Cell phones are so common that we often don’t think about how they work. They just do it, very profitable for us and sometimes annoying. But the key to their function is a large number of radio transmission towers. These cell towers cover a large percentage of the earth’s surface, especially in densely populated areas, and continuously transmit microwave signals.

With all these cell towers emitting all these radio signals, an interesting question to ask is whether these signals can be detected by an alien civilization. The answer to this question was published recently. Monthly Notices of the Royal Astronomical Societyand worth reading. The article begins with how radio signals entering space change. The majority of radio broadcasts in the 20th century were commercial radio and television stations.

Now these transmissions are smaller than mobile communications. Military radar transmissions remain the strongest source of radio leaks on Earth, but cell towers now rank second.

Each cell tower emits a radio signal with a power of 100-200 W. Given the number of towers and the amount of radio communications leaks, that’s equivalent to several gigawatts thrown into space. Assuming the alien civilization had advanced radio astronomy similar to our Event Horizon Telescope, then our broadcasts should be visible within about a dozen light years.

But it depends on where the aliens are in our sky. Cell towers radiate most of their radio power parallel to the Earth’s surface, so the tower’s signal is strongest when it rises or sets when viewed from a distant star. Since most of the towers are in the northern hemisphere, an alien star in the northern hemisphere will receive a stronger signal than in the southern hemisphere.

Another challenge is that the tower’s signals are all different and overlap in such a way that an alien civilization cannot discern any particular message. You don’t have to worry about aliens eavesdropping on your private phone calls.

But they can still use the signals to learn some interesting things about Earth. Because the distribution of the towers roughly fits the distribution of our population, the aliens can obtain measurements of Earth’s rotation and axial tilt.

They will also have measurements of the distribution of land on Earth and will be able to examine how the distribution of our population has changed over time. As an example, the team simulated signals from three nearby stars. Alpha Centauri is in the southern hemisphere, but only 4 light-years away, so it should receive a measurable signal from us.

Barnard’s Star (6 light-years away) and HD 95735 (8 light-years away) are in the northern hemisphere and will also receive good radio data from Earth. All three of these star systems are known to have planets, but none of them have a potentially habitable world.

As humanity transitions to more advanced mobile technologies like 5G, the towers’ signals will get stronger, meaning more nearby stars will have a detectable signal from Earth. It may be just a matter of time before our phone signals reach and touch alien minds.