Repeat Efficiency | by Brian Koberlein



21 June 2022

The location of the Wow Signal in the sky.
Bob King/Stellarium
The placement of the Wow Sign within the sky.

Within the decades-long seek for extraterrestrial intelligence, there has by no means been confirmed proof of an alien sign. There have, nevertheless, been a couple of tantalizing mysteries. Maybe the best of those is named the Wow Sign.

An aerial view of the Big Ear telescope. / NAAPO
An aerial view of the Large Ear telescope.

Noticed on 15 August 1977 by the Large Ear radio telescope at Ohio State College, the sign was a robust, steady, slim band radio sign lasting at the least 72 seconds. Our data of the sign is restricted given the design of Large Ear. Moderately than having the ability to observe radio indicators like most trendy radio telescopes, Large Ear was set to a selected elevation and relied on Earth’s rotation to scan throughout the sky. The explanation the Wow Sign lasts 72 seconds is as a result of that’s how lengthy it took the supply to brush throughout Large Ear’s statement vary.

Large Ear was additionally a passive telescope. Astronomers merely set it up, and it will run by itself, recording the energy of indicators because it goes. Due to this, the sign was solely found days after the occasion when recorded observations had been reviewed. By the point astronomers may return to look at the supply, the occasion was lengthy over.

A data fit to the Wow Signal observations.
Maksim Rossomakhin
An information match to the Wow Sign observations.

However regardless of having only one statement, the Wow Sign is taken into account the strongest candidate for an extraterrestrial sign. A number of pure origins have been proposed, however all of them are a bit missing. Essentially the most primary concept is that the sign was of terrestrial origin, maybe a airplane passing overhead, or a radio sign scattered off house particles. However a airplane wouldn’t be in vary for greater than 72 seconds, and there’s no file of such a flight. A scattered sign is feasible, however the energy of the sign could be uncommon, and the frequency of the Wow Sign is inside a spread the place transmissions are restricted.

A number of years in the past it was proposed that the sign may need been attributable to comets that had been close to the noticed space of the sky, however this has since been disproven. Whereas two comets had been near the supply location, they weren’t actually inside the noticed vary. And comets aren’t prone to emit such a robust narrowband sign.

A map of 21cm emissions in the Milky Way.
A map of 21cm emissions within the Milky Means.

One fascinating side of the sign is that its frequency was very near that of the so-called 21-centimeter line. It is a faint radio emission attributable to impartial hydrogen within the universe. As a result of hydrogen is the commonest aspect within the cosmos, any radio astronomers within the universe would make observations at that frequency. If you happen to needed to get the eye of alien astronomers, a robust sign close to that frequency could be a great way to do it.

Given the tantalizing nature of the Wow Sign, there have been a number of makes an attempt at repeat observations. A number of radio telescopes have been aimed on the supply over time, however to no luck. Each statement in that space since has turned up nothing. So what’s an astronomer to do? Properly, one approach to deal with the issue is to have a look at what your observations exclude. That’s the concept behind a current paper on the arxiv.

On this work, the authors argue that the supply might be some sort of stochastic repeater. Most repeating sources are periodic. Issues like variable stars or quick radio bursts can have a predictable variability. Astronomers have thought of this concept, and have made observations that rule out a supply with an everyday periodicity. A stochastic repeater is a bit totally different. Moderately than having a measurable interval, stochastic repeaters repeat considerably randomly. instance could be earthquakes. We all know the place they often occur, know they are going to occur once more, however predicting precisely when is nigh on unimaginable. Astrophysical processes could be stochastic in an identical method.

Applying Bayesian statistics to a likely outcome.
Making use of Bayesian statistics to a possible consequence.

On the face of it, this looks as if a foolish concept. We’ve by no means seen the Wow Sign repeat, and we’ve proved it could’t be repeating periodically, however possibly it’s been repeating non-randomly such that we’ve by no means noticed it. It sounds just like the authors are arguing that it should be a non-random repeater as a result of we’ve by no means noticed it repeat. However the concept isn’t as foolish because it sounds. The authors have a look at when an unobserved burst may need occurred, and apply Bayesian statistics to calculate when a future burst may happen.

Bayesian statistics is refined however highly effective. It’s extra than simply calculating the chances of a possible occasion. It appears to be like on the sample of occasions to foretell particular outcomes. It takes under consideration not simply how typically one thing has occurred, however how these occasions modified over time. So, figuring out of 1 burst occasion, and figuring out when different burst occasions haven’t occurred, the authors calculate the instances at which future occasions are most definitely. That is good to know since we are able to now particularly observe the areas in the course of the most definitely occasion intervals. If the Wow Sign was a stochastic repeater, then we’ll probably catch a brand new occasion. If we don’t see one other occasion, we are able to rule out stochastic repeaters as a possible trigger.

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