Specifically, time and time travel, one of my favorite subjects. Some highlights:
Space and time, some quantum gravity theorists say, are most likely a sort of illusion - or less sensationally, an "approximation" - doomed to be replaced by some more fundamental idea. If only they could think of what that idea is.
"By convention there is space, by convention time," Dr. David J. Gross... said recently, paraphrasing the Greek philosopher Democritus, "in reality there is. ... ?" his voice trailing off.
Looked at closely enough, with an imaginary microscope that could see lengths down to 10-33 centimeters, quantum gravity theorists say, even ordinary space and time dissolve into a boiling mess that Dr. John Wheeler, the Princeton physicist and phrasemaker, called "space-time foam." At that level of reality, which exists underneath all our fingernails, clocks and rulers as we know them cease to exist.
So far, so good. But here's where it gets really wacky:
Most scientists, including Einstein, resisted the idea of time travel until 1988 when Dr. Kip Thorne, a gravitational theorist at the California Institute of Technology, and two of his graduate students, Dr. Mike Morris and Dr. Ulvi Yurtsever, published a pair of papers concluding that the laws of physics may allow you to use wormholes, which are like tunnels through space connecting distant points, to travel in time.
These holes, technically called Einstein-Rosen bridges, have long been predicted as a solution of Einstein's equations. But physicists dismissed them because calculations predicted that gravity would slam them shut.
...Dr. Thorne and his colleagues imagined that such holes could be kept from collapsing and thus maintained to be used as a galactic subway, at least in principle, by threading them with something called Casimir energy... which is a sort of quantum suction produced when two parallel metal plates are placed very close together. According to Einstein's equations, this suction, or negative pressure, would have an antigravitational effect, keeping the walls of the wormhole apart.
If one mouth of a wormhole was then grabbed by a spaceship and taken on a high-speed trip, according to relativity, its clock would run slow compared with the other end of the wormhole. So the wormhole would become a portal between two different times as well as places.
Some mysterious "dark energy," astronomers say, is pushing space apart and accelerating the expansion of the universe. The race is on to measure this energy precisely and find out what it is.
Among the weirder and more disturbing explanations for this cosmic riddle is something called phantom energy, which is so virulently antigravitational that it would eventually rip planets, people and even atoms apart, ending everything. As it happens this bizarre stuff would also be perfect for propping open a wormhole, Dr. Lobo of Lisbon recently pointed out. "This certainly is an interesting prospect for an absurdly advanced civilization, as phantom energy probably comprises of 70 percent of the universe," Dr. Lobo wrote in an e-mail message....
...Dr. Lobo suggested that as the universe was stretched and stretched under phantom energy, microscopic holes in the quantum "space-time foam" might grow to macroscopic usable size. "One could also imagine an advanced civilization mining the cosmic fluid for phantom energy necessary to construct and sustain a traversable wormhole," he wrote.
In another recent paper, Dr. Amos Ori of the Technion-Israel Institute of Technology in Haifa describes a time machine that he claims can be built by moving around colossal masses to warp the space inside a doughnut of regular empty space into a particular configuration, something an advanced civilization may be able to do in 100 or 200 years.
The space inside the doughnut, he said, will then naturally evolve according to Einstein's laws into a time machine.
Mmm... Einsteinian time doughtnut...
Dr. Ori admits that he doesn't know if his machine would be stable. Time machines could blow up as soon as you turned them on, say some physicists, including Dr. Hawking, who has proposed what he calls the "chronology protection" conjecture to keep the past safe for historians. Random microscopic fluctuations in matter and energy and space itself, they argue, would be amplified by going around and around boundaries of the machine or the wormhole, and finally blow it up.
Dr. Gott and his colleague Dr. Li-Xin Li have shown that there are at least some cases where the time machine does not blow up. But until gravity marries quantum theory, they admit, nobody knows how to predict exactly what the fluctuations would be.
I recommend reading the full article - it's long, but it has all kinds of nifty stuff about time paradoxes and quantum physics and string theory and stuff.