Theodor Hänsch: A Lifetime of Curiosity



Professor Dr. Theodor Hänsch celebrates his 80th birthday on 30. Oct, 2021. What does the life of a Nobel Prize Winner look like? Science journalist Hubert Filser sat down with Prof. Hänsch to talk about a lifetime of scientific curiosity, his encounters with Steve Jobs, and why his work is not yet finished.

A lively buzz arises as Angela Merkel enters the Max Planck Institute of Quantum Optics in Garching to talk about visions for the future. Quantum computers are the next great hope – the beginning of a new era of computing. In the middle of it all, Theodor Hänsch films the chancellor with a small hand-held camera equipped with a special stabilizer as she is led through a tour of the high-tech setups in the lab. "How often do you get the opportunity to meet Ms. Merkel in our institute?" says Hänsch a few weeks later from his office. The Nobel laureate was there as director of his active emeritus group. "She was the only one who saw the small camera. She looked at me briefly in irritation, then at the camera, then at me again, and laughed." For a Nobel laureate, this can be permitted. Hänsch laughs at this story; one of his typical laughs. His eyes narrow, and you can see the small gap between his incisors. He looks young in that moment, as young as the soon-to-be 80-year-old researcher may still feel.

This laugh was probably already there when he began experimenting as a boy in his parents' house on Bunsenstraße in Heidelberg. His research career began curiously: "I wanted to know what you had to do to get a street named after you," Hänsch says. That's when his father brought him a Bunsen burner as an answer. It was the start of his chemistry experiments. Hänsch would sometimes get "interesting chemicals" at the pharmacy and hide them under the bed in his parents' bedroom. "They had no idea what I was up to at the age of twelve," Hänsch says. That is, until one concoction got a little out of hand. The ensuing explosion singed his eyebrows off, and he couldn’t hear very well for the next few days. "It's a good thing I hadn't already put the powder in a tube," Hänsch says, laughing again. It was doubly good, not only because he came out unscathed, but because the event spurred a consequential decision. He became a physicist instead.

Ⓒ Professor Theodor W. Hänsch (Private) Ⓒ Professor Theodor W. Hänsch (Private)
Stories about Nobel Prize winners cannot always be told so cheerfully. Theodor Hänsch’s story leaves one amazed at how eventful a life can be, how one can be driven towards a goal yet somehow also by chance, before ending up in the Olympus of researchers. Being a Nobel Prize winner in physics is extraordinary enough, but what about standing with Steve Jobs and Paul Wozniak, the founders of Apple, in their famous garage in Mountain View, and remarking on their first soldered-together computer: "It's no good yet, you need a memory extension"? Or visiting the Homebrew Computer Club almost weekly to see how Bill Gates presented his latest software and sold it on punched paper rolls? And somehow, managing to remain curious throughout his entire life?

And so, a few days before his 80th birthday, in his laboratory on Schellingstraße in the LMU physics building in Munich, stands a man who still likes to try out new things. "I'm still driven by my inventive spirit. I follow my instincts and do experiments without knowing exactly where they will lead," he says. It's a fairly large room full of high-tech equipment, with lasers, lenses, frequency combs, and high-precision measuring devices standing on large tables, while monitors display measured frequencies in hertz. Hänsch is working on a new high-precision microscopy technique for molecules, with a publication on the horizon. "I need a space like this to test an idea quickly," he says. "I'm mostly just concerned with proof-of-principle; fortunately, I have good collaborators who can pick up new ideas and elaborate on them." There are delicate setups that require a steady hand. "Look – thank God I can rely on these," he says, holding out his hands. No trembling. Only for very fine soldering work does he use a pair of glasses with eyepiece attachments on the lenses, like those used by dentists.

Theodor Hänsch had initially wanted to become a nuclear physicist, but he became drawn to a new light source that had been invented in the early 1960s and which remains with him today. "I was always interested in lasers," Hänsch says. "They produce this grainy-looking light, it's a magical pattern." But you couldn't buy good lasers in the beginning. So he started building his own helium-neon gas lasers. With these lasers, he investigated how spectral lines in a gas could be observed with high resolution despite Doppler broadening. However, the lasers were nearly impossible to tune, limiting experiments to laser lines or a few randomly resonant molecular lines. "I find it exciting to see problems for which there is no good solution," Hänsch says. It's a comment that perhaps goes some way toward explaining Theodor Hänsch's work.

He was never concerned with a grand vision. "I never planned for the long term," he says. And yet his life's journey seems like a sequence of clear decisions. After completing his doctorate in Heidelberg, Hänsch applied to Stanford University in Palo Alto to work with Arthur Schawlow, a laser expert who later won the Nobel Prize. "Schawlow was my hero," Hänsch says. In his LMU office there are two pictures of the physicist. In one, Schawlow holds a Mickey Mouse balloon in one hand and a pink toy gun in the other. "There was one of those ruby lasers built into it, and he used it to shoot the balloon," Hänsch says. His colleagues recently got him a pink plastic gun just like the one in the picture. "But unfortunately only with a green laser pointer in it," Hänsch says.

Hänsch's career in research has also always been about having fun and being playful. "Schawlow was a humorous man with great life experience," Hänsch says. "That's another reason I wanted to join him." Which is not to say that research is not also precise craftsmanship with constant experimenting, thinking, and improving. "Never measure anything other than frequencies," Schawlow advised him at the time.

The fact that Hänsch found himself in the exciting founding era of Silicon Valley is also one of these coincidences in the Nobel Prize winner’s life. The American years were turbulent, and not just politically. Hänsch met many later greats of the high-tech industry, Steve Jobs and Paul Wozniak, who were soldering away on the first Apple-1 at the time. The young Steve Jobs sat in on his lectures, and Hänsch, who himself spent his free time soldering computer boards and testing all kinds of innovations from Silicon Valley garage companies, also visited Jobs in the famous garage in Mountain View where today's global corporation has its roots. "I was just curious," Hänsch says. "I also wanted to have my own computer at that time." At one point, Jobs asked him for advice. Jobs had a meeting scheduled with investors, but still had hippie-length hair. So he asked Hänsch where he could get his hair cut.

Ⓒ Professor Theodor W. Hänsch (Private) Ⓒ Professor Theodor W. Hänsch (Private)

"I'm still driven by my inventive spirit. I follow my instincts and do experiments without knowing exactly where they will lead."

A visitor is lucky to hear such stories, which catapult you back into one of the most important phases in the history of technology, of which Hänsch is a first-hand witness. He almost participated in this boom himself, since he had founded a software company with Schawlow and even developed and successfully marketed a type of graphics software. "If I had said back then, we're doing a software graphics company, who knows, maybe we'd be something like Adobe now," Hänsch says.

Scientifically, Hänsch was also wildly successful while still at Stanford. He developed the first color-pure but broadband-tunable dye laser, with which he succeeded in resolving the red Balmer-alpha line of the hydrogen atom so well, by means of so-called saturation spectroscopy, that the famous Lamb shift could be seen directly in the optical spectrum. He was the first to formulate the principle of laser cooling, for which the Nobel Prize was later awarded to the American physicist Steve Chu, Bill Phillips, and Claude Cohen-Tannoudji. "It wasn't until the mid-1980s that laser technology had advanced to the point where the American physicist Steve Chu was able to put our original idea of 3-dimensional Doppler cooling into practice," Hänsch says. "I was busy moving to Munich at the time, so I didn't pursue it. It was a conscious decision at the time, and I didn't take it lightly."

On the one hand, California offered an exciting climate for new technologies, and on the other hand, the Max Planck Institute offered great opportunities for basic research. The decision against the U.S. and in favor of Germany ultimately led to the Nobel Prize in Physics in 2005. In California, Hänsch pursued the goal of measuring the lines of hydrogen as precisely as possible, using continuously improved methods of laser spectroscopy to determine universal constants, such as the Rydberg constant, and to critically test the predictions of quantum theory. Currently, the hydrogen laboratory at MPQ has achieved a measurement accuracy of 14 decimal places. Colleagues therefore jokingly christened him "Hydrogen Hänsch.” Precision became his trademark. His "Nobel Prize invention," the frequency comb generator, made it possible for the first time to precisely determine the quadrillions of oscillations that a light wave performs per second. In doing so, he had actually heeded his mentor Schawlow's advice to only ever measure frequencies. "Counting is ultimately a simple activity," Hänsch says. The frequency comb is a ruler for light. The potential of the device is still not exhausted. Twenty years ago, Hänsch and two of his former doctoral students founded his own company, Menlo Systems. He is currently working on new approaches in molecular spectroscopy.

When talking to Hänsch, it's hard to imagine that he could ever get angry, as patiently and openly as he answers all questions. But it did happen shortly after he was awarded the Nobel Prize. Not because of the hustle and bustle, which quickly became too much for him in the hours after the announcement, and during which he was glad to have to return to the airport for a long-planned appointment. In fact, despite his Nobel Prize, he was supposed to retire at 65, the year after the award ceremony. He threatened to go back to the United States. The matter triggered frantic activity, and the Bavarian Minister of Science came by personally to change his mind. He stayed. And apparently that was the right decision for everyone, because Hänsch is still one of the most cited researchers at LMU today. "I have published more highly-cited papers in the years since the Nobel Prize than before," he says.

You can tell how much the recognition of the Nobel Prize also pleased him, even if he didn't like how it also "made him visible all of a sudden," as he says. He can probably stand fuss about his work better than fuss about his person. This is one of the reasons why he is spending his 80th birthday in Florence, where he has also been an honorary citizen for fifteen years. Theodor Hänsch will certainly celebrate a bit there, with a view of the cathedral and the roofs of the old town. And who knows, maybe he'll do a little filming there, too.

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