Black and white Americans have always had vastly different experiences within their country’s justice system. You see it in so many different data sets, from police violence to incarceration and sentencing. It's impossible to understand without reference to America’s history of institutionalised racism. But understanding is one thing; the real challenge is how to change it. Stephen Sackur speaks to Bryan Stevenson, civil rights lawyer and founder of the Equal Justice Initiative. Will equality ever be more than a dream?