"Leggett, who won the Nobel Prize in Physics in 2003 for his work in superfluidity, was in Hyderabad on February 1 to receive the Lifetime Achievement in Science Award at the B.M. Birla Science Center. Afterward, delivering his lecture "Why Can't Time Run Backwards?," Leggett made the following assertion:
Reversing the time or speeding it up is not possible... The universe is hot and compact, so it is slowly in the process of cooling down. Time, therefore, is unidirectional.
Leggett's remarks nod toward the "arrow of time" model, which holds that because entropy only occurs in one direction — i.e. things become less organized over time, not more — time can be thought of as moving one way and not another. (Pick your spatial metaphor: time flows from left to right, or front to back, or from Disneyland to Disney World, but never the other way.) There's a classic breakfast-related example used to illustrate entropy's role in the arrow of time: you can turn an egg into an omelet, but you can't turn an omelet back into an egg.
It's worth noting that Leggett's comments don't rule out the possibility of time travel, at least the kind where an individual jumps around from point to point in the timestream — it's just that we can't produce a scenario where time itself runs in a different direction or at a different rate. The question of whether time travel might be accomplished via wormholes, or some other method that introduces a shortcut between two time coordinates, is still generally thought to be an open one."
