Bethe showed missile defence could be fooled by decoys

Person: Hans Bethe
Lived: 1906–2005
Field: Theoretical physics
From: German-born American

The story

From the late 1950s the United States and the Soviet Union both worked on anti-ballistic missile systems designed to shoot down incoming nuclear warheads. By the mid-1960s American research had advanced far enough for the government to plan a limited system called Sentinel, and in 1967 it announced that Sentinel would be scaled down into a smaller, less expensive system known as Safeguard.

Hans Bethe, the Nobel Prize-winning physicist who had led the theoretical work at Los Alamos during the Second World War, had served on the President's Science Advisory Committee and knew the technical issues well. In March 1968 he and the IBM physicist Richard Garwin published a detailed critique of the proposed defence in Scientific American. They argued that nearly any defensive measure the United States could take would be easily thwarted by an attacker deploying relatively simple decoys alongside real warheads.

The argument went to the heart of the problem. A defence needs one interceptor for every incoming object it cannot rule out, while an attacker can add decoys and extra warheads at a fraction of the cost. Later developments bore this out: missiles carrying several warheads, together with decoys, could overwhelm a defence with sheer numbers. Bethe and Garwin's analysis shaped the public and political debate, and Bethe helped persuade the Nixon administration to sign the 1972 Anti-Ballistic Missile Treaty with the Soviet Union.

The treaty limited each country to two defensive sites, one around its capital and one around a field of missile silos, with no more than 100 interceptor missiles at each. In 1974 the limit was cut to a single site. The treaty's purpose was to reduce the pressure on both sides to build more nuclear weapons simply to keep deterrence working, and it remained in force for 30 years, until the United States withdrew in 2002.

Bethe returned to the same arguments in the 1980s, when he and other physicists opposed President Ronald Reagan's Strategic Defense Initiative. In testimony to the Senate Foreign Relations Committee in 1982 he argued that, at the existing level of armaments, superiority in numbers of weapons had no real meaning.

Why it matters

By limiting missile defences, the ABM Treaty removed a major driver of the arms race and helped make later reductions in nuclear arsenals possible.

About Hans Bethe

Refugee physicist who explained how the stars shine, led theory at Los Alamos, cracked the Lamb shift and spent half a century campaigning against the nuclear arms race; Nobel Prize 1967.

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Hans Bethe

1906–2005 · Theoretical physics

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