A Window of Opportunity in the Sky


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Prof. Eli Waxman. A boundary for neutrinos



One needs a good reason to go to the South Pole. The night there lasts six months, and the annual temperature averages -50°C. The Pole has long been a prized goal of Arctic explorers, but why would a scientist interested in outer space, like Prof. Eli Waxman of the Weizmann Institute, want to travel to this forbidding frosty desert? Surprisingly enough, Waxman and a handful of astrophysicists in other countries believe the South Pole may become the next frontier in space exploration. They consider turning the 3,000-meter-thick plateau of ice hugging the Pole into a giant trap for elusive cosmic particles called neutrinos that may help reveal the secrets of the universe.

From time immemorial people have sought to understand the universe by gazing at the sky - first with the naked eye, then through optic telescopes. Next came more advanced telescopes, which examined celestial bodies by capturing the electromagnetic radiation they emit - from radio and infrared waves to ultraviolet radiation, X-rays, and gamma radiation. More information about celestial objects, such as black holes and galactic nuclei, can be gleaned from cosmic radiation - the rays of elementary particles, including protons, neutrons, electrons, alpha particles, and neutrinos - emitted by celestial bodies. However, because most of these particles have an electric charge, their path toward the

Earth is affected by various magnetic and electric fields whose magnitude and location in the universe is unknown. This makes it very difficult to trace these particles back to their sources, which in turn significantly reduces the amount of information they can provide about the universe.

Neutrinos may offer a solution. Lacking an electrical charge and with only a tiny mass, neutrinos behave in an 'unsociable' manner: they hardly interact with other particles of matter and travel to Earth in a straight line. Therefore, by observing the universe as it emerges from the rays of neutrinos one can perhaps gain valuable insights into the location and physical properties of the celestial bodies emitting the neutrinos. Physicists believe that these sources, such as active nuclei of galaxies affected by a black hole, could become enormous 'physics laboratories.' But realizing this fantastic prospect won't come easy.

Tracking down a loner


Physicist Wolfgang Pauli was the first to propose the existence of a particle later to be called the neutrino, back in the 1930s. Pauli was examining the law of energy conservation, which seemed to be violated by certain radioactive processes. However, many years passed before neutrinos were discovered - once again, mainly because these particles interact so sparsely with their surroundings. In fact, they interact with matter only via the weak force, which governs various radioactive processes such as the splitting of the neutron. But so weak is this force that the neutrinos hardly leave any tracks. For example, a neutrino can travel through the Earth in a split second without slowing down. During this journey very few neutrinos will forge any traceable 'connections' with other particles.

To spot neutrinos and learn about the celestial bodies that emitted them, physicists build giant detectors, each containing thousands of tons of matter, with which one out of the trillions of neutrinos is likely to collide, leaving a tiny flash of detectable light.

In this way scientists, including the Weizmann Institute's Prof. Israel Dostrovsky, have succeeded in detecting neutrinos spewed out by the sun - an observation that confirmed the theory about the way in which stars produce the energy they release. Other experiments led to the observation of additional neutrinos originating in the relatively close supernova of the Magellanic Cloud - a satellite galaxy of our own galaxy, the Milky Way. With these successes in hand, the appetite for neutrino sightings grew. Astrophysicists started observing the universe via detectors that can spot high-energy neutrinos emitted by the most distant and energetic sources. Equipment based on the absorption of electromagnetic radiation is unsuitable for studying these sources, since it does not allow the passage of photons - which is why physicists believe that the neutrino telescope may offer the best opportunity for exploring the universe.

Yet how realistic is it to build such a system? To answer this question, it's necessary to know how many neutrinos reach the Earth from different sources during a given period of time. This is where Prof. Eli Waxman of the Weizmann Institute's Physics Faculty enters the picture. Together with Prof. John Bahcall of Princeton University, he performed calculations showing that there is an upper boundary to the neutrino flux. Existence of the so-called Waxman-Bahcall bound means that detecting high-energy neutrinos would require a transparent detector containing at least a trillion tons of liquid.

This transparent detector would have to be surrounded by an array of light detectors, which would register the tiny flashes of light produced by the collisions of cosmic neutrinos with particles on Earth.

Waxman is a member of an international committee of scientists examining the possibility of building such a giant detector in Antarctica, inside the entirely transparent ice cap covering the South Pole. Another possibility is to set up a detector for high-energy neutrinos on the floor of the Mediterranean Sea, whose waters at great depth are also transparent. In any event, the project's cost is estimated at some $100 million - a sum that, astronomical as it may sound, is far lower than the cost of building the advanced particle accelerators needed to study elementary particles.

What can be learned with the help of giant neutrino detectors? 'When a proton of cosmic radiation hits a photon, a neutrino particle is emitted,' explains Waxman. 'If we could detect these neutrinos, we would be able to chart the sites of these collisions and use them to trace cosmic radiation back to its source - today one of the greatest mysteries of astrophysics.

'Another mystery that may be solved with the help of high-energy neutrino astronomy is the source of gamma-ray bursts occasionally occurring in the universe. Waxman and Bahcall, together with Prof. Peter Meszaros of the University of Pennsylvania have shown in theoretical studies that these gamma-ray bursts may originate in streams of matter bursting out when a black hole sucks in the remains of matter from a star that initially gave rise to it during the collapse of its core. High-energy neutrinos are emitted during this process, and the ability to detect them may lead scientists to the sources of gamma-ray bursts.

Neutrino relativity

Giant neutrino detectors may also make it possible to examine several underlying principles of the general relativity theory, by comparing the speed of the energetic neutrinos with the speed of photons coming from the same source. In this manner it will be possible to identify, among other things, the slowing of particles passing through gravitational fields. A neutrino detector that allows scientists to study this phenomenon was built recently near the South Pole. Sunk deep in the Antarctic ice, the installation is about twice the height of the Eiffel Tower. But according to Waxman's calculations, astrophysical studies of high-energy neutrinos would require an installation a hundred times larger. Hopefully, says Waxman, the international committee studying the feasibility of such a detector will reach a decision to open this new window of opportunity for astrophysics.

Prof. Waxman's research is supported by the Minerva Stiftung Gesellschaft fuer die Forschung m.b.H.