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When we change our point of view, we can often see things not previously apparent. Something similar holds true at the subatomic level: Getting our sights on particles we have not previously been able to “see” can expose us to a new and different “world view.” So, for instance, most of our observations of the Universe are based on the absorption of light waves in different lengths (some visible and some outside the visible range). But looking for tiny particles called neutrinos that come from outer space can provide unusual insights about the cosmos.
That has already been the case with neutrinos detected from relatively close sources: a supernova in a neighboring galaxy and our own Sun. In the first, the neutrinos emitted from the explosion confirmed that the star underwent a gravitational collapse, transforming it into a neutron star. And neutrinos measured from our Sun helped reveal the energy-production mechanisms at the Sun’s core, as well as enabling researchers to determine that neutrinos are not massless. The fact that neutrinos almost never interact with matter makes studying them a challenge, but it presents new possibilities as well: They are emitted from deep inside the core of such bodies as the Sun, where even photons can’t escape.
Theoretical studies have hinted that these two examples are just the tip of the iceberg: Neutrinos are thought to arrive at Earth from far across the Universe, bringing with them new insights that are not possible to attain with mere light-based observations. And that point of view could present us with a completely new sort of astronomy. For example, the Universe contains natural particle accelerators hundreds of millions of times more powerful than the biggest accelerator on Earth. We know these accelerators exist because we see their byproducts: When the accelerated particles hit the upper atmosphere with great force, they generate a shower of other particles that move down through the atmosphere, where they can be measured with earthly instruments. This finding has raised a host of questions: Where, exactly, are these accelerators located? What particles do they accelerate – protons or heavy nuclei, for instance? How do the particles attain their extreme velocities?
Prof. Eli Waxman of the Particle Physics and Astrophysics Department believes that these distant accelerators exist near young black holes approximately the mass of our Sun. These, he has proposed, are also the source of certain mysterious, high-energy gamma-ray bursts. Waxman and the late Prof. John Bahcall of the Institute for Advanced Study, Princeton, had developed a model that predicted how the particle acceleration process would produce energetic neutrinos.
Neutrinos are not the easiest particles to capture and measure: They have no charge, barely any mass, and they rarely interact with other matter. As they zip through the Universe, they pass though anything in their way – including Earth and its inhabitants – and keep going off toward their unknown destinations. And, if that did not make things difficult enough, Waxman and Bahcall, in another collaborative study, added an upper limit to possible neutrino numbers: One neutrino passing through a square kilometer of Earth per second. At that rate, with a billion-ton detector on Earth, one could record, over the course of a year, no more than a dozen or so energetic neutrinos arriving from the far reaches of the cosmos.
The detector that was eventually built, of course, was not a billion-ton device. Rather, its designers made use of a natural structure: the Antarctic ice cap. This detector is made up of many smaller detectors buried deep under the ice. Though this giant experiment, called IceCube, began recording before the setup was completed, it is the data from the last two years – around and after the end of its construction – that have yielded interesting results. A few dozen neutrinos were detected in that time – a number close to the upper limit set by Waxman and Bahcall. This suggests that these neutrinos are, indeed, coming from the far-off reaches of the Universe, and that the model is valid. Among the model’s assumptions that are supported by the experiment’s findings: The still unknown accelerators are equally distributed across the Universe; the particles accelerated are protons, in a process that generates neutrinos; the different types of neutrinos reaching the Earth are equally represented; and the pace of acceleration increases with distance. The last point implies that the more ancient the accelerator (i.e., the more distant from us), either the more particles it produces or the accelerators were closer together in the ancient Universe. In any case, it is clear that a large part of the accelerators’ energy goes into neutrino production.
Waxman expects that the data that will be collected from the Antarctic detector in the coming years will help clarify the nature of these neutrinos and may even help point to their still unknown source. “We hope the unique properties of these high-energy neutrinos, like those we detected from nearby sources, will enable us to draw valuable conclusions,” he says. “In particular, neutrinos produced close to the black holes – the most likely candidates for being the accelerators – in a region from which photons may not reach us, carry unique information on the workings of these objects.” Among other things, he says, the findings will provide new tools and data sets for developing the new astrophysics. These could, for instance, be used to test the theory of relativity with much more precise methods than those used today. It all depends, as they say, on one’s point of view.
Prof. Eli Waxman heads the Benoziyo Center for Astrophysics; his research is supported by the Friends of the Weizmann Institute in memory of Richard Kronstein. Prof. Waxman is the incumbent of the Max Planck Professorial Chair of Quantum Physics.
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