A universe full of dead aliens

seamus

It is worth noting that larger dinosaurs had been on a decline for millions of years before the K-T event. It's pretty likely that without it the large non-avian dinosaurs would have become extinct naturally. The changing global climate was better suited to the small feathered flyers than the classic terrible lizards. Would mammals still have dominated? Very hard to say. Large swathes of life from all kingdoms were wiped out in the K-T event, mammals just had that slight edge.

The intelligence levels of many birds tells us that human-level intelligence is not necessarily limited to strict and specific circumstances, but could arise at any time really.

We also don't know if sentient intelligence never existed on earth before. If a meteor event like K-T were to happen again tomorrow, any civilisation that appeared 66 million years from now would likely know nothing of our existence, all of our structures crumbled and blown away, all evidence of our existence having been wiped out by the geological timescale.
So perhaps some evidence might exist - like the odd satellite or buried nuclear waste. But if we had been wiped out in the 1800s, humanity's existence would never be known about.
So we actually can't assume that an intelligent civilisation has never existed on earth before.

Maximus Alexander

seamus wrote: »

We also don't know if sentient intelligence never existed on earth before. If a meteor event like K-T were to happen again tomorrow, any civilisation that appeared 66 million years from now would likely know nothing of our existence, all of our structures crumbled and blown away, all evidence of our existence having been wiped out by the geological timescale.
So perhaps some evidence might exist - like the odd satellite or buried nuclear waste. But if we had been wiped out in the 1800s, humanity's existence would never be known about.
So we actually can't assume that an intelligent civilisation has never existed on earth before.

Interesting perspective. I'd say it's safe enough to rule out a technological species because you would expect there would be clues about their tools and machines in the fossil record. However, it would probably be hard to rule our something more akin to early man in the cave painting phase having existed 10s of millions of years ago. Never really thought about it like that before.

seamus

Even on the simplest scale thinking about human evolution and the patchiness of our own fossil record; for some species we have only part of a single individual as confirmation that an intelligent species (and society) ever existed. How many evolutionary dead-ends (like Neanderthals) occurred in the homind tree that we will never know about? But nonetheless were likely to be intelligent species.

And considering that these species existed yesterday in geological terms, it's quite possible that hundreds of distinct species with human-level intelligence could have evolved and died in the last 500 million years, and we'd never know.

PMBC

Thanks Max and Seamus.
I didn't appreciate how frequent such meteor strikes were or that the large dinos were already on the wane. I don't have any 'position' on intelligent design versus chance except one of constantly wondering.

Capt'n Midnight

PMBC wrote: »

Would we have eveloved if the dinos had not been wiped out? Was that event not very small statistically?

Perhaps. But very unlikely. The ancestors of mammals split from the ancestors of the dino's about 275 million years ago. So we had 210 million years to make it to the top before the dinos died out. Birds took over for a while.



As for intelligent dino's winning out ?

Comparing brain sizes even the smartest dinos don't compare well to dumb birds. So any surviving species could have won the brains race. Birds, other dinos, mammals, other mammals, other reptiles, whatever.

Note that if the dinos had survived and become intelligent then they would not look like us despite all the bad SciFi. The only excuse for saying so is to save money on costumes and that's no longer a good excuse given how cheap computer graphics are these days.


Just look at the other candidates for intelligent life here. Most are quadrupeds with tails. The bipeds have wings. Dolphins don't have legs.

And they all very similar to us compared to an octopus. And even they shared a common ancestor for most of the time life existed on this planet. So apart from streamlining don't expect ET look like anything us.

ps200306

_Tombstone_ wrote: »

​What If We Haven’t Found Aliens Because Humans Came First?

http://motherboard.vice.com/read/what-if-we-havent-found-aliens-because-humans-came-first.

Sorry for quoting your post a second time but something else just occurred to me about it. Red dwarf stars may last thousands of times longer than our Sun, but their planets may not last that long in a habitable state. I'm thinking in particular of plate tectonics. Won't all mantle activity in a rocky planet have ceased long, long before trillions of years have elapsed?

We can't be certain how vital plate tectonics is, but it seems to have played a major role in regulating our atmosphere through volcanoes and mountain weathering, altering mantle chemistry with subducted water, concentrating vital mineral veins, making fertile land, creating environments for allopatric speciation, and perhaps a host of other things we don't know about.

Planets vastly older than our own might not then be such good candidates for life.

Capt'n Midnight

ps200306 wrote: »

We can't be certain how vital plate tectonics is, but it seems to have played a major role in regulating our atmosphere through volcanoes and mountain weathering, altering mantle chemistry with subducted water, concentrating vital mineral veins, making fertile land, creating environments for allopatric speciation, and perhaps a host of other things we don't know about.

Overall solar wind would be worse from a red dwarf ?

Other than that lower temperature star would mean less photo-dissociation ? so maybe a longer lasting atmosphere, or at least more able to support an atmosphere with lower gravity. So would make it easier for the inhabitants to leave the planet. most photosynthesis here uses red light anyway.

ps200306

Yes, worse solar wind. And a planet in the habitable zone after star formation would apparently be in a pretty hot zone for an extended period before an M dwarf reaches the main sequence. Photo-dissociation of the water with all the hydrogen escaping could leave only oxygen.

Taking this from a Wikipedia page I hadn't realised existed: Habitability of red dwarf systems

Another good article: Red Dwarf Planets Face Hostile Space Weather Within Habitable Zone/

It all sounds a bit bleak.

froog

regarding the possibility of intelligent life in the galaxy (or universe) i like to think of it this way - in the absolutely miniscule sample size of space we have explored and the handful of planets we've seen up close out of many many billions, we have absolute proof of one intelligent civilization - us.

Chromosphere

Considering we're only 50 years, if even into the level of technology to detect anything, I think we're still not really able to conclude anything other than we haven't found anything yet.

For all we know we may have stumbled upon radio waves, while others have stumbled upon something fundamental that we haven't hit upon yet and we are not actively detecting.

We should definitely keep looking though. We've only barely scratched the surface.

Maximus Alexander

Chromosphere wrote: »

For all we know we may have stumbled upon radio waves, while others have stumbled upon something fundamental that we haven't hit upon yet and we are not actively detecting.

Or maybe we've observed loads of things that we assume are natural phenomena, and we've gone and built our theories of the universe on that basis, but in actual fact we are completely wrong and the universe is teeming with life while we misinterpret the byproducts of their activities. :pac:

Chromosphere

We also make huge assumptions on scale and how biology might work exclusively based on what we see around us as that's all we have as a template to work from.

The reality is that the variables are so broad that we literally have no idea what we are looking for until we stumble across it.

intheclouds

Im inclined to go with the analogy of an ant hill next to a highway.

The ants are not aware of the existence of the road, even though they are right next to it. They didnt perceive us while the road was being built, they dont notice the cars passing.

We could be the ant hill. Alien life could be all around us, but we just dont perceive it.

Life Jim, but not as we know it.

ps200306

I don't think alien life will be vastly different to what we expect. It's from another planet, not another universe with different laws of physics. Entropy is winding down the universe, and life has to harness energy to reverse that on a local scale in order to exist. Every biosphere must include some kind of autotrophs. They have to built from the available raw materials and so does anything that eats them. We know what the available raw materials are -- they're the ones manufactured inside stars -- and we know their universal abundances, as well as their likely distribution on a rocky planet. We also know their chemical affinities, and that one of the most common -- carbon -- is overwhelmingly more versatile than all the rest.

We assume that complex life (and especially intelligent life) must be evolved life, since stable complexity doesn't spontaneously and instantaneously arise. Evolution requires reproduction and descent with modification. Reproduction requires stable inheritance of traits and the only mechanisms of sufficient fidelity for this are digital.

So it's unlikely we're going to find life consisting of gases or pure energy. It'll probably be on rocky planets like our own, will probably be carbon-based like us, will probably consume food and excrete waste, will probably have chemically encoded digital genes like us, and may even have evolved mechanisms akin to sexual reproduction for more rapid evolutionary experimentation.

Body forms will have taken a completely different evolutionary track to ours, but we won't be surprised to see some examples of homoplasy (a.k.a convergent evolution) due to our common requirements of locomotion, sensory perception, and so on.

This all suggests to me that alien life might be very different, but not altogether unrecognisable.

froog

ps200306 wrote: »

I don't think alien life will be vastly different to what we expect. It's from another planet, not another universe with different laws of physics. Entropy is winding down the universe, and life has to harness energy to reverse that on a local scale in order to exist. Every biosphere must include some kind of autotrophs. They have to built from the available raw materials and so does anything that eats them. We know what the available raw materials are -- they're the ones manufactured inside stars -- and we know their universal abundances, as well as their likely distribution on a rocky planet. We also know their chemical affinities, and that one of the most common -- carbon -- is overwhelmingly more versatile than all the rest.

We assume that complex life (and especially intelligent life) must be evolved life, since stable complexity doesn't spontaneously and instantaneously arise. Evolution requires reproduction and descent with modification. Reproduction requires stable inheritance of traits and the only mechanisms of sufficient fidelity for this are digital.

So it's unlikely we're going to find life consisting of gases or pure energy. It'll probably be on rocky planets like our own, will probably be carbon-based like us, will probably consume food and excrete waste, will probably have chemically encoded digital genes like us, and may even have evolved mechanisms akin to sexual reproduction for more rapid evolutionary experimentation.

Body forms will have taken a completely different evolutionary track to ours, but we won't be surprised to see some examples of homoplasy (a.k.a convergent evolution) due to our common requirements of locomotion, sensory perception, and so on.

This all suggests to me that alien life might be very different, but not altogether unrecognisable.

ps200306

Maybe not that recognisable :pac:

Capt'n Midnight

Chromosphere wrote: »

Considering we're only 50 years, if even into the level of technology to detect anything, I think we're still not really able to conclude anything other than we haven't found anything yet.

For all we know we may have stumbled upon radio waves, while others have stumbled upon something fundamental that we haven't hit upon yet and we are not actively detecting.

The whole point of quantum encryption is that you could send a signal that couldn't be eavesdropped without your knowing.

If ET is using long distance comms then like us they'll be using focused beams with as little power as necessary to get the job done. The days of spark gaps radiating across the Atlantic are gone as are picking up Band I TV from Crystal Palace down as far as South Africa.

The only way we'll pickup signals is if they beam them at us.



Fibre to the home is a real thing now and in the future there'll be less leaky copper radiating signals.

Capt'n Midnight

Chromosphere wrote: »

We also make huge assumptions on scale and how biology might work exclusively based on what we see around us as that's all we have as a template to work from.

The reality is that the variables are so broad that we literally have no idea what we are looking for until we stumble across it.

We are getting very excited about levels of some gases in the atmosphere of Mars because they aren't in equilibrium.

If the permafrost melts more here there'll be relatively large amounts of methane in our atmosphere of oxygen which will stand out like a sore thumb.

Chromosphere

Given that we can't actually find a large aircraft that got lost on planet earth, I wouldn't assume we haven't missed a lot in space.

There's still a bit of a notion out there that anyone assuming there are aliens out three somewhere are a bit off the wall, which doesn't really help things like SETI to get funding.

Anything is possible and we just don't know enough to draw any collisions at all.

Maximus Alexander

Chromosphere wrote: »

There's still a bit of a notion out there that anyone assuming there are aliens out three somewhere are a bit off the wall, which doesn't really help things like SETI to get funding.

I don't know about that, SETI is generally held in good regard because they take a skeptical approach. It's the nutters who see a light in the sky and think it's an alien mothership who are seen as off the wall; rightly so.

I think the funding issue is more that with limited budgets available, money tends to go to projects with a higher chance of producing useful results. SETI is an open ended search that may never find anything, and it doesn't have an end point; by not finding things you're not learning anything, which is not usually the case with scientific research.

Capt'n Midnight

Chromosphere wrote: »

Given that we can't actually find a large aircraft that got lost on planet earth, I wouldn't assume we haven't missed a lot in space.

The atmosphere is very roughly equivalent to 10 meters of clear water. We'd have no problem finding a plane if it was only that far down.

On the other hand space is big, really big. Things bigger than Pluto have been found recently in what is essentially our back yard.

ThunderCat

My money is on other civilizations being detected through analysis of their exoplanets atmosphere as our telescope technology progresses. I think that would be more likely than any signal as such reaching us. Then again the photons from their atmosphere reaching our telescopes is a signal really isn't it. Roll on the JWST I say!

ps200306

ThunderCat wrote: »

My money is on other civilizations being detected through analysis of their exoplanets atmosphere as our telescope technology progresses. I think that would be more likely than any signal as such reaching us. Then again the photons from their atmosphere reaching our telescopes is a signal really isn't it. Roll on the JWST I say!

I definitely agree exoplanet atmospheres are the next frontier in the search for life, given our ability to do spectral analyses is going to improve immensely. But I've always been a bit confused about which gases we're looking for. Venus has carbon dioxide, Mars has methane, Jupiter has water vapour, yet we don't think any of those are of biotic origin. Oxygen would be an obvious one, since it is reactive and we don't expect it to last long in the atmosphere unless it is being replenished. But does that mean we expect photosynthesis to evolve on other planets just as it has here? Is there something obvious about those chemical pathways that make it uber-likely, or is it just that it gives us the best shot of being able to definitively say we've found life?

JupiterKid

My personal feeling is that life is abundant throughout the universe, but that highly evolved, intelligent life is very, very rare.

If you look at Earth's history, simple unicellular life forms emerged pretty quickly in our 4.6 billion year history, but complex life did not evolve until about 600 million years ago. It then took another 599 million years for intelligent life capable of modifying its environment - basically us humans - to come on the scene.

That history indicates to me that life may take a foothold relatively easily on terrestrial planets in the "goldilocks zone" on their solar systems but that a very improbable series of events leads to advanced, multicellular life.

And we've only been listening out for radio signals from other civilisations for less than 60 years. of course, advanced alien civilisations have probably long since moved beyond radio waves as a means of communications.

There is a very good online course on Coursera that explores exobiology and the issues pertinent to possible extraterrestrial life.

mickmackey1

JupiterKid wrote: »

If you look at Earth's history, simple unicellular life forms emerged pretty quickly in our 4.6 billion year history, but complex life did not evolve until about 600 million years ago. It then took another 599 million years for intelligent life capable of modifying its environment - basically us humans - to come on the scene.

But that still leaves hundreds of millions of years when intelligence could have appeared, before the Sun begins to expand. And there's no reason to assume the Earth is particularly hospitable, there should be multitudes of planets with better conditions.

Ultimately with a case study of one, we can't decide anything.

Myrddin

JupiterKid wrote: »

My personal feeling is that life is abundant throughout the universe, but that highly evolved, intelligent life is very, very rare.

If you look at Earth's history, simple unicellular life forms emerged pretty quickly in our 4.6 billion year history, but complex life did not evolve until about 600 million years ago. It then took another 599 million years for intelligent life capable of modifying its environment - basically us humans - to come on the scene.

That history indicates to me that life may take a foothold relatively easily on terrestrial planets in the "goldilocks zone" on their solar systems but that a very improbable series of events leads to advanced, multicellular life.

And we've only been listening out for radio signals from other civilisations for less than 60 years. of course, advanced alien civilisations have probably long since moved beyond radio waves as a means of communications.

There is a very good online course on Coursera that explores exobiology and the issues pertinent to possible extraterrestrial life.

Don't forget though there's been quite a few near-complete extinction events in those years, meaning a lot of evolutionary progress would have been lost. Without those events, things could have been very different here on Earth

Gwynston

That poses some interesting questions:

What kind of event would it really take for a complete extinction of all life?

Or is it inevitable that once life has taken hold, pretty much any catastrophe could be survived by some life? (Even if it's low level, it could be enough for evolution to get going in some new direction with the inevitable result of complex life again.)

Mass extinction events in the past haven't really come that close to wiping out all life, and one could argue were entirely necessary to get to where humans are today. So maybe they're just part and parcel of the evolutionary cycle (albeit on the extreme side)?

seamus

We only really need look towards the rest of the solar system to see what kind of conditions become functionally incompatible with life - certainly any life that has evolved on earth anyway.

While extremophiles abound on earth, the actual range of environments they can survive in is still relatively narrow on a cosmological scale.

The tardigrade is often quoted as indestructible, but only really in its ability to hibernate. It doesn't actually live and reproduce in harsh conditions. We could send a bag full of tardigrades to Mars and dump them on the surface. They'll survive the trip, but won't "live" and reproduce and will eventually die.

In effect any event where the surface temperature of the earth dropped below -100C permanently would be enough to functionally kill everything on earth. Some organisms may "survive" in a crystallised or fossilised state, but that's about it. If it was a sudden freezing period, followed by a warming period again, life could easily restart. Not least from consuming all of the decaying plants and animals on the surface.

Cooking the planet is far more likely to actually destroy everything. A large meteor strike can send a shockwave that will basically tear up the earth's crust and turn the outer surface into molten material. All life would be gone (and without a trace too).

Of course, life could evolve again once the surface had cooled sufficiently, but it wouldn't be the same life we see now.

Gwynston

seamus wrote: »

A large meteor strike can send a shockwave that will basically tear up the earth's crust and turn the outer surface into molten material. All life would be gone (and without a trace too).

But how likely is that, since the Solar System has been fairly stable for billions of years now? The only meteor impacts that have occurred since life evolved weren't big enough for that. I would suggest that there aren't any big enough bodies left on any possible collision courses.

So my question remains: What could happen (or could have happened in the past 500m years) that would completely wipe out life?

Maximus Alexander

Gwynston wrote: »

But how likely is that, since the Solar System has been fairly stable for billions of years now? The only meteor impacts that have occurred since life evolved weren't big enough for that. I would suggest that there aren't any big enough bodies left on any possible collision courses.

There could easily be a moon mass object out there lurking at 5000 AU in the Oort Cloud on a 350,000 year elliptical orbit just waiting to strike. :P

seamus

Gwynston wrote: »

So my question remains: What could happen (or could have happened in the past 500m years) that would completely wipe out life?

To be fair, you asked "what kind of event would it take", rather than "what's likely to happen"

It's true to say that the solar system is far more stable than it was previously.

But there are still plenty of life-ending-sized objects floating around; bearing in mind that a relatively small object at huge speeds can do just as much damage as a city-sized object at a slow speed.

We've seen a number of impacts on Jupiter in the last century which would be devastating in an earth context. Notwithstanding Jupiter's ridiculous gravity well, the kind of objects which could wipe out everything are still out there.

Earth tends to see massive events every few hundred million years. When the last one is ~60mya ago you might talk about a new one being "due". But in the context of the existence of the human race which might be at best a million years in total, the odds of such an event occurring while humanity exists is very small. The odds of it occurring while you or I are alive are practically nil. But the odds of one occurring are basically certain in a geological timescale.

Gwynston

seamus wrote: »

To be fair, you asked "what kind of event would it take", rather than "what's likely to happen"

OK, fair enough...! :P

seamus wrote: »

Earth tends to see massive events every few hundred million years. When the last one is ~60mya ago you might talk about a new one being "due".

Yeah, but my point is that these kind of events aren't big enough to completely eradicate life - even that last one wasn't as big, or as devastating to life as earlier ones.

So it would take something bigger than we've seen in the last 500m years to have a bigger affect on life than we know it can survive.

I would say it's a lot less inevitable in the future than it was when the solar system was less stable. And things hitting Jupiter might well be one of the reasons the inner solar system is so stable.

josip

i would look for the presence of U-235 or Pu-239 in the exoplanet atmosphere as a sign that intelligent life once existed.
Half life of half a billion years for U-235 gives us a good window of opportunity but does exist naturally.
Half life of 25,000 years for Pu-239 is quite short but unlikely to have been produced naturally so any quantity of it in the atmosphere would be telling.

Given how often the issue of intelligent life (usually us) wiping themselves out before they can get off the planet is raised, i think it's a good bet that more planetary civilizations nuke themselves than send "Nous sommes ici" signals to the rest of the cosmos.

Maximus Alexander

josip wrote: »

i would look for the presence of U-235 or Pu-239 in the exoplanet atmosphere as a sign that intelligent life once existed.
Half life of half a billion years for U-235 gives us a good window of opportunity but does exist naturally.
Half life of 25,000 years for Pu-239 is quite short but unlikely to have been produced naturally so any quantity of it in the atmosphere would be telling.

Given how often the issue of intelligent life (usually us) wiping themselves out before they can get off the planet is raised, i think it's a good bet that more planetary civilizations nuke themselves than send "Nous sommes ici" signals to the rest of the cosmos.

A rather depressing approach which, while confirming that other intelligent life had been out there, would only be likely do so once it had extinguished itself.

josip

Maximus Alexander wrote: »

A rather depressing approach which, while confirming that other intelligent life had been out there, would only be likely do so once it had extinguished itself.

The uber positive among us would take hope from the fact that now that we had proven we weren't the only form of intelligent life, we might someday detect a species before they hit the big red button.
Given the time/distances usually involved eg. 100,000 light years, there's not really any difference between them being alive or dead at detection time.
A lot could have happened to them since then. They might have grown an extra head.

Capt'n Midnight

Gwynston wrote: »

That poses some interesting questions:

What kind of event would it really take for a complete extinction of all life?

Your common or garden Supernova.

The team discovered that at some time in their lives, the majority of stars in our Galaxy will be bathed in the radiation from a nearby supernova, whereas around 30% of stars remain untouched or unsterilized. “Sterilization occurs on a planet that is roughly [at a distance] between 6.5 to 98 light years, depending on the supernovae,” says Gowanlock. “In our model, the sterilization distances are not equal, as some supernovae are more lethal than others.” - See more at: http://www.astrobio.net/news-exclusive/living-in-the-galactic-danger-zone/#sthash.WJpDPKXM.dpuf

You don't see that mentioned much by the Drake fanboys.

_Tombstone_

The Sixth Mass Extinction Will Be Like Nothing In Earth's History

_Tombstone_

The Sixth Mass Extinction Will Be Like Nothing In Earth's History

ps200306

ps200306 wrote: »

ThunderCat wrote: »

My money is on other civilizations being detected through analysis of their exoplanets atmosphere as our telescope technology progresses. I think that would be more likely than any signal as such reaching us. Then again the photons from their atmosphere reaching our telescopes is a signal really isn't it. Roll on the JWST I say!

I definitely agree exoplanet atmospheres are the next frontier in the search for life, given our ability to do spectral analyses is going to improve immensely. But I've always been a bit confused about which gases we're looking for. Venus has carbon dioxide, Mars has methane, Jupiter has water vapour, yet we don't think any of those are of biotic origin. Oxygen would be an obvious one, since it is reactive and we don't expect it to last long in the atmosphere unless it is being replenished. But does that mean we expect photosynthesis to evolve on other planets just as it has here? Is there something obvious about those chemical pathways that make it uber-likely, or is it just that it gives us the best shot of being able to definitively say we've found life?

I stumbled across an answer to my own question. Apparently the telltale sign is not one particular unique gas, but a combination of gases that are out of kilter with the "thermochemical equilibrium". In other words, the temperature and insolation will create an environment in which oxidation and reduction (redox) reactions go in a particular direction at particular rates -- Le Chatelier's Principle from my dim recollection of chemistry class.

So for example, the 1.6 parts-per-million of methane in our atmosphere in combination with high oxygen levels is a biomarker because it represents a thermochemical disequilibrium. Unfortunately, 1.6 ppm is very difficult to measure with transmission spectroscopy in an exoplanet atmosphere. And in a high UV environment, biosignature gases can be quickly broken down and reduced to undetectable levels.

Telescopes of the calibre of the JWST will be needed to have any hope of detecting such gases even in planetary systems within a couple of dozen light years of us. And, of course, the number of transiting exoplanets within that distance on which transmission spectroscopy can be used is very low. So we need to advance to direct exoplanet imaging, which needs a whole new class of space telescope again, not to mention "star shades".

Loads more info in this fascinating paper: The future of spectroscopic life detection on exoplanets.

josip wrote: »

i would look for the presence of U-235 or Pu-239 in the exoplanet atmosphere as a sign that intelligent life once existed.
Half life of half a billion years for U-235 gives us a good window of opportunity but does exist naturally.
Half life of 25,000 years for Pu-239 is quite short but unlikely to have been produced naturally so any quantity of it in the atmosphere would be telling.

After reading above that 1.6 ppm of methane would be seriously difficult to measure, I was wondering what sort of concentrations of gases we are talking about from nuclear fallout. I know there is no rule of thumb since concentration and strength of a line in a spectrograph are not directly correlated, but a ballpark idea would be useful.

Detections of nuclear fallout on Earth don't depend on spectroscopy, but use radiation counters to measure concentrations of daughter products, especially radioactive noble gases that can be sampled from the atmosphere by suitably equipped aeroplanes. Different isotopes of Xenon can indicate Plutonium or Highly Enriched Uranium fission, and they can be detected both after nuclear tests and in the vicinity of nuclear reactors (refs: [1], [2], [3], [4], [5]).

"Interesting" levels of radioxenon are typically in the range of tens to hundreds of mBq/SCA. That unit of measurement is milli-Becquerels per Standard Cubic meter of Atmosphere. A Becquerel is one radioactive decay per second.

All of the isotopes of radioxenon involved have half-lives in the range 0.5 to 12 days. The longest half lives produce the lowest number of decays, and therefore need the biggest concentration for a given decay count. So we can calculate the number of xenon atoms of half-life 12 days in a population that would produce one decay per second. First we convert the half-life into a decay constant:



The decay constant is defined as the number of decays per unit time for a given quantity, N:



So by setting the left hand side to -1 per second (i.e. 1 Becquerel), and rearranging, we can calculate the number of atoms of radioxenon required to produce that rate:



We would need one tenth of this number to produce 100 mBq. Finally, we can calculate the number of molecules in a cubic metre of standard atmosphere using the molar gas volume and Avogadro's number:





So the concentration of radioxenon that would give 100 mBq/SCA is:



... in other words, about five parts per billion trillion. It sounds like the sort of concentrations of radioactive gases even in close proximity to nuclear reactors and bomb tests on Earth would be undetectable on exoplanets by spectroscopy.

josip

ps200306 wrote: »

... in other words, about five parts per billion trillion. It sounds like the sort of concentrations of radioactive gases even in close proximity to nuclear reactors and bomb tests on Earth would be undetectable on exoplanets by spectroscopy.

What about a nuclear winter?

ps200306

josip wrote: »

What about a nuclear winter?

I guess you might see some kind of dust or soot signature (lots of infrared). Possibly indistinguishable from a volcanic winter or a space impact winter, but in any case only lasting a few years before the dust falls out so the chance of catching one in the act would be very small.

Gwynston

Interesting article discussing the role that Earth's geological activity plays in life on the planet, and the repercussions for finding extraterrestrial life:

The unexpected ingredient necessary for life

ScumLord

seamus wrote: »

"Similar level of evolution" is something of a misnomer. There are no "levels" of evolution, nothing is "more evolved" than anything else.

Evolution doesn't have a set path that it follows where it is trying to achieve peak intelligence. Intelligence is an emergent property of evolution under certain conditions.

I'm sure crocodiles and sharks would say to wait another 10 millions years and if we still look the same as we do now we can start talking about being better evolved.

Another thing I don't hear mentioned much is the fact that the universe probably wasn't capable of supporting life for the first few million/billion years.

We are told we are made from star dust, which is true, so that would mean the first solar systems probably didn't have the required elements to make even the most basic forms of life.

When the second round of stars came about would their solar systems still be pretty low on heavy elements making life still very difficult or unlikely? For all we know our star which I heard somewhere is a third generation star is the first star that had enough star junk orbiting it to allow life to form.

We could be at the start of biologies life span rather than half way through.

Even though life has been around for billions of years on this planet it barely clung onto existence in the early stages of this planet. Life didn't get properly complex until the dinosaurs came along. Humans where a complete accident of climate change. Without that accident there's no reason to think another intelligent animal would have evolved to fill our place.

For all we know earth provides very specific conditions that allow life to thrive, and complex life can only form under the exact conditions that happened on earth. We don't even know how essential the moon was to that evolution. It could be that we will find basic single celled life all over the place but only earth sized planets with a moon will create complex life.

It all depends on what we find on the other planets, but even then we can't say that any life we find in our own solar system isn't just contamination from earth.

mickmackey1

ScumLord wrote: »

We are told we are made from star dust, which is true, so that would mean the first solar systems probably didn't have the required elements to make even the most basic forms of life.

When the second round of stars came about would their solar systems still be pretty low on heavy elements making life still very difficult or unlikely? For all we know our star which I heard somewhere is a third generation star is the first star that had enough star junk orbiting it to allow life to form.

Stars that end up as supernovae go through their lifetimes extremely quickly, on the order of around 10 million years. So several generations could come and go in the blink of an eye, astronomically speaking. Therefore plenty of opportunity for life to exist billions of years before now.

Capt'n Midnight

ScumLord wrote: »

Another thing I don't hear mentioned much is the fact that the universe probably wasn't capable of supporting life for the first few million/billion years.

We are told we are made from star dust, which is true, so that would mean the first solar systems probably didn't have the required elements to make even the most basic forms of life.

Yes you need a star going through the CNO cycle to make Carbon Nitrogen and Oxygen, and you need that star to go supernova to get those elements expelled from the star.



Silicon dioxide makes up most of the crust of this planet. And most animals and plants ignore silicon. You are most likely to encounter it in the stings of nettles. Similarly Iron , Aluminium and Magnesium make up most of the rest of the crust and again are only used as trace elements. Apart from Oxygen and Hydrogen we only use rare elements from the crust. Calcium we use in bones and things but bacteria don't really need it.

But biological systems are very good at concentrating stuff they do need.

So scarcity of elements isn't a show stopper. And once you leave the top rows of the periodic table elements can substitute to some extent. Arsenic for phosphorous , Strontium for Calcium. We use iron to carry oxygen, other organisms use copper. Tunicates use vanadium for something. Life can find a way, once it gets a toehold.

josip

Capt'n Midnight wrote: »

Yes you need a star going through the CNO cycle to make Carbon Nitrogen and Oxygen, and you need that star to go supernova to get those elements expelled from the star.



Silicon dioxide makes up most of the crust of this planet. And most animals and plants ignore silicon. You are most likely to encounter it in the stings of nettles. Similarly Iron , Aluminium and Magnesium make up most of the rest of the crust and again are only used as trace elements. Apart from Oxygen and Hydrogen we only use rare elements from the crust. Calcium we use in bones and things but bacteria don't really need it.

But biological systems are very good at concentrating stuff they do need.

So scarcity of elements isn't a show stopper. And once you leave the top rows of the periodic table elements can substitute to some extent. Arsenic for phosphorous , Strontium for Calcium. We use iron to carry oxygen, other organisms use copper. Tunicates use vanadium for something. Life can find a way, once it gets a toehold.

But they do need toes first?

tac foley

All this thread has provided me with some good reading, but it really poses far more questions than it could possibly answer.

The REAL problem is that our electronic 'message to the stars' is actually getting fainter, rather than stronger, as we use ever shorter wavelength for communication and other electronics prone to atmospheric leakage into space. In the early days of radio, we were happily using huge wave-forms and low frequencies and even TV and RADAR when in their infancy were powerful emitters.

Not so now.

Remember, too, that the shorter wavelengths are more prone to distance attenuation, and as the signal disappears into space it is as much subject to the inverse square law of radio propagation as any other form of electromagnetic radiation. Add to that that we have only been pumping this stuff out for a little over a hundred years, and that alone limits us to a hypothetical hundred light-year sphere of detection.

What is there within that range?

Not a lot, folks.

And even with the latest forms of propulsion, getting to the nearest star would take around 70,000 years, and by the time we got there, the twin stars that make up the Alpha Centauri binary would have moved so that what had been the nearest to us is the furthest away....

Shouting, whether it be with radio or light, is never going to work in our favour - we really are situated in the backwater slums of an average galaxy among a trillion others.

tac

ps200306

Interesting new paper by Sandberg, Drexler, and Ord -- pop science version here, actual paper here. They review various parameters of the Drake Equation, looking at ranges cited in the scientific literature. But their main argument is a mathematical one: estimates of the likelihood of extraterrestrial intelligence are wildly optimistic because of the use of point estimates.

To illustrate the idea, they take a toy model in which the Drake Equation has nine parameters, each with an estimated range of probability from 0 to 20%. The naive approach is to choose a median value of 10% probability for each of the nine parameters, giving a combined probability of one in a billion. With a 100 billion stars in our galaxy, that gives us 100 stars with ETIs. Moreover, the chance that all 100 billion systems come up empty, giving us zero ETIs, is a vanishingly small .

But instead of taking the median probability for each parameter, let's draw them at random from the range 0 - 20%. If we have no other information to guide us this is a better representation of our a priori knowledge. Now the chance that we get an empty galaxy is better than one in five, times more likely than computed with the naive point estimate approach.

Expanding on this line of reasoning with more realistic numbers, they conclude that we shouldn't be surprised to find no intelligent life in our galaxy or even in the observable universe. They are not saying this is likely, just that it is well within the range of probabilities as we know them so far. The "Fermi Paradox" is then not a paradox but merely an observation -- one which serves as disconfirmatory evidence.

Best we don't get off this planet if you ask me. We as a species will sweep through any new planet like a plague, eventually. How long before we're talking about conserving the last few of whatever alien species are left on the new planet? As we've done here. How long til our good intentions turn to "we should dig up every mineral on the new planet for the benefit of business interests?" As we've done here. The rest of the universe will be better off if we live and die on Earth.

In that same vein we might also be better if alien species like ours never make contact.

6541

Sergio Georgini wrote: »

Best we don't get off this planet if you ask me. We as a species will sweep through any new planet like a plague, eventually. How long before we're talking about conserving the last few of whatever alien species are left on the new planet? As we've done here. How long til our good intentions turn to "we should dig up every mineral on the new planet for the benefit of business interests?" As we've done here. The rest of the universe will be better off if we live and die on Earth.

In that same vein we might also be better if alien species like ours never make contact.

The whole point of space exploration is to take more resources, this should be a strategic aim of the human race.