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Anonymous Coward writes:
https://www.businessinsider.com/alien-civilizations-may-have-already-colonized-galaxy-study-2019-8
The Milky Way could be teeming with interstellar alien civilizations — we just don't know about it because they haven't paid us a visit in 10 million years.
A study published last month in The Astronomical Journal[$] posits that intelligent extraterrestrial life could be taking its time to explore the galaxy, harnessing star systems' movement to make star-hopping easier.
The work is a new response to a question known as the Fermi paradox, which asks why we haven't detected signs of extraterrestrial intelligence.
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Fermi Paradox Paper: Aliens Could Take Their Time Settling the Galaxy
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Croll's Query:
If tin whistles are made of tin, what are foghorns made of?
(Score: 2) by Immerman on Wednesday September 11 2019, @01:27AM (4 children)
I think you're grossly overestimating how much interstellar dust there is. The Local Cloud - a gas cloud about 60 ly across that our solar system is currently passing through, has an average density of about 1 atom of hydrogen per 10 cubic centimeters, or 100,000 atoms per cubic meter. Assuming you're traveling atr roughly light speed, with no shielding to guide material harmlessly around your ship, then that means every 1m^2 of cross sectional area of your ship is going to hit ~10^21 atoms per year, or about 3.3 milligrams of material.
Even assuming total mass-energy conversion in the impact, that amounts to only 83MWh/year/m^2, or 9.5kW/m^2 of instantaneous radiation. A cylindrical ship 1km in diameter would be emitting only 7MW of radiation, and the overwhelming majority of that wouldn't be directed at us.
To put that in perspective, Pluto receives 0.9W/m^2 of solar radiation, with an albedo a bit over 0.5. Which means it's 1188km radius is glowing with about 2 million MW of reflected sunlight.
Pluto is also over about 6,000x closer than the nearest star, meaning it appears 36 million times brighter than it would if it were shining as brightly from the nearest star. We couldn't begin to see it there with current telescopes . And our 1km diameter relativistic ship passing that star would be another 600,000x dimmer than that.
(Score: 2) by HiThere on Wednesday September 11 2019, @03:16AM (3 children)
You've got a very low average dust, 1 atom per ?, but it's not evenly distributed. Some of it's in the form of grains of sand, some in the form of meteors, etc. And I doubt that we know enough to say how sparse the average dust is. (Well, unless you are really only counting dust particles small enough to fuzz light reception.) Some of it's even going to be in the form of wandering planets, and failed stars that are too small to count as a brown dwarf.
For that matter, at 0.01C it's going to be quite difficult to dodge a baseball sized chunk of rock by the time you detect it.
0.01C = 6.706e+6 mph
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(Score: 2) by Immerman on Wednesday September 11 2019, @03:12PM (2 children)
Sure, but traveling at near light speed the density will likely average out pretty well - you're sweeping through a LOT of volume every second, 235,000 cubic km for a 1km diameter ship. There may be more dim stretches with bright flashes, but my point is that even assuming massive ships and total mass-energy conversion on impact (almost certainly several orders of magnitude more energy release than in an actual impact), the total power output would be many, many orders of magnitude lower than we could detect across interstellar distances - even from a ship as close as the nearest star.
And I would assume that any species engaging in such travel would probably have worked out how to avoid hitting the softball-sized chunks, to say nothing of wandering planets and failed stars.
(Score: 2) by HiThere on Wednesday September 11 2019, @06:36PM (1 child)
As to not detecting it...well, OK. I think you'd get things like tracks in cloud chambers, but I could well be wrong.
As to avoiding them...this I find quite dubious. I don't even know of any hypothesis that says you could do that, bar things like hyperspace.
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(Score: 2) by Immerman on Thursday September 12 2019, @02:57PM
You might get some slight trace of particles reaching Earth as a result but they'd be totally lost in the massive onslaught of particles we're constantly being bombarded with from stellar-and galactic-scale particle accelerators scattered across the universe.
Avoiding debris is relatively easy - you have an active scanning and defense system vaporizing or deflecting bigger debris in you path, and an ionization+magnetic field shielding deflecting gas and dust around you. Takes a lot of energy, but if you can accelerate to those sorts of speeds in the first place that's probably not a problem. Avoiding wandering planets is even easier - you map their location long before you get anywhere close, and then don't chart a path through the space they're going to be in. If you can travel interstellar distances at relativistic speeds, then building gravitational-lens telescopes powerful to map your course ahead of time is a pocket-change endeavor.