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A 1990 experiment to test whether we could discern life on Earth remotely
- WalterBright 3y agoThis is why Sagan was a great scientist. BTW, an oxygen atmosphere cannot be a requirement for life, as life on Earth created the oxygen atmosphere.
- PinkRidingHood 3y agoI heard that he turned Galileo around for publicity: NASA got more funding because there was this picture that showed exactly how small we were compared to the universe, inspiring people for exploration.
- InitialLastName 3y agoThat was Voyager 1 [0]. [0] https://en.wikipedia.org/wiki/Pale_Blue_Dot https://en.wikipedia.org/wiki/Pale_Blue_Dot
- virgildotcodes 3y agoI think it’s precisely because life created the oxygen atmosphere that it’s interesting as a potential sign of life, no?
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- kortex 3y agoOxygen is an indicator of life, but not a strong one, as there are many processes which could create an oxygen atmosphere. What is a strong indicator is the presence of both oxidizing (oxygen, ozone) and reducing (hydrogen, methane, dimethyl sulfide) molecules, in sufficient quantities. That suggests something driving the equilibrium away from all-oxidizing or all-reducing atmospheres.
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- roydivision 3y agoNote not "intelligent" life. You'd be hard pressed to find that living on the surface.
- matthewfelgate 3y agoIt must be hard to extrapolate from just one example of a planet with life.
- acqq 3y agoThe physics and chemistry must work the same, the same limitations determine biochemistry too.
- simpaticoder 3y agoThat's true, but we can make some educated guesses based on the general characteristics of life. In particular, the binding energy is a goldilocks problem (too much you can't recombine too little you don't stay together) that favors water and carbon. A good overview of the conventional wisdom on this is presented in this video "the aliens will not be silicon" by acollierastro, https://www.youtube.com/watch?v=2nbsFS_rfqM https://www.youtube.com/watch?v=2nbsFS_rfqM
- testfoobar 3y agoPerhaps sometime in the far distant future, kids will do the equivalent of the MNIST classification tutorial with planetary data: life or no life.
- HenryBemis 3y agoI never thought about this but now... Hmm.. satellites? A planet with 'advanced civilization' would have satellites or other objects orbiting. Also devices that fly - so some radar can pick a plane in the sky? its Transponders? Scan the sea for large floating objects? On the dark side one should be able to observe lights, and the same locations during the day should have cities (and not i.e. forest fires or volcanoes)? On the light side one would be able to 'zoom in'/scan the surface and see structures or other shapes that would imply some manipulation of the surroundings on large scale. How about antennas picking up multiple signals (radio, tv, etc.?) For the case that it's pre-cities/pre-industry, good luck... Good luck zooming in and spotting 10 'planet-lings' (from Earthlings) with their planet-goats crossing a valley. Having played A LOT of Starcontrol II though I can imagine that sending small crafts to the surface would help verify suspicions.
- freeone3000 3y agoThe planets tend to be very far away. Seeing individual continents is not probable, much less any features. Heck, often the planet itself is visible only as a speck of shadow. SETI is our attempt at the radio wave interpretation. The Sagan paper gives us non-radio detection methods as well (spectography, interpretations of reflected ligjt, and so on).
- extragood 3y agoIt may be possible to get much higher resolution images of exoplanets in the not too distant future by taking advantage of gravitational lensing and our own sun [1]. There are a few related advancements required before we can do that, although the thought that it could happen in our lifetimes is really exciting. [1] https://news.stanford.edu/press-releases/2022/05/02/gravity-telescope-image-exoplanets/ https://news.stanford.edu/press-releases/2022/05/02/gravity-...
- floxy 3y agoHere's a fun video on the concept of using gravitational lensing from the sun: https://www.youtube.com/watch?v=NQFqDKRAROI https://www.youtube.com/watch?v=NQFqDKRAROI ...along with a different paper, where they are going for 1km resolution: https://arxiv.org/abs/1802.08421 https://arxiv.org/abs/1802.08421
- extraduder_ire 3y agoSurprised this didn't happen earlier, at least conceptually. I can imagine people giggling to themselves coming up with the most understated name possible when publishing their results. Something like; "Strong indications of biological activity on rocky planet sol-3 detected by probe."
- TomK32 3y ago> “A strong case can be made that the signals are generated by an intelligent form of life on Earth,” Sagan’s team wrote, rather cheekily. Go dolphins!
- yieldcrv 3y agothink about it another way: it happened halfway through the current life of the space program and the chronology also makes sense: they sent probes straight to mars and straight to venus on the expectation of life and everything, but then dashing hundreds of years of fantasy tropes and imagination upon arrival, the new goal was to expand the search to much further destinations pending ongoing public interest and location within orbit of planets with much larger orbits like Jupiter and everything beyond so timing wise, there have only been a few years wasted in waned funding, and it was about time for the happenstance of that test to occur, logically
- robg 3y agoKarl Ziemelis, now chief physical sciences editor at Nature, handled the paper as a rookie editor. He says it remains one of his favourites — and one of the hardest to get in. Editorial approval for the paper was far from unanimous, because it was not obviously describing something new. Lesson that the blinders of consensus rarely see what’s new. I love this example so much because the method was so new and spoke to a key truth. If methods are useful then mundane results are to be expected but still need to be tested. One mentor once said to me that the whole point of an experiment is to test what you think you know. Testing ground truths helps mitigate confirmation biases.
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- wolverine876 3y agoBased on the OP, Ziemelis didn't think Sagan's paper was new but that it was a valuable control for other experiments (if I understood correctly).
- kogus 3y agoI half expected this article to essentially say "we wouldn't know life if it hit us in the face", but happily the conclusion was that yes, we could see strong evidence for the presence of life on earth from Galileo. Life in other places might be radically different from our own, and we might be looking for the wrong things - but it is good to know that our search is at least able to find what we are looking for.
- marcosdumay 3y agoPeople weren't that bullish on the conclusiveness of those results at the time. (Except for the radio signals.) Even though we know that our atmosphere is maintained by life, nobody did really know if something else could create oxygen and methane. And betting on a specific light absorption spectrum is quite a hard thing to justify. Today we know that those things are rare. And they are orders of magnitude more intense than the unexplained phenomenon we see on other planets. But at the time, we didn't have proof.
- robomc 3y agoAnd the radio signals are a pretty unhelpful indicator because you could have looked at earth with life on it at any point within the last three billion years and there have only been radio emissions for the last 30-millionth of that time. And they might become pretty rare or completely gone in another 30-millionth. For mammals, the radio-emitting time is still only a 2-millionth of the total.
- DaiPlusPlus 3y agoThe thing about radio-waves is inverse-square-law. I don’t understand why we’d expect our nondirectional radio/TV broadcasts to be distinguishable from background radiation once you get past, say, a light year or two from our solar system.
- goodcanadian 3y agoI don't have a reference for you, but I recall a study a decade or two ago that suggested that we would only be able to detect our strongest radio signals out to a distance of about 10 light years with existing telescopes (I may have that number wrong, but it was not large). That said, with new telescopes, particularly things like the Square Kilometre Array, we have higher sensitivity, and more importantly, we have the angular resolution to measure whether an excess of radio emission is coming from a star or from a planet around that star. So, even a weak signal can be detected, and if we were to find such a signal originating from a planet rather than another object, we could make a pretty good guess that it is artificial in origin.
- tantalor 3y ago> To use our own home to test... was an extraordinary suggestion at the time Was it? I dunno, it seems kind of obvious? Like, that should have been the plan from the very beginning of the mission.
- MightyBuzzard 3y ago[dead]
- _marlowe_ 3y agoCan someone explain to me in minimally technical terms how scientists can determine the molecular composition of an exoplanet's atmosphere? It boggles my simple mind.
- 49531 3y agoI thought it was just light? Light bounces off of different molecules differently?
- malfist 3y agoKinda, it's not that it bounces, it's that it passes through and is filtered by it. For example, passing light through a hydrogen gas cloud will remove some red/IR wavelengths from the end result. We can measure those missing wavelengths and figure out what gasses it's passed through
- not2b 3y agoWhen a planet with an atmosphere eclipses its star, light passes through its atmosphere and the molecules in the atmosphere will absorb specific frequencies of light. See https://exoplanets.nasa.gov/discovery/how-we-find-and-characterize/ https://exoplanets.nasa.gov/discovery/how-we-find-and-charac...
- mywittyname 3y agoThey use a technique where they do spectral analysis on the light reflected from the planet. Gasses absorb different wavelengths of light. So the light reflected will have holes at specific wavelengths, and scientists can determine which combination of gasses will result in those specific absorption lines.
- roelschroeven 3y agoI would guess spectral analysis of the light coming from the planet's star. Look at the spectrum when the planet is behind the start, look at the spectrum when the planet is in front of the star (in which case part of the light passes through the planet's atmosphere). The difference between the two indicates which frequencies are absorbed by the planet's atmosphere, from which its composition can be determined. Don't take my word for it though, I'm not an astrophysicist. In any case it boggles my mind that we can do things like that for planets thousands of light-years away from us.
- jebarker 3y agoThe spirit of this experiment reminds me of the paper "Could a Neuroscientist Understand a Microprocessor?" [1] [1] https://pubmed.ncbi.nlm.nih.gov/28081141/ https://pubmed.ncbi.nlm.nih.gov/28081141/
- dmbche 3y agoThat's fascinating! Do you know more about the solutions they offer the the end of the abstract?
- helloworld 3y agoYou can read the whole paper here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5230747/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5230747/
- colonwqbang 3y ago> High-resolution images of Australia and Antarctica obtained as Galileo flew overhead did not yield signs of civilization. They should have taken photos of the night side of Earth instead. It would have been hard to miss the bright lights everywhere.
- digging 3y agoFor earth yes, because we know that human civilizations generate tons of light pollution. Perhaps an alien civilization would be less careless, which means a dark night-side wouldn't prove anything. Which means that looking at night is not as effective as looking at day.
- colonwqbang 3y agoSo radio wave emissions are valid proof of life, but not emissions of visible light? Seems a bit arbitrary.
- digging 3y agoThat's the opposite of what I'm saying. Checking the night side of a planet for visible light isn't useful because a technological civilization won't necessarily create large amounts of light pollution the way we do. If we happen to see nighttime light pollution that would be useful, but planning a mission around looking for it specifically would not make sense.
- wolverine876 3y agoI wouldn't assume it's carelessness for the alien civilization. The night-side view is merely looking for surface-generated radiation with less interference from external radiation.
- digging 3y agoMost of our light pollution is caused by "carelessness". We put on way more lights than we reasonably need, and we put on the wrong kinds that are more confusing to wildlife. Aliens might not do that. Their cities might be fairly dark at night.
- takinola 3y agoI wonder what assumptions are baked into our search for extraterrestrial life. Would we recognize a life-form that is gaseous? One that is planet-sized? One that communicates so slowly that it takes a thousand years to make a single sentence?
- mensetmanusman 3y agoThe assumptions are life looks like is. Also, in regards to life using electrical signals to process information, gaseous clouds would have to generate signals with lower entropy than white noise from exploding stars.
- jcul 3y agoIn Greg Egan's diaspora, they touch on things like this. I can't remember specifically but at one point they encounter a planet where there is some process that is turning complete, and they realize an entire universe is simulated in the process with sentient beings that they can never communicate with. I can't remember the details so I'm butchering it a little, but it's a great read. Diaspora and Permutation City also touch on the topic of speeding up or slowing down conscious so that millenia pass in subjective seconds, or a subjective second passes in a millisecond, etc.
- sohkamyung 3y agoI believe you are referring to Wang's Carpets [1], which is part of Diaspora, but also as a short story. [1] https://timeteam.github.io/fiction/2016/12/21/wangs-carpets.html https://timeteam.github.io/fiction/2016/12/21/wangs-carpets....
- jcul 3y agoYes this is it, thank you. I didn't realize it was also a short story.
- RetroTechie 3y agoThere's another interesting possibility: a rogue planet. https://en.m.wikipedia.org/wiki/Rogue_planet https://en.m.wikipedia.org/wiki/Rogue_planet Under specific conditions, such a planet may even retain an atmosphere, have moderate surface temperatures (and thus, perhaps even liquid water), and life could exist on it if chemically-powered rather than using photosynthesis. Such a planet would be an excellent vehicle to carry life from one star system to another. And another plus: out of a star system's influence sphere (while free-wandering through space), it could have very stable conditions over long periods of time. Which doesn't hurt in the formation or evolution of life. True, there's a lot of (big) if's there. But the universe is biiiiiigggg so I wouldn't rule it out.
- bluGill 3y agoI like to ask a different question. Imagine an arbitrarily advanced alien civilization. How close to earth must they get before they can detect life. Of course we know a few hundred million light years are needed before there was life at all on earth, but could you detect it? Does intelligent life help or not (the first radio transmission was in the 1880s, so 140 light years - but could you detect those transmissions that far out)?
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- kjkjadksj 3y agoI would think that a truly advanced civilization wouldn’t even need to leave their system to find life in the universe. They could just develop a model of how life should emerge and use imagery to identify targets that are most likely to have life. Maybe this model is occasionally validated with a remote operated or autonomous probe.
- bluGill 3y agoThe speed of light is too slow: remotely operated probes are not possible. Autonomous probes are, but good luck building something that survived for the millions of years needed to get anywhere interesting, and good luck having your civilization still existing - by the time the signal returns.
- kjkjadksj 3y agoIf these organisms live for orders of magnitude longer than humans, maybe this isn’t so large of an issue as it is to us with our own frame of reference.
- giantrobot 3y agoDetecting life on Earth could be done with a telescope able to take spectra of Earth from the aliens' home system. They could look at Earth compared to the Sun and other planets in the system and see Earth has a bunch of biosignatures that would be broken down by the Sun's UV radiation if not constantly regenerated. One or two markers would be inconclusive but a bunch of different ones would give a high probability of life existing here. It's why the "Dark Forest" concept is fundamentally silly. There's no hiding Earth's biosignatures. If the arbitrarily advanced aliens have a long enough measurement baseline and are within 150 or so light years they could also see the technosignatures of the Industrial Revolution with different air pollutants. If they're within about 60-70 light years they'd start seeing evidence of radioactive isotopes from above ground nuclear testing.
- tempodox 3y agoAs already mentioned in the movie Mars Attacks, the more interesting question is whether there is intelligent life on Earth.
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- streakfix 3y agoVery unlikely
- canjobear 3y agoAlthough they detected life on earth, I don't see how they could have evaluated the false positive rate of their methods. Better would be some data from Earth, paired with data of the same resolution from a matched lifeless planet, and then the scientists have to figure out which one has life without knowing which is which.
- amelius 3y agoStill too easy to overfit.
- wizofaus 3y ago"High-resolution images of Australia..did not yield signs of civilization." I'd been living here my whole life when it flew over, and hadn't had much luck detecting such signs either. More seriously though, I wonder if the same tests could have detected anything a few 1000 or even million years after life first evolved?
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- rnk 3y agoHa, I was going to post the same comment, mostly to make fun of my friend from Australia. ;-) I'd like to see a followup to this article in the current time - that considers what we could detect on nearby star systems. What can we see at 5 ly, 10, 20, 100. I think someone must have already published some description of what we could see, but haven't come across it. Seems like I read they were looking for signs of air pollution, figuring they might follow our path? We seem to barely be able to see anything through the atmosphere from the planets that pass at just the right point between their sun and the earth.
- murphyslab 3y ago[dead]
- m3kw9 3y agoWhat if “life” may not need the stuff that we see commonly on Earth?