May 27, 2024


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Observing Tokyo or New York from space? Checking if you can see the light reflecting off the city |  sorae space portal site

Observing Tokyo or New York from space? Checking if you can see the light reflecting off the city | sorae space portal site

Are we the only intelligent life in a civilized world? Or are there other civilizations?

As science continues to discover celestial bodies that may harbor life, we may not be alone. However, even if an extraterrestrial civilization exists, we have no way of knowing what kind of civilization it is because we only know of an terrestrial civilization.

However, some civilizations may build structures on the surface of celestial bodies in the same way as humans. The structure will be covered in metal and glass like that of a human being, and will shine by reflecting the light that falls from the fixed star.

Thinking about it, I can only think of one possibility. Can we reveal her brilliance?

[▲ الشكل 1: سطح كرة الديسكو بالكامل مغطى بالمرايا ، لكن مساحة صغيرة فقط مرئية. وبالمثل ، حتى لو كانت هناك مدينة ضخمة تغطي كامل سطح الجسم السماوي ، فسيكون هناك حد لمقدار الضوء المنعكس الذي يصل إلى الأرض (Credit: Southend-on-Sea City Council (Public Domain))]

Even if there is an object reflecting light on the surface of the celestial body, the light reflected by the object must be from a limited area. Since the angle at which the reflected light reaches the Earth is very narrow, only a small part of the light reflected from the surface of the celestial body, which is a sphere, should reach the Earth. Science fiction films sometimes depict advanced civilizations where the surface of a celestial body is completely covered by cities, but even if there were such an orb, the amount of reflected light reaching Earth would be negligible.


[▲ الشكل 2: الحجم الحرج للضوء المنعكس من مدينة على جسم بحجم الأرض هو جزء صغير من حجم منطقتي طوكيو ونيويورك الكبرى. تعد المباني الطويلة أيضًا أحد أسباب زيادة الضوء المنعكس (Credit: Vitor Coelho Nisida (المجال العام))]

Bhavish Jaiswal of the Indian Institute of Science calculated the size of the city observable on Earth. First, assuming an Earth-sized planet, the critical size for a light-reflecting city comes to 5.4 ppm (0.00054%, 54 ppm) of the planet’s total surface area, or about 2,800 square kilometers. Rice field. That isAround part of the Tokyo metropolitan area and the New York metropolitan areawhich means that smaller cities will suffice.

[▲ الشكل 3: رسم تخطيطي يوضح انعكاس الضوء النجمي من المدينة. من الضوء الذي يصل إلى الكواكب (يمينًا) من النجم (يسارًا) ، من المتوقع أن ينعكس بعض الضوء الذي يصيب المدن (A1-A3) باتجاه الراصد (مصدر الصورة: Bhavesh Jaiswal)]


So, can light reflected from cities really be detected? Or, conversely, is it possible to know the existence of Tokyo and New York from far space? Depending on the circumstances, this may not be possible.

The slower the celestial body’s rotation speed, the longer the reflected light takes to reach the Earth. Depending on the latitude where the city is located and the planet’s orbital inclination, the light reflected from the city may arrive more frequently for each rotation. Skyscrapers also reflect light from the sides, so the reflected light may reach you even if the angle isn’t perfect. Also, if the building is covered with glass, the reflected light is not much, but if it is covered with aluminum, the reflected light of the city may be several times the light reflected by the planet itself.

The easiest way to detect, Jaiswal says, is light reflected from cities of planets orbiting stars with spectral classification M (red dwarfs). The light reflected from the planets, with or without cities, is much weaker than the light coming directly from the stars. In the case of a G-type star, which is similar to the Sun, the light from an Earth-sized planet is only about 1/10 billionth. On the other hand, for an M-type star, which is much fainter than the Sun, the light coming from the planet is about 1/100 millionth. The important thing is that the light reflected from the city is also considered to be just as bright. Briefly,If there was a city on a planet orbiting a red dwarf star, the light reflected from the planet could be instantly brighter.That’s what it means. 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 a water surface.

Of course, we have not yet found any evidence of extraterrestrial civilizations, nor have we found any such reflected light. However, as technology for directly detecting light from exoplanets continues to advance, there is a good opportunity to 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 this is not realized immediately, it is likely that in the near future we will be able to detect light reflected from cities built by extraterrestrial civilizations.


  • Bhavesh Jaiswal. “Specular reflections from artificial surfaces as a technology signature”. (arXiv)
  • Scott Alan Johnston. “We can see the glitter of megacities on strange worlds.” (the universe today)

Text: Rare Aya

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