Thread: "The Great Silence"

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.

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.

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.

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.

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?

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.

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.

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).

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.

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.