Visible from space, yes… but not visible from some telescope in another star system.
I was thinking in this direction myself. I doubt you can take a single “snapshot” of Earth and conclusively determine anything. I wonder, though, if it is possible that extended observation over a period of time might reveal changes that would be indicative of a technological society.
I can think of one possibility, given the right instruments with enough sensitivity. The Kepler telescope is finding planets by looking at the brightness of stars and seeing that the brightness is eclipsed by a planet orbiting it. Perhaps the same is true of Earth and its satellites. If all new satellites appear and disappear, that would be pretty conclusive evidence that someone was launching them.
I have a lot of trouble believing that this could be seen from another star. But is some aliens were in the neighborhood and doing a survey…
True. But realistically, a telescope in another star system isn’t going to see Earth at all. So when the OP didn’t specify how close the aliens were, I assumed a more reasonable distance.
There was an article in New Scientist about this recently (link here, behind paywall I’m afraid).
One of things they suggested is that detection of synthetic chemicals such as CFCs may indicate industrial civilizations.
One interesting fact they mentioned (that I never knew) is that at one point they turned the camera on the Galileo probe back to earth to see what a planet with life would look like from millions of miles away.
Ah, sorry, it was a perfect match for the Martian canals theory.
[/squint and fantasize mode off]
Actually, it is possible to image the Earth from a nearby stellar system. As I linked to above, NASA had a plan to examine terrestrial-sized planets with a cluster of space telescopes using interferometry. This would give the same angular resolution as a telescope as large as the distance between the telescopes (6,000 km IIRC). The even more ambitious Terrestrial Planet Imager would have been able to resolve Earth-sized planets as a disc. However, such a telescope does not have the light-gathering ability to match it’s resolution.
I’m not sure. Hopefully someone understands this better will chip in on what is and isn’t possible or feasable. I imagine detecting something like CFCs or lead in the atmosphere would be very difficult due to the low abundances.
And as standingwave mentioned there’s the idea of stationing a telescope out at the edge of the solar system and using the sun’s gravity as a gigantic lens. From what I recall reading about the idea years ago that would supposedly be theoretically able to resolve planets out as far as Andromeda.
Ah, found what I was barely remembering… The Sun as a Gravitational Lens: A Target for Space Missions Reaching 550 AU to 1000 AU by Claudio Maccone:
http://www.spaceroutes.com/astrocon/AstroconVTalks/Maccone-AstroconV.pdf
Meh…it’s only made of tree farts.
Dyson Spheres are an intriguing concept, but I wonder if the reason we haven’t seen them is because they’re way too grandiose to serve any useful purpose? It’s difficult to imagine any civilization would consume THAT much energy; it would be like having the world’s largest nuclear power plant dedicated solely to charging your iPhone.
I believe there was extensive deforestation in the industrial revolution which would be able to be seen with a good enough optical telescope.
In principle, it is possible to distinguish between isotopes spectroscopically: The differing masses make a slight difference in the reduced mass of the electron for individual atoms, and in the oscillation frequencies for molecules. In practice, though, it’s really, really hard to do for anything but the hydrogen isotopes (which we haven’t mucked about with much), and even that requires very good wavelength resolution and signal-to-noise. I don’t think you could detect C14 vs. C12, or U235 vs. U238.
The extra energy would be used in things we don’t do today, or maybe can’t even dream up. There are some things we can’t do today largely because of energy limitations. Production of antimatter on a large scale, for example. Or getting to Mars in less than 5 years. Then there are things we aren’t even sure are possible, like creating wormholes and using warp drives, that would consume more energy than we can fathom at our current production levels.
One interesting thing about Dyson spheres is that they really aren’t meant to be a single, solid sphere encircling a star completely. The idea is that you’d start with one or two light-gathering satellites each year. Over enough years, you’d eventually have the star surrounded by a cluster of satellites. In that respect, it’s much like our modern cities - no one set out to cover square miles of earth with buildings and asphalt; it just accumulated as the result of thousands of individual efforts.
A pale blue dot if they are close enough. Otherwise we do not exist.
Fascinating, love the concept of using the sun as a giant lens, but I doubt we’ll see this in my lifetime.
Thanks. How about detecting CFCs or lead in the atmosphere, how difficult would that be?