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An Interlude with Dirac

A geodesist remembers Paul Dirac’s visit to the University of Hawaii Lunar Laser Ranging Observatory

William E. Carter

The road up Mount Haleakala winds through forests of mostly eucalyptus trees before climbing into zones of grasslands, cacti and, finally, nearly barren lava. As we ascended through the grasslands, above the Park Service headquarters, Dirac asked, “How long has your lunar laser ranging observatory been operational?”

“The observatory is not yet even complete,” I said. “We are waiting for the receive telescope, which is being built at the University of Colorado. It’s a fly’s eye design, composed of some 70 8-inch refractor telescopes bundled together, each with optics to route the light collected to a common focus on the photomultiplier tube.”

“Then how are you ranging to the Moon?” he asked.

“We have been working with a temporary receiving system, using a telescope at the nearby DARPA observatory.” The Maui scientific community was small, and our friends at the Avco Everett Laboratory operated an observatory to image and study satellites for the U.S. military’s Defense Advanced Research Projects Agency. When they learned that delivery of our receive telescope had been delayed, they offered us time on their telescope.

“How many quanta do you receive back per laser shot?”

“It varies quite a bit, depending on atmospheric conditions, and judging from the temporal clumping of the returns there are probably errors in pointing at the reflector packages, but typically we get between a twentieth and perhaps a tenth of a photoelectron per shot.”

After several seconds of silence: “Perhaps I did not state my question clearly. A fractional quantum of light is not permissible, as it is the smallest unit of light.”

Dirac was, of course, correct. I had used the jargon that our team members used among ourselves. I quickly clarified. “Sorry. It appears as though virtually all of the returns we record are single photo-electron events, and we get about one return for each ten to twenty shots that we fire.”

Another pause, then: “And you are able to detect such a faint signal against the background light from the Moon?”

“Yes. Well, we know the range to within a few meters, so we use a range gate of about ten nanoseconds, or fewer. Combined with a field stop of about five seconds of arc, and a spectral filter of about two Angstroms, we are able to cut out most of the background light. We also cool the photomultiplier tube to reduce its dark noise.”

Once more a few seconds’ delay, then: “Quite remarkable!”

As we neared the summit of Mount Haleakala, Dirac asked where the crater was and whether we would be able to see it. Within a few minutes we reached a turnoff that led to an overview of the crater. We all climbed out of the car, and Mrs. Dirac insisted that “Father” put on his raincoat, as the air was chilly and a strong wind was blowing. When we got to the overlook Dirac leaned far out over the railing to see the wall of the crater immediately below. Mrs. Dirac, a.k.a. “Mother,” asked me to take hold of him to make sure that he did not fall. So there we were: Margit Dirac and Marilyn standing well back from the edge of the overlook platform, and me holding the Nobel laureate by the scruff of the neck (actually the collar of his raincoat) to make sure he did not fall into Haleakala crater.

The tour of the LLR observatory went well—both Mother and Father Dirac seemed to enjoy it. Afterward we returned to the Silversword Inn and dropped them off for an hour of rest before supper, for which we had invited them to our house. I returned to the inn to retrieve them, and when we arrived, our three daughters were home from school and were helping Marilyn prepare the table for dinner. The Diracs and I took a brief walk to see the fields of flowers, mostly carnations, that were grown commercially along the slopes of Haleakala, in the moderate temperatures and rainfall typical of that elevation. At dinner, Dirac asked each of the girls about their school work, life on Maui and their favorite pastimes. Dinner ended a bit later than usual, and after helping clear the table, the girls went to their rooms to get ready for bed. Dirac and I sat in the matching overstuffed swivel rockers that Marilyn and I usually sat in, and our wives sat on the couch. The four of us chatted about life on Maui as we finished our iced teas.

Soon our youngest daughter, Pam, returned in her nightgown, smelling of bubble bath, with a book in her hand. She was nine years old at the time, but she could easily have been mistaken for closer to six or seven. She went directly to Dirac, placed the book on his knee, opened it, and began to point to and talk about one of the pictures. After a minute or so, Dirac asked if she would like to sit with him so they could read the book together. She squeezed into the chair beside him, and the two of them spent the next 20 minutes reading the book and looking at the pictures, from cover to cover. When they completed it Marilyn said that it was getting late and that Professor Dirac might be tired from his long day of travel, so it would probably be best if Pam said goodnight. But Dirac said that he was fine and asked if there might be time for just one more book. He seemed to be thoroughly enjoying himself, so Marilyn agreed and Pam ran for another book. The two spent another half hour or so reading together.

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