Volume 24, Number 04, Summer, 2006


What would someone else’s SETI see?

By Tamara Wilhite

We imagine aliens coming to Earth with their views of Earth shaped by “I Love Lucy” reruns and 1930s radio broadcasts. Yet we fail to realize the irrationality of aliens seeing our TV signals light years away if we can’t hear FM stations 200 miles away. This then raises the question: What can they learn about Earth? And if they cannot receive our TV and radio signals, what can they receive?

Assumptions we would have to make for Reverse SETI are that a group like Amateur Radio Operators on Earth would be using a 100 Meter antenna dish on their world. We’ll assume the objective (Earth) is 10 light years away.

Would aliens be able to determine if Earth is populated by intelligent beings? They could tell that we have technology. You have to have electricity and signal generation technology to send a signal into space. Intelligence, however, is a whole other question. There are humans who don’t see other humans as intelligent. But they could see that we do have technology, which is a start. What else could they learn about us from our signals?

What exactly would they be able to determine about Earth? Let’s look first at what they could not determine. What is not detectable over interstellar distances?

Forget TV stations.If you can’t watch TV stations across the country, aliens will NOT see it. Forget Radio Stations: If you cannot get FM or AM once you are over the horizon, ET can’t, unless he’s in orbit. If he’s made it to Earth orbit, then they don’t need our signals to learn what they want about us; they could send probes or collect samples.

Let’s now look at the primary question: What is detectable over interstellar distances? The answer: Military radar output and satellite uplinks.

What do our signals look like from outer space? Imagine a transmitter as one of the dots on the surface of the Earth. If it is radiating out into the air—and space—it will project out in a disc in a straight line in tangent to the Earth’s surface. This means that with fewer transmission sites on the Southern Hemisphere, anyone “below” us while looking “up” at Earth with a radio telescope will see less than anyone a straight line view of the Northern hemisphere.

The RADAR’s can be identified by frequency and PRF (Pulse Rate frequency). Many radar systems exceed 180 degree sweep, so they can be detected on both edges of the planet. This would mean these signals are viewable twice every 24 hours, at sunrise and at sunset, coming and going.

A 24 hour rotation period is easy to determine via variations in signal strength as the Earth turns. By observing the polarization changes every 12 hours (Doppler shift) the position of the transmitter on the sphere (Earth) can be estimated. The northern hemisphere sure is busy! Observers along the line of the Southern Cross would detect little because there are few RADAR of multi megawatt levels south of the equator.

Measuring the Doppler shift between the rising and setting signals as the Earth rotates, along with their position on the sphere, would allow them to measure the Earth’s rotational speed. Knowing the day length of 24 hours and Earth’s rotational speed gives them the diameter of the planet. Extended observation produces the 365-day year. Extend Doppler measurements produces the orbital velocity of the planet. (Red shift/blue shift of radar frequency).

Direct observation of the Sun will give the mass of the Earth. Stellar mass along with the orbital velocity of the planet, or the Orbital Period, inserted into Keppler’s laws of planetary motion will give us the distance between the planet (Earth) and the Sun. Distance from the Sun and strength of the Sun’s solar output gives a rough idea as to temperature range. They will know we are not as hot as Mercury or as cold as Pluto. They will now know our length of day, length of year, that we have seasons, our gravity, and type of orbit.

They will ask: Is there air out there? Can they find that out? YES! Atmospheres distort radio signals. This is how weather radar works, be measuring the distortion of the radar signals to determine the density of the air above the surface. This is how we know when a cloud is a thunderstorm versus a fluffy cumulus cloud. Observers could not see our hurricanes, but there is enough atmosphere to tell them we have an atmosphere—and details about it.

Atmospheres bend radio signals. Air is thick near the surface, thinner as you go higher. Thus like light deflected going though a prism; the radio waves are bent downward slightly. If there were no air on a planet, the Radar signals would be detected every 12 hours.Due to atmospheric bending, the signals would be detected at 11 hours 55 minutes, and then at 12 hours 5 minutes.With some variation with the wavelengths used.

Atmospheric bending data, especially taken over several frequencies, gives observers the density of the atmosphere. Much of what they would learn would be negative data. While 80% Nitrogen with 20% Oxygen could not be determined, the detection of signals on absorption frequencies of other gases could rule them out. They could see hydrogen frequencies, so they would know our atmosphere was not a majority hydrogen (water vapor does not count). They would see methane frequencies, and hence learn that we are not like Titan.

Question for humans: What do satellite uplinks do? What can they tell observers?

Geosynchronous satellites (those that remain in stationary orbit above the Earth) used to relay satellite television signals. They are stationed in the Clarke belt, which is three Earth radii out. To send a signal out that far, satellite dishes send signals out to those satellites at ½ a Gigawatt. Think 500,000,000 Watts of radiated power per satellite dish sending out its signal. Now multiply that by several hundred TV stations sending up satellite signals. (Only a few hundred of the old network analog systems ran those levels. Cost a lot of money, big electric bills, so they run only what they have to.)

Satellite uplinks are very high power, some in the 0.5 Gigawatt Class. However, only about 20% of the sky is illuminated with such signals. The signals are short; sweep or observation time about 60 seconds per 24 hours. This is not long enough to provide much information except to see what frequencies we are using for our own communications. And, perhaps, to see a brief commercial for “I Love Lucy” reruns.

What Do We Know They Could Know

If aliens decided to make contact, how would they do it? The advantage of someone looking at the frequency ranges of radar is that we put out Gigawatts of signal. We have been sending out these signals in increasing strength since the 1950s. They can see us if they’re within 50 light years. We could see the same for them if we were looking for it. SETI/Argos currently scans all stars it searches for ALL frequencies. If they are radiating radio frequencies or radar frequencies, we’d see it. This means that we don’t have to wait for aliens to see us and send a signal back. They could already be visible, if they are radiating signals in a range and strength that we can receive.

Do I believe that intelligent life exists anywhere else in the universe? Can we prove it exists on Earth? Imagine if early humans did go extinct as they almost did 80,000 years ago. Dolphins would still be prospering. But they wouldn’t be obvious even if aliens sent a probe to Earth and mapped the planet as we have other worlds in our solar system. How about ultra-smart squid? Or a pre-technology hunter-gatherer species. How could you tell chimps from squirrels via a probe?

There are almost certainly single-celled organisms on untold numbers of planets in the solar system. There may be smart squid around some planet 50 light years away. They may be 2 billion light years away and will last another billion years—but they’ll never hear us, because our signals only travel at the speed of light.

If we’re going to meet intelligent life in any form, it is most likely of our own making. But if aliens are out there, and if they can listen for us, now you know what they’ll be able to learn about us.

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