This might be one of Fredric Brown's short-short stories, but I've tried and could not find a reasonably quick way to look up summaries of all his stories.
A group of scientists in a lab decide to experiment with backward (in time) causation. The experiment goes disastrously wrong and the world ends.
The details, as far as I remember them: A brick suddenly materializes on a lab bench. This is just as expected. An hour passes. At this point in time the scientists are supposed to press the button that will send the brick back in time. Dr. X says, "Hey, let's see what happens if we don't do it." Instantly, Dr. X, his colleagues, the lab, the city, the planet, and the entire universe vanish. Only the brick remains.