This Hubble infrared image is of a dense star cluster that surrounds the center of our Milky Way galaxy. That's a lot of stars, around 500,000 in this image alone! Its currently estimated that our galaxy contains somewhere between 100 to 400 billion stars.
We live in a barred spiral galaxy that is roughly 100,000 light years across and about 2,000 light years thick. Our Sun, the Earth, and everybody we know is believed to be within the inner edge of the Orion-Arm approximately 27,000 light years distance from the center. In other words, the light from the center we see today took 27,000 years to reach us. Putting this into perspective, the light from our nearest neighboring star, Proxima Centauri, takes 4.2 years to reach us, and the light from our Sun which is 93 million miles away takes 8.3 minutes.
Just as the Earth rotates around the Sun, our Sun rotates around the galaxy with a period of ~230 million years. The last time our Sun was in a similar position in the galaxy the Earth was at the end of the Triassic period with dinosaurs roaming the planet. What will the Earth be like the next time around? Hopefully we will still be around in some form or another, and if so we will very likely have evolved into a different species. What we end up becoming is driven by evolutionary pressures we as humans face today.
I'm going to take this moment to acknowledge the scientists that I work with who utilize the telescopes here in Hawaii and elsewhere to perform a wide variety of scientific studies. Two of the most recent and noteworthy are the imaging of the supermassive black hole (SMBH) in M87 galaxy followed by the imaging of Sagittarius A* SMBH in the center of our Milky Way. Maybe less exciting but no less important are the scientists who perform astrometric measurements to map out the galaxy we live in. In my mind they are akin to the navigators for the seafaring explorers of the past.
It's been estimated that our Milky Way galaxy hosts at least 300 million stars with Earth sized planets in the habitable zone. That's a pretty big number! But just because there are planets similar to Earth doesn't mean there is life on board. Look at Venus and Mars.
The question of how life got started on Earth is still unknown and will probably remain so until we discover other forms of life elsewhere. Our Sun and its planets including Earth were formed about 4.5 billion year ago. It was a hostile place for the first billion years or so due to constant bombardment from comets and asteroids and the coalescence of material into what would become our planet. As the number of impacts tapered off and the surfaced cooled there may have been a point in time when the Earth was a pristine world with oceans, land, an atmosphere, but devoid of life. This didn't last long because the first fossil evidence of life dates back to around 3.5 billion years ago.
One could fathom that there are other worlds with similar environments as our planet Earth. How long could a world with oceans of water and atmosphere exist without the formation of life? My intuitive feeling is that if such a world exists then some form of life will take hold of it.
Most technical people with whom I interact with, OK I admit that its an odd crowd, believe that life such as bacteria and possibly even simple multicellular life such as plants and jellyfish may be common outside of the Earth. But beyond that peoples opinions differ greatly because we simply don't know.
It's my opinion that given the right environment and enough time, it is inevitable that life evolves to become more complex due to evolutionary pressures to compete for resources. Here on Earth plant and animal species have continued to change to adapt and optimize to their environment. Mutations that are advantageous are passed on while less successful ones are suppressed. This constant evolution, however, doesn't necessarily have to continue forever. Take for instance sharks, turtles, and ants which have maintained their basic physical forms for millions of years.
On the other hand homo sapiens have only recently evolved to become what we are today. A million years ago we were quite a different species. What sets us apart from other creatures is our level of intelligence but it comes at the heavy price of carrying around a big brain that requires a lot of energy. Many people, including some evolutionary biologists, believe that the development of intelligence capable of manipulating atoms to constructing megastructures is a fluke and likely won't happen again either here or elsewhere.
I am of the mindset that intelligence emerged on Earth to fill a specific niche and that if other worlds present similar niches they will be filled just the same. But in order for this to take place there needs to be a stable environment for an extended period of time which raises another question. How is it that our planet Earth has remained stable for so long without turning into a frozen ice ball or becoming a hot furnace devoid of liquid water? An interesting but controversial theory is the Gaia hypothesis that posits the Earth itself is a self-regulating complex system maintaining the global temperature and atmospheric composition. We homo sapiens have thrown a wrench into the works but it wouldn't be the first time a disruptive event occurred on our planet. If anything, we are here because our species is resilient.
SETI has been around since 1960 and focused on radio emissions from potential civilizations similar to ours. We are still searching the radio spectrum today but with technology that permits several orders of magnitude increase in sky and frequency coverage.
Where does 60+ years of SETI null results leave us? At first glance it may appear that we alone in this big cosmos, or at least extremely rare. Several theories are abound explaining the rarity of Advanced Technological Civilizations (ATCs) such as ours. Two of these are the Great Filter and the Rare Earth hypothesis that to me are fairly elaborate conjectures (no disrespect to the authors!) to explain away our current lack of evidence (of ATCs).
Probably the most compelling argument that we are alone or very rare as a technologically advance species is the Fermi Paradox. If ATCs are common why haven't any colonized the galaxy or a significant fraction of it? One explanation from the above link is - "The Fermi Paradox is a very large extrapolation from a very local observation. You might just as well look out your window and conclude that bears, as a species, couldn’t possibly exist because you don’t see any. This, despite the fact that, in the history of North America, the bears have had plenty of time to shamble into your yard."
Though it may sound far fetched there are a subset of scientists who believe the search for alien megastructures and artifacts such as Dyson Spheres or probes might yield evidence of past or present ATCs. A discovery of a Dyson sphere would be monumental to say the least but alas the probability of an ATC constructing such a structure must be far less than emitting some form of communication signal.
What about communication through the use of gravitational waves? If such technology were possible it would require the manipulation of gravity and by association the mass of matter. Wouldn't it be cool if we could someday focus a gravity beam in a particular direction and change its intensity at will? Better yet if we could produce repulsive gravity! But alas back to our current reality, those technologies either don't exist or are far far into the future for us so I'll leave this for our future selves to ponder.
Quantum communications? It's mind boggling to think that this "spooky action at a distance" phenomenon really exists but I haven't been able to connect the dots as to how one could use this to send a message from point A to point B.
Neutrinos? Modulating the emission from a pulsar? I'm sure I've missed some other exotic forms of potential communication modes. But putting these aside for later (few hundreds of years) this leaves us with looking at the electromagnetic spectrum for signatures of ATCs. But haven't we been doing this all along since Frank Drake peered the heavens using Green Bank's 85-foot dish in 1960? Yes, but the EM spectrum is huge place covering wavelengths from radio to gamma rays. If anything, we've only explored a minuscule sliver of the EM spectrum focusing on radio and microwave.
I'll start off by admitting that I don't know the answer to the "Where". The sky is a big place and peering through a high gain telescope is akin to looking through a straw. Perhaps a good place to start would be a catalog of candidate stars with potentially habitable exoplanets. If I ever get around to building some SETI specific hardware I'll have to consult with someone who actually thinks about these things for a living. From a practical perspective most people who think about these things do it on the periphery as there is not much demand for this type of speculative research. However, one confirmed detection will change the landscape in ways that I probably can't image.
As far as "When", there are some who believe that civilizations come and go similar to the Roman Empire (~1100 years) or the ancient Greek Civilization (~400 years). If there are ATCs present in the galaxy how long do they persist? This ties in with the Great Filter theory where advanced civilizations will eventually destroy themselves, hence none are around to be discovered. Or that ATCs reach a nirvana level of being and simply exist without further exploration and expansion. Picture in your mind the image at the top of this page with a thousand ATCs but each lasting only 100,000 years. How often would two nearby civilizations overlap during their existence?
It turns out that our atmosphere is fairly opaque at most frequencies with just a few windows available to peer through from sea level. One of those transparent windows coincides with the visible spectrum (380 to 700 nm) and is what we and other terrestrial creatures rely on for vision. But by far the most transparent window is the radio spectrum ranging from approximately 30 MHz to 10 GHz and is and has been the focus for the majority of SETI research. But just because our atmosphere is transparent in this regime doesn't mean or imply that this is an optimal frequency range for deep space communications.
The largest challenge to establishing a deep space communication link is overcoming the free space loss incurred from the enormous distances involved. As a mental experiment let's calculate the link losses for a pair of 100 meter diameter antennas separated by 5 LY distance at three different carrier frequencies of 3 GHz, 30 GHz, and 300 GHz. Since antennas are not perfect reflectors I'll assign efficiencies of 85%, 65%, and 50% for 3 GHz, 30 GHz, and 300 GHz, respectively. Entering the above parameters into this gain and FWHM beam width calculator we arrive at: 69.2 dB/224"; 88.1 dB/26"; and 106.9 dB/3" for the respective frequencies. Note that as we increase the frequency we see a corresponding reduction in angular beam width and results in higher directivity and gain. If you were to shine a light with beamwidths of 224, 26, and 3 arcseconds from the Earth onto the moon it will illuminate a spot with diameters of 259, 30, and 3 miles, respectively.
The total free space loss across 5 LY distance with 100 meter diameter antenna apertures at each end are 237 dB, 220 dB, and 202 dB, for carrier frequencies of 3 GHz, 30 GHz and 300 GHz, respectively. An important concept is that for a fixed aperture size, directivity and its associated gain improves with increasing frequency. In fact we can extend this to include transmission at infrared wavelengths which is currently being investigated by NASA for future communication links. Knowing that higher frequencies provide the advantage of narrower beams, is it possible that constraining SETI observations to the 30 MHz to 10 GHz window may be one explanation for our null results to date?
Assume for this example that we may eventually be capable of constructing a 100 meter reflector in space, and even more challenging the transportation of a terminal across a distance of 5 LY. How much transmission power would be required to establish a 5 Mbps data link? I'll assign receiver noise temperatures of 5 K, 25 K, and 100 K for 3 GHz, 30 GHz, and 300 GHz, respectively. A received Eb/No of 10 dB produces a theoretical uncoded error rate of 4e-6. The required transmission power levels to achieve this are 3.47 MW, 347 kW, and 21.9 kW for 3 GHz, 30 GHz, and 300 GHz, respectively. These are tremendously high power levels that will certainly pose a technical challenge especially in space.
Modulation is the method used to impart information onto a EM carrier signal that can range from RF to optical wavelengths and beyond. Given a carrier at a fixed frequency:
S(t) = A*cos[(2*Pi*f)t + phi],
where A is the amplitude (V), f is frequency (Hz), and phi is phase (radians), we can modify or alter any one or a combination of parameters. If you listen to the radio in your car you would be familiar with amplitude modulation or AM used for commercial broadcast over 530 to 1600 kHz. Similarly frequency modulation is used for commercial broadcast over 88 to 108 MHz in the US. Both AM and FM are old legacy systems that modulate the analog signal (voice or music) directly onto the carrier. If you happen to have a SiriusXM radio in your car and subscribe to their service then you are listening to a digitized signal modulated using QPSK onto an S-band carrier operating from 2320 to 2332.5 MHz. More on BPSK and QPSK later.
As previously mentioned, deep space communication faces the major challenge of overcoming the large free space losses incurred over the distances involved. This can be partially remedied by using large antenna apertures, high transmission power levels, and low noise receivers. In an application where every dB counts the selection of a modulation format that can operate reliably in low signal-to-noise ratio (SNR) conditions is very important and I'll get to that in the next section.
Video: Real-time spectrum capture of KNWB B95 FM in Hilo using RTL-SDR V3 USB plugin device
Phase shift key (PSK) modulation is a method that utilizes the carrier phase to impart digital information for transmission. Its primary advantage over other forms of modulation is that it provides constant envelope power independent of the information content. Binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) are two common modes of PSK that are used by NASA's Mars Reconnaissance Orbiter spacecraft. The spectrum shown above is for a 5 Mbps BPSK transmission, black represents the unfiltered spectrum and navy blue is with a spectral mask applied. Note the relatively low spectral efficiency of 0.8 bps/Hz (5 Mbps bits occupies 6 MHz RF bandwidth) and is one of the downsides of this format. QPSK provides twice the spectral efficiency, i.e., 1.6 bps/Hz and is utilized for higher SNR conditions. A brief tutorial on PSK modulation can be found here.
Another subset of constant envelope modulation format is frequency shift keying (FSK). The FM video shown above is an example of analog frequency modulation with constant envelope but varying spectral content. 4-ary FSK also known as minimum shift keying (MSK) is similar to QPSK in terms of performance. A brief tutorial on FSK modulation can be found here.
A key item to note for PSK and FSK modulation is that they are of suppressed carrier formats. There is no reference carrier for the receiver to lock to which adds some complexity to the receiver architecture. With today's modern digital technology, however, carrier recovery and subsequent demodulation is performed fairly easily even under low SNR conditions. One could imagine a scenario where our cosmos is littered with PSK and FSK transmissions but we don't notice them because they appear as broadband bumps or interpret them as molecular emission lines. Or since these signals are not intended for us they are simply buried in the thermal noise floor.
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