Fermi Paradox solved?

[Warren Platts]

This SETI spreadsheet tells me that a 1 GW hydrogen-line signal [10HZ bandwidth] can be detected at almost 1,000 ly with a 100-meter dish at both ends. Does not seem unachievable by a type II civilization, the kind I expect will be transmitting or receiving [since the probability that we´re at the same technological level is negligible]. In fact it is doable by Earthlings in the present time.

You must be talking about directional broadcasts. That changes the whole equation, of course, but such a set up takes two to tango. That's the problem. How do you find each other in the first place?

Since we humans only have access to a data point of ONE technological species, we may as well use that data point. Historically, we did, in fact, send the Arecibo message. It was NOT sent to a destination a mere 25 or 500 light years away. It WAS sent to a destination 25,000 light years away, and part of the reason was fear of a hostile alien response.

Was this a "rational" thing to do? Maybe, maybe not. But it's something that actually happened, which counts for more than mere speculation.

Good points. I had never heard that fear of an alien invasion was the reason for aiming at a globular cluster, though. I thought it was to maximize the number of stars that would be hit by the beam (although globular clusters would be about the last place to look for stable planetary systems, or so I would think.) You wouldn't have a reference for that would you?

How do you know which planets to test for the oxygen signature?

When they get a proper interferometer going, the first step will be to just go through the entire catalogue of known planets and see what you can see.

I like your point about the likelihood of space travel possibly rendering the whole SETI debate moot. Maybe the aliens are already here. Or maybe we are the first. But I respect your rocket science skills. Is it possible to build interstellar craft without fusion or antimatter that can travel between stars in a reasonable time?

[timb]

You must be talking about directional broadcasts. That changes the whole equation, of course, but such a set up takes two to tango. That's the problem. How do you find each other in the first place?
Good points. I had never heard that fear of an alien invasion was the reason for aiming at a globular cluster, though. I thought it was to maximize the number of stars that would be hit by the beam (although globular clusters would be about the last place to look for stable planetary systems, or so I would think.) You wouldn't have a reference for that would you?

It was a publicity stunt ("symbolic" is the polite word). M13 won't even be there when the (incredibly weak) signal arrives. I assume Isaac was just confabulating when he claimed they aimed at M13 out of fear of retaliation.

[eburacum45]

It's kind of tautologous to say that we will only recognize the life that we can recognize. I'm doubtful about the existence of these "different biochemistries". AFAIK nothing comes close to carbon chain compounds for complexity.

That's why I think an important and achievable early step in identifying worlds likely to bear life is the identification of terrestrial planets with optically thin atmospheres and surface water.

We know for certain that a biosphere can persist without an atmosphere rich in free oxygen; our own planet had one for billions of years. Similarly we know that photosynthesis can occur using pigments other than chlorophyll.
From here
http://www.livescience.com/environme...ple_earth.html
""I think when most people think about remote sensing, they're focused on chlorophyll-based life," DasSarma said. "It may be that is the more prominent one, but if you happen to see a planet that is at this early stage of evolution, and you're looking for chlorophyll, you might miss it because you're looking at the wrong wavelength."

So we don't even have to look beyond the Earth to see other possibilities. The great majority of life-bearing planets might be anaerobic retinal-using photosynthesisers, especially if a Great Filter in evolutionary development prevents the emergence of oxygen based biospheres.

On the other hand, planets with a thin atmosphere, surface water and no life might be common, so we might get a lot of false positives from Williams' method.

[timb]

We know for certain that a biosphere can persist without an atmosphere rich in free oxygen; our own planet had one for billions of years. Similarly we know that photosynthesis can occur using pigments other than chlorophyll.
From here
http://www.livescience.com/environme...ple_earth.html
""I think when most people think about remote sensing, they're focused on chlorophyll-based life," DasSarma said. "It may be that is the more prominent one, but if you happen to see a planet that is at this early stage of evolution, and you're looking for chlorophyll, you might miss it because you're looking at the wrong wavelength."

So we don't even have to look beyond the Earth to see other possibilities. The great majority of life-bearing planets might be anaerobic retinal-using photosynthesisers, especially if a Great Filter in evolutionary development prevents the emergence of oxygen based biospheres.

So you don't think looking for oxygen is worthwhile? what do you propose as an alternative?

On the other hand, planets with a thin atmosphere, surface water and no life might be common, so we might get a lot of false positives from Williams' method.

I don't recall anyone proposing it as a test for life, just as an indicator that one should keep looking.

[eburacum45]

A fairly wide range of biomarkers might be useful, falling into two camps; firstly, atmospheric gases that are highly reactive, so they are unlikely to persist in the presence of a planetary crust to react with. This group might include oxygen and methane, already considered biomarkers, but also less likely reactive gases such as hydrogen sulphide, carbon monoxide, chlorine, fluorine, any of which might be produced as byproducts of biochemistry. Basically anything that shouldn't be there because it is too reactive. For each of these possible biomarkers there may be a range of possible (abiotic) geological or astrophysical processes which might have produced them instead, so it is likely that no biomarker is definitive.

The second class of biomarker would rely on the reflective characteristics of the planet, particularly the biosphere, as in the red edge reflectivity, or any equivalent tints produced by retinal photosynthesis or similar. Would these reflection characteristics be definitive? Only for life as we know it, if that. And the reflection and polarisation of light from water bodies would also be a possible biomarker, but not an exclusive one.

The best way to detect life is to go there. But the range of telescopes will always exceed the range of physical probes; so telecopic detection would become increasingly important, even if it were possible to send fly-by probes to check the candidate planets out at close range. Information from fly-by probes of nearby planetary systems would complement and refine the process of detecting life at long range by telescopy, not replace it.

[Disinfo Agent]

Perhaps David Bowie got it right:

There's a starman waiting in the sky
He'd like to come and meet us
But he thinks he'd blow our minds

[timb]

A fairly wide range of biomarkers might be useful, falling into two camps; firstly, atmospheric gases that are highly reactive, so they are unlikely to persist in the presence of a planetary crust to react with. This group might include oxygen and methane, already considered biomarkers, but also less likely reactive gases such as hydrogen sulphide, carbon monoxide, chlorine, fluorine, any of which might be produced as byproducts of biochemistry. Basically anything that shouldn't be there because it is too reactive. For each of these possible biomarkers there may be a range of possible (abiotic) geological or astrophysical processes which might have produced them instead, so it is likely that no biomarker is definitive.

The second class of biomarker would rely on the reflective characteristics of the planet, particularly the biosphere, as in the red edge reflectivity, or any equivalent tints produced by retinal photosynthesis or similar. Would these reflection characteristics be definitive? Only for life as we know it, if that. And the reflection and polarisation of light from water bodies would also be a possible biomarker, but not an exclusive one.

The best way to detect life is to go there. But the range of telescopes will always exceed the range of physical probes; so telecopic detection would become increasingly important, even if it were possible to send fly-by probes to check the candidate planets out at close range. Information from fly-by probes of nearby planetary systems would complement and refine the process of detecting life at long range by telescopy, not replace it.

Your suggestions sound a lot like mine, except I don't think going there will happen soon enough, if ever, to be an option worth discussing. Making sense of remote imagery without any ground truth is difficult, and interpretations are always subject to ambiguity, but that's what we're stuck with.

[eburacum45]

Here's a chapter on the subject from The Limits of Organic Life in Planetary Environments
http://books.nap.edu/openbook.php?re...=11919&page=80
Chirality- that's a good one (unless, for some reason some biospheres do not have any preference for handed molecules).

[zhamid]

Originally Posted by IsaacKuo
I actually see it the other way. We sent the Arecibo message to a destination some 25,000 light years away partly BECAUSE it's so far away. If the aliens hear our message and decide to attack us because of it, we have some 50,000 years to improve technology and colonize star systems to prepare for it.

If we discovered an Earth twin 25ly away, I think we'd stay silent and got a hurry on developing space colonization and space warships. We wouldn't want to draw attention to ourselves before we were prepared for the consequences.

But 500ly away? I could see someone sending a message and then telling the world--"People of Earth--the aliens have been alerted to our presence. We have one millenium to prepare for their possibly genocidal response." That would put a kick start into space technology developmen

Arent you making the assumption that aliens have the same lifespan as we do? If they lived for thousands of years then they'd totally want to communicate with species 1000 ly away. That said, I dont think we have anything to fear from aliens 25 ly away. Even if we ignore the muniscule probability of having advanced war-mongering aliens just at the right time in their civlisation's evolution at the right distance, we still can't ignore physics. They can't travel anywhere close to the speed of light (assuming they have mass and their space ships have mass), so unless they've mastered the art of transfering their consciences to a computer and then restoring it later, they aint going anywhere.

[sanman]

Maybe after a certain amount of technological advancement, every intelligent civilization converts itself to machine intelligence. That's a lot easier and more feasible than figuring out some way to surpass lightspeed, etc. Once you're in a machine intelligence form, then your pickiness about finding an "habitable world" could radically diminish. Because your machine form would be so much more robust and adaptable, that you could make almost any kind of place your home. You wouldn't even need a planet -- you and your pals could float around in open space, as long as you can harvest enough nearby solar energy, or cosmic rays, or free-floating hydrogen to fuse.

Once you're happy in your machine form, then perhaps the specialness of organically habitable worlds becomes only marginally more interesting than any other kind of world. Maybe you even lose interest altogether, since "interest" and "curiosity" are merely just electrochemical states of mind, which might not predominate in a machine mind.

[ichatfilipina]

The Fermi Paradox is the apparent contradiction between the high probability extraterrestrial civilizations' existence and the lack of contact with such civilizations.

How to detect other civilizations?
Since the presence of planets around other stars was demonstrated, that natural processes under adequate conditions appear to be able to generate life and that life generally evolves in the direction of an increasing complexity, it seems natural to imagine that there is are certain number of intelligent civilizations around other stars.

If such extraterrestrial civilizations do exist, what is the best way of discovering them or to communicate with them?

To send a spaceship or a probe would require a gigantic time and energy due to the enormous distances. In fact, the nearest star to the solar system, Proxima Centauri, is at a distance of 4,3 light-years, that is to say 41 thousands of billion kilometers. Getting there would take 40,000 years for a probe equipped with the current technology. Even for a very advanced civilization, the energy cost of a faster travel would be enormous. Moreover, as we do not know where to search, sending multitude probes would be necessary, which would multiply the costs.
http://www.fermisparadox.com/terrest...net-finder.jpg
NASA's Terrestrial Planet Finder

Radioastronomy
The best solution, in the current state of our knowledge, consists in resorting to electromagnetic waves in the radio field. The goal being to intercept a message which would be intentionally sent to us by another civilization, that is to say to intercept some of their internal transmissions, just like other civilizations could theorically intercept our radio, televiosion or satellite communications.
http://www.fermisparadox.com/arecibo-radiotelescope.jpg

[sanman]

Fine but what if there is pre-Radio-communicating life out there? How would we detect that?

[Sticks]

ichatfilipina, please take time to read the forum rules

Especially the part regarding hotlinking of images and I quote

8. Hotlinking

Try to avoid putting in links to images directly from someone else's website. This can add a lot to their bandwidth, and then the host has to pay for it. In other words, if you see an image you like on an astronomy site, put in a link to that site, but don't use the [IMG] tags so the image loads into your post from their site directly. If you really want an image in your post, put it in a public site someplace and link to it from there. One exception would be from sites like NASA or universities, where bandwidth is not such an issue. If you own the image, then you can upload it to the board yourself.

I have converted your images to standard links

[chrissy]

I have banned ichatfilipina, due to many reasons, one for posting copy right material and it's representing a spam site which has hit us before . I am closing this thread also, if other members wishing for it to be reopened please feel free to contact a moderator and give reasons why it should be reopened again.