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When scientists talk about the probability of us being the only intelligent species in the universe, they always say that "we should see the galaxy filled with life". I never really understood how they meant that.

Let's assume we're observing Earth from, let's say, 1 light year away. Is there a way to tell that life exists here and it is indeed intelligent? I mean how would you tell if an alien civilization is having 1000+ settlements in multiple planetary systems? What would be the symptoms?

They'd need a huge spaceship crossing in front of a star for us to notice them or something.

Is there a proper explanation on this?

Thanks

DrSheldon
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user35472
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  • When a huge object is crossing in front of a star, how do you know it is an alien psaceship and no natural body? – Uwe Apr 09 '20 at 13:17
  • It seems you're mixing up two different things. When some scientists talk about life being all over the galaxy, they're not talking about intelligent life. Some scientists still make a reasonable (though not indisputable) case that there should be lots of intelligent life out there too, but that "lots" is a whole lot sparser than they suppose life in general would be. – called2voyage Apr 09 '20 at 13:32
  • Most plausible designs for interstellar spaceships involve absolutely massive amounts of power -- "setting off a nuke every second, for hours on end" is on the low end for these types of schemes. Such spacecraft can potentially be detected over interstellar distances. – ikrase Apr 10 '20 at 01:58

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The most obvious evidence of life on Earth is probably spectroscopic -- there are combinations of gases in the atmosphere which are very hard to explain any other way. That could be observed from 1 light year with modern scientific instruments.

The most obvious intelligence of intelligent life is probably our radio (and TV and especially radar) broadcasts, which could be easily picked out of the background from 1 light year with a radio telescope.

In terms of the wider galaxy though, what we would really expect is that at least a few civilisations have developed the means, and inclination to either flood the galaxy with self-replicating slower-than-light robot probes (which would be expected to take no more than a few million years once you have built one) or to be making serious efforts to send messages to every star in the galaxy by some form of electromagnetic radiation. Either of these would presumably be very detectable, and yet we see neither.

Steve Linton
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  • So if I understand you correctly, it is basically the lack of electromagnetic radiation (that is different from generic background radiation). There could be intelligent civilizations on those planets which have the right ingredients in the air but they are only in 1700 without TVs and radars and such, and we couldn't tell that they are there in the first place, right? Is it a rule that you have to use radios if you are an intelligent, space-traveling, colonizing species? What if there are billions of these robots on billions on planets already that we don't see? – user35472 Apr 09 '20 at 14:39
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The 'galaxy teeming with life' statement generally starts with the Drake Equation looking at the number of observed stars, the number of stars with observed planets, the number of planets that could have earth like life (liquid water, enough light but not to much radiation) and guesses for how long life takes to develop -- and ends with the Fermi Paradox: "... but where is everybody?"

Simple life on earth appears to have developed reasonably quickly once conditions on proto earth allowed, and the sun is relatively young, which suggests that basic life probably has existed on many many planets since before our solar system existed.

A key point here is the difference between basic single cell life, tool using life(us) and life capable of crossing between stars or building structures we could see. Therefore current observations suggest that planets with slime molds are common, planets that have produced life capable of cheap faster than light travel is zero and planets with life somewhat like us is somewhere between the two. Many commentators are not explicit which case they are referring to.

In terms of remote detection of life, one plausible technique is looking at the spectra of planets looking for chemistry suggestive of life. This is beyond the resolution of current generation telescopes but is mathematically possible at least for nearby stars without implausible scaling up, and the questions one light year from earth is much closer than the nearest star (4.3 ly) so under ideal conditions (transit in front of the sun) current very large telescopes could probably identify earth as having liquid water, and probably the presence of on oxygen in the atmosphere, which would suggest some form of plant life going on. Detecting "tool-using life" is probably not possible at one light year unless something truly unnatural has been done to the atmosphere.

amI
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GremlinWranger
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Humans are currently a Kardashev level 0 civilization, according to the Kardashev scale. In a nutshell, this means that the total energy consumption of our species is less than is possible on planet Earth. A level 1 civilization is able to harness the power of an entire planet, a level two civilization harnesses the power of an entire star, and a level three civilization harnesses the power of an entire galaxy. An alien species with interstellar capabilities would at least be approaching a level 2 civilization, at least as long as our modern interpretation of physics is correct.

If there are any intelligent aliens, it is likely that they are either more technologically advanced than us or far inferior to us (cavemen essentially): our "modern" history is only like a couple centuries old, which is less then a blip on cosmological timescales. If there are any alien civilizations, we would expect them to be at least level 2, if not level 3 and those types of civilizations would be easily detectable by modern day telescopes, in the from of dyson spheres or dyson swarms. These megastructures should be quite obvious to our infrared telescopes, yet we don't see any. Therefore, the fermi paradox. Galaxy-spanning advanced alien civilizations should be easily detectable with modern telescopes due to their presumed massive energy consumption, yet we don't see any.

Dragongeek
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    "If there are any alien civilizations, we would expect them to be at least level 2, if not level 3" Why? Because of the time scale? Doesn't this assume civilization always progresses without setbacks? – Organic Marble Apr 10 '20 at 13:58
  • @OrganicMarble Admittedly, we only have a sample size of one, but growth and expansion seems to be a fundamental part of life as we know it. One explanation for lack of progress, could be setbacks or "great filters" which wipe out civilizations before they get very far. Our current technological progress is only speeding up, every decade more things are discovered and invented than before, and this pattern has been consistent throughout history. Even periods of "regression" like the "dark ages" didn't stop technological progress – Dragongeek Apr 10 '20 at 14:08
  • Makes sense, thanks for the reply. – Organic Marble Apr 10 '20 at 14:39