These cell towers cover a large percentage of the earth’s surface, especially in densely populated areas, and they constantly transmit microwave signals. Considering that all these cell towers are emitting all these radio signals, it’s a funny question whether these signals can be detected by an alien civilization.
The answer to this question was recently published in the Monthly Notices of the Royal Astronomical Society. The article talks about how radio signals entering space have changed over time.
In the 20th century, the bulk of radio broadcasts were from commercial radio and television stations. Now these transmissions are negligible compared to mobile communications. Military radar transmissions are still the most powerful source of radio leakage from Earth, but cell towers are now in 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 amounts to several gigawatts radiated into space. If we assume that an alien civilization has advanced radio astronomy, then our transmissions should be detected within a dozen light years or so.
But it depends on where the aliens are. Cell towers radiate most of their radio signal power parallel to the Earth’s surface, so the tower’s signal is strongest when it’s going up or down.
And since most of the towers are in the northern hemisphere, an alien star in the northern hemisphere will receive a stronger signal than a star from the southern hemisphere.
Another complication is that all the signals from the towers are different, and they overlap each other in such a way that an alien civilization would not be able to distinguish any specific messages.
You don’t have to worry about aliens listening in on your private phone calls. But they could still use the signals to learn some interesting things about Earth.
Since the distribution of towers roughly corresponds to the distribution of our population, aliens could measure the Earth’s rotation and axial tilt. They would also have a measure of the distribution of land on Earth, and over time they could study how our population is distributed.
As an example, the team simulated the signals seen from three nearby stars. Alpha Centauri is in the southern hemisphere, but only 4 light years away, so it should be receiving a tangible signal from us.
Barnard’s Star (6 light years away) and HD 95735 (8 light years away) are in the northern hemisphere and could also receive good radio data from Earth. All three of these star systems are known to have planets, although none of them has a potentially habitable world.
As humanity transitions to more advanced mobile technologies such as 5G, tower signals will become even stronger, meaning more nearby stars will “see” the signal from Earth. It’s only a matter of time before our phone signals reach and touch alien intelligences.
