If aliens have visited the solar system, here's how to find clues they left

Searching for ET phoning home or tiny, microorganisms on other worlds or in our solar system, other than Earth is a common theme. However, so far, no necessary demonstration from nature shows the biological life anywhere other than here on Earth. At some point the continued testing and searching must give way to factual, verified evidence that supports the paradigm in use. At present, in my opinion, reports like this look like hand waving efforts, still waiting to be shown to be science vs. a belief system.
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Sure, lots of us believe life exists outside of Earth and it is just a matter of time before it is found. The paper summarizes the current approaches to locating it. We can find radio transmissions, we might find alien trash somewhere, we might find microbes on Europa. There are probably other avenues to explore.
No amount of failure can prove aliens don't exist. Can't prove a negative.
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"No amount of failure can prove aliens don't exist. Can't prove a negative."

This view of science allows unlimited failed test to be conducted and the paradigm still held as science vs. a belief system yet to be shown true in nature. I prefer the standard where observations using nature must demonstrate that the paradigm is true. The same standard and requirement the heliocentric solar system astronomers were required to meet. Until this takes place in science, the paradigm presented continues to show failed test after failed test in nature, that includes observing abiogenesis taking place in nature today.
That is just the way logic works. And, we don't stop looking for something that we think probably exists, just because it has not already been proven to exist.

Neither abiogenesis not technological life elsewhere in the universe has had enough "looking" to seriously diminish the expectation.

Even on Earth, abiogenesis could be repeatedly starting, and just getting consumed by already present life forms before anybody can distinguish that it is happening. We don't really know what conditions it requires or how long it takes. We do know that it seems to have occurred on Earth when there was virtually no oxygen in the atmosphere and the water chemistry was probably much different. Also, no UV protection by an ozone layer. Did it start in a shallow surface pool bathed in UV light, or a deep ocean vent with no light at all?

The search for signs of life on Mars should provide us with some real advancement in knowledge. Did it ever exist there? Does it still? If neither, then we can lower our expectations for finding it elsewhere, but nowhere near to zero probability.

Likewise, if no sign of present or past life is discovered on Mars, then the probability of life existing elsewhere and evolving technological capabilities is going to be estimated to have a much lower probability.

At some point, if no life, or some life but no technological species are discovered within something like 100 light years from Earth, we will probably decide to not look so hard for it in the future.

But, at this point, I am thinking there is a high probability of finding some sign of life on Mars, but still a very low probability of finding a technological species in any of the star systems in our local part of our galaxy.
It is "virtually certain" there is life outside the Earth.
When we look at a sample of 1 from a population of 10^18 and it shows something we are justified in saying it is "virtually certain" there is at least one other.
When we actually find it then we can say it is "certain".
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Personally, I don't care if space aliens/life exists. My concern is how much of my tax money will be devoted to the looking task on a seemingly endless basis? My other concern is: if found would such life forms be ruthlessly exploited for all of those H. Sapiens reasons replete through History?
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I can understand sam85geo comments in post #7 about tax money spent on the search to show biological life exists or existed in the universe other than here on Earth. It does seem an endless search. Consider the geocentric vs. heliocentric solar system science. Tycho Brahe attempted to refute Copernicus because he understood the science showed Mars at opposition could at times, come closer to Earth than the Sun. This is something fully observable and verifiable compared to comments like "When we look at a sample of 1 from a population of 10^18 and it shows something we are justified in saying it is "virtually certain" there is at least one other.", post #6 or even the foundation belief, abiogenesis must be true despite zero observations from nature showing abiogenesis is true. Consider what Tycho Brahe did when confronting the heliocentric solar system science.

"Historians of astronomy have generally assumed that the Ptolemaic and Copernican systems give equivalent predictions of planetary positions, but Tycho Brahe knew that in the Ptolemaic arrangement Mars' distance was always greater than the sun's, whereas in the Copernican system Mars at opposition approached to half the sun's distance. Because Tycho accepted the traditional solar distance scale, 20 times too small, he expected to measure a Martian diurnal parallax of 4.5' at opposition if the Copernican system was true. (In reality the horizontal parallax was too small to measure by naked-eye observations.) Hence, during the golden decade of the 1580s at Hven, Tycho undertook a major campaign to find Mars' parallax...."

TYCHO Brahe's Copernican Campaign, TYCHO Brahe's Copernican Campaign - NASA/ADS (harvard.edu)

The paradigm claiming life is out there somewhere beside what we observe in nature here on Earth, and we will eventually prove it is true in science is tantamount to hand waving. Nothing like the specifics we see in the geocentric vs. heliocentric debates, this includes the phases of Venus too for testing.

If I use the 10^18 value and it cost 1$ per test for each of 10^18 tests to show life is somewhere other than on Earth, that will get very expensive, perhaps $10^18 or more :)
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Rod, with all due respect, that $10^18 is the type of hand-waving that you are trying to criticize.

Before we get into how much tax money is going into various sorts of efforts that might find extraterrestrial life or extraterrestrial technology, we do need to ask what else those efforts might be of value to.

For instance, going to Mars: Is that only about finding out if life ever occurred on Mars? Or, does it provide a deeper understanding of many factors that have importance to how we behave here on Earth? And does the technology developed in the process have other benefits here on Earth?

One could argue that any attempt to discover anything about distant stars or the beginning of the universe is "wasted money", and demand that politicians show what benefits Hubble, Webb, etc. have brought to feeding and housing our burgeoning human population here on Earth.

But, what that question misses is the "spin off" effect of how addressing questions like that advances both our understanding and our technological capabilities in ways that definitely benefit our species. Without the technology that our species has developed by trying to better understand our observations, we would still be hunter-gatherers and unable to support anything like the population of about 8 billion humans now on Earth.
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My observation here. At 10^18-dollar cost for searching for life somewhere out there, even if we never find any, we should see plenty of "spin off" benefits then :)
Thanks for the insights. Personally, I'm grimly cynical. The "hope" attributed to extra-terrestrials/technology seems to me to be a form of Deus Ex Machina. I think that either humanity, (H. Sapiens or an evolved species), will have to leave Earth to survive/avoid extinction or alternatively, significantly reduce the human population, and/or resource consumption/re-use to survive on Earth. It just may cost too much in money, resources, political and biological demands for either alternative to be achieved. I'll admit factoring in today's politics, especially the financial implications, only heightens my markedly less than cheery outlook.
I think it is pretty clear that it is not an either/or proposition that humans could survive by leaving earth or reducing population on Earth.

First, it would not be possible to use spacefaring technology to take people off Earth even as fast as the population on Earth is increasing, now.

Second, without some sort of overall recognition that human populations create consequences for the ecosystems and climate systems in which we live, we should expect any colonies of humans elsewhere to also increase their own populations rapidly into creating negative consequences wherever humans call "home".

We already know that we are our own biggest problem. But, any attempts to reduce populations run into intraspecies conflicts and the evolved sense that security comes from greater numbers of "us" to fend-off the aggressions by "them".

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