Freakish One-Offs in Astronomy

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Today’s video is sponsored by The Great Courses Plus, thank-you to ours sponsors. Discovery is the fuel of the scientific progress. But how do discoveries happen? In 2012, the world rejoiced in the announcement by CERN that the Large Hadron Collider had finally detected the Higgs Boson. It was a momentous day, the crowning achievement of modern particle physics. The Higgs boson had been predicted half a century beforehand, and indeed had represented the central goal of the $5 billion LHC. Decades of tedious planning, fundraising, building and analyzing. But, when the day finally came, there was also a tinge of disappointment, because this was exactly what we expected, as technically impressive as the detection was, it didn’t fundamentally alter our view of the universe. In a way, we learned very little that day. But discovery doesn’t always happen that way. Sometimes, they happen without such long term planning, or indeed any planning at all - just open minds being in the right place at the right time. One of the most famous examples happened in 1967, when graduate student Jocelyn Bell was studying the flickering of so-called radio stars with her advisor Anthony Hewish. To resolve this flickering, they had built an array of aerials sensitive to variations of a second or faster. And it was in within their data that Bell discovered unexpected - a series of narrow shaped pulses repeating once every 1.3 seconds. Bell has just discovered the first pulsar, for which her advisor, but not her, would pickup the Nobel prize seven years later. Pulsars are now understood to be rapidly spinning neutron stars aligned to our line of sight, like spinning lighthouse beacons scattered amongst the cosmos. But such stars are very rare, making up just 0.3% of the galactic population. And yet, despite their rarity, Bell’s serendipitous discovery ushered in monumental insights into how stars live and die. Pulsars teach us that sometimes rare, one-off events in science can have profound impact. A black swan event. You see black swans, just like pulsars, weren’t always widely known to exist, as we know today. In the 2nd century AD, the Roman poet Decimus Junius Juvenalis wrote that a perfect wife is “rara aris in terris nigroque simillima cycno" - a rare bird in the earth and most similar to a black swan. At that time, black swans were thought to not exist, and this phrase propagated accordingly into English language, becoming a common saying in Tudor London as an expression of something being impossible. It wasn’t until 1697 that Europeans first documented black swans when Dutch explorers encountered them in Western Australia. After this, the term black swan came to take on a new meaning, generally signifying a rare event with profound consequences. For me, what we might call astronomical black swans have always held a particular fascination - one that has culminated in a new research paper on this topic that I’ll discuss today. Research directly supported by many of you watching right now. You see there’s something captivating about these events, who knows what we might discover tomorrow if we just keep an open mind? It’s what makes science so exciting, in any given day, you could stumble across something that might change the world. As a recent example, the 2017 discovery of Oumuamua, an interstellar asteroid passing through the Solar System, shocked many at the time. Calculations predicted much lower rates of such objects, so much so that they generally weren’t expected to be found for another decade or so. As a result, Oumuamua has catalyzed new thinking into the formation and transport of asteroids between the stars. Dozens of papers have been triggered to explain its origins and space agencies are now taking seriously the idea of intercepting the next interstellar asteroid that comes our way. Perhaps no field embodies the quest for black swans more than SETI - the search for extraterrestrial intelligence. Here the idea is to monitor the sky continuously for years, even decades, with persistent, careful patience, on the hope that one day, in perhaps just one brief moment, an alien radio signal might be detected. Such a discovery would encapsulate a black swan event, an inherently rare occurrence yet one that would fundamentally transform our perception of the universe around us. But really, a one-off radio signal isn’t enough. Because it’s only fair and rational that the world would have a lot of skepticism about a signal that shows no signs of repeating. There’s an endless list of ways in which the signal could be spurious or faked. If we’re talking about something as profound as intelligent life - then one black swan isn’t enough, we need more. Indeed, that philosophy is self-evident in the astronomical literature. The first hot-Jupiter discovered, 51 Pegasi b, was met with considerable skepticism amongst the astronomical community at the time - few predicted Jupiter-sized planets could get so close to their stars. It wasn’t until the analogs rolled in that skepticism subdued and the exoplanet hypothesis became canonical. Similarly, after Jocelyn Bell’s discovery of the first pulsar signal, the origin was deeply unclear and appeared at first blush plausibly artificial. In fact, Bell and Hewish nicknamed it Little Green Man 1, a term which was used both playfully but also with a profound undertone. Here too, it took a second discovery, dubbed CP 1919 at the time, to establish that these were natural phenomena scattered across the sky. So as remarkable as the first black swan was, in both cases it was the follow-up detections that elevated these detections from mere curiosities to Nobel-prize winning discoveries. To illustrate this, we’re going to explore one of the most fascinating counter-examples, a case where no repeats were ever found and so, not surprisingly, no Nobel prizes were ever awarded. Again, we’re going to turn to SETI here because of its intimate relationship with black swan events. Let’s talk about the greatest Black Swan in the SETI literature, indeed perhaps the greatest Black Swan in all of science - the famous Wow Signal. Before we do, I want to thank the sponsor of today’s video, that’s The Great Courses Plus. It probably won’t surprise you to hear that I love learning. In fact, there’s a special thrill when diving into subjects that I didn’t take at college, history, art, philosophy, anthropology. The Great Courses Plus is a subscription on-demand video learning service with lectures and courses taught by experts from many of our greatest institutions, like top tier universities and The Smithsonian. With a subscription you get access to over 11,000 video lectures where you can learn just about anything, from the nature of time to the history of cooking. For example right now I’m taking the Masters of Greek Thought course by Dr Bartlett, exploring the ideas of Socrates, Plato and Aristotle and how they shape our modern world. As a special deal to our viewers, you can start a free trial using the link TheGreatCoursesPlus.com/CoolWorlds. So click on the link in the description to start your free trial today. Wow The so-called Wow signal is the kind of rare astronomy story that has gone from scientific curiosity to a part of widespread public discourse, a quick Google and you’ll find dozens of YouTube videos on the topic. One of the best ones is Event Horizon’s interview with the discoverer Dr Jerry Ehman that I’ll link to down below. Ehman was a SETI volunteer studying computer print outs from the Big Ear radio telescope in Ohio. In August 1977, one of the prints out he was looking at showed a highly significant signal, characterized by digits 6EQUJ5, against which Ehman scribbled Wow. Each digit here represents a signal to noise measurement taken every 12 seconds, with letters symbolizing numbers greater than 9. What makes the signal exciting is that it was narrow band, certainly less than 10kHz, suggestive of an artificial transmitter. Yet more, it occurred close to the hydrogen line, a frequency SETI astronomers expected other civilizations might use for communication. In total the signal lasted 72 seconds but that’s purely because the telescope was moving across the sky and any given star only stayed in view for that long. In reality the signal could have persisted for much longer. But critically the signal wasn’t detected in real time, it was found by Ehman several days after the observations. To date, there is no accepted natural explanation for the Wow signal, it’s origin continues to perplex us and it closely resembles the kind of signal we’d hypothesize by an alien intelligence. The Wow signal is perhaps the ultimate black swan, but one shrouded in doubt and skepticism since despite many efforts to re-observe it, no other detections have been recorded. So does the absence of any subsequent detections allow us to reject the idea that this is an alien transmission? METI president Douglas Vakoch concludes it has little credibility since any putative SETI signal must be replicated for confirmation. Afterall, this could simply be a classified spy satellite passing overhead at the time. But an absence of evidence is not evidence of absence - whilst we might not be able to confirm the Wow signal as alien, does the lack of repetition allow us to reject it? This is really where my new paper comes in, for really this is a question of probability theory - a topic we have been fleshing out into astrobiological domain over the last few years here. Put succinctly, how likely is that our subsequent observations might have just been unlucky and not caught a second Wow transmission? What this really comes down to is repeatability. At a very basic level, we have to assume that if the Wow signal does indeed originate from an artificial transmitter outside of the Solar System, then it repeats on some unknown timescale with some unknown pattern. If that’s not true, we have a kind of bizarre situation where an alien civilization transmitted a signal towards us once and once only in their entire lifetime and it just so happened that the Big Ear telescope swept across that precise patch of the sky at precisely the right moment. It’s a rather contrived scenario, and a far more reasonable starting point is that this some kind of transmitter that intermittently sends signals our way. Further, if this transmitter is some kind of hello beacon, then its transmission pattern and rate are most reasonably stable over time. For example we might imagine some kind pre-programmed sequence that simply repeats sending signals towards different stars. So that means that rate at which it was sending our signals our way in 1977 isn’t really any different than today. Using just these assumptions, we can now make mathematical progress because what we’re describing here is known as a uniform rate process in statistical parlance. Let’s say that you conduct a survey for a time t1 and then you get your black swan event. We’ve been thinking a lot about the Wow signal but really it could be anything, Oumuamua, Boyajian’s star, the first pulsar. And really we can substitute time t1 for a sample size n1, or effort level e1. We’ll stick to time in what follows, but the point is the maths is the same and this is a very general formalism for thinking about black swan events. Ok, so all we know is that it took a time t1 to get our black swan. Perhaps one of the first questions we might ask then is how long should we expect until we get a second signal, we can say occurs at some unknown time t2 To save time, I’m going to skip over the derivation, as well as other results in my paper like how to infer the repetition rate. I’ll link to the paper below for those who want the gory details. Instead, let’s just jump right into the result - how long until the Black Swan repeats, what is t2? It’s turns out t2 follows the highlighted probability distribution. So we have a steadily decreasing function as we go out to larger and larger t2 times. At first this might look confusing because it peaks at zero. But with continuous probability distributions like this, the y-axis is depicting a density, not an actual probability value. To get a probability out of this, you have to take the area under the curve between two values. So, for example the probability that t2 is some number between 0 and t1 is this shaded area, which equals 0.5, a 50% probability. Immediately this seems like great news, if you observed for 10 days and bagged a Black Swan, just observe for another 10 days and you have a 50% of getting another. Sounds like a good deal. Now naively, we might think that if double the observing time, so go out to 2*t1, we should expect the probability to double. So we’d have a 100% chance of detecting a second black swan. But looking at the curve, we can see that’s clearly wrong, in fact the probability only goes up to 2/3, 67%. OK, fine, let’s just be more aggressive and observe for, say, 20 times as long as we did the first time round, 20 t1. Once again though, the shape of the distribution defies intuition and the probability only goes as far as 95%. Now 95% might seem pretty good to the non-scientist, but really what it means is that there’s a 1-in-20 chance the Black Swan repeats but you just didn’t happened to miss it. And when we’re talking about a question like alien life in the universe, 1-in-20 confidence is hardly conclusive. What this function teaches us is that Black Swans demand patience - yes you might get lucky and bag a second one pretty quickly, but its also quite plausibly you have to wait much, much longer than how long it took you the first time around. So let’s come back to the Wow signal and see what our formula has to say here. If we add up how long was spent observing the region after the first detection, how likely is that we just missed a repetition? The Wow signal was initially detected in a 9-day run, where it visited each part of the sky just once. So we can say that t1 = 9 separate attempts. After that, the Big Ear observed it again somewhere between 50 to 100 times more with the same strategy, but no further detections. Speaking in 2016, Ehman said that considered these 50 failed attempts enough for him to broadly reject the alien hypothesis [“We should have seen it again when we looked for it 50 times. Something suggests it was an Earth-sourced signal that simply got reflected off a piece of space debris.”] But is Ehman right? If t1 = 9 attempts and t2 = 50 attempts, or 5.6 times t1, then the new formula reveals there there’s a 15% chance that Wow is repeating but Big Ear just missed it. I think Ehman’s conclusion might be a little a premature then, there is a plausible chance this signal is ongoing. Now you might say perhaps the repeat time is just really long, maybe many years, but that’s inherently unlikely given the time it took Big Ear to bag the first detection and is indeed built into this statistical calculation. But Big Ear isn’t the only telescope to look for the Wow signal in the years that followed with amateur astronomer Robert Gray spearheading several campaigns. The most comprehensive study was published just last year, in which Gray and colleagues used the far more sensitive Allan Telescope Array. In total, the team collected a staggering 100 hours of observing time monitoring the region and this represents the most comprehensive search to date. They published their results just last year and sadly found no repeating signals in that field. Now if we want to use these numbers, it’s a little bit tricky because Gray’s observing strategy was somewhat different from that of Big Ear. Let’s start by assuming that the Wow signal is a sporadic signal that lasts not much more than 72 seconds. Some support for this comes from the fact that the signal was only detected in one of Big Ear’s two horns, meaning the signal either abruptly started or ceased in the 3 minute time interval between the two horns observing the same part of the sky. In this case, we can say that Gray collected 100 hours of data, which is 280 times longer observed than the time it took Big Ear to obtain their initial detection. Plugging this into our formula, the probability that Gray could have missed a repeating Wow signal is pretty small, just 0.4%. But another way we can think about the problem, although somewhat contrived, is that the Wow signal lasts many hours, say a whole day. This is pretty unlikely because it means the 3 minute window the Big Ear looked at it was the exact end-point to this prolonged signal. But if this were true, it’s not so much the 100 hours of integrated observing time we care about but rather how many unique days did Gray observe for? From correspondence with Gray, he shared that they observed on 41 unique days, which when combined with the Big Ear follow-up means no repeats for 91 days. Here, the odds look much more favorable, giving a 9% chance that the Allan Telescope Array could have missed it. However, as I said, I think is a pretty contrived setup and I’d say the 0.4% calculation is the much more reasonable one here. So black swan theory doesn’t prove that Wow can’t be aliens, but it does put significant pressure on the idea that Wow is a repeating beacon. Perhaps then, it was, as Ehman suggested, just a terrestrial radio signal that somehow entered the Big Ear horns. But before we give up on alien signals, you might have heard that the privately funded Breakthrough Listen team recently found their own Wow-like signal from a completely different part of the sky. We still don’t have an official report or paper from the team about their signal, but after I contacted the team directly, they shared that the source, Proxima Centauri, had been observed for a total of 1550 cumulative minutes over a span of 1.75 years before their signal was found. Obviously follow-up is ongoing and its unclear how much has been obtained, but we can use our formula to estimate, how much data would it take to put pressure on the repeating beacon hypothesis, similar to as we did for Wow? The answer is that to reach 95% confidence, they’d need to collect 29,450 minutes, which corresponds to 20.5 days of round-the-clock monitoring. Remember that 95% is not conclusive, but it means that in just intensive observing season we could at least put pressure on the idea that this is a repeating beacon. Once again then, this highlights the enormous patience that Black Swans demand. For those wanting to learn more about this candidate, be sure to check out our previous video on the topic, as well a live stream by the Breakthrough team themselves where they cast their own doubts on the reality of the signal. Black Swans will surely continue to be discovered from time to time, both within SETI and astronomy more broadly. Louis Pasteur once famously wrote that “Chance favors the prepared mind” and in this work, we can lay down some of the statistical tools so that next time we’re ready. Black Swans remind us that often good things come to those who wait, that through perseverance and patience, extraordinary events can and will sometimes be found, and those one-offs can often be the most remarkable discoveries in science. All we have to do is listen, persevere, and keep an open mind, for in that approach, lies the possibility to unimaginable discoveries. So until next time, stay thoughtful and stay curious. Thanks so much for watching everybody. This research was supported by donors to the Cool Worlds Lab, this is a pretty wild experiment where our YouTube fans can actually financially support real research occurring here at Columbia University. In fact three of our Executive Producers are celebrating their anniversary as donors, that’s the awesome Laura Sanborn, Mark Sloan, and Tom Widdowson. Give them a big thumbs up in the comments and consider joining us using the link above.
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Channel: Cool Worlds
Views: 252,964
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Keywords: Astronomy, Astrophysics, Exoplanets, Cool Worlds, Kipping
Id: u9dQwsSc5NU
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Length: 24min 40sec (1480 seconds)
Published: Fri May 21 2021
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