Contents
Optical Seti
If we use a high-intensity pulsed laser coupled to a moderate sized
transmitting telescope, we create an efficient signaling device. Such a
laser transmitter with its slender beam would appear a thousand times
brighter than our sun in broadband visible light to a distant observer. Even
at distances up to a 1000 light years away, a single nanosecond laser
pulse would transmit a thousand photons to a 10-meter receiving
telescope.
Using research grade equipment, detecting signals from distances up to
1000 light-years are feasible and this is the motivation behind the Optical
SETI project which scans the skies for pulses of laser light in or near the
visible portion of the light spectrum.
Merits of Optical SETI over Radio SETI:
1. The higher frequency of light waves allows lasers to produce carrier signals at much higher frequencies allowing extremely fast data transfer.2. Visible light is typically not affected by interference as compared to microwaves.
3. Nanosecond pulses of light of natural origin are not known to exist.
4. Dispersion, which spectrally broadens radio pulses, is completely negligible at optical frequencies.
5. Intensive computer analysis required in sensitive microwave searches today is not needed for optical SETI.
The analysis of microwaves suffers from false triggers created by spark plugs, etc. However brief spikes or pulses of visible light due to natural origin are rare and the detection of a laser pulse signifies a higher probability of the signal originating from intelligent life and not from a natural astronomical event. In fact, nanosecond spikes of light are thought to be nonexistent in the universe.
Procedure of detection:
We are looking for pulses of visible light, several nanoseconds in duration. The amplitude of the pulse must significantly exceed the strength of illumination from the actual star. Each star is scanned for a certain duration of time which should exceed the time period of the expected pulses. Also we do not seek any specific frequency of light since even the lasers made out of current day technology, if sampled at a nanosecond pulse without filtering, still outshine the parent star by 30 times.False Triggers: Any strong flash of light in nature could be a potential source for a false trigger. However every such known source can be compensated for as elaborated below:
Cerenkov Flashes: A Cerenkov light flash is emitted when objects exceed the speed of light in media other than vacuum, such as air or water. These particles subsequently emit Cerenkov radiation, which is visible as a flash of blue light. However, these flashes release only a single photon in a nanosecond which is insignificant for detection purposes and thus do not affect OSETI readings. Lightning too does not affect readings as it cannot generate pulses in the nanosecond range.
Electrical arcs across high voltage terminals can trigger detectors. To compensate for this, we use two detectors, and reject readings that are not common to both simultaneously. This gets rid of the problems created by electrical arcing in the instruments. Cosmic rays entering the atmosphere can cause discharge of high energy photons that could trigger the OSETI detection setup. To prevent this, data from various adjacent detectors is compared.
