I'm not sure how many of you have read this before. I just read it and found it very interesting. There were some good points and it offers a different perspective on the search for life. However, I was really bothered by the fact that he did not even begin to consider the fact that his "great filter" might not exist. He shows little evidence to support his hypothesis.

There are two options he gives us: We are either alone in the universe because life and subsequent evolution is improbable, or we are fated to die alone because there is no way for civilizations to contact each other.

For one, he brings up the fact that we haven't been able to synthesize life. But there is so little we know about the early Earth! To say that just because the Miller-Urey experiment didn't do much, it was freak chance that over hundreds of millions of years of largely unknown conditions some process that seeded life began? We don't know very much about abiogenesis, but current evidence suggests that life is not improbable in the least bit, and a lot of people will argue that it is quite common.

For the other point, he brings up how aliens have never visited us, and SETI has never detected anything but noise. I think this is ridiculous. SETI has only been scanning for so many years with limited technology at limited portions of the sky for only a fraction of the electromagnetic spectrum. As for aliens visiting us, who's to say they haven't? We don't think they have, but they could have. Or not. If they exist, that doesn't make it certain that they will visit us. It's a huge galaxy.

Another bit is how civilizations tend to collapse, and how there are always things that could wipe out life. But it's difficult to make it seem like every civilization out of potentially billions would just collapse or die. Plus, even if they do implode, they always grow back bigger than before in some form, and by nature they spread outward.

Heck, I think it's quite unlikely that his "great filter" exists. I'm not trying to discredit this essay or say it's rubbish, it makes some very important points and has credibility to it, but I have to disagree with some of the pessimism in there.

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What should be evidence for such a "natural law" of spreading? Is it not speculative?

I completely agree. All we can say at this point is that there aren't any close civilizations transmitting strongly on the bands we're looking at. But why would a civilization? I doubt most would transmit ... they'd probably try to hide.

Well, at least human nature is to spread outward, but that may not be alien nature. Still, when you go down to the level of basic diffusion, things tend to spread out.

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Quite a long thead on this essay here
Why I hope the search for extraterrestrial life finds nothing.

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Interstellar communications in the radio wavelength ranges are problematic. You would need a gigantic antenna aperture and a ridiculous amount of power just to be heard at the nearest star-systems. Diffraction would limit your ability to collimate a signal severely.

I think if you wanted to communicate over interstellar distances, you would need to use something of much shorter wavelength, like a visible or UV laser.

Also, you would have to be pointing at the intended recipient. Even assuming space is jampacked with intelligent communicating life, why would we rate as interesting enough to point a signal at? I think a hypothetical alien would want to know if we were capable of recieving before dedicating a communications station to broadcast towards us.

And I very much doubt the universe is full of communicating life, going off of the single data point we have. Life maybe. But of the 3.5 billion years earth has had life, it has had intelligent life for only a million, technological life for only 500 years or so, and spacefaring life for maybe half a century. 50/3500000000 is a pretty small fraction.

If the sky is filled with earths, who do you expend your gigawatts trying to talk to? If it isn't filled with earths, then there isn't likely anyone who can hear on the few that are there.

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And adding to this they would have to be technological capable of sending or receiving a signal at roughly the same time as us, at the moment say around the last 100 or so years (unless we receive a long distance signal which was sent a very long time ago). This little window of opportunity which is now open for us is minuscule on the cosmic scale, a nano second of time in comparison to the age of the observable universe. I would not be surprised if we find nothing for the next 1000 years with those odds.

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It's simple really, humans are alone in the universe... except for other humans. You see, in the future we learn to travel through time (and by extension, space) but don't do it until we approach the heat death of the universe. Then we go back in time to a new planet and re-seed it with life and replant adamses and eveses and then use a memory dump as the population increases. That way we get to re-live life without contaminating the past, hence the silence. The future uses don't want to mess up the timeline.

Could be an interesting short story. Maybe it already is.

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What odds? How do you know the window is that small? They had to have started transmissions at least a few decades or so ago in order for us to hear them, probably centuries or millennia, but what's your limit on how long they can keep transmitting?

Or they may take a slower, but more energy-efficient approach. Find planets with oxygen atmospheres or other signs of life, and wait for radio noise or laser signals indicating a civilization before sending anything. In that case, their first signals may well be on their way here, due to arrive in a matter of decades.

Boredom, budget cuts. I think the most likely reason is that they'll only be using electromagnetics and operating a civilization even remotely understandable to us for a few centuries or millenia at the outside.

If we planted a time-travelling bronze age civilization on our own planet 2 billion years in the past, it could rise to modernity, fall to barbarism, rise again, fall again, a hundred times or so, and if they left gigantic ceramic monuments or vast works of engineering, we wouldn't be able to tell today, as it would have all been ground to dust less than 100kyears after they diverged from mankind-normal. I don't think we'll be able to converse intelligently with our own descendents 500, 1000 years from now.

Space is vast. Time is vaster. Space-time is mind-numbingly enourmous. Civilizations and empires (at least as we use the term, on human scales) are ephemeral pan-flashes compared with geological time. That's just my perspective, I don't necessarily "know" anything in the sense of having more than Earth to go off of.

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Yes, but when a civilization gets to a certain point at which they are capable of interstellar travel, which could happen to several civilizations at a time in a given galaxy, they will spread out and gradually colonize the galaxy, at which point they are sustained. Eventually the unified systems would achieve technological equilibrium. I think it is quite likely that large interstellar or even galaxy-wide civilizations exist. They just haven't payed attention to us yet.

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Well, if they are restricted by lightspeed, then maybe their detection systems are probes that try to pass close enough to stars to detect either albedo signatures of life on planets or technological emissions. But even if they find one, they may not be able to stop or divert to investigate. They might only pass the report back to the next nearest probe and hope it gets relayed all the way back to the home planet that is tens, hundreds or thousands of lightyears away. Then, that civilization has the option of sending a dedicated probe to visit the planet in question, then wait for that probe's response. Only then they might order it to attempt contact.

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Our window is that small, because we have only been capable of transmitting and receiving radio signal for around a century.

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Then where are they?

If relativity is correct and the speed of light is the universal speed limit then i can't see how more advanced technology could enable interstellar travel for life forms that would follow the form of any thing like what we know as life. If they achieve close to light speed travel then unless they aged per life time on their home world in thousands or even millions of equivalent earth years then inter stellar travel would most likely be a one way ticket.

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Yes, it would be a one-way trip. What's wrong with that?

BTW, if they got to a high enough speed it might not take millions of years to get here, from their point of view.

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Thats my point about relativity and reference frames. If they could achieve near light speed then the journey regardless how many earth light years long, could take days, even hours from their point of view. The problem is anybody measuring that time and distance from a different reference frame would most likely measure it to be very large. If for example the alien home planet was 1000 light years away, from our and the alien's back home time frame the round trip journey would take just over 2000 years. For the aliens on board the spaceship dependent on their relative speed it could only take a couple of hours each way from their reference frame. A lot of things can happen in 2000yrs. Certainly for any human, the trip would mean leaving behind here on earth all that they know and love, then returning to what might have become an alien world to them.

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One way or another, interstellar travel is a one-way trip.

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Yeah, that's my point too. Even if things change "back home" They'll probably not ever go back. Not unless they are near immortal and can install an industrial base here to prepare the fuel systems they would most likely use, based on realistic designs based on near current tech or slightly speculative tech.

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...ahem, pardon, at the risk of repeating myself
excuse me, i don't know how else to link it

Siguy, i love your OP and all that's following...

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clear skies

If you want to make an apple pie from scratch, you must first create the universe. CARL SAGAN

Mak: Pass the pepperoni please.
Fazor: "Hail, Bautainia! We pledge our hearts to thee! Science and woo, some babbling too, and astron-oh-meee!"
slang: And it made ash out of yew and tree.

Interesting theory. But the problem I see for the time travelers is each time they can only go back in time to the point they created the first temporal transporter and they have to travel back to point that they left from first or the bubble burst...ah now we travel into other quantum realities and alternate timelines. Someone or something has to remember the first time we time traveled so we do not make a mistake and send back two or three missions and we have to go to the same planet too. Well of course there is not proof of this but it is just my theory.

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Only of the author writes the story that way.

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau

It's purely a matter of distance.

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"There is absolutely no law in physics that prevents time travel." -Dr. Michio Kaku

This line from the piece caught my attention:
*"Another indication that an evolutionary step was very
improbable is*that it took a very long time for it to occur even after its prerequisites were in place"
I don't know that a few hundred milion years is nessesarily a "very long time" in cosmic terms. It would seem to me a few hundred million years ammounts to a few moments in perspective, given the relative age of the known universe, I say "known" universe because we aren't even sure ours is the only universe and if it isn't that they were even formed at the same time. That fact alone, that ours may not be the only one, increases the odds of life being out there somewhere exponentialy.

The other problem I have with the author's theory, is he bases the idea that civilizations eventualy become self destructive, on human nature only. Who is to say another intelligent species will have evolved the same self destructive nature or tendencies to violence, neglect or even the same moral code humans operate under?

Welcome xfahctor!

I like the name by the way, did you watch the series unfold?


Back to your posts, i agree with you on both especially this one I've quoted. The biggest mistake people make is to assume aliens will have the same tendencies as humans like you have stated. Heck, they might not even remotely reassemble any form of life as we know it or even possibly could imagine!

Hey, thanks for the welcome. The name isn't based on the show actualy and the "h" in it is to give it a jamacan/rastafarian accent (no, I'm noeither), LOOONG story.Anyway, the author just seemed to base his whole presumption on human nature instead of broader criteria. I'm no scientist, in fact I'm actualy pretty stupid, but, i would say if you are posing a theory on a cosmic level, you should base it on cosmic foundations.

To be blunt, Bostrom's concerns have the same weight whether the aliens he is looking for (and failing to find) are human-like in psychology, or sapient bacteria, or anything else. The great problem is that they aren't here; we can tell that by molecular genetic analysis. All life on Earth evolved here; no sapient bacteria from the stars have ever passed this way long enough to leave descendants.

It may be that Bostrom's populist treatment of this problem has misled some people; but in fact, the Great Filter hypothesis makes no assumptions about the physical or psychological nature of any hypothetical aliens; it simply observes- they are not here, and they are probably not everywhere. What has prevented them from filling every ecological and social niche in the galaxy?

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In my extremely humble opinion.......size.
Like 2 individual plankton in an ocean.

That would mean that any other intelligent species has evolved on a planet far, far away; that is fine, because that implies that life is rare, or at least that life rarely evolves into an intelligent species. In other words, the great filter is in the past, either between non-living and living matter, or between non-intelligent and intelligent life. Or both.

That is the best case scenario, since it does not require a filter that is waiting to catch us in the future, before we can expand to colonise the galaxy.

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