I graduated from Harvard with a degree in mathematics in 1951 and got my PhD in physics in 1955. I needed a job and a friend made a suggestion: on the Harvard campus there was a relatively modest cyclotron, simple enough for graduate students to operate. There was a position open for a ‘house theorist’. My friend recommended me and I got the job. My only formal duties were to try to answer questions put by the experimenters; otherwise I could do my own work. My main interest then was interpreting the recent findings at Stanford, where electrons were being scattered from protons, and from deuterons and other nuclei. I had written my thesis on the deuteron. But after two years my appointment was over and I had to look for a new job. I applied to the Institute for Advanced Study of Princeton and was accepted for autumn 1957. This left the summer.

The Livermore and Los Alamos weapons laboratories were actively recruiting. In the spring of 1954, while I was still working on my thesis, I’d had an unsuccessful interview in Washington with Edward Teller, who was recruiting for Livermore. He made an odd comment about preferring physics to politics. It was only much later that I realised my interview had taken place the day after he testified against Robert Oppenheimer. Another thing I didn’t know in 1957 was the very active role that many of the academics I knew had played in creating the bomb. At Harvard, Norman Ramsey had been involved in selecting the plane that delivered the bomb over Japan. He also signed off on the plutonium device that destroyed Nagasaki. Roy Glauber, an assistant professor when I first knew him, had been the youngest member of the technical staff at Los Alamos, recruited even before he got his bachelor’s degree. His roommate Ted Hall was the second youngest staff member. Hall was also one of the three known Russian spies at Los Alamos. Of course, the chemist James Conant, who was president of Harvard, had been one of the people who ran the whole enterprise. I was closer to Ken Bainbridge, who had selected the test site and was in charge of arming the device that was successfully tested at Alamogordo on 16 July 1945. Now he was chair of the Harvard physics department.

None of these people ever discussed their wartime activities so I was surprised when Bainbridge called me into his office and asked if I would like a summer job at Los Alamos. He said he would recommend me. Los Alamos for me had an almost mystic quality – and some extra money would come in handy when I got to Princeton. I spoke to a Los Alamos recruiter, who told me that I could have the job if I was able to get the relevant security clearance. In 1947 the Atomic Energy Commission introduced a Personnel Security Questionnaire to determine levels of clearance. The levels were ‘P’, then ‘S’, then ‘Q’. A person with Q clearance was entitled to information about nuclear weapons on a ‘need to know’ basis. I had to supply the FBI with a list of everywhere I’d lived for the last ten years. Much later I managed to get my file thanks to the Freedom of Information Act, but it was so heavily redacted that I didn’t learn anything. I’d been rather worried about a great-aunt. She was a subscriber to the Daily Worker and spoke in dark tones about the ‘bosses’. Either they overlooked her or decided she was harmless because I got my clearance.

I arrived at the guard station at Los Alamos and was fitted out with credentials. You needed a pass with your picture on it to enter any of the technical areas. The Los Alamos of 1957 was a much more closely guarded place than it had been during the war, when Oppenheimer had assigned Robert Serber and his wife to go into Santa Fe on a Saturday night and plant rumours at bars that what was happening on the hill had to do with submarines. No one showed the slightest interest. By 1957 everyone knew what was happening on the hill.

Being single, I was assigned quarters in a dormitory left over from the war. I acquired a bicycle, which became my principal means of transportation for the rest of the summer. The next day I rode it to the building where the T-Division – T for Theory – was located. I showed my pass to a series of guards and found my way to my office. I discovered that I was sharing it with Ken Johnson, whom I had known since graduate school. He had written a first-rate thesis and had been kept on as a post doc in the department. He was scheduled to go to the Niels Bohr Institute in Copenhagen in the autumn. It soon became clear that no one had any work for us, so we were free to do whatever we wanted. I had come with a problem of how to determine the parity of the pi zero using aspects of its two photon decay. I was stuck on the mathematics, so we decided to do it together. Ken was a great mathematician and he proved results with a high degree of generality. We wrote a paper and submitted it to Carson Mark, the director of the theoretical division, to see if we could publish it with a Los Alamos imprimatur. Mark was pleased: he wanted Los Alamos to have a reputation as something other than a bomb factory.

Of course, it was a bomb factory. Los Alamos and Livermore were churning out designs for devices small enough to fit into intercontinental missiles. These were being tested above ground in Nevada in a series that was called Operation Plumbbob. We used to have afternoon tea – something Oppenheimer had introduced during the war – and I’m sure that most of the people who attended were working on weapons. Since I had no ‘need to know’, no one told me anything. I have one ineluctable memory of those teas. James Tuck was a physicist who had been part of the British delegation to Los Alamos during the war. He had come back to work on controlled nuclear fusion, a classified subject in 1957. The first time I saw him he said: ‘The days of the great Los Alamos teas are over.’ Once upon a time you could have tea with Niels Bohr or Fermi, and now he had to make do with the likes of us.

Francis Low was a consultant to the controlled nuclear fusion programme. I had met him briefly when he’d been a visiting professor at MIT; after the summer he was returning there permanently. He was a hero of mine. He and Murray Gell-Mann had made a study of quantum electrodynamics at short distances that introduced techniques which are still basic to quantum field theory. Francis was a devoted tennis player and we played regularly until the middle of August, when he announced he was going to be away watching bomb tests in Nevada. Surprised, I asked if he had been working on weapons. He said no but that Carson Mark had invited him to observe a test. I asked if there was any chance I could go too. Francis said I would have to ask Carson, who told me I could come along provided I paid my way. On the morning of 30 August the three of us took off from the small airstrip at Los Alamos on a commuter flight to Albuquerque.

I was about to enter the ‘need to know’ world. I decided that under no circumstances would I ask any questions. I had no legitimate need to know. I had no idea of our itinerary. I knew that we would have to get from Las Vegas to Mercury, Nevada, the location of the test site, some 65 miles north-west of Las Vegas. That nuclear weapons were being exploded above ground – dumping thousands of kilocuries of radiation into the atmosphere – so close to a major city shows the craziness of the time. I knew that blackjack was part of the Los Alamos culture. In 1956 four American soldiers stationed at the Aberdeen Proving Ground near Baltimore had published a paper in the Journal of the American Statistical Association entitled ‘The Optimum Strategy in Blackjack’. They explained how to optimise your chances by using the casino rules. The theorists at Los Alamos programmed the Maniac computer to run tens of thousands of hands to see if the strategy actually worked. They were satisfied that it did, and gave a little card showing how to play the game to Los Alamos people who went to Mercury. Francis had made a study of the method and concluded that if you were lucky you might match the federal minimum wage. After we landed at Las Vegas and were met by a small delegation of Los Alamos people in a government car, a casino was our first stop.

The casino must have had a lot of business from people at the test site because there was a light that was turned on if the test scheduled for the next morning was on. The light came on, and we drove to Mercury for a few hours’ sleep. The tests were scheduled for 5.30 a.m. Carson woke us up, and we walked over to a place where a meteorologist was checking the wind. That morning a device called Smoky was to be tested. Carson explained that it was a Livermore device. You could tell because they named their devices after mountains; Los Alamos devices were named after scientists. Galileo was in a tower being readied for a test in two days. The winds were strong so we went to a concrete bunker to await the explosion. I was surprised to find there an old friend of mine from graduate school called Al Peaslee. He had got his degree and then disappeared, but here he was at the test site. His job was to escort a British delegation. ‘We don’t tell them anything,’ he said. He advised me to face away from the explosion and count to ten. I was also given some very dark glass to put over my own glasses. Even the reflection from the bunker walls could damage your eyes. I don’t know how far away from the explosion we were but we were close enough to see the 700-foot tower that had the bomb on top of it. I noticed a hill behind the tower with a grove of Joshua trees. They looked as if they were praying. A loudspeaker counted out the minutes until the explosion and then counted down the last sixty seconds. I had turned my back and covered my eyes with the dark glass but the bright flash still made me shut them. I counted to ten and then turned round.

The horizon in front of me was in turmoil. In the centre was a livid red-orange cloud. The hugeness of it was what impressed me. I had had no idea of the sheer scale of a nuclear explosion. Peaslee had prepared me for the next step. I felt a sharp and slightly painful click in my ears. This was the supersonic shock wave. At Hiroshima it produced a wind stronger than any known typhoon: it knocked over the kerosene cookers the Japanese used to make breakfast and caused most of the fires at Hiroshima. Then came the sound: a sort of rolling thunder. The cloud had turned purple and black and hung in the air like a radioactive cobra about to strike. There was talk of taking cover, but it didn’t move in our direction. I stood there mute. We went back to the dormitory to get a little more sleep.

Sometime around mid-morning I heard the sound of helicopters. Carson had a government car at his disposal. He was going to drive to the 500-foot tower where the next device, the Los Alamos Galileo, was going to be exploded. We went along. On the way there were spots in the desert where the sand had fused into glass. Signs were posted warning of high radioactivity from previous tests. We got to the base of the tower. You could ride most of the way to the top in an open elevator. From that point on there was only a rickety steel ladder. The desert looked a long way down. I had a moment of panic but then it occurred to me that at the top of the ladder there was a nuclear device with a yield comparable to the bomb that flattened Hiroshima. And I was worried about climbing a ladder?

The top was a flat space with just about enough room for Galileo and its attendants. It was a big device with various wires coming out and looked more like a diving bell than a bomb. There was a clicking noise from a vacuum pump. I had no idea why it was there and didn’t ask. Carson spoke to the crew and we went back down the elevator. I thought the tour might be over, but Carson drove to a concrete blockhouse at the far edge of the site. He offered no explanation.

He walked in without knocking or ringing and we followed. Neatly arranged on shelves were the plutonium pits of a considerable number of atomic bombs, probably enough to destroy many cities. I stepped back towards the door. I had read enough about Hiroshima and Nagasaki to know what I was looking at. When he saw me retreat, Francis said that being a few feet further away wouldn’t make much difference if one of them went off. Carson picked up one of the pits, handed it to me, and told me not to drop it. It was warm to the touch – alpha particles – and about the size and weight of a bowling ball. I didn’t know enough to ask the obvious question: why was it so light? A solid sphere of plutonium this size would have weighed a couple of hundred pounds. I’m sure that had I asked I wouldn’t have got an answer. Even asking would have been viewed unfavourably. It was then that I noticed her.

At the other end of the building there was a large workbench where a man was filing something that looked to me like white putty. I had read enough to know that what looked like white putty was a high explosive which was going to be attached to the pit to cause the implosion of the plutonium sphere. Next to him a woman was knitting a green sweater. I don’t mean to sound pretentious, but I at once thought of Eliot’s ‘This is the way the world ends.’ What was she doing there? I didn’t dare ask. Some years later I wanted to write about this, so I sent what I had written to Francis. He said that there had been no woman: I had made it up. Then I wrote to Carson. All he said was that there had been more than one married couple on the site. I was distressed. A week later, Francis called. All he said was: ‘She was there.’

The next morning Galileo was tested. I now knew what to expect but was still overwhelmed. Then we returned to Los Alamos. None of us talked about the tests. Francis, who was on his way to Massachusetts, drove me as far as Lake Forest, Illinois, and I now regret that I didn’t talk to him then. He might have brought me to my senses. Somehow I felt the experience had made me part of the secret world. If you like, I had learned to love the bomb. During my visit to Lake Forest I was introduced to Adlai Stevenson. I somehow conveyed my feelings about the bomb to him. Before I could say much, he looked at me with contempt and walked away. A moratorium on nuclear tests had been part of his unsuccessful 1956 presidential campaign. Eisenhower rejected the idea publicly but was considering the possibility in private. I had said the wrong thing to the wrong man.

Over the next years I came to realise how foolish I had been. The Plumbbob series, to which Smoky and Galileo belonged, were the biggest and longest series of tests ever done in the continental United States. There were 29 tests beginning on 28 May and ending on 7 October. The highest explosive yield was Hood, the test that took place on 5 July – the equivalent of 74 kilotons of TNT. The Nagasaki bomb was about 20 kilotons. Smoky was the second highest, with 44 kilotons equivalent. The series, during which the total yield was about 306 kilotons – something like a tenth of the yield of one hydrogen bomb – released about 58,300 kilocuries of radioiodine into the atmosphere. This fallout was distributed all over the United States and is estimated to have caused about 32,000 cases of thyroid cancer. Twelve hundred pigs were exposed to the explosions in blast-effect studies, and 18,000 servicemen also participated. Roughly 1200 watched the Smoky explosion from a distance of about 13 kilometres. A unit was flown to ground zero some 15 minutes later. They declared that it was safe to occupy so the rest were flown in twenty minutes after the explosion. The exercise was completed at 9.45 a.m., which was when I heard the helicopters. Some of these men later contracted leukaemia.

The plutonium pit I was given to hold was so light because it was hollow. The weapons being tested that summer were ‘boosted’: deuterium and tritium gas were injected into the cavity just before the explosion. I believe the vacuum pump I heard when we visited Galileo was connected to this. When the pit is imploded, and the density is increased enough to reach a supercritical mass, the fission chain reaction begins. When about 1 per cent of the plutonium has been fissioned, the temperature is raised to the point where the fusion reactions of the deuterium and tritium take place. These produce a blast of very high energy neutrons which boost the subsequent fission efficiency. That is what accounted for the large yields in some of the bombs tested that summer. There is no end to the ingenuity that was being applied to weapons design.

The last above-ground test by the United States took place in 1962, and the last above-ground test anywhere was conducted by China in 1980. This is certainly a good thing. But I have only one misgiving. No one has seen a nuclear explosion in more than thirty years and the number of people who have ever seen one is dwindling. Of the people I was with in Nevada in 1957, Peaslee died in 1976, Carson died in 1997 and Francis died in 2007. For most people, nuclear weapons are an abstraction. Perhaps there should be one more explosion in the desert of Nevada to remind us.