Today, on Martin Luther King, Jr., Day, a lot of us will be watching King’s “I Have a Dream” speech, on YouTube or elsewhere. I’ve often wondered what it would be like to have been there in 1963, and to have encountered the speech as a surprise—to have seen it without knowing how great it would be, or how famous it would become.

In 1963, the physicist Freeman Dyson was working in Washington, D.C., for the Arms Control and Disarmament Agency, a government agency tasked with improving national security through arms control. His office was a few blocks from Constitution Avenue, where, on August 28th, crowds were gathering for the March on Washington for Jobs and Freedom. On August 20, 1979, Dyson recalled that day in The New Yorker, as part of his memoir, “Disturbing the Universe”:

I took another half day off from my job at A.C.D.A. and strolled down from the State Department Building onto Constitution Avenue, a few blocks away…. Black people from all over the United States were marching. A quarter of a million people were marching. It was quiet. No music and no stamping of feet. I walked to the end of the avenue where the marchers were assembling and marched with them to the Lincoln Memorial. Each group of people carried banners saying where they came from. Occasionally, there would be cheering and shouting from the crowd when a group came by from one of the really tough places: Birmingham, Alabama, or Albany, Georgia, or Jackson, Mississippi—the battlegrounds of the early Freedom Riders. The people from the Deep South were very young—hardly more than children. The Northerners were older, many of them husbands and wives marching together, or union members brought to Washington by their unions. In those days, in the Southern towns where the battles for civil rights had been raging, black people with family responsibilities could not afford to take chances. From the toughest places, only young people came. Most of these children from the Southern battlegrounds had never been away from their homes before. They had been fighting lonely battles. They had never before had anybody to cheer for them. They had never known that they had so many friends. They sang their freedom songs while the Northerenrs listened, and they looked like the hope of the future as they danced and sang, with their bright faces and sparkling eyes.

From two until four, Dyson writes, a number of people spoke “with the huge figure of Lincoln towering over their heads”; James Farmer, the organizer of the original Freedom Ride, “sent a message from his cell in Louisiana jail.” Then Martin Luther King stood up to speak.

Martin Luther King spoke like an Old Testament prophet. I was quite close to him, and I was not the only one listening who was in tears. “I have a dream,” he said, over and over again, as he described to us his visions of peace and justice. That night, in my letter to my parents in England, I wrote, “I would be ready to go to jail for him any time.” I did not know then that I had heard one of the most famous speeches in the history of mankind. I knew only that I had heard one of the greatest. I also did not know that Martin Luther King would be dead within five years.

King’s speech was of great importance to the Civil Rights movement, obviously. But, to Dyson, it had additional significance within the context of the Cold War. “On the one side, the gospel of non-violence that Jesus and Gandhi and Martin Luther King preached and practiced,” he wrote. “On the other side, the madness of hydrogen bombs and the doctrine of ‘mutual assured destruction’ with which we are now precariously living.”

“Disturbing the Universe” is available in The New Yorker’s archive, which goes back to our first issue, in 1925.

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