By Paul Halpern

Acclaimed physicist Stephen Hawking once speculated that there is a law in the universe that forbids time travel. Apparently, no one has told that to Hollywood. With summer's heat inducing audiences into air-conditioned escapes, yet another movie makes slipping into the past or future seem as effortless as walking through the cinema doors.

The Time Traveler's Wife is the latest addition to a motion-picture genre that dates at least as far back as the 1949 Bing Crosby comedy A Connecticut Yankee in King Arthur's Court, which is based on the Mark Twain novel and depicts the crooner contending with Merlin. Intended to be more serious - but hilarious in retrospect due to its cheesy special effects - George Pal's 1960 science-fiction thriller The Time Machine infuses the classic H.G. Wells novella with Cold War angst.

It would take another quarter-century, however, for the time-travel theme to hit its stride with the wonderful Back to the Future trilogy. Can anyone look at a vintage DeLorean again without remembering how Michael J. Fox's character used one to skedaddle into the doo-wop era, interact with his high-school-age parents, and tamper with the threads of time? In the end, not only did Marty McFly undo the mistakes he made out of inexperience with time travel; he also managed to achieve every teenager's dream by actually making his parents cool.

Correcting otherwise indelible errors is the time-travel aficionado's dream. Who among us hasn't taken the wrong turn at some point in his or her life and longed to press the "reset" button? In an age in which any blunder is instantly broadcast over the Internet and stored in archives forevermore, I'm sure many politicians would welcome such a chance. In parallel universes without Watergate or Monica Lewinsky, for example, Richard Nixon and Bill Clinton could enjoy reputations unsullied by scandal.

Could we ever hope to scurry through time in real life, wiping the slate clean of our most regrettable mistakes? In recent decades, many speculative schemes have popped up in the pages of physics journals. Most would require special kinds of material assembled into extraordinarily massive objects called wormholes and cosmic strings.

Wormholes are hypothetical gateways between different regions of space and time. Cosmic strings are energetic fibers spanning large sectors of the universe. Each could allow what researchers call "closed timelike loops": time twisted into pretzel-like configurations.

Given that all this is well above the pay grade of any terrestrial scientist, the prospects for this sort of time travel would hinge on the development of a civilization far more advanced than our own.

However, in a far-flung conjecture, Russian mathematical physicists Irina Aref'eva and Igor Volovich speculated that minuscule wormholes could be spawned within the energetic conditions of the Large Hadron Collider, the mammoth particle smasher beneath Switzerland and France that is scheduled to restart operations in November. Such a wormhole could allow the transfer of minute bits of information from the future to the time of the wormhole's birth.

For example, suppose a Large Hadron Collider scientist in the year 2012 wanted to send a winning lottery number back to the past. She could turn that data into a stream of particles that somehow interact with a wormhole and affect its conditions in 2009. If she had already planned out the code, the earlier version of herself could take that information, save it for three years, and then run off to a Swiss lottery agency and win the jackpot.

These wormholes would be far too tiny to accommodate a DeLorean. But, if they were real and subject to manipulation, they could be used to create paradoxes just the same.

But many physicists doubt that real-life time travel is possible. Hawking posited a universal principle called the "Chronology Protection Conjecture" that would preclude intrusions from the future. Other physicists have pointed out that the cosmic rays constantly pouring down on Earth are even more energetic than the Large Hadron Collider, so any phenomena that could be created by the device already would have popped up in that rain of radiation. So perhaps wormholes suitable for time travel are just a pipe dream.

Forbidding time travel may be well and good for the fabric of reality. But what about Hollywood?

For the sake of supporting the film industry and its part in reviving the economy, perhaps scientists should keep mum. I'm sure the governor of California, who is rumored to have seen the future in a previous incarnation, would heartily agree.

Paul Halpern is a physics professor at the University of the Sciences in Philadelphia and the author of "Collider: The Search for the World's Smallest Particles." He can be contacted at p.halper@usp.edu.