This blogger does not intend to use humor to diminish the impact of water pollution or the need for water conservation – keep water clean (just don’t drink it raw)!
Animals socialize at the waterhole and require one without pollution (see Figure 1). In his 1979 paper, Bernard Oliver argued advanced societies might do the same. In this case, the “waterhole” meant a relatively quiet region of the electromagnetic spectrum and “pollution” referred to noise and interference from satellites and other man-made objects. The waterhole is defined by the Cyclops team as the region bounded at the low end by the hydrogen line (at 1420 MHz) and at the high end by the hydroxyl lines (~1662 MHz) (see Figure 2). It is this region, Oliver argues, that should be thoroughly searched for interstellar signals. Oliver can be considered a conservationist of sorts, as he makes a political plea to retain the lack of navigational satellites and other systems operating within this band and provides a logical argument for it. The premise for this policy decision is that any advanced society with a desire to communicate will choose the least expensive means to communicate.
Everyone likes a clean water hole. We are asked by Oliver to stop and consider (like the animals above) the implications of a clean water hole, albeit as an astrophysical concept (see Figure 2 for more information). In his words: “Where shall we meet our neighbors? At the waterhole, where species have always gathered.” Source: Disney’s The Lion King
Above, we see the “waterhole” in the microwave. It represents a small region where the noise is limited. This noise includes contributions from the galactic background, the cosmic microwave background, and our atmosphere. It’s a small window that could allow for extraterrestrial communication, according to Oliver. Source: Advances in Passive Microwave Remote Sensing of OceansOliver first considers the most fundamental question: how do we send information over interstellar distances? He immediately rebuffs the notions of sending probes or spaceships, on the ground of energy requirements and instead proclaims “we are not going to find other intelligent life by hurling tons of matter through space but by receiving – and possibly some day sending – some form of radiation.” As per Oliver, an effective form of communication must satisfy the following:
- the number of particles received must significantly exceed the natural background count;
- the signal must exhibit some property not found in natural radiations;
- require the least radiated power;
- not be absorbed by the interstellar medium or atmospheres;
- not be deflected by galactic fields;
- be readily collected over a large area;
- permit efficient generation and detection;
- travel at high speed; and
- normally be radiated by technological civilizations.
Any particle with mass is an unlikely candidate as it requires a lot of radiated power to be generated. Charged particles are ignored due to absorption and deflection while neutrinos are difficult to generate and detect. Ergo, photons are our best means of communication. Oliver then describes which region of the electromagnetic spectrum should be used. He considers the detected signal/noise power ratio (DSNR). The DSNR has contributions from galactic noise and the relic cosmic background (see Figure 2). He also considers the terrestrial microwave window. When taken together, the quietest part of the spectrum is the waterhole.
Oliver finally addresses the political considerations regarding the waterhole. Ignoring the chauvinism inherent to his preference for water-based life, he presented a sound case for a region in the electromagnetic spectrum where noise was minimized. This blogger agrees with Oliver’s insistence on keeping the waterhole free from man-made noise. The dearth of scientific data and dubious claims have largely stigmatized SETI, and this small window holds promise for some experimentation and data collection. Oliver went on to sign Sagan’s international petition three years later. If the argument presented by Oliver was not enough, this international petition for SETI reiterated his argument: we must create a policy to protect the most likely channels for extraterrestrial communication. The SETI Institute currently views the water hole as a region for ongoing sky surveys. The National Telecommunications and Information Administration also has a passing reference to SETI and allocates some of this region to radio astronomy and passive space research (i.e. listen but don’t talk):
“5.341 In the bands 1400-1727 MHz, 101-120 GHz and 197-220 GHz, passive research is being conducted by some countries in a programme for the search for intentional emissions of extraterrestrial origin”
These are small steps but at least some credence is given to SETI with regards to policy. To this blogger, the importance is the collection of data and science that can be performed. While we have yet to meet another society at the waterhole, it is important to continue to search before relegating SETI to the same level of pseudoscience as alchemy.