Physicists duplicate starlike super-dense matter
Berkeley, Calif. — Super-dense matter - the stuff of neutron stars and black holes - has been created artificially on Earth. A neutron star is a dead sun that has been compressed to the size of an asteroid. It consists of matter so dense a matchbox-full weighs a billion tons. A black hole is the end state of a star that has collapsed even farther, until its gravitational field becomes so strong that nothing, not even light, can escape.
For more than a decade, astronomers have found the idea of these collapsed stellar cinders fascinating. They have located a number of unusual objects in the heavens that they feel must be one or the other.
For about the same length of time, physicists using the world's atom smashers have been trying to re-create the highly compressed nuclear material from which these objects must be formed. But only now has a team of scientists from GSI Laboratory in West Germany and Lawrence Berkeley Laboratory (LBL) in the United States reported direct evidence that this goal has been achieved. Their findings are published in the current issue of Physical Review Letters.
''This is a major breakthrough they have achieved. For the first time we can simulate in the lab what goes on in the heart of these stars,'' says Horst Stocker, a theoretical physicist at Michigan State University, who predicted these results. His work is published as a companion paper to the GSI-LBL findings.
The actual experiments on which these results are based took place a year ago at LBL's Bevalac accelerator, explains a Lawrence Berkeley scientist, Arthur Poskanzer. He, along with GSI physicists Hans Gutbrod and Hans-Georg Ritter, headed the scientific team that made the breakthrough.
Bevalac is unique because it has been modified to accelerate the nucleus of heavy atoms. Most particle accelerators are limited to light-weight nuclei. It was this limitation, explains Dr. Stocker, that frustrated previous efforts to create compressed nuclear matter: ''Very light nuclei are not suited for modeling neutron stars.''
The experiments that succeeded in creating this state of matter involve smashing together particles with tremendous force and recording the results. The experimenters choose a material and fabricate it into a small foil target. Then they take more of the same stuff, vaporize it, and put it through strong electromagnetic fields to strip off its electrons. They spin the remaining nuclei around in the evacuated heart of the accelerator at a faster and faster clip. When the particles have reached the proper speed, typically 70 percent of the velocity of light, the scientists direct them into the waiting target.
In this case, the goal of the resulting subatomic mayhem is to create a few atoms' worth of extraordinarily dense nuclear matter for an inconceivably short period of time.
Over the years, scientists have tried this with a number of progressively heavier elements. But it wasn't until they reached niobium, an additive in high-strength steel alloys, that they found what they were looking for.
Normally, when particles are smashed together this violently, subatomic particles are spewed about randomly. With niobium, however, the researchers detected streams of particles shooting to the side. This effect is thought to be the result of the brief formation of matter as dense as that found in the core of a neutron star. Almost as soon as this is formed, it reexpands to its normal volume and in the process deflects particles to the side.
Currently, the GSI-LBL team is analyzing the results of more recent experiments with gold. More than twice as heavy as niobium, they feel these should yield even stronger evidence of this effect.
Already, this finding is stimulating a flurry of scientific activity around the world. At Brookhaven National Laboratory on New York's Long Island, an accelerator with greater power than Bevalac is being modified so it can also fling heavy nuclei. And one of the accelerators at the European Laboratory for Particle Physics in Geneva is gearing up for compressed-matter research.
''The problem we are working on now is how to get information about the properties of this material. It is very difficult, because it exists for such a very short time,'' Dr. Stocker explains.
Theoreticians speculate that compressed nuclear matter may exist in a number of forms. Ordinary matter takes on four basic states: solid, liquid, vapor, or plasma (an electrically charged gas). Super-dense matter, they theorize, may act in a similar fashion. But because this is a totally new realm of physics, no one really knows what to expect, Stocker acknowledges.
Understanding the behavior of matter in these extreme conditions will have a number of scientific benefits, these researchers believe.
For one thing, these conditions are thought to exist when a star explodes in a supernova. Besides being the source of black holes and neutron stars, these stellar explosions provide the material from which new stars are formed. Thus, additional knowledge about the process should add to the understanding of the evolution of stars and galaxies.
In addition, studying compressed matter could shed new light on the formation of the universe. According to the commonly accepted ''big bang'' theory, the universe originated some 10 to 15 billion years ago in a giant explosion.
For the first few seconds of its existence, all the matter in the universe is thought to have been in a super-dense state. So, determining the precise nature of this condition should allow a further refinement of current cosmological theories.