I had a bit of trouble trying to figure out if this is a GQ, GD, IMHO, or whatever. I put it here as it seems to fit more aptly than the others (as there is no true FACTUAL answer), but if I’m wrong, my apologies.
So, a band of humans builds a bonfire 750,000 years ago and the evidence still remains. My question is simple: if humanity were to disappear (lets be kind and say we transcended, ala Clarke’s Childhoods End), how long would it take before natural geological and astronomical forces wiped the evidence of modern humanity from the Earth? 50 million years? 500 million? Never?
Next, the debate shifts to How many angels can dance on the head of a pin, the concluding segment to tonights episode of ‘Truly Unanswerable Questions.’
Radioactive waste with astronomical half lives, huge networks of mines carved miles deep into the earth, nuclear bomb test sites with enormous craters. I say never.
So, a band of humans builds a bonfire 750,000 years ago and the evidence still remains. My question is simple: if humanity were to disappear (lets be kind and say we transcended, ala Clarke’s Childhoods End), how long would it take before natural geological and astronomical forces wiped the evidence of modern humanity from the Earth? 50 million years? 500 million? Never?
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But when you consider the earth has been around for billions [4.6 or so] of years, 750,000 years ago seems like yesterday. I think it depends on the geological forces that will happen for billions of years beyond the human timespan. Some things may remain, whether or not anyone will be able to find it is another thing.
We’re lookling at hundreds of millions of years at a minimum. Some of the things we’ve done (quarries and such) are truly permanent until subduction takes those parts of the continents back into the mantle.
Even then sufficiently talented alien scholars might be able to determine our existence from the lack of fossils fuels in the ground and other such things.
Well, in the geologically medium-short term, we’d always be obvious because of the presence of non-naturally occuring elements such as Technetium with a half-life of 4.2 million years. These are produced by nuclear fission such as in atomic weapons and nuclear power generation.
If it can be told that a group of cavemen stood around a fire 790,000 years ago (which I think is amazing) then certainly signs of modern civilization will probably always be around. Also, it is almost certain that someone is going to come up with another caveman’s weiner roast that is even older.
The contents of the sarcophagus at Chernobyl are likely to remain obvious artifacts for a half a billion years or so, after they have been subducted into the nearest plate juncture, which is not that nearby, even in geologic terms. Call it two or three billion years.
How long will it be before the contents of the proposed Yucca Mountain depository pass out of the recognizable artifact stage? Even the decay products of artificial elements are recognizably different than the natural ores of radioactive materials.
Come to think of it, the “transuranic decay products layer” in Earth’s fossil record may be presumptive proof of our existence for longer than the previously mentioned three billion years. Probably a controversial theory, at any rate.
You don’t really have to rely on radioactive waste, or even on something particularly large. The peculiar concentrations of minerals or their degradation products would tip any interstellar archeologist off (e.g., without even getting into cities, a nice-sized automobile graveyard). On the reverse end, the unconformities caused by mining would be a dead giveaway. I’d have to go with billions of years, if ever.
I don’t think the “never” answer is plausible. Earth itself is finite. The planet will be destroyed in a spectacular collision with another body or it will be swallowed by the sun or the planet will fall into a black hole. In any event, Earth will not last forever.
If Earth is taken out of the equation (just extrapolating the OP a little) traces of our civilization will be around for a very, very long time and may well outlast the planet itself. So far two space probes have left the solar system and at least two more are on their way. Even counting on collisions with debris and such, the Pioneer and Voyager probes are likely to be around for billions of years on some recognizable form or another (“Hey, Thrakomozogg - check out this bit of junk - it’s refined metal! Wonder where it came from…”) and it will be possible to trace their trajectories back to this solar system or what’s left of it. All an alien culture would need to do is trace the trajectory, factor in any major collisions and estimate when and how they happened, and factor in the rotation of the galaxy and the effects stars have on each other as well as a few thousand other factors…and they’d have a short list of candidates for where the spacejunk originated.
There will also be traces of probes in “pristine” environments that will likely survive the death of the Sun - Jupiter and Saturn’s moons (probes with landers are planned for Titan and Europa, more will probably follow), and Pluto (there’s one probe to Pluto in the works, more certainly will follow).
