Dear friends,
Many SETI enthusiasts have heard of the WOW! signal, which was detected in 1977 at the Ohio State University Radio Observatory. The signal lasted for 72 seconds and was observed in a 10 kHz bandwidth centered at a frequency of 1420.4556 MHz, about 50 kHz above the hydrogen spin-flip line at 1420.4057 MHz. It has never been observed again and there has been no convincing explanation to date. This week, a possible explanation for the signal was posted on the arXiv preprint server by Abel Méndez and collaborators: "We postulate that the Wow! signal was a superradiance event that produced a maser-like flare from a small cold HI cloud." Let's delve into this statement. HI is astronomical jargon for neutral hydrogen atoms. Small, cold hydrogen clouds are regions of the interstellar medium that contain on the order of 1–10 hydrogen atoms per cubic centimeter (cc), considerably less dense than the air we breathe (about 2e19 molecules per cc), but a million times denser than the average density of the universe. By small, we mean 1–10 times the average Earth-Sun distance. By cold, we mean less than 100 degrees above absolute zero or 100 kelvins (K). Temperature is a measure of the energy associated with motion, and even at 100 K, there is sufficient motion of the hydrogen atoms for the HI line to be substantially broadened by the Doppler effect (see our May 2, 2016 newsletter). The Doppler broadening is in fact on the order of 10 kHz, consistent with the bandwidth of the WOW! signal. What the authors propose is that a large fraction of the hydrogen atoms in the cloud may have been excited to a higher energy level, where the spins of the proton and electron are parallel, by a nearby astrophysical source (e.g., a magnetar). In this configuration, the atoms can in principle experience the phenomenon of superradiance and revert quickly to their low energy state, where the spins of the proton and electron are antiparallel, in a coherent manner, resulting in a transient, high-intensity pulse of light. This "maser-like flare" (in the authors' words) is distinct from spontaneous emission, where the atoms would revert to their low energy state independently of one another, with a low intensity and a mean timescale of 11 million years. It is also distinct from the stimulated emission phenomenon of lasers and masers, where the emission is normally continuous. To learn more about the fascinating quantum mechanical differences between superradiance and maser emission, consider reading the excellent 2016 paper by Rajabi and Houde.
|
|
|
|
|
|
Copy of the computer printout showing the Wow! signal. Each row represents 12 seconds. Each column represents a frequency channel. The intensity of the signal is encoded with a single character.
|
|
|
|
|
A hydrogen atom with proton and electron spins aligned (top) undergoes a flip of the electron spin, resulting in emission of a photon with a 21 cm wavelength (bottom). Image and caption from Wikipedia.
|
|
|
|
|
|
It is a pleasure to credit Nathan Myhrvold for encouraging me to look into superradiance. Nathan recognized that searching for sources of superradiance "might help funding agencies get over their bias against technosignature searches." It is a sad reality that the SETI community has been burdened by Freeman Dyson's (passing?) thought that "Every search for alien civilizations should be planned to give interesting results even when no aliens are discovered." This notion has been unfortunately repeated in several other SETI contexts and it appears to influence some reviewers of grant and observing proposals. However, Dyson's belief is misguided. While it may be nice to have collateral results from SETI programs, it would be foolish to adopt an inferior search strategy just to produce collateral results! When I relayed this sentiment to Nathan, he reminded me of the tale of the drunk searching for his keys under the lamppost and gave me a good laugh: "If there is any chance that the keys could be under the lamp, it's smart to look where you can see rather than where you can't. But if there is no chance then it is stupid. You're saying Freeman is the drunk!" Indeed, in perhaps less colorful language, I am saying that, despite all his brilliance, Freeman Dyson lost sight of the primary goal and was off the mark on SETI. Every search for alien civilizations should be planned to maximize the probability of a technosignature detection, whether the search strategy yields collateral results or not.
|
|
|
Last month, the International Astronomical Union held its triennial General Assembly in Cape Town, South Africa. I presented three talks remotely, including proposals to improve the definition of "planet" that are motivated in part by the results of an unsupervised clustering analysis.
|
|
|
|
Unsupervised clustering of solar system bodies yields two distinct clusters, with a 3-order-of-magnitude gap between the clusters. Our proposed planet definition honors the huge gap that nature created.
|
|
If you would like to know more about our planet definition proposals, please read our open-access paper or an explainer on the web. The American Astronomical Society also produced an audiovisual version of the paper.
Warm regards,
Jean-Luc Margot
|
|
|
|
|
