A new study led by the University of Manchester suggests that if any alien civilizations are turning their radio telescopes toward Earth, they may be able to not only detect our mobile phone signals, but could deduce a lot about our planet and even produce crude maps of it.
Since the 1960s, scientists have been looking for signs of advanced technological civilizations beyond our Solar System. Because we know nothing about these hypothetical extraterrestrials, these searches have operated on the assumption that the best bet is to listen for powerful radio signals beamed directly at the Earth in the narrow radio bands associated with the element hydrogen.
In 1978, another approach emerged as researchers realized that Earth was inadvertently beaming radio waves into the galactic neighborhood in the form of military radar, television broadcast signals, and other powerful transmissions. In other words, these techno-signatures not only indicate how we could detect other civilizations, but how they could detect us.
However, things have changed in the 45 years since then.
Today, television signals are fading as audiences abandon broadcast television for cable and internet streaming services, but the team from Manchester and the University of Mauritius have found a new radio source for aliens to zero in on in the form of mobile phone transmissions.
In 1973, mobile phones were a technological curiosity. In 2023, there are an estimated 7.6 billion mobile phones and 10.9 billion mobile connections. Since the total population of Earth is only 7.9 billion, that is a lot of radio signals. According to the Manchester team, these phones and the towers that service them aren't very powerful individually, but cumulatively they add up to a very strong signal.
To determine how powerful this signal is and whether it could be detected by any interstellar eavesdroppers, the team developed a computer model based on publicly available information about mobile phone usage and by dividing the Earth into different regions representing the distribution of phones and towers.
The team then chose three stars within 10 light years of Earth, HD 95735, Barnard’s star, and Alpha Centauri A, which are located in the southern sky, the equator, and the northern sky. They also assumed that orbiting these stars is a radio telescope equivalent to the Green Bank Radio Telescope in West Virginia.
What they found was that the global mobile phone network leaks a peak power of about 4 GW into space, with the mobile phones leaking about an order of magnitude less. Their calculations indicated that a Green-Bank-type telescope couldn't detect these signals. However, a more advanced version could do so and Earth's mobile signal is growing and moving toward more powerful broadband systems, meaning it is becoming easier to detect each year.
One interesting conclusion of the study is that, because Earth's mobile devices are not evenly distributed and they are designed to transmit parallel to the horizon, if mobile signals could be detected they would allow an observer to make deductions about the Earth, including the distribution of seas and land masses, vegetation, and even mapping of the surface.
“I’ve heard many colleagues suggest that the Earth has become increasingly radio quiet in recent years – a claim that I always contested,” said Professor Mike Garrett, Team Leader of the project and Director of Jodrell Bank Centre for Astrophysics at the University of Manchester. “Although it’s true we have fewer powerful TV and radio transmitters today, the proliferation of mobile communication systems around the world is profound. While each system represents relatively low radio powers individually, the integrated spectrum of billions of these devices is substantial.
“Current estimates suggest we will have more than one hundred thousand satellites in low Earth orbit and beyond before the end of the decade. The Earth is already anomalously bright in the radio part of the spectrum; if the trend continues, we could become readily detectable by any advanced civilization with the right technology."
The research was published in the Monthly Notices of the Royal Astronomical Society.
Source: University of Manchester