Mobile phones are so ubiquitous that we usually don’t think about how they work. But the key to their operation is a vast array of radio transmission towers.
These cell phone towers cover much of the Earth’s surface, especially in heavily populated areas, and transmit microwave signals constantly. Given that all these cell towers are emitting all these radio signals, a question that must be asked is whether these signals could be detected by an alien civilization.
The answer to this question was published in Monthly Notices of the Royal Astronomical Society.
The work begins by looking at how radio signals traveling in space have changed. In the 20th century, most radio broadcasts were from commercial radio and television stations. Now, these transmissions are eclipsed by those of mobile communications, he writes ScienceAlert.
Cell phone towers cover much of the Earth’s surface
Military radar transmissions are still the strongest source of radio leakage on Earth, but cell phone towers now take second place.
Each cell tower emits a radio signal with a power of 100 – 200 watts. Given the number of towers and the amount of radio leakage, this means several gigawatts sent into space.
If we assume that an extraterrestrial civilization has sophisticated radio astronomy similar to our Event Horizon Telescope, then our transmissions should be detectable out to about a dozen light years.
But that depends on where the aliens are in our sky. Cell phone towers emit most of their radio power parallel to the Earth’s surface, so a tower’s signal is strongest when it is rising or setting, as seen from the alien star.
The strongest source of radio leakage on Earth
And since most towers are in the northern hemisphere, a star in the northern hemisphere will receive a stronger signal than one in the southern hemisphere.
Another complication is that the towers’ signals are all different and overlap in such a way that an alien civilization would not be able to distinguish any specific message. However, such a civilization could use the signals to learn interesting things about Earth.
Since the distribution of the towers roughly corresponds to the distribution of our population, aliens could get a measure of the Earth’s rotation and axial tilt. It would also have a measure of the distribution of land on Earth and over time could study how the distribution of our population is changing.
- Simulation of the Earth’s radio-leakage from mobile towers as seen from selected nearby stellar systems: https://academic.oup.com/mnras/article/522/2/2393/7028804
- Alien Civilizations Could Map Earth Using Signals From Mobile Towers: https://www.sciencealert.com/alien-civilizations-could-map-earth-using-signals-from-mobile-towers
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