Time travel is easy. The trick is finding a way to choose your route. Quirks of relativity aside, we never get to see the clock spin in any direction but forward and at a fixed and unvarying rate. (Illusions born of daylight savings time, inter–time zone travel and medication don’t count.) However much we might like to stop, divert or roll back time, our sense of it remains linear and progressive. Yesterday always recedes. Tomorrow draws closer.
Only in fiction do characters sometimes have the liberty to experience time differently. One of writer Martin Amis’s best known novels is Time’s Arrow, in which the narrator perceives the flow of time in reverse. For him, people enter the world in coffins disinterred from the soil; once animated, they grow younger and younger and eventually disappear inside their mothers’ wombs. That backward perspective turns the most hideous catastrophes into carnivals and the most joyous occasions into tragedies.
Billy Pilgrim, the hero of Kurt Vonnegut’s Slaughterhouse-Five, is doomed to live the moments of his life in random order. He hops unwillingly between childhood, Dresden during World War II, his drab middle-class existence and his captivity among the aliens of Tralfamadore. In Vonnegut’s comic vision, the chaos of 20th-century life seems senseless in every frame of reference.
In Einstein’s Dreams, by physicist Alan Lightman, a fictional Albert Einstein nightly imagines a month’s worth of variations on time: worlds where time is circular, backward, motionless, individualized, altitude-dependent, and so on. One of those visions (can you recognize it?) corresponds to the relativistic spacetime all around us.
But even physicists without literary predilections have been intrigued by time’s stubborn insistence on moving one way. Symmetric principles of physics suggest that the universe ought to run forward and backward easily. Yet entropy, the tendency of isolated systems to become more disordered, imposes a bias. Something unbalances the equation and lets entropy increase. Physicists locate that “something” in how the cosmos originated—and, by extension, how it will eventually subside into a nearly empty nothingness.
Physicist Sean M. Carroll reconciles the inherent symmetry of time with its observed asymmetry and concludes that there are parallel universes where Amis’s reversed arrow is not just a narrative device. Readers will not find a formula for reversing time in Carroll’s article, but they will surely find an intellectually enriching way of spending it.
If one could go back in time, surely many meteor buffs would visit Tunguska, Siberia, 100 years ago on June 30. That was when something exploded in midair and flattened uninhabited forest for dozens of miles. Identification of that object as a small asteroid or comet has been hindered by the failure to find any fragments or impact craters from it. Now, however, Luca Gasperini, Enrico Bonatti, Giuseppe Longo and their colleagues suspect that a small lake near the center of the explosion site may be a crater in disguise. Their report, recounts how their work peeked into the past.
This story was originally printed with the title, "Escaping From Time".