Ever worry about shadowy forces tapping into your phone calls and listening in on your private conversations? Well, astronomers have some good news for you: It won’t be aliens with their ears (or whatever auditory sensory organs they evolved) to the speaker getting into your business.
At least, not yet. Unless they’ve done a lot better than we have funding radio astronomers. And only if they’re really close by.
Scientists with SETI—the search for extraterrestrial intelligence—have long pondered how to detect life outside Earth. Assuming there are technologically advanced aliens out there, they might be trying to communicate with us. Or they might just be leaking radio energy into the cosmos by accident. Either way, can we pick up that signal?
One way to tackle this question is to turn it around: We know how much energy we’re broadcasting into space. Given our own level of technology, could we detect such a signal from light-years away? If so, then maybe we can hear ET, too.
SETI scientists have mostly focused their efforts on radio waves because they’re easy to make; any young technological civilization will figure that out pretty quickly—after all, we did. They can be beamed with a lot of power, have lots of information encoded in them, and can travel easily through the myriad dust and gas clouds littering our local space environment. They’re ideal for cross-galactic communication.
This kind of study has been done in the past; research published in the journal Science in 1978 looked at our TV signals and military radar, the most powerful transmissions we sent into space at the time. Then-current radio telescopes could detect such emanations out to 25 and 250 light-years away, respectively, a volume of space that encompasses several hundred thousand stars.
In the decades since, our broadcast TV signal has waned as we turned to cable and the Internet to deliver our shows. The days of wondering if aliens loved Lucy as much as we did are behind us, I’m afraid.
But other communication methods are on the rise, and they could prove more fruitful for any aliens looking for another lonely civilization with which to chat. New research by SETI scientists, published in the Monthly Notices of the Royal Astronomical Society, looks at how our cell phone usage might be detectable at other stars.
Without going into too much technical detail—the FCC has a decent page explaining how this works—cell phones emit a weak signal that can be detected by a nearby tower, which in turn emits a much more powerful signal to send the transmission along. Coverage for a given phone company is divided into small areas called cells, each populated with one or perhaps a few towers that can pick up nearby phone signals.
While an individual phone only produces a signal strength of a fraction of a watt, a tower emits a couple of hundred watts—about the same as a bright light bulb. That’s not much, but there are a lot of them in total. OpenCelliD, an open database of cell locations, has tens of millions of cells listed globally. The total power emitted by cell phone transmissions can be measured in gigawatts.
What an alien detects when pointing a radio telescope at Earth depends on more than just the combined signal strength of all those towers, though. The direction the towers transmit is also important. Most human cell phone users are located near the Earth’s surface, so the tower antennae are configured to send their signals parallel to the ground, covering it like lawn sprinkler spraying out water. If you’re on the ground near the tower you’ll get a strong signal from it, but if you’re above it, you get only at best a weak signal.
Tower locations matter as well. Very few towers are in the Pacific Ocean nations, compared with a huge number in the U.S. Moreover, there are more towers in the Northern Hemisphere than the Southern, so our alien friends would see a different signal depending on where their home star is located in the sky.
Putting this together, the scientists modeled what aliens would see from hypothetical planets orbiting three nearby stars: HD 95735, Barnard’s Star, and Alpha Centauri A. All of these are less than eight light-years away, practically in our galactic back yard, maximizing the snooping capabilities of any nosy aliens. The stars are also widely spread in declination (the measure of latitude on the sky), to see how Earth appears from different directions.
The conclusion? If the alien tech is the same as ours—with a radio telescope as big as the 100-meter Green Bank Telescope in West Virginia—our overall cell phone signal is still far too weak to detect from any of the three stars. The next-generation Square Kilometer Array, currently under construction in Australia and South Africa, will be more sensitive, but still only has about 1 percent of the sensitivity needed to detect Earth’s transmissions from tens of trillions of kilometers away.
If they’re anything like us, then, we’re safe from eavesdropping. On the other hand, judging from my time spent in airports and other public places, a lot of people don’t care at all who overhears their calls. I wouldn’t go so far to saying I hope aliens abduct them, but I’m not not saying that.
But what if our galactic neighbors are more advanced technologically? Telescopes detecting interstellar radiation are like buckets set outside in a rain shower: the bigger the bucket, the more water it collects. It’s technically feasible to build far larger radio telescopes than we have now. There are even serious proposals to build huge radio telescopes on the moon. These would be far more sensitive than what we have today, perhaps capable of picking up such mobile transmissions even from interstellar distances.
So it’s still possible ET could listen in on our cell phones, provided they’re close enough, in the right part of the sky, and have slightly better tech on hand (or tentacle or pseudopod) than we do now.
You can pick whichever part of that last sentence is the most far-fetched. But either way that’s a lot of ifs. The longest odds are that they’re so close to us; if their home world is 1,000 light-years away, they’d need a telescope the size of a moon to pick up our transmissions. Possible, but a lot of effort.
Still, the scientists note that the population of terrestrial cell phone towers is on the rise, and we get brighter in radio waves every day. They also plan on expanding their work to include more powerful 5G towers, radar, satellite services, and more, getting a better handle on just how loudly we’re announcing our presence in the galaxy.
Remember, too, that all of this is to answer the more pertinent question of whether we can hear them. That’s still a tentative “maybe,” an ambiguous if somewhat maddening conclusion. And of course this still all rests on the biggest question of all: Are they even out there?
If so, the E.T. please phone Earth: we’re eagerly awaiting your call.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.
Editor’s Note (5/24/23): This article was edited after posting to correct the descriptions of cells listed at OpenCelliD and total power emitted by cell phone transmissions and to clarify the comparison between the numbers of cell phone towers in Pacific nations and in the U.S.