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Why Time Might Not Be an Illusion
Einstein’s relativity pushes physicists towards a picture of the universe as a block, in which the past, present, and future all exist on the same footing; but maybe that shift in thinking has gone too far.

The Complexity Conundrum
Resolving the black hole firewall paradox—by calculating what a real astronaut would compute at the black hole's edge.

Quantum Dream Time
Defining a ‘quantum clock’ and a 'quantum ruler' could help those attempting to unify physics—and solve the mystery of vanishing time.

Our Place in the Multiverse
Calculating the odds that intelligent observers arise in parallel universes—and working out what they might see.

Sounding the Drums to Listen for Gravity’s Effect on Quantum Phenomena
A bench-top experiment could test the notion that gravity breaks delicate quantum superpositions.

March 19, 2018

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Stephen Hawking (1942-2018)
By ZEEYA MERALI • Mar. 14, 2018 @ 14:32 GMT

Credit: NASA
Many of you have woken to the sad news of the passing of Stephen Hawking — a towering figure in theoretical physics and cosmology, who inspired so many within science and beyond, with his intellectual insights into the nature of the universe, his wit, and his zest for the life that doctors once told him he would never have.

It’s difficult to overstate Hawking’s influence on the physics community. Roger Penrose, writing in the Guardian, and Martin Rees, in Nature, have written extremely accessible descriptions of the impact of his work. In the 1960s, with Penrose, George Ellis, and others, Hawking’s calculations helped to elucidate the mathematics of how the big bang marks the beginning of not just space, but time itself, from an infinitely small, dense “singularity.” Such singularities represent the breakdown of our best theories of physics and are also thought to lie at the heart of black holes, shrouded by an event horizon. It was believed that these event horizons act as one-way membranes that allow nothing that passes, not even light, to escape a black hole’s clutches. But, in the 1970s, Hawking’s work examining black holes with both general relativity and quantum theory, led him to propose that black holes can, in fact, slowly radiate particles, through the process of what has become known as “Hawking radiation.” This, in turn, suggests that black holes will slowly evaporate away — raising profound puzzles over the fate of information about the objects that fell in to the black hole.

This black-hole information paradox has yet to be resolved to everyone’s satisfaction; in fact, in recent years it has only become more confounding. But, as is often the way in science, posing questions can sometimes be as fruitful as offering solutions — and researchers are using this paradox, and its offshoots, as a handle to try to understand which of the two cornerstones of physics, Einstein’s general theory of relativity or quantum theory, has to give. Many FQXi members are today absorbed in projects that have their roots in Hawking’s work.

But Hawking’s influence goes deeper. As the bestselling author of A Brief History of Time, he directly inspired so many of us to study physics and cosmology, in the first place. I’ve lost count of the number of FQXi members who cite reading the book as a defining moment in their lives. I myself read natural sciences at Cambridge University, where Hawking was based and held the Lucasian Chair of Mathematics for many years. My fellow students and I never ceased to be excited to hear Hawking, or bump into him on the streets (quite literally, in the case of a friend, whom I recall returning to college one afternoon in a flustered state because she had cycled into his wheelchair). And while there might have been a danger that his celebrity status would give him an aura of unapproachability, he undercut this with his humour and the twinkle in his eyes. Hawking laced his public talks with jokes, managing to display comic timing despite speaking through a voice synthesiser. I personally loved him playfully claiming that the Spice Girls were his favourite group, for instance (or possibly that was entirely sincere, I’m not sure).

Later, as a science journalist, I was honoured that Hawking agreed to be interviewed by me for New Scientist and Nature, both about his own work, and to give his thoughts on that of others, even though providing comments clearly took a physical toll. But, of course, in addition to his scientific accomplishments, he will be remembered and admired for his commitment to engage the public and his refusal to be defined by the motor-neurone disease that struck him at just 21 years of age. That legacy will remain for generations to come.

Our thoughts at FQXi are with Hawking’s family, friends, and colleagues.
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The Quantum Hourglass—How a Quantum Time-keeper Can Replicate Continuum Temporal Events
By MILE GU • Mar. 1, 2018 @ 20:17 GMT

This post has been co-written by Mile Gu and Thomas Elliott.

Artist's impression of a quantum hourglass
In 2006, Oxford University Press set out to determine the most commonly used nouns in the English language. Topping the list was ‘time,’ closely followed in 3rd and 5th by ‘year’ and ‘day’ respectively, both delineating periods of time. This highlights how deeply embedded the concept of time is within the human psyche.

This prevalence of time is perhaps not too surprising—observing and tracking time are integral to our daily lives. Our work days are carried out according to schedules, meetings are planned with set durations. We synchronise with others by fixing times to meet, and we plan each day with the knowledge that they contain a set number of hours. To prepare for what is going to happen in the future, time-keeping is essential. Indeed, one can argue that one of the foundations modern civilization is built upon is our capacity to track time and anticipate future events.

In this context, the hourglass is distinguished for its role as one of the first accessible means of tracking time to the accuracy of seconds in medieval society. Indeed, their function is so familiar that ‘the sands of time’ has become a popular idiom, referring to the visual metaphor that the passage of time appears to flow like falling sand, steadily and irreversibly progressing from past to future.

Zoom in close enough on an hourglass, and one will see the individual grains of sand. At this level, the flow is not smooth, but inherently granular. At any moment, a finite number of grains of sand will have fallen, incrementing temporal progress in discrete packets. Time itself however appears, at any observable level, to be continuous. The hourglass analogy thus extends only so far.

This limit illustrates a pertinent observation—objects with a finite configuration space can only mimic the passage of time to finite precision. This isn’t particularly surprising: a finite configuration space can support only a finite memory. Thus, irrespective of any other physical limitations, such a system can only store the current time to a finite precision.

Indeed, this limitation is all too familiar to those in scientific fields that involve digital modelling or simulation. Theoretical models almost always assume time operates on a continuum. Whether modelling neuronal spike trains or dynamics of quantum systems, time is generally represented by some parameter t that takes on real values. Digital simulations of the resulting systems must however inevitably approximate such dynamics by discretising time.

As an illustration, consider the simulation of a particularly simple delayed Poisson process. This consists of a single system that emits only a single type of output at probabilistic points in time. The probability of an emission occurring during each infinitesimal time-interval δ is constant, with one catch: no output is ever emitted within a fixed relaxation period τ after an emission. For a device to replicate these statistics correctly, it would need to record whether it is in such a relaxation period, and if so, precisely how much time has elapsed since the last emission. Let’s call this elapsed time t. Only by storing t can the simulator know precisely much longer it must wait (i.e., the value of τ – t) before it is okay to emit the next output.

However, the variable t is real, and can take on a continuum of values. The more decimal places we wish to store about it, the more memory is required. In the limit where we want to faithfully predict the next emission with accuracy up to an arbitrary level of precision, this memory becomes unbounded. In practice, when writing computer code to perform the simulation, we would approximate t by discretising time into granular packets. For example, we could take some sufficiently small Δ and call it a day, provided we are happy to track time only to the nearest kΔ, for some integer k. A typical program for simulating such a process would have a pseudocode along the lines of:

if t is less than τ,

increment time by setting t = t + Δ


emit an output and set t=0 with probability p Δ

end repeat from start

Each iteration of the code simulates one timestep Δ. As Δ goes to zero, the output of this simulator will become statistically identical to the original delayed Poissonian process. The cost though, is that the number of values t can take scales as 1/Δ, growing to infinity as Δ goes to zero.

Thus, the more accurately that we wish to simulate the process, the more memory we need to invest. There is an ever-present trade-off between temporal precision and memory, and perfect statistical replication requires the allocation of unlimited resources. An hourglass, with a finite amount of sand, can thus never achieve exact replication.

While this trade-off is of clearly of practical relevance, it is fascinating also from a more foundational perspective. Many scientists seriously consider the possibility that we live within a simulatable reality, wherein everything in nature can be thought of as information processing. If we assume the memory capacity of this underlying computer is finite, running a program like the pseudocode above, how would it simulate processes that operate in continuous time? One may be tempted then to conclude that either we live in a computer with unlimited resources, or that continuous time exists only as a theoretical idealisation. Perhaps our universe is itself like sand in an hourglass—zoom closely enough, and everything appears granular.

While this could be a valid possibility, is there perhaps a way to avert this conundrum?

What we have not yet considered here is the quantum nature of information. The key element is that every bit of data represents some physical system with two different configurations—one which we label |0>, the other |1>. Provided the system can be isolated sufficiently well from the environment, we can also steer it into quantum mechanical degrees of freedom that possess coherence: superposition states represented by α|0>+β|1>, simultaneously |0> and |1> with specific weights dictated by α and β. The difference now is that α and β are intrinsically continuous degrees of freedom. Thus, this quantum bit—a finite physical system—contains within it a continuous parameter. Could this be leveraged to encode the continuity of time?

In our latest article, published in npj Quantum Information (4, Article number: 18 (2018)), we show that this ingredient gives us exactly what we need. Instead of using a classical hourglass where each grain of sand has either fallen or is yet to fall, we employ a quantum time-keeper where the grains of sand are in a weighted superposition of both possibilities. By deforming the weights continuously with the passage of time, we are able to prove that the delayed Poisson process could be modelled with perfect precision using finite memory.

Pragmatically, this result could immediately lead to memory savings in continuous time simulations. Numerical evidence indicates that our results apply to much more general cases, where the waiting time distribution between successive emissions is arbitrary. Such general processes, known as renewal processes, are relevant in many diverse fields of study—from modelling the firing of neurons to arrival times of buses. Thus a means to simulate such systems with less memory could have direct practical use. Similar advantages can be found when considering other continuous variables, such as position, as was shown in a companion article recently published in New Journal of Physics (2017).

The foundational consequences, however, are perhaps more exciting. Let us again entertain the scenario where we live in a simulated reality. Would the architects of this reality prefer the use of classical or quantum information? Our work shows that if they are intent on constructing a universe where time flows smoothly, then quantum mechanics may be the only feasible method. Time, should it be continuous, could well necessitate an underlying quantum reality.

Mile Gu is a physicist at Nanyang Technological University and the Centre for Quantum Technologies, Singapore. Thomas J. Elliott is a physicist at Nanyang Technological University, Singapore. Their research was supported in part by FQXi.
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Joe Polchinski (1954-2018)
By ZEEYA MERALI • Feb. 6, 2018 @ 22:29 GMT

As many of you will have heard by now, Joe Polchinski sadly passed away on Feb 2, 2018, after a long battle with cancer. For FQXi, this marks not only the loss of a brilliant mind, but a dear friend.

There have been many tributes from Joe’s colleagues celebrating his life and work. You can learn more through those links about his pioneering work on string theory, in particular, his role in the discovery of D-Branes (with Jin Dai and Rob Leigh), objects to which the ends of open strings can become tethered. (Matt Strassler has a wonderful discussion about the significance of this in relation to a description of black holes in string theory, and to the discovery of fundamental dualities between descriptions of physics using strings, black holes and gravity to those using quantum fields and particles.) But Joe’s achievements go beyond that, including his work on the string landscape (with Raphael Bousso) and more recently, articulating the blackhole firewall paradox, as the “P” in the AMPS team (with Ahmed Almheiri, Donald Marolf and James Sully). He won numerous awards, including a share of the $3-million Breakthrough Prize in 2017.nI am sure that in the coming days, many FQXi members who worked closely with Joe Polchinski will add more tributes, here and elsewhere.

Speaking as a science writer, I am sure many journalists would join me in saying that Joe was exceptionally friendly and supportive when it came to explaining his own work, and that of his colleagues. And I wanted to add one anecdote based on my many interactions with him from that perspective. The last time that I met with Joe in person was while I was researching my book. We spoke about his work on D-Branes, and the series of related discoveries by others around that time that make up the “second superstring revolution.” He was truly animated, scribbling on the blackboard and talking through how the realisation came aboutin the mid-90s. I asked him how exciting it was, back then. Was he aware of his work’s significance? Proud of this immense accomplishment?

Joe stopped, chalk in hand, and thought for good while, reaching back to how he must have felt at the time. “I’ll tell you something funny, just as a personal note,” he said. One of his sons, he explained, played roller hockey, and Joe was asked to coach the kids’ team. His first season of coaching coincided with the D-Brane discovery. “It was weird…I was much more emotionally involved in the coaching than I was in the D branes.” And it was the roller hockey with his son, he noted, that was the real highlight of his year.

Our thoughts are with Joe’s family and friends.

Joe Polchinski's: "Memories of a Theoretical Physicist"
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Review of "Foundations of Quantum Mechanics: An Exploration of the Physical Meaning of Quantum Theory," by Travis Norsen
By IAN DURHAM • Jan. 10, 2018 @ 20:45 GMT

I remember being out to dinner once with some folks from the American Physical Society's Division of Quantum Information (at the time it was only a Topical Group) at which we were discussing some of the sessions that we sponsor at the annual March Meeting. In particular, I remember discussing the various foundations-related sessions which are often controversial and frequently include a wide range of viewpoints. At some point, in a rather exasperated response to something someone had said, Matt Leifer remarked "foundations is hard." I don’t remember the exact context or the person to which he was responding, but that statement – foundations is hard – has always stuck with me. It might be better to amend that statement to "foundations is hard to do well" because anyone can do foundations but not everyone can do it well, but hopefully you get the idea.

Part of the problem is that foundational questions are so deeply subtle that merely understanding them (let alone answering them) has eluded the grasp of some of the most famous physicists in history. Now consider attempting to explain these questions to an undergraduate student. It’s a task that Travis Norsen has taken up in his book Foundations of Quantum Mechanics: An Exploration of the Physical Meaning of Quantum Theory (Springer, 2017, $44.99). Norsen is a lecturer in physics at Smith College in Northampton, Massachusetts in the US and is a member of FQXi. He has published numerous papers on the de Broglie-Bohm pilot wave, Bell's theorem, and more. He is one of those physicists who can do foundations research well.

One of the traps of foundations work is an illusion of simplicity. While I am a firm believer in Occam's razor, it is nevertheless too easy to fall into the trap of oversimplifying when it comes to foundations. In that regard, I think Norsen has succeeded in that he has eschewed overly simple explanations in favor of more rigorous but complex ones. That's not to say that I always agree with his assessment or even his pedagogy, but simply that he has chosen to take the approach that foundations is hard to do well and students should, at the very least, understand that point.

That being said, I think Norsen may have slightly overestimated the average undergraduate physics major. While it is true that this book grew out of notes from an advanced undergraduate course that he teaches at Smith, it's hard to deny that Smith is known for having exceptional students with very strong backgrounds. In his description of Bell's formulation of locality, which is referred to repeatedly throughout the book and which is thus of crucial importance to the book's overarching aims, is perhaps a tad overly abstract. One of the subtle things I have only recently started to understand about how the human mind works, is that on average it can only abstract so far. Even some pure mathematicians, for example, find category theory too abstract. But the point at which we lose a lot of people is, oddly enough, in the symbols and their definitions. This was actually noticed nearly eighty years ago by Arthur Eddington who wrote,

"If in a public lecture I use the common abbreviation No. for a number, nobody protests; but if I abbreviate it as N, it will be reported that "at this point the lecturer deviated into higher mathematics"." (A.S. Eddington, The Philosophy of Physical Science, Cambridge University Press, Cambridge, 1939, p. 137.)

The point is not that we should ditch the symbols (obviously). The point is that subtle differences in notation and definition can have an oddly outsized effect on understanding. Here is where I think Norsen's expectations may have been a bit on the high side. While his notation describing the probabilities associated with spacelike-separated events isn't, by itself, necessarily confusing, the explanation of the symbols seemed a bit obtuse and convoluted, as did some of his diagrams. Far too often, I find that physicists trying to explain a complex idea to non-specialists end up sounding like Yoda (from Star Wars). As heretical as this may sound, I think Bell was one of the worst offenders in this regard even though his works are masterpieces.

At any rate, I think Norsen's brief review summary of quantum physics in the next chapter is excellent (though it might pay to define a few terms such as "ansantz" and "gauge" just in case students have not encountered them before). Likewise I found his conceptual description of the measurement problem to be quite good, but things start to get a bit muddy in the formal treatment. Several times he falls into the infamous trap that gets nearly every textbook author at some point when he says things like "it is very easy to see…" I can already see students cursing him under their breaths. While it may not be worth working out the details, as a rule statements of this ilk are best avoided.

Norsen is at his best when dissecting the historical record and summarizing the existing state of some of the challengers to the Copenhagen orthodoxy. In particular, he does an excellent job of setting the record straight about Einstein's actual concerns vis-à-vis the EPR problem. In the broadest of senses, he also does a good job getting across the differences between the various theories he discusses and how each of them deals with the measurement problem, locality problem, and ontology problem (though I would have liked to have seen a discussion of the PBR theorem and I thought his discussion of Schrödinger's Cat missed the crucial difference between superposition states and mixed states). But he struggles a bit to make the more technical discussions approachable.

For any reader who is an avid proponent of spontaneous collapse or many-worlds theories, a word of caution is warranted: Norsen is an unabashed proponent of the de Broglie-Bohm pilot wave theory and sometimes his conclusions seem tailor-made for it. In other words, while he certainly tries to be even-handed in his analysis, one gets the impression that his conclusions are designed to agree with a pilot wave theory. For example, in his chapter on Bell's theorem, he concludes that faster-than-light causal influences really do exist in Nature. While that is certainly one way to interpret the results of experimental tests of Bell's inequalities, it is certainly not the only way. In addition, he fails to mention that such influences (if that's what they really are) nevertheless cannot be used for superluminal signaling in the practical sense of the term.

Despite these concerns, I do think this is a book worth buying for anyone interested in the foundations of quantum mechanics. I also think it would make an excellent supplemental text for a course on the subject. My hesitancy in recommending it as the sole text for such a course is largely due to its clear bias toward the pilot wave theory. But it contains a lot of deep, meaty ideas ripe for classroom discussion. In addition, the chapters include "Projects" (more like lengthy homework problems) to stimulate further discussion.

In summary, while I do think it has its issues (what book doesn't?), I think Foundations of Quantum Mechanics is an excellent addition to the library of physicists and philosophers working on these problems, and makes a very good supplemental text for related advanced undergraduate courses.
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The Year Everything Changes
By WILLIAM OREM • Dec. 31, 2017 @ 18:38 GMT

Let’s take the lesser extraterrestrial threat first: The New York Times, in the last month of the truly strange 2017, ran a much-attention-getting article on “The Pentagon’s Mysterious UFO Program,” detailing how 22 million of those tax dollars you’re working right now to generate were given to “Advanced Aerospace Threat Identification”—which is to say, flying saucers.

Which is really to say, “Most of the money went to an aerospace research company run by a billionaire entrepreneur and longtime friend of [Senator Harry] Reid’s, Robert Bigelow,” who is “absolutely convinced” aliens exist, and have visited earth—settling one of the biggest foundational issues right there. And, indeed, resolving the Fermi Paradox in the bargain, although—to get sidetracked for a minute—I was reflecting recently on how the Paradox would still exist even in the demonstrable presence of an alien visitation to Earth, because everything we know about stellar evolution, planetary accretion, the evolution of life in habitable zones, the age of the galaxy, and so on, leads to the conclusion that there should not just be one confirmed case of aliens, but lots of them, obvious ones, in every direction. But I digress.

Anyway, UFOs have long been a staple of media outlets where science isn’t, shall we say, a high priority. (Here’s Megyn Kelly asking the foreign desk, in all earnestness, whether a UFO over the Dome of the Rock means Jesus is back.) Even when LGM get mention in credible media, it’s usually a popular interest piece starring the same cast of characters: someone interviewed the good-hearted but gullible Edgar Mitchell; there’s mention of Jimmy Carter having seen something in the sky he couldn’t personally identify at the time, and so on. It’s a bit like the way Francis Collins is always mentioned in articles on how science and religion don’t really conflict, because, you know, Francis Collins.

This article was different. The United States’ DOD itself has been running this Advanced Aerospace Threat Identification Program for years, and with “black money,” no less; the spending committee that backed it included Reid, Ted Stevens (R, AK) and Daniel Inouye (D, HI); and I suppose the Nimitz video making the year-end rounds may indeed show something important, though it actually looks pretty sketchy as evidence, the aerial equivalent of a Bigfoot sighting. A new generation of Chinese drone? Unidentified, as the article rightly notes, means just that. Despite public enthusiasm, the options are not alien spacecraft or YOU GOT ANY BETTER SUGGESTIONS? In any event, whatever we’re looking at here, I, and clearly a lot of other folks, were intrigued.

Then things get sketchy: “Under Mr. Bigelow’s direction, [Bigelow Aerospace] modified buildings in Las Vegas for the storage of metal alloys and other materials that Mr. Elizondo and program contractors said had been recovered from unidentified aerial phenomena. Researchers also studied people who said they had experienced physical effects from encounters with the objects and examined them for any physiological changes.” ( . . . )

“We’re sort of in the position of what would happen if you gave Leonardo da Vinci a garage-door opener,” said Harold E. Puthoff, an engineer who has conducted research on extrasensory perception for the C.I.A. and later worked as a contractor for the program. “First of all, he’d try to figure out what is this plastic stuff. He wouldn’t know anything about the electromagnetic signals involved or its function.”

Let me just interrupt the responsible reportage here to giggle. Come on, Bigelow Aerospace; no you don’t. I’ll bet 22 million tax-payer dollars right now that the number of tech objects built by aliens in those Las Vegas hangars is exactly zero.

Anyway, the weird eventually takes over altogether. The DOD program was cancelled in 2012 when “It was determined,” according to the Pentagon, “that there were other, higher priority issues that merited funding,” but Mr. Elizondo, a true believer, kept at it. Now he and a few others, including the guitarist from a band called Blink-182, are somehow involved in a “public benefit corporation” called To The Stars Academy of Arts and Sciences that has plans to make great profits for you, investor, and all your investor buddies off alien tech, just invest now!

I admit I can’t quite get my head around To The Stars, but they appear to be FQ(x)’s evil twin. “What if scientists were given resources to investigate the boundaries of traditional theory?” the pop star inquires in their home-page pitch. Hear, hear! I say: FQ(x) has been trying to fill this gap for years. What follows, though, is a puzzling melange of foundational science issues, pseudoscience, actual names evidently associated with the CIA and DOD, and ready-made History Channel filler. Honestly, it’s hard to understand quite *what* this is. The Pyramids at Giza are shown as voice-over describes “mysteries of the universe,” ancient astronauts come into it, and we are told the company’s ambitions include such things as “[pulling together] unified study from religious scholars” and funding “warp drive metrics.” ESP, telepathy—it’s all in there. (“Quantum theorists” get the usual nod, though for some reason they are listed separately from “physicists.”) In any event, with my mysterious powers of precognition, I will now predict the actual scientific output from this venture . . .

But, break my heart, New York Times! I read your article and thought: Is this it? We have talked since I was young, at the dawn of the “space age,” about the tantalizing possibility of visitors—is this what it feels like, the morning of that day when it actually happens? That day when everything changes, and the new year will be truly, profoundly new?

Not yet. Though we did get also get a taste, late in 2017, of just how such a day will likely feel. Someone on the news will say something a lot like this: “It’s definitely from outside our solar system, it’s not shaped in any way you’d expect, indeed it’s cigar-shaped, it has no comet tail, it’s big, it’s bigger than a skyscraper, it’s half a mile long, actually we don’t know what it is . . .”

At the beginning, it wasn’t at all outrageous to wonder whether ‘Oumuamua was something manufactured. (Its name means “forward scout” or “first messenger.”) A generation ship? The planet killer? Lots of people called it Rama, in honor of Arthur C. Clarke’s centennial, but I prefer the Hawaiian name, having just recently been to the beautiful and cold Haleakala Observatory myself (as a tourist). Everyone pointed telescopes, including FQ(x) familiar Avi Loeb, and SETI with the Allen Array: nope, not broadcasting, apparently covered by a foot and half of reddish crust and showing signs of millions of years of cosmic ray bombardment. It’s just (“just”) a truly weirdly shaped piece of planetary debris that happened to be brushing our neighborhood.

So as we ring in the New Year on earth, we wish ‘Oumuamua well on its long flight through the emptiness. That fuzzy video of something being chased by fighter jets left me intrigued, but finally unmoved. But, just for a second, seeing the first artist’s renditions of ‘Oumuamua, I felt myself in the position of a Wampanoag tribesman seeing some kind of impossible ocean-crossing vessel emerge on the horizon, and wondering whether this was it—the moment when everything was about to change.

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