## Biggest Telescope

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lonestar998
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Joined: Mon Mar 04, 2019 9:21 pm UTC

### Biggest Telescope

Would it be possible that people on another exoplanet build such a big telescope that it can watch the earth many years ago. So not only the outer shape of the earth also the ground and can for example watch our History like watching TV?
I asked my physics teacher once but she didn't know the answer maybe you can help

Tub
Posts: 472
Joined: Wed Jul 27, 2011 3:13 pm UTC

### Re: Biggest Telescope

What is your desired spatial and temporal resolution?

Angular resolution is strictly limited by aperture size and wavelength. An internet search turns up the Dawes' limit. It's not perfect, but it seems to be good enough for an approximation, and the formula is simple enough that you can plot your own desired values. Getting a resolution of 1 meter from 1 light year distance would require an aperture size of ~1/1000 light year; that's on the order of magnitude of a planet's orbit.

The second limit is exposure time. You need to catch enough photons to get a picture. Too few and you get noise, even fewer and the picture just stays black. As earth keeps moving and spinning, you'd need to bring your exposure time waaaay down. You are moving >100 meters per second simply due to earth's rotation, so that 1m resolution requires an exposure time of <<10ms. Given that earth is comparatively dark, that may require another increase in aperture size, but I couldn't find formulas with a quick search.

Considering that any alien civilization will be further away than 1 light year, I'd say that your telescope can't be done. The amount of materials and energy required for construction is beyond anything available to them, not to mention the technical challenges to keep a giant glass disc aligned, dust-free and not shattered. There might be workarounds with gravitational lensing, but having a bright star in the way of the dark thing you want to watch is probably not helpful. It would certainly be cheaper and faster to send a ship that snaps a couple of pictures from a nearby orbit.

There is one thing that alien civilizations might have seen though. They might know we exist. When the sun, earth and their telescope align, the color of earth's atmosphere can be detected. If someone has watched earth for long enough with a better telescope than ours, they know about the rising CO2 levels, which is a strong indicator of life.

Soupspoon
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### Re: Biggest Telescope

Tub wrote:There might be workarounds with gravitational lensing, but having a bright star in the way of the dark thing you want to watch is probably not helpful.
Solution: Dark Matter lens, and/or a currently 'dieting' black hole with no appreciable accretion disk!

It may be possible to aggregate a Dyson swarm of (omnidirectional?) observations with statistical analysis to account for relative motion (planet surface in daily rotation, in its annual orbit around its star, the proper motion of the star through the 'background', the background through the 'true background', and of course the individual swarm elements' positions over time and of course mapping any undulations or distortions of the intervening space) to establish a processed mosaic image, but it might welk generate Schiaparellian artefacts, and the chaos of our atmosphere would probably be far more significant at tumpty-tump LY range where even low angular deflection diverges co-originating photons on the way to a wider appature of atmospheric departure before then reconverging in the detector width, and blurs things much more than with an Earth-orbit observation (e.g. a spysat).

Sufficiently Advanced Technology or not, they probably won't be reading the headlines if the newspaper you left sat on that park bench (even if you leave it there for days or a year, and keep it visible enough to count for each pass, for some damnfool reason), and even the park itself would be lost to such supreme masters of remote photonic aggregation.

But a rough and ready map of continental and land-type swathings might be noted (and qualified by spectroscopic determination that the green bits are whatever-they-think-chlorophyll-is and that the atmospheric mix indicates something is actively producing nominally short-lived gasses to a state of dynamic equilibreum. It'd be worth some speculation.

Or note the VHF TV emissions, rather than the visible light, and scrape together enough data to be able to decode and then logically reconstruct our own images (and sounds) of Earth, leaked out there. A different kind of problem, probably just as unlikely, but if they can tune past the snow (not even going to consider DVB!) it's probably easier to aggregate in the first place than a purely optical lens/mirror/whatever arrangement.