Backstitching Time with Cosmic String
Cosmology is physics on the grandest scale, the study of inconceivably vast distances and unbelievably long durations. Its subject matter is the universe; its findings are brain bending.
Consider, for example, a topic now challenging cosmologists: the theoretical possibility of traveling backward through time. The debate, as it appeared in Science magazine, begins with the work of Einstein. In his theory of relativity, space and time are faces of the same thing. Einstein even wrote it as one word---spacetime. Physicists often speak of the fabric of spacetime, as if it were a kind of universal bedsheet.
Imagine that sheet, the fabric of spacetime, as the surface of a trampoline. An ant could scurry across the trampoline without even dimpling the fabric. A bowling ball, on the other hand, would roll into a Pit of its own making. That's a crude but fair way of depicting the effects of mass---otherwise known as gravity---on spacetime. By principles of relativity, the faster something moves through space, the slower its time passes. Astronauts' watches confirm that, and greater speeds would make for slower passing of time until---at the speed of light---time stops. Theoretically, if the astronauts' ship flew faster than the speed of light, the hands of the astronauts' watches would run backward. Time's direction would reverse.
Reversing time leads to all manner of paradoxes and problems; the idea has produced many enjoyable pages of science fiction, but most physicists loathe it. They prefer a universe where cause follows effect, with no chance of effect preceding cause. Einstein and his intellectual heirs were untroubled by the possibility, because the speed of light is the cosmic speed limit. Nothing can go faster.
However, Einstein also had been untroubled by another theoretical entity, the cosmic string. Well after his lifetime, astrophysicists came up with mathematical and logical evidence for the existence of these thin streams of pure energy. They are---if they exist---left over from the theoretical "Big Bang" in which the universe is said to have exploded into being.
Princeton physicist J. Richard Gott contemplated the effects of cosmic strings on spacetime, and decided they'd be prodigious. Recall (from Einstein again) that energy and mass are faces of the same thing, with their relationship fixed by the famous equation of E=mc2. The pure energy of a cosmic string translates into a mass equivalence of 40 million billion tons an inch. (No, I didn't work that out. I trusted the fact-checkers of Science magazine.) Ponderous mass puts one whale of a warp in the fabric of spacetime.
For those of you now shouting, "Warp eight, Mr. Scott!"---right, the potential distortion could be so great that a spaceship could find shortcuts thanks to the warp. It could reach a destination before a light ray following the usual route through spacetime. (Think of the ant on the hypothetical trampoline; by avoiding the dent made by the bowling ball and crossing near the still-flat edge, it would have a time-saving shortcut.)
That interesting possibility wasn't enough for Gott. He wanted to see if something could open a shortcut so effective that a traveler could go back in time. He played with the equations of general relativity, he tuned the cosmic strings, and he finally found a warp so severe it produced a potential time machine.
The requisite circumstances sound a little unusual: Gott's equations demanded two cosmic strings heading in opposite directions and zipping past one another at very near the speed of light. But unusual isn't the same as dismissible, and even the most remote possibility that time's arrow could be reversed caused an uproar among the experts. They continue to muster powerful arguments against it, but none has been definitive yet---at least, Gott isn't convinced by them.
I admit one of them convinced me. The astrophysicist Stephen Hawking said that strong evidence against time travel was, "We have not been invaded by hordes of tourists from the future."
