Lasers blaze a new path for studying stars in lab

Wednesday, June 21st 2000, 12:00 am
By: News On 6

ROCHESTER, N.Y. – It may seem odd to find an astronomer working in a windowless laboratory. But that's where some of the best new views are coming from.

Instead of gazing at exploding stars, giant planets and interstellar blast waves, some scientists now replicate those phenomena in the lab. Using powerful lasers, astronomers can make the equivalent of a supernova in a bottle.

"For the first time," said astronomer Adam Frank, "we can bring the stars down to Earth."

It's a new field called laser astrophysics, where astronomers and laser physicists collaborate to study hydrogen gas in Jupiter, powerful bursts of gamma rays, and the peculiar edges of black holes. The field is exploding as new and powerful lasers come online, bringing the heavens to Earth.

"This is really just the right time for this kind of science," said Edison Liang, an astrophysicist at Rice University. He spoke this month in Rochester at a meeting of the American Astronomical Society.

The research should heat up even more within a decade, when a $2 billion laser facility comes into operation. The National Ignition Facility will replace the Nova laser at the Lawrence Livermore National Laboratory in California, where laser astrophysics was pioneered.

For the next few years, laser astrophysicists must instead use the huge Omega laser at the University of Rochester. Similar machines, like the giant X-ray generator at Sandia National Laboratories in New Mexico, can also create astronomy in a can.

A laser blast can make a minuscule version of astronomical environments – just millimeters across rather than light-years, said Dr. Frank. But the equations that describe a laser explosion also describe an atom bomb blast or a stellar explosion.

"In some sense, we're building scale models of nature," he said.

Like a movie special-effects wizard, lasers also create some bizarre physical conditions. Laser experiments can produce plasma jets speeding at high Mach numbers, where the speed of flow is much greater than the speed of sound. Lasers can also mimic how light moves ahead of a shock wave in space, changing the structure of interstellar material before the blast wave reaches it. Only a high-powered laser can properly simulate these motions, said Dr. Frank, of the University of Rochester.

Scientists are beginning to reproduce the entire environment around blast waves and exploding stars, said Rice's Dr. Liang.

"No longer do we make just static measurements," he said at the meeting. "Now we try to create the whole enchilada."

The whole enchilada means different things to different scientists. Dr. Frank, for example, uses the Omega laser to try to reproduce gases flowing out from young stars. Recent work has revealed how conical flows of gas, pointed at each other like two fire hoses meeting, can remain stable instead of spraying off in a chaotic mess. The discovery could help explain why stellar outflows look the way they do, Dr. Frank said.

At the University of Michigan, Paul Drake shoots lasers into layers of plastic and foam to replicate what happens when a star blows up. The plastic explodes, just as the star does, while the lower-density foam is pushed away, just as the gas surrounding the star is. The foam eventually slams into a metal endplate, just as the stellar gas finally strikes a rim of material that had been blown off by the star in its dying days.

Laser astrophysicists are also studying planets. At the meeting, Robert Cauble of the Livermore lab described new experiments aimed at understanding how hydrogen behaves when it's densely packed into giant planets like Jupiter.

Dr. Cauble and colleagues zapped some liquid deuterium – a heavy version of hydrogen – with the Nova laser. They found that deuterium gets compressed more by a shock wave than scientists had suspected. In fact, the deuterium gets so squeezed that it begins conducting electricity, as a metallic fluid will.

The work suggests that Jupiter's interior may behave differently than scientists had suspected, or that there might not be a clear-cut transition between its metallic core and its gaseous atmosphere, Dr. Cauble reported. His team published the findings last week in Physical Review Letters.

Meanwhile, other scientists are working on theories of what they might see with lasers even bigger than Omega. Dr. Liang, for instance, has calculated how certain laser blasts might resemble gamma-ray bursts, high-energy blasts that occur somewhere in the universe at the rate of roughly one a day.

By aiming a powerful laser at both sides of a thin gold target, Dr. Liang showed, the blast could create electrons and their antimatter counterpart, positrons. When the laser is turned off, the electrons and positrons would then zoom outward, creating a shell that expands at nearly the speed of light. Astronomers think a similar shell might fly away from exploding stars, creating a gamma-ray burst.

If scientists could make such a laser fireball, Dr. Liang said, "we can only speculate what kind of physics we may learn out of that."

Similarly, Pisin Chen of the Stanford Linear Accelerator Center has proposed using lasers to study black holes and their environs.

A black hole's event horizon marks the point beyond which nothing can escape its gravitational pull – nothing except a faint leakage of radiation, called Hawking radiation after its discoverer, British physicist Stephen Hawking. An electron traveling at high speeds – such as that in a laser – leaks a similar kind of radiation, called Unruh radiation, Dr. Chen said. He wants to set up a laser experiment to detect Unruh radiation, and then analyze it to learn about Hawking radiation and black holes.

The work is still theoretical: No one has ever seen Unruh radiation, and even if they could, it might not reveal much about Hawking radiation or the event horizon, Dr. Chen said. But he has proposed building such a laser experiment at Stanford.

In the meantime, most scientists are looking to the National Ignition Facility for the next stages of research. The first of its 192 laser beams are scheduled to begin firing in 2004. After that, laser astrophysicists hope to do experiments with those initial beams, while other fusion researchers have to wait until all the beams are working.

For now, though, astronomers and laser physicists are just trying to learn more about each other's work. Combining the two fields could open up new ways of understanding the cosmos, Dr. Frank said.

"We can do things that have never been possible before," he said. "We're just starting this game here."