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Technical Background |
In the SCETI literature, and the SCETI community at large, it is widely thought that even if we detect another civilizations TV transmissions, we would not be able to watch their shows.
We've only been receiving significant electromagnetic signals for less than 100 years. Therefore, if we can pick up alien signals at all, they are less than 100 ly away. From this distance, we can easily reconstruct TV programs. So if aliens exist, and we know where to look, we can probably watch their TV if we want to.
The earth is at the centre of an expanding bubble of electromagnetic radiation. The bubble, expanding at the speed of light, contains all of the man-made electromagnetic transmissions of the earth - radio, TV, radar, and so on. In theory, an alien civilization could receive these signals, and form their opinion about the earth by analyzing them.
What if we were the aliens decoding the signals of a civilization far, far away? Could we form an opinion of an alien civilization from their TV transmissions. Decoding the transmissions are much harder than detecting them so the chances of this happening have traditionally been seen as very slim.
“We will probably not be able to see their home with visible light, but we might, with equipment somewhat more sensitive than what we have so far possessed, be able to pick up their television carrier signals.” – Dr. Taniguchi
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TECHNICAL BACKGROUND
Detection of broadband signals from Earth such as AM radio, FM radio, and television picture and sound would be extremely difficult even at a fraction of a Light-Year distant from the Sun. For example, a TV picture having 5 MHz of bandwidth and 5 Mega Watts of power could not be detected beyond 0.01 Light-Years of the Sun even with a radio telescope with 100 times the sensitivity of the 305 meter diameter Arecibo telescope.
Why does TV reception from astronomical distances seem so difficult? It is much, much harder to decode an analogue TV signal than to merely detect that it is present. And since even detecting the carrier is hard, then demodulating and watching the show must be nearly impossible. For example, let's look at how well we could do with our radio technology. A powerful UHF TV transmitter in Costa Rico has about 5 MW effective radiated power (typically 170 KW power with an antenna gain of about 30). Most of this power is concentrated in the lowest megahertz, leading to a spectral power density of about 5 W/Hz. The carrier, though, is about 16 times stronger in spectral terms. Perhaps 10 percent of the power goes into the carrier, and the bandwidth is about 17 times smaller (0.1 Hz). Thus the spectral power density is about 5 MW/Hz. So once you detect the carrier, you need another factor of 1 million more signals to watch the video. Alternatively, the distance at which the carrier can be detected is 1000 times greater than the distance at which the same equipment could decode the video.
The dismissal of the possibility of decoding TV begins with the Cyclops report 2 published by SCETI which in chapter 6 discussed how ETs could glean information from the detected carrier. Not explicitly stated, but clear from the discussion, is that if they could actually watch the show, then they would not need to rely on careful measurements of the carrier to determine the signal was artificial (though they might need this to tell if our civilization is intelligent, since this cannot be easily determined from most TV content.)
How far away could our traditional technology receive a high power UHF TV signal? A state of the art radio telescope has a noise temperature of about 25 degrees K. The biggest existing telescope is Arecibo, with a diameter of 305 meters. The feed pattern means this area is about 70 percent utilized. Plugging in the numbers, we find that even for a very marginal signal to noise ratio of 1, we can only detect the carrier out to 0.81 light years. We can only watch the program out to a range of 0.00081 light years, or about 51 AU. So our traditional technology can't even detect our strongest television signals out to one measly light year, and can only receive our signals out to a little past the orbit of Pluto. |
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WHAT WE DID
We have constructed radio telescopes a few orders of magnitude better than ever before. These telescopes, across interstellar distances, can detect UHF TV carriers but not decode the modulation. Decoding the modulation out to a distance of 1000 light years, for example, would require 12 orders of magnitude better telescopes. This seems at present impossible.
However, let's look at the possible improvements. These include better receivers and feeds, bigger antennas, signal processing, and stellar focusing. The combined improvements are quite impressive!
The conventional view considerably underestimates the technologies that AREA already made available to us. By looking at other technical improvements – developing better receivers and feeds, bigger antenna, better signal processing, and utilising stellar focusing, we can detect alien radiations and be able to decode it as well. Thus we can form our impressions of their alien realm from their version of News broadcasts.
These are straightforward improvements we did ourselves in a few years. First, improve the system noise temperature to 3 degrees K (about the best possible because of 2.7 K cosmic microwave background). Next, we improved feed efficiency to near 100 percent. The result is a signal to noise improvement of nine for a range improvement of three. Range is now up to 0.0028 ly.
The two factors limiting bigger radio telescopes are physics and economics.
First, look at the physics. From a physical perspective, using antenna tolerance theory, we need about 1-10 cm stability to coherently receive UHF radio signals. A planet can easily provide this stability over 10000 km distances (as shown by VLBI astronomy). However, a body such as the moon might well be a better site, since it is not big enough to have significant internal heating, and so is seismically dead. It also has no atmosphere, another advantage for radio astronomy. So if we restrict ourselves to moon sized bodies, we could easily have (from a physics perspective) an antenna about 1000 km on a side.
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USING GEOLOGICAL FEATURES AS AN ANTENNA
It is certainly possible that the biggest antennas are best built in space, with an absence of wind loading and much smaller gravitational forces. It has been held that it is currently economically impractical for earth. However, with easily foreseeable technology, we might well build an Arecibo style antenna in a crater on the back of the moon, or perhaps in a crater on an asteroid such as Mathilde. There are many suitable craters of size 30 km and up.
At least one of these methods is surely possible for an advanced civilization, so for the rest of this paper we will assume the aliens are using either a small (30 km) antenna or a large (1000 km) antenna.
The 'small' 30 km antenna has a collecting area 10,000 times that of Arecibo, for a demodulation range of 0.28 ly, and a detection range of 280 light years. Since we have only been sending out UHF TV signals for 50 years, this is more than enough range. This antenna, aimed at earth, could easily see our UHF TV carriers from any location the signals have gotten to so far.
So then it is only a matter of finding and locating a suitable location with the right combination of location and size. This can be found in one of the largest impact craters on Earth, an undersea 180-kilometer wide and 900-meter deep crater. Chicxulub, located off the coast of Puerto Rico. It eluded detection for decades because it was hidden (and at the same time preserved) beneath a kilometer of younger rocks and sediments. Size isn't the only thing that makes Chicxulub special. Most scientists now agree it's the "smoking gun" -- evidence that a huge asteroid or comet indeed crashed into Earth's surface 65 million years ago causing the extinction of more than 70 percent of the living species on the planet, including the dinosaurs. This idea was first proposed by the father and son team of Luis and Walter Alvarez in 1980.
Though the buried giant can't be seen, the impact crater has left subtle clues of its existence on the surface. Like a bowl with a sheet over it, all that’s now visible of the bowl is a subtle depression. It is not a big hole anymore, but if you look at the rim of the depression, you'll see that it is still in the same position as the rim of the bowl beneath. That's how surface expression allowed us to interpret something about the buried structure
The view from space let scientists see some of Chicxulub's surface features that are not nearly so obvious from the ground. Satellite images showing a necklace of sink holes, called cenotes, across the Puerto Rico's northern tip are what first caught the attention of SCETI researchers Drs. Kevin Pope and Adriana Ocampo in 1980. They were among the first to propose Chicxulub as the impact site linked to the mass extinctions that occurred at the end of the Cretaceous and beginning of the Tertiary geological ages, called the K/T boundary. They also realised that they now had access to the largest naturally formed antenna in the world.
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SIGNALS
For any particular source, we can gain still more range by locating the antennas at the point of gravitational focusing the sun. This focuses all the energy from a ring extending entirely around the sun onto an antenna. The width of the ring is half the diameter of the antenna. The sun is about 1.4 × 1E6 km in diameter, so for a 1000 km diameter antenna, we would focus all energy going through a ring of width 500 km and length À× 1.4 × 1E6 km. This is a gain of about 2000 in collecting area. This is good for about another factor of 45 in range. Now we are up to about 16300 light years for decoding the signal.
The smaller antenna gets a higher relative gain from focusing, about 9300 in collecting area. This is enough to demodulate the signal out to about 2600 light years.
Note that stellar focusing is good only for one specific target star. In this case this was not a serious disadvantage since we could already detect the carrier from a much larger distance than usual without using focusing. After we found the source of the signals, we moved the antenna to the focusing point and began to decoding the transmissions.
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DECODING
At first we received the same show many times, without knowing it. Like an alien civilization somewhere in the plane of the earth's equator, receiving signals from earths North America. First, the civilization would get the eastcoast stations - on many different channels and almost all of them at the same time. Then the Midwest stations would rotate into view. Because the shows are delayed for each time zone, the alien civilization would see the same shows again, delayed by 1 hour, again from many stations. This will happen yet again for Mountain Time, then Pacific Time. Individual networks often have more than 100 affiliates in the USA alone, all broadcasting the same show, so the aliens might record this many copies of the same show during one observing session.
Could we actually make use of all these copies? Although our improved signal processing and storage it is only slightly better than usual; we might well record all the signals from the stars for later signal processing. This is a lot of data (say 2 GB/sec if we want to cover all UHF TV channels), but on just exactly practical even for traditional technology. The major advantage of this approach is that near optimum signal processing can be applied, off line if need be, and all the historical data can be used.
Once we know that they are getting the same show again and again, on many different channels and at different times, we can use this to improve their signal to noise ratio, and hence range. Unfortunately we cannot use coherent summation of the voltage waveforms. (Only the modulation is the same - the actual waveform is not since the programs are broadcast on different channels, and the carriers are not locked to the frame rates in any case.) We can, however, average in the power domain. The received signals have several characteristics that made coherent summation of the modulation easy, such as strong synchronization signals.
With the methods outlined above, (plus re-runs), we accumulated 1000 copies of the same broadcast. When we summed up 1000 copies, so that our S/N goes up by 31, and our range by 5.5. Then we received the signals out to 1.5 ly with the small antenna, and 58 ly with the big one.
Furthermore, there appeared to be lots of internal redundancy in the received transmissions. Frames appeared to be the same as proceeding frames; scan lines similar to preceding lines. Compression using these ideas achieved 100:1 compression without smuch loss of quality. This was similar to getting 100 copies to average over. In the long run, we modelled the underlying objects and lighting. Then fiddled with the coefficients to get the best match to the data (similar to maximum entropy methods in astronomy). This yielded at least another factor of 10, for a total redundancy of least a factor of 1000. And increases the signal to noise by another factor of 31, for another range increase of 5.5. Now we can demodulate the signals out to 8.6 ly with the small antenna, and 327 ly with the big one.
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