Recent activity in sending signals to the stars has caught the attention of plasma physicist Jim Benford. The CEO of Microwave Sciences and chairman of the Sail Subcommittee for Breakthrough Starshot, Jim has more than a few doubts about the efficacy of these signals, and questions the rush to send them. Is the recent EISCAT signal detectable at interstellar distances? A look at the science of such signals follows, and thoughts on the caution with which we ought to proceed.
By James Benford
Yet another ‘Message’
Recently, advocates of METI (Messaging to Extraterrestrials) sent a ‘message’ consisting of prime numbers followed by 36 music pieces to Luyten’s Star. It was a collaboration of METI International, led by Doug Vakoch, with the Catalonia Institute of Space Studies. This star is 12.4 light years from Earth and has a potentially habitable exoplanet (GJ 273b).
This was sent from the EISCAT facilities near Tromsø, Norway, using a microwave antenna. The music pieces are 10 seconds long, therefore contain only 1500 bits, so are quite simple.
Can this deliberate transmission from Earth be detected at the distances of nearby stars? What is the reality of claims that the low power messages sent to date are ‘practically detectable’? Such qualitative statements are not useful in a quantitative science. We will see that the message is faint and very unlikely to be detected, even if aimed at nearer stars.
How detectable will the message actually be?
Image: EISCAT Tromsø site with the EISCAT3D test facility in the foreground. Credit: Craig Heinselman.
Can it be heard?
No. This group is in fact whispering at the stars.
I base this on what we know about Vakoch’s transmission: power 2MW, 32 meter dish, frequency 929-930.2MHz, bit rate 125 bits/s, encoded to 8 bits PCM and to a frequency of 6.4 kHz, repeating the 33 minute signal three times over three days.
Analysis of this EISCAT (European Incoherent Scatter Scientific Association) METI, accounting for differences in power, aperture and frequency, is that the power density at long range (EIRP, effective isotropic radiated power) is only 1/100th of Arecibo, and a 10th of that used by Alexander Zaitsev in his messages. Therefore, the several observations and conclusions that John Billingham and I made 3 years ago about the unobservable Zaitsev ‘messages’ are true in spades for this failed transmission as well (“Costs and Difficulties of large-scale METI, and the Need for International Debate on Potential Risks”, John Billingham and James Benford, JBIS 67, pg. 17, 2014).
The conclusion is: This will not be detectable as a message by radio telescopes such as we have on Earth. The energy might be detected with radio telescopes larger than any we have, integrating the signal, but there isn’t much integration time, and integration would destroy the content of Vakoch’s transmission. So it will not be recoverable as a message by ETI if their radio telescopes are comparable to or substantially greater than ours. (One can of course assume a Supercivilization only 12 light years away with vastly larger radio telescopes. But if our leakage were detectable by them, as the METI-ists claim, then their greater leakage radiation would surely be detectable by us. But we do not see it.)
Dave Messerschmitt, who is in METI International’s Advisory Council, but wasn’t consulted about this message, observes:
“This METI signal is a simple on-off keying scheme, which dates to the 1837 invention of the telegraph. It has the virtue of extreme simplicity and transparency. However, there are modulation and coding techniques known today that operate near the fundamental limits of data-rate vs energy, such as is described in my paper “Design for Minimum Energy for Interstellar Communications.
“For the same average power (and energy consumption) such signal designs can considerably increase the distance over which information can be reliably extracted. They have other benefits, such as easier discovery and less susceptibility to local sources of radio-frequency interference. However energy-efficient signals will be essentially unobservable by long-term spectrum analysis. Rather, transient (short-term energy) analysis is more effective for such signals. They also require a transmitter implementation capable of high peak-to-average power ratios.”
Note also that the following must all occur for ETI to detect this weak signal:
1) Their system must stare in the very small part of the sky where our sun is, i.e., they must be interested in our system. (To get high sensitivity, the antenna area must be large, so the targeting angle is very small.) This could be because they’ve detected our out-of-equilibrium atmosphere, thus possible life here. This has been true for billions of years.
Or they could have detected our leakage radiation. But the bandwidth of incoherent leakage sources, such as TV and radar, is too wide and the power too unfocused for such signals to escape the Solar System, let alone reach other stars, before it’s indistinguishable from noise.
2) They would have to guess the bit rate of the message. Processing the stored signal with successive assumed rates and seeing which gives the best signal could do this.
3) They would have to deduce that we’re using simple on-off keying instead of another of our many modulation methods, so must analyze the received signal against a list of such stratagems.
While all the above could occur, this is by no means certain.
A decent respect for the opinion of mankind…
They are Star Whisperers. They show no decent respect for the opinion of mankind, to quote a certain historical document, in speaking for Earth.
What we see here is yet another attempt by the METI-ists to announce ourselves to the stars with weak signals that have no serious possibility of being received and interpreted. After several such transmissions in the last decade, they continue to make false claims and send silly signals, paying no attention to the scientific fact that their messages cannot be heard. And they continue to advertise such matters as the following:
“I would say, on behalf of the Klingons, that I prefer to listen to some good music than to the empty whistle of SFO’s radar.” – Seth Shostak
Seth Shostak surely knows that the San Francisco Airport short-range radar, being weak in power with a very low gain antenna, cannot be heard beyond the moon. He certainly knows this if he does any quantitative calculation. They claim, quite falsely, that we have announced ourselves by leakage radiation or intentional transmissions in recent years. This is not true. Therefore these are simply claims to excite the public. This is not an intellectually defensible position.
I advise the METI-ists to restrain themselves from trying to signal ETI. They are not being given access to seriously high power facilities such as Arecibo because they have no rationale for sending messages. They have no claim to speak for Earth.
In 2014 John Billingham and I made several suggestions in our paper referenced above. The time has come to address the METI issue on an international scale by establishing international symposia on transmitting from Earth to ETI. I advocate a moratorium on METI until an international consensus has been reached about announcing ourselves to the stars.