In a dream I had somehow traveled back in time a couple of years and was wandering about one of the towers of the World Trade Center. It may have been early morning; the building was deserted or nearly so, and most of the halls and rooms were dark, but gray, misty light came in through the windows.
I was on a vaguely defined research mission. The notion of changing history never entered my mind. I became increasingly nervous as I moved about, wondering about structural failure, looking for weaknesses in the walls, imagining the whole tower buckling and coming down on me, and thinking to myself: It won't happen today; it can't happen today. You already know the exact day and hour.
In mounting, irrational terror I took the high-speed elevator down to the ground-floor lobby and rode out the dream lying flat on my back on a bench down there, thinking only of the unimaginable deadly mass of the entire tower suspended above me, and the infinitely greater, deadlier bulk of unchangeable fate hanging above me in spacetime.
And awoke, to only the coldest of reassurances: There is no such thing as a time machine.