"The Great Silence"

So, I know that most AI threads deal with social/political issues or vague philosophical questions instead of science, and maybe we don't have many posters who are interested in scientific questions in general, but I hope at least a few people find this topic intriguing even though it may seem a bit speculative.

"The Great Silence" is a term used to refer to the fact that we (humans) don't seem to be picking up any radio transmissions, or other forms of communication, from any extraterrestrial civilizations. This is probably something most people take for granted, thinking that it would be far more bizarre if we WERE to receive such transmissions, but it does make some scientists wonder why this is the case. Is it because life itself is very rare in the universe? Or because intelligent (tool-using, technological) life rarely arises even though life itself may be common? Or because such intelligent life as does exist in the universe tends to keep quiet for some reason? If so, why? This question is related to the Fermi Paradox (http://en.wikipedia.org/wiki/Fermi_paradox, which in its basic form asks why we haven't encountered alien spaceships/probes if extraterrestrial civilizations are as probable as has sometimes been estimated), but is perhaps more basic; before you can ponder the Fermi Paradox you have to take for granted that technologically sophisticated extraterrestrial civilizations are indeed probable.

Also of related interest is the Drake Equation (http://en.wikipedia.org/wiki/Drake_equation), a mathematical formula used to guesstimate the number of communicating civilizations in the Milky Way galaxy at any one moment. This equation has been called meaningless, but it was never supposed to actually "tell us" how many such civilizations exist or to yield a certain predictively correct result. Anyone who likes can plug in their own values, at least some of which are necessarily pulled out of thin air, and see what result they get. It's value is thus to simply tell us the hypothetical implications of different combinations of values for those terms, allowing us to see how many communicating civilizations the galaxy might contain IF a certain set of values is accurate, and from there to provoke thought and perhaps guide research. Frank Drake never claimed that the particular set of values he plugged into his equation were definitely correct, and nor should anyone else working with the equation. Think of it as a visualization tool for examining where certain assumptions lead to.

Anyhow, there are a lot of possible explanations for the "Great Silence", many of which are discussed on the Wikipedia page for the Fermi Paradox I linked to above. The major ones are probably that other civilizations are rare (and thus unlikely to be nearby, or extant at all, at the same time we're listening) or nonexistent for some reason; technological civilizations tend to somehow destroy or radically transform themselves comparatively soon after achieving the ability to communicate; we're not looking for them in the right way; or they aren't interested in communicating with us for some reason. All of these broad explanations can break down into many different specific ones. Again, this question may seem speculative, but I think it's an interesting one, and it's answer has potentially deep significance to many fields of science as well as to religion, humanity's view of itself, and so forth. It also also probably raises many more questions, ethical and moral and religious (and perhaps practical) as well as the obvious scientific ones. In fact the existence of any extraterrestrial life at all, communicating or otherwise and intelligent or otherwise, raises those questions. Speculative as it may seem, there are also serious scientists still involved today in searching for evidence of extraterrestrial civilizations. The recent explosion in the rate at which planets are being discovered in other solar systems--increasing Earthlike ones, too--also makes this question a bit more interesting and maybe a bit less pie-in-the-sky.

I know none of us here are scientists and I suppose I'm mainly just interested in hearing opinions and talking them over. What do you think? What explanation(s) for the "Great Silence" strike you as reasonable/likely or unreasonable/unlikely? Why? In general, what do you make of the idea of extraterrestrial life itself, extraterrestrial life that's "intelligent" (tool-using), or extraterrestrial life that might possess the ability to communicate over interstellar differences? Hopefully I'm not the only one here who finds these questions intriguing.

Alien life is certainly more than likely in some form and I believe that some intelligent lifeforms may exist. As one of the sections of the Fermi paradox wiki page states, scale is a huge factor in any communication between the two. The Aricebo Message was only sent out in 1974 and will take 25000 years to reach its intended destination. A reply will take some time after that, depending on the level of technology of the recipients to send a reply, and that is only after they have decoded the message and figured out where it has come from. The level of alien technology is very important as it can either be too adanced for them or not advanced enough (imagine people in 300 years time trying to play a VHS, there is a possibility that such "primitve" technology may have been totally lost). If we do get a reply, it will have been a very lucky shot with a massive coincidence in the technology the respective communicators have.

However if we consider ourselves as the aliens in the view of other beings, if we were to recieve a message first (which i think is the most likely situation) and if we were able to decode it, even with what is considered a highly advanced level of technology, we would not be able to go and visit for a very long time, probably not without the help of the group that contacted us anyway. Even the idea that we would be able to reply is questionable.

Good points, just bear in mind that the Arecibo message is not actually much of a serious attempt to communicate with aliens; it was more of a publicity stunt to celebrate Arecibo's reopening after having some new equipment installed. If it had been a serious attempt to send out signals that might be picked up by extraterrestrials, the transmission probably would have been aimed at more than one target, definitely at targets that were much closer, and probably at targets that were estimated to have a higher likelihood of harboring life. Plus it would have been repeated more than once! The Arecibo message was a single 167-second shout into the sky, pointed in a direction that was basically picked because it was convenient for the scheduled time of the facility's reopening ceremony. So it's not necessarily a good example if we're trying to consider this question. If a civilization (ours or alien) is seriously interested in sending out communications to other civilizations, the method we/they use won't bear much resemblance to the Arecibo message.

Thats true but I was just using as an example of poor effort. One burst of encoded information that has no real value to the recipient is never going to be successful.

My main problem with any message though is understanding.You could phone a guy in Brazil and say all you wanted, but he isnt going to get any information out of you or reach a conclusion of where the message is from (unless he speaks english, but its a needle in a haystack)

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Originally Posted by Syme

I know none of us here are scientists

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uuuuuuuummmm excuse me said the phd student waiting for the vacuum chamber to pump down so he can do e-beam lithography?

i mean i completely admit to being the worst graduate student in the world, but still...

(ps please don't tell the people who will be on my thesis defense committee i said that kthnx)

and no i'm not an exobiologist either. exobiology is hillarious: it's the only field of "science" whose object of study hasn't been proven to exist (well, exobiology and string theory, though string theory doesn't have any "experimental string theorists.")

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Also of related interest is the Drake Equation (http://en.wikipedia.org/wiki/Drake_equation), a mathematical formula used to guesstimate the number of communicating civilizations in the Milky Way galaxy at any one moment. This equation has been called meaningless, but it was never supposed to actually "tell us" how many such civilizations exist or to yield a certain predictively correct result. Anyone who likes can plug in their own values, at least some of which are necessarily pulled out of thin air, and see what result they get.

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yeah, the problem people who don't understand science think it's suppose to be predictive, people like reporters and those who listen to them. i remember having a long argument with a person about how some of those factors, like the chance for a planet that can develop life to actually develop life, are completely unknown. they still didn't believe me afterwards and were convinced that the drake equation was something more than a device to spur thought and discussion.

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Originally Posted by sailor jack

Alien life is certainly more than likely in some form and I believe that some intelligent lifeforms may exist. As one of the sections of the Fermi paradox wiki page states, scale is a huge factor in any communication between the two. The Aricebo Message was only sent out in 1974 and will take 25000 years to reach its intended destination. A reply will take some time after that, depending on the level of technology of the recipients to send a reply, and that is only after they have decoded the message and figured out where it has come from. The level of alien technology is very important as it can either be too adanced for them or not advanced enough (imagine people in 300 years time trying to play a VHS, there is a possibility that such "primitve" technology may have been totally lost). If we do get a reply, it will have been a very lucky shot with a massive coincidence in the technology the respective communicators have.

However if we consider ourselves as the aliens in the view of other beings, if we were to recieve a message first (which i think is the most likely situation) and if we were able to decode it, even with what is considered a highly advanced level of technology, we would not be able to go and visit for a very long time, probably not without the help of the group that contacted us anyway. Even the idea that we would be able to reply is questionable.

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first of all, we have been transmitting radio signals to space for the last 90 to 100 years. that's how long we've had radio communications used on a moderately large scale (thought the first signals were wireless telegrams and not audio signals). so we've sent out radio waves out to a distance of 90 or 100 light years. still, that only covers something on the order of 10^4 stars (i think) out of the 200 to 600 billion stars in our galaxy.

also, all that matters is that we find a radio signal with structure. it doesn't matter if we or the hypothetical aliens can decode it. in fact, when radio signals from pulsars were first detected, people thought they might be radio signals from an alien civilization. it turned out that it was just a magnetic rotating remnant of a star that went supernova which appears from earth to send out pulses of electromagnetic radiation periodically (it's actually a "beam" of EM radiation that rotates like the light in a lighthouse, so we can see it when it's facing earth).

there are a number of reasons that i could think of as to why we haven't heard anything from other intelligent life, if indeed there is life in elsewhere in our galaxy. i have to go, so i'll post them in a few.

If we received some type of vague alien communication whose validity, meaning, and purpose we would struggle immensely to discern, I'm certain we would hardly bat an eye, much less make any dedicated, meaningful effort to establish any type of contact.

I would say the most likely outcome is that any message we send would fall on deaf hearing appendages, even though something exists which could receive it.

Edit: Also, consider the level of strife, conflict, and technological disparity amongst our own species. The odds of a message even being received and recognized for what it is seem slim.

Also, this is more of a technological question, but how are these signals transmitted? From my rudimentary understanding of physics and space, there shouldn't be any theoretical limit that a radio or similar signal could travel, right? But isn't there a practical limit formed by objects and gravity wells through which a signal could not pass, ultimately resulting in large shadows that our signal wouldn't be able to reach?

Thus, aren't any of the odds of contact hopelessly optimistic because, while we have infinite possibilities in terms of planets on which life has developed, isn't there a pretty small physical limit of locations that we can contact?

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Originally Posted by sailor jack

My main problem with any message though is understanding.You could phone a guy in Brazil and say all you wanted, but he isnt going to get any information out of you or reach a conclusion of where the message is from (unless he speaks english, but its a needle in a haystack)

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Hmm, I don't mean to sound nit-picky, but this is a somewhat flawed analogy. The "language barrier" problem is a pretty obvious one, and so, as you might guess, it's been considered at length by scientists involved with this question. The Arecibo message, though it wasn't a serious communications effort in terms of target or transmitting time, does provide a pretty good illustration of how the "language barrier" issue can be addressed, and how the likelihood that the recipient will make sense of the message can be maximized . The best solution has to do with mathematical and numerical patterns, based on the assumption that any species which can build a high-sensitivity radio receiver (and all the other technological gizmos that go along with that level of sophistication) will have to understand math in some way, and that mathematics is universal. For instance, they will probably know about prime numbers too. So the Arecibo message, which in it's "natural" form is just a squirt of radio waves with the frequency hopping slightly up and back down several times per second, will hopefully have a certain significance to anyone who understands mathematics (i.e., presumably, anyone who builds radio equipment). The number of bits (frequency shifts) in the Arecibo message is the multiple of two prime numbers. Arrangement of these bits as visual elements on a plane, with the number of elements (pixels) along each edge corresponding to one of these two prime numbers, produces an image. There are two possible ways to do this but one of them has noticeable symmetry and other traits that mean it may well stand out and lead an alien observer to recognize that it's the correct way, while the other is gibberish. From there, at least some of the parts of the image probably have a fighting chance of being interpreted correctly by any organism that relies on visual sensing.

Obviously this still rests on some assumptions that may not hold true. The aliens MAY not understand prime numbers, but if they can receive the message in the first place, they probably will. It may never occur to them to arrange the bits as visual elements on a two-dimensional plane, but on the other hand, we can imagine how we would respond if we received a recognizably artificial radio signal from space: Great numbers of scientists, with access to a huge budget and all sorts of equipment and resources, would probably spend night and day for months subjecting the signal to every sort of analysis and processing imaginable, looking for any patterns or meaningful properties whatsoever. Another species that would construct devices to listen for signals from space is probably reasonably likely to have a similar response. For various reasons, there's a decent chance that arrangement of the bits as visual elements on a plane would be one of the basic things they'd try. From there, they may not recognize the significance of structured symmetry vs. random noise, but that frankly seems nearly as unlikely as their being able to build radio receivers without understanding mathematics. They may not rely on visual sensing, but there is good reason to believe that there's at least a decent chance they do. And they may not be able to make sense of all of the image's components even if they do use vision, but at least some of them will probably suggest certain things to them for various reasons (I'm talking specifically about the contents of the Arecibo message here, but the same would presumably hold true of the messages sent in more realistic attempts at communication). At the end of the day, there's no guarantee that whoever detects the message can make sense of it, but there is at least a fighting chance they can extract some meaning from it even if not all of it's meaning. Probably more of a chance than your "needle in a haystack" comment might suggest, since that would seem to suggest that the chance would be vanishingly minuscule. There are probably better-than-minuscule chances that radio-receiver-building aliens understand math, would at some point arrange the message's bits as a two-dimensional grid, would recognize the significance of symmetry, and would understand the concept of visual information and thus an image.

Also, while two people on a telephone babbling at each other in different languages may not be able to reach a conclusion about where the other is calling from, the same rules don't quite hold true for radio transmissions. Even if we (or an alien civilization) couldn't make any sense whatsoever of a received radio signal, even if it was pure and total gibberish to us/them, one thing we/they could still tell would be the direction from which the signal came and thus, probably, it's origin. That's not all that the signal reveals even if its actual meaning/contents are totally mysterious to the recipient. At a bare minimum, such a signal tells the recipient not only where it came from, but that it was sent by a tool-using civilization which possesses the technical capability and understanding of physics/math/etc. to construct radio transmitters of the type needed to send that signal, and, presumably, the inclination to use that technology to attempt contact with other such civilizations (which reveals a mindset that is at least somewhat outward-looking and inquisitive, disposed to dream up and consider hypotheticals like the existence of life elsewhere in the universe, and attempt to discover whether they are correct). Furthermore the receipt of a message may tell the receiver something about what the sender considers likely places to find other intelligent life, and thus perhaps something about the sender's own planetary environment, star system, and so forth.

That may not be a huge quantity of information but it is some very significant basic knowledge about the sender of the signal, which can be gleaned without even beginning to understand the signal itself. Such a signal also implies several things about the evolution of intelligence, the incidence of life in the universe, the frequency of life-bearing planets and technological civilizations, etc., based on how far away the signal's source is, and what is known about that source in astronomical terms. So even a message that goes totally "untranslated" tells it's recipient a lot and will probably stimulate a great deal of research, speculation, and other activity in the recipient civilization. Certainly I think that would be the case if such a message was received here on Earth. A message from another species would carry a lot of significance totally apart from it's actual content (and again, understanding that content may actually be a good bit easier than "finding a needle in a haystack", if the message is properly designed).

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Originally Posted by Atmoscheer

If we received some type of vague alien communication whose validity, meaning, and purpose we would struggle immensely to discern, I'm certain we would hardly bat an eye, much less make any dedicated, meaningful effort to establish any type of contact.

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Well sure, if even the communication's "validity" was very difficult to discern (i.e. if it was hard to determine whether the signal was really of artificial and alien origin), then it might not be a big deal. That's not saying much, though; it's essentially saying "there wouldn't be huge excitement over an alien message if we didn't know it was alien", which is an obvious and also kind of pointless observation. Assuming the message could be recognized as having a clearly artificial, extraterrestrial origin, I think it would be a pretty huge deal even if it's meaning and purpose were totally unknown. You might not bat an eye but a lot of scientists and a lot of people in general would flip their shit. Whether we would/could make a serious effort to send a reply message of our own would of course depend on how far away the message came from.

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Originally Posted by Syme

Well sure, if even the communication's "validity" was very difficult to discern (i.e. if it was hard to determine whether the signal was really of artificial and alien origin), then it might not be a big deal. That's not saying much, though; it's essentially saying "there wouldn't be huge excitement over an alien message if we didn't know it was alien", which is an obvious and also kind of pointless observation. Assuming the message could be recognized as having a clearly artificial, extraterrestrial origin, I think it would be a pretty huge deal even if it's meaning and purpose were totally unknown. You might not bat an eye but a lot of scientists and a lot of people in general would flip their shit. Whether we would/could make a serious effort to send a reply message of our own would of course depend on how far away the message came from.

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It might be a pointless observation in terms of considering our response, but there's a huge assumption here that, even if all the other probabilities work out, that an alien civilization would both recognize our signal and, more importantly, care about it.

Consider, too, that if we had received such a transmission in the midst of, say, World War II, it likely would've fallen on deaf ears and been forgotten.

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Originally Posted by Atmoscheer

It might be a pointless observation in terms of considering our response, but there's a huge assumption here that, even if all the other probabilities work out, that an alien civilization would both recognize our signal and, more importantly, care about it.

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The question of whether other species would recognize our signals (or vice versa) has been raised many times in the past and in fact is mentioned in the Wikipedia page I linked to as one of the possible explanations for the Great Silence, so I wouldn't really say there's an assumption here that they would.

When I responded to what you said about a signal whose validity we'd struggle to discern, I didn't mean to imply that I'm assuming alien signals will automatically be easily recognizable. I don't assume that. But it seemed as if you were implying that you assume such signals will tend to be unrecognizable, or that any possible messages from aliens will fail to arouse much interest because they will tend to be hard to identify as such. I was simply pointing out that this may not be the case, and that at any rate we can obviously only concern ourselves with those that are recognizable. The fact is that an alien signal might or might not be readily identifiable as artificial, and we really can't guess too meaningfully at the odds that it would or wouldn't be. But we can't do much about the ones that we'll never pick up because they're sent at one bit every five years, or whatever. Again, you're obviously right that an unrecognizable signal would go unrecognized. And one explanation for the "Great Silence" is that all alien signals that reach us are unrecognizable. It kind of strikes me as less reasonable than some of the other explanations, though.

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Originally Posted by Atmoscheer

Consider, too, that if we had received such a transmission in the midst of, say, World War II, it likely would've fallen on deaf ears and been forgotten.

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Do you mean to say that the signal might not have been detected at all because there would be no, or less, non-military research (such as the use of radio telescopes to detect radio waves from space) going on during wartime? Or that it would have been detected but that no-one would have cared because there was a war on? I think that both suggestions are dubious, though the first is probably somewhat more defensible.

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Originally Posted by Atmoscheer

Also, this is more of a technological question, but how are these signals transmitted? From my rudimentary understanding of physics and space, there shouldn't be any theoretical limit that a radio or similar signal could travel, right? But isn't there a practical limit formed by objects and gravity wells through which a signal could not pass, ultimately resulting in large shadows that our signal wouldn't be able to reach?

Thus, aren't any of the odds of contact hopelessly optimistic because, while we have infinite possibilities in terms of planets on which life has developed, isn't there a pretty small physical limit of locations that we can contact?

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We have no problem directly observing stars and other astronomical features that are tens of thousands of light years distant in our own galaxy, or even further off outside of it; their emitted radiation (including radio waves as well as light) travels in exactly the same way as manmade radio signals would. Yes, objects cast "shadows", meaning it would be hard to send a signal to a star system that's located precisely behind another star/object from our point of view, but space is most empty and the portion of the starfield occluded by those objects will be comparatively quite small. Think of yourself standing outdoors on a clear day; if there were golf balls hovering in the air around you at various distances and altitudes, but each individual ball was separated from it's nearest neighboring ball by hundreds of feet, they wouldn't obscure your vision much.

The more real problem is signals that become excessively weak after traveling only a short distance (in interstellar terms) due to the inverse-square law. The solution is simply wattage and collimation, meaning tightly focused signals of high power. Lasers would be ideal because they aren't subject to the inverse-square law. For instance, hypothetical aliens in the Alpha Centauri system, the nearest star, would need an incredibly huge and sensitive radio telescope to pick up TV and radio broadcasts from Earth, since those signals are weak and poorly collimated. So aliens who are, say, 50 light years away probably aren't listening in on our TV programming from 1960. It's far too weak for them to pick up without fantastically sensitive instruments.

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Originally Posted by Syme

Lasers would be ideal because they aren't subject to the inverse-square law.

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Our best lasers don't stay collimated for an infinite amount of distance, and they certainly wouldn't stay collimated for astronomical distances.

At essentially infinite distance you'd have the same result as a dipolar emitter: something that looks basically like a plane wave (i.e. extremely large radius of curvature) with very low intensity.

Also, I have never heard of a "RASER." You, however, must be familiar with one of them, so please, tell us about it and its operation.

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Originally Posted by sycld

Our best lasers don't stay collimated for an infinite amount of distance, and they certainly wouldn't stay collimated for astronomical distances.

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Hrmm, maybe you know something I don't, but I was given to understand that collimation means the rays are close to parallel and thus the light doesn't spread out quickly as it travels. I guess the point at which light can be called "collimated" thus somewhat arbitrary? Anyhow, as I understand it, collimated light, whether it comes from a laser or anything else, will stay collimated forever unless the rays start hitting things that change the direction they're traveling. This doesn't mean it doesn't spread as it travels, but it remains collimated (i.e. having a small angle of divergence between individual rays) as it does so. My degree isn't a scientific one and I'm getting my understanding of this from layman-type sources though, so I could easily be wrong.

Anyhow, I wasn't saying that it's collimation which makes laser light uniqutely desirable for long-distance communication; both lasers and non-laser light sources can produce collimated light (and some lasers produce uncollimated light). What makes lasers uniquely desirable is that laser light is not subject to the inverse-square law, which non-laser light is subject to even if quite well collimated (I think).

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Originally Posted by sycld

Also, I have never heard of a "RASER." You, however, must be familiar with one of them, so please, tell us about it and its operation.

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Sure you have heard of them, they just aren't usually called "rasers". They are usually just called "RF masers". Many masers emit on radio wavelengths. Although I have read (can't recall where) that lasers for interstellar communication might actually be visible-light.

Syme we are talking about hypothetical infinite distances and hypothetical infinitely collimated signals. Just the same as we're turning miniscule probabilities into measurable fact because we're approaching limits, if you have infinite objects randomly distributed throughout infinite space, eventually there will be a finite distance after which all signal has been obscured.

Obviously there's no way to know where that line gets drawn or how many life forms could exist within it at this point, but its worth acknowledging that we aren't dealing with a truly unlimited system. There is a limit to which even an everlasting signal emitted from Earth can actually travel.

Though sure, today, I think we can all agree that the system constraint is the signal, not the interference.

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Originally Posted by Atmoscheer

Syme we are talking about hypothetical infinite distances and hypothetical infinitely collimated signals. Just the same as we're turning miniscule probabilities into measurable fact because we're approaching limits, if you have infinite objects randomly distributed throughout infinite space, eventually there will be a finite distance after which all signal has been obscured.

Obviously there's no way to know where that line gets drawn or how many life forms could exist within it at this point, but its worth acknowledging that we aren't dealing with a truly unlimited system. There is a limit to which even an everlasting signal emitted from Earth can actually travel.

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Right, I'm definitely not saying that electromagnetic radiation can travel for an infinite distance through anything less than a perfect vacuum (which even the intergalactic medium isn't). The SETI question is generally limited to the Milky Way galaxy anyhow, though. Which is the context in which I've been considering it ITT so far. And within that context, barring occultation of certain regions by the hub and dusty areas, you can consider the range of signals more or less unlimited (although again, the range at which they can be detected may be much less based on signal strength and receiver sensitivity).

EDIT: I've seen SETI FAQs claim that the Arecibo dish can detect narrowband signals from a range of many thousands of light years, though I can't remember what was said about power, if anything. Presumably gigawatts at least. Arecibo is not the best radio telescope on Earth, either. Some multi-dish arrays are more sensitive. At any rate, again, that's only detection range, which is not the same as propagation range.

Are these probabilities even relevant within the limits of the Milky Way?

I'd like to point out that most people like to assume that if intelligent life is indeed "out there", it is more technologically advanced than that on Earth. But what if all nearby civilizations are developing technologically at relatively the same pace? Or what if we are the smartest kids on the block?

Intelligent life could be quite common in our galaxy. After all, the ancient Egyptians were intelligent but they weren't transmitting radio signals.

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Originally Posted by Atmoscheer

Are these probabilities even relevant within the limits of the Milky Way?

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You mean the probabilities reflected in the terms of the Drake Equation?

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Originally Posted by KT.

I'd like to point out that most people like to assume that if intelligent life is indeed "out there", it is more technologically advanced than that on Earth. But what if all nearby civilizations are developing technologically at relatively the same pace? Or what if we are the smartest kids on the block?

Intelligent life could be quite common in our galaxy. After all, the ancient Egyptians were intelligent but they weren't transmitting radio signals.

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You are quite right. However, the Drake Equation specifically deals with civilizations that send detectable evidence of their presence into space (radio transmissions, for instance), not just with intelligent life or alien civilizations in general. From the point of view of the Drake Equation, a "civilization" doesn't count as such until it starts creating some sign that an observer in another star system could use to detect it's existence. But yes, if we built an interstellar space vehicle and flew around checking out planets that are home to tool-using aliens or alien civilizations, we would probably be just as likely (or more likely) to find civilizations that are less sophisticated than us, rather than more sophisticated. It really depends on how long a species survives on average after attaining our current level of technology, which of course we don't know. As Arthur C. Clarke famously said, we'll find "apes or angels" in most cases, rather than anything close to us in technological terms (and even that assumes that technological civilizations tend to develop into some transcendent state and then last for evolutionarily significant time scales; if this isn't the case, then it's just apes that are likely, rather than "apes or angels"). Hominids started using tools about 2.5 million years ago, but for more than 99% of the time since then, the tools didn't even involve metal. An alien spaceship visiting Earth at some random point during that 2.5 million years would most likely find stone-age savages, would have a quite slim chance (less than 1%) of finding bronze-age societies, an even slimmer chance of finding iron-age societies, and a vanishingly tiny chance of finding industrial societies. Even within the context of industrial societies, a couple centuries went by before we started building radio transmitters and receivers. Who knows how long we will keep it up. If we simply looked at planets that are home to life in general, without any stipulation of "tool-using" or "intelligent" or "civilizations", we would be stupendously unlikely to find even apes or angels, going by Earth's biological history. Just unintelligent animals (or whatever).

At any rate, yes it's true that tool-using life or intelligent life won't necessarily be more technically sophisticated than we are at the moment, but the Drake Equation takes that into account. It's an equation for calculating the number of communicating civilizations in the galaxy at a given time, not simply the number of intelligent or tool-using species.

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Originally Posted by Syme

Sure you have heard of them, they just aren't usually called "rasers". They are usually just called "RF masers". Many masers emit on radio wavelengths.

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Uh-huh. You might want to check the wavelength range of rf lasers and masers. And yeah, a true "Radio Amplification by Stimulated Emission of Radiation" device would be called a "RASER."

Sure, masers might emit what are technically radio waves, but they are just on the border between radio waves and microwaves. They aren't the sort of radio waves we're talking about here. It's not like radio waves are as narrow a band of wavelengths as visible light.

Okay, I just saw an experimental 128 km "laser" that's been made, but if that is really a radio-aser, then it is hardly ready for practical use.

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Although I have read (can't recall where) that lasers for interstellar communication might actually be visible-light.

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Well that's fine. The fact is that no laser, maser, or blank-aser can stay collimated for interstellar distances. If you can find one that can, I think you'd be a candidate for a Nobel Prize.



But since you're obviously expert on all this, I'll just leave this thread.

Did I say something to piss you off? I'd be happy to have your input here, you almost certainly know more about some of this stuff than I do. I explained my understanding of collimation, but also freely admitted that it's a layman's understanding and I could easily be wrong. It sounds like you are telling me that laser beams do in fact de-collimate over distances. Is this correct? I did not know that but certainly don't pretend that means it isn't true.

And yes, I know that "raser" would indeed be the proper name for an RF maser. Along with grasers and irasers and all that. But in practice you are much more likely to see an RF maser called just that, an RF maser, rather than a "raser" (as far as I can tell it's rare for anyone to actually use those frequency-range-specific laser terms). I don't know anything about the wavelength range of RF masers; if you do, please tell me. I said what I said because I do know that there are RF masers, distinguished from 'normal' masers because they emit in what's normally considered the true radio spectrum rather than microwaves, HF radio though it may be. You asked about rasers, I told you that. If you had meant "rasers operating below a certain frequency", you should have said so.

EDIT: Also, if you poke around on google a bit, you will find plenty of serious discussions about the advantages of using lasers for interstellar communication.
http://adsabs.harvard.edu/full/1992lbsa.conf..637S
http://www.coseti.org/ross_02.htm
http://oreilly.com/catalog/alien/chapter/ch07.html

In fact apparently there is a whole parallel SETI effort, called OSETI, focused on looking for optical communications from other star systems instead of radio signals. Again, the lasers considered are usually actually visible-light lasers, so the problems of making a raser that operates on long wavelengths is probably irrelevant.

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Originally Posted by KT.

I'd like to point out that most people like to assume that if intelligent life is indeed "out there", it is more technologically advanced than that on Earth. But what if all nearby civilizations are developing technologically at relatively the same pace? Or what if we are the smartest kids on the block?

Intelligent life could be quite common in our galaxy. After all, the ancient Egyptians were intelligent but they weren't transmitting radio signals.

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Well I'm not sure if this is what Syme was getting at because I'm sitting at a bar and I'm currently simple jacked about to bring home cougars, but...

While life has existed on earth for significantly longer than "intelligent life" has, the way that intelligent life has accellerated so quickly here would indicate that, were we to find a planet with life, the odds of having both intelligent life, and intelligent life capable of receiving communication are pretty much the same.

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Originally Posted by Atmoscheer

While life has existed on earth for significantly longer than "intelligent life" has, the way that intelligent life has accellerated so quickly here would indicate that, were we to find a planet with life, the odds of having both intelligent life, and intelligent life capable of receiving communication are pretty much the same.

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That's still quite an assumption. Behaviorally modern humans have existed between 200,000 years and 50,000 years (depending on which theory you agree with) while radio waves were only theorized and discovered about 200 years ago and mass radio broadcast only began in the 1920s.

Therefore about 99.9% to 99.6% of the time intelligent life has existed on this planet, it hasn't been able to send or receive radio transmissions.

Please divide 200,000 by 4.6 billion and then divide 200 by 4.6 billion and recognize that the orders of magnitude are meaningless at that point.

That still doesn't mean that the odds of finding "intelligent life" are nearly the same as finding "intelligent life capable of receiving communications". The former is still, based on human history thus far, about a thousand times more likely than the latter; going by that guide, for every thousand planets that are home to intelligent life, only one or a couple will be capable of receiving radio messages. That fact isn't changed by the fact that the chances of finding intelligent life at all, radio-receiving or otherwise, are probably minuscule if you show up at a random point during a planet's lifetime.

Of course, again, who knows how much longer we will last as a civilization capable of receiving radio signals. If we end up lasting many millions of years, then yeah, it will be more defensible to say that finding communications-receiving life is nearly as likely as finding intelligent life in general (again, based just on the example of human history which of course might not be anything like a representative norm).

I finally found 2 extremely relevant articles after digging through my Stumble history.

The first one, http://www.thesun.co.uk/sol/homepage...to-aliens.html, is about how satellites are cutting down/eliminating our radio waves that we send into space.
"Dr Frank Drake said transmission of signals into space was being cut by the digital revolution.

As a result, any little green men on other planets would not be able to detect them.

Dr Drake said Earth was surrounded by a 50 light year-wide "shell" of radiation from analogue TV, radio and radar transmissions. The signals had spread far enough to reach nearby star systems but are vanishing.

He explained that while old-style TV transmissions could generate one million watts, digital transmissions are much weaker."

This means, to me, that other advanced civilizations (who would likely use satellites in one form or another) may have reached this stage long ago. This would make them very hard to detect for us.

The second article is about the age of Earth compared to other planets, especially ones that would likely harbor life. "...that the median age of terrestrial planets in the Milky Way is about 1.8 gigayears greater than the age of the Earth and the Solar System, which means that the median age of technological civilizations should be greater than the age of human civilization by the same amount." Basically this article is about the technology gap this could have created and what that would mean. http://www.dailygalaxy.com/my_weblog...y-classic.html

Just thought I'd throw those out there. They seem like decent explanations for the silence.

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Originally Posted by mrbazoun

I finally found 2 extremely relevant articles after digging through my Stumble history.

The first one, http://www.thesun.co.uk/sol/homepage...to-aliens.html, is about how satellites are cutting down/eliminating our radio waves that we send into space.
"Dr Frank Drake said transmission of signals into space was being cut by the digital revolution.

As a result, any little green men on other planets would not be able to detect them.

Dr Drake said Earth was surrounded by a 50 light year-wide "shell" of radiation from analogue TV, radio and radar transmissions. The signals had spread far enough to reach nearby star systems but are vanishing.

He explained that while old-style TV transmissions could generate one million watts, digital transmissions are much weaker."

This means, to me, that other advanced civilizations (who would likely use satellites in one form or another) may have reached this stage long ago. This would make them very hard to detect for us.

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Well, it's a common supposition that the evidence by which we could notice extraterrestial civilizations (or they could notice us) would be the radio emissions created by normal wireless communications activity on the planet in question, but that's not really accurate. It's not normal megawatt-range radio and TV transmissions that will be probably provide the evidence, but much more powerful and probably more tightly focused transmissions. As mentioned above, this is because of the inverse-square law to which radio transmissions (and other forms of EM radiation) are subject. For instance, there's a SETI FAQ which says that even using an instrument as sensitive as Arecibo, normal TV/radio transmissions from Earth would only be detectable out to a range of 0.3 light years--less than a tenth of the way to Alpha Centauri, our nearest neighboring star. This means that at ten times that distance, 3 light years (still not all the way to Alpha Centauri), the signals would be a hundred times weaker still, and thus would require an instrument 100 times more sensitive than Arecibo to detect. At 30 light years, they would require an instrument ten thousand times more sensitive than Arecibo to detect. At 300 light years, the instrument would have to be a million times more sensitive than Arecibo. And so forth.

Put simply, TV and radio signals are probably much too weak to be detected in other star systems even before satellite telecommunications starts reducing the strength of the transmissions. Unless aliens are building really ridiculously huge radio telescopes, our TV and radio transmissions can't be detected at a range of even five light years, much less fifty. Any signal which can be detected in another star system would have to be much more powerful and directional. Terawatt-range transmission powers (1 million times stronger than a megawatt-range TV/radio signal) are often mentioned in discussions of interstellar radio communications.

Alien use of satellite technology is therefore not a compelling explanation for the Great Silence: Because even if they didn't use satellites, we would nevertheless be unable to detect the transmissions that their civilizations use for ordinary communications on and around their planet(s). And they wouldn't be able to detect ours unless they are building radio telescopes with 500-mile-wide dishes (which isn't strictly not out of the question, we must admit).

After learning about all the short-comings of sending messages via radio, "communication is impossible due to problems of scale" and "communication is impossible for technical reasons" both sound like the most reasonable explanations for the Great Silence. Although, optimistically and hopefully, the reason we haven't had any sort of contact yet is because humans have not been searching long enough. I think I read that scientists so far have only scanned a small portion of the sky for alien transmissions; If this is true, we still have a long ways to go before we can conclusively say that the sky is silent.

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Originally Posted by Syme

That still doesn't mean that the odds of finding "intelligent life" are nearly the same as finding "intelligent life capable of receiving communications". The former is still, based on human history thus far, about a thousand times more likely than the latter; going by that guide, for every thousand planets that are home to intelligent life, only one or a couple will be capable of receiving radio messages. That fact isn't changed by the fact that the chances of finding intelligent life at all, radio-receiving or otherwise, are probably minuscule if you show up at a random point during a planet's lifetime.

Of course, again, who knows how much longer we will last as a civilization capable of receiving radio signals. If we end up lasting many millions of years, then yeah, it will be more defensible to say that finding communications-receiving life is nearly as likely as finding intelligent life in general (again, based just on the example of human history which of course might not be anything like a representative norm).

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And when there are an infinite number of thousand-planet groups, the limit on the equations both approach infinity.

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Originally Posted by Syme

And they wouldn't be able to detect ours unless they are building radio telescopes with 500-mile-wide dishes (which isn't strictly not out of the question, we must admit).

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First, you wouldn't need a single 500 mile wide dish. You can construct arrays of much smaller radio telescopes that combined have effectively enormous collecting areas. In fact, LOFAR is going to have an effective diameter of more than 500 miles: http://en.wikipedia.org/wiki/Lofar


Also, if what you're saying is the case, then using a continuous laser won't buy you much either.

The advantage of using a continuous beam from a laser is to transmit a signal is that its energy subtends a relatively small solid angle compared to a dipole emitter, whose radiation is emitted in all directions.

However, this is at most three or four orders of magnitude increase in intensity, I believe...

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EDIT: Also, if you poke around on google a bit, you will find plenty of serious discussions about the advantages of using lasers for interstellar communication.
http://adsabs.harvard.edu/full/1992lbsa.conf..637S
http://www.coseti.org/ross_02.htm
http://oreilly.com/catalog/alien/chapter/ch07.html

In fact apparently there is a whole parallel SETI effort, called OSETI, focused on looking for optical communications from other star systems instead of radio signals. Again, the lasers considered are usually actually visible-light lasers, so the problems of making a raser that operates on long wavelengths is probably irrelevant.

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Well, first of all, the reason we use radio waves for long distance communication is because there's a window of wavelengths to which our atmosphere is transparent.

So I read one of your links:

http://oreilly.com/catalog/alien/chapter/ch07.html

and now I get why lasers could be useful in interstellar signalling. Even more important than focusing the beam energy into a small solid angle is that lasers can release their energy in very intense pulses. Consider that the "habitable zone" of most stars is considered to be within a radius of 2 million miles and a radius of 1 billion miles. This paper claims that we can make such an intense laser pulse over that gigantic annular region that the pulse can seem appear brighter than nearby stars when observed by an alien whose planet lies within this annulus. That seems really hard for me to believe, but that's what they're claiming....

The second link you provided:

http://articles.adsabs.harvard.edu/c...IF&classic=YES

is talking about making entire planets into enormous, extremely high intesity lasers using the CO2 gas in the atmospheres of either Mars or Venus as the gain medium for microwave lasing. A satellite orbiting the planet would be synchronized with the beam so that the beam bounces off of it at just the right moments to effectively form a resonant cavity for it. Its orbit and mirrors would have to be extremely tightly controlled. Very interesting, but purely speculative.

The third link looks like it's more about what frequency of carrier wave would be best to use and pulse width/period between pulses. I think it's more or less indirectly advocating a pulsed laser scheme to send interstellar messages.

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Originally Posted by sycld

First, you wouldn't need a single 500 mile wide dish. You can construct arrays of much smaller radio telescopes that combined have effectively enormous collecting areas. In fact, LOFAR is going to have an effective diameter of more than 500 miles: http://en.wikipedia.org/wiki/Lofar

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Yes I know, you'll note I have already mentioned multi-aperture (multi-dish array) radio telescopes earlier in this thread. I was being broad when I spoke of aliens constructing 500-mile-wide dishes. Obviously they would actually use distributed arrays like we do.

I don't really want this thread to become bogged down into the technical merits of laser vs. radio for interstellar communications. Both methods are possible (whether you believe it or not). What I'm interested in talking about in this thread is the "Great Silence" problem and the question of why, if life is common throughout the universe (which many people think it should be), we don't see any evidence of alien civilizations.

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Originally Posted by Atmoscheer

And when there are an infinite number of thousand-planet groups, the limit on the equations both approach infinity.

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I'm not quite sure what you're getting at. There aren't an infinite number of thousand-planet groups. Large, but not infinite. Again, using human history thus far as a model, intelligent life in general will be vastly more common than intelligent life capable of engaging in interstellar communications. For every alien civilization out there that's shooting radio waves into space and listening to radio waves from space, there are ~1,000 that aren't. How does that not put the lie to your claim that, if we were to find a planet that has life, the odds of finding communication-capable life there should be "pretty much the same" as finding intelligent life in general there?

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Originally Posted by Syme

Both methods are possible (whether you believe it or not).

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You obviously didn't even bother to read my post. I never said optical communication isn't possible or feasible, so don't say I did.

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Again, using human history thus far as a model, intelligent life in general will be vastly more common than intelligent life capable of engaging in interstellar communications

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That seems to be a wild claim to make. As a species, humans are only 200,000 years old. Agriculture is only 15,000 years old, and what we consider to be the history of civilization is only aroun 7 thousand years old. We have only had a conception of "interstellar communication" at all for a little over 100 years. As a species we are young, and as a highly organized, "civilized" species, we are even younger, so there is very little reason to believe that most civilizations would be as young as we are.

Also, we have no idea if humans developed civilization faster or slower than other intelligent species would, nor do we know how common it would be for intelligent species to develop civilization. Maybe the probability for an "intelligent" (or rather "sapient") species to produce civilization given enough time is unity, or maybe it is far less than that.

And we really don't know how long a civilization survives on average after it has any ability to achieve interstellar communication. Drake and his contemporaries estimated 10,000 years (rather arbitrarily), and so we, by comparison, are a barely nascent "interstellar" civilization, an arguably we haven't even entered this phase since we don't really have as of yet a firmly established protocol for interstellar communication.

We haven't been to mars, and we haven't discovered FTL travel.

/discussion

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Originally Posted by Syme

Well, it's a common supposition that the evidence by which we could notice extraterrestial civilizations (or they could notice us) would be the radio emissions created by normal wireless communications activity on the planet in question, but that's not really accurate. It's not normal megawatt-range radio and TV transmissions that will be probably provide the evidence, but much more powerful and probably more tightly focused transmissions. As mentioned above, this is because of the inverse-square law to which radio transmissions (and other forms of EM radiation) are subject. For instance, there's a SETI FAQ which says that even using an instrument as sensitive as Arecibo, normal TV/radio transmissions from Earth would only be detectable out to a range of 0.3 light years--less than a tenth of the way to Alpha Centauri, our nearest neighboring star. This means that at ten times that distance, 3 light years (still not all the way to Alpha Centauri), the signals would be a hundred times weaker still, and thus would require an instrument 100 times more sensitive than Arecibo to detect. At 30 light years, they would require an instrument ten thousand times more sensitive than Arecibo to detect. At 300 light years, the instrument would have to be a million times more sensitive than Arecibo. And so forth.

Put simply, TV and radio signals are probably much too weak to be detected in other star systems even before satellite telecommunications starts reducing the strength of the transmissions. Unless aliens are building really ridiculously huge radio telescopes, our TV and radio transmissions can't be detected at a range of even five light years, much less fifty. Any signal which can be detected in another star system would have to be much more powerful and directional. Terawatt-range transmission powers (1 million times stronger than a megawatt-range TV/radio signal) are often mentioned in discussions of interstellar radio communications.

Alien use of satellite technology is therefore not a compelling explanation for the Great Silence: Because even if they didn't use satellites, we would nevertheless be unable to detect the transmissions that their civilizations use for ordinary communications on and around their planet(s). And they wouldn't be able to detect ours unless they are building radio telescopes with 500-mile-wide dishes (which isn't strictly not out of the question, we must admit).

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Well Syme, you certainly seem to have thought this through. Your explanation makes a lot of sense, but I have a question, or four. How sensitive is the equipment we're using to detect radio transmissions? Can we detect the waves if they enter our solar system for sure? What about our galaxy? If not, then aliens would have to deliberately target us, correct? Depending on the sensitivity of our instruments, it could be that aliens have no interest in contacting us. That would certainly explain everything. Sorry if you have already discussed this, I don't really have time yet to read everything.

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Originally Posted by sycld

You obviously didn't even bother to read my post. I never said optical communication isn't possible or feasible, so don't say I did.

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Let's recap how this started: I made a comment that lasers would work well for interstellar communications, you have done nothing since then except snarkily try to harp on all the ways that this isn't true ("they won't stay collimated", "they would perform essentially the same as dipole emitters", "who ever heard of a raser" even though I didn't say anything about rasers and was talking about normal visible-light lasers, then "RF masers aren't the kind of rasers I was asking about because their radio emissions aren't long-wave enough", etc.).

If you aren't trying to claim that interstellar communications by laser isn't impossible/infeasible/pointless, then great, we agree! But then if that's the case, I also don't know why you've been posting the things you've been posting thus far. I'm content to drop it, though. Again, I didn't start this thread to hash out which specific method of interstellar communications is best.

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Originally Posted by sycld

That seems to be a wild claim to make. As a species, humans are only 200,000 years old. Agriculture is only 15,000 years old, and what we consider to be the history of civilization is only aroun 7 thousand years old. We have only had a conception of "interstellar communication" at all for a little over 100 years. As a species we are young, and as a highly organized, "civilized" species, we are even younger, so there is very little reason to believe that most civilizations would be as young as we are.

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Right, hence my earlier statement: "Of course, again, who knows how much longer we will last as a civilization capable of receiving radio signals."

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Originally Posted by sycld

Also, we have no idea if humans developed civilization faster or slower than other intelligent species would, nor do we know how common it would be for intelligent species to develop civilization. Maybe the probability for an "intelligent" (or rather "sapient") species to produce civilization given enough time is unity, or maybe it is far less than that.

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Right, hence my earlier statement: "Again, based just on the example of human history which of course might not be anything like a representative norm."

Quote:

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Originally Posted by sycld

And we really don't know how long a civilization survives on average after it has any ability to achieve interstellar communication. Drake and his contemporaries estimated 10,000 years (rather arbitrarily), and so we, by comparison, are a barely nascent "interstellar" civilization, an arguably we haven't even entered this phase since we don't really have as of yet a firmly established protocol for interstellar communication.

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Right, hence my earlier statement (again) that we really have no idea how much longer human civilization will last with the ability to communicate between stars.

I've never once used human civilization as a guideline to guess about other civilizations without also clearly qualifying those guesses with the points you have needlessly reiterated here. Speaking of not bothering to read posts....

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Originally Posted by mrbazoun

How sensitive is the equipment we're using to detect radio transmissions? Can we detect the waves if they enter our solar system for sure? What about our galaxy? If not, then aliens would have to deliberately target us, correct? Depending on the sensitivity of our instruments, it could be that aliens have no interest in contacting us. That would certainly explain everything. Sorry if you have already discussed this, I don't really have time yet to read everything.

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We can (of course) only detect a signal if it hits the dishes of our radio telescopes. Which means that, yes, alien civilizations in other star systems would most likely have to be deliberately squirting high-powered radio, laser, etc. transmissions at other star systems (including ours) in order for us to detect them. And yep, one of the explanations for the "Great Silence" is that maybe other civilizations exist but aren't interested in doing that sort of thing (perhaps for practical/strategic reasons; there is a fairly sound argument that an intelligent species revealing it's presence to other such species is inviting an attack aimed at exterminating them). Certainly we ourselves haven't done it very seriously, yet. So, a good point.

Actually that's quite an interesting side-discussion, which I might start a thread on later (or anyone else is welcome to of course) once I get through reading and maybe commenting on this thread: The question of what we do about the discovery of an alien species.

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Originally Posted by MrShrike

Actually that's quite an interesting side-discussion, which I might start a thread on later (or anyone else is welcome to of course) once I get through reading and maybe commenting on this thread: The question of what we do about the discovery of an alien species.

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Definitely an interesting question, I hope you do start such a thread and I'd be eager to participate.

Or perhaps its because they are so far away that by the time they get our signals the human race will have been extinct for years.

I realize someone else may have pointed this out, but i'm tired and i'm not reading every single post.