In the opening line of his “I Have a Dream” speech, delivered at the Lincoln Memorial on Wednesday, Aug. 28, 1963, the Rev. Martin Luther King Jr. predicted that the March on Washington for Jobs and Freedom would “go down in history as the greatest demonstration for freedom in the history of our nation.”

Fifty years on, we know he was prophetic.

But at the time, it was a bold statement, for there had been many examples in our country’s history when Americans had screwed their courage and protested for a noble cause. One could point to the actions of the Sons of Liberty that led to the signing of the Declaration of Independence. One might also point to the anti-slavery movement that culminated in the American Civil War and the liberation of the slaves with the ratification of the 13th Amendment. Indeed, there had been many other protests, picket lines and parades up until that point in our history, to be sure — some of them, like the suffragist movement, with far-reaching consequences that could redefine the roles of president and first spouse in 2016, with the potential election of our first female president.

But because of what radio and television were able to transmit in late August 1963, the March on Washington was witnessed by far more Americans than any previous demonstration, and from the deep vaults of American history, now flung open with a few taps on a touchscreen, images and sounds from that day are easily sampled as part of the stream of signal events that define our nation’s memory.

That these images are black-and-white and crackle only enhances their mystique — and thus their power to move and inspire our awe at “the majestic heights of meeting physical force with soul force,” as King said during his speech. And so we are left to wonder: What was it like to be there? What could they see, smell and hear that we today cannot, beyond the camera’s frame?

We know the temperature in Washington reached a high of 82, but how “sweltering” and packed-in were the throngs gathered around the reflection pool? And how did King’s voice carry, both up-close and as far away as the Washington Monument? What was said in the Oval Office, as President John F. Kennedy and his attorney general, brother Robert, watched along with the rest of the nation on TV?

And with such numbers — some 250,000 to 300,000 people squeezed together, many of them with signs, buttons and folded white hats — how did it remain so peaceful and calm, with the sounds of respectful clapping, a murmur here and there, the call and response of the invigorated and engaged crowd serving as backdrop and frame for King’s stirring words and enhancing their power, just like black church during a very special sermon?

The March on Washington is not only part of our commonplace book of American history. A signal chapter, it changed the physical and spiritual landscape of our country, beginning with the signing of the Civil Rights Act of 1964 and then the Voting Rights Act of 1965 within the next 24 months. Because of them, the march and the other “searing” images of the civil rights movement — Emmett Till’s mutilated body; Bull Connor’s snarling dogs; King’s poignant “Letter From the Birmingham Jail”; the murder of our innocents, those beautiful “Four Little Girls” — there is now a black man in the White House, who, on the eve of his first inaugural, returned to the scene of the march to take in the full weight of the history he was about to make; who, in October 2011, helped dedicate a memorial to King a short walk from Lincoln’s at 1964 Independence Ave.; and who will again speak at the anniversary of the march at the Lincoln Memorial.

We are living through challenging times with a mix of pride at what we have accomplished and despair at the facts that tell us that despite the formal smashing of “the manacles of segregation,” as King called them, too many black men, women and children 50 years on from the march still dwell “on a lonely island of poverty in the midst of a vast ocean of material prosperity,” while others are “still languishing in the corners of American society” feeling like “exile[s] in [their] own land.”

As of last month, the unemployment rate among African-Americans was more than 13 percent and almost double the national average.

The same is true of the poverty rate: More than 27 percent of black Americans dwell in poverty, compared with the nation’s average, 15.

The poverty rate among African-American children is especially alarming, as it was in 1968, the year of the King assassination — both at more than 30 percent.

The black male prison population remains the highest of any demographic — 38 percent of all inmates, state and federal — despite the fact that blacks make up only 13 percent of the U.S. population.

These are hard numbers, numbers that the March on Washington explicitly sought to change, along with eradicating de jure segregation, and while we have come so far, and crossed many more rivers since then, we have so much more work to do to realize the “dream” that King so beautifully and so memorably articulated at the culmination of his speech.

The memory of the march today, like any family event, is filled with more emotions than fact: anticipation, nostalgia, reverence, worship, disappointment, exaggeration, wistfulness, poignancy and pride. It was all these things then, too, because as King evidenced in his speech, the protesters who gathered in August 1963 were very conscious of place, of where they had traveled and where they hoped to go, and of the fact that, while there had been other nonviolent mass gatherings — from Detroit, in June of that summer, where King had delivered an earlier version of his “Dream” speech, to Los Angeles — this was Washington, D.C., the capital of the nation, the perfect setting for measuring progress since emancipation.

In the distance stood monuments to George Washington and Thomas Jefferson, reminders of the “bad check” that had been written out to our enslaved ancestors at the founding and, for their descendants, was still marked “insufficient funds.”

Behind King, seated and imposing, was the Great Emancipator himself, Lincoln, whose memorial had become that “hollowed spot to remind America of the fierce urgency of now” and a reminder that in the 100 years since emancipation, little measurable progress had been made, in practical terms, for those attempting to replace the badges of slavery with the rights of citizenship; with equal access to opportunity and place; with “brotherhood,” the elimination of “police brutality” and a decent-paying, respectable job.

The March on Washington for Jobs and Freedom was the emotional summit of the civil rights movement. Fifty years on, let it inspire you, just as it did all of us who heard it live, whether on the Mall or in our living rooms.

Let it challenge you to continue pursuing the arc of change that King and his devoted followers risked and sacrificed their lives to effect, so that this generation of African-Americans and the chronically poor would be the first in our country’s long history of race relations to have equal access to the promise of America. Let it be your opportunity to “let freedom ring.”

Henry Louis Gates Jr. is the Alphonse Fletcher University Professor and the director of the W.E.B. Du Bois Institute for African and African-American Research at Harvard University. He is also the editor-in-chief of The Root, an online magazine.