Call Me Maybe

Ervins Strauhmanis, via Flickr

Mobile phones are so ubiquitous that we typically don’t think about how they work. They just do, much to our benefit, and sometimes annoyance. But the key to their function is a vast array of radio transmission towers. These cell towers span a large percentage of Earth’s land surface, particularly in heavily populated areas, and they transmit microwave signals all the time. With all those cell towers emitting all those radio signals, a fun question to ask is whether those signals could be detected by an alien civilization.

The answer to this question was recently published in the Monthly Notices of the Royal Astronomical Society, and it’s worth the read.¹ The paper starts by noting how the radio signals streaming into space have changed. In the 20th century, the bulk of radio transmissions were those from commercial radio and television stations. Now those transmissions are dwarfed by those of mobile communications. Military radar transmissions are still the most powerful source of Earth’s radio leakage, but cell towers now fill the second position.

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, that amounts to a few gigawatts beamed into space. If we assume an alien civilization has sophisticated radio astronomy, similar to our Event Horizon Telescope, then our transmissions should be detectable within a dozen light-years or so.

Saide, et al

But that depends on where the aliens are in our sky. Cell towers emit most of their radio power parallel to the surface of the Earth, so a tower signal is strongest when it is rising or setting as seen from the alien star. And since the majority of towers are in the northern hemisphere, an alien star in the northern hemisphere will get a stronger signal than one in the southern hemisphere.

One other complication is all the tower signals are different, and they overlap in such a way that an alien civilization wouldn’t be able to distinguish any specific messages. You don’t need to worry about aliens listening in on your personal phone calls. But they could still use the signals to find out some interesting things about Earth. Since the distribution of towers roughly corresponds to our population distribution, the aliens could get a measure of Earth’s rotation and axial tilt. They would also have a measure of Earth’s land distribution, and over time could study how our population distributions change.

As an example, the team modeled signals as seen from three nearby stars. Alpha Centauri is in the southern hemisphere, but just 4 light-years away, so it should get 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 would likewise get good radio data from Earth. All three of these star systems are known to have planets, though none are known to have a potentially habitable world.

As humanity transitions to more modern mobile technology such as 5G, tower signals will become even stronger, which means even more nearby stars would have a detectable signal from Earth. It may be just a matter of time before our phone signals reach out and touch alien minds.