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June 2, 2022
Physicists Built the Bomb, Urged Restraint Too
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September 2023
By Lisbeth Gronlund

As the film Oppenheimer documented, many Manhattan Project scientists were concerned that use of the weapons they built would lead to a U.S.-Soviet arms race. J. Robert Oppenheimer, who headed the Los Alamos Laboratory during the Manhattan Project, and his scientist colleagues repeatedly argued that the U.S. nuclear weapons monopoly gave the United States a unique opportunity to prevent a world-threatening outcome by briefing Soviet scientists and policymakers on their work and proposing a treaty prohibiting nuclear weapons. On July 17, 1945, Leo Szilard and 68 other members of the Manhattan Project Chicago laboratory petitioned President Harry Truman directly, making the case against using these weapons on Japan because doing so would make the United States responsible for “opening the door to an era of devastation on an unimaginable scale.”1

University of Chicago professor Leo Szilard, shown testifying before the U.S. Congress in 1945, joined 68 other Manhattan Project scientists in writing to President Harry Truman to argue against dropping the first atomic bomb on Japan during World War II. (Photo from Bettmann Archives via Getty Images).Although none of these early efforts to influence the government was successful, during the Cold War and afterward, Soviet and U.S. physicists and other scientists, sometimes working together, repeatedly made a positive impact on international and national policies. Two cases in particular highlight the essential role of scientists: achieving limits on nuclear explosive testing and working to keep defenses against long-range nuclear-armed ballistic missiles from triggering an arms race. Academic physicists were particularly active and remain concerned about and engaged with nuclear weapons-related issues today.

Nuclear Explosive Testing

All five of the established nuclear powers (China, France, the Soviet Union, the United Kingdom, and the United States) started out testing nuclear weapons in the atmosphere, releasing large amounts of radiation that spread around the globe and fell to the ground. Beginning in 1954, when the United States tested a very powerful hydrogen bomb—the 15-megaton Castle Bravo test that was equivalent to 15 million tons of TNT—in the South Pacific, there was an international outcry about the environmental and human effects. The Soviet Union and the United States, however, continued testing with abandon.

The UK physicist Joseph Rotblat, the only scientist to leave the Manhattan Project on moral grounds after Germany was defeated, began researching the radioactive effects of nuclear testing in 1954. His calculations showed that the United States greatly understated the radioactivity released by these nuclear tests. Widespread media coverage of his findings increased public outrage.

Nevertheless, the Soviet Union, the UK, and the United States continued atmospheric testing until 1963, when they negotiated and signed the Limited Test Ban Treaty, banning all but underground tests. Neither China nor France signed the treaty, and their last atmospheric tests were in 1980 and 1974, respectively. In 1957, chemist Linus Pauling started a scientists’ appeal for a complete ban on nuclear testing.2 Within two weeks, 2,000 U.S. scientists, including Albert Einstein, had signed. Within a few months, the list had grown to 11,000 scientists around the globe. Many U.S. scientists actively promoted the test ban. They engaged the public, met with their government representatives, spoke to the media, and several years later, could share in the treaty’s success.

Another significant development was the founding, also in 1957, of the Pugwash Conferences on Science and World Affairs (named after Pugwash, Nova Scotia, where the first conference was held) by Joseph Rotblat and the mathematician and philosopher Bertrand Russell. These international conferences continued until 2020 and brought together scientists, including some who advised their governments, and other experts.

They were the first contemporary Track 2 meetings, where participants interacted as individuals, not as representatives of their governments. In this Cold War environment, they could have open, frank discussions. These meetings influenced many international treaties and agreements, including laying the groundwork for the Limited Test Ban Treaty. They also facilitated strong personal relationships between Soviet and U.S. scientists and, eventually, Chinese and U.S. scientists, which proved essential to progress on arms control.

The Limited Test Ban Treaty was followed in 1974 by the Soviet-U.S. Threshold Test Ban Treaty, limiting the yield of underground nuclear tests to no more than 150 kilotons. Concerns about verification stalled ratification until 1990 because the two nations did not agree on the means for verifying the treaty. The Soviet Union insisted that remote seismic measurements could determine the yield of a test explosion. The United States insisted on using an on-site method that required placing a cable in a shaft near the shaft to be used for the nuclear weapons test, which would measure the shock wave at close range.

In the late 1980s, a group of Soviet and U.S. physicists—Frank von Hippel at Princeton University; Evgeny Velikhov, director of the Kurchatov Institute; Roald Sagdeev, director of the Space Research Institute; and especially Tom Cochran of the Natural Resources Defense Council—were responsible for breaking the deadlock. They proposed that each country conduct a nuclear test whose yield would be measured by teams of scientists from both nations, with both teams using both methods of estimating the yield.

The governments agreed, and in 1988 they conducted the “Joint Verification Experiment,” which demonstrated that seismic verification was effective. The two countries subsequently ratified the treaty with verification provided by seismic monitoring and hydrodynamic monitoring under certain circumstances. It is not an overstatement that the efforts of a few scientists were responsible for ratification.

In 1994, negotiations began on the Comprehensive Test Ban Treaty (CTBT), which prohibits all nuclear explosive testing. Verification again stood in the way. It is more difficult to verify a yield of zero than one of 150 kilotons. Some U.S. opponents of the CTBT argued that countries could cheat by testing a small-yield explosive inside a large underground cavity, which would reduce the seismic signal by decoupling the explosion from the surrounding rock.

U.S. chemist Linus Pauling, who won a Nobel Prize in Chemistry and the Nobel Peace Prize, poses with his alpha-helix model in front of a chalkboard at the Linus Pauling Institute, Menlo Park, Calif., in 1983. In 1957, he started a scientists’ appeal for a complete ban on nuclear testing. (Photo by Janet Fries/Getty Images)Many U.S. scientists, especially seismologists, became involved in this debate. They debunked the large cavity argument and, as in previous cases, influenced opinion by engaging the public, policymakers, administration officials, and the media.

Finally in 1995, a study by the JASON group of high-level scientists that advises the U.S. government played an important role in resolving the CTBT debate. At that time, JASON members were mainly physicists. They considered the technical details relevant to the CTBT and concluded that there were no reasons that the United States should not sign a treaty of enduring duration, provided it included the standard statement that a nation could withdraw in the event that a nuclear explosive test was necessary to protect its “supreme national interest.”

This study had a large effect on President Bill Clinton’s decision to sign the CTBT in 1996. Although the Senate refused to ratify the treaty, the United States and 186 other signatories have continued to abide by it.

Long-Range Ballistic Missile Defenses

Throughout history, people have built defenses against armaments, so it is not surprising that the Soviet Union and the United States sought to protect their populations from nuclear weapons. Beginning in the late 1950s, the two nations deployed nuclear-armed intercontinental-range ballistic missiles, which hurl their warheads into space, which then fall to the ground under the influence of gravity. Shortly thereafter, both countries began deploying anti-ballistic missile interceptors. In 1962, the Soviet Union began placing such interceptors around Moscow. Because the interceptors were not accurate enough to destroy warheads with conventional explosives, they were armed with nuclear weapons.

In the late 1960s, the United States began preparations to deploy the Sentinel anti-ballistic missile system, which was billed as a limited defense against an accidental Soviet or Chinese attack, despite the fact that China had no nuclear-armed intercontinental-range missiles and would not for decades. The Sentinel interceptors carried megaton-level nuclear warheads. The Army’s decision to place 13 of the 17 planned interceptor sites near major cities enraged local populations, leading to large demonstrations in some areas.

After scientists at Argonne National Laboratory learned that one of the Sentinel sites was to be built near Chicago, they engaged very effectively with activists, providing fact sheets and other materials, giving numerous presentations, and talking to the media. When Department of Defense officials subsequently came to brief the local public about the project, they found an angry audience armed with facts and arguments provided by the Argonne scientists. These officials regarded the meetings as a disaster. Similar efforts took place in other cities, and in many cases, physicists played major roles in the opposition. Because of this widespread opposition, the Sentinel program was canceled in March 1969, after only 18 months.

Beginning in the 1960s, some U.S. physicists and other scientists understood that missile defenses against nuclear weapons were a terrible idea because building defenses would prompt an adversary to build more missiles and lead to a destabilizing arms race. Limits on offensive weapons would be possible only if defenses also were limited.

Initially, Soviet scientists did not embrace this logic. Their eventual acceptance was partly a consequence of meetings of the Soviet-American Defense Study group, which spun off from the Pugwash meetings in 1964 and consisted of a smaller select group of Soviet and U.S. scientists, some of whom advised their governments.

Soviet scientists also were influenced by a prominent 1968 Scientific American article by Hans Bethe, director of the theoretical division of the Manhattan Project, and Richard Garwin, who helped develop the H-bomb.3 It laid out the technical and political arguments against these defenses for the public and experts alike. It also discussed the myriad ways in which defenses could be defeated, making them useless and provocative.

Such activities by scientists led to the first Strategic Arms Limitations Talks treaty, which reduced offensive weapons to 6,000 for each country and was coupled to the Anti-Ballistic Missile (ABM) Treaty. The ABM Treaty prohibited essentially all missile defenses, but allowed research on defensive technologies, which ultimately led to its demise.

The beginning of the end came in March 1983 with President Ronald Reagan’s infamous “Star Wars” speech in which he announced the Strategic Defense Initiative (SDI) to deploy a defense using satellite-based interceptors and lasers that would render “nuclear weapons impotent and obsolete.” This goal was absurd given the 6,000 intercontinental warheads that treaties allowed the Soviet Union and United States each to maintain.

The program infuriated the physics community, which characterized it as nonsensical. Around the country, physicists again became active and reached out to the public, policymakers, and the media. Their activities were key to creating a small but influential movement against the program.

The Case Against SDI

The case against SDI was first articulated by the influential April 1984 report “Directed Energy Weapons in Space” by Ashton Carter, a physicist working for the Congressional Office of Technology Assessment who eventually became secretary of defense under President Barack Obama.4 The case was bolstered by the October 1984 Scientific American article “Space-based Ballistic-missile Defense,” authored by Bethe, Garwin, Kurt Gottfried of Cornell University, and Henry Kendall of the Massachusetts Institute of Technology (MIT), the latter two of which had helped found the Union of Concerned Scientists.5

Ashton Carter, who eventually became President Barack Obama’s defense secretary, wrote an influential report against the Strategic Defense Initiative in 1984. (Photo by Saul Loeb/AFP via Getty Images)In 1985, SDI began giving grants to academics, declaring that “this office is trying to sell something to Congress. If we can say that this fellow at MIT will get money to do such and such research, it’s something real to sell.” The idea that the program wanted to use scientists to sell the program further enraged the physicists.

In response, physicists at Cornell and the University of Illinois-Urbana wrote a pledge of nonparticipation for scientists and engineers, stating that they would not apply for or accept funding from SDI program and why. By the time the pledge results were released in May 1986, it had been signed by 6,500 academic scientists and engineers around the country.6 The protest received significant media attention and hammered home that scientists believed the program was technically unworkable and unwise. Finally, the professional organization of physicists, the American Physical Society, released an authoritative study in 1987 on the science and technology of directed energy weapons, such as lasers, concluding that SDI was unworkable.

Politically wounded by this surge of expert opposition, SDI was canceled in 1993 by Clinton. The program never progressed beyond research and development, so the ABM Treaty remained intact.

Even so, interest in missile defense continued. Clinton’s Pentagon replaced SDI with the National Missile Defense (NMD) program, which relied on ground-based “hit to kill” interceptor missiles that would destroy an incoming warhead by slamming into it.

In response, the Union of Concerned Scientists issued a report in April 2000 titled “Countermeasures,” showing that even if this system worked perfectly, it could be defeated in numerous ways. Five months later, Clinton announced that he would not deploy the system, citing, among other issues, its vulnerability to countermeasures.

President George W. Bush changed the name of the program to the Ground-Based Missile Defense program and, to allow its nominal deployment, withdrew from the ABM Treaty in 2002. Nevertheless, scientists have continued to point out the system’s shortcomings and critique the intercept tests, with the result being that the system’s effectiveness is widely doubted.

Although Russian-U.S. relations are again strained and the only remaining arms control agreement, the New Strategic Arms Reduction Treaty (New START), is teetering, the research and activities of these physicists and other scientists who have questioned and criticized U.S. nuclear and missile defense policies have mattered, sometimes quite a lot. For decades, scientists willing to challenge Pentagon programs and Washington orthodoxy helped produce stabilizing outcomes in U.S. nuclear and missile defense policy. There is still a CTBT, for instance, and the United States abides by it. Missile defense deployments are limited and widely viewed as ineffective.

Unfortunately, further progress has been hampered by the continuing U.S. commitment to deploying missile defenses. Some physicists and other scientists have remained actively engaged, but the expert technical analysis and engagement of the wider scientific community are needed more than ever in this time of growing geopolitical tensions.



1. “A Petition to the President of the United States,” https://www.atomicarchive.com/resources/documents/manhattan-project/petition.html (petition dated July 17, 1945).

2. Linus Pauling, “An Appeal by American Scientists to the Governments and People of the World,” Bulletin of the Atomic Scientists, Vol. 13, No. 7 (May 15, 1957).

3. Richard L. Garwin and Hans A. Bethe, “Anti-Ballistic-Missile Systems,” Scientific American, Vol. 218, No. 3 (March 1968): 21.

4. Ashton B. Carter, “Directed Energy Missile Defense in Space,” U.S. Office of Technology Assessment, OTA-BP-ISC-26, April 1984, https://www.princeton.edu/~ota/disk3/1984/8410/841001.PDF.

5. Hans A. Bethe et al., “Space-based Ballistic-Missile Defense,” Scientific American, Vol. 251, No. 4 (October 1984): 39-49.

6. Lisbeth Gronlund, et al, “A Status Report on the Boycott of Star Wars Research by Academic Scientists and Engineers,” May 13, 1986, https://www.dropbox.com/scl/fi/yzojaywaqq84tb0jj0g0a/SDI-Pledge-Report-1986.pdf?rlkey=vfuvznu8rg4buw9g8cx8hlwhr&dl=0

Lisbeth Gronlund is a visiting scholar with the Laboratory for Nuclear Security and Policy at the Massachusetts Institute of Technology Department of Nuclear Science and Engineering.