Humans have wondered whether we share this cosmos with other sentient species since the birth of civilization. We've been asking this question for millennia. Philosophers and priests to provide us with a solution to this topic. Answers have always been given, been forthright and reflected the person's belief system offering the solutions. The second half of the twentieth century saw the emergence of the investigation. For the first time in human history, scientists have discovered Engineers and scientists have the possibility to organize and carry out experiments and, that just might provide a solution to this age-old and crucial question.
SETI (Search for Extra-terrestrial Intelligence) is a scientific discipline that has been around for 40 years. While Frank Drake was prepared to conduct Project Ozma, the first observational program utilizing a typical radio astronomical apparatus, Cocconi & Morrison (1959) published the first scholarly study on the subject in Nature. This exploratory scientific effort may take another 40 years, 400 years, or perhaps 4000 years to locate what it seeks or to conclude that there is nothing to uncover.
In the next two decades, we have a lot to look forward to. The search for extrasolar planets, a better knowledge of how the Earth and our own solar system arose, and whether our system is typical are all pertinent to the question of life elsewhere in the cosmos because life as we know it is a planetary-phenomena. As a result, efforts are focused not just on identifying planets (especially terrestrial planets) close to Earth so that we can investigate them for potential biomarkers, but also on compiling a list of planets and solar systems linked to vast populations of stars.
What are search strategies SETI has been focusing on, is it probes, charged particle/photons or near/far search? Innumerable questions come into our mind, right?
Let’s look at some of strategies which are being used:
1) Artificial signals: Signals that are compressed in the frequency and/or time domains beyond what astrophysics allows—signals whose time-bandwidth product approaches the limiting value imposed by astrophysics—are examples of the first category. By the concept of uncertainty Coherent signals are obvious examples of this type of signal. Since the earliest SETI sightings, the exact frequency of choosing for these narrowband signals has been a point of contention (Blair & Zadnik 1993). The allure of a neutral hydrogen 1420 MHz magic frequency first hypothesized in 1959 has faded as more astrophysical lines have been identified. Magic frequencies are still used to limit the range of hypotheses tested by a given experiment in searches with limited access to telescope time or limited financial resources that prevent the construction of large spectrometers with the millions (or even hundreds of millions) of narrowband channels required for systematic searching. There will be relative acceleration between the sender and receiver unless the narrowband transmitter is targeted at our receiver and both the transmitter and receiver correct for their respective motions within some shared rest frame. A search for narrowband signals thus necessitates not just a frequency space search, but also the ability to detect signals with unknown Doppler frequency drift. A signal's frequency drift rate due to natural accelerations dv/dt will be proportional to, and the signal will drift through an individual spectrometer channel of width B in time = B/(d/dt). The appropriate channel width B should rise as B increases in an instrument with B = 1 (to fully sample the frequency-time domain) (Morrison et al. 1977). As a result, the resolving power required to search for narrowband optical signals is far lower than that required to search for microwave signals. If these narrowband artificial signals are generated in the neighbourhood of a star [as is the case for any civilizations populating planets inside their planetary system's habitable zone (Kastings et al. 1993)], our existing technology will be unable to resolve them geographically. To be detected, the artificial signals must therefore outshine the star. This is a simple task at microwave frequencies. A weak radio source, such as our sun, produces just 1 kW/Hz. As a result, radio astronomy can be done 24 hours a day. On a regular basis, our rudimentary technology outshines the sun. The incorporated narrowband carriers are 106 to 109 times brighter than the silent sun at the frequencies of broadcast FM and TV transmitters. Because the highest output of stars like the sun is at optical frequencies of 4 1011 W/Hz, the power requirements for a detectable transmitter at those frequencies are higher. The signal must be detectable above the stellar fluctuations (rather than the total power) in a long integration, and Rather (1988) argues that the transmitter power requirement is manageable for sufficiently narrow lines broadcast in the depths of the Fraunhofer absorption bands of the stellar spectrum. Narrowband continuous wave (CW) transmissions or extended duration microwave pulses, as well as broadband, extremely brief optical pulses, appear to be the most likely possibilities for manufactured signals.
2) Quasi-Astrophysical Signals: An sophisticated technology attempting to attract the attention of a developing technology, such as ourselves, might do so by emitting signals that are detectable during routine astronomical studies of the cosmos. Emerging technology will eventually create the necessary devices to observe their surroundings and collect the signal. Although it may take some time for the distinctions between the purposeful, quasi-astrophysical beacon and natural emissions to be noticed, the transmitting technology can be confident that detection will occur without any SETI-specific effort on the receiver's behalf. Some candidates for such signals include (a) pulsars whose spin-down rate remains precisely zero for an extremely long time, (b) pulsars that periodically "glitch" between two fixed values of rotation rate, (c) impossible emission lines in the spectrum of an otherwise normal solar-type star (possibly as a result of nuclear fissile waste disposal by dumping it into the star), and (d) microwave emission at the tritium line frequency coming from a location far away. Many more suggestions have been made in the peer-reviewed and popular literature, and there will likely be many more. Those previously mentioned have deserved some limited observation time and archival research (see Appendix). If an extraterrestrial transmitter has chosen this technique, the best search strategy is to keep studying the universe in as many ways and with as many instruments as we can think of and implement, resulting in a robust observational astronomy program.
If the signals being generated are not intended for receipt by a distant technology, but rather for use by the generating technology, predicting the likely nature of such signals becomes more challenging. We can and have used present terrestrial technologies to predict what such leakage may look like, but the situation is rapidly evolving. Within the blink of a cosmic eye, broadcast television as we know it now will have transformed into something altogether new. In terms of measuring the completeness of any particular search program or justifying the plausibility of a new strategy, comparisons with current technology are more useful. At the very least, it is plausible to assume that signals generated for local consumption will be weaker than signals designed to be detected over interstellar distances due to some type of resource conservation.
3) Targeted Searches: After deciding on a preferred frequency for the search, one must determine whether to look in all directions or to focus on a defined subset of directions with a higher a priori likelihood of success. The objects of choosing are frequently, but not always, solar-type stars, and these two methodologies are known as sky survey and targeted searches. There are hybrids of these two approaches that examine a small (but desired) portion of the sky in both area and depth. The (now unknown) density of extraterrestrial civilizations and the luminosity function of their transmitters determine which technique has the best chance of succeeding. If weak transmitters exceed strong transmitters, the most detectable ones will be closer together. Even though sky surveys are normally less sensitive than their targeted equivalents, if there are enough strong, continually broadcasting transmitters, a sky survey is more likely to detect one at a longer distance. Sky surveys are also a better match for signal sources that aren't related with any recognized item type. These strategic decisions were well-understood from the start. Kardashev (1964) proposed a system for categorizing technologies based on their energy consumption and, as a result, the quantity of power available for transmission. Type I civilizations are thought to spend ∼ 4*1012 Watts (Earth-like), Type II civilizations consume ∼ 4*1026 Watts, and Type III civilizations can manage the energy production of their galaxy and consume ∼ 4*1037 Watts. To locate anything resembling a Type III beacon, low-power sky surveys might suffice, but if these are fleeting phenomena, continuous sky coverage would be preferable. The luminosity function of remote transmitters could be described as a power law, similar to astrophysical sources, such that the spatial density of detectable alien transmitters with transmitter power in the range P to P + dP is (P) = P. (Drake 1973). The number of distant, detectable, powerful transmitters will outnumber the number of lesser local emitters in 5/2, making a sky survey the preferred method. We have no idea what it is or if it even exists. Because manufactured transmitters are not the outcome of a random, naturally occurring astrophysical event, the output power they produce is likely to be determined by one or more design parameters.
A power law may be an inadequate representation in this scenario; a delta function may be more appropriate (J.W. Dreher, personal communication). If advanced technologies decide to illuminate every target with the same power flux density, emerging technologies will be able to detect all such transmitters once they are capable of detecting the closest one. The sensitivity of our search and the likelihood that we are included in the list of objects to be illuminated by distant technologies will determine our success. This chance could be distance dependent, and thus indirectly reliant on transmitter strength.
4) Temporal Clues: There could be a literal timing in SETI. Because the only observable signal is one that is "on" when we gaze in the right direction, it has been proposed (Makovetskii 1980, Lemarchand 1994) that any transmitting civilization will synchronize its transmissions by using cosmically brief and unusual occurrences. An ellipse passing through the target (with its imagined transmitter) and having the earth and the cosmic transient event as its two foci can be used to calculate the best time for us to observe any specific target.
What to expect in near future?
Larger collecting areas could improve all targeted optical searches, without requiring high imaging skills. As a result, preliminary conversations with the Whipple high-energy Gamma-ray telescope's architects are underway to see if a commensal OSETI project may make advantage of this massive light-bucket and focal-plane photodiode array while searching for Cherenkov radiation. The key will be to catch single photodiode events from the focal plane array that are now being discarded in the search for longer air showers. By requiring coincidence from several telescopes in the proposed VERITAS array, the event rate might eventually be decreased to an acceptable level. Because OSETI is still in its early stages, using modest targeted and sky survey telescopes, there has been no long-term planning. The Terrestrial Planet Finder may be able to detect signals from other planets (Howard & Horowitz 2001). Because OSETI does not require outstanding picture quality, big, dedicated antennas at optical and IR wavelengths may become inexpensive in the not-too-distant future. The importance of synchronicity between distant sites must be better recognised, and the ability to conduct IR searches 24 hours a day will be fully realised as soon as the technology becomes inexpensive. The IR spectrum, rather than the optical spectrum, will allow targets to be seen at larger distances through galactic dust. Computing prices should continue to fall over the next few decades. Moore's Law advances are expected to continue for several more generations, while quantum computing's promise is unwavering. At least in the low frequency region of the terrestrial microwave window, this should allow the construction of an Omni-directional Sky Survey telescope for SETI observations on temporally sparse signal occurrences. The capacity to work around the ever-increasing RFI environment will be critical to the success of SETI using an Omni-Directional Sky Survey Telescope or the Square Kilometer Array. The establishment of SETI observing facilities on the lunar farside may be required for microwave SETI success in the end. Although it appears unlikely that this could be funded only for SETI, it is possible in conjunction with a multidisciplinary, remotely operated research station on the far-side.
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