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Where did the dBs Go?

  

The key difference between the microwave and optical approaches to SETI are the assumptions about the directivity (beaming capabilities) and continuous wave or peak powers that ETIs might transmit in our direction. So much transmitter gain (dBs) have been needlessly thrown away by unreasonable assumptions concerning the technical prowess of ETIs in their ability aim narrow beams towards nearby stars, that the emphasis of present-day terrestrial SETI research on microwaves is likely to be wrong. Surely, the advanced and sophisticated sending civilization would make it easier for us to detect their transmissions by ensuring that their signals were strong. In this way, we wouldn't have to build expensive, vast arrays of telescope mirrors in order to collect their weak stream of photons. See also:

Energy Cost Per Photon

Optical SETI 101

Gain of Arecibo-type microwave dish (300 m aperture) uplink at 1.5 GHz = 73.5 dB

Gain of full Cyclops array (3 km aperture) uplink at 1.5 GHz = 93.5 dB

Project Cyclops

Gain of an infrared telescope (10 m aperture) uplink at 10,600 nm (10.6 microns) = 129.4 dB

Gain of a visible telescope (10 m aperture) uplink at 656 nm = 153.6 dB

Difference in gain between visible telescope and Arecibo dish = 80 dB (100 million)

Difference in gain between visible telescope and full Cyclops Array = 60 dB (1 million)

Arecibo dish with a 1 MW (60 dBW) continuous wave transmitter (late 20th century technology)

Effective Isotropic Radiated Power (EIRP) = 73.5 dB + 60 dB = 133.5 dBW

Equivalent of a 10 m visible telescope with 1 GW (90 dBW) continuous wave laser (ETIs' futuristic technology)

Effective Isotropic Radiated Power (EIRP) = 153.6 dB + 90 dB = 243.6 dBW

EIRP advantage of ETIs' optical uplink to that of the 20th Century terrestrial microwave uplink for continuous wave beacons = 243.6 dBW - 133.5 dBW = 109 dB (nearly 100 billion times!)

See also Viewgraph 9308-001

Example

 

Array equivalent of a 10 m visible telescope with laser beacon of 1 GW (90 dBW) mean power, sending out pulses of 1 ns duration, once every second.

Peak power = 180 dBW

Peak Effective Isotropic Radiated Power (EIRP) = 153.6 dB + 180 dB = 333.6 dBW

Solar-type star's (Sun's) Effective Isotropic Radiated Power (EIRP) = 266 dBW

During each pulse, the intensity of the pulse exceeds that of the ETIs' star by 333.6 dBW - 266 dBW = 67.6 dB

See also Viewgraph 9308-002

 

Thus, while the average brightness of the visible laser is only 1% of the star's brightness, during each 1 ns pulse, it is nearly 10 million times brighter than the ETIs' star! There would be no problem in detecting this pulse with very fast (wideband) optical receivers. Such receivers would be many orders of magnitude faster than anything employed by astronomers today - comparable to those employed in the fiber-optic industry, albeit the latter being at a somewhat longer wavelength (1.3 and 1.5 microns).

Improvement of the ETIs' advanced pulsed visible laser system over the continuous wave terrestrial 20th Century Arecibo microwave system = 333.6 dB - 133.5 dB = 200 dB (one hundred million, trillion times)! All the numbers here speak volumes. The improvement margin in favor of lasers is so many orders of magnitude, whichever way one massages the numbers, that it is illogical to continue to claim that microwaves are superior. Even the quantum noise sensitivity penalty of 36 dB that visible receivers have with respect to microwave receivers hardly makes a dent in these numbers. See:

Quantum Noise & Effective Noise Temperature (9006-016)

 

Elsewhere on this site we consider the issue of interstellar scintillation and dispersion effects that can corrupt a wideband electromagnetic signal. It is suggested that while such effects are very severe at microwave frequencies, severely limiting the modulation bandwidth, they are insignificant over ranges of several thousand light years at optical frequencies. For this reason alone, lasers have the capability to support an Optical Interstellar Information Superhighway - microwaves do not.

Can there be any doubt now that if advanced ETI civilizations are predisposed to use electromagnetic communications to contact emerging civilizations like ourselves, they would use lasers rather than microwaves? We cannot say that the lasers employed by ETIs would be in the visible spectrum or at a wavelength for which our atmosphere is essentially transparent - only that lasers are vastly superior for free-space interstellar communications for point-to-point links over distances up to several thousand light years. But you won't get that impression from popular terrestrial SETI literature or from the bulk of the peer-reviewed literature!


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