I read the paper and I find it highly unserious how frivolously they assigned some probability to the question how many intelligent civilizations there are in the universe.
Their key assumption is: "We assume that if an SP remains in the circumstellar Habitable Zone (HZ) for a time
equal to the current age of the Earth it will develop
intelligent, communicative life." (page 5 at https://arxiv.org/ftp/arxiv/papers/2004/2004.03968.pdf)
Why? Why do you assume a planet will develop intelligent life when it remains in the Habitable Zone long enough? Just because it happend on Earth?
And they continue with: "[...] according to the Copernican Principle – we propose that the likelihood of the development of life, and even intelligent life,
should be broadly uniformly distributed amongst any suitable habitats"
We have no clue how likely it was at Earth that life developed at all. Maybe it was just a chain of many lucky coincidences that happen once in 10 billion years. How is distribution any relevant then?
One thing that bothers me is that papers like this always assume alien life will look exactly like life on Earth. It must be the same distance from the sun as us, have water and proteins like us, etc.
If we effectively limit the definition of "life" to "the types of organisms we see on Earth", then of course we will never find life anywhere else.
Maybe there are aliens that live on a gas giant and they "eat" the lighting rather than food, and their bodies look more like circuitry than the chemical life on Earth.
Maybe there is a planet with a very strong magnetic field, and the life there is kind of like an electromagnet. They have bodies high in copper and the current of the ocean moves them through the magnetic field and generates electrical energy.
It makes sense that we restrict our search for life forms we know to be possible while we are stuck with limited tools at our disposal. But I don't want us to overlook some amazing discoveries because we have put blinders on.
The start of life is basically a statistics game waiting for a system to spontaneously enter the right self-reproducing state, and it is relatively easy for a system combining water and various mostly-carbon-based molecules to enter into that state.
We thus suppose that life looks like life on Earth because life on Earth has several properties which allowed it to come into existence. By contrast, there are no known states that cause a strong planetary electromagnetic field to rearrange copper into self-reproducing patterns: if anything, such electromagnetic fields are more likely to disrupt any spontaneous order that happens to arise.
So it's true there may be other systems of life out there, but too much handwaving about that fact can obscure the fact that the other candidates for ordered systems arising from chaos are weak candidates. It would be unwise to rely on this wild speculation while trying to responsibly and conservatively estimate how much life there is.
I think the real question with the probability of life starting is eactly how easy it is to create life or even a good reproducing state given the proper conditions. What makes it dificult is that probabilities shrink very rapidly. If you cover every planet in the universe with billions of monkeys each typing randomly on typewriters, starting a new try every week for billions of years, they will never write Hamlet. They won't even get the first line. The probability to type the right n characters is 63^n, which grows exponentially. We can think of giant volumes of planets, but the number of planets will only grow linearly with the volume. The probability of creating even basic constructs is very small.
Even if you do manage to create a basic reproducing construct, that doesn't mean you are finished. The reproducing construct will only have a limited number of instances on its single planet. So once again, it is difficult to get to the next step beause your numbers are small relative to the the imaginable probability (of course what this is is not known).
On the other hand, the probability of us observing intelligent life is 1, since the intelligent life is the observer. No matter how few planets develop intelligent life, each will look at itself and say, "There it is." Our own existance or the history on our planet says nothing about the probability of life evolving without knowing the number of "tries" that failed to evolve life.
People who agree with what I have said so far may disagree with this next part, and that is ok. In counting the number of tries, we don't know that there is only one universe in which those tries exist, meaning we could be the only ones in this universe, and ours is one of the relatively small number of universes that have life.
Of course, I can't say the probability will be small. But I think it is also unfair to just assume the probability of life evolving is big.
Right now there is a lot of active search for life based on the habitable zone theory. I also think that we should open up our ideas. We might find something of intellect and of interest closer than 17000 light years. If nearest carbon based life if so far there is not much to be learned. If we can find even a mildly interesting self supporting system in our own solar system we may not be able to play golf but we might be able to communicate and learn.
I also think we are foolish to not look for intelligence on earth. We killed off the neanderthals. There is more hope of bringing them back and asking for an opinion than finding little green men.
We might indeed find something of intellect closer! It would indeed be interesting! It is not, however, something we have reason to expect would be particularly probable.
I am unsure what variety of "opinion" you hope to extract from your hypothetical Neanderthal, and take the opportunity to note that, as social mammals with large and adaptable brains, the opinion systems of intelligent hominids are substantially shaped by their upbringing and how they relate to the society around them, particularly as regards the more complex systems of thought.
I think this very article disproves your hypothesis.
Life like us is probably so far away and so short-lived that we will never be able to communicate. An intelligence that is near enough to reply & able to survive millions of years (so its lifespan crosses ours) might provide some results.
If we start sending out signals and ask how long will it take for a reply? 17,000 years from something like us. Thats no use.
Might be better to phone every star in the neighbourhood starting with the nearest, and see if anyone picks up.
Our understanding of the universe and physics is still young. We're reasonably sure about 'normal' matter, things like you, a beer, or a star. Unfortunately, that's only about 5% of the universe that we know of today.
Things like Dark Matter, well, we know it falls down. That's about it, really. There's a lot of theories about it though. More unfortunately, we think Dark Matter is about 20% of the universe.
Then there's Dark Energy. It makes things fall up (?!). And we're not super sure about that, either. Most unfortunately, Dark Energy is about 75% of the universe.
We don't have a clue if any intelligence is even using photons to communicate anymore. Most of the universe doesn't seem to really interact with them anyways. And that's with this limited understanding of 'Dark' things. We're still standing at the shore of a vast ocean.
With other assumptions, they would arrive at 0 intelligent civilisations in the galaxy. That would make the rest of the paper void. :-)
Nobody knows how life is distributed in the galaxy. Assuming that we are not special (the Copernican Principle) is a valid prerequisite. As it neither can be proved or disproved, it's not unserious to just state what the unproven assumptions are.
> We have no clue how likely it was at Earth that life developed at all. Maybe it was just a chain of many lucky coincidences that happen once in 10 billion years. How is distribution any relevant then?
Habitable for who anyway? We shouldn't assume that life can only develop in the habitable (for us) zone. But we do anyway because we don't know anything else.
Why would other sentient beings even want to communicate with those self-centered carbon-breathers? Why not just laugh about their hubris and that woefully incomplete definition of life? Telling them would spoil it!
Dude - Mars billows out CO2 and methane during it's seasons, indicating that there is microbial life on that planet as well.
The media has told us that life is rare and that WE ARE ALONE IN THE GALAXY. I saw it with my own eyes of Neil deGrasse Tyson explaining in an authoritative way that we as a human species have to accept that we are alone in this galaxy.
Yet another lie pushed by the mainstream media. Also a glaring breakdown in logic to assert to a definitive negative, especially on the concept of life existing on one of the 100 billion or so star systems in the galaxy.
They can't even get their story straight. The US military is already acknowledging UFO's and the POTUS has hinted that the truth is a really big deal.
How many times will the story change and the goal posts be moved until we acknowledge that ET's have visited earth and have been for a very long time?
Personally, I think they are just overreacting to their excessive optimism about the prevalence of life in our solar system before any of the probe missions to Mars and Venus. If you read old astronomy books from that period they were full of speculation and hope that we would easily establish contact with other forms of life. They were burnt badly by the discovery that all other planets and moons in our solar system are much harsher environments than we initially expected and they received a lashing from the media after headlines full of starry eyed promises. It seems to be a similar effect to an AI winter where initial progress and optimism is met with challenges and limitations that turn fickle public opinion against the science, the researchers then internalize these beliefs or they become fringe pariahs. Thankfully some brave souls persevere and eventually prove the field isn't worthless after all.
Cetaceans are considered sentient, the way humans are sentient, by a wide scientific consensus, e.g. [1]. Some degree of sentience is also assumed by the scientific community to be present in a lot of other animals [2].
Still the successes in contacting these nonhuman sentient beings, even if they gladly support such contacts, like dolphins, is very limited at best. And those are our close relatives, fellow mammals, sharing the planet with us.
Now imagine an attempt to contact a sentient being which developed and lives in an environment totally alien to humans.
But Cetaceans do have language? And we do have communication to some level.
> Experiments have shown that they can learn human sign language and can use whistles for 2-way human–animal communication. Phoenix and Akeakamai, bottlenose dolphins, understood individual words and basic sentences like "touch the frisbee with your tail and then jump over it" (Herman, Richards, & Wolz 1984). Phoenix learned whistles, and Akeakamai learned sign language. Both dolphins understood the significance of the ordering of tasks in a sentence. [0]
Maybe this definition of sentience is too broad, but it is worth evaluating what you can hope to achieve, by looking at the closest approximation available and what you are achieving there. And in this case, I would say "not much". We know e.g. dolphins have a language and communicate, we can very coarsely decipher it, and they can learn to broadly decipher a language we engineered for them. But the most we are achieving with this communication are party tricks in a circus.
The only reason that most people do not accept the obvious sentience of many animals is that they want to keep killing and eating them. It's as simple as that.
These sorts of discussions are bedeviled by the problem that, in English and other languages, ‘language’ has a sufficiently broad usage that one could make a case for calling cetacean vocalizations a language in some sense. I am pretty sure, however, that if they had a human-like capability for language, it would be as obvious as if chimpanzees used power tools in the wild.
It only has to be able to express abstract ideas, with a power at least close to ours. Presumably, we are not going to communicate with alien species unless they have a sufficiently developed science and technology, and it is hard to imagine that without a language.
And by “a language like ours”, do you mean English? (The one language that we know you and I both have in common.) Noam Chomsky has spent a career looking for a common deep grammar for all human language, and it turns out that there is not much in common, except at the most fundamental and abstract level. Despite this, there are no human populations divided by having mutually incomprehensible languages, and maybe that is so throughout the galaxy.
In order for someone on another planet to be detectable by us humans with our present tech they would have to broadcast radio signals. We developed this tech a little over a century ago. And as a civilization we had plenty of opportunities to self destruct, from nuclear and biological warfare, to destroying the environment. We also have the chance of developing a "better than radio" communication method that's completely obscure to us at this point.
The window of of that civilization being detectable is squarely between them developing radio and them dying off or using some tech that we cannot discern as artificial signals. The window for us detecting them has been ~50 years so far. Given the size of the galaxy and the minuteness of that window overlap it's pretty clear the chances of communication are slim. And indeed even if we do detect something, calling it communication might really be pushing our definition of that.
Are we? Most people have no clue that their two cats hate each other (which they are most likely to do if they did not grow up together) or that their dog has chronic backpains (which many breeds do because they have overly long backs like the Dachshund) or difficulty breathing (like all breeds with short noses) or that their horse is bored to death (which they tend to be because they are kept in a much to small lot). We are pretty terrible with our pets. We just like to imagine we are not since we consider ourselves emphatic and don't hear a complaint so all must be good.
The authors of the paper do various calculations to arrive at 36.
How do they determine f_l, the probability of life originating on a suitable planet?
They follow the "Principle of Mediocrity", ie life exists on Earth, and we are not special, so probably it exists anywhere it can exist and f_l = 1.
This is the exact opposite of the Anthropic Principle, which says (correctly) that we cannot deduce anything about the probability of life originating from the fact of our own existence.
If f_l isn't 1, and it's some smaller number, like 1^-1000, then there won't be 36 alien civilisations in the galaxy - there will only be zero.
Even in the observable universe, there will be zero alien civilisations.
> This is the exact opposite of the Anthropic Principle, which says (correctly) that we cannot deduce anything about the probability of life originating from the fact of our own existence.
Your definition of the Anthropic Principle seems to clash with the Wikipedia definition [1], which states:
> The anthropic principle is the philosophical premise that any data we collect about the universe is filtered by the fact that, in order for it to be observable at all, the universe must have been compatible with the emergence of conscious and sapient life that observes it.
Well, even within the wikipedia article you see quite a few definitions under 'variants'. It's also nearly a word for word copy of how it is defined in contrast to the mediocrity principle ( https://en.wikipedia.org/wiki/Mediocrity_principle )
Either way, his point is fundamentally pretty close to even the WAP definition in the summary.
I think it's fairly self evident, though. If there is only one set of observers in the entirety of the observable universe, those observers would be inaccurate in assuming that they are unlikely to be special because they exist and therefor others likely to as well. We just can't make a deduction about the probability of life based on the fact we exist, because for us to make that observation we have to exist, regardless of (im)probable it was. You can reach the same conclusion just thinking about selection bias - we have selected for intelligent life out of necessity, because if we hadn't, we couldn't be around to make the argument either way.
If having a huge moon was an essential driver enabling development of advanced life and not, e.g., turning into Venus, then the expected number of others drops from 36 to well below 1.
If any exist (i.e., if other origins are possible, such as on Endor), we would only encounter expansionist cultures, because the others don't get out. If they all have controlled fusion (and how could they not?) they should have little interest in inner rocky planets, never mind the extreme primitives on them.
Look to the Kuiper belts, well supplied with frozen gases and extreme cold. Of the two, the latter is the far more valuable.
That’s not quite what the Anthropic Principle states. For instance, we can conclude from the fact that we exist that the universe is a place where life can exist, and that it is more likely we live in a universe where life arises frequently. However, the fact that we haven’t observed any aliens decreases the probability that life arises frequently.
> Of course since no one knows how long intelligent civilizations last, (given our current trajectory, some might guess not very long)
These people must have a very short sighted view of history? Our current trajectory for almost everything that matters is positive. Longer life spans, fewer wars, better medicine and technology, average base-level knowledge, etc. Barring some global unavoidable catastrophe, there is no reason to think we are going anywhere very soon.
Cosmic timescales are different. Do you think humans will be around for the next say 100,000,000 years? I think we'd be lucky to survive another 10,000.
We've only had organized societies for somewhere around 20,000.
There could have been say, intelligent life on Venus, then they triggered a runaway greenhouse effect, everything died, the oceans boiled, and then 700,000,000 years pass before we started talking pictures of the dead planet with all signs of previous life completely erased.
If we burned this planet down and waited 700,000,000 years, would remnants of our civilisation remain? On the one hand, we have structures that are massive blocks of concrete and metal that I find hard to imagine fading away. We've got stuff in space and on the moon. On the other hand, there's weathering and erosion which would be amplified by chaotic weather patterns. And 700 million years is a long time.
I'm leaning towards us leaving some evidence behind, but it's such an inconceivable amount of time that I'm not really sure.
In the case of Venus there's likely little left due to extreme volcanic activity[0]. Even if we didn't create a hellscape like Venus on Earth, over 700,000,000 years geological processes would grind those structures to dust. Repeated glaciation every 41,000 years or so would act the fastest but over hundreds of millions of years the surface would be significantly altered by tectonic movement. Add to that all the insults of rain, oceans, rivers, wind, volcanism, and biology. Practically nothing would be left except for maybe the fossilized remains of some junk thrown into rivers that managed to sink into the mud and then ride the tectonic waves up onto some high escarpment. Even the orbits of satellites decay. The few artifacts we left on the Moon would stand the best chance. This is why the only hope we have is to spread out into the solar system and beyond. If we stay in our tiny ecological niche on this island we're doomed to extinction and erasure.
Near-Earth satellites, yes; the thinnest wisps of the edge of the atmosphere slow them down. Further out, however, in geostationary orbit, satellites are not subject to such forces. They may be disrupted long-term by the gravitational influence of other distant bodies, but I think I'd worry more about the potential impact from micrometeorites.
> If we burned this planet down and waited 700,000,000 years, would remnants of our civilisation remain?
I've seen this discussed a bunch over the years, and the informed answer to me seems to be that various isotopes and such would remain in trace amounts as clear evidence of technology.
I wonder how much our chances of surviving as a species change if we drastically increase the average lifespan of an individual. I think this might also be a solution for interstellar travel as well. If we're going to conquer the depths of space we need to take control of our biology first.
You're only looking at the positive side of things and ignoring well known issues that will lead to global catastrophe. Consider climate change, loss of biological diversity, soil erosion, mineral resource exhaustion, pandemics, etc. The possible solutions to these issues require global cooperation which we haven't seen yet. Market solutions don't seem to be panning out well.
Market solutions work when applied to the correct areas, same thing with global cooperation. Too much interdependence in the wrong areas can be a hinderance. There is no panacea, we have to be smarter than that.
> Barring some global unavoidable catastrophe, there is no reason to think we are going anywhere very soon.
If only we didn't live during a huge ecological disaster that is threatening to destroy the livelihoods of hundreds of millions of people directly, with unforeseeable consequences for the rest of the world; which the major powers of the world are unwilling to act in any significant way to curtail; and which seems to require a re-structuring of the entire world's economy to actually solve...
Looking at the world right now, we know one thing for sure: none of the trends we see today will be relevant in 100 years. One way or another, the world will go through massive changes whose scope can't be predicted.
One possibility is global warming will continue unabated until we have hundreds of millions of people migrating from rapidly submerging areas (Bangladesh being one of the largest concentrations of population in the world living a few meters above sea levels), most likely followed by some kind of global war to prevent or redirect the massive migrations.
Another is that the global economy will be completely re-created to stop the production of greenhouse gases dead in its tracks, almost certainly in a non-capitalist system, since capitalism simply can't deal with externalities. The required political and economic shifts are hard to imagine, and the resulting world is equally hard to predict.
And of course, we are still always a few madmen away from nuclear war, with many of the world's most tense regions having nuclear weapons aimed at their enemies. Could global warming start a resource war between India and Pakistan that would lead to them using their arsenals? Could Israel decide to use its illegal nuclear weapons to attack Iran in a "defensive first strike"? Could the US decide that its interests are important enough that, as its soft power wanes, it could launch a bomb at some country to remind everyone of its hard power? Could the Europeans spark a new war, either among themselves or against some common threat, and feel the need to use their nuclear arsenal? Could the Chinese? As long as nuclear weapons exist, they are an ever growing threat.
Remember that just in the last few years, the long standing US-Russian non-proliferation treaty was unilaterally ended by the US, signalling a vast increase in the risk of nuclear weapons actually seeing use again.
First of all, there's no need for 7 billion people to exist to fulfill being a civilization. Surely our civilization could exist of just 1 million people if it comes to that.
Secondly, the climate change "catastrophe" isn't what we need to be worried about. Rather one of those events that has occurred before on earth, wiping out almost all existing life. But considering some non-intelligent life forms, or at least life forms without our superior intelligence, survived those events I'd bet that we would survive them too. Considering we can detect them, and (somewhat) protect against them.
Nuclear War is unlikely to happen since any instigator know it would come at tremendous cost. Also, the more countries that have nukes the better, since it levels the playground. Even if nuklear war happens, it wont wipe out all people, all communities.
If in a group of a million people you had enough people interested in and capable of doing the science necessary for making progress interstellar communication or travel, we would have many many many times more people working in those fields today.
A civilization with only a million people in it would have priorities vastly different than our own.
The human species doesn't need to be wiped out for civilization to end,at least at the level of being interested in interstellar communication. It's true tho that civilization may spring up again after some time, so perhaps that should be factored into the equation.
And nuclear war had been only narrowly avoided on around 13 documented occasions in the last 70 years [0]. Some of these events were avoided by happenstance, not any kind of systematic protection. This kind of luck will not continue forever. And if it does break out, right now we don't really understand what may happen to the planet if the US or Russia unleash their full nuclear arsenal, rather than just bombing one or two cities.
> right now we don't really understand what may happen to the planet if the US or Russia unleash their full nuclear arsenal
Nothing will happen. Planetary nuclear arsenal yield is dwarfed by a single large vulcanic eruption, and those happen regularly. In fact, modern nuclear arsenal is barely enough to destroy military targets and major cities of both parties. Direct casualties will be in millions, but probably in single digits of them.
In other words, it will be a major humanitarian catastrophe, but nothing humanity (or even a major world power like US) can't survive. Probably the biggest change would be due to large areas rendered uninhabitable for decades, but then again, plenty will be left and "uninhabitability" is a matter of life quality standards. People live in Hiroshima and Chernobyl just fine.
You're free to believe what you want, but the peer reviewed studies that have been conducted with current climactic models predict significant global cooling that would lead to at least global famine. The crux is not the power of the explosion, but the amount of soot generated by detonating high - yield weapons in huge cities.
And people do not live in Chernobyl at all. However, it's not really relevant to a discussion of nuclear weapons, as nuclear plant meltdowns are a completely different problem. For sure if the Chernobyl core had melted down and exploded nothing would be living in Ukraine at all, and possibly a much larger area. Hopefully though, that is one area though where technological progress has actually significantly reduced the chance that it would ever happen, in a systematic manner.
Not matter the opinion, "nuclear winter" concept is a belief, you are totally right here. It has never been a scientific question in the first place, the idea was largely accepted for political reasons. Wikipedia [1] has a short overview of the discussion on the topic, but in reality there is no debate on that for a long time: one can't really argue in favor of milder consequences because he'd be labeled as a militarist and all. And I agree that this is one of the few discussions that we probably shouldn't have.
But in general: all the articles greatly exaggerate the volume of nuclear arsenal (tenfold), the number of actual nuclear detonations, the territory of fire and ash yield of the fires. And even then, the planet has tolerated all those at larger scale in volcanic eruptions and forest fires.
Napkin math: US/Russia arsenal is about 1500 warheads on 500-1000 carriers each, which is about 500MT combined. Out of those optimistically maybe a half will detonate on each side (they will be destroyed in preventive strikes/intercepted/etc.), which leaves you with about 125MT total yield - about the yield of a couple of Tsar Bombs that was detonated in 1961 without any consequences whatsoever. The majority of the strikes will be on military targets in the middle of nowhere, where there won't be any fire at all. The rest will fall on modern concrete cities that don't have much fuel either.
> And people do not live in Chernobyl at all
They do, there are about 3000 people working there at any given time and even more living there illegally. Also, animals have no problems living there whatsoever and in fact the ecosystem is in a better shape due to low human activity. Also, you are aware, that Chernobyl power plant itself was operational until year 2000, right?
> nuclear plant meltdowns are a completely different problem
I absolutely is, and in terms of ecological consequence Chernobyl is far more severe than thermonuclear warhead detonation. The core HAS melted down and exploded, pieces of graphite rods were found as far as hundreds of meters away from the building.
I'm not saying nuclear war is nothing to think about - this would be a major catastrophe. But it's very unlikely to end the civilization.
The Chernobyl facts you bring up are very interesting. I had no idea that the plant kept operating for so long. I knew that there were people who stayed behind, and that there are workers around the exclusion area, though I admit that I would have imagined significantly lower numbers.
> Also, the more countries that have nukes the better, since it levels the playground.
That's the American "best thing against a bad guy with a gun is a good guy with a gun" attitude. Empirically this does not hold true. With the density of lethal weapons in the U.S. and the statistically lower safety from gun violence compared to other developed countries, it's safe to assume that the world would be a safer place with a reduced number of countries having nukes (or ideally none of them) rather than an increased number.
I'm not sure that it's safe to assume that the dynamics of international relations among nuclear and nonnuclear states can be so tidily extrapolated from the dynamics of society and interpersonal relations among gun-owning and non-gun-owning humans acting in their individual capacities.
Maybe not. But it's at least an attempt to a somewhat informed hypothesis, which a simple statement like "since it levels the playground" definitely is not.
Forget travel. Even radio communications seem unlikely. I recently got depressed trying to calculate how much energy would be required to establish a radio link with a colony on a planet orbiting the nearest star (about 5 light years).
Even if we use the entire output of a nuclear reactor to power the transmitter, even if we tighten the beam ten times beyond what's currently possible and even if the receiving collector is square kilometers wide, the signal is still going to be very weak.
I can't even fathom how to make this work at 10K light year scale.
The receiver would be much, much bigger than a square kilometer by many (>6) orders of magnitude. Any civilization capable of interstellar travel would be very capable of setting up a planetary or solar system sized astronomical interferometer (aka hypertelescope [1]) using a large fleet of synchronized satellites. These satellites can be prototyped with today's technology and any interstellar civilization would be capable of both receivers and transmitters with far more power than we have attempted today.
That's if you're trying to reconstruct a "picture" of what the interferometer is pointed at. If you control both the transmitter and receiver, you can use many different techniques to pack more information into the same amount of power. 2G cellular radios, for example, were 2 watts max power while 3G and 4G radios are 500 milliwatts max power despite being orders of magnitude faster thanks to better encoding due to radio sensitivity and spatial resolution due to MIMO - the cellular equivalent of an astronomical interferometer.
The technical term is space-time block coding [1]. The speed negotiation would take at least 3x the distance between the two start systems though, until which time they'd be limited to a conservative estimate based on the channel impairment between the star systems, aka max expected noise between the two astronomical phased arrays.
It's nowhere near that bad. Arecibo could communicate with a similar installation out to 21,000 LY, at least.[1] That's with a megawatt of power, less than many analog TV stations.
Seth Shostak, head astronomer for the SETI institute, thinks the actual number is 400 light years, not 21000 light years. And James Benford, President and Founder of Microwave Sciences (and twin brother of science fiction author Gregory Benford), thinks that 400 light years would be the range for detection, not for communication, pointing out that the ground-based receivers wouldn't be able to integrate the signal over enough time due to planetary rotation.
If we're still factoring in ground based solutions for interstellar communication I think we're assuming the wrong level of technological development. I'd like to see how it works out for a few dozen receivers positioned as far apart as possible in Solar orbit.
Keep in mind you can increase the energy per bit by increasing symbol duration.
I had a problem like this on my wireless systems exam recently, where the setup was two Arecibo style antennas with a reasonable 20ly distance, a low 48bps, an Eb/N0 of at least 10dB, and a few other reasonable assumptions. The transmit power needed turned out to be less than 250kW.
Interesting! Clearly something is wrong with my calculations. Given the effects of the inverse square law over 20ly, how is a antenna of any size able to collect a reasonable amount of photons from a 250kW source? Even a beam that is parallel for all terrestrial purposes will spread out quite a bit over such a huge distance.
Would be really grateful if you could link me to some learning resources!
I'd be interested to see your calculations, because I'm just doing it according to the book, blindly following Friis transmission equation. It sounds like you are applying more physics based thinking to the problem. I have no clue how many photons the antennas are spitting out or receiving, just the gains and losses associated with the antenna size and distance.
I'm writing out the basics of the solution below, but the physical reason for the non-impossibility is that the problem statement assumes the lowest possible amount of noise to overcome, long symbol durations, and high gain antennas.
For learning resources to recommend I'm lacking a little, since I mainly relied on lectures etc, but if you're interested in terrestrial wireless communications you could check out Stanford's class by Andrea Goldsmith. [1] There's very little about antennas, satellites, long distance communications and all that, but a draft of her book is available and all the lectures, so it's very comprehensive even though it's only tangentially related. You can also check out the link budget Wikipedia page [2] for an overview of things affecting these kinds of transmissions, as well as many links to interesting related topics, like the voyager program and their unique struggles (300+dB path loss and still hanging in there!)
The path loss is L = (4 * pi * d/lambda)^2, where lambda is wavelength. At 1.5GHz carrier frequency this results in L = 1.41e38 = 382dB, which definitely is a monstrous number, and with no antenna gain and terrestrial noise powers this would be practically impossible to overcome.
But with an equivalent receiver noise temperature of 5K (meaning the noise we are receiving is only the faint radio noise from space [3]) and a bandwidth of 100Hz the signal is only competing with noise with power N_0 = kBT = 6.90e-21W = -202dBW giving us some hope of establishing communications.
The gain of the 300m diameter antenna is G = 4 * pi * Area/lambda^2 = 2.2e7 = 74dB, and we have one transmitting and one receiving so we can double that dB gain.
Given the Eb/N0 = 10dB requirement and the rate of R = 48bps we can figure out the necessary received power after the antenna gain. Energy per bit is N_0 * 10, so the received power has to be Pr = N_0 * 10 * R = 3.3e-18W = -175dBW.
The transmit power plus gains minus loss has to be greater than the receive power limit, so it should be Pt >= Pr - 2G + L = -175dBW -(2 * 74)dB + 382dB = 59dBW = 800kW
I'm getting a different answer here because the exam had some additional pointing/polarization loss, which I apparently flipped the sign on in my solution. So the correct answer for my problem should be in the low MW, but still not outlandish. I have not received the corrected exam yet, so there might be other errors too.
[3] I'm a little unclear on how this would be achieved, since the earth antenna is, well, earth based and at some 290K. This is however what the problem assumed, so maybe they're counting on cooled space based antennas.
Incredible! Thanks for putting this together. Yes, I'm using a more first principle based approach (merely applying the inverse square law to the transmission power). Clearly your approach is better (because it accounts for background noise). I shall have a look at all this!
Some civilizations may be able to populate multiple planets in their solar system and increase the chances that they will survive for perhaps thousands of years at a technologically higher level. During this time they may even be able to reach other solar systems. If they do that and can repeat it then they may be able to keep going millions of years. This may already be happening somewhere.
If you look at our solar system, humans can survive on Earth, Mars, Venus and likely some larger moons without too much fictional technology. The universe seems to be full of rocky planets. The only insurmountable problem to living on a rocky planet with hydrogen, oxygen and carbon in some form really is gravity, if the planet is too big. But species that can travel interstellar distances must have the technology to live in space and only send robots to nearby planets or asteroids to mine raw materials. That increases the range of suitable systems considerably.
We have a pretty good idea of the environment on Mars and safe for potential long-term effects of the different gravity there there is no reason to believe that humans can't survive there using technology we already have, or could likely build.
Depends what qualifies as suitable. With enough engineering you could establish a colony practically anywhere with access to energy and matter. More near term, you could build self contained environments on or below the surface of planets and moons or in orbit. Longer term (slightly?), you could modify your biology to thrive in harsher conditions. Even further out, just bring your "planet" with you it would be a true wanderer then.
I wouldn't find it surprising if self destruction were a common occurence after intelligent life changes its environment enough to develop long range communication. Unless the technology develops at a comparable rate to evolution, there will always be a sharp discontinuity associated with industrialization that suddenly grants the ability to manipulate things on a planetary scale. The lifeforms will be placed in a completely novel situation without evolved mental hardware needed to see the danger. Just like how posting to facebook while driving doesn't feel dangerous in the way standing near the edge of a cliff does. I often wonder if this is where the great filter is located.
That jumped out at me as well. It seems to me highly unlikely that humans will wipe themselves out in the next hundred years, and if they don't, even less likely beyond that. Maybe, but I doubt it.
So their calculation is actually for the number of civilizations in the galaxy that are at exactly the same level of technological development that we are. Even assuming that some percentage of civilizations annihilate themselves beyond recovery there must still be a very long tail of ones that only do so at a later date as well as those that do not and those that recover at some later time.
Given that we only have ourself to compare with and we can't look into the future, I think that is looking at it based on facts. Sure, we can identify this as one of the big unknowns in the calculation, but 100 years is what we know.
Explode a number of nukes in space, which forms a prime number pattern. For instance by cadence.
That should get the attention of any listeners out there. There should be no natural process which could explain that.
Bonus:
An incredibly strong, directional and short pulse could be directed by placing a mylar parabolic shape behind the bombs. Just before being vaporised, the mylar would reflect the initial energy.
The same could be done with the radio energy by a thin copper mesh.
Edit - downvoter thinks it’s untenable or just a bad idea in case it works? :-)
Or one better: build Dyson spheres around your sun, or ideally around several local suns. This would be a clear signal of a very mighty civilization indeed! But even this signal would propagate outward at a snails pace in comparison to the size of the galaxy, let alone the universe. At that point I think it's interesting to wonder what such long-lived and powerful civilizations would want to talk about, and indeed how the conversation would take place. The time-scales are unimaginable; it would require something like a slowed-down AI living in a physical substrate maintained by short-lived AIs and/or biologicals, somehow maintaining a stable (or at least meta-stable) social structure for millions of years. It would be an incredible achievement, but I would worry that the "human cost" would be immense -- I'm thinking of the very stable but stagnant (and totalitarian) Egyptian civilization. Were the pyramids worth the cost?
You need some kind of shutter that orbits the sun, which can be opened and closed rapidly to modulate the existing light. At least we can send the message that we were here and you’re not alone.
I don't believe this is necessary. First, the construction of such a sphere is itself a kind of "blinking" - over time, imagine the stars blinking out, replaced by a dull red glow of infrared. Second, I believe it is possible to infer the existence of stars by gravity and other effects, such that it's lack of emitted light in the visible range would be a clue to an observer that they are, in fact, observing an artificial phenomena.
The energy released by a nuke explosion is utterly insignificant compared to the light emitted by the sun so it would not be noticeable at all by a faraway observer looking at our star.
A directional pulse may be an option, but then you have to pick a particular star at which to aim it.
My own thoughts: intelligence is something which you need to observe predicting the future, ie something proactively reacting to the future in order to achieve a goal. This means ideally you have a bit of interaction so that you can see that you're interacting with is capable of learning in response to you
In an alternate reality where we observed prime numbers in space, I suspect we’d be telling ourselves they must have a natural cause in the same way we’re looking at dark matter and arguing what type of particle it is made of.
And if we did decide it had to be aliens, the point is they don’t have to be intelligent to do prime numbers — they might be space cicadas who evolved to spread around Oort clouds by gathering enough uranium to go prompt-critical[0].
From the point of view of aliens looking at us if we detonated nukes in a prime sequence, the same arguments apply.
[0] this is a reference to a novel or short story which I can’t remember enough details of to find on Google or DDG.
Meh. According to https://en.m.wikipedia.org/wiki/Intergalactic_travel, it takes a mere 28y to get to Andromeda if you have the tech to do constant 1g acceleration. You could say that is not "communication" from a civilization point of view, but if they can send us a live messenger it would sure be interesting?!
That's 28 years on-ship time, but about 2.5 million years for an external observer in Andromeda or the Milky Way. If you want to make a return journey, then 5 million years would have passed on Earth by the time you get back.
The problem is that at something like 0.4c the radiation from hitting the very rarefied hydrogen atoms that fill the interstellar void (something like 1 atom per 1m³) becomes so hard it will kill you without a colossal radiation shield.
One way to avoid that is to project a powerful beam of electromagnetic radiation to ionize these atoms, and then swipe them away with magnetic field. But it adds significant energy requirements, and adds noticeable "friction" to your spaceship.
Why couldn't you slingshot around the sun and planets a few times to pick up speed before leaving the solar system? In the way the slingshot racer reached super high speed on his run to the gate in The Expanse?
The rocket doesn't know, but an outside observer will. If you're on the rocket, you experience a constant 1g force. The rocket doesn't "feel" that it's harder to accelerate, but an observer that remains at rest relative to the rocket's starting point will see it speed up less and less the faster it's going as it approaches the speed of light.
They continue to accelerate at the same rate for their frame of reference, but relative to earth or Andromeda their time moves slower and with their time moving slower their acceleration slows down to
It certainly going faster each time relative to the speed of light, which increases the mass. At first this is minuscule, but it starts to add up by the time you're going a decent fraction of c. It will asymptotically approach infinity, which is why nothing with mass can be accelerated to c. It also means that at some point, you won't have enough energy to continue accelerating.
Isn’t that galaxy also rotating? So if you wanted to hit the far right corner of that galaxy, then don’t you have to calculate where it actually is now, since it took so many light years from that part of the galaxy to reach us.
Then, you’d have to calculate where that part of the galaxy will be, by the time you reach it. Then, you aim yourself at those coordinates, and hope you don’t have any mechanical issues that will prevent you from gravitationally locking on to your destination star system.
The plan seems to be "to find intelligent civilizations, look for Earth-like planets that can harbor life." How terribly narrow minded.
> you'd think the chance that there is intelligent extraterrestrial life in our galaxy seems enormous
"And the Earthling rocket ships departed Planet X3, having found no life, only an advanced civilization of extraterrestrial robots."
> Another study out of the University of British Columbia looked at the number of sun-like stars in the galaxy and estimated that one in five of them could have an Earth-like planet
On Earth, Life has embarrassed us by being found in the most inhospitable, acidic, saltiest, hottest places. And then embarrassed us again, by turning up in the cold black energy-poor ocean depths, sipping sulfur from vents. Life is surely bigger than Earth-like planets. And Intelligence is surely bigger than Life.
In Sagan's book Contact, the hard problem was not finding intelligence, but recognizing it. By the end, we learn that the arrangement of the stars and the digits of pi itself carry a message. This is the level of thinking that our search calls for.
> On Earth, Life has embarrassed us by being found in the most inhospitable, acidic, saltiest, hottest places. And then embarrassed us again, by turning up in the cold black energy-poor ocean depths, sipping sulfur from vents.
And yet, all life on Earth has the same basic structure. The most likely reason for this is that life only originated once, and in one "place". This observation significantly reduces the probability we should ascribe for life to appear, and/or the specificity of the conditions required. For example, we know for sure that a planet like the Earth today is completely inhospitable to the emergence of life (there has not been an abiogenesis event for billions of years).
The other option is that life has emerged in many places/times on Earth, but with the exact same structure. While unlikely, this would suggest that life must have a very specific structure, that at least on Earth-like planets we should also expect RNA and DNA based life at least. This would also suggest that the likelihood of other kinds of life existing is smaller than initially could have been conceived. And even with this assumption, it would still not be clear that life is appearing on Earth today anymore, as the life forms we've looked at seem to have a phylogenetic link with other life (though our methods may be weak if the possibility of abiogenesis were correct). And even if abiogenesis was still occurring on Earth, it would have to be in some remote place in extreme conditions, as we have tried in all regular places with no success.
Overall, my belief is that the emergence of life is likely a vastly improbable event, and that we shouldn't expect to "see" a living (or formerly living) being that is not related to life on Earth in the lifetime of the Earth.
> And Intelligence is surely bigger than Life.
This seems a mystical belief, given all we know of the world.
> This seems a mystical belief, given all we know of the world.
if you assume that intelligence comes from the brain and if you assume that the brain is just a very highly optimized computer, there's no need for an intelligent entity to be alive. it just needs an adequate computer to exist.
Sure, but the computer must have been built by some form of life ultimately (possibly through a long line of other computers). If your point was only that artificial life created by a form of life could vastly outlast that form of life, then I agree that that is a very good point.
There are certain realities that limit where life can be established and flourish, and while different kinds of life might be possible, they're extremely unlikely due to basic physical/chemical constraints, amount of other elements in the universe compared to hydrogen and carbon and oxygen, etc. And even the most inhospitable place on Earth is pretty okay by galactic standards - Earth's magnetic field, characteristics of our star and our planetary configuration being the main reasons.
I don't think your advanced civilization of extraterrestrial robots is doing enough work to establish this particular point.
That said I would think looking for earth like planets is not so much just for finding life, but also for the point of making it easier to recognize it as life, and finally hopefully having enough points in common to communicate.
Why do they assume that an alien civilisation could only hold communication technology for a hundred years? Surely there’s nothing really holding them back from it unless they wiped themselves out, and is there any reason to believe that at least some of them couldn’t last for thousands or even millions of years?
> "The search for intelligent life is only expected to yield a positive observation if the average life-span of [communicating extra-terrestrial intelligence] within our Galaxy is 3,060 years. That is to say, our communicating civilization here on Earth will need to persist for 6,120 years beyond the advent of long-range radio technology (approximately 100 years ago) before we can expect a [search for extra-terrestrial intelligence] two-way communication."
The problem with long lasting alien civilisations is that if they do exist, why aren't they here already? Colonizing the galaxy only takes a few millions years even with 0.1c.
So far, we only have proof that civilisations can exist for at least several thousand years if you consider the Human civilisation.
But so far, the data on civilisations capable of interstellar communication doesn't look too promising. The "unless they wiped themselves out" is the big question. Are we behind or in front of the Great Filter.
> The problem with long lasting alien civilisations is that if they do exist, why aren't they here already? Colonizing the galaxy only takes a few millions years even with 0.1c.
Suppose it turns out that there is nothing faster for interstellar travel than rockets or light sails. No wormholes stabilized with exotic matter. No warp drives. No other tricks.
I'm not sure in that case that those long lasting civilizations would bother trying interstellar colonization. The round trip transport time between the home world and colonies would be so long that it is hard to come up with any economic benefit establishing such colonies provides to the home worlds.
The problem with these discussions is everyone seems to assume colonising a galaxy is just like colonising a planet - so you have an Age of Exploration, and then you set up colonies, and stuff happens, and you have an Empire or a Federation, or something.
This is really, really unlikely to be true, for all kinds of reasons.
There's a huge difference between colonising a galaxy - which assumes some kind of uniform culture, and common strategic goals - and simply throwing more or less living/sentient stuff around a galaxy and hoping some of it sticks.
We've gone from barely being noticeable on Earth to being in danger of being responsible for a planetary extinction event in around 12,000 years.
We know nothing about building a stable culture that can last for millions of years. It's unlikely any sentient species will be able to learn that skill without making a lot of mistakes - some of which will be terminal.
So galactic colonisation requires a lot more than solving the mechanics of transportation. And the "few million years at 0.1c" estimate is going to be Not Even Wrong.
You don't need to build a stable culture for a galaxy wide colonisation.
We colonized the whole planet without that. Likewise a locust-like swarm of colonizing ships without any central control or common culture would also be a possibility.
> it is hard to come up with any economic benefit establishing such colonies provides to the home worlds.
If medium human life were only 4 weeks, nobody would find funding for a 1 year colonization trip across a different continent. You wouldn't find a crew when it'd take one generation just to cross the atlantic once.
I think it's similar here. With future (bio-)technological improvements, we might live for hundreds or thousands of years. Suddenly individuals would start thinking really long term...
At one point every niche of the solar system will either be declared a national park/treasure, or mined, filled with real estate developments, etc. We'd run out of space and some rich folks would meet on one of the private moons of Jupiter (like how there are private islands today) to discuss a venture towards another star.
> The round trip transport time between the home world and colonies would be so long that it is hard to come up with any economic benefit establishing such colonies provides to the home worlds.
Let’s just hope, for the sake of any beings already on other planets, that you are right.
History here on Earth suggests that long journeys are not a sufficient obstacle for exploitation.
> The problem with long lasting alien civilisations is that if they do exist, why aren't they here already? Colonizing the galaxy only takes a few millions years even with 0.1c
The kind of colonization you’re describing requires exponential growth, but at least on Earth we’ve seen that birth rates dropping in developed nations.
Exponential growth doesn't inherently require population expansion. This would especially be true for a far more advanced civilization. You could dramatically amplify your colonization efforts with machines, reducing your need for traditional population expansion (productivity expansion in tandem with population contraction is exactly what we're going to see on Earth this century).
Without population growth you lose the motivation. Communication gets harder every time you add a star to your civilization, unless you somehow break the speed of light. Resource use is unlikely to be a motivator - a civilization sufficiently advanced to be multistellar will no doubt have cracked either fusion or the stain sphere problem.
A sufficiently advanced civilisation would be able to control their birth rate to whatever rate they would need at the moment. Think of an artificial womb (if that civilisation is mostly biological, if it's mostly robotic, controlling the birth rate becomes even easier).
It's also likely that a self sufficient, long-term thinking planetary civilisation would limit their population to a certain size, to not needlessly exploit the resources that they have available.
Of course, my point is that a high birth rate is something we can’t assume past a certain threshold of development. I can see the argument for being in two, three star systems - it gives you redundancy. Beyond that you add complexity with no inherent benefit.
In fact, the only motivation that makes sense w/o exponential population growth is a militaristic/defense oriented one, which would also favor the dark forest theory.
By definition there has to be a first intergalactic civilization, if there is to be at least one.
The universe is ~15B years old, and we expect it to keep forming stars (as good as any of a metric for “hospitable to life”) for another trillion years. In other words, we’re in the first percent or so of the lifespan of the universe.
Doesn’t seem too crazy that if we start conquering the stars, we might be the first ever species to do so.
(And if history is any indication, we’ll probably cause some sort of galactic global warming in the process, that all starfaring species after us will curse us for)
> (And if history is any indication, we’ll probably cause some sort of galactic global warming in the process, that all starfaring species after us will curse us for)
The only serious theoretical FTL paper (Alcubierre drive) I know of requires exotic matter (negative mass) or energy equivalents of entire planets so if we ever manage to build it, I think that's a fair bet.
Another paper calculated that the space-time bubble created by the Alcubierre drive would capture radiation and particles along its path, creating a gamma ray burst at the destination in a large cone in the direction of travel. Meaning, we'd be sending deadly bursts of radiation all over the galaxy that would wipe out all carbon based life for tens or hundreds of light years every single jump. A rogue actor or a miscalculation could wipe out significant fractions of our own civilization.
There might be not a lot sense in doing that. Maybe infinite growth isn't desirable. Maybe becoming a hundred trillion people across a galaxy isn't so great.
There might be some crossover between civilizations that take dark forest precautions and the type of civilization that doesn’t kill itself on accident.
Or it could be possible that a civilization pessimistic and risk-adverse enough make dark forest precautions a significant part of their culture wouldn't have the risk-taking needed to advance their technology to interstellar levels. Technological advancement and the possibility of self-destruction might go hand in hand.
Such a great trilogy! I hope we'll send out noticeable signals into space at some point down the line, and I wonder if our descendants in a few tens of thousands of years will look back on it as self-destructive hubris in the same way we look as burning coal now.
At this point of theoretical speculation, it's perhaps just as creatively interesting to visit sci-fi on first contact situations and evaluate their approaches to see how they mesh with our astronomical observations.
The book that came to mind is a classic, "The Mote in God's Eye." Though there are dozens of delightful variations and imaginative situations, such as with "The Saturn Run." There's an entire spectrum that's been imagined and explored.
I worry that other far more advanced civilizations out there routinely stamp out any new "pest" civilizations....much like how I handle ant colonies behind my house. They get too large or too close...then I knock them out.
Isn't the who point of the drake equation to discuss the factors? And most of the factors can't actually be measured until you've met a few dozen other civilisations and compared histories?
How can this article, as well as the hundreds of comments here, not acknowledge that all they are doing is solving the Drake Equation for new values they are estimating?
Best chance of anything hypothetically being possible in the very very far future would be not going faster than light, but contracting and expanding space. There's plenty of problems with that theory, one of many would be that it would hypothetically have an event horizon at the front while "moving" faster than light, which would be a big problem because the Hawking radiation from it would kill stuff. It's called an Alcubierre drive and it's a fun thought experiment at the very least. Due to the energy constraints if it was ever hypothetically possible it would need to envelop the vessel in a bubble that's downright infinitesimally small. You could pass right through a planet and not hit anything.
Paradigm shifts don't make physical phenomena possible. At best, they take an observed natural phenomena, isolate and amplify it. AFAIK nothing has been observed moving faster than light and I don't believe there is anything even predicted to. Of course, the details of black hole physics, and early cosmological physics, are very strange and full of contradictions. To relate back to the original topic, to last for a very long time, I would imagine a civilization would require some sort of central urge even beyond simple survival, and argue that studying black holes would be one of the best, and perhaps universal motivators. (The goal of survival in-and-of-itself at the civilizational level is ironically quite nihilistic!)
As far as I know (not a physicist), it takes infinite energy to reach FTL, so out of the question with modern science as you've said.
As a non scientist, I'm also confused with some of the relativistic implications of FTL. I think motion through space and motion through time add up to the speed of light. If so, time would freeze when hitting the speed of light, but does that happen to the traveler moving at C, or to the outside observer? Also, if traveling faster than light (FTL), do I go back in time? Again, is that from the traveler or outsiders perspective?
As far as getting FTL, you've probably heard of the Alcuberrie (spelling) drive which expands spacetime behind a ship and contracts in front of it to push a ship through spacetime via a bubble? Sounds great, but I think it requires exotic materials which don't exist. So we're stuck. I'm pointing this out in the chance you don't already know this.
My hope is that within the next century or millennia, our understanding of math and physics will advance to a point to where things like wormholes become possible, but it'll be long after I'm dead. What worries me is if we're nearing the end of what the human brain is capable of realistically uncovering. Like if my brain's network was significantly denser and able to think better, maybe we could come up with something? I don't think the solutions to the next problems will be a single equation like some physicists like to claim (note they're far more qualified than I to speculate on this). I just see things like the proof for Fermat's Last Theorem that is like 80 pages and one man took like a decade to figure out and other mathematicians took like a year to check. Someone like me has zero chance of intellectually following that. If we have indeed "picked all the low hanging fruit" we can expect very small marginal gains in the upcoming years even with legions of scientists.
My hope is that we're just in another phase waiting for another Newton or Einstein to come up with a single beautiful idea that revolutionizes the field and leads to an explosion of progress.
Their key assumption is: "We assume that if an SP remains in the circumstellar Habitable Zone (HZ) for a time equal to the current age of the Earth it will develop intelligent, communicative life." (page 5 at https://arxiv.org/ftp/arxiv/papers/2004/2004.03968.pdf)
Why? Why do you assume a planet will develop intelligent life when it remains in the Habitable Zone long enough? Just because it happend on Earth?
And they continue with: "[...] according to the Copernican Principle – we propose that the likelihood of the development of life, and even intelligent life, should be broadly uniformly distributed amongst any suitable habitats"
We have no clue how likely it was at Earth that life developed at all. Maybe it was just a chain of many lucky coincidences that happen once in 10 billion years. How is distribution any relevant then?