What an Alien SETI Program Would Learn from Earth’s Signals
We’ve been listening for alien signals for several decades, though the silence has led as many to wonder if we are listening for the right things as those wondering if there are any aliens at all. What many fail to understand is how many signals a civilization like ours produces that are lost to the background noise before anyone outside our system would hear it, much less what they might learn if they were able to pick up our signals?
What signals would an alien civilization detect? And what could they learn about Earth from what they could receive from human transmissions?
What Won’t Aliens Pick Up by Listening for Us
Aliens won’t be watching standard television broadcasts. After all, these signals are lost over a hundred miles away unless they are satellite TV signals. And satellite signals sent down from satellites can’t be detected, because they are aimed at the Earth.
Aliens won’t be able to listen to our standard AM or FM radio signals, either, since those signals suffer distortion and fade in the ionosphere. They’ll only be able to listen in if they were in orbit, and at that point, they’d be able to see everything we’ve built.
Wifi may be ubiquitous on Earth, but it has a short range and suffers interference in its own frequency band from other wifi sources. Factor in the RF radiation put out by planets like Jupiter on that frequency range and many others, and most signals generated by people are lost by the noise of our local neighborhood. And that’s not taking into account the radiation we produce that is overwhelmed by our sun, so light pollution won’t make a difference when aliens study the light dimming caused by our planet traversing the Sun.
What Could Aliens Detect From Earth?
There are two major signals aliens could detect at a reasonable distance of ten miles away. These are radar sources like radar and weather radar and satellite uplinks. Let’s look at why aliens could detect these signals and what aliens would learn from each.
What Could Aliens Learn by Listening to Our Signals
Military radar (and traffic control radar) has several advantages over other types of signals. First, it is put out by massive transmitters to look for threats and aircraft – that’s enough power to actually cross the distance when combined with other signals. Second, these frequencies aren’t competing with RF output created by the solar and Jovian electromagnetic fields. So the military radar signals from the Cold War looking for incoming ICBMs are still traveling through space and stand out to any aliens listening on those frequencies.
Air traffic radar, military radar and similar applications are still using these frequencies, too, so our planet continues to radiate them. Take this in contrast to the shift to internet communications from television, causing demand for TV broadcasts to go down, and satellite radio and internet radio potentially killing the radio transmissions we once thought would cause aliens to discover Earth by listening to old rock songs.
Aliens who detect radar would see that we are here. They would also be able to tell by regular periodic variations in the signal strength that our planet is rotating every 24 hours. Observing the polarization changes or Doppler shift of the radar signals lets them estimate where these signal generators are on the planet, such as the density of them on the Northern Hemisphere. They won’t know that the blank spaces are oceans or simply uninhabited, but they’ll know where the radar transmission hubs are, approximately. And they can use this information to estimate how large the Earth is, since no planet could have a smaller diameter than the distance between the farthest two signal broadcasters.
Doppler radar can travel over the horizon, so aliens in almost any direction could gather this information, though aliens directly in line of sight of Antarctica and the Southern Cross would see very little. They could study the Doppler shift between the rising and setting signals as the planet rotates to know Earth’s rotational speed, as long as they are close enough to have this resolution. Knowing the rotational speed and 24 hour day length gives you a direct answer to the diameter of the planet.
They would be able to tell by Doppler frequency shifts that the planet has seasons, altering its angle with the seasons even as the signals remain relatively unchanged day to day. And by studying the location of the signals relative to the Sun, they’ll know how long our year is. Extended Doppler measurements tell you the orbital velocity of the planet, which relative to our position from the sun, gives them an idea of our orbit.
Direct observation of the Sun and its “wobble” due to gravitational interactions with the Earth will tell them how large the Earth is, though this is hard to detect given the much greater influence of large Jovians like Jupiter. If they can determine how much Earth causes the Sun to wobble gravitationally, they’ll be able to determine our stellar mass. Using Kepler’s laws of planetary motion, inserting the orbital velocity or orbital period tells them exactly how far we are from the Sun. If you know how far the planet is from the sun, they’ll know how much radiation it receives from the Sun barring the Albedo effect, so they can estimate our planet’s temperature or at least how much solar energy it receives. And from the planet’s size and mass, they’ll be able to estimate our gravity.
Radio frequency output gets distorted by the atmosphere. This is actually how weather radar works, by measuring the distortion of radar signals to determine the density of the air and thus cloud density. Aliens couldn’t see major weather phenomena, but they’d be able to see that we have an atmosphere. By studying the atmospheric bending over several frequencies, they’ll be able to see what frequencies are absorbed by our atmosphere and what is absent. The gas chromatography we use in astronomy today is based on this concept. Aliens could study the electromagnetic spectrum and know that our atmosphere has oxygen and nitrogen. They couldn’t see water directly, but they would see hydrogen frequencies, which means our atmosphere isn’t mostly hydrogen.
This means that from radar alone, they’ll know we are here, how long our day is, how long our year is and be able to estimate our seasons. They’ll know we have an atmosphere, and if they understand the physics and have more than a little information, they’ll know what our atmosphere is made up of. If they get enough resolution, they’ll be able to determine our gravity, approximate temperature and whether our orbit is circular or elliptical.
They won't know if we have a runaway greenhouse effect or Ice Ages, only whether our world is certain to be overheated like Mercury or frozen like Ceres.
Geosynchronous satellites provide a wealth of information as well, to aliens who can see them. Geosynchronous satellites sit in a stationary orbit about three Earth radii out. To send signals to these satellites you need a lot of power – around half a Gigawatt to start, and more powerful signals are sent. If it is powerful enough to make it out of the atmosphere, it is powerful enough to continue traveling through space unimpeded. And these signals are multiplied by the thousands of TV stations sending their data to satellites.
The downside to these signals is that they are only seen for about sixty seconds per 24 hour period. Someone watching Earth would see the equivalent of a very long lighthouse, silence on these frequencies followed by a daily flash. If they are at a good angle to the Earth, they’d see a rotating pattern of flashes like a disco ball, the sign someone is transmitting. It would be hard to receive much information from such flashes, such as a minute of a soccer game, but they’d be able to garner some of the same information those looking at radar frequencies would, such as how often the Earth rotates and our time to travel around the sun. They’d be able to garner some information on the size of the planet by studying satellite signal rotation time and our planet’s effects on the sun, and thus back to Kepler’s laws to get the size of the planet and our distance from the star.
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