SearchLites - Vol 12 No 1

Magazine Overview

Title: SearchLites
Issue: Vol. 12 No. 1, Winter 2006
Publisher: The SETI League, Inc.
Document Type: Quarterly Newsletter

This issue of SearchLites, the quarterly newsletter of The SETI League, Inc., focuses on the ongoing discussions and technical aspects of the Search for Extraterrestrial Intelligence (SETI).

Active Assessment of Active SETI

In his lead article, H. Paul Shuch, Ph.D., Executive Director of The SETI League, addresses the long-standing debate surrounding Active SETI, which involves deliberately transmitting signals into space. Shuch acknowledges the arguments for reciprocity made by proponents, who believe it's hypocritical to search for signals without being willing to transmit them ourselves. Conversely, he presents the counter-argument that broadcasting signals could be dangerous, likening it to 'shouting in the jungle.' Shuch notes that the debate often assumes the existence of technological extraterrestrial intelligence (ETI) and centers on their intentions. He criticizes the tendency to seek blanket policies for or against transmissions, arguing that the risks and benefits of any interstellar transmission are not equal and depend on factors like power, duration, directionality, bandwidth, and information content.

Shuch introduces the 'San Marino Scale,' a proposed analytical tool developed by Ivan Almar. This scale aims to quantify the transmission risk of Active SETI proposals on an integer scale from 1 to 10, based on specific, measurable characteristics. He urges SETI League members to familiarize themselves with this tool to conduct a quantifiable risk/benefit analysis for both promoting and opposing transmissions.

About SETI Range and Sensitivity

Lieven Philips contributes an article discussing the achievable range and sensitivity in SETI searches. He explains that while traditional SETI searches focus on detecting narrow-band Continuous Wave (CW) signals, the recovery of energy is key. However, for data communication, coherent tracking and carrier phase recovery are necessary. Philips highlights that increasing the range can be achieved by reducing the data rate (and thus bandwidth) at constant transmission power.

He elaborates on the detection of narrow-band CW signals using multi-million point Fast Fourier Transforms (FFTs). The accumulation of energy in each frequency bin over a limited time (e.g., 100 seconds) helps average out noise fluctuations. However, Philips proposes that by attempting to track the CW signal over extended durations, even signals deeply buried in noise could be detected. This 'coherent tracking' could extend the recovery range significantly beyond non-coherent energy detection methods. While direct phase synchronization is impossible for a buried CW signal, running numerous hypotheses on phase evolution, similar to Doppler compensation hypotheses used in SETI@home, could achieve this. This approach, though computationally intensive, could eventually become practical with advances in grid computing.

Philips illustrates this with examples using FFTs of varying lengths (512, 4096, and 8192 points) to detect a 1500 Hz signal buried in noise. The figures demonstrate that longer FFTs (longer time windows) result in a higher signal-to-noise ratio (S/N), enabling the detection of weaker signals. He clarifies that this method does not contradict standard sensitivity definitions or Shannon's theorem, as it focuses on detecting the presence of a signal rather than its content, and the S/N can be arbitrarily low for a narrow bandwidth signal at the expense of computation time.

This strategy, he suggests, could be applied to search globular clusters or galaxies, compensating for the vast computation with the number of stars scrutinized. He references the hypothesis that life might be rare, necessitating searches for 'very powerful, but very far away' beacons.

Event Horizon

This section lists upcoming conferences and meetings relevant to SETI. It includes events such as Philcon 2005, the SETI League Ham Radio QSO Party, SETICon06, various VHF conferences, World Science Fiction Conventions, and International Astronautical Congresses, spanning from late 2005 through proposed events in late 2008.

How Old is ET?

Ray Norris, from the Australian National Telescope Facility, explores the probable age of a civilization that might be detected in a SETI search. Norris identifies two key factors: the length of time since intelligent life first appeared in our Galaxy and the median lifetime of a civilization.

He assumes stars like our Sun have been forming for about 10 Gyr, with our Sun being 5 Gyr old. For the first 5 Gyr, there wasn't enough time for civilization to develop. Between 5 and 10 Gyr, the number of civilizations would increase linearly. Assuming civilizations die at the same rate they form, the number levels off. This suggests the median age of a detected civilization is likely around 1.7 Gyr more advanced than us.

Norris then considers cosmic hazards that could shorten a civilization's lifespan, focusing on quantifiable events: asteroids, supernovae, and gamma-ray-bursters (GRBs).

• Supernovae: An explosion within 50 light-years (ly) of Earth can cause catastrophic ionization, depleting the ozone layer for years and leading to high mortality. While supernovae occur about every 5 years in our galaxy, one within 50 ly is expected every 5 Myr, and closer ones every 200 Myr. The fact that Earth life has survived suggests these events haven't occurred nearby recently.
• Gamma-Ray Bursters (GRBs): These are even more energetic events, potentially releasing 10^45 J in seconds. A GRB at the Galactic center could cause a lethal blast of ionizing radiation followed by intense cosmic rays, potentially wiping out most life.

Norris notes that GRBs are expected roughly every 200 Myr, and such an event would likely cause extinction. The fact that Earth has survived these potential catastrophes leads to two possibilities:

1. The calculations are wrong: The timescales or severity of effects are overestimated.
2. We are lucky: The probability of survival is extremely low, implying we might be alone in the Galaxy.

If we are simply lucky, the median lifetime of ET is meaningless as we will never detect them. If the calculations are broadly correct, and we have survived, then ET civilizations are likely to be at least a million years, and more probably a billion years, older than us.

Justifying Janskys

Roy Norris explains the units used to specify the strength of electromagnetic waves received from non-terrestrial sources. He differentiates between units used for SETI signals and those used by radio astronomers for natural sources.

For SETI, which targets weak, narrow-band signals, signal strength is typically specified in Watts per square meter (W/m²). This is because the signal's energy is concentrated, allowing for efficient rejection of noise by narrow-band filters, thus improving detectability. The ratio of signal to noise is crucial.

Radio astronomers, however, often study 'continuum sources' that radiate over a very wide band of frequencies. To capture as much of this dispersed signal power as possible, they use wideband receivers. For these sources, signal strength is specified in Janskys (Jy), where 1 Jansky = 1 x 10⁻²⁶ Watts/meter²/Hertz. This unit accounts for both the signal strength and the receiver's bandwidth.

Norris explains that the actual signal power available to the receiver depends on the capture area of the antenna. He then provides a practical example of calculating the power level in dBm (decibels relative to 1 milliwatt) for a SETI station using a 10-foot dish antenna and a 25 Jansky continuum source. The calculation shows that the station can detect signals as weak as -150.4 dBm, demonstrating good performance.

He also touches upon the use of dBm for expressing very low power levels, noting that negative dBm values indicate power levels below 1 milliwatt.

SETI League Wins Twice in Bird Charity Auction

This short news item reports that The SETI League received a unique piece of electronics equipment—a Bird® Model 43 Thruline® Wattmeter (meter number 300,000)—and a substantial cash contribution from a charity auction sponsored by Bird® Electronic Corporation. The wattmeter will be used to monitor the operation of their Lunar Reflective Calibration Beacon.

Recurring Themes and Editorial Stance

The issue consistently emphasizes the technical and scientific aspects of SETI. There's a clear focus on the practical challenges of signal detection, range, and sensitivity, as well as the theoretical underpinnings of these endeavors. The debate on Active SETI is presented with a call for reasoned, quantifiable analysis rather than blanket policies. The exploration of cosmic hazards and their implications for civilization longevity highlights the long-term perspective of SETI research. The SETI League positions itself as a grassroots organization supporting privatized SETI efforts, providing resources and information to its members through publications like SearchLites and events like SETICon.