Reflections on the 70th Anniversary of the Manhattan Project: Questions and Answers

I began my professional life by obtaining degrees in physics and entering a conventional academic career in teaching and astronomical research, but I had always been curious about the physics of the Manhattan Project and its role in ending World War II. With grants, publications and tenure established, I began to indulge this interest as a legitimate part of my work and about 20 years ago, to explore it in depth.

As anybody that comes to this topic in more than a casual way will attest, it can grow into an obsession. I have now published two books on the Project, well over two dozen articles and book reviews in technical, historical, and semi-popular journals, and have made a number of presentations at professional conferences. Over this time I must have looked at thousands of archived documents and held hundreds of real and electronic conversations with other scientists, historians, and writers whose interest in this pivotal event parallels my own. While my knowledge of the Project is certainly not and never will be complete, I have learned much about it over the last 20 years.

To my surprise (and pleasure) I am frequently asked questions about the Project by students, family members, guests at dinner parties, colleagues at American Physical Society meetings, and even casual acquaintances at my favorite coffee shop. Typical queries are:

“Why did we drop the bombs? Were they necessary to end the war?”

“Did President Truman and his advisors really understand the power of the bombs and the destruction they could cause?”

“Have nuclear weapons helped deter subsequent large-scale wars, and do we still need a deterrent?”

“What about the ethical aspects?”

“In studying the Manhattan Project, what most surprised you? Do you think it or something similar could be done now?”

At first I was awkward in trying to answer these questions but with passing years, increased knowledge, and much reflection I now feel more comfortable addressing them. With accumulating experience in a scientific career, you often learn that the questions you and others initially thought to be important may not be the ones that the facts address and that there may be much more interesting issues behind the obvious ones. In this spirit, I offer in this essay some very personal reflections on the Project and the legacies of Hiroshima and Nagasaki, framed as responses to questions like those above. In some cases a “yes” or “no” along with an explanation will do, but for many issues the nuances involved obviate a simple response.

I begin with the issue of the “decision” to use the bomb and the state of President Truman’s knowledge. In the spring of 1945, Secretary of War Henry Stimson assembled a committee to consider and advise upon immediate and long-term aspects of atomic energy. This “Interim Committee” comprised eight civilians, including three scientists intimately familiar with the Manhattan Project: Vannevar Bush, James Conant, and Karl Compton. In a meeting on May 31 which was attended by Army Chief of Staff General George C. Marshall, Stimson opened with a statement as to how he viewed the significance of the Project1)Minutes of Interim Committee meetings can be found at

The Secretary expressed the view, a view shared by General Marshall, that this project should not be considered simply in terms of military weapons, but as a new relationship of man to the universe. This discovery might be compared to the discoveries of the Copernican theory and of the laws of gravity, but far more important than these in its effect on the lives of men. While the advances in the field to date had been fostered by the needs of war, it was important to realize that the implications of the project went far beyond the needs of the present war. It must be controlled if possible to make it an assurance of future peace rather than a menace to civilization.

For his part, President Truman had been thoroughly briefed on the project by Stimson and General Leslie Groves, director of the Project, soon after he became President in late April. In late July, Truman recorded his reaction to the Trinity test in his diary2)R.H. Ferrell, Harry S. Truman and the Bomb. High Plains Publishing Co., Worland, WY (1996), p. 31:

We have discovered the most terrible bomb in the history of the world. … Anyway we think we have found the way to cause a disintegration of the atom. An experiment in the New Mexico desert was startling – to put it mildly. Thirteen pounds of the explosive caused the complete disintegration of a steel tower 60 feet high, created a crater 6 feet deep and 1,200 feet in diameter, knocked over a steel tower 1/2 mile away and knocked men down 10,000 yards away. The explosion was visible for more than 200 miles and audible for 40 miles and more. … The target will be a purely military one and we will issue a warning statement asking the Japs to surrender and save lives. I’m sure they will not do that, but we will have given them the chance. It is certainly a good thing for the world that Hitler’s crowd or Stalin’s did not discover this atomic bomb. It seems to be the most terrible thing ever discovered, but it can be made the most useful…

I have no doubt that Stimson, Marshall and Truman were well aware of the revolutionary nature of the bomb and the possibility (indeed, likelihood) that a postwar nuclear arms race would ensue. Any notion that Truman was a disengaged observer carried along by the momentum of events is hard to believe in view of the above comments. These men were making decisions of grave responsibility and were fully briefed as to both the immediate situation of the war and possible long-term geopolitical consequences: the “mature consideration” that Franklin Roosevelt and Winston Churchill agreed in 1943 would have to be carried out before use of the bombs was authorized. Perhaps Truman did not so much make a positive decision to use the bombs so much as he opted not to halt operations that were already moving along when he became President, but I have no doubt that he realized that atomic bombs would be a profoundly new type of weapon. Further, let us not forget that it was Truman who personally intervened after Nagasaki to order a halt to further atomic bombings when the Japanese began to signal a willingness to consider surrender negotiations.

As much as I am convinced that Truman took his duties with the greatest sense of responsibility, I cannot answer “yes” or “no” as to the necessity of the bombings: the question is always loaded with so many unstated perspectives. If the Japanese could not be convinced to surrender, then Truman, Stimson, and Marshall faced the prospect of committing hundreds of thousands of men to a horrific invasion followed by a likely even more horrific slog through the Japanese home islands. After 70 years it is easy to forget the context of the war in the summer of 1945. Historians know that the Japanese were seeking a path to honorable surrender and might have given up within a few weeks, but the very bloody fact on the ground was that they had not yet surrendered; thousands of Allied and Japanese servicemen were dying each week in the Pacific. Military historian Dennis Giangreco has studied Army and War Department manpower projections for the two-part invasion of Japan scheduled for late 1945 and the spring of 19463)D. M. Giangreco, “Casualty Projections for the U. S. Invasion of Japan, 1945-1946: Planning and Policy Implications.” Journal of Military History 61, 521-582 (1997).. Planning was based on having to sustain an average of 100,000 casualties per month from November 1945 through the fall of 1946. The invasion of Kyushu was scheduled to begin on November 1, 1945. Had this occurred, the number of casualties might well have exceeded the number of deaths at Hiroshima and Nagasaki, let alone those which would have occurred in the meantime. From the perspective of preventing casualties, perhaps it was unfortunate that the bombs were not ready at the time of the battle for Iwo Jima, one of the bloodiest protracted battles from February 19 to March 26, 1945, during which more than 25,000 were killed on both sides.

Even if they believe that the Soviet Union’s declaration of war on the night of August 8, 1945, against Japan was the most significant factor in the Japanese decision to surrender, most historians allow that the bombs had at least some effect on that decision. The Soviet invasion came between the two atomic bombings on August 6 (Hiroshima) and August 9 (Nagasaki). These two bombings would convince the Japanese that Hiroshima was not a one-shot deal: America could manufacture atomic bombs in quantity. The impact of the bombings was alluded to by Emperor Hirohito in his message to his people on August 15, 1945, in which he stated that “ … the enemy has begun to employ a new and most cruel bomb,” which was one of the motivations for his government’s decision to accept the terms of the Potsdam Declaration. But there are certainly political aspects that muddy this story, namely justifying the immense resources poured into the Project and sending a message to the Soviets that at least for a while America was the ascendant postwar power in the world. I give a qualified “yes” to the question of necessity.

The necessity debate often overlooks a corollary issue which I have come to think of as “nuclear inoculation.” Had the bombs not been used in 1945 and world leaders made aware of their frightening power, what far more awful circumstances might have unfolded in a later war when there were more nuclear powers armed with more powerful weapons? I am absolutely convinced that the bombings have had a significant deterrent effect and that they may well have prevented the outbreak of further major wars since 1945. Indeed, we know that there were occasions such as the Cuban missile crisis when national leaders looked into the maw of a possible large-scale war and backed away.

The “inoculation” issue leads to the question of whether or not America continues to need a nuclear deterrent. To this I say: “Yes, but for not entirely rational reasons.” Even very conservative military planners estimate that a few hundred warheads would be enough for any conceivable nuclear-mission scenario and that the thousands still stockpiled are a waste of resources and budgets. But the deterrent issue seems to me to be more psychological than mission-driven. With potentially unstable or irrationally-led states pursuing weapons and possibly encouraging proliferation, what “established” nuclear power would consider unilaterally disarming itself?  If America and Russia engage in further rounds of treaties and draw down their numbers of deployed and reserved weapons from thousands of warheads, a time may come when these numbers will get down to those held by powers such as Britain, France, China, India and Pakistan4)Hans M. Kristensen and Robert S. Norris, “Worldwide deployments of nuclear weapons, 2014,” Bulletin of the Atomic Scientists 70(5),96-108 September 2014.. How then will negotiations proceed? Even if rigorous inspection regimes are agreed to, it seems to me that it will take decades until we might get to a level of trust where we won’t feel compelled to rationalize: “They could be slipping a few weapons into their arsenal under the table; we had better keep some in reserve.” In the meantime, I encourage students and acquaintances to question their elected representatives regarding the Comprehensive Test Ban Treaty and a possible Fissile Materials Cutoff Treaty.

What about the ethics of the bombings? To my mind the answer is: “The war had rendered this issue irrelevant.” Even against the “standards” of present-day terrorist acts, the ferocity of World War II seems almost incomprehensible. Deliberate atrocities against civilians and prisoners by the Axis powers were beyond the ethical pale, but how does one classify the Allied fire-bombings of Coventry, Dresden, and Tokyo even if there were arguable military objectives? The vast majority of victims at Hiroshima and Nagasaki succumbed not to radiation poisoning but to blast and burn effects just like the victims of these other attacks. I do not see that the bombs crossed an ethical threshold that had not already been breached many times before.

What have I learned about the Manhattan Project that especially surprised me? Well, practically everything. I approached the Project as a physicist, and it was a revelation for me that much of the physics involved is entirely accessible to a good undergraduate student. Computing critical mass involves separating a spherical-coordinates differential equation and applying a boundary condition: advanced calculus. Estimating the energy released by an exploding bomb core is a nice example of using the Newtonian work-energy theorem of freshman-level physics in combination with some pressure/energy thermodynamics. Appreciating how a calutron separates isotopes is a beautiful example of using the Lorentz force law of sophomore-level electromagnetism. Estimating the chance that a bomb might detonate prematurely due to a spontaneous fission invokes basic probability theory. These are exotic circumstances which require wickedly difficult engineering to realize, but the physics is really quite fundamental.

Everybody knows that the Manhattan Project was a big undertaking, but I now realize just how truly vast it was. At first, one’s attention is drawn to the outstanding personalities and dramatic events and locales associated the Project: J. Robert Oppenheimer, Enrico Fermi, Groves, Los Alamos, Trinity, Tinian, Hiroshima and Nagasaki. Then the  appreciation of the complexity of the production factories at Oak Ridge and Hanford, facilities designed by unappreciated and now largely-forgotten engineers of outstanding talent. Hundreds of contractors and university and government laboratories were involved, staffed by hundreds of thousands of dedicated employees. Also, bombs are not transported by magic to their targets; bombers had to be modified to carry them, and training of crews to fly the missions was initiated well before the final designs of the bombs and choice of targets were settled. The magnitude of the feed materials program to source and process uranium ores is rarely mentioned, but without this there would never have been any bombs (or any later Cold War).

While physics, chemistry, and engineering were front-and-center, I have also come to appreciate that the organization and administration of the Project was equally important. This is a hard thing for an academic scientist to admit! The Project was incredibly well-administered, and there is a lesson here for current times. Yes, the Project had its share of oversight and consultative committees, but they were run by scientists, engineers, government officials and military officers of superb competence and selfless dedication to the national good. These people knew what they were doing and knew how to get things done through the bureaucratic channels involved. An existential threat is always good for getting attention focused on a problem, but somebody has to actually do something. Of course there were security leaks and some inefficiencies, but what else would you expect in an undertaking so large and novel?

Could a Manhattan-type project be done now? I do not doubt for a moment that American scientists, technicians, engineers, and workers still possess the education, brains, dedication, and creativity that characterized Manhattan. But I do not think that such success could be repeated. Rather, headlines and breathless breaking news reports would trumpet waste, inefficiency, disorganization, technically clueless managers, and publicity-seeking politicians. The result would likely be a flawed product which ran far over-budget and delivered late if at all, no matter how intense the motivation. Do the words “Yucca Mountain” require further elaboration?

General Groves’ official history of the Project, the Manhattan District History, can be downloaded from a Department of Energy website, and I encourage readers to look at it5)The Manhattan District History can be found at It is literally thousands of pages, and is simply overwhelming; I doubt that anybody has read it from end-to-end. Click on any page and you will find some gem of information. Beyond the MDH lie thousands of secondary sources: books, popular and technical articles, websites and videos. But I have not one iota of regret that I plunged in. The Project was vast: many aspects of it have yet to be mined, and there are lessons to be had for scientists, engineers, biographers, historians, administrators, sociologists, and policy experts alike.

My research on the Project has made me much more aware of the world nuclear situation. Belief in deterrence aside, I am astonished that there has not been an accidental or intentional aggressive nuclear detonation over the last seventy years. We now know that on many occasions we came very close and that we have been very lucky indeed. While I see the chance of a deliberate nuclear-power-against-nuclear-power exchange as remote, the prospect of a terrorist-sponsored nuclear event does cause me no small amount of concern.

Nuclear energy is the quintessential double-edged sword, and those of us who have some understanding of the history, technicalities and current status of nuclear issues have a responsibility to share our knowledge with our fellow citizens in a thoughtful, responsible way. The stakes are no less existential now than they were seventy years ago.


Notes   [ + ]

1. Minutes of Interim Committee meetings can be found at
2. R.H. Ferrell, Harry S. Truman and the Bomb. High Plains Publishing Co., Worland, WY (1996), p. 31
3. D. M. Giangreco, “Casualty Projections for the U. S. Invasion of Japan, 1945-1946: Planning and Policy Implications.” Journal of Military History 61, 521-582 (1997).
4. Hans M. Kristensen and Robert S. Norris, “Worldwide deployments of nuclear weapons, 2014,” Bulletin of the Atomic Scientists 70(5),96-108 September 2014.
5. The Manhattan District History can be found at