In Ray Bradbury's vision, reality was a fabric so delicate that the crushing of a butterfly could ripple up through 65 million years to change the results of an election.

Bradbury painted that scenario in his 1952 story, "A Sound of Thunder." The butterfly was victim of a misstep by a big game hunter who travelled back in time to pursue the thundering prize known as tyrannosaurus Rex.

The dinosaur had been fated to die, but the insect's untimely demise had haunting consequences that confronted the hunter upon his return to his departure date of 2055. Not only did he learn that a more dictatorial candidate had won a recent election, but nothing was quite the same, including written English. A sign read: SEFARIS TU ANY YEER EN THE PAST. YU NAIM THE ANIMALL. WEE TAEK YU THAIR.YU SHOOT ITT.

When Bradbury died this month at the age of 91, more than half the time had elapsed between the writing of the story and the futuristic date in which it was set. In the intervening 60 years, physicists have reconsidered our understanding of time and the plausibility of Bradbury's classic tale.

"The story is interesting because of the whole concept of changing history, and that tiny change in the past could have enormous repercussions in the future," said physicist Paul Halpern, who discusses the story in a Nature of Time class he teaches at the University of the Sciences.
"This idea that reality is so fragile and just a very slight tweak will lead to big differences—that's connected to chaos theory," said Halpern, whose newest book "The Edge of the Universe" is coming out this fall.

The term, "The Butterfly Effect" is often connected to Bradbury's story, Halpern said, but the phrase originated with meteorologist Edward Lorenz, who proposed in the 1960s that the beat of a butterfly's wings on one side of the world would eventually cause a storm on the other. This was meant to illustrate chaos theory and the impossibility of predicting the weather more than a few days or weeks in advance.

Chaos theory is well accepted in scientific circles, at least when time goes the expected direction, but in the Bradbury story, time travel led to two contradictory versions of reality following the butterfly's death.

Physicists disagree on whether someone travelling into the past could have any impact on the unfolding of events, said Halpern. "A lot of physicists say there has to be some self-consistency." Like other time travel tales, Thunder raises some serious questions about the nature of time, such as why it has a direction at all, why it seems to be ripping along like a tidal current, and whether we can get out.

For Caltech physicist Sean Carroll, the arrow of time is defined by the irreversible progression from order to disorder, which manifests itself in such mundane events as a glass breaking, an egg scrambling or cream dispersing into coffee. If you ran a film of any of those processes in reverse, they would look improbable if not impossible.

Carroll, author of a popular book on time called From Eternity to Here, says he believes erosion of order—technically an increase in entropy—underlies the arrow of time. Entropy, he believes, separates the past from the future. But there's no widespread agreement on that idea yet.

"Does entropy increase with time or does it make time?" asked Drexel University physicist David Goldberg, author of A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty. The inexorable increase in entropy is labeled the second law of thermodynamics, but Goldberg thinks since it's statistical in nature, it's more like a really good suggestion.

Despite the way disorder is dragging us unwittingly into the future, Caltech's Carroll said there are no laws of physics that absolutely rule out a trip into the past. "If Newton had been right about space and time, then time travel would be impossible." But Einstein's theory of general relativity allow space and time to act like a fabric that can curve and loop back around on itself. Some particularly convoluted routes through space-time can, in theory, lead you to a time before you left.

But whether you can actually have any influence on events is another matter, he said, on which physics is frustratingly vague. You can't create a paradox, by, say, going back and killing one of your parents so that you could never have been born.

There is one possible loophole, the physicists say. That's the notion that if you change something in the past, you create a fork in the road of time and a new universe branches off in which the consequences of the crushed butterfly play themselves out.

That idea, said Halpern, is a consequence of what's called the "many worlds" interpretation of quantum physics—a view that allows different versions of reality to branch off like twigs on an a tree. If that's the case, you could kill or parents before you were born or otherwise prevent your birth and that would create a new branch of the universe in which you don't exist. Of course, you'd have to return to the universe in which you do exist.

Drexel's Goldberg warns that there's no evidence these other universes exist or that if they did exist, we could get there from here.
He suspects there's just one reality. When the dinosaur hunter stepped on that butterfly, the act would necessarily be part of the one and only history of time. There would always be tracks or some other sign that a time traveler had always crushed the butterfly. There would be no reality in which the unfortunate creature got to live any longer.

That was how writers of the original Terminator movie envisioned time travel, Goldberg said. The time travelling rescuer, Kyle Reese, had to save Sarah Connor so that her son would be born—and Reese also fathered the son—but all this had to happen to create the destiny that led to the existence of the time travelers in the first place. In this view, history is painted on a stiffer canvas.

"To be fair, this is a matter of belief," he said, "But it's a belief that's informed by the laws of physics that we know." In other words, these guys aren't blindly guessing, but they're out far enough on the frontier of knowledge that respectable physicists can agree to disagree.
If there's only one universe and you can't change the present from the past, then how can we change the future in the present? This raises the contentious possibility that we can't. Goldberg says this may be the way it is. Others see loopholes that would allow us to have the kind of free will we seem to be experiencing. "The right way to think about the future is that it's changeable," said Carroll.

Arguments against free will often invoke determinism: The idea that if we know where everything is, the laws of physics determine where everything is going.  Carroll says it's impossible to know exactly where every particle is at any given time, and this impossibility saves us from the fatalistic idea that our futures have already been written.

In Bradbury's story, Carroll thought it more likely that tweaking reality 65 million years ago would either have no effect, or the ripples would have grown so large that all civilization would be different, or perhaps humanity wouldn't have evolved at all. But all agree that nobody knows for sure. The questions Bradbury raised about time remain unresolved. And so A Sound of Thunder remains provocative and influential, and its impact will send ripples through time for decades to come.