It was Tuesday, December 4, 2012 and I was having an ice-cream for lunch. The temperature was pretty warm in College Park, a few metro stops from Washington D.C. and I had not yet tasted the famous local ice-cream of the University of Maryland. It was a few days before I would leave the U.S. at the end of my postdoc stint and I wanted to soak in all I could of that place, the last where I would spend time doing research in theoretical physics.
My plane back to Rome was due in a couple of weeks and I had decided to spend that remaining time traveling with a friend. Before that I had one more trip waiting for me: later that afternoon I would visit a very special place, the past. My guide was Oscar Wallace Greenberg, Professor of Physics at the University of Maryland. “Wally”, as he wants to be called, had agreed to time-travel together to let me meet Einstein and other giants of physics who had shaped the last century outside their domain of expertise.
My Virgil had already told me a few anecdotes over lunch once, when he had joined me and other scientists working on gravitational waves, while his fellow particle physicists had gone eating out and he found it more convenient to stay inside. He told us he had been friends with Joe Weber, the first who had tried to build a microphone to listen to gravitational waves, the sounds of the universe caused by massive astrophysical objects, when they hit on the fabric of space and time as mallets on a drum membrane. The two of them used to go on mountain hikes but, out of respect for a friend, Wally would never ask Weber about his controversial research results. I already knew that Weber’s findings had never been replicated by anyone else but listening to a more personal side of the story made me want to know more. This is one of the reasons why I wanted to talk further with Wally; soon after I realized that the man who used to take the stairs to our department on the fourth floor wasn’t just a walking trove of treasures from the history of physics: he had his own story, too and I wanted to know it.
I then asked him if he’d let me interview him and he agreed to do it on that extraordinarily warm day of December 2012. Since then I have gone back and forth to the idea of writing about this personal encounter that I cherish a lot. Because life has a funny sense of humor, I have just found out the iPhone screenshot above in this post, which brought me back to that day and the desire to tell you something about Professor Greenberg.
He, too, has been a pioneer of physics in the 20th century: he was the first to see that the subatomic world had to be a colorful place. In much the same way as green, red and blue conjure to give you white when superposed, Greenberg understood that something like it was at work for quarks making up neutrons and protons. Even though elementary particles are not colored in the sense we think, this analogy is common parlance in physics: protons and neutrons are colorless conglomerates of quarks because their constituents combine their green, red and blue color charges in such a way as to neutralize them. That this mechanism was at play in the invisible world of microscopic particles had dawned on Wally when he was visiting Princeton in the 60’s as an assistant professor, on leave from the University of Maryland. He had already been at Princeton during his formative years as a student, the same Princeton where Einstein worked once he fled Europe due to the persecution against the Jews.
On a Spring afternoon of the early 50’s, together with other students, Wally had the chance to meet Einstein and ask him some questions; surprised by the humbleness of the great scientist and filled with emotion for the encounter, he later wrote a poem to celebrate this life changing experience. It is called “Mercer Street”, from the name of the road where Einstein used to live, and synthesizes the feelings and the physics that shaped that afternoon:
A Spring afternoon
A line of nine walk though the town
A musty house, the shutters drawn
A sage lives within
His key turned the lock
For twenty years to unify
Electric field, magnetic field
Space-time matter, too
A calm beyond time
A humble man received his guests
To talk, to feel the breath of youth
To hand them the key
The day turned to dusk
The parting time. Advice was sought
For these young men who start the path
He lost long ago
He shrugged. Scratched his head,
Discomforted, at sea, he sent
Them out with “Who am I to say?”
Cool air cleared their heads
Unfortunately the experimental evidence necessary to do justice to Wally’s intuition and prediction took years to come; when it did, he received only a few citations but not the full credit he deserved. In spite of this, the words Wally spoke to me did not contain resentment or accusations: he was just happy about the journey through the physical world that his whole life has been … and still is! When I asked him if he was satisfied with what had been his personal exploration of science through physics he answered proudly:
“Yes but I’m still doing it! What I’m working on right now is muonic hydrogen: if you do very accurate spectroscopy of the hydrogen atom you can infer the charge radius of the proton…”
As Wally explained to me, the data on muonic hydrogen were very different from the theoretical expectations and he thought he could help clarify the situation because he had experience in a calculation tool that particularly suits the problem. If this sounds technical, it is; suffice it to say that I’ve recently found out that Wally is participating in an international collaboration to deepen the investigation of muonic hydrogen. This attachment to physics as a life mission is something I love to recognize because it resonates with me a lot; seeing it at play in an 80 year old man, to me, is as joyful as witnessing my niece coming to grips with stairs: the pleasure of the novelty and finding things out is never enough.
My conversation with Wally went on for an hour or so. He told me about that time he was waiting for a cab in the blistering cold with a soon-to-be Nobel Prize awardee and they shared a chewing gum to have some sugars in the blood. He told me about that time when, as a student, he fell asleep during a class given by another giant of physics because he was under medications. He told me about that time at lunch when a rising star in theoretical physics started throwing bread crumbs at the table to call for attention. If you want to know more about Wally, you can listen to an account in his very own voice here:
I will end by saying that going through my remembrances for writing this piece made the long wait very worthy. One day, if I live long enough to grow old, I’d like to be asked about the tale of my own journey through physics: that day I will start by telling of the time I sat with Wally.