On Saturday morning, my PhD advisor and friend Professor Emil Wolf passed away at the age of 95.  He was a singularly gifted scientist as well as an extraordinarily kind and wise person.  It is fair to say that I would not have made it to through my own PhD, and onward to being a full professor, without his support, patience, and guidance.

I’m sure that much will be said about Emil’s life and legacy in the near future; I thought I would share some of my personal experiences from being advised by him, working with him, and being his friend.

I should begin with some words on his life and accomplishments, for those unfamiliar.  Unfortunately, there isn’t a good biography of him — he was rather humble, and resisted having anyone document his life — so some of the stories I share are my recollections of what he shared.  Emil was always happy to tell a story, and he had a near limitless supply of them.  As he got older, he forgot which stories he had shared, and with whom, which he was aware of.  But it was never a problem — it was always fun to hear them again.

Born in Czechoslovakia in 1922, Emil’s youth was transformed by World War II. Forced to flee ahead of the advancing Germans, he ended up in Paris, working for the Czech government in exile. At first they had him working as a bicycle courier, but Emil found the hectic traffic in Paris too difficult to maneuver; fortunately, the government officials liked him enough that they offered him an office job when he explained his predicament! When the Germans moved into France, Emil fled ahead of them,  taking a boat to England in 1940.  In an incredible twist of fate, he ended up on the same boat as his brother! They had lost contact when the conflict began.  Emil often marveled at the coincidence and wondered if they would have found each other again after the war without this happy meeting.

He earned his PhD in Mathematics from Bristol University in 1948.  In 1951, the famous quantum physicist Max Born was looking for someone to help him write a new English version of his famed 1933 optics book Optik.  Emil was recommended for the job, and he joined Born’s group to work on the project.  There was a perversely ironic reason for the new edition, outside of the obvious one of incorporating new discoveries.  Born, a Jewish scientist, had the rights to his book seized by the nazis when they came to power in 1933.  After the war, things did not get better: the Allies took intellectual property as a “spoil of war” and Born found that his book was now owned by an American company!  So a completely rewritten book was necessary.

Emil worked with Born in person from 1951 to 1954, but the book project would continue for a number of years, up until 1959, when the first edition of Principles of Optics was published.  The book became known over the years as “The Optics Bible,” as it contains incredible detail on almost all aspects of physical optics.  It contained one of the first descriptions of holography in a book, which made Dennis Gabor, the inventor of holography, very happy.

The book also contained the first detailed description of what is known as optical coherence theory, the merging of optics and statistics. His contributions to coherence theory are what Emil is most known for, and he is often referred to as the “Father of Coherence Theory.”  All light sources possess some degree of randomness, and the light that they emit possesses random fluctuations. Lasers possess relatively small fluctuations, while light bulbs have lots of fluctuations.  We do not see these fluctuations, because they happen much too fast; our eyes, and most detectors, only see the average properties of a light wave.  These average properties can be described using statistical methods.

Before the work of Emil Wolf, it could be said that most researchers viewed the fluctuations of light as uninteresting “noise.” In 1954, however, Emil discovered and published a simple pair of optics equations, now known as the Wolf equations, that show that the statistical properties of a light wave also propagate as a wave.  This was a demonstration that the fluctuations of light were not just noise to be accounted for, but a physical phenomenon worth studying in their own right.

These equations are the foundation of optical coherence theory, and of fundamental importance.  In 2012, the Optical Society of America honored Emil’s 90th birthday by giving out bookmarks with the Wolf equations at their annual Frontiers in Optics meeting.

The chapter on coherence theory was the last part of Principles of Optics to be written, and it was delaying publication; Max Born impatiently told Emil to leave it out of the book and send it off!  Emil did not, of course, and this was the right decision: in 1960, the first laser was built, and coherence theory turned out to be crucial to understanding the nature of the light emitted by the novel light sources.  In the end, Born was delighted that coherence was included in the book, and the chapter helped propel it into being the most important optics text for decades.  In 1999, the 7th (expanded) edition of Principles of Optics was published by Cambridge University Press.

Emil had an incredibly prolific career, writing about coherence theory and a variety of other topics in optics.  He published well over 300 papers — I honestly have lost track of how many — and the last one, “Creating von Laue patterns in crystal scattering with partially coherent sources,” was published in Physical Review A in 2016.

In the 1980s, already in his 60s, Emil made another fundamental discovery in coherence theory, the phenomenon of correlation-induced spectral changes; also now known as the “Wolf shift.” In short: Emil and colleagues demonstrated that the statistical properties of light (its coherence) can cause the spectrum of light (the distribution of colors) to change as the light propagates.  In the process, Emil developed a new way of working with the statistical properties of light — emphasizing the spectrum — that became the standard way for folks like myself to study coherence theory to this day.

It was around this time that I first crossed paths with Emil. I entered graduate school in 1994, and one of the first classes I had was Emil’s Mathematical Methods  in the physics department, which he had already been teaching for decades.  Emil was very much an old school teacher, and the classes, which began at 8 a.m., were very intense.  In fact, I have previously blogged about one wacky incident that my classmates and I got involved in thanks to the stress of taking Emil’s midterm exam!  He was a very kind teacher, though, and very supportive of students and approachable.

When I entered the University of Rochester, I was set on getting a degree in experimental particle physics. After passing my written preliminary exam, however, I was less convinced that I wanted to continue down that path, for a variety of reasons.  I happened to be having lunch one day at Taco Bell with my classmate Scott, who was working with Emil, and Scott said that Emil was still taking on students — maybe I should talk to him?

So I did.  The first thing that Emil said when I met him, and this is from memory: “Well, you should keep in mind in working with me that I’m getting old and I could die at any time.  However, my doctor says I’m in good health right now, so…”

And then Emil dumped a huge pile of preprints and reprints on my lap, and told me to look through them and see if the work seemed interesting to me.  It was interesting, but the thing that really convinced me that he would be good to work with — and not to worry about his health?  All of the papers he gave me to look at had been written within the past 5 years.

Emil was loved by everyone who worked with him; within a year of starting my PhD work with him, current and former students of his organized a 75 year birthday celebration and scientific workshop at the Optical Society Annual Meeting in Long Beach, California. My classmate Scott also got “1997 Year of the Wolf” t-shirts printed for the celebration, and a good time was had both at the workshop and during the dinner afterwards.

One thing that I always appreciated about being a student of Emil’s is that he treated all of us like family.  I really had the feeling that he was happy to spend time with us and worried about our well-being as people, and not just students.

A couple of examples are worth mentioning.  Around 1998, I believe, Emil was given an honorary degree at Laval University in Quebec City.  In a lovely act of generosity, he invited Scott and me to come to Quebec City, as well.  I think he simply liked us and wanted us to be there with him for his honor.  So that we could have the trip expense covered, we turned it into a working trip and met with colleagues at Laval to discuss optics.

At about the same time, I had broken up with my girlfriend.  I was devastated, and Emil ended up being aware of it.  One day, a few weeks before the trip, he came up to me and said, “I’m sorry your relationship ended — it happens to all of us sometime! But I expect you’ll be dating again soon, and I want you to bring along your new girlfriend on the trip with us!”

For the record, I didn’t end up getting a new girlfriend for the trip, but I really appreciated Emil’s sympathy and his ability to make me smile even in trying times.

Another situation, more serious, happened perhaps a year later.  I had been in an extended bout of depression, which had been affecting my work.  I had not yet recognized as clinical depression, but Emil had.  One day, he took me aside privately and told me that he could tell I was struggling and said that there were medications available to treat such problems, and that I might consider looking into it.  At the time, all I could think of was: my boss thinks I need to take drugs.  But he was right, and when I finally went on antidepressants a year later, his encouragement helped me make that decision, which changed my life immensely for the better.

Emil was a great mentor.  He really drummed into my head the importance of well-written papers and clear scientific writing.  All symbols in a paper needed to be defined, and everything needed to be described as clearly as possible.  When we were working on our third paper together, I decided to keep a stack of all the revisions we went through; I think the stack ended up being somewhere around 20 versions high, if not more!

Emil had a great sense of humor, and was not afraid to laugh at himself a bit.  He often said that he had been told he had great “ego strength,” and it showed in his ability to not take things personally.  Everyone who worked with Emil on research inevitably had a near-shouting argument with him about some scientific problem, but Emil would always emphasize, explicitly, that these arguments were all in good fun and that we were always friends, during and afterwards.

Emil was always very fond of spending time with the spouses and girlfriends of his students and colleagues as well, and he was quite a charmer. When he first met my then wife Beth back around 2008, we had a nice dinner at an Italian restaurant in Rochester.  With Emil. all of his students followed the very old school tradition of calling him “Doctor Wolf” or “Professor Wolf” until such time as he asked us to do otherwise; by the time of the dinner, I hadn’t earned that distinction yet.  Beth was chatting with him, calling him “Professor Wolf,” and suddenly he said, “Please, that’s much too formal: call me Emil.” Then he looked over at me and said, “Oh, it’s time that you should call me Emil, too.” It’s been a long-running joke between Beth and me that she earned his respect some 30 seconds before I did, and only after about 1/2 hour of knowing him!

Emil was devoted to his wife Marlies, however, and the two of them were inseparable.  She also treated us as part of the family, and we treated her the same: students in Emil’s Mathematical Methods class, or other classes, were often treated to cookies at the end of the semester from her.  Those of us in Emil’s research group would occasionally be honored with a meal at the Wolf home, where we would get one of Marlies’ wonderful home-cooked desserts. Emil and Marlies were obviously happy together and very much in love.

I defended my PhD in 2001, and Emil and Marlies both came to my graduation ceremony, joining my family; it was a moment to have my entire “extended family” together to celebrate my accomplishment.

Emil gave me a lovely gift on the occasion of my PhD: a collector’s edition reprint of a classic unpublished Einstein paper on special relativity.

I loved spending time with Emil.  So much so, that I agreed to help him work on the index of the expanded 7th edition of Principles of Optics before it came out in 1999.  The book had been completely retypeset, so all the entries had to be rechecked.  It was grueling and tedious work, but it gave me the opportunity to spend more time with Emil and Marlies at home.  I insisted that we take a photo to commemorate the completion of the index; neither of us looked our best, as we were exhausted, but it was a great feeling to have finished it.

My last paper with Emil came out in 2004, co-authored with Emil’s long-time collaborator and friend, and my good friend, Professor Taco Visser. I was certainly eager to do more research with Emil, but he was so in demand and beloved by colleagues that I stepped aside to let other people have a chance to work with him.  Our collaboration didn’t end there, however; I also volunteered to draw figures for Emil’s 2008 book Introduction to the Theory of Coherence and Polarization of Light.

Emil influenced and inspired my career in good and unexpected ways.  For years, he regularly gave a talk on “Scientists who created the world of optics,” a historical survey of the lives and accomplishments of the most famous optical scientists.  This inspired me to investigate the history of science myself, which led to this very blog and many of the popular science writings that I have done over the past decade.

After our last book collaboration, I visited Emil anytime I could find an excuse to be back in Rochester.  In 2016, I had the good fortune to be in town over the summer for his birthday, and was invited to the birthday festivities.  It was a beautiful day, and I’ll treasure that time I was able to spend with him there.

At this point, Emil was somewhat forgetful and using a wheelchair to get around.  The photo doesn’t do him justice because, although he was infirm, he was still in good spirits and giving us all a good-natured hard time.

At the OSA Frontiers in Optics meeting in late 2016, I had the chance to see Emil one more time.  OSA honored him with a celebration to unveil his official portrait that will hang in the Institute of Optics in Rochester.  They also had on display an ice sculpture of a wolf howling at the moon, based on an identical image that Emil had put on the back cover of Introduction to the Theory of Coherence and Polarization of Light, again showing his sense of humor.

Emil was a great scientist, a great mentor, and a great friend.  I will miss the times we spent arguing good-naturedly about physics and the enthusiasm that he could infect you with while working on a problem.  He was supportive and kind, and a role-model not only as a scientist but as a human being.

I will miss you, Emil.  I believe we all will.

(Note: many of these photos I only have available in physical copies, and my scanner is broken, so I took a photo of a photo to share. Will replace these with improved scans when I have the opportunity.)