Yuri Orlov, a Russian-born physicist who designed particle accelerators, studied the foundations of quantum mechanics and — turning from theory to practice in his boldest experiment of all — created the Moscow Helsinki Group to expose human rights violations in the Soviet Union, died Sept. 27 at his home in Ithaca, N.Y. He was 96.

His wife, Sidney Orlov, confirmed the death but did not give a precise cause.

Like his contemporary Andrei Sakharov, a father of the Soviet hydrogen bomb who later won the Nobel Peace Prize, Dr. Orlov was both an eminent physicist and a champion of free speech and human rights. He examined the building blocks of matter, helped launch the Soviet branch of Amnesty International and organized what is widely considered Russia’s oldest human rights organization, the Moscow Helsinki Group.

As a result of his advocacy efforts he was harassed, jailed and sentenced to hard labor and exile in Siberia, where he endured illness and isolation before being freed as part of a prisoner swap orchestrated by President Ronald Reagan and Soviet leader Mikhail Gorbachev. He came to the United States having lost weight, partly because of hunger strikes, and having picked up a new habit, smoking, from his time in jail.

“I did it to poison my interrogator,” he told the Washington Times. “And I told him so.”

Dr. Orlov came to prominence in the 1970s, at a time when the Soviet protest movement “was hopeless and desperate,” said Lev Ponomarev, a fellow Soviet-era physicist and dissident who now leads the For Human Rights movement. “Any attempts by young people to unite in protest groups were monitored and defeated, and people went to the camps without achieving a noticeable result.”

In an email interview, he added that Dr. Orlov “was looking for an idea for unification, an understandable and meaningful program of action. And he found it. He came to the conclusion that it is necessary to unite people not just against totalitarianism, but in constructive activity that would not formally look anti-Soviet and therefore could not be immediately suppressed.”

Formed in May 1976, the Moscow Helsinki Group reported on regime crackdowns and abuses, publishing its reports on official letterhead — rather than in underground pamphlets — and distributing them to journalists and embassies. The organization inspired groups such as Helsinki Watch in New York City, which evolved into Human Rights Watch.

Dr. Orlov was well aware that his activism could end badly. His protest efforts dated back to 1956, when he gave a speech at his Moscow physics institute denouncing Stalin’s reign of terror as “a shameful page in the history of our party.” Nikita Khrushchev had recently delivered an address along similar lines, attacking his late predecessor’s “cult of personality.”

But while Khrushchev was the Communist Party’s first secretary, Dr. Orlov was a 31-year-old junior scientist. Soviet leaders condemned his “hostile outbursts” and expelled him from the party. He was fired from his institute, criticized in the pages of Pravda and forced to continue his scientific career in Armenia.

Soon after returning to Moscow in 1972, he became a founding member of the Amnesty International office and lost his physics job once again. Physicists were being recruited to sign a letter denouncing Sakharov for his human rights work, but Dr. Orlov refused, instead writing a cheeky letter to Soviet leader Leonid Brezhnev.

Known as “13 Questions to Brezhnev,” the document criticized the campaign against Sakharov while suggesting political and economic reforms, including “abolition of censorship of the press, free exchange of information, glasnost,” a term that Gorbachev would later use as a political slogan.

Dr. Orlov’s activism expanded in the wake of the 1975 Helsinki Accords, which were signed by 35 states in the Soviet bloc and the West, covered a sweeping array of political and economic issues and included a section noting that “the participating states will respect human rights and fundamental freedoms.”

The document was nonbinding and angered dissidents such as Natan Sharansky, a future Israeli politician. “We felt that the West betrayed our interests,” he said in a phone interview. “The Soviet Union got what it wanted, and in exchange, it gave lip service” to human rights.

Dr. Orlov’s solution was to create an organization that would report on rights violations, force the West to pay attention and bring together dissidents including Sharansky, Lyudmila Alexeyeva, Alexander Ginzburg and Yelena Bonner.

“He said, ‘The result is clear: We will be arrested,’ ” Sharansky recalled. “ ‘But that will make it much more difficult for the Soviets to ignore.’ ” Dr. Orlov, he added, was like a theoretician suddenly practicing experimental physics. “He tried to predict the result of the experiment, and he happened to be right.”

After less than a year, Dr. Orlov was arrested by the KGB in February 1977, then convicted of “anti-Soviet activity” following a three-day trial.

“Ideological tolerance is necessary for peaceful coexistence,” he told the court in his final remarks, according to the news service United Press International. The statement was met by shouts from the audience — “Traitor! Spy!” — and he was sentenced to hard labor for seven years and to internal exile for five.

Sent to a labor camp in the Ural Mountains, he slept on bare planks, dined on hot food every other day and was diagnosed with tuberculosis. He protested mistreatment at the camp, only to be punished further and later moved to far eastern Russia, where he melted river ice for water in a village just below the Arctic Circle.

His confinement generated outrage from scientists and the U.S. government, and in 1986, he was stripped of his Soviet citizenship and deported to the United States.

Dr. Orlov resumed his scientific career at Cornell University in Ithaca, published a memoir, “Dangerous Thoughts” (1991), and campaigned on behalf of imprisoned Soviet dissidents. They were released slowly, he wrote in his memoir, “in small numbers, like drops of blood squeezed from a stone.”

Yuri Fyodorovich Orlov was born in Moscow on Aug. 13, 1924. His father was a lathe operator, and his mother sewed leather handbags and later worked as a secretary.

Dr. Orlov’s high school education was interrupted for six years by World War II, when he helped manufacture tanks and served as a Red Army artillery officer. He graduated from what is now the Moscow Institute of Physics and Technology and later received doctoral degrees from physics institutes in Armenia and Siberia.

His marriages to Galina Papkevich, Irina Lagunova and Irina Valitova ended in divorce. In 1988, he married Sidney, a Cornell writing instructor who helped teach him English. Their first lesson, he later wrote, turned into “six hours of fierce debate on Chekhov in my twenty words of English and her two hundreds words of Russian. Day by day my English got better and her Russian got worse. So we bought a house in the country and married.”

In addition to his wife, survivors include two sons from his first marriage; a son from his second; eight grandchildren; and two great-grandchildren.

Dr. Orlov retired from Cornell in 2015 and later did theoretical work in cosmology, examining the origins and development of the universe. But his interest in Russian politics never wavered.

“Russia is flying backwards in time,” he told the Washington Times in 2004, long after the collapse of the Soviet Union. “Putin is like Stalin, and he speaks in the language of the thug, the mafia.”