Michal and Hod Lipson have synchronized their careers. And that's not a metaphor.

"We have to keep working to stay together," Hod says. "A dual career is unstable. The moment one person gets ahead they accelerate. The other one stays more and more behind, the gap opens and it's irrecoverable. You see that happening all the time."

From the time they met and married, Hod and Michal planned to stay together in everything they did. They timed their education and postdoctoral research to stay in step. They applied for faculty positions "synchronously," as engineers would put it. And they screened their offers to find the school that really wanted them both. That was Cornell, where in 2001 Hod became assistant professor of mechanical and aerospace engineering and Michal, assistant professor of electrical and computer engineering.

"Cornell is one of the few places where neither of us had to compromise," Michal recalls. "Other places were better for me and worse for Hod or the other way around. We wanted to be appreciated independently."

Synchronicity started long before they met. Both were born in the same neighborhood in Haifa, Israel; both were children of physicists, raised to value education and love science. Both had parents on the faculty of Technion, the prestigious Israel Institute of Technology. But when Michal was 8 years old, her family moved to Brazil.

Ten years later Hod, then 21 and about to graduate from Technion, planned a trip to South America. Michal, then 18, was visiting Israel. "I heard there was this woman from Brazil, and I thought she could tell me something about South America," Hod recalls. "Things followed. I forgot all about the trip."

After his last final, Hod followed Michal back to Brazil. Five months later they were married and back at Technion. Michal pursued a bachelor's degree in physics at Technion, while Hod completed his mandatory military service in the Israeli navy. Michal, technically Brazilian, was exempt, which allowed the couple to get in phase.

After the navy Hod returned to Technion. "I was a year behind him, doing a master's when he started his Ph.D.," Michal explains. "When he finished his Ph.D., he waited six months for me to finish."

Faculty positions are hard to come by in such a small country. The unwritten rule was that you first had to do a postdoc in the United States. Michal landed a job at the Massachusetts Institute of Technology (MIT), working with light-emitting silicon. Hod got a joint appointment at MIT and Brandeis University.

Thoughts of returning to Israel were quickly set aside. While Technion has some of the greatest scientific minds, the resources available in the United States were eye-opening. Michal recalls asking a professor: "What material should I use for this?" He opened the periodic table of the elements and told her anything was possible. "In other parts of the world they say, 'These are the tools you have, what questions can you answer?'" Hod says.

And Michal realized that they could do work that would have an impact. "We saw really quickly that people were changing the world," she notes.

Michal now works with photonic circuits, where beams of light flitting through tiny waveguides on a chip replace electrons in wires. She has created innovative devices to manipulate light beams in the ways transistors manipulate electric currents, such as switches that let one light beam turn another on or off, and developed techniques to connect photonic chips directly to optical fibers, which seem hairline thin to our eyes but are gigantic beside the waveguides on chips.

Hod works in "evolutionary robotics." In computer science, "evolution" is about telling a computer what you want and letting it try different approaches until one emerges as the most successful. He builds robots that can adapt to injury, make copies of themselves or even design new, improved versions of themselves.

The Lipsons have even joined forces to create "evolutionary photonics," in which a computer evolves a design for a photonic device. So far, Michal says, the evolutionary approach has come up with a couple of ideas that work better than anything she would have thought of without the computer's results.

And, Hod and Michal remain synchronized, both coming up for tenure review this summer. "I can count several times in our lives when one of us could have given up for the other," Michal admits. "Now we are very glad we didn't."

With two careers and now two sons, ages 9 and 2, they say there is no time for hobbies. "We put the kids to bed and go downstairs and work together. When the kids are off to Hebrew school or with a babysitter, we go off to the coffee shop; we both bring our laptops and work," Michal says.

But, she adds, "We are working, but we are holding hands. We are very lucky."