The Telescope in the Ice

by Mark Bowen

  The prize carries 750,000 Swiss francs, equivalent to about $770,000, and half the money is to be spent on research, “carried out preferably by young humanists and scientists.” He used it to set up a Balzan Fellowship in Madison.

  At the end of his brief speech, Francis mentioned the late John Bahcall’s two secrets about physics: that it does not proceed logically and that “we would do it even if we were not paid for it.”

  “The same is true even without awards,” he said in closing, “but I must admit that this one is very much appreciated.”

  * * *

  Indeed, in an essential sense, the accolades were beside the point. The collaboration was too caught up in the thrill of the chase to pay them much mind. Even before their epochal paper was accepted by Science, Nathan and Claudio had run their starting-track analysis on a third year of data and found nine more high-energy events. This pushed them over the five-sigma threshold, so that they could now claim “observation” of astrophysical neutrinos (and Francis got his “five sigma in five years”). About a year later, a student of Albrecht’s named Chris Weaver found “evidence,” at 3.7 sigma, for extraterrestrial muon neutrinos pouring in from the northern sky. This was not a starting-track analysis, it was a vanilla-flavored search for up-going muon tracks. Since it meshed in nearly all details with the starting-track analyses, it served as separate confirmation—a difficult thing to do in expensive, one-of-a-kind experiments like this one.

  When the excitement finally wore off, it dawned on Francis that their discovery was more momentous than they had realized. They had been so fixated on finding a point source—and disappointed that they hadn’t—that they had not stopped to consider the implications of what they had found. Their measured flux of high-energy neutrinos was both “diffuse,” meaning there were no real hotspots in the sky, and “isotropic,” meaning the neutrinos seemed to arrive more or less evenly from all directions. Thus the majority were not only extraterrestrial, they were extragalactic: they originated outside the Milky Way. Had they been born inside our disk-shaped galaxy there would have been a hotspot in the direction of the galactic center, perhaps, or along the line of the galactic plane. On further consideration, Francis realized that the flux was “exceptionally high by astronomical standards” (even though twenty-eight in two years doesn’t seem like a lot) and that it dovetailed well with the diffuse flux of high-energy gamma rays measured by the Fermi satellite telescope—and to a lesser extent that of ultrahigh-energy cosmic rays. Standard particle physics led him to surmise that all three of these cosmic messengers may originate from the same array of sources, be they starburst galaxies, galactic clusters, active galactic nuclei, gamma ray bursters, a combination of them all, or whatever else. In other words, the extragalactic universe appears to be awash in high-energy photons, neutrinos, protons, and other nuclei, generated by untold numbers of what Francis calls “mega-LHCs,” or Large Hadron Colliders.

  * * *

  It should come as no surprise that what he was actually doing when he came to these realizations was thinking ahead. He was planning for the next generation of the instrument, which they now call IceCube-GEN2. It wasn’t that new an idea, actually; he had presented it to their NSF managers during the exciting collaboration meeting that had culminated in taking their breakthrough discovery public. But there are few things better than writing for getting the gears of the mind in motion, so he had taken deeper stock of their findings as he and Markus Ahlers had been working together on a GEN2 white paper.

  This is the way it goes. GEN2 will be to IceCube as IceCube was to AMANDA—with one important difference: there won’t be much increase in cost. Thanks to the extraordinary clarity of deep Antarctic ice, they can expand the size of the instrument to ten cubic kilometers (a factor of ten) by adding roughly the same number of strings, eighty or so, but placing them much farther apart, something like 250 meters. This would result in a disk-shaped instrument with about the same “thickness” as IceCube, about one kilometer, but a footprint on the ice sheet of ten square kilometers. (Jim Haugen will have a lot more running around to do if he wants to plant another set of flags after the last GEN2 string is dropped in the Ice. He’ll also be a lot older. The present plan calls for GEN2 to be completed in 2031.)

  Owing to its shape, GEN2 will be able to resolve smaller celestial objects in the horizontal plane than IceCube does. And owing to its size, it will not only detect ten times the number of astrophysical neutrinos per year, its reach will extend to higher energies. It is more likely that Aya Ishihara will find the quarry she has been seeking for more than ten years now, cosmogenic or GZK neutrinos. And just as it took the step in size from AMANDA to IceCube to detect their first cascades, so will the next step increase their chances of detecting the double-bang signatures of high-energy tau neutrinos, in which one cascade is generated when the neutrino dies to produce a tau particle and a second bursts forth some distance away when the tau decays.

  The little particle continues to be a coy mistress, giving up its secrets slowly. As they contemplate the meaning of the extraterrestrial neutrinos that continue to trickle in, the collaboration has come to understand that the reticence of the particle, the very quality that allows it to bring news from distances and times that the photon cannot, compounds the difficulty of seeing a “neutrino star.” Owing to the particle’s extreme penetrating power, when you look at the neutrino sky, you see almost to the edge of the universe, which is equivalent to the beginning of time. Since all the sources from here to there are “visible,” they act as a diffuse background, not unlike the nearby diffuse background from neutrinos created in the atmosphere. Reminding us that IceCube’s resolving power is some fraction of a degree, Eli Waxman points out that “with this acceptance angle, the detector window is open to sources from more than 10,000 galaxies.” Thus, for a high-energy neutrino star to be “visible,” it will need to be relatively nearby and relatively bright. I don’t know about you, but in me this line of reasoning evokes an immense sense of space.

  Their latest search for a neutrino star employs seven years of IceCube data, reaching back to the days of their forty-stringed instrument, and they still haven’t found one. GEN2 would make this holy grail about ten times more attainable.

  * * *

  Perhaps the most fundamental implication of the birth of this new astronomy is that it employs a previously unemployed messenger to bring us news from cosmological distances and times. And this seems to be where astronomy as a whole is headed. The biggest breakthrough in the field thus far this young century has been the detection of the first gravitational wave, a disturbance in space-time first predicted by Albert Einstein a century ago, by the Laser Interferometer Gravitational-Wave Observatory, in September 2015. LIGO is in some sense the sibling of IceCube, since it went through its birthing pains at the about same time and was similarly midwifed by the National Science Foundation. Unnoticed in the worldwide and well-deserved acclaim for the LIGO discovery was the fact that IceCube released a joint study with LIGO and several other instruments simultaneous to LIGO’s announcement, revealing that the instrument at the South Pole had detected no neutrinos arriving in tandem with the gravitational wave.

  A friend of mine recently pointed out that these two instruments stretch the idea of what a telescope is altogether. “Wait!” he asks. “Where is the imaging plane? What does the picture ‘look’ like? How exactly does ‘looking’ work in this new sense?” It is exceedingly odd that the IceCube scientists never have and never will see their instrument, that there is no imaging plane as there is in a camera or standard telescope, and yet, thanks to the wonders of modern computing, their instrument is quite visual and sometimes even intuitive to use. LIGO is even weirder. It measures the stretching and contracting of two perpendicular laser beams, four kilometers long.

  LIGO’s gravitational wave seems to have been caused by just the sort of cosmic earthquake that should produce high-energy neutrinos in abundance: the distant merging of two relatively
large black holes. Several years ago, LIGO and IceCube signed an agreement that allowed them to share so-called alerts with each other. When one detects an event of interest, they will share it privately with the other, so that they can look over their data to find out if they saw something that might have been related. In fact, one of the central initiatives in IceCube since the completion of their instrument has been to make similar arrangements with a variety of instruments: gamma ray telescopes like Fermi, cosmic ray instruments like Auger, and so on. Astronomy is becoming a “multi-wavelength” or “multi-messenger” endeavor. Every wavelength of photon is being employed, from the radio band through the infrared and visible to the ultraviolet and on through X-rays to gamma rays. Now neutrinos and even gravitational waves have been enlisted. The latest thing is to broadcast alerts from these different messengers in real time, so that if IceCube detects what seems to be a gamma ray burst, for example, it can alert the Fermi satellite to point in that direction and see what it sees. What if LIGO were to detect a gravitational wave at the same time? The possibility of understanding the many violent creatures that roam the universe is greatly enhanced.

  In April 2016, NSF director France Córdova held a retreat with her senior managers aimed at developing a list of grand ideas to inform the course of the agency over the next several decades. Multi-messenger astrophysics was one of the five overarching initiatives they identified. The head of the foundation’s math and physical sciences directorate observed that they’ve been hoping to combine optical, particle, and gravitational observations for a long time, but that now they “have all the pieces.”

  * * *

  Meanwhile, the Fellowship of the Cube is having a grand time. There are plenty of games in their playground as it is. If GEN2 or a big multi-messenger initiative comes along it will only add to the fun. In 2015 alone they did multi-messenger studies with four different instruments. They carried out a joint search with the ANTARES neutrino telescope in the Mediterranean, which is still at the AMANDA stage but gaining momentum. They searched for magnetic monopoles and dark matter. Let us not forget that they’ve had an increasingly sophisticated supernova watch in play since the early days of AMANDA, and they’re still hooked up to SNEWS, the SuperNova Early Warning System.

  Very sadly, Per Olof Hulth succumbed to cancer in February 2015. “The model of a kind but tough Swedish gentleman,” in Buford Price’s words, the outpouring of affection from the rest of the collaboration demonstrated that Peo was widely admired and loved. He had led the first European group to join AMANDA. He had husbanded the project through the bubbles. His student, Adam Bouchta, had detected the first up-going neutrino candidates with AMANDA-B4. He had helped launch IceCube as its first spokesperson, “with the same style and flair characteristic of his science,” as Francis put it. And his vision for DeepCore continues to bear fruit. It has proven capable of world-class dark matter and oscillation studies, and the idea of extending it to even lower energies has now been proposed.

  PINGU, the Precision IceCube Next Generation Upgrade, an even denser infill array within DeepCore, stands to be competitive with several multi-billion-dollar accelerator experiments in the study of neutrino oscillation, where the hottest topic today is the so-called mass hierarchy. The discovery of oscillation indicates that each neutrino flavor is actually a mixture of three neutrino “mass states,” and the hierarchy question has to do with the relative masses of these states. Not only could this line of inquiry lead to a theory of neutrino flavor, it might help answer Ettore Majorana’s age-old question about the neutrino and its antiparticle. It also has implications for theories of the early universe and how it evolved.

  This is pure particle physics, not astronomy, and IceCube made a splash in that field as well in 2015, by coming very close to ruling out the existence of a fourth neutrino flavor, the sterile variety. (Here, one of the smallest things one can imagine has implications for the largest, since it turns out that the existence or non-existence of sterile neutrinos has implications for the large-scale structure of the universe.) There was more than the usual internal strife around this finding, so they didn’t get the paper out until 2016. When they finally did, to Francis’s tongue-in-cheek irritation, “not seeing sterile neutrinos attracted as much attention as cosmic neutrinos.” There was even a prominent article about this non-discovery in the Wall Street Journal. Since they have another, more stringent sterile neutrino study coming fresh on the heels of this one, he can’t wait for the theorists to figure out ways to get around their first result, so IceCube can give them a “left-right” with the second … sort of like they did with gamma ray bursts.

  * * *

  At the age of seventy-two, Francis is as enthusiastic as ever. I haven’t seen him this excited since the days of the Peacock events (which he still considers the most exciting discovery they have made). He’s working with new students, he’s brimming with ideas, and he looks forward to leading the way on GEN2. Indeed, it’s hard to imagine who could take his place.

  In the traditional year-end e-mail he sent to the collaboration in December 2015, he made sure to reiterate that the prizes he had received actually recognized the collaboration as a whole and thanked them all for saving him from the daily life of a theoretician. After enumerating the many accomplishments of the year, he referred to IceCube as “the experiment that keeps on giving” and apologized: “I got carried away, but who wouldn’t.”

  Francis has a strong hunch that they’ll find a point source with IceCube. There are two very plausible candidates in the running right now. I think I’ve been hearing him say this for about fifteen years now, and he’s saying it with the same joy and lack of irony that he did the first time. He often points out that the people in his family tend to live into their nineties and that he doesn’t plan on retiring until he reaches a hundred. That gives him another twenty-eight years.

  * * *

  You can find out more about IceCube and this book at telescopeintheice.com.

  Acknowledgments

  In the nineteen years that it has taken me to write this book, nearly everyone associated with AMANDA and IceCube, both at the South Pole and in the “real world,” has supported me in large ways and small. This came in part from innate generosity and in part because they knew this story needed to be told. There have been scores of you. I cannot possibly name or even remember you all. But thanks.

  Francis Halzen has been my most constant and generous ally. It began with his welcoming response to a phone call I placed to his office in March 1998 and continues even now that the book is written. No writer could hope for more helpful or entertaining a source of information and insight. I count eighteen lengthy, transcribed interviews with Francis, and part of the reason they took so long was that we laughed so much. That doesn’t include meals together, phone calls, and innumerable e-mails. He has been remarkably open and large-minded. I know Francis well enough to know that he hasn’t told me everything, but he has never turned down a request for information or refused to answer a question, and I can think of only one instance in which he restricted anything he told me in any way (it was inconsequential). Usually, he has given more than I have asked. As Serap Tilav once said of him, “The big man has no worries. A big man has no fear.” The remarkable example he has set ethically and in rising above all pettiness is largely responsible for the success of IceCube and has been an inspiration to me, both personally and in the writing of this book. I shall miss having this excuse to talk to him. I hope we can keep it up in some way.

  Bob Morse has also been a tremendous help, giving numerous interviews, sharing valuable documents and photographs, embodying “Wisconsin-style physics” and the “glory days” at Pole, and telling fabulous stories. Not least, he arranged for me to work at Pole in 1999. Many thanks, Bob.

  The late Bruce Koci introduced me to this project and, I am proud to say, became a friend. This book benefits greatly from his generosity, his remarkable memory, his quiet insight, and his “disheveled wisdom.”
I would also like to thank his widow, Ann Guhman, for her gracious support.

  Gary Hill has been my go-to guy for all kinds of facts and information—about Pole, about the science, about the history. He was especially helpful in uncovering the story of the “first nus.” John Jacobsen provided brilliant insights from a unique vantage point—and marvelous photographs. Christian Spiering was a generous host in Zeuthen and Berlin many years ago and shared his first-hand knowledge of Baikal and the Russian side of things, as well as his comprehensive knowledge of the history of neutrino astronomy in general.

  What can I say about John Learned? Aside from the fact that there would not have been a story here without him, he always seemed to drop in—in Madison, Zeuthen, or wherever—at propitious times, and was always willing share the overall perspective and long view of the field that only the “pied piper of neutrino astronomy” could. John has even more stories than Bob Morse. They could not all be told here. Perhaps they deserve another book.

  Buford Price, Terry Millar, Jim Yeck, Albrecht Karle, and Jim Haugen have given extended interviews and gone the extra mile in other ways. “Queen” Kim Kreiger, Megan Madsen, and Catherine Vakhina have been the glue that has held things together for me on my many visits to Madison. Of special help in Antarctica were Stuart Klipper, Martin Lewis, Darryn Schneider, and John Wright.

  Special thanks to Tan-Quy Tran of the MIT Physics Reading Room for providing me with dozens of scientific articles over a period of about six years.