It's said that Nostradamus gained inspiration for his prophecies by staring into a copper bowl filled with wa ter.
Today scientists, like the modern prophets they are, seek to gain insight into the Universe---into the life and death of stars---by staring into huge underground tanks of ultrapure water. They watch for elusive blue flashes that signify the passage of neutrinos, subatomic particles that tell the tale of the violent death of stars: supernovas.
The story of supernovas and neutrinos reads almost like a great religious epic: it spans countless eons, but the moment of truth is counted in milliseconds; involves titanic forces struggling for dominance, yet the central character is humble and "weak;" and promises from violent death the gift of life.
This epic begins with the quiet, pastoral lives of
Almost all stars start out by burning hydrogen, the most abundant element in the Universe, to helium. Eventu ally the star consumes all the hydrogen fuel. If the star is large enough, the weight of the star will compress the helium ashes until they ignite and burn to carbon and oxygen, which in turn burn to silicon and neon, and eventually to iron.
(Incidentally, our Sun is too small to burn anything much beyond hydrogen as a major fuel source. This sounds boring but is fortunate. Massive stars live life in the cosmic fast lane and die after only a few tens of mil lions of years. Our little yellow G-class star has been around for five billion years and will live for another five billion. Small is beautiful!)
The evolution of the star goes no farther than iron, which simply won't "burn" in a thermonuclear reaction. So a core of iron accumulates, growing larger and larger.
The only thing supporting the iron core against the enormous and inexorable force of gravity is the pressure of the electrons in the matter; this electronic pressure is not inconsiderable, as it is the same that supports you and I here on the surface of the Earth. Still, grav ity slowly and surely wins out, as the iron core eventu ally becomes so massive---a little more massive than our entire sun!---that the electrons cannot balance the gravitational force.
And then the iron core collapses under its own weight.
As the core collapse proceeds, the tremendous pres sure squeezes many of the electrons into the atomic nuclei, turning protons into neutrons. It is the strong nuclear force, a thousand times stronger than the electromagnetic force, which repels the neutrons and halts the collapse. The collapse doesn't just screech to a halt, mind you. It "bounces."
The bounce redirects the energy of the infalling mate rial outward into a powerful shockwave. This shockwave moves outward through the star and in a few hours reaches the surface, blowing off the outer layers of the star. Most of the energy of the shockwave goes into the kinetic energy---the velocity---of the expelled stellar atmosphere, but enough of it heats the material to produce a brilliant burst of radiation. The optical light radiated shines brighter than a hundred thousand stars and is seen as a supernova, one of the most spectacular of astrophysical events.
This is almost the end of the saga of the supernova. What is the history of the remnants of the explosion? Left behind is a neutron star---the kind favored by sci ence-fiction writers---whose dense matter weighs battle ships to the spoonful. If enough material is left in the neutron star, or if later more matter falls onto the neutron star from a companion star, then even the strong nuclear force cannot withstand the accumulated might of gravity. Then the hand of Nature pulls the plug and the neutron star collapses down a cosmic drain to a black hole. Black holes are lots of fun, but we'll have to save their story for another day.
Perhaps of more direct relevance to us is the fate of the material expelled from the supernova. The explosion blows off the outer layers of the star---which contain the accumulated isotopes manufactured in the star's his tory, such as iron, silicon, carbon, and oxygen. Other elements, such as the fluorine in your toothpaste, are forged in the supernova explosion itself. These elements seed the cosmos, paving the way for planets, life, civilizations, and readers of MindSparks. Supernovas are not merely spectacular, they are necessary.
Consider that a typical galaxy shines with the light of hundreds of millions of stars. At its peak, a single supernova appears nearly as bright as an entire galaxy. What is truly amazing, however, is that this visible flash is less than a fraction of a percent of the total energy emitted in a supernova! Ninety-nine percent of the energy escapes in the form of neutrinos, the shyest of all subatomic particles.
Neutrinos are neutral particles that are nearly mass less, at least ten thousand times lighter than electrons, which in turn are two thousand times lighter than protons and neutrons, which compose the nuclei of atoms. Neutrinos nteract with other particles only through the "weak" nuclear force, which is more than a thousand times weaker than the electromagnetic force. For several decades neutrinos were only guessed at, a ghost necessary to bal ance the energy accounts in certain kinds of radioactive decay. A neutrino can easily traverse a light-year of lead without being deflected or absorbed.
It is exactly this property of "weakly interacting" that makes neutrinos key players in a supernova. As the collapse of the core proceeds, the matter becomes incredibly dense, for a moment even denser than a neutron star. The collapsing star desperately tries to radiate away the excess energy generated by the gravitational collapse. But it can't radiate electromagnetic radiation (light, radio, x-rays, gamma-rays) because the matter is so dense it immediately absorbs that radiation again. Only neutrinos, which interact the weakest with matter, have a chance of escaping the dense and hellish conditions at the heart of a supernova. And so nearly all of a supernova's energy is emitted as neutrinos.
The news quickly spread to the rest of the world. When it reached two groups of experimental physicists in the U.S. and Japan, they sat up, checked their experi ments, and soon announced that they had actually detected neutrinos from the supernova---a grand first.
The experiments deep underground at the Kamioka mine in the Japanese Alps (known as Kamiokande II, for Kamioka Nuclear Decay Experiment, version 2) and in a Morton- Thiokol salt mine in Ohio (IMB, for Irvine-Michigan- Brookhaven collaboration) had not been originally de signed to detect neutrinos. Instead they had been look ing for the end of the Universe in the form of proton de cay. Protons are an essential and apparently stable build ing block of matter. In some reasonable but as-yet unproven theories, however, protons are predicted to de cay.
(The average lifetime is very much longer than the age of the Universe, so you need a large collection of protons, say a tank of water, to see one or two premature decays a year.)
If a proton decayed, it would fall apart with a great deal of energy, emitting smaller particles at high veloci ties approaching the speed of light in vacuum. In fact, they would be so energetic that they would surpass the speed of light in water. (Light is slower in water and even air than in vacuum. Einstein's relativity only requires you go slower than the speed of light in vac uum.) At that speed, a particle builds up an electromag netic shockwave, not too dissimilar to a sonic boom, and emits a cone of blue light, known as Cerenkov radiation. Cerenkov radiation is a useful phenomenon commonly used by high energy physicists to detect their prey and to gauge its speed.
What happened at Kamiokande and IMB was a series of twelve and eight blue flashes. In each case, a neutrino-- or, to be exact, an antineutrino, the antiparticle of a neutrino but having the exact same aloofness---crashed into a proton, creating a neutron and a positron. The positron, which is an anti-electron by the way, shot through the water, emitting Cerenkov radiation: the blue flashes.
(It's amusing to note that the human body, being mostly water, can function as a Cerenkov detector. In fact, one can estimate that out of the population of the Earth, one would expect one to five thousand people to have had a neutrino "event" in their body from the 1987 supernova, and perhaps one or two to have had an event--- a blue flash---in their eye!)
Why are more detectors of supernova neutrinos being built? The supernova story is not over, of course. We only understand the broad sketches of the plot. There are two large questions that remain: we don't know ex actly how supernovas explode, and we don't know how often they occur.
"Wait a minute," says the attentive woman in the front row. "You told us already about the "bounce" of the core and all that." That's true. And that's more or less how astrophysicists believe the story happens. How ever, when researchers try to do a careful simulation of a supernova explosion on a computer, it doesn't explode. It's a dud. A failure. But we know the stars must ex plode and spew off the heavy elements they cooked in their cosmic cauldrons, because without that you wouldn't be reading this article.
It really isn't surprising that a careful calculation can still get the "wrong" result. Remember that only about one percent of the total energy in the supernova goes into the physical explosion and shockwave, the rest being emitted as neutrinos. This means one must be able to calculate the processes to one percent. Usually, how ever, an astrophysicist is gleeful if he or she can calcu late a quantity with only a ten percent error. Therefore it is a very hard calculation to do and there are many pitfalls.
The problem is that the shockwave from the bounce, which is supposed to eject the outer layers of the star in a brilliant explosion, stalls on the way out. As the shockwave travels outward from the point of "bounce" in the inner core, it must struggle against the outer layers of the iron core which are still falling inward. Most of the energy of the shockwave goes into "boiling" the iron nuclei into individual protons and neutrons. Too much of the energy, according of the calculations, is spent here, so that none remains to eject the stellar atmosphere. In the United States there are two main groups of re searchers trying to make a supernova "explode" on a com puter, and each has their own "solution" to the problem. The East Coast researchers, centered around the State University of New York at Stony Brook, assume that the dense nuclear matter formed in the gravitational collapse of the star's core is in fact more elastic than generally believed. The elasticity or stiffness of nuclear matter is a difficult question, and nuclear physicists are cur rently building RHIC---the Relativistic Heavy Ion Collider---at Brookhaven National Laboratory on Long Island, in part to explore this question. By colliding two heavy nuclei together, say two uranium nuclei, they hope to reproduce in miniature the conditions of squashed nuclear matter found in the heart of a supernova.
The West Coast group, led by James Wilson of the Lawrence Livermore National Laboratory, instead invokes our friend the neutrino. Suppose the shockwave stalls. During this time, and in fact for a period of about ten seconds following the initial collapse, a tremendous num ber of neutrinos are being emitted from the center of the supernova. It's true that most of the neutrinos do not interact with the matter as they pass through. But if enough of them do, and deposit just a fraction of the en ergy they carry, this could "jump-start" the shockwave enough to continue it all the way out to an explosion.
The most recent calculations, using the most detailed modelling of neutrino production and transport inside the supernova, appear to finally be successful in creating an explosion. In this game, however, one has to be careful this result is not just a numerical accident created by a fortuitous approximation. At any rate the question of how supernovas explode is not completely settled, and computer simulations can only take one so far. The score of neutrinos seen in 1987 could only confirm the gross characteristics of a supernova. What is needed now is a careful study of the neutrino flux from a supernova; such a study could provide clues as to the nature of the monstrous explosion.
To this end the detector at Kamioka has been upgraded and other experiments have been planned. Most of them, which I will survey in a moment, are actually designed primarily for purposes other than the detection of super nova neutrinos. The reason for this, of course, is no one knows when the next nearby supernova will blow.
The rate at which supernovas occur is still an open question. Most astronomers and astrophysicist estimate that between one and ten should occur in our galaxy every century. Those numbers are based on many different esti mates, from model calculations of star populations, to extrapolations from other galaxies.
A factor of ten uncertainty is not unusual in astro physics, but it is critical when it comes to supernova re search. Waiting ten years for a galactic supernova is not bad---the gold mine of information from the neutrinos is worth the wait. But a century? No one really knows.
In stark contrast, neutrino observatories are buried deep underground. Only neutrinos can penetrate deep in side the Earth; the shield of thousands of meters of rock eliminates the pernicious background of high-energy cos mic rays. Far from the surface, scientists wait pa tiently as their tanks quietly gather information from the far-flung reaches of the universe.
The first astrophysical neutrino detector was not a water Cerenkov counter, but instead of a class known as radiochemical. The so-called "chlorine" experiment was constructed in the Homestake gold mine in Lead, North Dakota, built to detect neutrinos from the nuclear reac tions in the Sun.
The Homestake experiment has been in existence for two decades. Kamiokande has been detecting neutrinos, supernova and solar, for the past four years. And now other neutrino observatories are being planned and built.
Many of these are Cerenkov detectors, like Kamiokande. The design is simple in essence: a huge volume of water in which high-speed positrons knocked out by neutrinos can emit Cerenkov radiation. Surrounding the water are thousands of photomultiplier tubes which can detect nearly imperceptible flashes of light. By reconstructing the path of the electron one can learn a great deal about the neutrino.
Several are to be located at Gran Sasso in Italy. There a long highway tunnel through the heart of a mountain pierces the Italian Alps; halfway through, a turnoff leads to a maze of tunnels for scientific experiments. One of the experiments at Gran Sasso, MACRO, is also designed to detect magnetic monopoles from outer space---sounds like a Larry Niven story, doesn't it?---which is predicted in some advanced but still untested theories of physics. Yet another detector will use tons of liquid argon.
A Canadian experiment, SNO (Sudbury Neutrino Observatory), will use thousands of tons of heavy water, the kind used in nuclear reactors. The deuterium, or heavy hydrogen, is sensitive to exotic "flavors" of neutrinos whose detection is critical to understanding the behavior of the Sun as well as of supernovas.
Finally, one of the most interesting and ambitious experiments is DUMAND, the Deep Underwater Muon and Neutrino Detector. Reasoning that one would like as large a volume of water as possible, this experiment will use deep water off Hawaii as the "target." Long strings of photomultiplier tubes will be anchored to the sea floor. By timing the light flashes, DUMAND will look for neutrinos coming from the other side of the Earth, essentially using the entire Earth as a shield from background radiation!
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