we'd probably know about it by now?

    Also, what is the greatest distance at which another civilisation of exactly our technical ability could (be reasonably expected to) detect our radio signals, such as radar and radio/TV broadcasts. If we were 50 light years away from Earth, would we be picking up 50 year-old TV programmes, or letting them slip by unnoticed? Is there a practical limit beyond which distance the signals would be too weak for anyone to detect?
  2. jcsd
  3. Feb 2, 2010 #2
    Good question, someone with SETI must have thought about this?
  4. Feb 3, 2010 #3


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    The most realistic answer is probably 'nothing'. Detection of EM signals of comparable strength to those generated by human activity is hugely difficult over interstellar distances [see, for example, http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html].
  5. Feb 3, 2010 #4
    Radio waves degrade pretty severely at interstellar distances. One really needs to send an extremely strong and highly focused radio signal for it to make it to another star system in strong enough shape to be received.

    I don't think SETI has ruled anything out, but it certainly hasn't added anything either, other than perhaps a greater understanding of some of the radio wave sources it has picked up over the years.

    But I think a more important reality is that the chance of another civilization sending radio signals during our society's technological age is pretty small. Intelligent life may be formed plus or minus billions of years for all we know, and we've only been sending radio signals for almost a hundred! The odds of both civilizations existing at the same time are incredibly small.

    With the recent technological improvements allowing us to detect exoplanets, and the new generation of telescopes that may even allow us to determine atmospheric compositions of some of these planets, SETI may be able to point its telescopes directly at planetary systems that have a better chance at having life, rather than pointing at random areas of the sky. This may give us a better chance at detecting something, however small.
  6. Feb 5, 2010 #5
    Intriguing, especially: "It is only the narrowband high intensity emissions from Earth (narrowband radar generally) that will be detectable at significant ranges (greater than 1 LY). Perhaps they'll show up very much like the narrowband, short duration, and non-repeating, signals observed by our SETI telescopes. Perhaps we should document all these "non-repeating" detections very carefully to see if any long term spatial detection patterns show up."

    So is the "wow signal" one of many such?
  7. Feb 5, 2010 #6
    Unless technology is enough of a survival advantage that technological civilisations tend to be astronomically long-lived.
  8. Feb 5, 2010 #7
    Well yes, that's true. This is probably a better answer for socialogists to answer. The pessimistic view is that technology will destroy us rather than let us live long enough to reduce the time differential between us and another civilization. The optimistic view is that technology will allow us to spread out among the stars and develop modes of interstellar travel and communication that allow us to live for eternity. Of course, what is an eternity? Is it 100,000 years from now? Even that, however, is a miniscule amount of time when we are talking about scales of +/- millions of years for civilizations to "meet up". And if technology does allow one to travel between the stars, why haven't we seen any other civilizations or heard their communication? Many have suggested that interstellar travel is just too difficult and expensive to ever become viable. So what's the right answer?

    This issue is also one of my favorite topics. The more I read, however, the more I think we might be alone. But I can still hope!

    Check out:

    If the Universe Is Teeming with Aliens... Where Is Everybody? Fifty Solutions to Fermi's Paradox and the Problem of Extraterrestrial Life



    Rare Earth: Why Complex Life Is Uncommon in the Universe


    There are a number of other books that are more optimistic, however. And I do think the focus on KNOWN exoplanets will improve SETI's chances of finding radio signals. But the odds are still long.
    Last edited by a moderator: May 4, 2017
  9. Feb 5, 2010 #8
    Technology allows us to travel between continents, but that doesn't mean that a random bug going about its brief life in a tiny corner of one continent is likely to ever come into direct contact with a human being. Even if interstellar travel was fairly widespread, might it not be directed mainly along established routes? It could be possible to travel between stars and yet still not be economically viable to go to every single star, on the off-chance of finding something interesting enough there to warrent the expense. And even with many exploration voyages, there are still a lot of stars, and a long way between them.

    But I'm sure people better informed than me have considered that answer, and probably tried to quantify it. The huge distances and times involved are enough to make me wary of my intuitions, quite apart from all the uncertainties.

    Thanks for the book suggestions.
  10. Feb 5, 2010 #9
    Mjabosca, thanks for the book recommendation also. They will be interesting reads.
  11. Feb 5, 2010 #10
    Glad you guys liked the recommendations. These happen to be two of the more pessimistic books you could read, but they are pretty thoughtful. There are another dozen I am meaning to read about the very subject!

    One thing is for sure... if we are alone in the galaxy, then I am one pretty unique guy!
  12. Feb 6, 2010 #11
    You should definitely read the book "Are we alone?" by Davies himself. In it, he states that basically given the distances between galaxies, it would take millions of years for any kind of signal activity to reach earth, so we might not be alone, but it will take millions of years until we hear from our nearest neighbor.
  13. Feb 6, 2010 #12
    Of course, that assumes we communicate using the old-fashioned speed of light as our limit. By the time we ever realistically travel the stars, it will probably be because we've learned to warp space, or figured out how to take shortcuts through other dimensions that connect regions of 3D space that are vastly far apart, or learned how to create wormholes. In that case, communications may be easier than expected because we'll set up communication buoys in all the warp interchanges to send signals through!
  14. Feb 6, 2010 #13
    I was struck by the similarity of Paul Davies' comments in Are We Alone? at the end of the chapter on consciousness--"We have a closed circle of consistency here: the laws of physics produce complex systems, and these complex systems lead to consciousness, which then produces mathematics, which can then encode in a succinct and inspiring way the very underlying laws of physics that gave rise to it"--to those of Roger Penrose in The Road to Reality where he presents this diagram:

  15. Feb 6, 2010 #14
    Rasalhague, great quote. This is exactly what Davies was trying to prove, that chaos doesn't necessarily lead to more chaos- when he was trying to describe self-ordering systems that arise from chaos. The chaotic universe doesn't rule out life elsewhere other than earth. If the universe we are in arose out of chaos, why can't it elsewhere as well, given the probabilities of planetary systems around other stars??
  16. Feb 7, 2010 #15
    Not clear. One thing that is the case is that while there does seem to be some fundamental limits on the speed that things can travel, there *don't* seem to be any fundamental physics limits on the human lifespan. There is no basic reason that I know of that people can't live thousands or even millions of years. My guess is that science will find a way to stop/slow/reverse human aging long before we develop wormholes, since wormholes may not exist, but our best knowledge of biology says that stopping or reversing aging is possible, and I'm guessing that it will happen within the next 200 years.

    One thing that *I* find strange is that science fiction stories about interstellar travel always assume a breakthrough in physics, when I think that what may make interstellar travel possible is a breakthrough in biology that stops aging. Now what a human society of people that are practically immortal would look like, that's an interesting question, and I'm surprised that people haven't written more science fiction stories about this.

    One question that I think would be interesting is suppose I could live for tens of thousands of years. I'm not sure that I'd want to, which gets into interesting questions. In human society, suicide and murder are considered horrible crimes because human life is so brief and precious. Suppose we were to meet aliens that never aged. Would *they* consider murder to be something even worse they we do, or would they not are about it?
  17. Feb 7, 2010 #16
    Also, I once read a book on "space law" most of the book was on things like treaties involving satellites and things like that, but the last chapter was on the legal status of aliens. It considered three possibilities. The first possibility was that we meet aliens that are more technologically advanced. The book spent one paragraph talking about that situation, and it's an easy situation. Basically we do whatever they want us to do because we don't have any choice. The second possibility was that we meet aliens that are equal in technology. There it gets harder, but there have been a lot of situations in where things like that happens on earth, and so we'd probably evolve something like international law.

    Now the really, really hard legal problem is if we go out in space and meet aliens that are *less* technologically advanced than us. Who would make decisions about what to do with them? It's a really hard problem.

    One fact that is interesting is that human beings have existed in our current forms for about 200,000 years, but it's only for a tiny, tiny fraction of that time (i.e. 10,000 years) that we have had anything like non-hunter gatherer societies, and we've only be capable of communicating over interstellar distances for about 100 years. What seemed to force people into agriculture was a freak set of climatic changes, and one weird idea that people are thinking about is that the earth was hit by a major comet which triggered major climate changes that forced humans to develop agriculture.

    One thing that's cool about SETI is that it combines a lot of different fields. To understand how aliens *might* behave we have to understand a lot more about how human beings behave.
  18. Feb 7, 2010 #17
    Rasalhague: It could be possible to travel between stars and yet still not be economically viable to go to every single star, on the off-chance of finding something interesting enough there to warrant the expense.

    But would space aliens have anything resembling the same sort of "economics" than we have? One thing that we do see in human societies is that people often take major journeys for non-economic reasons. There are people that go around to every village that they can find so that they can spread the word of God.

    I can imagine space aliens going to every planet in order to spread the word of Zardoz the Great. I can imagine waking up one morning and finding a message about the Good News of Zardoz. Unfortunately, based on what has happened in the past, I can also imagine waking up one morning and finding a message about the Good News of Zardoz which we must accept OR ELSE DIE AS ZARDOZ AS COMMANDED!!!!!

    One thing that sort of depresses me is that in 1965, people thought that space was the "final frontier" and that we'd just naturally end up spreading out into space. It's 2010, and it's not clear any more if we are ever going to get off this rock. One problem is that we really went to the moon for non-economic reasons (i.e. to beat the Russians), and once we did it there was no other reason to go anywhere.

    One other scary thought is that it may be that civilizations that are "nice" tend to stay on their planets, and if we meet anyone out there, they would be aliens with "not nice" motives. I'm thinking about a planet where their equivalent of the Nazis won World War II, and instead of wanting to be the master race on their planet, they feel that it is their destiny to be the master race on all planets.
  19. Feb 7, 2010 #18


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    A shared opinion, twofish. First contact would likely be last contact for humanity. Any critter bright enough to travel here would be ungodly powerful. Our best hope would be it is an exobiologist wishing to study primitive societies, and have their government declare us a galactic park. I doubt we would have anything they truly need they could not find elsewhere, so it is a realistic possibility.
  20. Feb 7, 2010 #19
    I wonder if this is because we're so used to steeling ourselves to the apparent inevitability of death that we almost daren't imagine that it could be otherwise. If that makes any sense... Like not wanting to let our guard down. Then again, throughout history people have entertained beliefs about the possibility of living forever, so I'm sure we could get used to it.

    I heard about a TV programme on which an advocate of life-extension asked the audience who wants to live forever. Not many hands raised. Then he asked: who wants to die now. No takers...

    Perhaps life would be considered more precious if we had more of it to lose, or after we'd had hundreds of years to get used to it in. If death wasn't accepted as something that was bound to happen sooner or later anyway, maybe it would seem like even more of an affront. As it is, we see it as especially bad if someone dies young "with their whole life ahead of them". Of course, there's the principle that rare things are worth more, but at any time we're faced with the immediate prospect of death, our own lives are rare!
  21. Feb 7, 2010 #20
    This raises the interesting question of what's universal in morality and what's parochial. For a hopeful view, try "Steven Pinker: A brief history of violence".

    He mentions Peter Singer's idea of an "expanding circle" of fellow-feeling. I recently heard a similar expression attributed to Albert Einstein. If rational beings tend to evolve culturally towards a kinder ideal, we might be in luck. And the harder it is to travel between the stars, the more time it gives for cultures to grow out of their more vicious aspects.
    Last edited by a moderator: Sep 25, 2014
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