Thursday, April 28, 2016

What can Large Telescopes See?

Asymptotic technology is nice. It means that what works for one alien civilization will work for another alien civilization. Telescopes fall under this rule. Let’s suppose an alien civilization was looking at Earth from their own solar system with a large telescope. They might be trying to decide if they should come here, or perhaps something else. For the purposes of making a few calculations, let’s assume they can build, in space, telescopes of a very large variety, and make them accurate to the usual resolving power of a lens. Space doesn’t suffer from the gravitational distortion of a land-based telescope, and there is little aberration from intervening gases, the atmosphere being the worst culprit.

Building some structure in space is something we have only very basic knowledge of. We can cobble together parts that were built on Earth. That has limitations arising from the need to launch it. Let’s assume aliens are long past that, and can build structures in space. In particular, they can build reflecting telescopes of 10, 100, 1000, or 10000 m in diameter. If the alien solar system were close to ours, say 100 light years, and they used visible light, the respective resolutions would be 60,000 km, 6000 km, 600 km and 60 km. This means that they can resolve continents easily with the biggest one, or even the 1 km telescope. No structure built by man could be resolved.

They would be able to figure out there were both ocean areas and land areas on the planet, and polar icecaps. Perhaps the difference between some types of land area could be resolved, deserts versus forest, for example. Figuring out life was present from vegetation detection would be a piece of cake. The moon would be detected.

The atmosphere itself is only about 5 km in thickness, so it would not be resolvable, but perhaps some interesting tricks could be done to get an idea of what it consisted of. It would be quite possible to track the Earth around its annual orbit, so that not only would they be able to determine something like seasons, from polar cap size, but also they might see the reflected light coming from the atmosphere. For most of the orbit, solar light would be reflecting from the Earth’s surface, obscuring any atmospheric signature. But at the part of the orbit closest to the alien solar system, only a crescent of solar illumination would be visible. With enough resolution to look at the opposite edge, the only photons coming through would be those reflected in the atmosphere. Running their detectors for long enough would give them a spectral content, and, lo and behold, they know what’s in the atmosphere.

There would be plenty more they could determine. The resolution at infrared wouldn’t be as good as at visible, but near and far infrared observations could be used to get temperatures at different points on our globe. Then come the modelers. With an assuredly huge amount of computational power, they could make a map of our world, and deduce some details that might not be directly observable. Could they get the depth of the oceans? Their model knows the impinging solar flux, and it knows the distribution of albedo, both atmospheric and surface. So if they could get accurate enough temperature readings from ocean surface areas in different parts of the globe, a planetary heat transfer model might give them a clue about the heat capacity of the oceans, hence some idea of the depth. Very likely there are many other ways they could determine that.

How about cities? Cities are heat islands, and some are about the size of a pixel with the aliens’ largest telescope. Would they understand they were looking at cities? What else might give that signature? Geological activity could be considered, but alien science understands tectonics and having that distribution of geological hot spots would be impossible. There would also be some clues in that many of the hot spots were located near coastlines.

If they watched the heat spots grow with time, they might get an idea of the state of our civilization. There has been a huge change over a period of a hundred years, and if they were watching that long, they would be able to estimate our civilization’s state. They would have to do alienology and figure it out, but it would be possible, given they were observing us at that very, very brief interval of time when these changes happened.

Let’s turn the tables around. Suppose we here on Earth keep going on with space exploration and space science and interplanetary shipping and use our resources to build a 10 km telescope, maybe out beyond Saturn. We would be able to see the same things as were listed above for the aliens viewing Earth. We would know all this stuff about their civilization, and could use it to figure out what options we had.

Granted, space might be a pretty empty place for life, and a hundred light years might not produce any alien civilizations at all. Going out the next decade, to 1000 light years, cuts our resolution by a factor of ten as well, and so even the ten kilometer dish wouldn’t figure out alien city locations. Maybe there are clever things that clever astronomers could figure out to beat the resolution limit, or maybe Earth’s resources would allow the building of a thirty kilometer dish. More than likely, putting an array of one to ten kilometer dishes around the solar system and doing some really amazing interferometry would beat the resolution limit of a single dish.

This takes us back to the question of what is the better thing to do, build a large (or multiple large) astronomical observatories in space to find out about alien planets, or send out probes? The dish could divide its time between multiple candidate planets, and would have an iteration working for it. Early, quick observations would eliminate most possible candidate planets for alien life, allowing the dish or array to focus observation time on the few best ones. This is very important, as rotating a dish this large would knock out a large chunk of observation time. Probably small dishes, 10 to 100 meter size, could be used to do this preliminary filtering, and then the large ones, 1 to 10 kilometer in size, would not have to spend much time in passing from one point to another.

Does it make any sense at all to do local observations with an interstellar probe? This is not a simple question, but the large dish’s ability to figure out much of what was on the surface of a near exo-planet means the benefit of a close-in observation would have to be examined very cautiously to avoid duplicating what was already known.

And there is another implication. This blog started off asking why aliens haven’t shown up here. Another explanation for this is that they can see whatever they want to with an array of dishes they could easily build at home, and there just isn’t much else worth knowing about Earth that would make a space probe or a space visit here worth the cost.

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