Igor Novikov

The Russian-born physicist Igor Novikov, an enthusiastic investigator into the subject of time travel, has suggested that the paradox doesn't apply because space-time is probably self-consistent. That is, I may be able to travel back in time and somehow become interwoven into a past of which I was already a part, but I will not be able to kill my grandfather, quite simply because I have not killed him already.

Novikov has also thought a good deal about the other time travel conundrum--the "bootstrap paradox." Suppose I travel to 2009, find a design for a zero-emission automobile engine and return with it to 2008 and patent it. Suppose further that the patent is developed into the design that I find in 2009.

The obvious question: Who would have invented the zero-emission engine? The answer is, no one would have invented it. The design would have been generated quite literally from nothing, courtesy of a time machine and (perhaps) a skirting of some yet-to-be-written intellectual property laws.

British physicist David Deutsch, invoking the "many-universe" interpretation of quantum mechanics, believes that "pastward" time travel would require travel to another, parallel universe--one in which I could kill my grandfather and in which I (therefore) would never be born. Via a time machine, I would have removed myself from this universe to take up residence in that one.

The idea has some interesting implications. Deutsch has suggested that one reason we have detected no extraterrestrial civilizations may be that, using time machines, they have left this universe, preferring to live in another.

Metaphysical and philosophical questions aside, exactly how realistic is the physics of pastward time travel? Each of the several schemes for making a time machine creates a region in which pastward time travel is possible and separates it from a region in which time travel is impossible. The boundary between these regions, the "chronology horizon," has remained a mystery, in part because its nature depends upon the characteristics of space-time on the smallest possible scales.

We have at best a dim understanding of these scales, and we will not have a real understanding until we have developed a full theory of quantum gravity. This is the holy grail of theoretical physics: the so-called "theory of everything" that would eliminate disparities between relativity (which explains nature on very large scales, where gravity becomes important) and quantum mechanics (which explains nature on very small scales, where quantum effects become important).

Some physicists think the theory of everything is 10 years away; others suspect it is a good deal further off. For the moment, then, the question of whether time travel is possible has been put on hold.

The recent (and, no doubt, temporary) decline of interest in traveling to the past is welcomed by physicists who argue that work in less fanciful areas might yield a greater intellectual profit. New Zealand physicist Matt Visser, himself the architect of a number of theoretical time machines, calls that attitude overly cautious and "boring."

More than two decades after Thorne's seminal work, we still don't know whether time travel is possible. But one thing is certain: Even as an idea, it's anything but boring.

David Toomey is an assistant professor of English at the University of Massachusetts Amherst. His most recent book is The New Time Travelers: A Journey to the Frontiers of Physics .