You Can't Travel Back in Time, Scientists Say
By: Sara Goudarzi / Source: Live Science
The urge to hug a departed loved one again or prevent atrocities are among the compelling reasons that keep the notion of time travel alive in the minds of many.
While the idea makes for great fiction, some scientists now say traveling to the past is impossible.
There are a handful of scenarios that theorists have suggested for how one might travel to the past, said Brian Greene, author of the bestseller, "The Elegant Universe" and a physicist at Columbia University."And almost all of them, if you look at them closely, brush up right at the edge of physics as we understand it. Most of us think that almost all of them can be ruled out."
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The fourth dimension
In physics, time is described as a dimension much like length, width, and height. When you travel from your house to the grocery store, you're traveling through a direction in space, making headway in all the spatial dimensions—length, width and height. But you're also traveling forward in time, the fourth dimension.
"Space and time are tangled together in a sort of a four-dimensional fabric calledspace-time," said Charles Liu, an astrophysicist with the City University of New York, College of Staten Island and co-author of the book "One Universe: At Home In The Cosmos."
Space-time, Liu explains, can be thought of as a piece of spandex with four dimensions. "When something that has mass—you and I, an object, a planet, or any star—sits in that piece of four-dimensional spandex, it causes it to create a dimple," he said. "That dimple is a manifestation of space-time bending to accommodate this mass."
The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.
Mathematically one can go backwards or forwards in the three spatial dimensions. But time doesn't share this multi-directional freedom.
"In this four-dimensional space-time, you're only able to move forward in time," Liu told LiveScience.Cosmic strings
Another popular theory for potential time travelers involves something called cosmic strings—narrow tubes of energy stretched across the entire length of the ever-expanding universe. These skinny regions, leftover from the early cosmos, are predicted to contain huge amounts of mass and therefore could warp the space-time around them.
Cosmic strings are either infinite or they're in loops, with no ends, said J. Richard Gott, author of "Time Travel in Einstein's Universe" and an astrophysicist at Princeton University. "So they are either like spaghetti or SpaghettiO's."
The approach of two such strings parallel to each other, said Gott, will bend space-time so vigorously and in such a particular configuration that might make time travel possible, in theory.
"This is a project that a super civilization might attempt," Gott told LiveScience. "It's far beyond what we can do. We're a civilization that's not even controlling the energy resources of our planet."
Impossible, for now Mathematically, you can certainly say something is traveling to the past, Liu said. "But it is not possible for you and me to travel backward in time," he said.
However, some scientists believe that traveling to the past is, in fact, theoretically possible, though impractical.
Maybe if there were a theory of everything, one could solve all of Einstein's equations through a wormhole, and see whether time travel is really possible, Kaku said. "But that would require a technology far more advanced than anything we can muster," he said. "Don't expect any young inventor to announce tomorrow in a press release that he or she has invented a time machine in their basement."
For now, the only definitive part of travel in the fourth dimension is that we're stepping further into the future with each passing moment. So for those hoping to see Earth a million years from now, scientists have good news.
"If you want to know what the Earth is like one million years from now, I'll tell you how to do that," said Greene, a consultant for "Déjà Vu," a recent movie that dealt with time travel. "Build a spaceship. Go near the speed of light for a length of time—that I could calculate. Come back to Earth, and when you step out of your ship you will have aged perhaps one year while the Earth would have aged one million years. You would have traveled to Earth's future."
Edited by: Lawyer Asad