If they are out there why don't we hear them?

[nottheone]

Actually, that is the problem.

We can not detect random signals from more than a few hundred light years away.
The cost of sending focused signals is very expensive and even those can not be detected more than a few thousand light years away.

Earth has sent only 1 or 2 focused signals and they each lasted less than a 1 hour.
Our Galaxy is 100,000 light years in diameter. So, at best we can only hope to detect
a signal within an small part of that.

I tend to agree.

 

[nottheone]

I wouldn't make the assumption that a moon (of our moons size or any size for that matter) is required for life to develope. It would seem that way when we look at life on our planet, but that is only because our life developed and evolved with our moon. If there was no moon, it would have evolved differently, but it still would have evolved.

The moon isn't required for life to develop. What it probably did was take a few hits for us. We might not be here today if some of the things that hit it were heading towards us and that is likely. It may save us in the future too. But we had better get off our *** because it is a mighty small shield and sooner or later we will be hit again.

 

[doublemint]

They do not want to interfere with a pre-warped civilization :tongue2:

 

[mplayer]

The moon isn't required for life to develop. What it probably did was take a few hits for us. We might not be here today if some of the things that hit it were heading towards us and that is likely. It may save us in the future too. But we had better get off our *** because it is a mighty small shield and sooner or later we will be hit again.

The moon could just as easily attract more material towards the earth-moon system or even speed it up before it hits Earth, causing more destruction. One argument that I've heard for the moon being vital to certain kinds of life is that its orbit around Earth stabilizes Earth's rotation. Apparently without it, our axial tilt would fluctuate wildly and could cause some pretty extreme variation in seasons and weather. I'm not sure if this is true, though.

 

[subliminal]

Nuclear Physicist Stanton Friedmans thoughts on SETI

http://www.v-j-enterprises.com/sfufovsseti.html

 

[mplayer]

Take a look at this article(.pdf):

Brin, Glen David. 1983. http://www.brin-l.com/downloads/silence.pdf" , Quarterly Journal of Royal Astronomical Society, 24: 283-309.

Summary

Recent discussions concerning the likelihood of encountering intelligent extraterrestrial technological civilizations have run into an apparent paradox. If, as many now contend, interstellar exploration and settlement is possible at non-relativistic speeds, then reasonable calculations suggest that space-faring species, or their machine surrogates, should pervade the galaxy. The apparent absence of evidence for extraterrestrial civilizations, herein called 'the Great Silence' places severe burdens on present models.

Many of the current difficulties are due to inadequate exploration of the parameters of the problem. A review of the topic shows that present approaches may be simplistic.

Conclusion

The quandary of the Great Silence gives the infant study of xenology its first traumatic struggle, between those who seek optimistic excuses for the apparent absence of sentient neighbors and those who enthusiastically accept the Silence as evidence for humanity's isolation in an open frontier.

Both approaches suffer greatly from personal bias, and from lack of detailed comparative study. In this article we have attempted to deal with a subject that, for all its great importance, is almost ghostly in its intangibility. We have broken the subject into it logical elements and attempted a morphological discussion of the possibilities. Table I represents an overview of many of the ideas discussed here and their respective effects on the factors in equations (1-3). It is up to the interested reader to look up the references cited and come to his/her own conclusions.

Some of the branch lines discussed here serve the optimists, while others seem pessimistic to an unprecedented degree. We have laid out only the outline of a full analysis of the problem. Further work should consider every experimental test that could be applied to this fundamental question of humanity's uniqueness.

This survey demonstrates that the Universe has many more ways to be nasty that previously discussed. Indeed the only hypotheses proposed which appear to be wholly consistent with the observation and with non-exclusivity - 'Deadly Probes' and 'Ecological Holocaust' - are depressing to consider.

Still, while the author does not accept that elder species will necessarily be wiser that contemporary humanity, such noble races might have appeared. If such a culture lived long, and retained much of the vigor of youth, it might have instilled a tradition of respect for the hidden potential of life in subsequent space-faring species.

It might turn out that the Great Silence is like that of a child's nursery, wherein adults speak softly, lest they disturb the infant's extravagant and colorful time of dreaming.

I typed out the summary and conclusion for everyone to read to hopefully generate interest in reading the full article linked above, its about 25 pages long, fairly in-depth. It is a speculative paper of course, but it is very interesting.

 

[BoomBoom]

The moon isn't required for life to develop. What it probably did was take a few hits for us. We might not be here today if some of the things that hit it were heading towards us and that is likely. It may save us in the future too. But we had better get off our *** because it is a mighty small shield and sooner or later we will be hit again.

Yes, it is a mighty small shield. If something was on a trajectory to hit Earth, it would be a small chance that it would hit the moon instead. As far as shields go, Jupiter is the best one we have...and it would seem these types of planets are rather common within solar systems.

 

[mgb_phys]

If we intercept a communication signal in the electromagnetic spectrum, no matter how different they are it will not look like a natural random phenomena. No matter how they communicate it will have to contain intelligent information by it's definition as a communication signal. Look at it the other way around. If this hypothetical species of yours intercepted an FM signal from us they wouldn't understand it but they would know it wasn't a natural signal.

Not necessarily, compressed digital data should ideally be completely random. From an information theory point of view any recognisable intelligent information in a signal is wasting bandwidth.
Efficient wide band spread-spectrum communications uses very little power and so wouldn't go across a galaxy in the same was a megawatt TV signal.

 

[nottheone]

The moon could just as easily attract more material towards the earth-moon system or even speed it up before it hits Earth, causing more destruction. One argument that I've heard for the moon being vital to certain kinds of life is that its orbit around Earth stabilizes Earth's rotation. Apparently without it, our axial tilt would fluctuate wildly and could cause some pretty extreme variation in seasons and weather. I'm not sure if this is true, though.

I base my comment on the apparent fact that the far side of the moon is much more heavily cratered than the near side and a little common sense. The moons gravity is low so it isn't going to be dragging too much towards us or itself. But a significant percentage of the things that caused craters on the moon's far side would have been heading towards us if they hadn't hit the moon's backside. They were probably already heading towards it, not deviated towards it.

 

[mplayer]

I base my comment on the apparent fact that the far side of the moon is much more heavily cratered than the near side and a little common sense.

A large majority of the craters on the moon's surface are from a period early in the solar system's formation called the Late Heavy Bombardment. Assuming equal age of a surface, crater density and distribution should be relatively uniform. This is one method used to date the surfaces of terrestrial-like planets. When there are differences in impact crater distribution, this implies a difference in the ages of the associated surfaces. The higher the concentration of impacts, the older the surface. This indicates nothing about a planetary body acting as a shield. The far side of the moon appears more heavily cratered due to the fact that the surfaces on the far side are in general older than the surfaces on the near side. The only region of the moon that would be expected to accrue higher than average impact events would be its western hemisphere, as viewed from Earth. This is in the direction of its orbit around the earth.

It is known that the crust on the far side of the moon is thicker than on the near side. Early in the moon’s history, but after the Late Heavy Bombardment, basaltic lava flows were heavier on the near side of the moon than on the far side, presumably due to the 40km difference in crust thickness. Approximately 31% of the surface on the near side is covered with these flows, compared to 2.5% of the surface of the far side.

http://csep10.phys.utk.edu/astr161/lect/moon/luna.gif
highlands – older surface, higher crater density, covers 97.5% of far side surface
maria – result of lava flows into lower elevations, younger surface, more abundant on near-side

But a significant percentage of the things that caused craters on the moon's far side would have been heading towards us if they hadn't hit the moon's backside.

Really? Think this spec blocks many things from hitting Earth? Size and distance to scale.

http://www.thenakedscientists.com/forum/index.php?PHPSESSID=0e993780182228e40cd1ab4c86ddd8ff&action=dlattach;topic=20637.0;attach=7122;image

Well, back to the topic, we could maybe build some radio antennas on the far side of the moon to reduce interference from earth, and listen for aliens there

 

[Borek]

As far as I remember I have read (somewhere back in seventies) that Earth was then one of the strongest radio sources in this part of the galaxy. Technology has changed, so it can be no longer true, still, even if we got much quieter, Earth - as a source - will have an interesting characteristics, that won't look natural. I will gladly read your opinions about:

Spectrum - whatever we emit now, does it look like a natural source? We use different bands for different types of communication, won't that mean that our emission has some fancy fingerprint? While expecting the same from alliens may be asking for too much, assuming they are doing everything to interfere with their own transmissions doesn't look more convincing

Periodicity - radio sources are not evenly spread on the Earth surface, they are concentrated in places were population & technology are concentrated. Thus places like Japan are much noisier than places like Atlantic. It requires more then 24 hours of observation to find this periodicty, but then it must look artificial - or at least hard to explain. Again - assumption that allien population is not spread evenly on the surface of their planet doesn't look very unlikely. Question is - have we tried to observe sky in periods long enough to find such signals?

 

[nottheone]

...

Really? Think this spec blocks many things from hitting Earth? Size and distance to scale.

If it hit the far side of the moon it certainly wasn't headed AWAY from us and if the moon wasn't there, the trajectory for many of those things would have brought them into our gravity field. I didn't say it blocked a lot of what was heading this way, I said that what hit it's far side was heading this way, big difference. And there are some really large craters on the far side too. Any one of which could have caused an extinction after the last one that did happen here which could have taken out whatever we are descended from.

 

[Borek]

If it hit the far side of the moon it certainly wasn't headed AWAY from us and if the moon wasn't there, the trajectory for many of those things would have brought them into our gravity field.

Looking at solid angles only about 0.02% of those things would be stopped by Moon instead of hitting Earth.

Unless my math is faulty.

 

[Karl G.]

As a possible answer to the question why we can't hear them... I remember seeing somewhere that all radio waves emmited froms Earth's TVs, radios, etc, become impossible to comprehend after traveling 2 ly. Maybe this has something to do with it?

 

[Borek]

Doesn't matter - we are still visible as a strong radio source, energy emitted doesn't dissappear.