"A Sound of Thunder" is a science fiction short story by Ray Bradbury, first published in Collier's magazine in the June 28, 1952, issue and Bradbury's collection The Golden Apples of the Sun in 1953.
In the year 2055, time travel has become a practical reality, and the company Time Safari Inc. offers wealthy adventurers the chance to travel back in time to hunt extinct species such as dinosaurs. A hunter named Eckels pays $10,000 to join a hunting party that will travel back 65 million years to the late Late Cretaceous period, on a guided safari to kill a Tyrannosaurus rex. As the party waits to depart, they discuss the recent presidential elections in which an apparently fascist candidate, Deutscher, has been defeated by the more moderate Keith, to the relief of many concerned. When the party arrives in the past, Travis (the hunting guide) and Lesperance (Travis's assistant) warn Eckels and the two other hunters, Billings and Kramer, about the necessity of minimizing the events they change before they go back, since tiny alterations to the distant past could snowball into catastrophic changes in history. Travis explains that the hunters are obliged to stay on a levitating path to avoid disrupting the environment, that any deviation will be punished with hefty fines, and that prior to the hunt, Time Safari scouts had been sent back to select and tag their prey, which would have died within minutes anyway, and whose death has been calculated to have minimal effect on the future.
Although Eckels is initially excited about the hunt, when the monstrous Tyrannosaur approaches, he loses his nerve. Travis tells him he cannot leave, but Eckels panics, steps off the path and runs into the forest. Eckels hears shots, and on his return he sees that the two guides have killed the dinosaur, and shortly afterward the falling tree that would have killed the T. rex has landed on top of it. Realizing that Eckels has fallen off the path, Travis threatens to leave him in the past unless he removes the bullets from the dinosaur's body, as they cannot be left behind. Eckels obeys, but Travis remains furious, threatening on the return trip to shoot him.
Upon returning to 2055, Eckels notices subtle changes – English words are now spelled and spoken strangely, people behave differently, and Eckels discovers that Deutscher has won the election instead of Keith. Looking at the mud on his boots, Eckels finds a crushed butterfly, whose death has apparently set in motion a series of subtle changes that have affected the nature of the alternative present to which the safari has returned. He frantically pleads with Travis to take him back into the past to undo the damage, but Travis had previously explained that the time machine cannot return to any point in time that it has already visited (so as to prevent any paradoxes). Travis raises his gun, and there is "a sound of thunder".
The story was aired on The Ray Bradbury Theater on October 8, 1989.
A film adaptation starring Ben Kingsley, Edward Burns and Catherine McCormack was released in 2005. Roger Ebert stated that while he "cannot endorse it, [he] can appreciate it" as a film that is bad because it "want[s] so much to be terrific that [it] explode[s] under the strain."
A Game Boy Advance video game based on the film was also released. It was finished in time for the film's planned 2003 release, delayed along with it and ultimately released in February 2005. Planned console ports were cancelled.
The story was parodied in the Time and Punishment section of The Simpsons episode "Treehouse of Horror V".
"A Sound of Thunder" is often miscredited as the origin of the term "butterfly effect", a concept of chaos theory in which the flapping of a butterfly in one part of the world could create a hurricane on the opposite side of the globe. The term was actually introduced by meteorologist Edward Norton Lorenz in the 1960s. However, Bradbury's concept of how the death of a butterfly in the past could have drastic changes in the future is a representation of the butterfly effect, and used as an example of how to consider chaos theory and the physics of time travel.