Catherine Asaro's faster-than-light (FTL) space drive really works -- at least on paper.

The physicist and science fiction writer has based the "inversion engine" of her Skolian Empire space opera series on her own thought experiments about new forms of space travel.

Besides using it to get her characters from place to place, she first wrote about the mathematics of the FTL drive in an April 1996 paper in The American Journal of Physics

Research into space travel has been a focus for Asaro's work as a consulting physicist.

The workshop was part of a program established by NASA in 1996 that explores new propulsion possibilities based on the latest developments in physics.

The theories researched by the program are a long way from being turned into practical applications, but Asaro is optimistic about the possibilities suggested by the new approaches.

The main problem with the theories under discussion, she says, is the difficulty of making them work in the real world.

"There's a lot of theories that are mathematically sound, but they all have something unrealistic about their physics."

That's an apt description of her own "inversion drive." The drive -- which first appeared in her 1995 debut novel, Primary Inversion -- does an end run around the speed of light by using imaginary numbers.

Imaginary Numbers

Imaginary numbers are based on the premise that even though no "real" numbers can be squared -- multiplied by themselves -- to yield a negative number, negative numbers can have square roots. The imaginary number i, therefore, is the square root of minus one.

Imaginary numbers provide a framework for complex equations involving vectors, forces and accelerations. They're like the y-axis in an x-y graph -- ifyou assigned the compass points "North, "South", "East" and "West" to i, -i, 1 and -1, you could represent three steps north and two east as "3i + 2".

Asaro uses imaginary numbers to think of speed -- normally a scalar variable, subject only to increasing and decreasing -- as the x-axis of an x-y graph. By adding imaginary numbers -- the y-axis -- to her speed equations, she can eliminate the relativistic difficulties caused by approaching light speed.

Once you take these "imaginary speeds" into account, she said, the mathematics of going faster than light "behave quite nicely."

"Unfortunately," she added, "in the real world we have no physical quantity that corresponds to imaginary speed."

Despite this problem, her equations bear a strong resemblance to those used in absorptive dispersion and resonance scattering theory, both of which are more closely tied to real-world phenomena. She's still studying her equations to learn if they can yield predictions that can be tested by experimentation.

Mathematically valid FTL drives aren't the only unusual element Asaro brings to her space opera. The Skolian Empire series has a strong romantic component.

The first book, Primary Inversion, follows the adventures of combat pilot Sauscony Valdoria, or "Soz." Soz is heir to the throne of the Skolian Empire, whose scionic royal family is the primary line of defense against the torture-loving "Trader" empire.

When Sauscony has an accidental encounter with Trader heir Jaibriol Qox, however, she discovers a kindred spirit. After a number of plot complications, they flee their respective heritages and settle down on an obscure planet together.

Asaro returns to their story in the fourth volume of the series, The Radiant Seas, but tends to jump around the saga of the Skolian Empire from book to book. Each volume features seemingly unlikely romantic combinations, though, such as the Earthwoman and far-future Skolian pilot of 1987's Catch the Lightning.

"My core readers are science fiction fans," she said. However, the reviews and e-mail she's gotten suggests that the mix of romance and hard SF is "a nice blend."

Asaro's interest in space is more than theoretical or literary -- it's keenly personal. Her husband, John Kendall Canizzo, is a NASA astrophysicist, and Asaro herself applied several times to be a NASA astronaut.

While she's disappointed at some of the recent failures of NASA's probes, she thinks the agency's approach is "realistic and optimistic." In addition, she said, "there's a lot going on in commercial space development."

Thanks to her scientific work and her writing, she's had the opportunity to meet many of the people currently working in the aerospace industry. She's impressed by their energy and creativity.

"The right technology [for better space travel] is developing," she said. "You couldn't keep us on this planet if you wanted to."

Asaro's next Skolian Empire novel, The Ascendant Sun, will be published in hardcover this month. Her most recent paperbacks are The Radiant Seas and the near-future thriller The Veiled Web.