Searching for Technosignatures: Detecting Reflected Light from Extraterrestrial Cities

2023-07-16 12:00:16
Are we the only intelligent life in a civilized universe? Or are there other civilizations? As science continues to discover celestial bodies that may harbor life, perhaps we are not alone. However, even if an extraterrestrial civilization exists, we have no way of knowing what kind of civilization it is because we only know the civilization of the earth. However, some civilizations may build structures on the surface of celestial bodies in the same way as humans do. The structure will be covered with metal and glass like the structure of mankind, and it will shine by reflecting the light that falls from the fixed star. Thinking about it, I can’t help but think of one possibility. Could we detect its brilliance?[▲ Figure 1: The entire surface of the disco ball is covered with mirrors, but only a small area is visible. Similarly, even if there were a mega-city that covered the entire surface of the celestial body, there would be a limit to the reflected light reaching the earth (Credit: Southend-on-Sea City Council (Public Domain)). Even if there is a reflecting object, the reflected light from the object should be from a limited area. Since the angle at which the reflected light reaches the earth is extremely narrow, only a small portion of the light reflected from the surface of the celestial body, which is a sphere, should reach the earth. Science fiction movies sometimes depict advanced civilizations where the surface of the celestial body is entirely covered with cities, but even if there were such a celestial body, the amount of reflected light reaching the earth would be negligible.[▲ Figure 2: The critical size of reflected light coming from a city on an Earth-sized celestial body is a fraction of that of the Tokyo and New York metropolitan areas. Long tall buildings are also one of the causes of increased reflected light (Credit: Vitor Coelho Nisida (Public Domain)) I calculated what was possible. First, assuming a planet the size of Earth, the critical size of a city that reflected light reaches was calculated to be 5.4 ppm (0.00054%, 54 millionths) of the total surface area of ​​the planet, or about 2800 square kilometers. rice field. This is a fraction of the size of the Tokyo and New York metropolitan areas, meaning that even smaller cities can suffice.[▲ Figure 3: Schematic diagram showing the reflection of stellar light from a city. Of the light that reaches the planets (right) from the star (left), it is expected that some of the light incident on the cities (A1-A3) will be reflected back towards the observer (Credit: Bhavesh Jaiswal)】 So, is reflected light from cities actually detectable? Or conversely, is it possible to know the existence of Tokyo and New York from distant space? Depending on the conditions, it may not be possible. The slower the rotation speed of the celestial body, the longer the duration of the reflected light reaching the earth. Depending on the latitude of the city and the orbital inclination of the planet, the reflected light from the city may reach the planet repeatedly during each rotation. Skyscrapers also reflect light from the sides, so the reflected light may reach you even if the angle is not ideal. Also, if the building is covered with glass, the reflected light is not so much, but if it is covered with aluminum, the reflected light of the city may be several times more than the light reflected by the planet itself. there is. Jaiswal says the easiest to detect is the reflected light from the cities of planets orbiting stars with the spectral classification M (red dwarfs). Reflected light from planets, with or without cities, is much weaker than light coming directly from stars. In the case of a G-type star, which is the same as the Sun, the light from an Earth-sized planet is only about 1/10,000,000,000. On the other hand, for an M-type star, which is much fainter than the Sun, the light from the planet is about 1/100 million. And the important thing is that the reflected light from the city is also considered to be about the same brightness. In other words, if there is a city on a planet that orbits a red dwarf, it is possible to capture how the reflected light from the planet instantly becomes many times brighter. By performing spectral analysis, it will be possible to distinguish whether it is a phenomenon caused by an artificial object or a natural object such as the water surface. Of course, we have so far found no evidence of extraterrestrial civilizations, nor have we found any such reflected light. However, as the technology for directly detecting light from extrasolar planets continues to advance, there is a good chance that we can distinguish whether the reflected light is natural or man-made. A space telescope has also been proposed to image the surface of planets, although it is still in the conceptual stage. Even if it doesn’t come true right away, it’s possible that in the near future we’ll be able to detect reflected light from cities built by extraterrestrial civilizations. SourceBhavesh Jaiswal. “Specular reflections from artificial surfaces as Technosignature”. (arXiv)Scott Alan Johnston. “We Could See the Glint off Giant Cities on Alien Worlds”.
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